Field and laboratory investigation of the durability ...

Field and Laboratory Investigation of the

Durability Performance of Geopolymer

Concrete

By

Kirubajiny Pasupathy

A thesis submitted in total fulfilment of the requirements for the

degree of

Doctor of Philosophy

Faculty of Science, Engineering and Technology

Swinburne University of Technology

Melbourne, Australia

2018

iii

Abstract

The use of geopolymer concrete (GPC) in construction industry has been extensively

investigated in recent decades, owing to the inherent merits of GPC such as reduced CO2

emissions to the environment and increased use of industrial wastes. Traditionally,

ordinary Portland cement (OPC) is used for concrete manufacturing, however, this

results in high embodied energy of production and hence, large amounts of CO2

emission to the environment. GPC uses supplementary cementitious materials

including, fly ash and ground granulated blast furnace slag (GGBFS), which are derived

as industrial waste by-products and the production of GPC shows very less emission to

the environment. GPC behaves similar to OPC concrete, and it possesses similar or

higher engineering and durability properties compared to OPC concrete.

Despite many decades of research on GPC, the widespread application of GPC in the

construction industry is limited. The major challenges of GPC relate to the wide range

of source materials to choose, lack of long-term durability studies and inadequate

standard methods of practice to assess the performance. Furthermore, the long-term

durability of concrete structures exposed to field environment, particularly in aggressive

environments including marine and saline environments, is crucial for the real-world

application of GPC. While the previous studies revealing a superior durability

performance of GPC compared to OPC, they were mainly conducted using accelerated

testing methods in laboratory-controlled environment with little or no relevance to field

environment properties. Therefore, this research study aims to assess the long-term

durability of GPC exposed to various field environments.

Experimental investigations were conducted on the concrete core specimens extracted

from GPC structures exposed to different environmental conditions and compared with

the OPC concrete structures in the same environment. The durability performance of

concrete structures exposed to the normal atmospheric environment was assessed by

studying the carbonation properties of core specimens extracted from these structures.

The tests were conducted on three different GPC structures having the different

constitution of fly ash and slag binder materials, aged at eight years. The results showed

that the carbonation resistance of GPC is lower than OPC concrete in the atmospheric-

iv

exposed environment. The formation of soluble carbonation products such as sodium

carbonate (Na2CO3) and potassium carbonate (K2CO3) in fly ash based GPC has found

to be washed out from the concrete surface with the contact of water, causing higher

porosity after the carbonation. The increased porosity of concrete with the washout of

carbonation products further exacerbated the carbonation diffusion and significantly

affecting the durability of fly ash based GPC structures. However, the fly ash-slag based

GPC showed higher resistance to carbonation compared to fly ash based GPC due to the

formation of insoluble CaCO3 products with the use of slag in such GPCs.

On the other hand, the durability performance of GPC exposed to aggressive

environment has also been evaluated. The combined effect of carbonation, chloride

penetration and sulphate attack were investigated on concrete core specimens prepared

using fly ash based GPC structure, and slag-fly ash blended GPC structure exposed to

the saline, marine environment for 6 years and 4 years, respectively. The test results

showed that the durability of GPC is lower than OPC concrete in the marine exposure

conditions. This was particularly elaborated by reduced resistance to carbonation,

chloride diffusion and sulphate attack of GPC, compared to OPC concrete.

Apart from the durability assessment on field exposed concrete structures, the testings

were also conducted on laboratory prepared specimens, subjected to accelerated testing

methods, to validate the test result obtained from field experiments. The geopolymer

mortar specimens prepared with different mix proportion of fly ash and slag were

subjected to accelerated wetting-drying analysis in different solution such as water,

chloride solution and, the combination of chloride and the sulphate solutions. The

compressive strength of the concrete specimens exposed to the accelerated environment

was measured with age. The test results suggested that the degradation in the GPC

specimens is higher than OPC mortar. The loss of compressive strength was, however,

found to be low with the increasing level of slag in the GPC. Furthermore, the corrosion

of rebar in the GPC has been examined when concrete specimens containing rebar

subjected to a concentrated CO2 environment of 1%. The influence of source materials of

GPC on the corrosion behaviour was studied. The test results showed that the corrosion

of rebar in fly ash based GPC was higher than the OPC specimens and, the corrosion rate

is reduced with the incorporation of slag in GPC. Also, the correlation of carbonation

rate with different source materials of GPC was studied with the development of

v

carbonation coefficient models according to diffusion equation based on the Fick’s law

and the use of empirical equations.

Overall, it was found that the use of slag in GPC well enhances the durability

performance, compared to fly ash based GPC in atmospheric and aggressive

environments. Under the same activation and mix proportional conditions, high slag

content leads to better durability performance of the resulting geopolymer concrete.

However, the durability of GPC is still lower than traditional OPC based concrete.

Therefore, investigation of suitable GPC chemistry and mix design is required to

enhance the durability of GPC for real-world applications.

vi

Declaration

I hereby certify that this thesis entitled “Field and Laboratory Investigation of the

Durability Performance of Geopolymer Concrete” contains no material which has been

accepted for the award to the candidate of any other degree or diploma, except where

due reference is made in the text of the examinable outcome. To the best of my

knowledge, the thesis contains no material previously published or written by another

person except where due reference is made in the text of the examinable outcome. Where

the work is based on joint research or publications, discloses the relative contributions

of the respective workers or authors.

_____________________

Kirubajiny Pasupathy

April 2018

vii

List of Publications

Published and Submitted Journal papers

1. Pasupathy, K., M. Berndt, J. Sanjayan, P. Rajeev and D.S. Cheema, “Durability

Performance of Precast Fly Ash–Based Geopolymer Concrete under Atmospheric Exposure

Conditions.” Journal of Materials in Civil Engineering, 2018. 30(3): p. 04018007.

2. Pasupathy, K., M. Berndt, J. Sanjayan, P. Rajeev and D.S. Cheema, “Durability of low

‑calcium fly ash based geopolymer concrete culvert in a saline environment.” Cement and

Concrete Research, 2017. 100: p. 297-310.

3. Pasupathy, K., M. Berndt, A. Castel, J. Sanjayan and R. Pathmanathan, “Carbonation

of a blended slag-fly ash geopolymer concrete in field conditions after 8 years.” Construction

and Building Materials, 2016. 125: p. 661-669.

4. Pasupathy, K., M. Berndt, J. Sanjayan and R. Pathmanathan, “Durability Performance

of Concrete Structures Built with Low Carbon Construction Materials.” Energy Procedia,

2016. 88: p. 794-799.

5. Pasupathy, K., M. Berndt, J. Sanjayan, DW. Law and R. Pathmanathan, “The effect of

chloride penetration on slag-fly ash blended geopolymer concrete in marine environment”

Submitted to Construction and Building Materials.

Journal papers under preparation

1. Pasupathy, K., M. Berndt, J. Sanjayan, and P. Rajeev, “influence of the source materials

on the alkali leaching and the degradation behavior of geopolymer binder in accelerated

wetting-drying environment.”

2. Pasupathy, K., P. Rajeev, J. Sanjayan, and M. Berndt, “Mathematical modelling to

determine the CO2 diffusion of geopolymer concrete”

Conference proceedings

1. Pasupathy, K., M. Berndt, J. Sanjayan and R. Pathmanathan, “Durability Performance

of Concrete Structures Built with Low Carbon Construction Materials”, Applied Energy

Symposium and Summit 2015 (CUE2015), Nov 15-17, 2015, Fuzhou, China.

2. Pasupathy, K., M. Berndt, J. Sanjayan, P. Rajeev and D.S. Cheema, “Evaluation of

Chloride Penetration into Geopolymer Concrete under Marine Environment” Concrete

2017 conference, Adelaide, Australia, 2017.

viii

3. Pasupathy, K., M. Berndt, J. Sanjayan and R. Pathmanathan, “Geopolymer Concrete-

Green Technology- Review”, 8th International Conference on Structural Engineering

and Construction Management 2017, Sri Lanka, 2017.

4. Pasupathy, K., R. Pathmanathan K., J. Sanjayan and M. Berndt, “Mathematical approach

to determine the CO2 diffusion coefficient of geopolymer concrete.” Will be presented in

The International Federation for Structural Concrete 5th International fib Congress,

2018.

ix

Acknowledgement

It would not have been possible to complete this PhD study without the guidance, help

and support of many people around me. I would like to express my sincere gratitude to

them for their support over the past 3 ½ years of my PhD travel.

I would like to thank and appreciate my supervisors for their patient guidance,

enthusiastic encouragement and useful critiques of this research work. Firstly, I wish

like to express my thank and appreciate to Prof Jay Sanjayan, for his valuable guidance,

advise and support to complete my research study on the prepared schedule. This thesis

would not be finished without his valuable supervision and assistance. Secondly, I

would like to extend my thanks to A/Prof Marita Berndt for her constant guidance as

well as for providing necessary information and the direction to complete my work. I

appreciate her support during the field testing period. I extend my sincere thanks to Dr

Pathmanathan Rajeev for all the advice, ideas, and moral support to progress well in this

research work

I would also like to thank the technical staffs in the smart structural laboratory,

Chemistry Laboratory and material research laboratory from Swinburne University.

Particularly, Michael Culton, Kia Rasekhi, Sanjeet Chandra, Kevin Nievaart and Firas

Al-Akeedi for providing the technical assistance to complete my experimental works

within the time frame. I also specially thank Dr James Wang and Savithri Galappathie

for access and support to use SEM, XRD and FT-IR equipment.

I would like to express my sincere thanks to A/Prof Arnaud Castel from University of

New South Wales, Australia, and Dr David Law from RMIT University, Australia for

their incredible advices, and feedbacks during my PhD programme. Their help and

support provide a path to complete my PhD journey in an easy way. I also thankfully

appreciate Dr D. S. Cheema Didar for his valuable help to conduct the field test in Perth,

Western Australia.

I am grateful to CRC for Low Carbon Living Ltd. (RP1020) supported by the Cooperative

Research Centres program, an Australian Government initiative and Swinburne

x

University of Technology for the financial support provided me throughout the 3 1/2

years of my PhD study period.

I would also like to acknowledge the support of Zeobond Pty Ltd and Antonello Precast

Concrete Pty Ltd for allowing access to the field test site in Campbellfield, Australia. I

wish to thank Main Road Western Australia for their support on field study assistance

in Perth, Australia. Furthermore, I gratefully acknowledge the Port of Portland for

providing a site for specimens for marine exposure in Portland, Australia

I would also be thankful to all staff, PhD students and friends at Swinburne University

for their encouragement, support and friendship, during my PhD journey.

Finally, I wish to thank my lovely family for their ongoing support and encouragement

to complete my PhD study. I am also greatly thankful to my loving husband Sayan for

his patience, encouragement and support throughout this PhD period. I would not be

able to complete my PhD thesis without his help and support during the field

investigations, experimental works and thesis writing.

xi

Table of Contents

Abstract ................................................................................................................................. iii

Declaration ............................................................................................................................ vi

List of Publications .............................................................................................................. vii

Acknowledgement ............................................................................................................... ix

Table of Contents ................................................................................................................. xi

List of Tables ....................................................................................................................... xvi

List of Figures .................................................................................................................... xvii

Chapter 1 Introduction .......................................................................................................... 1

1.1 Background ............................................................................................................ 1

1.2 Problem statement ................................................................................................. 3

1.3 Aim and objectives................................................................................................. 4

1.4 Significance of the research ................................................................................... 5

1.5 Thesis outline ......................................................................................................... 5

Chapter 2 Literature review .................................................................................................. 8

2.1 Introduction ............................................................................................................ 8

2.2 Overview of geopolymer....................................................................................... 8

2.3 Geopolymerisation mechanism and sources of geopolymer binder ................. 9

2.3.1 Geopolymerisation mechanism..................................................................... 9

2.3.2 The precursor of geopolymer binder ...........................................................12

2.3.3 Activators .......................................................................................................14

2.3.4 Curing method ..............................................................................................15

2.4 Mechanical properties of hardened geopolymer concrete.................................17

2.5 Permeation properties of hardened geopolymer concrete ................................20

2.6 The durability of geopolymer concrete ...............................................................22

2.6.1 Efflorescence and leaching of geopolymer ..................................................23

2.6.2 Carbonation resistance of geopolymer concrete .........................................27

2.6.3 Chloride penetration .....................................................................................31

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2.6.4 Sulphate attack .............................................................................................. 35

2.7 Porosity and pore structure of geopolymer concrete......................................... 37

2.8 Corrosion of reinforcement .................................................................................. 41

2.9 Studies related to durability of geopolymer concrete structures in field

environments .................................................................................................................... 46

2.10 Current application of geopolymer concrete in the construction field............. 48

2.11 Motivation for the study ...................................................................................... 51

Chapter 3 The durability of geopolymer concrete exposed to the atmospheric

environment ......................................................................................................................... 54

3.1 Introduction .......................................................................................................... 54

3.2 Field Description .................................................................................................. 54

3.2.1 Description of concrete structures ............................................................... 54

3.2.2 Mix design of concrete structures ................................................................ 56

3.3 Testing methods ................................................................................................... 58

3.3.1 Carbonation depth measurement ................................................................ 58

3.3.2 pH profile measurement .............................................................................. 58

3.3.3 Water absorption (Ai) and apparent volume of permeable voids (AVPV)

59

3.3.4 Sorptivity analysis ........................................................................................ 61

3.3.5 Fourier Transform Infra-red (FT-IR) analysis ............................................. 62

3.3.6 Mercury intrusion porosimetry (MIP) test .................................................. 63

3.4 Test results and discussions ................................................................................. 64

3.4.1 Carbonation resistance of geopolymer concrete ......................................... 64

3.4.2 pH profile measurement .............................................................................. 69

3.4.3 Volume of permeable void test results ........................................................ 71

3.4.4 Sorptivity analysis test results ..................................................................... 73

3.4.5 FT-IR analysis ................................................................................................ 77

3.4.6 Mercury Intrusion Porosimetry (MIP) analysis .......................................... 82

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3.4.7 Corrosion of reinforcement in fly ash based geopolymer concrete ...........88

3.5 Concluding remarks .............................................................................................89

Chapter 4 The durability of geopolymer concrete exposed to an aggressive environment

92

4.1 Introduction ...........................................................................................................92

4.2 Field Investigation ................................................................................................93

4.2.1 Description of concrete structures, exposure condition and mix details ..93

4.3 Testing methods ....................................................................................................96

4.3.1 Carbonation depth measurement ................................................................96

4.3.2 Chloride penetration measurements ...........................................................96

4.3.3 Sulphate content measurements ..................................................................97

4.3.4 pH profile measurement ...............................................................................98

4.3.5 Sorptivity analysis .........................................................................................98

4.3.6 Fourier Transform Infra-red (FT-IR) analysis .............................................98

4.3.7 Mercury intrusion porosimetry (MIP) test ..................................................99

4.3.8 Scanning Electron microscopy (SEM) and Energy dispersive X-ray (EDX)

analysis 99

4.4 Test results and discussions ............................................................................... 100

4.4.1 Carbonation resistance of geopolymer concrete in an aggressive

environment ................................................................................................................ 100

4.4.2 Chloride penetration ................................................................................... 104

4.4.3 Sulphate attack in an aggressive environment ......................................... 113

4.4.4 Scaling effect of geopolymer concrete ....................................................... 115

4.4.5 pH profile measurement ............................................................................. 117

4.4.6 Test results from the sorptivity analysis .................................................... 120

4.4.7 FT-IR analysis .............................................................................................. 122

4.4.8 Pore size distribution analysis with MIP test ............................................ 126

4.4.9 Microstructural analysis by SEM/EDX method ....................................... 130

xiv

4.4.10 Corrosion of reinforcement in fly ash based geopolymer concrete ........ 135

4.5 Concluding remarks ........................................................................................... 139

Chapter 5 Study on alkali leaching, wet and dry cyclic resistance of geopolymer....... 141

5.1 Introduction ........................................................................................................ 141

5.2 Materials and Methods ...................................................................................... 141

5.2.1 Materials ...................................................................................................... 141

5.2.2 Testing methods .......................................................................................... 143

5.3 Results and discussions ...................................................................................... 145

5.3.1 Alkali leaching test...................................................................................... 145

5.3.2 Concrete Resistance in Wetting-drying Cycles in water .......................... 149

5.3.3 Concrete Resistance in Wetting-drying Cycles in chloride solution ....... 153

5.3.4 Concrete Resistance in wetting-drying Cycles in a chloride+ sulphate

solution 156


Chapter 6 Accelerated carbonation and corrosion test on geopolymer concrete ......... 161

6.1 Introduction ........................................................................................................ 161

6.2 Materials and methods ....................................................................................... 161

6.2.1 Materials ...................................................................................................... 161

6.2.2 Testing methods .......................................................................................... 164


6.3.1 Carbonation depth measurement by universal and phenolphthalein

indicators .................................................................................................................... 168

6.3.2 Carbonation depth measurement of field-exposed fly ash based

geopolymer concrete samples by universal solution .............................................. 175

6.3.3 Evaluation of corrosion of reinforcement ................................................. 176


Chapter 7 Mathematical models for carbonation of geopolymer concrete ................... 187

7.1 Introduction ........................................................................................................ 187

xv

7.2 Materials and methods ....................................................................................... 187

7.2.1 Materials ...................................................................................................... 187

7.2.2 Testing methods .......................................................................................... 188

7.2.3 Mathematical approach on the carbonation of geopolymer concrete ..... 189


7.3.1 Compressive strength test results .............................................................. 191

7.3.2 Accelerated carbonation test results in 1% CO2 environment ................. 193

7.3.3 Carbonation model with diffusion theory and mathematical approach 194


Chapter 8 Conclusions and Recommendations ............................................................... 198

8.1 Summary ............................................................................................................. 198


8.3 Recommendations for future work ................................................................... 202

References ........................................................................................................................... 204

xvi

List of Tables

Table 2-1 pH values of geopolymer mortar specimens in carbonation environment [111]. ....... 30

Table 2-2 IUPAC pore size classification [139] ................................................................................ 38

Table 3-1 Mix compositions of concrete (kg/m3) [18, 167] ............................................................. 57

Table 3-2 Mix composition details of fly ash-slag blended geopolymer concrete ......................... 58

Table 3-3 K values obtained for the OPC concrete and the two geopolymer concretes (in

mm/yr0.5) ........................................................................................................................................... 68

Table 3-4 Water absorption and AVPV values ................................................................................ 72

Table 3-5 Water absorption and AVPV values ................................................................................ 73

Table 3-6 Initial rate of water absorption of FGPC- A and OPC-A concretes ............................... 75

Table 3-7 Initial rate of water absorption of FGPC- A and OPC-A concretes ............................... 77

Table 3-8 Porosity and pore size distribution of atmospheric exposed concrete specimens ........ 84

Table 3-9 The pore characteristics details of both types of geopolymer concrete specimens ....... 88

Table 4-1 Mix compositions of concrete (kg/m3) [18, 167] ............................................................. 94

Table 4-2 Mix proportions of OPC-M concrete ............................................................................... 95

Table 4-3 Carbonation depth measurement of OPC-S specimens ................................................ 102

Table 4-4 Apparent diffusion coefficient and surface chloride content values .......................... 108

Table 4-5 Chloride diffusion coefficient and surface chloride content of concrete. .................... 113

Table 4-6 Coefficients of sorptivity values ..................................................................................... 121

Table 4-7 Coefficients of sorptivity values ..................................................................................... 122

Table 4-8 Pore size percentages (based on IUPAC classification) ................................................ 127

Table 4-9 The pore characteristics details of both types of concrete specimens. ......................... 130

Table 5-1 Leaching ions from the mortar specimens in de-ionised solution ............................... 149

Table 6-1 Mix composition details of geopolymer and OPC concrete mixes .............................. 162

Table 6-2 The relationship between resistivity and risk of corrosion [221]. ................................ 167

Table 6-3 The relationship between resistivity and corrosion rate .............................................. 167

Table 7-1 Mix compositions of concrete (kg/m3) .......................................................................... 188

xvii

List of Figures

Fig. 2-1 Conceptual model of geopolymer reaction [30] .................................................................10

Fig. 2-2 Conceptual model of geopolymer mechanism [12] ............................................................11

Fig. 2-3 (a) Typical appearance of the geopolymer from fly ash, (b) Detailed character of the

geopolymer [31] .................................................................................................................................11

Fig. 2-4 SEM micrograph of efflorescence crystals and associated X-ray spectrum (backscattered

electron image, 20 kV, the sample is not coated) [97]. .....................................................................24

Fig. 2-5 XRD patterns of efflorescence from ambient temperature cured samples [97]. ...............25

Fig. 2-6 Efflorescence on the surfaces of geopolymer samples with different proportion of fly ash,

slag content, activator type and different curing regimes [100]......................................................26

Fig. 2-7 Carbonation depth measurements of concretes after 1000 h of exposure to a 1%

CO2 environment [89]. .......................................................................................................................29

Fig. 2-8 Chloride penetration depth of concrete specimens at the end of the Nord Test (A)

100 wt.% slag, (B) 75 wt.% slag/25 wt.% fly ash, (C) 50 wt.% slag/50 wt.% fly ash, (D) OPC [22]

.............................................................................................................................................................33

Fig. 2-9 Chloride profiles of geopolymer and OPC concrete after 5 weeks of immersion test [29]

.............................................................................................................................................................34

Fig. 2-10 Geopolymer mortar after the 1.5-year exposure to the solutions of NaCl and MgSO4 salts

[134]. ...................................................................................................................................................37

Fig. 2-11 Cumulative pore size distribution of geopolymer and OPC concrete [140] ..................39

Fig. 2-12 Pore volume distributions of geopolymer paste [141].....................................................40

Fig. 2-13 Relationship between porosity and curing duration for fly ash-slag geopolymer systems

[142] ....................................................................................................................................................41

Fig. 2-14 Corrosion of reinforcement bar in (a) fly ash based geopolymer concrete, (b) OPC

concrete [115] .....................................................................................................................................43

Fig. 2-15 Photographs of geopolymer concretes/ steel interfaces (A), (B) low Ca fly ash based

geopolymer (C) high Ca fly ash based geopolymer after 450 days of accelerated carbonation. [90]

.............................................................................................................................................................45

Fig. 2-16 Photographs of steel rebar in (A) high-Ca fly ash (B), (C) and low-Ca fly ash based

geopolymer after 450 days of accelerated carbonation. [90] ...........................................................45

Fig. 2-17 Half-cell potential measurement with age of concrete in real field environment [157] .47

Fig. 2-18 (a) casting of the precast beam, (b) installation of precast geopolymer beams [163] .....50

xviii

Fig. 2-19 (a) geopolymer concrete footpath (25 MPa) along Westgate Freeway extension Port

Melbourne, Australia, (b) 55 MPa precast panels across Salmon Street bridge, Port Melbourne,

Australia [166]. .................................................................................................................................. 51

Fig. 3-1 (a) FGPC-A box culvert, (b) OPC-A concrete box culvert ................................................. 55

Fig. 3-2 Fly ash-slag blended geopolymer concrete structures in the atmospheric exposed

environment ...................................................................................................................................... 56

Fig. 3-3 Aqua pH meter ..................................................................................................................... 59

Fig. 3-4 Water absorption and AVPV test: (a) oven dried samples, (b) water immersed samples,

(c) boiled samples, (d) suspended weight measurement of samples. ............................................ 61

Fig. 3-5 (a) schematic diagrams of the sorptivity test, (b) experimental arrangement of the

sorptivity test ..................................................................................................................................... 62

Fig. 3-6 FT-IR spectroscope ............................................................................................................... 63

Fig. 3-7 Mercury porosimeter ........................................................................................................... 64

Fig.3-8 Carbonation depth measurements of core specimens using a phenolphthalein indicator

(a) FGBC-A, (b) OPC-A concrete ...................................................................................................... 66

Fig.3-9 Carbonation depth measurements of core specimens (a) & (b) FSGPC-1 specimen before

and after applying phenolphthalein, (c)&(d) FSGPC-2 specimen before and after applying

phenolphthalein ................................................................................................................................ 67

Fig.3-10 pH variation with depth of concrete from the exposed surface (8 years old) ................. 70

Fig.3-11 pH value versus depth in fly ash-slag blended geopolymer concrete from the surface 71

Fig.3-12 Sorptivity curves of FGPC-A and OPC-A concretes ......................................................... 74

Fig.3-13 Sorptivity curves of fly ash- slag blended geopolymer concretes .................................... 76

Fig. 3-14 FT-IR Spectra of FGPC-A concrete .................................................................................... 78

Fig. 3-15 FT-IR Spectra for -A concrete ............................................................................................ 79

Fig.3-16 FTIR spectrum of FGPC-A concrete samples (Field and Laboratory) ............................. 80

Fig.3-17 FTIR spectra for both type specimens (Top layer) ............................................................ 81

Fig. 3-18 The cumulative intrusion of atmospheric exposed concrete specimens ........................ 83

Fig. 3-19 Differential pore size distribution obtained for atmospheric exposed concrete specimens

............................................................................................................................................................ 83

Fig.3-20 MIP test results for laboratory carbonated FGPC-A concrete .......................................... 85

Fig.3-21 Cumulative intrusion of atmospheric exposed fly ash- slag blended geopolymer concrete

specimens ........................................................................................................................................... 86

Fig.3-22 Differential pore size distribution obtained for both types of geopolymer concrete ...... 88

Fig.3-23 Corrosion of embedded steel bars at a cover depth of 45 mm in FGPC-A and OPC-A

concrete at 8 years exposure in an atmospheric environment ....................................................... 89

Fig.4-1 Concrete culvert structures exposed to the saline environment (a) FGPC-S culvert, (b)

OPC-S culvert. ................................................................................................................................... 94

xix

Fig.4-2 (a) SFGPC-M and OPC-M concrete blocks in the marine environment, (b) concrete coring

work after marine exposure. .............................................................................................................95

Fig. 4-3 profile grinder to obtain the powder samples from core specimens .................................97

Fig. 4-4 SEM test (a) Cutting of concrete specimen into thin slice using a precision diamond saw,

(b) Scanning Electron Microscopy equipment. ................................................................................99

Fig.4-5 Carbonation depth measurements of core specimens using a phenolphthalein indicator

(a) FGPC-S specimen before applying phenolphthalein, (b) FGPC-S specimen after applying

phenolphthalein, (c) OPC-S specimen before applying phenolphthalein, (d) OPC-S specimen after

applying phenolphthalein ............................................................................................................... 101

Fig. 4-6 Carbonation depth measurements of core specimens by phenolphthalein solution, (a)

SFGPC-M specimen before applying phenolphthalein, (b) SFGPC-M specimen after applying

phenolphthalein, (c) OPC-M specimen before applying phenolphthalein, (d) OPC-M specimen

after applying phenolphthalein ...................................................................................................... 104

Fig. 4-7 Carbonation depth values after 02 years and 04 years of exposure. ............................... 104

Fig.4-8 Chloride penetration depth measurements of core specimens using an AgNO3 solution (a)

FGPC-S specimen before applying AgNO3, (b) FGPC-S specimen after applying AgNO3, (c) OPC-

S specimen before applying AgNO3, (d) OPC-S specimen after applying AgNO3 ..................... 105

Fig.4-9 Free chloride variation with depth values from the saline environment......................... 106

Fig.4-10 Total chloride variation with depth values from the saline environment. .................... 107

Fig.4-11 Chloride penetration depth measurements of core specimens using an AgNO3 solution

(a) SFGPC-M specimen before applying AgNO3, (b) SFGPC-M specimen after applying AgNO3,

(c) OPC-M specimen before applying AgNO3, (d) OPC-M specimen after applying AgNO3 .... 110

Fig. 4-12 Free chloride profiles of both SFGPC-M after 4 year of exposure and OPC-M concrete

after 04 years of exposed time in marine environment. ................................................................ 111

Fig. 4-13 Total chloride profiles of SFGPC-M and OPC-M concrete after 04 years and 06 years of

exposure in the marine environment ............................................................................................. 112

Fig.4-14 Sulphate concentration versus depth for FGPC-S and OPC-S concrete ......................... 114

Fig.4-15 Sulphate concentration versus depth for FGPC-S and OPC-S concrete ......................... 115

Fig.4-16 Visual appearance of concrete structure (a) FGPC-S concrete culvert, (b) the outer surface

of FGPC-S showing exposed aggregate, (c) OPC-S concrete culvert, (d) the outer surface of OPC-

S with no visual evidence of deterioration ..................................................................................... 116

Fig. 4-17( a) appearance of the SFGPC-M concrete surface after four-year, (b) appearance of the

OPC-M concrete surface after six-year. .......................................................................................... 117

Fig.4-18 pH variation with depth for FGPC-S and OPC-S concrete ............................................. 118

Fig. 4-19 pH variation with depth of the concrete from the exposed surface .............................. 119

Fig.4-20 Capillary absorption of FGPC-S and OPC-S concrete ..................................................... 120

Fig. 4-21 Test results from the sorptivity analysis ......................................................................... 121

xx

Fig.4-22 FTIR spectrum of FGPC-S concrete samples with various depth intervals................... 123

Fig.4-23 FTIR spectrum of OPC-S concrete samples with various depth intervals..................... 124

Fig.4-24 FTIR spectrum of (a) SFGPC-M concrete samples, (b) OPC-M concrete with various

depth intervals ................................................................................................................................. 125

Fig. 4-25 Cumulative intrusions of aggressive exposed FGPC-S and OPC-S concrete specimens.

.......................................................................................................................................................... 126

Fig. 4-26 Differential pore size distribution obtained for aggressive exposed concrete specimens

.......................................................................................................................................................... 127

Fig. 4-27 Cumulative pore size distribution obtained for both types of concrete at the surface level

and the mid-depth level. ................................................................................................................. 129

Fig. 4-28 Differential pore size distribution obtained for both types of concrete at the surface level

and the mid-depth level. ................................................................................................................. 130

Fig.4-29 SEM micrograph of (a) Top, (b) Middle and (c) Bottom part of FGPC-S concrete core

specimens with corresponding EDX analysis ............................................................................... 132

Fig.4-30 SEM micrograph of chloride deposit on FGPC-S concrete specimens with corresponding

EDX analysis .................................................................................................................................... 132

Fig.4-31 SEM micrograph of (a) Top, (b) Middle and (c) Bottom part of OPC-S concrete core

specimens with corresponding EDX analysis ............................................................................... 133

Fig.4-32 SEM micrograph of (a) FGPC-S, (b) OPC-S concrete specimens .................................... 134

Fig.4-33 SEM micrograph of SFGPC-M and OPC-M concrete specimens (0-3 mm depth) with

corresponding EDX analysis .......................................................................................................... 135

Fig.4-34 SEM micrograph of (a) SFGPC-M, (b) OPC-M concrete specimens .............................. 135

Fig.4-35 Rebar interface and reinforcement bar after six years of exposure, (a) typical rebar

interface of FGPC-S concrete specimen, (b) reinforcement bar in leg part of FGPC-S culvert, (d)

typical rebar interface of OPC-S concrete specimen, (e) reinforcement bar in the leg part of OPC-

S culvert ........................................................................................................................................... 137

Fig.4-36 SEM micrograph of (a) FGPC-S; (b) OPC-S at the rebar/matrix interface with

corresponding EDX analysis .......................................................................................................... 138

Fig. 5-1 Mortar mixer (Hobart mixer) ............................................................................................ 143

Fig. 5-2 ICP Spectrometer ............................................................................................................... 143

Fig. 5-3 Compressive strength testing equipment ......................................................................... 144

Fig. 5-4 pH of the solutions after continues immersion of the mortar samples .......................... 146

Fig. 5-5 visual observation of mortar sample after 1 week of immersion, (a) M1, (b) M2, (c) M3, (d)

M4, and (e) CT .................................................................................................................................. 147

Fig. 5-6 Visual observation of solutions after 1 week of immersion, (a) M1, (b) M2, (c) M3, (d) M4,

and (e) CT ........................................................................................................................................ 148

xxi

Fig. 5-7 Visual observations of the mortar specimens after subjected to wetting-drying cycles in

water for 6 months period, (a) M1, (b)M2, (c)M3, (d) M4, and (e) CT ............................................ 150

Fig. 5-8 The weight changes of the mortar specimens after subjected to wetting-drying cycles in

water for 6 months period. .............................................................................................................. 151

Fig. 5-9 28 days Compressive strength of the mortar specimens .................................................. 152

Fig. 5-10 Compressive strength loss of the mortar specimens after subjected to wetting-drying

cycles in water .................................................................................................................................. 153

Fig. 5-11 Visual observations of the mortar specimens subjected to wetting-drying cycles in 3% of

NaCl solutions for 6 months period, (a) M1, (b) M2, (c) M3, (d) M4, and (e) CT ........................... 154

Fig. 5-12 The weight changes of mortar specimens after subjected to wetting-drying cycles in 3%

of NaCl solutions for 6 months period. .......................................................................................... 155

Fig. 5-13 Compressive strength loss of the mortar specimens after subjected to wetting-drying

cycles in 3% of NaCl solutions for 6 months period. ..................................................................... 156

Fig. 5-14 Visual observations of the mortar specimens subjected to wetting-drying cycles in 1.5%

NaCl+1.5% of MgSO4 solutions for 6 months period, (a) M1, (b) M2, (c) M3, (d) M4, and (e) CT.

........................................................................................................................................................... 157

Fig. 5-15 The changes of the weight of the mortar specimens after exposed to 1.5% NaCl+1.5% of

MgSO4 solutions............................................................................................................................... 158

Fig. 5-16 Compressive strength loss of the mortar specimens after exposed to 1.5% NaCl+1.5% of

MgSO4 solutions............................................................................................................................... 159

Fig. 6-1 Concrete preparation process, (a) mixing of dry components in a concrete mixer, (b)

concrete after mixing with water, (c) mould for accelerated corrosion test, (d) concrete specimen

after casting. ..................................................................................................................................... 163

Fig. 6-2 Cutting of concrete specimens by using a table saw ........................................................ 164

Fig. 6-3 Environment chamber for Carbonation test ..................................................................... 166

Fig. 6-4 Colour chart of the universal solution with a pH value.................................................. 166

Fig. 6-5 Resipod resistivity meter ................................................................................................... 167

Fig. 6-6 Application of universal solution on the fresh geopolymer and OPC concrete surfaces

(before carbonation) ......................................................................................................................... 168

Fig. 6-7 Carbonation depth measurements of the concrete samples after exposed to 1% CO2

environment for 6 months period ................................................................................................... 170

Fig. 6-8 Carbonation depth measurements of the concrete samples after subjected to wet and dry

cyclic exposure in 1% of CO2 + water environment for a 6-month period .................................. 172

Fig. 6-9 Carbonation depth measurements of the concrete samples after subjected to wet and dry

cyclic exposure in 1% of CO2 environment + chloride solution for a 6-month period ................ 174

Fig. 6-10 carbonation measurements of core specimens from aggressive and atmospheric exposed

environment. .................................................................................................................................... 176

xxii

Fig. 6-11 Visual observation of the concrete specimens after 6 months of exposure in (a) 1% of the

CO2 environment, (b) wet and dry cyclic exposure in 1% of CO2 environment + water, (c) wet and

dry cyclic exposure in 1% of CO2 environment + chloride solution ............................................ 177

Fig. 6-12 Electric resistivity measurements before exposed to the CO2 environment................. 179

Fig. 6-13 Electric resistivity measurements of concrete specimens subjected to the CO2

environment..................................................................................................................................... 181

Fig. 6-14 Electric resistivity measurements of concrete specimens subjected to CO2 + water

environment..................................................................................................................................... 181

Fig. 6-15 Electric resistivity measurements of concrete specimens subjected to CO2 + chloride

environment..................................................................................................................................... 182

Fig. 6-16 Reinforcement bar from the concrete exposed in 1% of CO2 environment for 6 months

.......................................................................................................................................................... 184

Fig. 6-17 Reinforcement bar from the concrete exposed in 1% of CO2+water environment for 6

months ............................................................................................................................................. 184

Fig. 6-18 Reinforcement bar from the concrete exposed in 1% of CO2+chloride water environment

for 6 months ..................................................................................................................................... 185

Fig. 7-1 Compressive strength test ................................................................................................. 189

Fig. 7-2 Compressive strength values of geopolymer and OPC concrete specimens. ................ 193

Fig. 7-3 Carbonation depth measurements in 1% CO2 environment ........................................... 194

Fig. 7-4 Carbonation models for S1 type concrete ......................................................................... 195



Fig. 7-7 Carbonation models for OT type concrete ....................................................................... 196

Chapter 1 Introduction

1.1 Background

Concrete is the most widely used construction material worldwide with the annual

production exceeding 1 m3 per capita [1]. Moreover, in recent years, the production of

concrete increased sharply due to increasing population growth and the rapid

developments of buildings and infrastructures. The large demand on concrete must be

catered by using the traditional cementitious materials (i.e. clinkers) which are produced

from calcination process operated at 1400° C. The production of clinker for cementitious

materials not only consume a large amount of energy, they also have detrimental effects

to the environment [2]. For instance, the production of 1 tonne of ordinary Portland

cement (OPC) releases approximately 1 tonne of CO2 to the environment, which

increases the greenhouse gas concentration in the atmosphere, causing the global

warming effects and climate change. Furthermore, Schneider et al. [3] forecasted that the

cement production would increase from approximately 2.54 billion tonnes in 2006 to 4.38

billion tonnes in 2050 based on the projected growth rate of 5%. However, most recent

observations revealed that the projected cement demand for 2015 has already been

reached by 2011 [4]. This indicates that the actual demand of cement production in 2050

would be much higher than the projected demand. Moreover, the Portland cement has

high-embodied energy, and the cement manufacture is accountable for approximately

5% to 8% of CO2 emissions worldwide [5-7].

It is therefore understood that alternative cementitious materials with low embodied

energy and less emission of harmful greenhouse gases to the environment are necessary

to ensure sustainable construction practices. In this regard, geopolymer binder has been

identified as a viable alternative cementitious material, owing to its inherent merits of

low embodied energy and less carbon emission to the environment [8, 9]. Geopolymer

binder is formulated by activation of industrial by-products containing supplementary

cementitious materials (i.e. clinker free minerals) with alkaline activators. Therefore,

these binders have lower CO2 emissions as they neither require elevated temperature


2

processes nor the formation of clinker products [10]. The previous research has shown

that 1 tonne of geopolymer concrete production releases approximately 0.184 tonnes of

CO2 [11]. The primary factor of CO2 emission in geopolymer concrete (GPC) is caused

by the production of alkalis, such as Na2O and K2O. Therefore, production of alkali-

activated binders is corresponding to 80% of CO2 emission reduction from the Portland

cement production [12, 13].

The replacement of geopolymer binder to Portland cement materials not only helps to

reduce the CO2 emission to the environment but also increases the utilisation of waste

materials. As a result, the landfill wastes can be drastically reduced with the prevention

of groundwater contamination by leaching heavy metals. With the consideration of all

these benefits, geopolymer binder has become a popular cementitious material in recent

years. In addition, GPC has been identified to have superior early age properties and

mechanical strength compared to OPC concrete.

Geopolymer binder varies substantially from Portland cement binder in terms of its

reaction mechanism to attain structural integrity. In Portland cement concrete, strength

development is achieved by the formation of calcium-silicate-hydrate (C-S-H) gel,

whereas GPC exhibits polycondensation reaction of silica and alumina oxides presented

in the supplementary cementitious materials with alkaline activator solutions. Precursor

material for the preparation of geopolymer binder can either be a by-product from

industrial processes (i.e. fly ash, slag, red mud and rice husk ash) or geological sources

like metakaolin, that contains a rich source of alumina and silica oxides [14, 15]. The

process of geopolymerisation starts with the dissolution of silica and alumina oxides

from the precursor materials in an alkaline media. The properties of reacted components

depend on the characteristics of precursor materials as to whether they are made of

aluminosilicate rich materials or calcium-rich materials, type of alkali activator, amount

of activator, curing conditions and mixing procedures. For example, geopolymer

reaction of 100% alumina-silicate rich source materials yields three-dimensional N-A-S-

H gels as a resultant product. On the other hand, the activation of calcium-bearing

alumino-silicate materials such as slag material results in two-dimensional, layer

structured C-S-H or C-A-S-H gel formation along with the N-A-S-H gel three-

dimensional products [16, 17].


3

1.2 Problem statement

Although the commercial implementation of geopolymer technology in the construction

industry has already begun, the widespread use of geopolymer binder in the

construction industry is very limited. There are many barriers preventing the application

of geopolymer materials in the construction industry. The major limitations include a

wide variety of precursor materials to choose from, limited studies on the short and long-

term properties of produced GPC and the limited availability of standard guidelines of

practice. Amongst, lack of long-term durability investigation on geopolymer concrete is

a major hindrance to the implementation in construction practice. While the main focus

of early stage geopolymer research was on the mechanical performance of GPC, more

recent research studies investigating the durability performance revealed a detrimental

effect on long-term durability of geopolymer concrete when exposed to the natural

environment [18, 19]. Furthermore, the early stage studies conducted on durability

performance were utilized laboratory scale experiments in a controlled environment (i.e.

accelerated durability testing methods), which cannot be directly interpreted to the

durability performance of real scale GPC structures exposed to the natural environment.

The durability of concrete structures is prime importance in buildings and

infrastructures considering their lifespan of 50 and 100 years, respectively. Thus, there

is an immediate requirement to investigate the long-term durability of real scale GPC

structures exposed to the natural environment in order to adopt the geopolymer

concrete in construction industry [20]. According to the reported literature, accelerated

testing methods conducted in the laboratory conditions revealed superior durability

behaviour for geopolymer concretes, whereas inferior durability characteristic was

obtained in some research studies conducted on natural exposure conditions [21-26].

Another major limitation preventing the application of GPC in the construction industry

is the lack of long-term durability assessment of GPC structures under natural exposure

conditions. The industrial survey conducted on GPC application methods in Australia,

identified that the lack of long-term durability of geopolymer concrete as one of the

significant issues for the usage of GPC in construction industry [27]. The short-term

laboratory testing methods are not suitable to predict the long-term durability behaviour

in the real environment. Therefore, as a new construction material, GPC must be

thoroughly assessed for its long-term durability performance before implementing into

construction practices. In addition, most of the standard laboratory testing methods that


4

assess the durability properties use the small size samples exposed to extreme

conditions. In particular, high CO2, and acid or salt concentrations are used for

laboratory-accelerated tests and these tests are carried out for very short periods.

Therefore, the results obtained from those accelerated testing methods would not predict

the actual durability behaviour of GPC.

Furthermore, it is well known that the mechanical and durability properties of the GPC

depend on the types of precursor materials. The durability behaviour of fly ash based

GPC differs from the slag-based GPC. This is due to the distinct types and proportions

of chemical components found in slag and fly ash materials. In addition, the material

properties and chemical compositions of fly ash- slag blended GPC would be different

from the GPC prepared with fly ash or slag materials solely. This leads to dissimilar

durability characteristic of fly ash-slag blended GPC compared to fly ash based

geopolymer or slag based GPC. Therefore, it is necessary to investigate the long-term

durability of the GPC with different precursor materials.

1.3 Aim and objectives

The aim of this research study is to understand the long-term durability of GPC

structures exposed to different field environmental conditions and to validate with the

experimental analysis conducted on laboratory prepared concrete specimens. The

specific objectives are:

1. To assess the durability of atmospheric exposed fly ash based and fly ash-slag

blended GPC structures in relation to the carbonation processes.

2. To investigate the durability of fly ash based GPC when subjected to a

combination of carbonation, chloride attack and sulphate attack, and compare

with the OPC concrete under same aggressive exposed conditions.

3. To assess the durability of slag-fly ash blended GPC in the marine environment

and compare with the OPC concrete under same aggressive exposed conditions.

4. To determine the leaching behaviour and the strength degradation of the

geopolymer concrete exposed to wet-dry cycle analysis and to study the effect of

source materials presence in GPC.

5. To examine the corrosion behaviour of reinforcement bars in geopolymer

concrete and to determine the suitable geopolymer mix to resist the corrosion


5

when it is subjected to accelerated carbonation condition with 1% of CO2

environment (near-natural exposed condition).

6. To develop a suitable carbonation model to predict the carbonation behaviour of

geopolymer concrete.

1.4 Significance of the research

While the mechanical performance of GPC has been the focus in previous research, there

has been little consideration given to the long-term durability performance of GPC,

which is affected by the type of precursor materials and exposure conditions. The

detailed durability investigation of the GPC was conducted under different exposure

condition, and the comparison of the durability behaviour has been made with OPC

concrete exposed to same environment. Therefore, this study will assist in identifying

the long-term durability issues in different geopolymer concrete compositions. This

research study will also be beneficial to develop the confidence about the geopolymer

concrete and enhance the adoption of the GPC in construction industry.

1.5 Thesis outline

The thesis is organized into seven chapters. The current chapter provides the

background to the research study, research problems, aim and objectives of this research

and the outline of this thesis. Chapter 2 presents a comprehensive literature review

related to this research study. The literature review on geopolymerisation mechanism

and source materials, mechanical and permeation properties of hardened GPC are

included in this chapter. Chapter 2 also provides the literature review related to

durability of geopolymer concrete in terms of carbonation, chloride penetration,

sulphate attack and corrosion of reinforcement, which has been determined from

accelerated laboratory testing methods.

Chapter 3 reports the durability performance of GPC structures exposed to the

atmospheric environment. Carbonation resistance of GPC after 8 years period in

atmospheric exposed condition was evaluated on core specimens extracted from

different types of GPC structures prepared with different mix proportion of precursor

materials such as fly ash and fly ash-slag blended binders. The description of GPC

structures, experimental methods and the results obtained from the experimental

analysis on the concrete core specimens are included in this chapter. The carbonation is


6

an important phenomenon causing the degradation of durability in an atmospheric

environment. Therefore, this study mainly focuses on the carbonation of GPC exposed

to the atmospheric environment for the 8-year period. pH of the concrete can be

considered as an indicator to measure the carbonation reaction in concrete, and

therefore, this study also investigated the variation of pH value along the depth of the

concrete. The apparent volume of permeable voids (AVPV) and the sorptivity tests are

conducted to determine the transport properties of aged concrete and compared with

the transport properties of freshly prepared similar concrete mixes. In addition, the

formation of carbonation products in GPC and the factors causing the formation of

carbonation products in the atmospheric exposed environment was assessed by FT-IR

analysis. Furthermore, porosity and the pore structure of the atmospheric exposed GPC

was determined with the aid of mercury intrusion porosimetry (MIP) measurements.

Chapter 4 evaluates the durability performance of GPC exposed to aggressive

environmental conditions for long-term periods. Under aggressive exposed conditions,

the durability of concrete is affected by a combination of carbonation and chloride

penetration. Therefore, the core specimens from GPC structures were subjected to the

experimental analysis to determine the carbonation resistance and chloride penetration.

This study conducted on the core-extracted samples from fly ash based GPC culvert

exposed to the saline environment for six years and slag-fly ash blended geopolymer

concrete block structures exposed to the marine environment for four years. The

durability behaviour of the GPC was compared with OPC concrete exposed to the same

environment. In addition, the sulphate attack is also most important for the durability in

the aggressive field and therefore, sulphate ingress was determined on the collected

concrete core specimens. The microstructure studies were conducted with XRD and SEM

analysis to determine the changes in microstructure due to the aggressive effects.

Furthermore, porosity and the pores size distribution of the samples were determined

by MIP analysis.

Chapter 5 presents the experimental investigations to determine the leaching effect and

the compressive strength variation using wet and dry cycle analysis. The geopolymer

mortar mixes were prepared with different proportions of fly ash and slag constituents.

The leaching ion from the GPC was studied with the pH measurements and Inductively

Coupled Plasma (ICP) test. The variation of compressive strength and the surface


7

deterioration were evaluated for concrete specimens immersed in 3% NaCl solutions

and the combination of 1.5% NaCl +1.5% MgSO4 solutions.

Chapter 6 of the thesis explains the effect of source materials on the corrosion behaviour

of geopolymer concrete. To conduct this analysis, GPC specimens were prepared with

different proportions of fly ash and slag constituents. Corrosion of reinforcement in

geopolymer concrete was investigated after six months in three different exposure

conditions such as 1% of CO2 controlled carbonation chamber, exposed to water and 1

% of CO2 environment, and exposed to chloride solution and 1 % of CO2 environment.

Since the higher concentrations of CO2 changes the carbonation reaction products in

geopolymer, 1% of CO2 concentration was used [28] to predict the natural carbonation

behaviour. Moreover, carbonation of GPC was determined using a new type of indicator

known as universal solution.

Mathematical approaches to determine the carbonation diffusion coefficient of GPC are

presented in chapter 7. The mathematical models were developed using the diffusion

equation based on the Fick’s law and the empirical equations to predict the CO2 diffusion

of geopolymer concrete. The test results from the carbonation test with 1% of CO2 was

used to determine the mathematical models.

Finally, Chapter 8 presents the conclusions of this research work and the

recommendations for future work.

Chapter 2 Literature review

8


2.1 Introduction

This chapter presents the background and overview of geopolymerisation mechanism,

binder materials and the development of geopolymerisation products. A brief

introduction about the mechanical and permeation properties of geopolymer concrete is

also presented in this study. Since the durability aspects are an important key parameter

for the concrete structure, this chapter also describes the past studies related to the

durability of the geopolymer concrete such as efflorescence, carbonation, chloride attack

and sulphate attack. The corrosion of reinforcement in geopolymer concrete also

discussed in this chapter. The final section of this chapter explains a summary of the

research requirement and research objectives of the thesis.

2.2 Overview of geopolymer

Geopolymer binder has been identified as a viable replacement for OPC binder

concerning green technology [8, 9]. The production of OPC binder is corresponding to

the significant amount of CO2 emission in globally. One tonne of concrete production

releases about one tonne of CO2 to the environment, whereas geopolymer binder is

produced with sustainable cementitious material and the CO2 emission from one tonne

of geopolymer concrete production is only 0.184 tonnes of CO2 to the environment.

Therefore, the use of geopolymer materials in concrete production is an effective way to

reduce the environment impact due to the CO2 emission. Geopolymer binders are

produced by the activation of aluminosilicate rich source of materials with the alkaline

activators. The most commonly used industrial materials in geopolymer concrete

preparation are fly ash, ground ash, slag, meta-kaolin and so on. The popular alkaline

solutions used as activators in worldwide are sodium hydroxide (NaOH), sodium

silicate (Na2SiO3) and potassium hydroxide (KOH). In addition, the solid type activator

such as sodium-meta silicate powder [24] and sodium metasilicate pentahydrate powder

[29] also used in the geopolymer binder preparation. The growth of research activities


9

on geopolymer have been developed very quickly due to low CO2 emission by

geopolymer concrete production compared to OPC and blended cement concrete.

2.3 Geopolymerisation mechanism and sources of geopolymer

binder

2.3.1 Geopolymerisation mechanism

The Geopolymer binders are created as a result of a chain of chemical reactions of

alumina-silicate oxides with alkali polisilicates. These reactions initially produce Si-O-

Al-O bonds which lead to formation aluminium and silicate monomers. The monomers

are then changed into oligomers and subsequently changed into silicate polymers.

Three-dimensional silico-aluminate geopolymer structures can be expressed as one of

the following three formula [14].

1. Poly(sialate) type (-SiO-Al-O-)

2. Poly(sialate-siloxo) type (-Si-O-Al-O-Si-O-)

3. Poly(sialate-disiloxo) type (-Si-O-Al-O-Si-O-Si-O-)

It should be noted here that the word Sialate is an abbreviation of silicon-oxo-aluminate.

Davidovits [11] also proposed that the geopolymerisation is an inorganic

polycondensation reaction. The end products of such geopolymer reaction produce a

three-dimensional tecto-aluminosilicate network product [11], and the general empirical

formula that geopolymer reaction can be explained as below:

Mn[-(SiO2)z-AlO2]n .wH2O [11] (1)

Where, M is alkali cation such as K, Na or Ca and n is a degree of polycondensation and

z is 1, 2, 3 or higher values.

In addition, Fernández-Jiménez et al. [30] proposed a conceptual model to describe the

geopolymer reaction, provided in Fig. 2-1. As explained in Fig. 2-1 (a) and (b),

geopolymerization reaction begun with the dissolution of the fly ash particles by

hydroxide ions. First, the reaction started at a point of the surface and then the reaction

is continued until the consumption of all fly ash particles into the reaction process. In

addition to that, during the initial stage of the reaction, smaller particles are filled the

large holes of the fly ash particle and yields bi-directional alkaline attack: i.e., from the


10

outside in and from the inside out. Subsequently, the alkaline cation also reacts with

these smaller particles and produce dense matrix (Fig. 2-1 (c)), and the reaction is

continued until the fly ash particle is completely consumed. The Fig. 2-1 (d) indicates

that the geopolymer reaction is not uniform, which contains reaction products and the

unreacted fly ash particles. After some period, the reactions of the smaller particles are

prevented with forming of crust layer by the reaction products and covering the smaller

particle (Fig. 2-1 (e)). Moreover, the geopolymerisation process is not uniform, and that

depends on the particle size distribution of the fly ash and the chemistry like pH values.

Fig. 2-1 Conceptual model of geopolymer reaction [30]

Furthermore, Duxson et al. [12] also developed a conceptual model to explain the

mechanism of geopolymer in a simple version. Fig. 2-2 revealed their model and

indicated that the geopolymer reaction in five stages of the process. Initially, the reaction

process started with the dissolution of amorphous aluminosilicate source material by

alkali hydroxide solution with a present of water and released silicate and aluminate

species. Secondly, these species become an equilibrium stage and water that is consumed

during the dissolution stage is released in this step. In the next stage, alumina silicate gel

is forming with the condensation of the solute species in the presence of high pH and

water released in this stage too. Thereafter, the pore size distribution and the

microstructure properties are created by rearrangement and reorganisation of the

system. In the final stage, three-dimensional aluminosilicate network formed after

hardening of the system.


11

Fig. 2-2 Conceptual model of geopolymer mechanism [12]

Škvára et al. [31] also reported that the geopolymer mechanism proceeds when the parts

of fly ash are firstly dissolved in a strong alkaline media and then new geopolymer

structures developed in that solution (Fig. 2-3 (a), and Fig. 2-3 (b)).

Fig. 2-3 (a) Typical appearance of the geopolymer from fly ash, (b) Detailed character of

the geopolymer [31]

It should be noted that the characteristics of geopolymer reaction products are controlled

by various factors including the characteristics of alumina-silicate source materials, type


12

alkali activator, amount of activator and the curing regime. Therefore, by changing of

these properties, the final product of geopolymer reaction can be varied widely.

2.3.2 The precursor of geopolymer binder

Geopolymer binder can be produced with any source materials, which contain

amorphous forms of Aluminium (Al) and Silicon (Si) components. The widely used

precursor materials are fly ash, slag, Metakaolin, ground ash, bottom ash etc. Among

them, fly ash, slag and Metakaolin are most commonly used to produce geopolymer

binder due to the availability and reactivity.

2.3.2.1 Fly ash-based geopolymer

According to ASTM C 618 [32], fly ash is divided into two subclasses such as class F fly

ash and class C fly ash. Class F fly ash is produced from burning of anthracite or

bituminous coal, and it contains minimum 70 % of the total constituents are silicon

dioxide (SiO2), aluminium oxide (Al2O3) and iron oxide (Fe2O3). Class F fly ash also has

a low amount of calcium oxide (CaO) whereas class C fly ash is the by-product of

burning of lignite or sub-bituminous coal and contains SiO2, Al2O3 and Fe2O3 minimum

50% of the total constituents. Class C fly ash has a high amount of CaO compared to

Class F fly ash. Between two types, Class F fly ash is determined as the most suitable

source material for geopolymer binder production due to the high amorphous

components, finer particle size, morphology and availability [33]. The primary reaction

component of class F fly ash is sodium aluminium silicate hydrate (N-A-S-H) gel [34].

However, due to the high amount of CaO content in class C fly ash, C-S-H and C-A-S-H

binding gels are also produced with N-A-S-H gel during the activation process.

Therefore, higher mechanical strength has been achieved with class C fly ash

geopolymer compared to class F fly ash geopolymer binder [35, 36]. However, drawback

in class C fly ash geopolymer binder is low setting time and this causes high voids, and

higher permeation properties have been identified in class C fly ash binder [37]. In

addition, it has also been reported that the high amount of Ca in class C fly ash would

be interfered the geopolymer reaction and changes the microstructure of the geopolymer

products [33, 38]. Conversely, compared to class C fly ash, high aluminium content in

class F fly ash produced more durable geopolymer matrix [39, 40]. Moreover, many

research studies have been confirmed that the class F fly ash based geopolymer binder

had good mechanical durability properties [21, 33, 41, 42].


13

2.3.2.2 Ground granulated blast-furnace slag (GGBFS) based geopolymer

GGBFS is another source of material for the preparation of geopolymer binder, which is

gaining from the by-product of blast furnaces in iron production. GGBFS mainly

contains CaO, silica and alumina components. In fly ash based geopolymer, N-A-S-H

gel is the primary reaction component during the geopolymerisation whereas in slag

based geopolymer binders C-A-S-H is a primary binder phase, which is amorphous to

partially crystalline and relatively highly cross-linked network [43]. In addition to the C-

A-S-H gel, N-A-S-H gel also forms during the geopolymerisation, and the formation of

both gels promotes the higher mechanical strength [43, 44]. The important benefit to

make geopolymer binder with slag material is that the concrete can be cured at room

temperature conditions. As opposed to that, higher temperature curing is required for

fly ash based geopolymer binder. Therefore, slag based geopolymer would be suitable

for many industrial applications in concrete industry, whereas fly ash based geopolymer

concrete can only be applicable in precast construction.

2.3.2.3 Metakaolin based geopolymer

Metakaolin also used to prepare the geopolymer binder by many researchers [45-49].

The major advantages of the usage of metakaolin in geopolymer binder preparation are

the dissolution rate of metakaolin is high in alkaline medium and can be easily control

of Si/Al ratio [33, 38]. However, the main barrier to the common application of

metakaolin is the cost of the material. Therefore, fly ash and slag are most suitable

materials in geopolymerisation.

2.3.2.4 Fly ash and GGBFS blended based geopolymer

Since the higher temperature curing method is favourable for fly ash based geopolymer,

the incorporation of slag into fly ash based geopolymer mix is an effective technique to

cure at ambient temperature, and this can enhance the usage of geopolymer binder in

concrete preparation. Li et al. [50] reported that the incorporation of slag with alumina-

silicate rich source materials in geopolymer production enhances the properties of

geopolymer binder at fresh and later stages. Blending of slag and fly ash binder produces

calcium aluminate silicate hydrate (C-A-S-H ) and calcium silicate hydrate (C-S-H)

phases with N-A-S-H gels and creates less porous structure [51]. During the

geopolymerisation process, initially, fly ash particles are dissolved in an alkaline


14

solution and produced alumina silicate binding gel. The dissolution rate of Ca from the

source of slag is slow compared to the dissolution of fly ash particles. Due to this reason,

C-S-H and C-A-S-H phases are formed at latter stage of geopolymerisation. Therefore,

slag blended fly ash based geopolymer binder produced higher strength at the later age

of concrete [52]. Furthermore, Nath et al. [53] reported that the incorporation of slag into

a fly ash mix up to 30% of total binders produced good quality geopolymer concrete at

ambient temperature by activation of sodium silicate and sodium hydroxide solution.

Moreover, Ismail et al. [54] stated that the end product of geopolymer reaction depends

on the ratio of slag to fly ash in the mix composition. They have reported that the

geopolymer mix with slag content above 50% of total precursors yields C–N–A–S–H

type gel, while more proportion of fly ash in the geopolymer mix produced N–C–A–S–

H gel with small pore size matrix. Ismail et al. [24] also conducted another investigation

to determine the durability performance of geopolymer concrete mixes with different

proportion of slag and fly ash materials. According to their study, they have proposed

that the geopolymer concrete with slag as the main precursor have better the durability

performance compared to the geopolymer mix with a higher proportion of fly ash

content.

2.3.3 Activators

Alkali activators are necessary to initiate the geopolymerisation reaction by dissolution

of source materials. The following types of activators are commonly used in the

geopolymer production:

Sodium hydroxide (NaOH) or potassium hydroxide (KOH) solution only

Sodium silicate (Na2SiO3) or potassium silicate (K2SiO3) solution only

Combination of NaOH+ Na2SiO3 or KOH+ K2SiO3 solution

Combination of NaOH+ Na2SiO3 + KOH solution

Na2SiO3 powder only

From the above types, a combination of NaOH and Na2SiO3 solutions are mostly used

in the geopolymer binder preparation. It should be noted that the ratio of Na2SiO3 to

NaOH is important in the production of geopolymer binder, which is influenced on the

geopolymer concrete properties. Chindaprasirt et al. [55] reported that the higher

mechanical properties in the geopolymer binder could be achieved when the ratio of


15

Na2SiO3 to NaOH in the range of 0.6-1.0. The activator modulus (SiO2/Na2O ratio) is also

controlled the reaction rate and influenced on the geopolymer binder properties. Law et

al. [21] identified that the increment of SiO2/Na2O ratio from 0.75 to 1 created the

strength enhancement for fly ash based geopolymer binder. However, they have

proposed that this strength increment is limited when the SiO2/Na2O ratio changed from

1 to 1.25. It was also mentioned that the durability of the geopolymer binder is improved

by the increment of SiO2/Na2O ratio up to 1 for slag based geopolymer binder system

[25].

The evidences are shown that the concentration of NaOH is predominantly influenced

on the rate of geopolymerisation reaction and the mechanical and durability properties

of the geopolymer binder. The increment of NaOH concentration is beneficial for

geopolymer production. The high concentration of NaOH accelerates the

geopolymerisation reaction and reduces the unreacted particles in geopolymer. This

attributes to the dense structure and enhances the strength of the geopolymer binder [33,

56]. It has also mentioned that the geopolymerisation reaction can be produced even

without soluble silicate when the concentration of NaOH is high [57]. There are some

researchers conducted experimental studies with NaOH as a sole activator [56, 58].

Compared to the Na2SiO3 solution, NaOH is commonly available and low viscous

material. However, using of NaOH as a sole activator for fly ash based geopolymer

produces more porous structure than NaOH activated GGBFS based geopolymer [56].

Incorporation of Na2SiO3 solution into NaOH solution promotes the gaining of high

strength by enhancing the formation of binding gel [59].

Moreover, Lee et al. [60, 61] reported that the combination of NaOH and Na2SiO3 or KOH

and K2SiO3 could be commonly used as the activator for geopolymer, which leads to

enhancing the properties of geopolymer. However, KOH is more expensive material

compared to NaOH and therefore, the usage of KOH is limited in the preparation of

geopolymer binder.

2.3.4 Curing method

OPC concrete is generally cured with water curing method at ambient temperature.

However, different types of curing methods can be used in the geopolymer concrete

preparation. Geopolymer concrete is prepared at a different range of curing temperature

with different curing regime. Generally, elevated temperature curing method is


16

beneficial for fly ash based geopolymer concrete. According to the study by Hardjito et

al. [33], the compressive strength of the geopolymer concrete increased with the increase

of curing temperature range of 30 °C to 90 °C. Sindutha et al. [62] mentioned that the

high-temperature curing accelerates the dissolution of precursor materials with the

expansion total pore volume and surface area. They have suggested the preferred

temperature for curing of geopolymer was in the range of 30 to 75°C, and the

temperature below 30°C curing produced precipitation of dissolved species instead of

poly-condensation of silicate and aluminate. Palomo et al. [63] also observed that the fly

ash based geopolymer concrete produced 60 Mpa compressive strength after cured at 85

°C temperature. Therefore, heat curing method is favourable for fly ash based

geopolymer concrete and can be achieved higher strength values [64]. However, in

another study, Chindaprasirt et al. [55] reported that the optimum curing temperature

for fly ash based geopolymer was 60 °C and higher temperature curing reduced the

strength of the geopolymer concrete. Therefore, it showed that it should be necessary to

keep an optimum temperature during the curing process of fly ash based geopolymer

concrete preparation.

On the other hand, ambient curing method is beneficial for slag based geopolymer

concrete and fly ash –slag blended geopolymer concrete. Ismail et al. [22] conducted the

study on ambient cured fly ash- slag blended geopolymer concrete with different mix

compositions. According to their research, geopolymer concrete samples displayed

higher compressive strength (65 MPa) compared to OPC concrete (60 MPa) after 90 days

of ambient curing period. Deb et al. [65] studied the properties of ambient cured fly ash-

slag blended geopolymer concrete and reported that the strength of ambient cured

geopolymer concrete is increased with age of the samples. This is because the reaction

rate is lower in ambient temperature compared to elevated temperature curing method.

Furthermore, curing period also influenced on the properties of geopolymer concrete,

especially during the heat curing process. The increment of the duration of heat curing

is providing significant enhancement of the compressive strength when the specimens

are curing at a higher temperature. Hardjito et al. [33] observed that the compressive

strength of fly ash based geopolymer concrete promoted by extended period of curing.

The compressive strength values of the geopolymer concrete increased with higher

curing period. On the other hand, Castel et al. [66] reported that the mechanical

properties of geopolymer concrete are decreased with prolong heat curing at a higher


17

temperature. Prolong curing could be attributed to the lower strength due to

deterioration of microstructure of the geopolymer concrete [55, 67]. Moreover, Subhash

et al. [68] proposed that the compressive strength of fly ash based geopolymer mortar is

increased with increasing the period of heat curing. However, they have mentioned that

the curing of more than 12 hr is not significant for the strength increment and they have

suggested that the increasing curing temperature with the less period of heating can be

developed high compressive strength for geopolymer. They have proposed that the

range between 60 °C and 90 °C temperature heat curing is suitable for fly ash based

geopolymer concrete. They also observed that the cracks formed on the geopolymer

mortar surfaces when it is cured at 120 °C temperature.

Moreover, some of the research works have been conducted with the geopolymer

concrete produced with steam curing method. Sarker et al. [69] used two different types

of stream curing methods to prepare the geopolymer concrete. One set of samples were

steam cured at 60 °C for 24 hr immediately after casting, and other sets of samples were

included in steam cured three days after casting at 80 °C for 24 hr. According to their

study, higher compressive strength was obtained for the geopolymer sample prepared

at 80 °C after three days of casting compared to the samples cured at 60 °C temperature.

Therefore, they have proposed that the compressive strength of geopolymer is increased

with a rest period and higher curing temperature. In addition, Olivia et al. [70] also used

steam curing method to prepare the fly ash based geopolymer concrete samples and

achieved higher mechanical properties for geopolymer concrete.

2.4 Mechanical properties of hardened geopolymer concrete

Previous research investigations have indicated that the geopolymer concrete exhibited

superior mechanical properties compared to OPC concrete. The compressive strength

value is an important parameter for the concrete materials. It should be noted that the

mechanical properties of geopolymer concrete depend on various factors including the

type of source material, activator type and the amount of alkali activator, curing regime,

curing temperature, water content and the admixture content used in the concrete mix.

To obtain high compressive strength of concrete, the high reactivity source of materials

are required [71]. The formation of high strength gel phase and the ratio of gel phase to

non-polymeric phases is important to achieve high compressive strength values.

Therefore, the parameters such as the type of source material, molar ratios of Aluminium


18

oxide, Silicon oxide in the source material, type, concentration and the pH value of the

alkaline solution and the solubility of source materials in the activator solution are

influenced on the compressive strength of geopolymer concrete.

As explained above, the type of source materials influenced on the strength of

geopolymer concrete. Compared to class F fly ash, higher mechanical strength was

obtained for the concrete prepared with class C fly ash geopolymer binder [35, 36] due

to the high Ca content. Diaz et al. [72] also studied compressive strength characteristic

of geopolymer concrete prepared with class F and class C type fly ash. According to that,

the compressive strength values obtained for class F fly ash based geopolymer concrete

was 45-50 MPa, while the concrete prepared with class C fly ash displayed in the range

of 50-80 MPa. Hardjito et al. [33] reported that the geopolymer concrete prepared with

class F fly ash exhibited higher strength values compared to OPC concrete. Sofi et al. [73]

examined the engineering properties of geopolymer concrete and reported that the

geopolymer concrete prepared with fly ash, slag source materials produces the

compressive strength values in the range of 47-60 MPa. Partha et al. [74] reported that

the mechanical properties such as compressive strength, splitting tensile strength and

the flexural strength of geopolymer concrete are increased with the increment of slag

content in the fly ash mixture. It was reported that the fly ash, slag blended geopolymer

concrete produced 100–160 MPa strength concrete after 28 days of curing [75].

Furthermore, Chi et al. [51] determined that the compressive strength of fly ash slag

blended geopolymer is greater than OPC mortar, whereas the geopolymer mortar

prepared with fly ash binder displayed lower strength values.

The alkaline solution also plays an essential role in the mechanical properties of

geopolymer. Usually, the higher concentration of the activator solution induces the

solubility of aluminosilicate particles present in the source material, and this increases

the high geopolymerisation rate, which produces a noticeable effect on the mechanical

properties of the geopolymer [76]. Hardjito et al. [33] obtained seven days compressive

strength values of 67 MPa for the geopolymer concrete prepared with 14 M concertation

of activator solution. In another study, the higher molarity of the activator (14 M)

produces a compressive strength of geopolymer mortar with 25 MPa strength after 48 hr

[77]. Moreover, Phoo-ngernkham et al. [78] studied the effect of activator solution on the

strength properties of fly ash, fly ash-slag blended mix and slag based geopolymer.

According to their study, they have reported that the geopolymer prepared with fly ash


19

and fly ash- slag blended materials with the activation of NaOH or Na2SiO3 solutions

shows lower strength at the ambient curing temperature. To achieve the better strength,

the combination of NaOH and Na2SiO3 activator solutions are required. They have also

reported that the use of Na2SiO3 solutions is favourable for the strength development in

slag-based geopolymer.

In addition to the type of source materials and the activator solution, the curing

temperature also influenced on the strength development of geopolymer concrete. As

explained earlier, curing temperature and the curing period play a vital role in the

mechanical properties of geopolymer. The heat curing method is favourable for fly ash

based geopolymer concrete. As reported in the previous studies, the fly ash based

geopolymer concrete cured at high temperature produced higher compressive strength

values [33, 62, 63]. However, people also reported that the compressive strength of

geopolymer concrete increased with the curing temperature and concrete cured at very

high temperature reduced the strength of the geopolymer concrete [66]. On the other

hand, the source materials rich with calcium can be cured at ambient temperature. Ismail

et al. [24] reported that the incorporation of slag in to fly ash based geopolymer mix

enhances the mechanical strength when it is cured at ambient temperature. They have

obtained 45 MPa and 65 MPa compressive strength values for the geopolymer concrete

prepared with 50% of fly ash and 50% of slag constituents in 28 days and 90 days

respectively. In addition to that Deb et al. [79] showed that the 80% fly ash and 20% slag

based geopolymer mix displayed 51 MPa compressive strength values after 28 days of

ambient curing method. Therefore, this indicates, ambient curing method is

advantageous when the slag is mixed with fly ash source materials to achieve the higher

mechanical strength values.

The curing period also plays another important effect on the strength development of

geopolymer concrete. Bakharev [64] reported that 6 hr cured fly ash based geopolymer

activated with Na2SiO3 activator displayed better strength values compared to the

geopolymer concrete cured for 24 hr period. Palomo et al. [8] stated that the mechanical

strength of geopolymer concrete is high when it is cured at 85 °C than 65°C temperature

curing method. However, compared to 5 hr curing period, the strength increment is

much small when the concrete was cured for 24 hr period.


20

Therefore, the past studies revealed that the better mechanical properties could be

achieved for geopolymer concrete by maintaining a suitable combination of the source

material, activator solution and the appropriate curing methods.

2.5 Permeation properties of hardened geopolymer concrete

Permeation is defined as a property of concrete, that allows the fluids ( both liquids and

gases) can enter into the concrete and move through the concrete [80]. The permeation

properties of the concrete are the critical parameters for control the durability of the

concrete structure. A permeability characteristic of the concrete is vital for the

penetration of the aggressive agents such as CO2, chloride ion and other gases or

chemical ions those promote the decaying the concrete and produce the corrosion of the

steel bar in concrete. Less permeable concrete can reduce the penetration of those agents

and minimise the deterioration effect on the concrete structure.

Permeation of fluid through the concrete can be divided into three transport mechanisms

such as absorption, permeability and diffusion [80, 81]. Absorption is a movement of the

liquid through the concrete by capillary suction to fill the pore space available in concrete

[80, 82]. Permeability is a parameter, which allows the fluids pass into the concrete under

the action of a pressure gradient. Finally, gases and chemical ions penetrate to concrete

surface by diffusion mechanism due to a concentration gradient. The diffusion rate of

the gases is very slow through the saturated concrete surface, and the diffusion rate is

high when the concrete is subjected to partially saturated conditions. Therefore,

diffusion is most important to the concrete structures that are exposed to field

environment with partially dry conditions. The diffusion of fluids such as chloride and

sulphate ions are most vulnerable to the deterioration of durability of the concrete

structures that are submerged in the water environment [25].

Many researchers have been investigated the permeation properties of geopolymer

concrete by using accelerated testing methods. However, considering the gas

permeation of geopolymer concrete, only a few studies have been conducted so far.

Sagoe-Crentsil et al. [83] reported that the gas permeability of steam-cured fly ash based

geopolymer was equivalent to OPC concrete with the similar compressive strength

values. However, they have not mentioned the method that was used to determine the

gas permeability coefficient. Gunasekara et al. [84] also compared the air permeability

coefficients of geopolymer concrete, which are prepared with various fly ash types,


21

available in Australia. They used Auto Clam Permeability System to determine the air

permeability characteristic of geopolymer concrete. They have determined that the air

diffusion characters of geopolymer concrete depend on the type of fly ash that is used in

geopolymer binder.

Compared to gas permeation, there are many studies have been carried out on

geopolymer concrete to determine the water permeation properties. The water

permeation properties of the geopolymer concrete are commonly measured with the

volume of permeable voids (VPV) test method and sorptivity analysis. According to the

previous studies, contradict conclusion have been derived for the water permeation

properties of geopolymer concrete. Ismail et al. [24] carried out VPV test on geopolymer

concrete samples with different proportion of fly ash and slag and reported that the VPV

values of slag based geopolymer concrete is higher than OPC concrete. They have also

stated that the VPV values of geopolymer concrete can be comparable with OPC concrete

by adding fly ash content into the slag binder [24] .

On the other hand, Deb et al. [65] mentioned that the addition of slag into fly ash binders

reduced the VPV values and showed lower values compared with the test results of OPC

concrete. This study is opposed to the test results of the previous investigation [24]. Chi

et al. [51] also proved that the geopolymer concrete had lower water absorption

compared to OPC concrete.

Sorptivity is a key index for the moisture transport into unsaturated concrete specimens,

which is identified as an important parameter for the durability of the concrete [85]. It

was confirmed that the ratio of SiO2/Na2O is essential to control the sorptivity

parameters of geopolymer binder. Qureshi et al. [86] revealed that the water absorption

and the sorptivity coefficient values of slag based geopolymer concrete are decreased

with the range of SiO2/Na2O ratio between 0.2 to 0.8 and the coefficients are increased

when the ratio is greater than the value of 0.8. However, in contrast with that, Adam et

al. [25] obtained that the optimum sorptivity coefficient value for slag based geopolymer

concrete with a SiO2/Na2O value of 1. In addition, Law et al. [21] mentioned that the

sorptivity value of fly ash based geopolymer concrete reduced when the ratio of

SiO2/Na2O is increased from 0.75 to 1.25.

Previous studies have shown that the slag-based geopolymer displayed higher

sorptivity coefficients compared to fly ash based geopolymer and OPC concrete


22

specimens due to the highly porous structure of slag based binder, which promotes the

absorption of water [22, 25, 87]. Furthermore, slag based geopolymer concrete showed

the higher coefficient of sorptivity values than OPC concrete and the absorption

coefficient of geopolymer concrete is reduced with the inclusion of fly ash to slag mixture

[24]. As opposed to that, Deb et al. [65] mentioned that the sorptivity parameters of slag

blended fly ash based geopolymer concrete is lower than OPC concrete, and the

sorptivity values are decreased with an increment of slag proportion into fly ash based

binders. Chi et al. [51] also stated that the sorptivity of fly ash based geopolymer binders

are higher than the values of slag blended fly ash binders and slag based geopolymer

binders. This is due to pore structure filled with fine particle size of slag and C-A-S-H

binding gel in slag rich geopolymer contain a substantial amount of water, whereas in

fly ash-based binder less amount of bound water content by the N-A-S-H binding gel.

More recently, Noushini et al. [88] conducted the study to determine the effect of heat

curing on the transport properties of fly ash based geopolymer concrete. They compared

the curing temperature with the range of 60°C to 90°C and the period for curing was 8

hr to 24 hr. They have reported that the sorptivity coefficients of geopolymer binders are

depended on the curing time and the temperature.

2.6 The durability of geopolymer concrete

It is well known that the concrete made with Ordinary Portland cement is a durable

material. However, in a field environment, the aggressive agents such as CO2

penetration, chloride diffusion, sulphate attack and acid attack affect the durability of

concrete. In OPC concrete, diffusion of these aggressive agents reacts with Portlandite

(Ca(OH)2) and calcium silicate hydrate (C-S-H) components in the cement matrix and

this induces the deterioration of the cement matrix. However, the reaction between the

aggressive agents and the geopolymer concrete varies from OPC concrete due to

different source materials and the different reaction products in geopolymer concrete.

Therefore, durability behaviour of geopolymer concrete is differentiated from OPC

concrete in an exposed environment. According to the previous studies, many

researchers found better mechanical properties [25, 89] and excellent durability

performance [22, 90]. However, some other researchers obtained inferior durability

performance considering carbonation resistance [26] and chloride penetration [25]. In

this section, the durability of geopolymer regarding the efflorescence and leaching

behaviour, carbonation, chloride penetration and the sulphate attack are discussed.


23

2.6.1 Efflorescence and leaching of geopolymer

In the past studies, the authors have reported that the efflorescence in geopolymer occurs

due to the open microstructure of some materials, which have a lower extent of reaction,

high alkali concentration in the pore solution and weak binding of Na in the geopolymer

structure [91-93]. For OPC concrete, efflorescence is normally harmless except for the

discolouration. However, for geopolymer concrete, efflorescence can be a significant

concern when the products are exposed to humid air or in contact with water due to

higher soluble alkali content than conventional OPC concrete. Because in geopolymer

concrete, alkali cations (Na+, K+) are movable within the pore network, when the

moisture presented in the concrete. Therefore, these alkali cations deposit on the surface

when the water evaporates from the concrete. The deposited alkali cations react with

CO2 from the atmosphere and produce white carbonate products on the concrete surface,

which is known as efflorescence [94]. However, this carbonation mechanism is varied

from the carbonation of the geopolymer binder, which is the reaction between the

sodium based geopolymer binder phases and atmospheric CO2 [95, 96]. Carbonation

causes pH reduction, degradation of binder, corrosion of embedded reinforcement bar

in concrete, which are harmful to the service life of the concrete structure, whereas

efflorescence makes the formation of visible deposit on the surface, which may not be

involved into the further degradation of the binder. The carbonation of geopolymer

concrete is explained in the section 2.6.2.

Temuujin et al. [97] reported that the ambient cured geopolymer concrete specimens

exhibited efflorescence products, whereas no such efflorescence was observed in the

samples cured at 70° C temperature. This is because the rate geopolymerisation is higher

when the sample cured at high temperature, which leads to complete incorporation of

the Na and P atoms into geopolymer structure and reduces the free movement of alkali

cation within the pore network. Fig. 2-4 shows the Scanning Electron Microscopic (SEM)

image of efflorescence crystals and the X-ray spectrum analysis on the efflorescence

crystal. According to that the presence of Na and P atoms in the efflorescence has been

confirmed.

Some of the research studies have been proved that the efflorescence crystals from

geopolymer concrete consist sodium carbonate hydrate (Na3H(CO3)·2H2O) or sodium

carbonate(Na2CO3) [98, 99]. However, the presence of sodium phosphate hydrate


24

(Na3PO4·12H2O) also identified in the efflorescence crystals of ambient temperature

cured samples [97]. According to the XRD test results (Fig. 2-5) presence of

Na3PO4·12H2O has been confirmed for ambient cured geopolymer sample. In those

different studies, different types of fly ash have been used to prepare the geopolymer

binder. Therefore, this indicates, the composition of the efflorescence strongly depends

on the chemical, and mineralogical composition of the fly ash [97].

Fig. 2-4 SEM micrograph of efflorescence crystals and associated X-ray spectrum

(backscattered electron image, 20 kV, the sample is not coated) [97].


25

Fig. 2-5 XRD patterns of efflorescence from ambient temperature cured samples [97].

On the other hand, Zhang et al. [100] proposed that the efflorescence of geopolymer

depends on the type of activator, the temperature of curing and the incorporation of slag

content in the geopolymer mix. Fig. 2-6 displayed the visible formation of efflorescence

on the surfaces of geopolymer samples with different proportion of fly ash, slag content,

activator type and different curing regimes. They have determined that the geopolymer

binder activated by NaOH displayed less and slower efflorescence compared to Na2SiO3

activated geopolymer specimens under ambient curing conditions. In addition, as

mentioned previously, these authors also reported that the high-temperature curing is

beneficial to reduce the efflorescence rate, due to the local reorganisation and

crystallisation of N–A–S–H gels. The incorporation of slag as a source of Ca into fly ash

based geopolymer binder reduces pore sizes and porosity with either ambient or high-

temperature curing, which leads lower efflorescence rate in geopolymer system.


26

Fig. 2-6 Efflorescence on the surfaces of geopolymer samples with different proportion

of fly ash, slag content, activator type and different curing regimes [100]

Some of the researchers have already suggested the suitable methods to reduce the

efflorescence in geopolymer concrete. Najafi et al. [94] proposed to incorporate high

alumina cement admixtures, which encourage to the extent of crosslinking in the

geopolymer binder. This reduces the mobility of alkalis in the geopolymer binder

system, which helps to minimise the efflorescence in geopolymer. Movement of alkalis

is the main reason for the efflorescence in geopolymer system. They also have

recommended using high-temperature curing method to reduce the efflorescence effect.

They have reported that the curing of geopolymer specimens at temperatures of 65 °C

or higher enhanced to minimize the efflorescence effect.

Considering the leaching behaviour of the geopolymer concrete, limited studies

available in the literature. Zhang et al. [101] investigated the leaching ions from red mud-

fly ash based geopolymer in sulphuric acid solutions and deionised water. They have

determined that the leaching of As, Cu, Cr and Cd from the geopolymer samples in both

exposed solutions and the amount of leaching ion can be comparable with OPC concrete.

They also revealed that the leaching behaviour of geopolymer sample does not depend


27

on the temperature. In addition, Zhang et al. [100] determined that the Na and K alkalis

from geopolymer binder are leached, and the amount of Na alkali leaching is much

higher than the leaching of K alkali. Furthermore, they also mentioned that the leaching

rate is decreased by an increment of soluble silica content in the geopolymer binder. On

the other hand, Izquierdo et al. [102] determined that the leaching amount of K and Si

ions are high, when the geopolymer samples cured in the open air conditions due to the

high and quick evaporation of water from geopolymer samples.

2.6.2 Carbonation resistance of geopolymer concrete

Considering the durability of the concrete structures, carbonation is an essential

parameter for the degradation of the durability in an atmospheric environment. In OPC

concrete, carbonation occurs when the CO2 from the atmosphere diffuses into the

concrete and reacts with calcium hydroxide (Ca(OH)2) and calcium silicate hydrates (C-

S-H) components [103]. Due to this reaction, calcium carbonate (CaCO3) component

produces in OPC concrete, and this reduces the pH level of OPC concrete.

It is well known that the pH of the pore solution in OPC concrete should be more than

12.6 [26] . The pH level is important to concrete. This is because the reinforcement bar in

OPC concrete is protected against corrosion activity by the alkalinity (Ca(OH)2)

component of the cement, which is forming a thin oxide layer (passivation layer) around

the reinforcement bar. However, this passivation layer will be de-passivated with the

carbonation process due to pH deduction (alkalinity deduction) to less than 9.0 in the

concrete [104, 105]. Therefore, the reinforcement bar will be started to corrode, and this

reduces the durability of the concrete structures when it is exposed to the field

environment. The moisture content of the concrete mainly influences the rate of

carbonation. The optimum relative humidity level for highest rate of carbonation of

concrete was 50%- 70% [106]. If the moisture level is too low or too high from the above

limit, then the carbonation rate should be less. Therefore, the effect of carbonation is

high, when the concrete structures are subjected to the partially dry environment. In

addition, the carbonation rate of concrete also depends on the binder phase, physical

properties, permeability characterises and the porosity of the concrete surface [107, 108].

It should be worth to mentioned here that the carbonation reaction in geopolymer

concrete is varied from OPC concrete due to the absence of Ca(OH)2 components and


28

the different reaction components in geopolymer concrete. Therefore, carbonation

mechanism and the carbonation reaction components in geopolymer concrete is

different than the OPC concrete. Unlike the Portland cement material, the amount of Ca

component is very low in the geopolymer source materials, especially when fly ash

material used as a precursor. Therefore, the availability of Ca(OH)2 in geopolymer

concrete is limited. On the other hand, geopolymer matrix has NaOH and KOH

hydroxides components from the activator solutions. Therefore, during the carbonation

reaction, the diffusion of CO2 reacts with NaOH and KOH, and produced Na2CO3 and

K2CO3 components.

There are various research studies have been conducted on the carbonation resistance of

slag based geopolymer concrete based on accelerated testing method. Adam et al. [25]

mentioned that the alkali-activated slag based geopolymer concrete consists lower

resistance to carbonation compared to OPC concrete. During the accelerated testing

period with 20% of CO2 concentration, they have found that the carbonation depth of

slag-based geopolymer concrete was around 35 mm after the eight weeks of exposure.

Conversely, OPC concrete showed only 10 mm carbonation depth at the same time of

exposure. Song et al. [109] also investigated the carbonation resistance of alkali-activated

slag based geopolymer mortar and reported that the rate of carbonation in geopolymer

binder is higher than OPC binder. They found that the C-S-H gel in slag-based

geopolymer is more vulnerable to carbonation than that in OPC paste and the

carbonation reaction in geopolymer attributes the change of C-S-H gel to silica gel, and

the aluminium compounds were completely disintegrated. Therefore, this produces

higher carbonation rate in slag based geopolymer compared to OPC binder.

Moreover, Bakharev et al. [26] also observed higher carbonation depth for slag based

geopolymer concrete. In addition, Bernal et al. [89] reported that the carbonation depth

of slag based geopolymer concrete is higher than OPC concrete and the resistance to

carbonation is improved with the increment of binder content in the mix. Fig. 2-7 showed

the carbonation depth measurements of slag based geopolymer concrete (AASC) and

OPC concreter (OPCC) after exposed to 1% of CO2 environment for 1000 hr period. As

shown that the geopolymer concrete produced with a low amount of binder content (400

kg/m3) exhibits higher carbonation depth. A further, according to Fig. 2-7, carbonation

depth of OPC concrete was lower than geopolymer concrete when the amount of binder

content is similar. It was also proved that the carbonation rate of geopolymer concrete


29

depends on the type of activator. Puertas et al. [110] observed that the carbonation depth

of Na2SiO3 activated slag based geopolymer mortar was 10 mm after four months of

exposure. However, according to their study, slag based geopolymer mortar activated

by NaOH as the sole activator showed only 3 mm depth after four months of the

exposure period. Therefore, they have concluded that the NaOH activator is better than

Na2SiO3 activator to reduce the effect of carbonation in slag based geopolymer concrete.

Fig. 2-7 Carbonation depth measurements of concretes after 1000 h of exposure to a 1%

CO2 environment [89].

Considering the carbonation of fly ash based geopolymer binder, only limited studies

are available in the past literature. The carbonation depth of slag-based geopolymer

concrete was determined by the application of phenolphthalein indicator, which is the

usual method to determine the carbonation of OPC concrete. Despite that, the previous

studies have shown that the phenolphthalein indicator did not provide clear carbonation

front for fly ash based geopolymer concrete [25, 90, 111]. Sufian Badar et al. [90] studied

the effect of fly ash type on the corrosion of steel bars induced by accelerated carbonation

methods. According to their study, the fly ash with high Ca content more susceptible to

carbonation and superior pH reduction compared to low Ca based fly ash based

geopolymer concrete. Besides that, Law.et.al [111] stated that the pH of low Ca fly ash

based geopolymer after the carbonation is sufficient to protect reinforcement bar against


30

corrosion. They have prepared the geopolymer mortar samples with different activator

modulus, and the accelerated carbonation test was conducted in 5% of the CO2

environment. The pore water from mortar sample was then extracted, and the pH value

of the pore solution has been measured.

Table 2-1 displays the pH value changes with the exposure time. According to that, pH

values are changed with a small reduction and the final pH values are higher than the

pH of carbonated OPC samples. The different mix details were prepared by changing

the amount of Na2O dosage and the activator modulus. For example, the mix notation

G7.5-1.0 represents a 7.5 % Na2O dosage and a 1.0 activator modulus in the mix.

Table 2-1 pH values of geopolymer mortar specimens in carbonation environment [111].

Mix pH

0 days 3 days 7 days 28 days

G7.5-0.75 11.86 11.88 11.01 10.88

G7.5-1.0 11.94 11.91 11.35 10.46

G7.5-1.25 11.73 11.71 11.39 10.73

G15-1 11.96 11.97 11.5 11.05

G15-1.25 11.99 11.88 11.5 11

G15-1.5 11.97 11.98 11.77 11.23

Therefore, according to the available literature studies, slag based geopolymer concrete

is more susceptible to carbonation compared to OPC concrete and the carbonation effect

in low Ca fly ash based geopolymer concrete is not that much of severe compared to slag

based geopolymer concrete. This conclusion has been derived with the higher

concertation of the CO2 environment. However, Bernal et al. [112] mentioned that the

carbonation product from natural carbonation process and accelerated carbonation test

method could be dissimilar. That is, unlike natural carbonation, sodium bicarbonate

products are mostly formed during accelerated carbonation testing at high CO2

concentration level (i.e. 3% CO2). This induces a higher pH reduction compared to

natural carbonation. Therefore, measuring the carbonation depth of geopolymer

concrete exposed in ambient conditions is the appropriate way to determine the

durability performance in real atmospheric CO2 environments.


31

2.6.3 Chloride penetration

Reinforced concrete structures in marine and saline environments are highly susceptible

to deterioration compared to normal atmospheric environments due to the chloride

attack. Evidence shows that the combination of carbonation and chloride penetration

compromises the durability of OPC concrete and causes higher corrosion of the

reinforcement [113, 114]. In addition, concrete is also susceptible to various chemical

reactions with the ions present in the seawater, such as sulphates and magnesium, which

are associated with deterioration of concrete [115-117].

As explained earlier, reinforcement bar in concrete resists against corrosion by a passive

layer of the oxide film. As similar to CO2 penetration, the diffusion of chloride ion also

de- passivates the passive film and creates the path to corrosion of reinforcement bar in

an aggressive environment with the presence of water and oxygen. Therefore, chloride

attack is an important phenomenon for the durability aspects of the concrete structures

in marine environment. In general, chloride ion is penetrated through the concrete

surface with a three-transport mechanisms such as capillary absorption, diffusion and

hydrostatic pressure [82]. Capillary absorption is a mechanism, which transports the

liquids by surface tension acting in capillaries. Capillary absorption is important for the

penetration of chloride ions to the concrete. When the concrete structures are subjected

to not permanently contact with water, then the chloride ions are diffused into the

concrete surface by capillary forces. The risk of chloride attack is high when the concrete

structures are subjected to wet and dry conditions (not permanently contact water).

Diffusion is a transfer of chloride ions as a result of net flow from higher concentration

regions to lower concentration regions. This mechanism is important for the concrete

structures exposed in a fully submerged condition. Hydrostatic pressure method is used

to transfer liquids or gases through the concrete by a pressure head. This type of

transport mechanism is applicable for the concrete structures, which are contacted with

water under a pressure head.

In OPC concrete, there are many factors contributed to the rate of chloride penetration

such as water-cement ratio, type of cement, mix constituents, mix proportions and the

porosity of the concrete. Among that, the rate of diffusion highly depends on the

porosity of the concrete in terms of pore size, pore distribution and interconnectivity of


32

the pore system. Therefore, concrete with lesser pore sizes and lower pore connectivity

is beneficial to reduce the chloride ingress through the concrete structure [118].

Although the chloride penetration is important for the durability aspect of the concrete,

past studies showed that only a few investigations have been conducted to determine

chloride diffusion of geopolymer concrete by using accelerated testing methods. Ismail

et al. [22] studied the chloride penetration of geopolymer concrete by using accelerated

chloride penetration test (Nordtest NT Build 492) and chloride ponding test (ASTM

C1543) methods. According to that, the slag based geopolymer exhibited lower chloride

penetration compared to OPC concrete. In addition, the experimental analysis showed

the diffusion of chloride is increased by the addition of fly ash binder into the slag based

geopolymer binder. Fig. 2-8 showed the chloride penetration depth measurements at the

end of chloride penetration test (Nordtest NT Build 492). As per that, the OPC concrete

exhibited higher chloride penetration compared to all geopolymer samples. The depth

of chloride penetration is increased with the increment of fly ash content in the

geopolymer mix. This showed that the replacement of fly ash materials by slag

constituents provided better benefits for the durability of geopolymer concrete and

compared to fly ash based geopolymer concrete, slag based geopolymer concrete is more

suitable for marine environment. Moreover, Olivia et al. [119] also confirmed the higher

chloride attack in fly ash based geopolymer concrete compared to OPC concrete.

However, despite of those studies, Adam et al. [25] revealed that the fly ash based

geopolymer concrete had lower chloride diffusion compared to OPC concrete and

higher diffusion observed in the alkali-activated slag based geopolymer concrete. Shaikh

et al. [120] also proposed that the geopolymer concrete contains the lower risk of

corrosion than OPC concrete under chloride environment by half-cell potential (HCP)

measurement based on CU/CUSO4 electrode.


33

Fig. 2-8 Chloride penetration depth of concrete specimens at the end of the Nord Test

(A) 100 wt.% slag, (B) 75 wt.% slag/25 wt.% fly ash, (C) 50 wt.% slag/50 wt.% fly ash,

(D) OPC [22]

Furthermore, Kupwade-Patil et al. [121] examined the corrosion of reinforcement in fly

ash based geopolymer concrete, which is associated with accelerated chloride testing

method. Geopolymer concrete samples were prepared with two types of fly ash such as

class F and class C fly ash, and all concrete specimens are exposed to wet and dry

chloride environment for 12 months period. The test results showed that the chloride

diffusion and the chloride content in geopolymer concrete are lower than OPC concrete.

Besides that, the concrete prepared with class F fly ash performed better than the

concrete produced with class C fly ash. It was proved that the fly ash based geopolymer

concrete contain higher resistance to chloride diffusion than OPC concrete [122].

More recently, chloride-induced corrosion in fly ash and slag based geopolymer concrete

have been evaluated with the accelerated testing method by Tennakoon et al. [29]. They

have prepared geopolymer mixes with different proportion of fly ash and slag

constituents, and the chloride diffusion test was conducted according to the Nord test

NT Build 443 method. According to their study, chloride diffusion of fly ash-slag

blended geopolymer concrete is lower than the diffusion in OPC concrete, and the

diffusion of chloride in geopolymer concrete is improved with the increment of slag

content in the mix. This is consistent with the test results obtained by Ismail et al. [22].

Fig. 2-9 shows the chloride profiles of concrete specimens after subjected to the


34

accelerated testing process. As shown in Fig. 2-9, OPC concrete had higher chloride

content throughout the depth, whereas the chloride penetration is decreased with the

substitution of fly ash by slag materials. They also evaluated the chloride-induced

corrosion after exposed to 2.826 M NaCl solution for 500 days and the test results

revealed that the corrosion activity started in OPC concrete reinforcement bar and no

sign of corrosion identified in the reinforcement bar in blended fly ash and slag

geopolymer concrete specimens. However, in contrast with all the above studies,

Ganesan et al. [123] determined that the chloride penetration of geopolymer and OPC

concrete are almost same.

Fig. 2-9 Chloride profiles of geopolymer and OPC concrete after 5 weeks of immersion

test [29]

Therefore, based on the past investigations, it is difficult to determine the risk of chloride

attack in geopolymer concrete due to the contradict results by various researchers. It is

important to consider that the above experimental studies have been conducted by an

accelerated testing method on freshly prepared geopolymer concrete specimens.

Therefore, different people have used different exposure conditions to conduct the

accelerated test, and this produces various results among different researchers.

However, the actual exposed environment condition should be varied from the

accelerated testing condition and due to this reason, geopolymer concrete will be

behaved differently in the real environment. Therefore, it is necessary to determine the

chloride penetration of geopolymer in the actual environmental conditions.


35

Moreover, it is well known that the chloride penetration of OPC concrete reduces with

the age of concrete (maturity factor) due to the continuous hydration reaction in OPC

concrete with time after the casting of structures [124]. Due to continuous hydration

reactions in OPC concrete, the porosity and pore connectivity system are decreased with

the time of exposure, and this reduces the chloride diffusion into the concrete with the

age of the structure. Therefore, the chloride penetration in aged OPC concrete is less

compared to the fresh stage. In contrast with the hydration reaction of OPC concrete,

geopolymerisation reaction in heat-cured fly ash based geopolymer concrete is very

rapid with little or no further reaction after a few days of curing. Therefore, even though

the chloride penetration is same or less in geopolymer at fresh stage, the ingress of

chloride ions in geopolymer concrete could potentially be higher than OPC concrete after

some years of exposure [21].

2.6.4 Sulphate attack

Sulphate attack is another important phenomenon for the durability of concrete

structures in marine environment. Sulphate attack on concrete is produced an expansion

in concrete which ultimately leads to cracking or softening of the concrete [125]. In OPC

concrete, the mechanism of the reaction between sulphate and hydration reaction

components are complicated. In OPC concrete, penetration of the sulphate ion reacts

with calcium hydroxide, C–S–H gel and the aluminate component (C3A) of hardened

cement paste [126, 127]. As a result of this reactions, concrete structure suffers

expansion, spalling, cracking and loss of strength [126, 128]. As a results of the reaction

with sulphate ion, gypsum (calcium sulphate dihydrate, CaSO4.2H2O) and ettringite

(Ca6Al2 (SO4)3 (OH) 12 .26H2O) components forms in OPC concrete [129]. The formation

of gypsum and ettringite are the reason for the volume increment of the concrete matrix,

which causes concrete structures undergo cracking and spalling. Moreover, concrete

surface becomes softening and disintegration due to the destruction of C–S–H gel by

sulphate ion. The penetration of magnesium sulphate (MgSO4) caused magnesium

hydroxide (brucite, Mg(OH)2) and gypsum are formed in concrete. It has been confirmed

that the deterioration of concrete structure under MgSO4 penetration is more vulnerable

than in Na2SO4 environment [130]. Under MgSO4 penetration, decalcification of C–S–H

gel is high, and Mg2+ ion replaces the Ca2+, and this caused magnesium silicate hydrate

(M-S–H) are formed.


36

On the other hand, mechanism of sulphate attack in geopolymer binder is varied from

OPC binder due to the different of geopolymer reaction products compared to hydration

components in OPC concrete. According to the past literature, only few research studies

were available in regard the sulphate resistance of geopolymer concrete. Bakharev et al.

[131] reported that the durability of the geopolymer concrete depends on the precursor

materials and the type of the sulphate solutions used in the accelerated testing methods.

The best performance was obtained for the concrete prepared using sodium hydroxide

and in the solution of 5% sodium sulphate+5% magnesium sulphate. In addition,

geopolymer concrete also displayed superior durability performance compared to OPC

concrete in sulphuric acid solution under accelerated testing method [132, 133]. Ismail et

al. [128] observed the microstructural changes of alkali-activated fly ash/ slag based

geopolymer paste under Na2SO4 and MgSO4 exposed conditions. They have reported

that the reaction between Na2SO4 and geopolymer binder varies from the reaction

between MgSO4 and geopolymer binder. The degradation of geopolymer paste in

MgSO4 is severe than in Na2SO4 solution, which is similar to OPC concrete performance

under sulphate environment. Under MgSO4 exposed environment, decalcification of the

Ca-rich gel phases in the blended fly ash/slag geopolymer binder due to the Mg ion,

which promotes more degradation of the geopolymer binder and formation of gypsum.

Despite that, the Na2SO4 solution was not degraded the geopolymer binder, and no

components with the presence of sulphate have been identified in geopolymer system.

Moreover, Škvára et al. [134] also determined that the heat cured low Ca fly ash based

geopolymer mortar consists higher sulphate resistance under MgSO4 and NaCl media.

Fig. 2-10 display the geopolymer mortar specimens after exposed to NaCl and MgSO4

solution for 1.5 years. As shown in Fig. 2-10, no significant changes, surface

deterioration, cracking or spalling have been identified after the exposure. In addition,

they found that the compressive strength values of geopolymer samples were not

changed after exposure to the sulphate solution.


37

Fig. 2-10 Geopolymer mortar after the 1.5-year exposure to the solutions of NaCl and

MgSO4 salts [134].

However, it should be noted here that the concentration of sulphate solution used in the

accelerated testing method should differ from the actual concentration of the sulphate

ion in the groundwater and soil. Therefore, the investigation of sulphate attack of

geopolymer in a field environment is an appropriate way to determine the sulphate

resistance of geopolymer concrete. Moreover, the porosity of the concrete is the main

parameter that controls the rate of penetration of the chloride ions, sulphate ions and

CO2 into concrete. Therefore, the concrete with less porosity is beneficial to resist

chloride, sulphate ingress [118] and carbonation.

2.7 Porosity and pore structure of geopolymer concrete

It is well known that the concrete durability depends on the porosity, the pore structure

and pore connectivity of the concrete. The pore structure of the concrete controls the

transportation of gasses, water and aggressive agents to the concrete surface, especially

the pore volume, pore size distribution, connectivity and shape of the pores [135]. It

should be noted that the pore structure of the geopolymer concrete differs from the OPC

concrete due to distinct reactions between the different sources of materials. The pore

size and microstructure characteristic of geopolymer binders depend on the physical and

chemical characteristics of source materials and the type and characteristic of the alkaline

solution [136]. Ryu et al. [42] determined that the number of smaller pore sizes in fly ash

based geopolymer increased with the high molarity of the alkaline solution. Nazari et al.

[137] proposed that the total pore volume depends on the particle size distribution of the


38

fly ash, the time of oven curing and the time of room curing methods. They found that

the pore volume is reduced with the finer size of fly ash particles and which is resulted

in the dense surface for fly ash based geopolymer concrete. Ma et al. [138] proposed that

the pore developments in alkali-activated fly ash are slower than cement paste, and the

heat curing method is helped to develop the pore system quicker than the normal

ambient curing method. They have also mentioned that using of high alkali content is

reduced the total porosity and enhance a finer pore system development.

Table 2-2 showed the classification of pore sizes according to the International Union of

Pure and Applied Chemistry (IUPAC) system [139]. As per that the pores are categorised

with the range of pore sizes. The pore sizes with less than 1.25 nm are classified as

microspores, and the pore size range between 1.25-25 nm is considered as Mesopores.

Macropores is measured with the pore size ranges in between 25-5000 nm, the pore size

range in between 5000-50000 nm is corresponding to entrained air voids, entrapped air

voids, and pre-existing microcracks in concrete. In OPC concrete, capillary pores include

both mesopores and macropores constitute the water-filled space existing between the

cement particles, while calcium- silicate- hydrate gel component creates micropores in

the Portland cement concrete.

Table 2-2 IUPAC pore size classification [139]

Pore description Pore size (radius) (nm)

Micropores <1.25

Mesopores 1.25-25

Macropores 25-5000

Entrained air voids, entrapped air

voids, pre-existing micro cracks

5000-50000

Collins et al. [140] determined the pore size distributions of alkali activated slag paste by

using MIP analysis. Cumulative pore size distribution of OPC (OPCP) and slag based

geopolymer (AASP) is provided in Fig. 2-12 [140]. The water to binder ratio of the

geopolymer system was 0.5. The results showed that the slag based geopolymer samples


39

has a finer pore size distribution than OPC sample. The higher proportion of pore sizes

of geopolymer samples in the range of mesopore limits ( 1.25–25 nm) [139].

Fig. 2-11 Cumulative pore size distribution of geopolymer and OPC concrete [140]

Moreover, Duxson et al. [141] determined that the Si/Al ratio in the geopolymer system

also influences on the pore structure of the geopolymer. They found that the

microstructure of geopolymer mix with Si/Al ratio ≤1.40 consists clustered dense

particulates with large interconnected pores, while the mix with Si/Al ≥1.65 shows

homogenous with porosity distributed in small pores. In between that two range (1.4 <

Si/Al < 1.65) displays the development of the microstructure with increasing silicon

content is rapid yet continuous within the small compositional region. As observed from

Fig. 2-12, the pore volume is decreased with the increment of Si/Al ratio.


40

Fig. 2-12 Pore volume distributions of geopolymer paste [141]

Moreover, it was reported that the substitution fly ash in slag based geopolymer mix

leads higher porosity compared to slag based geopolymer mix [142]. The water to binder

ratio (w/b) of geopolymer system was 0.40 As shown in Fig. 2-13, 100% slag based

geopolymer mix exhibits lower porosity compared to slag-fly ash based geopolymer and

100% fly ash based geopolymer. This is due to the different reaction products in between

those types of geopolymer systems. In geopolymer with slag rich systems, C-(A)-S-H gel

is the primary reaction component, which is significant bound water content, whereas

fly ash based geopolymer system produces N-A-S-(H) gel, with a lower bound water

content [143]. Therefore, the presence of more bound water provides more pore-filling

capacity and that reduces the porosity in slag based geopolymer system. Furthermore,

the Fig. 2-13 also showed that the porosity decreased with the time of curing. Zhu et al.

[19] also confirmed that the substitution of fly ash by the slag content reduces the pore

size and total porosity of the geopolymer system.

On the other hand, it was also determined that slag based geopolymer concrete exhibits

higher porosity compared to OPC concrete. Al- Otaibi et al. [144] stated that the porosity

values of slag-based geopolymer concrete are greater than OPC concrete.


41

Fig. 2-13 Relationship between porosity and curing duration for fly ash-slag geopolymer

systems [142]

2.8 Corrosion of reinforcement

Corrosion of reinforcement is an important problem in the concrete structures. Usually,

the reinforcement bar in concrete is protected by a thin and stable oxide layer, which is

called a “passive film” [145]. This passive layer is started to form as soon as the initiation

of the hydration reaction in cement due to the pH value of the concrete rises and then

this layer is become stabilised in the first week period to protect the reinforcement from

active corrosion [146]. Then, the passive layer remains around the reinforcement bar and

protect against corrosion under the high pH environments for the service life period of

the structures [146]. The thickness of the passive film would be in the order of 5 nm.

However, in the field exposed environment conditions, several factors contribute to the

deterioration of this passive layer. Among that, carbonation and chloride diffusion are

two important causes for the deterioration of this oxide layer when the concrete exposed

to the field environment. Ingress of chloride ions at the reinforcement bar level reduces

the pH at that point to the lower value. The corrosion at reinforcement bar will initiate

when the sufficient amount of chloride penetrates at the reinforcement bar level [147].

Penetration of chloride ions activate the surface of the steel to form an anode, and the


42

passivated layer becomes cathode [124]. As the results of this electrochemical reaction,

the passive layer will break down and causes that the reinforcement bar will start to

corrode.

Carbonation is also another important factor for the corrosion of reinforcement in

concrete. As explained earlier, the pH level is maintained at a higher level in OPC

concrete by the hydration reaction components such as Ca(OH)2 and C-S-H gel and [108].

The ingress of CO2 reacts with the hydration reaction products in cement concrete, and

this reduces the pH value of OPC concrete. When the carbonation depth reaches to the

reinforcement bar level (that means CO2 diffusion at the reinforcement bar level), pH

level at that point will drop to the value of 9. Therefore, this will de-passivate the

protective layer and this causes; corrosion will initiate at the embedded reinforcement

bar in concrete.

Even though the corrosion activity is important for the durability of concrete structures,

only a few studies have been conducted on corrosion activity of the rebar exposed in

geopolymer concrete. Most of the previous studies revealed that the reinforcement bars

exposed in the fly ash based geopolymer concrete are less susceptible to corrosion effect

under chloride-exposed environment. Kupwade-Patil et al. [121] examined the corrosion

of reinforcement bar in fly ash based geopolymer concrete under chloride solution for

12 months and determined that only micro level of corrosion products at the matrix-

rebar interface of geopolymer concrete specimens, whereas the matrix-rebar interface of

OPC concrete exhibited multiple gross corrosion products. Moreover, Reddy et al. [115]

also reported that there is no corrosion effect in the reinforcement bar embedded in the

geopolymer concrete, whereas reinforcement bar in OPC concrete exhibited severe

corrosion damage after exposed to seawater solution. Fig. 2-14 shows the corrosion of

reinforcement bar in geopolymer and OPC concrete after the exposure condition. They

have used an accelerated laboratory electrochemical method to induce the corrosion in

the reinforcement bar. Shaikh et al. [122] stated that the fly ash based geopolymer

concrete had superior resistance to corrosion compared to OPC concrete in chloride

solution. More recently, Babaee et al. [148] reported that the test results of corrosion

potential and polarisation resistance after de-passivation of the reinforcements indicated

that the risk of corrosion in fly ash based geopolymer concrete is similar to Portland

cement concrete under chloride environment. Miranda et al. [149] reported that the steel

bar embedded in fly ash based geopolymer mix containing 2% chloride does not have a


43

protective passive layer which displays similar corrosion behaviour to OPC concrete.

Furthermore, Saraswathy et al. [150] also studied the corrosion resistance of fly ash based

geopolymer concrete in a chloride environment. According to their study, it was stated

that the corrosion resistance of reinforcement bar in fly ash based geopolymer concrete

is comparable with corrosion in OPC concrete at the same time of exposed condition.

Moreover, it was also reported in recently, fly ash based geopolymer concrete exhibited

more protection against corrosion compared to Portland cement concrete [151]

Fig. 2-14 Corrosion of reinforcement bar in (a) fly ash based geopolymer concrete, (b)

OPC concrete [115]

All studies mentioned above are confirmed that the risk of corrosion of reinforcement in

fly ash based geopolymer concrete is lower than OPC concrete in a chloride

environment. However, in contrast with above studies, Olivia et al. [119] reported that

the risk of corrosion in fly ash based geopolymer concrete is high compared to OPC

concrete. They have conducted the corrosion study on the concrete samples immersed

in chloride solution by using half-cell potential (HCP) method.

Considering the corrosion activity of reinforcement bar in slag based geopolymer

concrete, Malolepszy et al. [152] studied the corrosion activity of embedded

reinforcement bar in OPC mortar and alkali-activated slag mortar, and they found that

no difference between the two type of systems up to 336 days. However, they suggested

that the long-term studies should be required to determine whether the corrosion


44

activity would maintain as same as the initial stage. More recently, Zainal et al. [153]

observed that the corrosion resistance of fly ash-slag blended geopolymer binder

displayed higher corrosion resistance compared to fly ash based geopolymer binder

systems in a chloride environment. Tennakoon et al. [29] also examined the corrosion

behaviour of fly ash- slag blended geopolymer concrete and compared with OPC

concrete after exposed to NaCl solution for 500 days. According to their report, the

embedded reinforcement bar in geopolymer concrete did not corrode , whereas the

reinforcement bar in OPC concrete was started to corrode after 500 days of exposure.

They mentioned that the protection against corrosion in geopolymer concrete is

provided by the oxygen around the reinforcement in the presence of slag in the binder.

In addition, they also have reported that the electrochemical measurements of

geopolymer concrete are not correlated with the corrosion activity as higher expected

corrosion risk was observed in both types of concrete. However, corrosion activity was

found in the OPC concrete rebar only, and no corrosion product was observed in the

geopolymer concrete rebar.

Therefore, the all above studies investigated the corrosion resistance of geopolymer

concrete exposed in chloride environment only. However, corrosion of reinforcement

bar is induced by the carbonation reaction too. Nevertheless, only one research study

available related to the corrosion of reinforcement bar with carbonation of geopolymer

concrete. Badar et al. [90] investigated the corrosion of steel bars induced by accelerated

carbonation (5% of CO2) in low calcium and high calcium fly ash geopolymer concretes.

They have reported that the low calcium fly ash based geopolymer concrete had a lower

risk of corrosion compared to high calcium fly ash based geopolymer concrete. As shown

in Fig. 2-15, corrosion products have been identified in high calcium fly ash based

geopolymer concrete interface, whereas no sign of corrosion has been observed in low

calcium fly ash based geopolymer. The reinforcement bar obtained from the geopolymer

concrete samples were shown in Fig. 2-16. According to that, the steel bar from high

calcium fly ash based geopolymer was corroded, whereas no sign of corrosion was

observed on the steel bar from low Ca fly ash based geopolymer samples. However,

further studies are required to investigate more about the corrosion resistance of

geopolymer concrete under CO2 environment. Since the accelerated carbonation test

with a higher concentration of CO2 is not suitable for geopolymer [28], corrosion study


45

of geopolymer concrete also needs to be revised according to the appropriate

carbonation, and chloride test methods.

Fig. 2-15 Photographs of geopolymer concretes/ steel interfaces (A), (B) low Ca fly ash

based geopolymer (C) high Ca fly ash based geopolymer after 450 days of accelerated

carbonation. [90]

Fig. 2-16 Photographs of steel rebar in (A) high-Ca fly ash (B), (C) and low-Ca fly ash

based geopolymer after 450 days of accelerated carbonation. [90]


46

2.9 Studies related to durability of geopolymer concrete structures in

field environments

Although many research papers have been published related to the durability evaluation

of geopolymer concrete, those studies have been conducted by an accelerated testing

method on freshly prepared concrete specimens. Besides that, contradict conclusions

were obtained regarding the durability of geopolymer concrete. Therefore, this indicated

that the accelerated testing methods are not suitable to predict the durability properties

of the geopolymer concrete and determining the durability of geopolymer in a real field

environment is more appropriate way compared to accelerated testing methods.

However, the past literature studies indicated that only limited research studies have

been carried out on the geopolymer concrete specimens that are exposed to real field

environment conditions.

In regarding with carbonation of geopolymer concrete, only few research studies are

available for geopolymer concrete exposed in a field environment. This is because the

carbonation rate in the natural exposed condition is very slow compared to accelerated

carbonation testing method. Therefore, more time required to determine the carbonation

of concrete. Bernal et al. [154] investigated the carbonation depth of slag based

geopolymer concrete exposed to real field environment for seven years. They have found

that the carbonation depth values are influenced by the activator and the slag contents.

They have also reported that the carbonation depth of field exposed slag based

geopolymer concrete is less than the test results obtained from accelerated carbonation

testing methods. In addition, seven years old slag based geopolymer concrete displayed

high dense interfacial transition zone and three types of binding gel such as C-A-S-H

and two types of (C, N)-A-S-H binding gels [155]. Therefore, slag based geopolymer

concrete revealed better long-term durable performance without any degradation of

concrete. Furthermore, Nedeljković et al. [156] studied the natural carbonation depth of

alkali-activated pastes after exposed to laboratory ambient environment for 12 months

periods. They have prepared the different geopolymer mixes with a different proportion

of fly ash and slag contents into the mix. The results indicated that the carbonation is

more susceptible to 100% fly ash mix and the carbonation depth is reduced with an

increment of slag component in the mix. Moreover, Cheema et al. [18] found that the fly

ash based geopolymer concrete is more vulnerable to carbonation compared to OPC

concrete in the field environment.


47

Moreover, Shayan et al. [157] conducted the experimental analysis on the core specimens

extracted from the different locations of slag based geopolymer concrete retaining walls

and conducted the compressive strength test and volume of the permeable void test on

the core samples. They have also monitored the corrosion rate of reinforcement bar for

three years by using half-cell potential measurements. Fig.2-17 explains the half-cell

potential measurements at various location of the reinforcement bar in the geopolymer

concrete structure. As shown in half-cell potential measurements, the risk of corrosion is

identified as low. In addition, they have reported that the geopolymer concrete core

specimens had the higher compressive strength. However, they have found the high

VPV values for geopolymer concrete core specimens.

Fig. 2-17 Half-cell potential measurement with age of concrete in real field environment

[157]

Considering the chloride penetration, laboratory studies demonstrated that the fly ash

based geopolymer concrete exhibits lower chloride diffusion and less corrosion effect of

the steel bar compared to OPC concrete [25, 121, 122]. However, as mentioned earlier,

the chloride penetration of OPC concrete reduces with the age of concrete due to the

continuous hydration reaction in OPC concrete with time after the casting of structures.

However, geopolymerisation reaction in heat cured fly ash based geopolymer concrete

is very rapid with little or no further reaction after a few days of curing. Therefore, the

ingress of chloride ions in fly ash based geopolymer concrete could potentially be higher

than OPC concrete after some years of exposure [21]. Compared to laboratory prepared

geopolymer concrete, only a few limited studies available on the durability evaluation


48

of geopolymer concrete exposed in a real chloride environment. Chindaprasirt et al. [23]

investigated the effect of the sodium hydroxide concentration of fly ash based

geopolymer concrete on chloride penetration and steel corrosion after three years of

exposure in the marine environment. They have reported that the concentration of

NaOH is influenced on the chloride diffusion to the geopolymer concrete. Chloride

penetration and the chloride diffusion coefficient decreased with increased NaOH

concentrations due to the increment of the geopolymer reaction rate by higher NaOH

concentration. However, they have not compared geopolymer concrete with OPC

concrete. In addition to that Zhu et al. [19] reported that the fly ash based geopolymer

paste and mortar mixes are more susceptible to chloride diffusion compared to OPC

paste and mortar mixes under non-saturated conditions.

Furthermore, Cheema et al. [18] determined the chloride diffusion of fly ash based

geopolymer concrete exposed to the saline environment for three years period. As

mentioned in their investigation, the chloride ion ingress into geopolymer concrete is

higher than OPC concrete, and geopolymer concrete displayed higher chloride diffusion

coefficient compared to OPC concrete.

Concerning the sulphate resistance of geopolymer under the field condition, only one

investigation was carried out so far. El-Didamony et al. [158] conducted the experimental

analysis to determine the durability of alkali-activated slag pastes immersed in the sea

water and reported that the geopolymer had good durability under sea water

environment. However, they have measured the compressive strength values after

immersed in seawater and there is no information provided about the amount of

sulphate content in geopolymer specimens after the seawater exposure.

2.10 Current application of geopolymer concrete in the construction

field

The application of geopolymer binder in building and civil infrastructure has begun

worldwide [159]. It should be noted that the geopolymer is not a new material in the

application of civil infrastructures. The evidences are shown that the usage of

geopolymer materials has been started about many decades ago. In very early stages,

geopolymer materials were used in Roman architecture to build an ancient Cister

concrete wall, Israeli Roliea Spa ancient baths and other elements [160]. After that, some

evidence available for the application of geopolymer materials in the mid-decade. In the


49

1980s, geopolymer materials have been used in France during the tile production [161].

Furthermore, high rise buildings with more than 20 storeys also built by using alkali-

activated slag based geopolymer concrete in Lipetsk, Russia between the period of 1986

and 1994 [34, 159]. All the building elements were cast with slag based geopolymer

concrete. The exterior walls were cast by in-situ mix, and the floor slabs, staircases and

other structural components were prepared with the pre-cast concrete method. It has

also been reported that the geopolymer concrete was used to prepare commercial scale

products in recent years. Geopolymer concrete also used to prepare the concrete

pavements and roads in Russia and Ukraine. These all geopolymer concrete were

prepared with the activation of slag materials. Geopolymer concrete was also used to

prepare reinforced concrete sewer pipes, railway sleepers and wall panels [162].

In recently, Australia has been started to use geopolymer concrete for many civil

infrastructures in the construction field. In recent times, a building was constructed with

geopolymer precast floor panels for the Global Change Institute at University of

Queensland, Australia [163] and this believed to be a world first use of suspended

modern geopolymer concrete in the building industry. The span of the precast floor

panels was 11 m, and the building contains 33 geopolymer precast floor panels. The

geopolymer concrete was prepared with the combination of fly ash and slag materials,

and the structural performance of geopolymer concrete was checked according to

Australian standards AS 3600. Fig. 2-18 shows the casting and installation procedures of

geopolymer precast beams.

In addition to that geopolymer concrete was also used in the airport construction in

Toowoomba, Queensland, Australia [164]. This is the first green-field public airport in

worldwide, and the geopolymer concrete was used to construct the heavy-duty

pavements in the aircraft turning node and apron areas. The geopolymer concrete was

prepared with the combination of fly ash and slag materials. The concrete was prepared

in a twin mobile wet mix batch plant and supplied to the site by tipper trucks. Curing of

the concrete was carried out in two stages. Initially, a water-based hydrocarbon resin-

curing compound was applied, and then the concrete was covered with a geotextile to

protect against any thermal shock. Moreover, geopolymer concrete was also used in

many other construction applications in Melbourne, Australia such as road pavements,

retaining walls, driveways, footpaths and house-slabs. Fig. 2-19 (a) shows the footpath

built with geopolymer concrete in Westgate Freeway extension Port Melbourne,


50

Australia and the geopolymer concrete precast panels across Salmon Street Bridge, Port

Melbourne, Australia are presented in Fig. 2-19 (b). Although geopolymer concrete

construction has been begun so far, the application is very low compared to OPC

concrete construction. According to the survey conducted form representatives of the

concrete and affiliated industries within Australia, there are many barriers that affect the

wide spreading of geopolymer concrete in the construction field. Among that the lack of

guidelines, lack of standards, lack of long-term durability data (particularly field

performance) and supply chain, availability issues are considered as more important

barriers for geopolymer concrete [165]. Therefore, overcoming from those barriers will

enhance the usage of geopolymer concrete, and due to this, the CO2 emission will be

significantly reduced in future.

Fig. 2-18 (a) casting of the precast beam, (b) installation of precast geopolymer beams

[163]


51

Fig. 2-19 (a) geopolymer concrete footpath (25 MPa) along Westgate Freeway extension

Port Melbourne, Australia, (b) 55 MPa precast panels across Salmon Street bridge, Port

Melbourne, Australia [166].

2.11 Motivation for the study

According to the detailed literature review discussed so far, it is clearly identified that

the application of geopolymer concrete in construction field is an effective way to reduce

the CO2 emission to the environment. The research studies have been conducted on

geopolymer materials in the past few decades. Recently, the commercial application of

geopolymer technology in the construction industry is also being started around the

world. However, the development of geopolymer materials application is very low due

to many technical challenges. The durability of geopolymer concrete in field condition

is one of the primary concerns to reduce the widespread of geopolymer concrete

application, and that needs to be clearly determined to minimise the barrier to the

geopolymer usage.

Based on the durability studies of geopolymer concrete discussed above, it is clear that

the durability behaviour of geopolymer concrete determined from accelerated testing

methods is providing contradict results about the durability of geopolymer materials.

Some of the studies displayed positive response for geopolymer concrete, and some

other studies reported negative responses for geopolymer concrete usage. In addition to

that, the factors controlling the durability of concrete is limited in the laboratory

environments. When the concrete structures are exposed in the real field environment,

it should face several deterioration factors at the same time. However, to produce that

same exposure condition in the accelerated testing method is very difficult and

impossible. Therefore, it is important that to determine the durability of geopolymer

concrete exposed in real field conditions. In addition, the past research studies proved

that only limited studies are available to determine the durability aspects in the actual

environmental conditions. Therefore, based on that, this research study was conducted

to determine the long-term durability of geopolymer concrete in the actual field

environment conditions.

It should be noted that the durability of the concrete structure exposed in the

atmospheric environment primarily affects by the CO2 diffusion from atmospheric

environment. The ingress of CO2 is mainly deteriorated the concrete structures in


52

atmospheric conditions. Therefore, carbonation of geopolymer concrete exposed in the

atmospheric environment for the long-term period will be investigated in this current

study. In addition, the past research works shown that the durability of geopolymer

concrete depends on many factors including the source of materials used to prepare the

geopolymer binders. Among the various source of materials fly ash and slag are two

leading sources, which have been widely used in geopolymer production. Therefore,

geopolymer concrete prepared with fly ash, slag or blended of fly ash and slag mixes

should behave in different ways, when they exposed to the field environment. Thus, it

is necessary to determine the durability behaviour of various types of geopolymer

concrete, and that will provide a better understanding of the durability aspects.

Therefore, in the present study, the durability of geopolymer concrete structures

prepared with fly ash based geopolymer mix and fly ash and slag blended geopolymer

mixes exposed in atmospheric environment condition will be investigated. The

investigation will be conducted by taking the core specimens from the concrete

structures, and the durability of geopolymer concrete will be compared with OPC

concrete from the same exposed environment. The test results obtained from this

investigation are provided in Chapter 3.

When we considered the durability of geopolymer in an aggressive environment,

carbonation is not only the factor influenced on the durability. In aggressive condition

such as marine and saline environment, chloride and sulphate diffusions are also mainly

affected the durability in addition to the carbonation reaction. Therefore, due to this

reason, durability behaviour of concrete structures exposed in the aggressive

environment should be varied from the durability of atmospheric exposed concrete

structures. Therefore, it is necessary to determine the durability of geopolymer materials

in aggressive environment for the commercial enhancement of geopolymer concrete in

the marine or saline exposed conditions. The combined effect of carbonation, chloride

diffusion and the sulphate attack on the geopolymer concrete will be investigated as part

of this research study. As explained in the above paragraph, different types of

geopolymer concrete mix included in this investigation too. The durability of fly ash

based geopolymer concrete and slag, fly ash blended geopolymer concrete exposed in

the aggressive environment will be evaluated. The comparison also will be conducted

with the concrete prepared with Portland cement from the same exposed environment.

The test results of this study are presented in Chapter 4.


53

Apart from the field investigations, laboratory-accelerating testing is required to validate

the test result obtained from field samples. In addition to that, there will be some

limitations presented to collect the information regarding the durability of concrete in

some cases from the samples collected from field investigation. Therefore, few

experimental tests will be carried out with the laboratory prepared samples. Alalki

leaching from the geopolymer binder will be studied after immersed of the speciemens

in de-ionised water. In addition, the strength loss of geopolymer in the accelerated

wetting-drying cyclic testing with water, chloride water and the combination of chloride

and sulphate solution will be studied with the laboratory prepared mortar specimens

and the effect of source materials on the strength loss of the mortar samples will be

evaluated. The experimental results from this investigation are provided in Chapter 5.

Corrosion of reinforcement bar is an essential parameter in the concrete building.

Therefore, corrosion of reinforcement in geopolymer concrete will be examined with the

concrete prepared with different proportions of fly ash and slag constituents. According

to this investigation, the influence of the source of geopolymer materials on the corrosion

of reinforcement bar in geopolymer concrete will be determined. The test results from

this relevant experimental method are presented in Chapter 6.

The coefficient of carbonation is key parameter to predict the carbonation behaviour of

the concrete. The CO2 diffusion of geopolymer concrete will be determined with the

mathematical models developed with the diffusion equation based on the Fick’s law and

the empirical equations. The analysis and the results from mathematical models were

presented in Chapter 7.

Chapter 3 The durability of geopolymer

concrete exposed to the atmospheric

environment

3.1 Introduction

This chapter experimentally investigates the durability of geopolymer concrete exposed

to the atmospheric environment. The investigation was conducted on the core specimens

from geopolymer concrete structures exposed to the atmospheric environment for the 8-

year period. Three different types of geopolymer mix compositions, such as fly ash based

geopolymer concrete, and two different types of fly ash-slag blended geopolymer

concrete, were included in this analysis. As identified in the literature review of this

thesis, carbonation of concrete is an important parameter that mainly influences the

durability of concrete when exposed to the atmospheric environment. Therefore, this

study primarily focused on the investigation of carbonation rates in different

geopolymer concrete structures and compared with OPC concrete structure exposed to

the same environment. The pH variation along the depth of concrete and Fourier

Transform Infrared (FT-IR) were conducted to study the formation and variation of

carbonation products in concrete. Further, transport properties (apparent volume of

permeable voids (AVPV) and the sorptivity coefficient) and pore structure (using

mercury intrusion porosimetry (MIP) test) were also studied.

3.2 Field Description

3.2.1 Description of concrete structures

3.2.1.1 Fly ash-based geopolymer concrete & OPC concrete culverts in atmospheric

exposure condition.


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55

The fly ash based geopolymer concrete (FGPC-A) and OPC (OPC-A) concrete structures

used for the durability assessment were constructed as box culvert structure in 2007 and

placed in Lake King, Western Australia. Western Australia has a temperate climate with

an average annual maximum temperature of 24.7°C and average annual rainfall of 727

mm. Fig. 3-1 shows the FGPC-A and OPC-A box culvert exposed to outdoor atmospheric

conditions. In 2015, core specimens were extracted from the concrete structures to

conduct an experimental assessment of its durability properties. The diameter of the

extracted core samples was 68 mm and 94 mm from vertical parts and top slab,

respectively. The manufacturing details of geopolymer concrete culvert have been

described in elsewhere [167]. The mix details of the concrete structures and the exposure

conditions are described in the following subsections

Fig. 3-1 (a) FGPC-A box culvert, (b) OPC-A concrete box culvert

3.2.1.2 Fly ash-slag blended geopolymer concrete in atmospheric exposure condition.

Durability behaviour of fly ash-slag blended geopolymer concrete exposed to the

atmospheric environment for eight years of periods was evaluated on two different

reinforced fly ash-slag blended geopolymer concrete slabs exposed to the ambient field

environment. Both slabs were cast in 2007 by Zeobond Pty Ltd and placed in

Campbellfield, Victoria, Australia. Campbellfield, Victoria has a mild temperate climate

with an average annual maximum temperature of 21 °C and average annual rainfall of

603 mm. The dimensions of the first slab (FSGPC-1) were 7.8 m × 4.07 m with the

thickness of 600 mm. The next slab, which was located adjacent to the FSGPC-1 slab, was

classified as FSGPC-2. The FSGPC-2 slab was submerged into the soil, and only the top

surface was exposed to the atmosphere. Two different GPC mixes were used to cast two

types of slabs. The thickness of the FSGPC-2 slab was 150 mm. Fig. 3-2 shows the


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locations of the slabs exposed in outdoor conditions. The diameter of the extracted core

samples was 94 mm.

Fig. 3-2 Fly ash-slag blended geopolymer concrete structures in the atmospheric exposed

environment

3.2.2 Mix design of concrete structures

3.2.2.1 Mix details of fly ash-based geopolymer concrete & OPC concrete culverts in

atmospheric exposure condition.

For the preparation of FGPC-A and OPC-A, Class F fly ash from the Collie power station,

Western Australia and Cockburn general purpose cement respectively were used. The

mix design details of both types of concrete are provided in Table 1. Geopolymer binder

was prepared with the activation of fly ash precursor by commercial grade alkaline

activators of sodium silicate (Na2SiO3) and sodium hydroxide (NaOH). NaOH solution

was prepared with the molarity of 8 M by dissolving NaOH flakes (98% purity) in water.

The ratio of Na2SiO3 to NaOH was chosen as 2.5. To maintain the workability of the

geopolymer concrete mix, high range water reducing superplasticiser (naphthalene

sulphonate-based) was used. The FGPC-A was subjected to steam curing at 60°C for 24

hours, whereas, the OPC-A concrete culvert was cured at ambient temperature. The mix

design of GPC culvert undergone a trial and error process and the optimum mix design


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57

(given in Table 3-1) has been derived with several trials. The GPC with similar mix

composition exhibited excellent mechanical properties of high compressive strength and

tensile strength [33]. This mix also showed lower shrinkage and low creep coefficient

values, and excellent resistance to sulphate attack during the accelerated laboratory

testing conditions [41]. After 1 year period, the creep coefficient and the shrinkage values

of the concrete were 0.6 and 100 micro strains, respectively. In addition to field extracted

specimens, laboratory specimens were prepared to check and compare the compressive

strength and permeable properties of aged concrete with early age concrete. The

compressive strength and water absorption values obtained for GPC concrete are 39MPa

and 4.7%, respectively [167].

Table 3-1 Mix compositions of concrete (kg/m3) [18, 167]

Materials Mass (kg/m3)

FGPC-A OPC-A

Coarse

Aggregates

14mm 554 920

10mm 702 300

Fine Sand 591 640

Fly Ash (Low Calcium ASTM Class F) 409 -

Cement 400

Sodium Silicate Solution (SiO2/Na2O =2) 102 -

Sodium Hydroxide Solution 41 (8 M) -

Superplasticiser (SP) 6 -

Water 22.5 170

3.2.2.2 Mix details of fly ash- slag blended geopolymer concrete slabs in atmospheric

exposure condition.

A combination of Bayswater type fly ash and ground granulated blast furnace slag

(GGBFS) were used as precursors to prepare geopolymer binders. The mix compositions

of both types of the slabs are provided in Table 3-2. FSGPC-1 geopolymer binder was

prepared with a blend of 75% of fly ash and 25% of GGBFS, and a combination of 70 %

of fly ash and 30 % GGBFS as precursors in FSGPC-2 concrete. A combination of 7 M (50

mol % Na cations and 50 mol % K cations) of sodium hydroxide (NaOH) and potassium

hydroxide (KOH) were used as hydroxide activator for both slabs. In addition, sodium


environment

58

silicate (Na2SiO3) was added to activator combinations for the FSGPC-1 slab, which

consisted of 2.5% SiO2 relative to the binder content. A commercial-grade Na2SiO3 with

D grade (29.4% SiO2 and 14.7% Na2O by weight from PQ Australia) was used. The water

to binder ratio used to prepare activator combination was 0.25. However, to achieve

sufficient workability, extra water was added to maintain total water to binder ratio at

0.3.

Table 3-2 Mix composition details of fly ash-slag blended geopolymer concrete

Materials(kg/m3) Slab 1 (FSGPC-1) Slab 2 (FSGPC-2)

Total binder 400 400

Fly ash 300 280

GGBFS 100 120

Fine aggregate 630 630

Coarse aggregate 1150 1150

NaOH pellet 14 14

KOH pellet 19.6 19.6

Na2SiO3 solution 34.48 -

Water used to prepare activator 81.04 100.32

Extra water 20 20

3.3 Testing methods

3.3.1 Carbonation depth measurement

Carbonation depths of the core specimens were tested immediately after extracting from

the concrete structures. A 1% solution of phenolphthalein indicator was sprayed onto

the surface of the core specimens, and the depth of the carbonation was measured by

observing the colour change of phenolphthalein solution. Phenolphthalein is used as a

pH indicator which changes from purple to colourless when the pH value of the concrete

surface drops below 8.3 due to carbonation [168]. Non-carbonated concrete surfaces

remained with purple colour.

3.3.2 pH profile measurement

The variation of pH with the depth of concrete surface was determined according to the

water suspension method recommended by Räsänen et al. [169]. In this method,


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59

powdered samples were first collected from the core specimens by drilling at various

depth intervals. The powdered samples from the core specimens of fly ash based

geopolymer concrete and OPC concrete structures were collected at 3 mm interval. For

the fly ash-slag blended concrete, samples were collected at 5 mm intervals from the

surface of the core specimens. Approximately 15 g of powdered samples were collected

for each depth interval.

The powdered sample was then mixed with distilled water with the weight proportion

of 2:3 and then the mixture was stirred for 15 min with a magnetic stirrer. The solution

was filtered using filter paper, and the pH value of the filtered solution was measured

with a pH electrode. The temperature of the solution was kept in the range between

20 ± 2 °C. The pH electrode used to determine the pH value of the solution was shown

in Fig. 3-3.

Fig. 3-3 Aqua pH meter

3.3.3 Water absorption (Ai) and apparent volume of permeable voids

(AVPV)

The permeable parameters of the concrete are of primary properties revealing the

durability of the concrete structures [170]. Concrete with less permeable can resist the

penetration of aggressive agents such as CO2, chloride, sulphate ions and other

aggressive agents. Therefore, determination of the permeability characteristics is

important to predict the durability of concrete. In this case, the AVPV measurement


environment

60

could be an indicator which explains the pore structure of the concrete including, the

capillary pores and other void systems.

The determination of water absorption (Ai) and apparent volume of permeable voids

(AVPV) measurement was carried out according to Australian standard AS 1012.21

(1999) [171]. For this test, specimens were prepared by cutting the core specimens into

equal pieces with the thickness of 50 mm, and the average value of three slices was used.

Water absorption (Ai) values were calculated using oven dry weight and saturated

weight measurements. First, the specimens were kept in an oven at 105 ± 5 °C

temperature for 24 hr and then weighed to determine the oven dry weight (M1). Then,

the specimens were immersed in water for 24 hr, and the saturated weight of the samples

(M2) was measured. The Eqn (2) was used to calculate the water absorption (Ai) values

with the weight measurements of M1 and M2. In addition to the above readings, the

weight of the specimens was measured after boiling samples as well as the suspended

weight of the water to calculate the AVPV values. The boiled specimen weight (M3) was

determined by keeping the samples at 100°C for 5.5 hr, and the suspended weight of the

specimens (M4) was measured by holding a track in water. Eqn (3) was used to calculate

the AVPV values with above weight measurements. All weight measurements were

carried out using a balance with an accuracy of 0.01 g. Fig. 3-4 illustrates the

experimental arrangement to determine the oven-dried, saturated, boiled and

suspended the weight of the specimens.

𝐴(𝑖) =

𝑀2 − 𝑀1

𝑀1× 100%

(2)

𝐴𝑉𝑃𝑉 =

𝑀3 − 𝑀1

𝑀3 − 𝑀4× 100%

(3)

To compare the changes of AVPV values with age, fresh concrete specimens were

prepared in the laboratory, and the same tests were conducted at the age of 28-days. The

cylindrical (100 mm dia. x 200 mm height) concrete specimens were prepared with

similar mix compositions provided in Table 3-1 and Table 3-2. All the concrete cylinders

were cured with the corresponding curing methods that are used for the curing of the

field exposed concrete structures. After 28 days of curing, all the specimens were cut into

four pieces with 50 mm thickness. Thereafter, all specimens were included into the same


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experimental procedures, which is described above to determine the Ai and AVPV

values.

Fig. 3-4 Water absorption and AVPV test: (a) oven dried samples, (b) water immersed

samples, (c) boiled samples, (d) suspended weight measurement of samples.

3.3.4 Sorptivity analysis

The sorptivity of aged concrete specimens were conducted according to ASTM C1585

[172], where, 50 mm thick concrete slices were considered. Core specimens from the field

exposed concrete structure were cut into 50 mm thick pieces. The pieces of 50 mm length

from the outside exposed surface and another disc was taken at the mid-depth level (50

mm to 100 mm) were included into the experimental analysis to evaluate the changes of

sorptivity parameters with increasing depth. In addition, 100 mm×200 mm size of

cylindrical specimens was prepared in the laboratory with the same mix composition of

field concretes and subjected to sorptivity analysis to study the sorptivity variation with

age. Cylindrical specimens were cut into 50 mm thick pieces, and the test was conducted


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62

on the pieces from top part (50 mm length from the outside exposed surface) and mid-

depth level (50 mm to 100 mm).

Fig. 3-5 illustrates the schematic diagrams and the experimental arrangement of the

sorptivity test. All specimens were initially stored in an environmental chamber with

50°C temperature and 80% of relative humidity for three days of the period. Then the

side surface of the specimens was sealed by using an epoxy coating. The top surface,

which was not in contact with water, was covered with plastic film to prevent moisture

transportation through the specimens. Then, the specimens were placed in water-bath,

and the mass gain due to water absorption was measured at time intervals, as per

standard testing method. The water absorption value (I) was calculated using the change

of specimen weight after being placed in water (Mt) for certain duration (t) and the

surface area of the specimen that was exposed to water (a) by using the following

equation:

𝐼 =

𝑀𝑡

𝑎 × 𝑑

(4)

Where, d = the density of the water

Fig. 3-5 (a) schematic diagrams of the sorptivity test, (b) experimental arrangement of

the sorptivity test

3.3.5 Fourier Transform Infra-red (FT-IR) analysis

The Fourier Transform Infrared (FT-IR) analysis was conducted to identify the

carbonated and un-carbonated products of different concrete specimens. This is a

qualitative method to identify the effect of carbonation on bonding characteristics of

concrete. A Nicola thermostate FT-IR spectroscope (Fig. 3-6) was used to measure the

FT-IR spectra of concrete at depths. Powder samples were extracted from the concrete


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63

specimens using a profile grinder and mixed with KBr to prepare pellets. The IR spectra

were recorded at 32 scans per sample collected from the range 4000 cm-1 to 525 cm-1 at 4

cm-1 resolutions.

Fig. 3-6 FT-IR spectroscope

3.3.6 Mercury intrusion porosimetry (MIP) test

Pore size distribution of the concrete was determined by using a Mercury intrusion

porosimetry (MIP) analysis. The test was conducted by using Autopore IV 9500 mercury

porosimeter (Fig. 3-7) with a peak pressure of 206 MPa. The sample preparation for MIP

test was conducted by collecting solid mortar particles (around 1 g) from the core

specimens at various depth level. Before starting the test, the samples were kept in an

oven at 80 °C for 24 hr to remove the moisture present in the samples. MIP

measurements were carried out using mercury with surface tension and the contact

angle of 0.48 N/m and 140 °C, respectively. The relationship between pressure and pore

diameter is expressed as follows:

𝑑 =

−4𝛾(𝑐𝑜𝑠𝜃)

𝑝

(5)

Where d is pore diameter, 𝛾 is the surface tension of mercury, ɵ is the contact angle

between the surface of the specimen and mercury, and p is the absolute intrusion

pressure.


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64

Fig. 3-7 Mercury porosimeter

3.4 Test results and discussions

3.4.1 Carbonation resistance of geopolymer concrete

The phenolphthalein solution application identified carbonation depth of the core

specimens. The phenolphthalein solution (1% phenolphthalein in 70% ethanol) was

sprayed on the core surfaces, immediately after taken from the concrete structures. The

depth of the carbonation was measured with the colour change of phenolphthalein by

using the measuring tape. From the carbonation depth measurement (Xc), the coefficient

of carbonation (K) was predicted by the following empirical relationship:

𝑋𝑐 = 𝐾√𝑡 (6)

Where Xc is the measured carbonation depth (mm), t is the exposure period (year), and

K is the corresponding carbonation coefficient (mm/year0.5). This formula is based on

the square-root-t-law, which has been used previously by many researchers to determine

the carbonation behaviour of the concrete [173-175].


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65

3.4.1.1 Carbonation resistance of fly ash based geopolymer concrete

Fig.3-8 shows the carbonation depth measurements of FGBC-A and OPC-A concrete

core specimens, which are extracted from the FGBC-A and OPC-A concrete culvert

structures exposed to the similar atmospheric environment for eight years. As shown in

Fig.3-8, the application of phenolphthalein solution resulted in the colour change to pink

in FGBC-A specimens. Therefore, this reveals that the clear carbonation depth

measurement is difficult by the phenolphthalein application method for fly ash based

geopolymer concrete. A similar observation has also been reported for fly ash based

geopolymer concrete by various researchers [25, 90, 111]. According to these studies, fly

ash based geopolymer concrete showed pink colour surface after the phenolphthalein

application. However, carbonation of fly ash based geopolymer concrete can be

classified into three different zones with the colour variation of phenolphthalein solution

such as fully carbonated zone (almost colourless), partially carbonated zone (light pink

colour) and un-carbonated zone (deep pink colour) [176]. Fig.3-8 also remarks such

classification of carbonation zones of both concrete core specimens. Accordingly, FGBC-

A exhibited 45 mm of the fully carbonated zone and 70 mm of the partially carbonated

zone after eight years of exposure. The carbonation depth measurements of OPC-A

concrete core specimens can be seen in Fig.3-8 (b), where an excellent response to

phenolphthalein indicator can be observed with the carbonation depths ranging

between 4-10 mm. This indicates carbonation resistance of fly bash based geopolymer

concrete was much less compared to OPC concrete in an ambient exposed environment.

The coefficient of carbonation (K), as measured from Eqn (6) indicates that the K value

for FGBC-A specimen was 15.9 mm/year0.5 and OPC-A concrete specimens was 1.06 to

3.54 mm/year0.5. It is worth to mentioning here that, for FGBC-A specimens, carbonation

depth measurement from fully carbonated zone was used to calculate the K value. This

illustrates that the rate of carbonation in fly ash based geopolymer concrete is much

higher than OPC concrete when exposed to the atmospheric environment and

significantly differs from laboratory carbonation measurements on fly ash based

geopolymer concrete. According to the accelerated carbonation testing methods

conducted previously, low Ca fly ash based geopolymer concrete displayed better

carbonation resistance compared to OPC and high Ca fly ash based geopolymer

concretes [90]. The carbonation rate observed for OPC-A concrete is comparable with


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the previous carbonation results of normal OPC concrete exposed in field environment

[177].

Fig.3-8 Carbonation depth measurements of core specimens using a phenolphthalein

indicator (a) FGBC-A, (b) OPC-A concrete

3.4.1.2 Carbonation resistance of fly ash-slag blended geopolymer concrete

Carbonation resistance of fly ash- slag blended geopolymer concrete was evaluated with

two distinct types of geopolymer concretes (FSGPC-1 and FSGPC-2). Carbonation depth

measurements of both type concrete specimens are shown in Fig.3-9. It can be seen from

Fig.3-9 that the carbonation depth of FSGPC-1 concrete is much higher than the values

obtained from FSGPC-2 concrete. The carbonation depth values for the FSGPC-1 and

FSGPC-2 concrete were measured as 23.5–27.5 mm and 8–14 mm respectively, after the

exposure of atmospheric environment for eight years. Such significant variations in

carbonation depths could be attributed to either different geopolymer concrete mix

designs or the location of the coring in the slabs. Indeed, the core specimens were

extracted from the different surfaces such as the vertical surface for FSGPC-1 concrete

slab and the horizontal top surface of the FSGPC-2 concrete slab. The moisture content

of the concrete could be different at the vertical and horizontal surfaces that may affect

the CO2 diffusion. In addition, moisture variation would also be occurred between both

slabs due to exposure conditions. As discussed before, the FSGPC-2 concrete slab was

submerged into the soil, and only the top surface of the slab was exposed to the

atmosphere, whereas the FSGPC-1 concrete slab was entirely exposed to the atmospheric

environment.


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Fig.3-9 Carbonation depth measurements of core specimens (a) & (b) FSGPC-1 specimen

before and after applying phenolphthalein, (c)&(d) FSGPC-2 specimen before and after

applying phenolphthalein

Carbonation resistance of fly ash-slag blended geopolymer concrete specimens is then

compared with previously published studies such as Castel et al. [177] and Ho and

Lewis [178]. In the first study, two OPC concrete beams were exposed to the ambient

environment for 13 years in South-West of France were considered [177]. On the other

hand, Ho and Lewis [178] considered 75 mm × 75 mm × 300 mm concrete samples

exposed to ambient exposure in Melbourne, Australia. The water to binder ratio used in

the mix was 0.55. The compressive strength of the OPC concrete measured by coring the

beams in French beams was 45 MPa and 56 MPa for 28 days and 13 years, respectively.

The carbonation depth values measured for these beams ranged from 7 to 13 mm. The

concrete specimens prepared by Ho and Lewis [178] were consisting of 20% fly ash as

supplementary cementitious material and showed the 28-day compressive strength of

46 MPa. The carbonation coefficients of one-year-old specimens were determined as

4.5 mm/y0.5 for north oriented vertical exposure and 3.0 mm/y0.5 for south inclined


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exposure. The variation in the carbonation depths between north and south exposures

was reported as due to lower rainfall on the vertical surfaces.

The coefficient of carbonation (K) values of the concrete specimens of this study, as

calculated using the Eqn (6), are given in Table 3-3. From Table 3-3, the carbonation rate

of FSGPC-1 concrete was found higher than OPC concrete in the ambient exposure

conditions. whereas, the carbonation rate of FSGPC-2 concrete is comparable with OPC

and 20% fly ash concrete.

Table 3-3 K values obtained for the OPC concrete and the two geopolymer concretes (in

mm/yr0.5)

OPC Concrete

(mm/yr0.5)

20% fly ash

concrete (mm/yr0.5)

FSGPC-1

(mm/yr0.5)

FSGPC-2 (mm/yr0.5)

2.0 – 3.6 3.0 - 4.5 8.3 – 9.8 2.8– 5.0

3.4.1.3 Discussion of geopolymer concrete carbonation resistance

The above test results reveal that the carbonation resistance of geopolymer concrete is

lower than OPC concrete. Given the same exposure conditions, CO2 ingress into the OPC

concrete surface decreases with time due to the formation calcite (CaCO3) layer, a

product of carbonation reaction. During the carbonation in OPC concrete, CO2 dissolute

in the concrete pore fluid and this reacts with calcium from calcium hydroxide

(Ca(OH)2) and calcium silicate hydrate. This causes the formation of solid, dense, and

water-insoluble CaCO3 layer on the concrete surface, reducing the carbonation rate with

time.

On the other hand, the carbonation process in geopolymer concrete produces sodium

carbonate (Na2CO3) and potassium carbonate (K2CO3) components. Particularly, in fly

ash based geopolymer concrete, primary carbonation reaction products are Na2CO3 and

K2CO3 due to the reactions of NaOH and KOH with CO2 in the atmosphere [179]. These

carbonation products are highly soluble and can dissolve in the contact water when the

concrete is exposed to outdoor environment. Thus, the porosity of the concrete increases

and further increase the penetration of CO2 into concrete. The porosity and the pore size

distribution of the concrete specimens are discussed in the later part of this chapter.


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While Bernal et al. [154] report that the fly ash-slag blended geopolymer concrete

produces CaCO3 as one of the carbonation reaction components, the amount of Ca in the

slag is low compared to Portland cement. Therefore, the formation of CaCO3 is also very

small, and this cannot assist in reducing the CO2 diffusion rate in fly ash- slag blended

geopolymer concrete. In addition, the formation of soluble carbonation components

such as Na2CO3 and K2CO3 also contributes to higher carbonation rate in fly ash- slag

blended geopolymer concrete.

Moreover, the selection of activator type in geopolymer concrete also significantly

influences the carbonation rate of fly ash-slag blended geopolymer concrete, as observed

from above experimental results. The incorporation of Na2SiO3 causes more carbonation

compared to the geopolymer concrete mix activated with NaOH solution only.


3.4.2.1 pH measurements of fly ash based geopolymer concrete

pH measurements have been conducted for core specimens (8 years old) extracted from

leg part of the culverts. Fig.3-10 shows the pH profile with increasing depth measured

from the external surface. As shown in Fig.3-10, the pH of FGPC-A does not vary

significantly with the depth; with the pH range from 9.92 to 10.41 from the exposure

surface to 30 mm depth level. The corresponding pH values for OPC-A concrete was

measured as 10.84 at a depth of 2.5 mm and 12.32 at the 30 mm depth respectively. In

addition, the pH value of the uncarbonated zone of the FGPC-A core specimen (120 mm

depth level from exposed surface) was measured as 10.5, revealing that the carbonation

does not have a significant effect on the pH of the fly ash based geopolymer concrete in

the atmospheric exposed environment. It must be underlined that the measured pH

values in this study for un-carbonated fly ash based geopolymer concrete is lower than

previously reported values [11, 111, 148]. The past research studies showed that the pH

of fresh geopolymer concrete is approximately 11.5 [11, 111, 148].

The difference in the pH measurements of both type of concrete can be explained as

follows. Considering the pH of concrete is directly related to pore solution occupied in

the open pores of concrete, OPC concrete contains a combination of Ca (OH)2 and C-S-

H gel. In contrast, the pH of fly ash based geopolymer concrete is influenced by the pH

value of the activator solution and the amount of residual activator that remains in


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geopolymer binder. Therefore, the pH of the FGPC-A specimen is mainly controlled by

the NaOH [111]. Thus, the pH of the carbonated and un-carbonated geopolymer

concrete vary from OPC concrete pH range.

Furthermore, pH of the OPC concrete is generally reduced to a value of 9.0 with a

complete carbonation reaction. However, this study shows that the pH value of OPC-A

concrete core specimens at 5 mm depth level was 10.84 and this indicates that the OPC

concrete has not been entirely carbonated at this depth. It should be noted that the

carbonation depth of the OPC-A concrete core specimen, which is used for the pH

measurement test was 4 mm. However, the first pH measurement was taken up to 5 mm

depth level from the exposure surface. Therefore, this indicates pH measurements were

conducted not only in the carbonated zone, but also accumulated in the partially

carbonated zone. The partially carbonated zone consists non-saturated carbonation

products including Ca(OH)2 and calcium silicate hydrate [180], and hence, retained a

higher pH value. FT-IR analysis confirmed the presence of partially carbonated zone

after 4 mm depth level, and further discussion is presented in the upcoming section.

Fig.3-10 pH variation with depth of concrete from the exposed surface (8 years old)

3.4.2.2 pH measurements of fly ash-slag blended geopolymer concrete

Fig.3-11 depicts the changes of pH value against the depth of the concrete surface for

both types of fly ash–slag blended geopolymer concrete specimens. The pH of the

FSGPC-1 concrete ranges from about 10.07 at the surface to 11.25 at 50 mm depth, and

the corresponding pH variation in FSGPC-2 concrete was obtained as 9.68 to 11.38,


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respectively. The carbonation front depths are also marked in Fig.3-11 for both type of

specimens. According to that, the pH values of the carbonated parts of both types of

concrete is less than 10.5, which is higher than the carbonated OPC concrete. It was

reported by Khan et al. [176], that the geopolymer concrete mix prepared with the high

amount of low calcium fly ash resulted in discoloured zones for the test method with

phenolphthalein indicator when pH is inferior to 10.5. Therefore, considering 10.5 as the

carbonation front, Fig.3-11 shows a good agreement between the pH profiles results and

the carbonation front obtained using the phenolphthalein indicator (23.5–27.5 mm for

FSGPC-1 concrete and 8–14 mm for FSGPC-2 concrete). Furthermore, it can be seen from

both types of specimens that the pH value of the carbonated geopolymer concrete

(produced with a higher amount of fly ash precursors) is higher than 10.

Fig.3-11 pH value versus depth in fly ash-slag blended geopolymer concrete from the

surface

3.4.3 Volume of permeable void test results

3.4.3.1 Volume of permeable void test results for fly ash based geopolymer concrete

The water absorption and apparent volume of permeable voids (AVPV) of the FGPC-A

and OPC-A concrete core specimens are shown in Table 3-4. To compare the aged

specimen’s results with fresh concrete, the test was conducted on the fresh concrete

specimens prepared with same mix compositions. The test results of fresh concrete

specimens are also presented in Table 3-4. As shown in Table 3-4, the variation of water


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absorption values with the age of the concrete was minor, which is in the range of 4.9%

to 4.97% for the FGPC-A concrete specimens and 5.35% to 5.22% for the OPC-A concrete.

These lower water absorption values are indicating lower porosity of the concrete.

Moreover, the AVPV values of FGPC-A specimens are increasing with the age of the

concrete. For instance, the fresh FGPC-A specimens showed the AVPV of 11.4%,

whereas, eight years old specimen had a value of 12.27%. On the other hand, OPC-A

concrete specimen showed trivial changes with age. The AVPV values obtained for 28

days and 8 years old OPC concrete specimens were 12.25 and 12.5%, respectively.

According to VicRoads Specification Section 610 [181], the durability of the concrete

types can be classified based on the measured AVPV value of core specimens. As per

specification, the concrete with AVPV values less than 12% is classified as excellent

quality concrete, and the concrete with the values in the range of 12%-14% is considered

as good quality. This study shows that the values obtained for both types of concrete are

lower than 13% and therefore, the quality of the concrete is categorised as good quality

range after the age of 8-year.

Table 3-4 Water absorption and AVPV values

Specimens No Water absorption (%) AVPV (%)

FGPC-A 28 days 4.90 11.4

FGPC-A 8 years 4.97 12.27

OPC-A 28 days 5.35 12.25

OPC-A 8 years 5.22 12.5

3.4.3.2 Volume of permeable void test results for fly ash-slag blended geopolymer

concrete

Table 3-5 depicts the water absorption, and AVPV measurements of fly ash-slag blended

geopolymer concrete specimens at the age of 28 days and eight years. As similar to fly

ash based geopolymer concrete specimens, the changes of water absorption value with

age of fly ash-slag blended geopolymer concretes is small and negligible. FSGPC-1

concrete specimens showed a variation from 7.15% to 7.27% after eight years of


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exposure, and the respective AVPV values for FSGPC-2 concrete varied from 6.02 to

6.39%. For these types of concrete, AVPV values were increased with age. The AVPV of

FSGPC-1 concrete is increased from 16.4% to 17.04% from 28 days to 8 years period, and

the AVPV of FSGPC-2 concrete is changed from 13.2% to 14.35% during the same period.

Furthermore, the water absorption and AVPV values of FSGPC-1 concrete are higher

than FSGPC-2 concrete specimens. This well correlates with the carbonation test results.

More precisely, FSGPC-1 type concrete displayed higher carbonation depth compared

to FSGPC-2 type concrete after 8 years of exposure.

Furthermore, according to the VIC roads classification Section 610 [181], concrete quality

of both types of fly ash-slag blended geopolymer concrete can also be identified. The

FSGPC-1 type geopolymer concrete is classified as marginal quality, and the FSGPC-2

concrete is classified as normal quality concrete.

Table 3-5 Water absorption and AVPV values

Specimens No Water absorption(Ai)% AVPV%

FSGPC-1 -28 days 7.15 16.4

FSGPC-1 -8 years 7.27 17.04

FSGPC-2 -28 days 6.02 13.2

FSGPC-2 -8 years 6.39 14.35

3.4.4 Sorptivity analysis test results

3.4.4.1 Sorptivity analysis of fly ash based geopolymer concrete

Fig.3-12 illustrates the sorptivity analysis results for fresh and eight-year aged concrete

specimens, as measured from the depth of water penetration with time. The rate of water

absorption of core specimens is compared with fresh concrete properties after 28 days of

curing. Here, ‘T’ and ‘M’ are denoted as top and middle part of the concrete specimens,

respectively. The test results indicated that the top part of the atmospheric exposed

geopolymer concrete specimens (FGPC-A, 08 years ‘T’) had the highest water absorption


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rate and the top part of atmospheric exposed OPC concrete (OPC-A, 08 years ‘T’) had

the lowest rate. The comparison of mid part of concrete specimens indicates that the 8-

year-old FGPC-A specimens shows a lower sorption rate, while the mid part of the aged

OPC-A concrete ( OPC-A 08 years ‘M’) specimens showed a higher rate compared to

their top part, indicating that the porosity of the FGPC-A increased when it is exposed

to the field environment.

On the other hand, the change in water absorption rate between Top and Mid part of

fresh concrete (28 days) is very small, indicating that the exposure to the atmospheric

environment has a significant influence on the water absorption rate. The reduction of

water absorption at the top part of atmospheric exposed OPC concrete is the result of

carbonation reaction [85]. The pore structure and the interconnectivity of the pore

systems of the top surface of OPC concrete changes with the formation of CaCO3 and a

solid densified layer formed by the carbonation reaction. As a result, water absorption

rate of the top part of OPC concrete reduces with the age of the concrete. In contrast, the

porosity increment in FGPC-A is attributed by the formation of soluble carbonated

products such as Na2CO3 and K2CO3, which induce a higher water absorption rate for

carbonated FGPC-A concrete samples. To further examine the porosity and pore size

distribution of both concrete types, a Mercury Intrusion Porosimetry (MIP) test was

carried out on the concrete specimens. The discussion corresponding to the pore

distribution is provided in section 3.4.6.

Fig.3-12 Sorptivity curves of FGPC-A and OPC-A concretes


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It is important to note that the AVPV values of FGPC- A are very close to that of OPC-A

concrete, whereas, FGPC-A had greater capillary sorption parameters compared to OPC-

A concrete sorption values. This is because the capillary sorption is related to total

porosity, tortuosity and the size of the pore network [24], while AVPV is associated with

open porosity values only. Here, the lower sorptivity coefficient of OPC-A concrete is

associated with the formation of a more densified pore structure, resulting in a more

tortuous network and reduced pore size. Therefore, measuring the sorptivity parameters

of the concrete specimens is an accurate method to predict the durability behaviour of

the concrete.

The initial rate of water absorption (mm/s1/2) determined from the slope of the line that

is the best fit to water absorption (I) plotted against the square root of time (s1/2), are

reported in Table 3-6. These results indicated that the FGPC- A concrete specimens are

showing higher sorptivity coefficient compared to OPC-A concrete specimens.

Therefore, concretes prepared with a cement binder exhibits excellent resistance to

incursive agents and are highly durable in field environment compared to fly ash based

geopolymer concrete.

Table 3-6 Initial rate of water absorption of FGPC- A and OPC-A concretes

Sample Type The initial rate of water absorption S1 (mm/s1/2)

Top part specimen Mid Part specimen

FGPC- A 28 days 0.0157 0.0127

FGPC- A 08 years 0.0242 0.0122

OPC-A 28 days 0.0102 0.0087

OPC-A 08 years 0.0017 0.0052

3.4.4.2 Sorptivity analysis of fly ash-slag blended geopolymer concrete

Fig.3-13 shows the sorptivity curves of both types of fly ash – slag blended geopolymer

concrete specimens. As similar to the previous section, the rate of water absorption of

core specimens are compared with fresh concrete properties after 28 days of curing.

Here, ‘T’ and ‘M’ denotes the top and middle part of the concrete specimens,

respectively. As similar to the observation made in section 3.4.3 for the volume of

permeable void test results, the sorptivity parameter of FSGPC-1 concrete was higher


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than the FSGPC-2 concrete. That is, the FSGPC-2 concrete contains less porosity with

denser structure compared to FSGPC-1. This was further ensured by conducting pore

size distribution analysis.

It was further noted from Fig.3-13 that the water absorption of top part of aged core

specimens is greater than mid part of core specimens. As explained earlier, this is due to

the carbonation reaction in an atmospheric environment. Although the rate of water

absorption of FSGPC-1 and FSGPC-1 are increased with age, the increment is lesser than

the increment obtained for fly ash based geopolymer concrete. According to the previous

section, sorptivity parameters of fly ash based geopolymer concrete in the atmospheric

environment is increased at a higher rate with the exposure period. Initial sorptivity rate

calculated from water absorption plot is provided in Table 3-7. These results indicated

that FSGPC-1 concrete samples depict higher sorptivity coefficient values compared to

FSGPC-2 concrete specimens. These results are well correlated with the carbonation test

results.

Fig.3-13 Sorptivity curves of fly ash- slag blended geopolymer concretes


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Table 3-7 Initial rate of water absorption of FGPC- A and OPC-A concretes

Sample Type The initial rate of water absorption S1 (mm/s1/2)

Top part specimen Mid Part specimen

FSGPC- 1 28 days 0.01238 0.0119

FSGPC- 1 08 years 0.0153 0.0132

FSGPC-2 28 days 0.0047 0.005

FSGPC-2 08 years 0.00594 0.00529

3.4.5 FT-IR analysis

3.4.5.1 FT-IR test results of fly ash based geopolymer concrete

The FT-IR technique is used to identify the effect of carbonation on bonding

characteristics of concrete. In this test, carbonation depth was determined from the

position of the C–O characteristic peaks relative to the baseline at wavelength range of

1410-1420 cm-1 [180, 182]. The FT-IR technique is used to identify the presence of

saturated (pH value < 8.3) and non-saturated (pH value > 8.3) carbonation products in

concrete [180, 183], whereas phenolphthalein indicator has the limitation of only

providing carbonation depth at the saturated zone. FT-IR analysis was conducted on the

samples collected from the FGPC-A and OPC-A concrete core specimens at 5 mm depth

intervals. The powder samples were collected up to 30 mm depth from FGPC-A core

specimen due to the higher carbonation behaviour in the atmospheric environment. The

powder samples collected from OPC-A concrete core specimens was limited to a depth

of 15 mm since the carbonation depth of OPC-A concrete was about 4-10 mm after eight

years of exposure in the atmospheric environment. Fig. 3-14 and Fig. 3-15 illustrate the

IR spectra of FGPC-A and OPC-A concretes, respectively. As shown in Fig. 3-14, there is

no evidence for the presence of C–O at the wavenumber of 1410-1420 cm-1, which

indicates carbonation reaction components are not present in FGPC-A concrete after

long-term exposure to the atmospheric environment. However, the phenolphthalein

application revealed that the carbonation of FGPC-A was higher than the carbonation in

OPC-A concrete culvert and this indicates the carbonation products from the GPC

culvert are removed in field exposed conditions.


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In contrast, the C-O bond has been identified in OPC-A concrete after the exposure in

the atmospheric environment. As shown in Fig. 3-15, the peak identified at the

wavenumber of 1420 cm-1 is corresponding to the CaCO3 formation in OPC-A concrete.

Compared to other depth levels, the peak of the collected sample near the exposed

surface of the concrete (5 mm) was identified with high intensity, indicating that the

carbonation rate at the surface of the concrete was very high compared to the inner part

of the specimen. In addition, the peak at 899 cm-1 relates to the stretching vibration of C-

O bond [182] and that was also found in the first layer of OPC-A concrete, indicating

more carbonation in the first layer compared to other layers. Moreover, it should be

noted that the C-O bond was identified in all OPC-A concrete samples (up to 15 mm

depth), whereas a maximum of 10 mm carbonation was identified by the

phenolphthalein indicator. As explained earlier, FT-IR technique is a powerful tool to

identify all carbonation products at any saturation level. Therefore, the minor amount of

CaCO3 content in the partially carbonated zone was also identified at 10 and 15 mm

depth levels.

Fig. 3-14 FT-IR Spectra of FGPC-A concrete


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Fig. 3-15 FT-IR Spectra for -A concrete

It is interesting to note that the FGPC-A specimen has not detected C-O bond, though

the previous study revealed the formation of Na2CO3 products in the laboratory

carbonated fly ash based geopolymer concrete samples at 1417 cm-1 [96]. Therefore, to

further investigate this phenomenon, an accelerated carbonation test was carried out on

the geopolymer concrete specimens. The fly ash based geopolymer concrete specimens

were prepared with 100 mm diameter, and an accelerated carbonation test was carried

out in a carbonation chamber at 21°C with 65% relative humidity after 28 days from

casting date. Since higher concentrations of CO2 yield bicarbonate components in

geopolymer concrete [112], the concentration of CO2 used in this experiment was

controlled 1% to produce the same type of products that would form under natural

carbonation conditions. Fig.3-16 shows the comparison of FT-IR results for the samples

collected from laboratory-exposed specimens (in carbonation chamber for four weeks)

and the field exposed FGPC-A culvert core specimens at a depth level of 5 mm. As

opposed to the field exposed FGPC-A, the C-O bond was recognized in laboratory

prepared FGPC-A samples at 1450 cm-1. This suggests that carbonation components are

formed in fly ash based geopolymer concrete specimens, and those products in field

exposed specimens appear to have dissolved in water from rain and other factors.

Moreover, FT-IR spectra also provide the peaks for a geopolymerisation reaction as well

as a hydration reaction in FGPC-A (Fig. 3-14) and OPC-A (Fig. 3-15 ) concrete,

respectively. The peak at 997 cm-1 in FGPC-A concrete is associated with asymmetric

stretching vibration of the Si-O-T (T is Si or Al) bond [54, 184] and the peak detected at


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997 cm-1 in OPC-A concrete specimens correspond to the stretching of Si-O from the C-

S-H phase. Finally, the peaks between 647 cm-1 and 777 cm-1 are related to the crystalline

phase of quartz components [98, 184].

Fig.3-16 FTIR spectrum of FGPC-A concrete samples (Field and Laboratory)

3.4.5.2 FT-IR test results of fly ash-slag blended geopolymer concrete

Fig.3-17 shows the FT-IR spectra of fly ash- slag based geopolymer concrete specimens

collected from top 10 mm depth of core specimens. As shown in Fig.3-17, the peak has

been identified at 1410 cm-1 and 1460 cm-1 for FSGPC-1 and FSGPC-2 specimens,

respectively, due to the stretching vibration of C-O in carbonate (CO32-) components

[182]. Additionally, carbonation bond has also been observed at the peaks of 871 cm-1

and 856 cm-1. It should be noted that the peaks due the carbonation bond have not been

identified in the fly ash based geopolymer concrete previously. However, this

investigation showed the presence of carbonation products in fly ash-slag blended

geopolymer concrete exposed to the atmospheric environment. This indicates the

incorporation of slag constituent into fly ash based geopolymer concrete produces

insoluble CaCO3 as the carbonation component in addition to Na2CO3 and K2CO3

components. Therefore, carbonation products remain on the concrete surface exposed at

the ambient environment.

Furthermore, FT-IR spectra shows the main intensity bands of FSGPC-1 concrete as

follows: 772, 871, 1000, 1410, 1630, and 3420 cm-1. The FSGPC-2 concrete contains main


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adsorption bands at 641, 856, 953, 1460, 1630 and 3360 cm-1. The peaks at 1000 cm-1 and

953 cm-1 are attributed to stretching vibrations mode of the Si-O-T bond (T is Si or Al) in

FSGPC-1 and FSGPC-2 concrete, respectively. It should be noted that the primary band

wave numbers were shifted to lower wavelengths in FSGPC-2 concrete compared to

FSGPC-1. This is likely due to the high fly ash content in FSGPC-1 concrete, which

increased the formation of alumina-silica geopolymer gel and the presence of large

amount of unreacted fly ash particles [54], or the contribution of silicate activator in

FSGPC-1 concrete.

Moreover, the peak at 3420 cm-1 in FSGPC-1 concrete and 3360 cm-1 in FSGPC-2 concrete

is associated with stretching vibration of OH and H-O-H groups from hydration reaction

products [58]. In addition, the peak at 1630 cm-1 in both types of concrete attributes to

the bending vibration of OH groups. Finally, the peaks at 698 cm-1 and 772 cm-1 in

FSGPC-1 concrete and the peaks at 641 cm-1 in FSGPC-2 concrete is due to the bending

vibration mode of Si–O–Si or Si–O–Al gel.

Fig.3-17 FTIR spectra for both type specimens (Top layer)


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3.4.6 Mercury Intrusion Porosimetry (MIP) analysis

3.4.6.1 MIP test on fly ash based geopolymer concrete

Mercury Intrusion Porosimetry (MIP) analysis is a method to investigate the porosity

and pore size distribution in the concrete, which has been used in many research studies

[42, 185]. In this study, the variation of the pore structure of concrete due to the

carbonation process has been investigated by using MIP analysis. Fig. 3-18 illustrates the

cumulative intrusion of carbonated and un-carbonated part of FGPC-A and OPC-A

concrete specimens, which are collected from the core specimens at different depth level.

According to Fig. 3-18, main increment in the cumulative intrusion of FGPC-A concrete

samples (both carbonated and un-carbonated samples) occurred in the pore diameter

intervals of 20 nm to 160 nm, and a similar change can also be observed in OPC-A

concrete specimens at similar intervals (25 nm -170 nm). However, the total intrusion of

FGPC-A concrete samples (both carbonated and un-carbonated samples) was higher

than the total intrusion of OPC-A concrete. This reveals that the porosity of FGPC-A

samples was higher than OPC-A concrete. This could be due to the formation of porous

geopolymerisation reaction products such as a three-dimensional network of N-A-S-H

gel by the activation of fly ash [24] .

Fig. 3-18 also shows that the carbonated FGPC-A specimen had higher total intrusion

than un-carbonated part of FGPC-A concrete specimen, whereas the total intrusion of

OPC-A concrete decreases with carbonation process throughout the age of concrete. This

further reinforces with the test results obtained from the sorptivity analysis.


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83

Fig. 3-18 The cumulative intrusion of atmospheric exposed concrete specimens

Fig. 3-19 depicts the differential pore size distribution of the carbonated and un-

carbonated concrete specimens. Table 3-8 also summarises the total porosity and the

proportions of different pore types according to pore diameters. Here, the pore types are

categorized according to the pore diameters. Pore diameter less than 20 nm is considered

as “harmless”, and pore size between 20 nm and 100 nm is classified as “minor harmful”

pores. A pore size between 100 nm and 200 nm is defined as “harmful” and greater than

200 nm is considered as “serious harmful” pores [186].

Fig. 3-19 Differential pore size distribution obtained for atmospheric exposed concrete

specimens

As given in Table 3-8, carbonated FGPC-A concrete showed higher porosity compared

to the respective un-carbonated specimen, where the total porosity of the carbonated

and un-carbonated part of the FGPC-A specimens are 20.2% and 16.1%, respectively.

While the porosity of OPC-A concrete specimen reduced from 12% to 10.1% with the

carbonation reaction when exposed to the similar environment. As explained

previously, this is due to the formation of different carbonation components between

two types of concretes. It is anticipated that the formation of soluble carbonation

components in fly ash based geopolymer concrete (i.e. Na2CO3 and K2CO3) are washed

out from the concrete surface in field exposure, causing higher porosity after the

carbonation. However, the formation of insoluble CaCO3 in OPC concrete fills the pores


environment

84

of the surface and reduces the porosity with the carbonation. Furthermore, in fly ash

based geopolymer concrete, increasing porosity with the carbonation process further

exacerbates the carbonation rate due to the enhanced CO2 ingress rate with porosity. As

a result, higher carbonation depth was observed in fly ash based geopolymer concrete

compared to OPC concrete after eight years of exposure in the ambient environment.

Table 3-8 Porosity and pore size distribution of atmospheric exposed concrete specimens

Specimens Porosit

y

(%)

Pore

diameter (0-

20 nm) (%)

Pore

diameter (20-

100 nm) (%)

Pore

diameter

(100-200 nm)

(%)

Pore

diameter

(>200 nm)

(%)

FGPC-A

Carbonated

20.2 5.1 70.3 5.1

19.4

FGPC-A - Un

carbonated

16.1 6.2 72 4 18.2

OPC-A -

Carbonated

10.1 6.3 36.53 21.8 35.3

OPC-A - Un

carbonated

12.0

10.13 26.7 29.3 33.9

It can be seen from Fig. 3-19 and Table 3-8 that the FGPC-A specimens contain fine pores,

and the majority of pores have a pore diameter of 20 -100 nm. By contrast, OPC-A

concrete displayed a range of pore diameters with a substantial percentage of pores

detected in the range of 20-100 nm, 100-200 nm and greater than 200 nm. This indicates

that the average pore size in OPC-A concrete was greater than the average pore size of

FGPC-A concrete.

This data indicates that the percentage of capillary pores (100 nm -200 nm) in FGPC-A

concrete increases with carbonation process. However, the carbonated OPC-A concrete

contains a lower percentage of capillary pores compared to the un-carbonated OPC-A


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85

concrete specimens. Combining these together, we can confirm that the porosity of

carbonated OPC-A concrete has been reduces since the pores in OPC-A concrete are

filled with carbonation reaction products. Conversely, carbonation increases the

porosity in fly ash based geopolymer concrete in an atmospheric environment. This can

be seen from MIP test results obtained for accelerated carbonated FGPC-A concrete

specimens with same mix composition. Fig.3-20 represents the MIP test results of the

accelerated carbonated samples after 3 months of exposure in 1% of CO2 environment (

temperature of 23°C and relative humidity of 65%) and control (un-carbonated) FGPC-

A concrete samples. The specimens size was used for the accelerated carbonation test

was 100 mm × 50 mm. It should be noted that the accelerated carbonation test was

carried out in the environment chamber without contact with water. The plotted graphs

indicated that there is a minor difference between the carbonated and control specimens,

and the total porosity obtained for both specimens were 15.4% and 14.9%, respectively.

This confirms the increase in porosity of FGPC-A concrete in the atmosphere by

dissolving carbonated components in running water. These results are again well

correlated with FT-IR spectra analysis.

Fig.3-20 MIP test results for laboratory carbonated FGPC-A concrete

Furthermore, the test results from MIP analysis are consistent with the sorptivity test

results, while the AVPV test provides contradictory results. This is because the AVPV

values of both FGPC-A and OPC-A concrete specimens increase with exposure period.

As explained earlier, AVPV test is suitable to determine the open pores only, whereas

MIP and sorptivity test methods are determining the total pores of the concrete


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86

specimens. As such, both MIP and sorptivity analysis are appropriate methods to

determine the pore size distribution of concrete samples.

3.4.6.2 MIP test on fly ash –slag blended geopolymer concrete

Fig.3-21 shows the cumulative intrusion of FSGPC-1 and FSGPC-2 geopolymer

concretes, which are collected from the core specimens at a depth level of 0-50 mm (Top)

and 50-100 mm (Mid). As given in Fig.3-21, the total intrusions of FSGPC-1 concrete

samples were higher than FSGPC-2 concrete samples. This indicates that the porosity of

the FSGPC-1 samples was greater than FSGPC-2 concrete porosity. According to

Fig.3-21, major increment in the cumulative intrusion of FSGPC-1 samples occurred in

the pore diameter range of 10-1000 nm, while cumulative intrusion of FSGPC-2 concrete

is changed between 3-25 nm. This reveals that the FSGPC-1 concrete contains a wide

range of pore size distribution, whereas the maximum amount of pores of FSGPC-2

concrete is in the smaller pore size range (3-25 nm).

Fig.3-21 Cumulative intrusion of atmospheric exposed fly ash- slag blended geopolymer

concrete specimens

The differential pore size distributions of both types of geopolymer concrete specimens

are shown in Fig.3-22. As seen in Fig.3-22, the top and mid layers of the same types of

concrete demonstrate similar pore size distribution characteristics. The overall pore

characteristic details are given in Table 3-9.

http://www.sciencedirect.com/science/article/pii/S0306261913004625#t0015

http://www.sciencedirect.com/science/article/pii/S0306261913004625#t0015


environment

87

Table 3-9 According to Fig.3-22 and Table 3-9, top and mid layers of FSGPC-1 specimen

contain higher pores compared to FSGPC-2 specimens. The top and mid layer of FSGPC-

1 concrete specimens have a total porosity of 17.6% and 17.2%, and the total porosity of

FSGPC-2 concrete specimens for the relevant layers are 14.9% and 11.2%, respectively.

Therefore, the porosity of the concrete specimens agrees with the carbonation results.

That is, the lower porosity concrete has a higher resistance to carbonation in an ambient

environment.

Although the porosity of top layer in FSGPC-1 concrete is high, it possesses lower total

pore area and higher average pore diameter values compared to the FSGPC-2 concrete

top layer. This suggests that the FSGPC-1 concrete contains a high amount of larger size

pores compared to FSGPC-2 concrete. It is also noted that the FSGPC-2 concrete contains

fine pores and the largest amount of pores occur at the pore diameters of 4.7 nm and 5.3

nm for the top and mid layers respectively. On the other hand, the top and mid layers of

FSGPC-1 concrete specimens possess large amount of pores at the pore diameter of 15.7

nm. This confirms that most of the pores in both types ambient-exposed geopolymer

concrete are harmless or less harm, as a significant proportion of pores occur at the

diameter less than 50 nm [187]. However, the FSGPC-1 concrete still shows a noticeable

amount of pores at the diameter range of 50 -150 nm (Fig.3-22), which are harmful pores.

Therefore, it can be concluded that the FSGPC-1 concrete contains a high amount of

harmful pores, which can promote the penetration of CO2. This was also confirmed by

the carbonation test results, where the penetration of CO2 was high in FSGPC-1

geopolymer concrete surface in the ambient environment.


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88

Fig.3-22 Differential pore size distribution obtained for both types of geopolymer

concrete

Table 3-9 The pore characteristics details of both types of geopolymer concrete

specimens

FSGPC-1-Top

layer

FSGPC-1-Mid

layer

FSGPC-2-Top

layer

FSGPC-2-Mid

layer

Porosity (%) 17.6 17.2 14.9 11.8

Total pore

area(m2/g)

15.252 18.966 22.8 16.621

Average Pore

diameter(nm)

23.6 22.9 13.5 12.5

3.4.7 Corrosion of reinforcement in fly ash based geopolymer concrete

The corrosion of reinforcement bar in the field exposed FGPC-A concrete and OPC-A

concrete was visually inspected after the split the core specimens. Special attention has

been paid on the rust development on the surface of the rebar. Such inspections could

not be performed for fly ash-slag blended geopolymer concrete structures due to the

absence of rebar in near-surface, and the rebar had been embedded in the depths higher

than the core size.

Photographs of embedded reinforcement bars in fly ash based geopolymer and OPC

concrete from the atmospheric environment are illustrated in Fig.3-23. These

reinforcement bars are collected from the leg parts of the concrete culverts with 45 mm

cover depth values. As shown in Fig.3-23, the reinforcement bar in FGPC-A has begun

to corrode, whereas no corrosion products can be observed in the reinforcement bar

embedded in OPC-A concrete. In OPC concrete, reinforcement bars are well protected

by the formation of a thin oxide layer around the reinforcement (called a passivation

layer) in the presence of alkalinity (Ca(OH)2) component of cement. The carbonation

process leads to the de- passivation of this oxide layer due to alkalinity deduction (pH

deduction to less than 9.0) in the concrete.

The pH range of FGPC-A specimens was measured as 9.92 to 10.41 from the exposure

surface to 30 mm depth level and 10.5 at 120 mm depth, as reported in section 3.3.1. This


environment

89

indicated that the steel bar in geopolymer concrete had not been protected even though

the pH greater than 9.0. The protective layer surrounding the reinforcement bar in

FGPC-1 is therefore compromised despite its high pH. This reveals that the fly ash based

geopolymer concrete is more susceptible to carbonation than OPC concrete.

Fig.3-23 Corrosion of embedded steel bars at a cover depth of 45 mm in FGPC-A and

OPC-A concrete at 8 years exposure in an atmospheric environment

3.5 Concluding remarks

This chapter investigated the durability of geopolymer concrete exposed to the

atmospheric environment for eight years. The fly ash based geopolymer concrete

structure, and two distinct types of fly ash-slag blended geopolymer concrete structures

were considered for durability analysis. Experimental works have been conducted to

determine the carbonation of geopolymer concrete in the atmospheric exposed

environment and compared with OPC concrete. The carbonation depth values and the

pH variation of the concrete were investigated after eight years of exposure, and the

carbonation components in the concrete samples were identified with FT-IR analysis.

Furthermore, the influence of carbonation on the transport properties and porosity of

the concrete core specimens were studied. The following conclusions can be drawn

based on this experimental works:

The effect of carbonation was more significant in fly ash geopolymer concrete

compared to OPC concrete. However, the carbonation rate of fly ash-slag


environment

90

blended geopolymer concrete highly depends on the mix design of materials.

FSGPC-1 geopolymer concrete, with 75% fly ash/25% GGBFS and additional

Na2SiO3 activator showed a poor resistance against carbonation compared to

OPC concrete. In contrast, the performance of FSGPC-2 geopolymer, with 70%

fly ash/30% GGBFS with hydroxide activator, was similar to OPC concrete.

FT-IR spectra revealed the absence of carbonation products in fly ash based

exposed to the atmospheric environment. On the other hand, the presence of

carbonation products such as calcium carbonate components and crystallised

CaCO3 in fly ash-slag blended geopolymer concrete were identified.

According to the sorptivity test results of fly ash based geopolymer concrete, the

sorptivity characteristic of geopolymer concrete increases with the age of the

concrete, while the sorptivity of OPC concrete reduces with age. Similar

behaviour also observed in fly ash-slag blended geopolymer concrete specimens.

The FSGPC-1 and FSGPC-2 specimens showed that the sorptivity parameters are

increased with the age of concrete.

The MIP test results of fly ash based geopolymer concrete confirm the porosity

increase with carbonation in field conditions. The carbonated part of the field

exposed FGPC-A sample shows higher porosity than un-carbonated FGPC-A

sample from field environment. However, laboratory prepared accelerated

carbonated FGPC-A specimens (without contact with water) displays similar

porosity compared to un-carbonated FGPC-A sample. This is due to the removal

of carbonation components from the FGPC-A concrete in the ambient

environment. Once again, these results validate the observations made in FT-IR

analysis. On the other hand, field exposed OPC concrete after the carbonation

had lower porosity compared to un-carbonated OPC samples.

The MIP test results of fly ash-slag blended geopolymer concrete also confirm

the porosity increment in geopolymer concrete in the ambient environment. This

is due to the soluble carbonation components with insoluble carbonation

products. In addition, MIP analysis illustrates the relationship between

carbonation and the porosity of the concrete. Specifically, the test results revealed

that the FSGPC-1 concrete contains higher porosity and larger average pore

diameter than FSGPC- 2 concrete.


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91

The visual inspection of the embedded reinforcement bar in fly ash based

geopolymer concrete shows the corrosion initiation after eight years of exposure

in the ambient environment, whereas the reinforcement bar in OPC concrete is

in good condition at the same exposure conditions.

Chapter 4 The durability of geopolymer

concrete exposed to an aggressive environment

4.1 Introduction

Concrete is less durable in the aggressive environment compared to the normal

atmospheric environment due to the more influences of aggressive agents such as CO2,

chloride ions, sulphate ions and so on. This chapter consists the experimental works

related to the durability of geopolymer concrete exposed in the aggressive exposed

environment. The investigation was conducted on the core specimens collected from two

distinct types of geopolymer concrete structures, such as fly ash based geopolymer

concrete culvert exposed to the saline environment for six years and the slag-fly ash

blended geopolymer concrete block structures exposed to the marine environment for

four years. The durability behaviour of the geopolymer concrete was also compared with

OPC concrete structures exposed to the same environmental conditions. This study

presents the investigation of the combined effect of carbonation and chloride ingress in

geopolymer concrete. Furthermore, sulphate attack also influenced on the concrete

durability in aggressive condition and therefore, the sulphate penetration to the concrete

also evaluated. Moreover, the durability of the concrete is validated by measuring the

other properties such as pH changes, transport properties and pore size distribution

measurements. Finally, the microstructural characterisation of the field exposed concrete

also studied to determine the deterioration of microstructure due to carbonation,

chloride diffusion and the sulphate attack.


93

4.2 Field Investigation

4.2.1 Description of concrete structures, exposure condition and mix details

4.2.1.1 Fly ash-based geopolymer concrete & OPC concrete culverts in the saline

exposure condition

Fig.4-1 depicts the exposure conditions of fly ash based geopolymer concrete (FGPC-S)

and OPC (OPC-S) box culverts in the saline lake environment for the six-year period.

The culverts were cast by Rocla’s precast concrete plant located in Perth, Western

Australia and placed in a saline lake environment (Lake King, Western Australia) in

2009. The chloride contents of the lake water are higher than the typical chloride amount

in the seawater. The chloride content and the total soluble salt content of lake water are

150,080 ppm and 468,390 ppm, respectively [18]. By comparison, seawater typically has

a chloride concentration of 19,000-19,500 ppm and total soluble salt concentration of

approximately 35,000 ppm. The legs of the culverts were exposed to wetting and drying

cycles associated with the changes in the lake water depth. The dimensions of the

concrete box culverts were 1200 mm length, 1200 mm width and 600 mm depth. The

details and the manufacturing methods of geopolymer concrete are presented in

previous studies [18, 167]. An experimental investigation was carried out on the core

specimens extracted from both culverts. Two cores were taken from the top slab and four

core specimens from the leg part of the culverts. The diameter of the core samples from

top slab was 94 mm and 68 mm for the leg part cores, and the length of the core

specimens was 135 mm and 90 mm, respectively.

Geopolymer and OPC binders were prepared using the mix details provided in Table

4-1. Similar mix details have been used to prepare the FGPC-A and OPC-A types

concrete, which are exposed in an atmospheric environment. The durability of FGPC-A

and OPC-A types concrete were discussed in Chapter 3. The FGPC-S concrete culvert

was cured by a stream curing method at 60°C for 24 hrs, and the OPC-S concrete culvert

was cured at ambient temperature.


94

Fig.4-1 Concrete culvert structures exposed to the saline environment (a) FGPC-S

culvert, (b) OPC-S culvert.

Table 4-1 Mix compositions of concrete (kg/m3) [18, 167]

Materials Mass (kg/m3)

FGPC-S OPC-S

Coarse

Aggregates

14mm 554 920

10mm 702 300

Fine Sand 591 640

Fly Ash (Low Calcium ASTM

Class F)

409 -

Cement 400

Sodium Silicate Solution

(SiO2/Na2O =2)

102 -

Sodium Hydroxide Solution 41(8M) -

Superplasticiser (SP) 6 -

Water 22.5 170

4.2.1.2 Slag- Fly ash blended geopolymer concrete slabs in marine exposure condition.

Fig.4-2 (a) illustrates the location of the slag-fly ash blended geopolymer and OPC

concrete structures in Portland, Victoria, Australia and Fig.4-2 (b) displays the coring

work on the concrete structures. The slag- fly ash blended geopolymer concrete (SFGPC-

M) blocks are exposed in the atmospheric zone of the marine environment for four years

of the period. In the same environment, OPC (OPC-M) concrete blocks are exposed for

six years of the period. The sizes of the concrete block structures were 600 mm ×600 mm


95

and the thickness of the blocks was 200 mm. Concrete core specimens were collected

from both SFGPC-M and OPC-M concrete structures, and the diameter of the extracted

core samples was 35 mm, and the average length was 100 mm.

Fig.4-2 (a) SFGPC-M and OPC-M concrete blocks in the marine environment, (b)

concrete coring work after marine exposure.

The SFGPC-M was a commercial geopolymer concrete mix with 400 kg of binder per m3,

the exact mix design is not given, but is known to be a predominantly slag-based slag-

fly ash geopolymer (approximately 80% slag and 20% of fly ash). The OPC-M (400

kg/m3) was made with 100% of general purpose cement. The mix details of OPC-M

concrete are provided in Table 4-2.

Table 4-2 Mix proportions of OPC-M concrete

Material OPC-M (kg/m3)

OP Cement 400

Coarse aggregate 20 mm 550



Coarse sand 370

Fine sand 275

WRA 1.22

Water 159


96

4.3 Testing methods

4.3.1 Carbonation depth measurement

As explained in Chapter 3, the carbonation of concrete core specimens was evaluated by

using 1% of phenolphthalein solution. The carbonation depth values were measured by

spraying of phenolphthalein solution on the fresh surface of the extracted core

specimens from the concrete structures after exposure to the aggressive environment.

4.3.2 Chloride penetration measurements

4.3.2.1 Chloride penetration depth

The chloride penetration depth of the core specimens was measured by using silver

nitrate (AgNO3) solution. The core specimens were split into two parts, and the AgNO3

solution was sprayed on the fresh split surface. Chloride penetration depth can be

qualitatively measured by the precipitation of white silver chloride (AgCl). The white

colour of the AgCl precipitation indicates the presence of chloride ions in the concrete

surface. The following equation explains the chemical reaction between the AgNO3

solution and chloride ions.

𝐴𝑔+ + 𝐶𝑙− → 𝐴𝑔𝐶𝑙 ↓ (𝑊ℎ𝑖𝑡𝑒 𝑝𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑒) (7)

4.3.2.2 Water soluble and acid-soluble chloride content measurement

The evaluation of chloride ingress to the concrete surface exposed to the aggressive field

environment was with the aid of free and total chloride profile measurements. Powdered

samples were collected with 5 mm depth intervals from the exposed surface by using

profile grinder. The experimental setup to collecting powder samples from the core

specimens is provided in Fig. 4-3.


97

Fig. 4-3 profile grinder to obtain the powder samples from core specimens

The water-soluble chloride (free chloride) content in the powder sample was estimated

according to ASTM C 1218 [188] standard. Prior to the test, powdered samples were oven

dried at 105°C to remove the moisture content in the samples. As per mentioned in the

standard, 10 g of powdered sample was mixed with 50 ml of distilled water and boiled

for 5 min. Then the solution was filtered by using filter paper. The free chloride content

in the filtered solution was determined by using Potentiometric titration method with a

1 N AgNO3 solution.

The amount of acid soluble chloride (total chloride) in the powder sample was evaluated

with ASTM C 1152 [189] standard. This method is commonly used by many researchers

[190-192]. Similar to the water-soluble measurement test, the powdered samples were

oven dried at 105°C temperature. Then, approximately 10 g of powdered sample was

mixed with 75 ml of distilled water, and then 25 ml of nitric acid was mixed slowly into

the solution. The solution was stirred with a glass rod to avoid forming of any lumps in

the solution. Thereafter, the solution was boiled for few seconds, and then the solution

was filtered by using filter paper. Potentiometric titration method was used to determine

the acid-soluble chloride content in the filtered solution by titrating 1N AgNO3 solution.

4.3.3 Sulphate content measurements

The sulphate content was measured on the collected powdered samples from the core

specimens up to 30 mm depth, at 5 mm depth intervals with Australian Standard AS

1012.20 [193]. As similar to chloride content measurements, 10 g of oven dried powdered


98

samples were collected and mixed with 75 ml of distilled water, and then 25 ml of nitric

acid was mixed slowly into the solution. During the mixing procedure, the solution was

stirred with a glass rod to avoid the lump formation. Then, the solution was boiled for

few seconds and allowed to cool for few minutes. Thereafter, the solution was filtered

by using filter paper. The sulphate concentration was determined by using the

gravimetric method.


The pH of the concrete in aggressive environment is affected by the carbonation and

chloride attack. Therefore, the pH of the concrete core specimens was determined by

using water suspension method [169]. As per that the powder samples were extracted

from the concrete specimens with 3 mm depth intervals. To conduct the test, the

powdered sample was mixed with distilled water with the ratio of 2:3 and then the

mixture was stirred for 15 min with a magnetic stirrer. Next, the solution was filtered

using filter paper, and the pH value of the filtered solution was measured with a pH

electrode. The detailed test methods are explained in the Chapter 3.

4.3.5 Sorptivity analysis

The capillary absorption parameter of the concrete is a key index to evaluate chloride

diffusion to the concrete surface. When the concrete is exposed to the marine or saline

environment, the chloride ion is transported at the concrete surface by capillary

absorption. Therefore, the capillary absorption parameters are important to predict the

chloride diffusion to the concrete. The sorptivity test was conducted according to ASTM

C1585 [172] standard method. The thickness of the concrete core specimens included in

this analysis was 50 mm from the outside exposed surface. The water absorption value

(I) was calculated using the change of specimen weight value after being placed in water

(Mt) subjected to the period (t) and the surface area of the specimens. The detailed

experimental methods are provided in Chapter 3.

4.3.6 Fourier Transform Infra-red (FT-IR) analysis

The Fourier Transform Infrared (FT-IR) spectroscope was used to conduct the FT-IR

analysis. Powder samples were collected from the concrete specimens by using profile

grinder at various depth levels. To determine the IR spectra, a powder sample was mixed

with a KBr pellet, and a Fourier-transform infrared spectroscope was used to ascertain


99

the spectrum with 32 scans per sample collected from the range 4000 cm-1 to 525 cm-1 at

4 cm-1 resolutions. The experimental set up is explained in Chapter 3.

4.3.7 Mercury intrusion porosimetry (MIP) test

The pore size distribution of the concrete was determined by using Mercury intrusion

porosimetry (MIP) analysis. The solid mortar particles were collected from the core

specimens, and the samples were included in an oven dried at 80 °C for a 24 hrs to

remove the moisture content present in the samples. MIP measurements were carried

out using mercury with surface tension and the contact angle of 0.48 N/m and 140 °,

respectively. The detailed experimental methods are provided in Chapter 3.

4.3.8 Scanning Electron microscopy (SEM) and Energy dispersive X-ray

(EDX) analysis

The deterioration of microstructure of the concrete specimens was studied by using

Scanning Electron Microscopy (SEM) and energy dispersive X-ray (EDX) analysis, at an

accelerating voltage of 3 kV (ZEISS Supra 40 VP SEM instrument is used). To conduct

the SEM analysis, concrete specimens were cut into 2 mm thickness by using the

precision diamond cutter. Before the test, the concrete samples were coated with a very

thin layer of gold coating to induce the electric conductivity. Fig. 4-4 (a) shows the

cutting of concrete specimens with a diamond cutter and Scanning Electron Microscopic

(SEM) equipment is presented in Fig. 4-4 (b).

Fig. 4-4 SEM test (a) Cutting of concrete specimen into thin slice using a precision

diamond saw, (b) Scanning Electron Microscopy equipment.


100

Moreover, the SEM test was also conducted on the samples collected from the

steel/concrete interface area to identify the corrosion products in that place.

4.4 Test results and discussions

4.4.1 Carbonation resistance of geopolymer concrete in an aggressive

environment

4.4.1.1 Carbonation resistance of fly ash based geopolymer concrete

Fig.4-5 illustrates the carbonation depth measurements of FGPC-S and OPC-S concrete

core specimens from saline exposed concrete structures (6 years of exposure) by the

application of phenolphthalein indicator. According to that carbonation rate of FGPC-S

specimen is much higher than OPC-S concrete under the same exposure condition. The

phenolphthalein application showed that the total length of FGPC-S type core specimen

was turned to colourless (approximately 90 mm of leg parts and 135 mm of the top slab

of the culvert), indicated that the FGPC-S culvert was completely carbonated after six

years of exposure in the saline environment. However, carbonation depth values

obtained from OPC-S concrete core specimens were less than FGPC-S, with a maximum

value of 20 mm. Table 4-3 shows the carbonation depth values of OPC-S concrete core

specimens extracted from various locations of the culvert structure.


101

Fig.4-5 Carbonation depth measurements of core specimens using a phenolphthalein

indicator (a) FGPC-S specimen before applying phenolphthalein, (b) FGPC-S specimen

after applying phenolphthalein, (c) OPC-S specimen before applying phenolphthalein,

(d) OPC-S specimen after applying phenolphthalein

As demonstrated in Table 4-3, carbonation depth values varied at different locations due

to the moisture variation throughout the culvert structure, which would affect the

diffusion of CO2 to the concrete surface. It should be noted that the culvert was exposed

to CO2 environment from both inner and outer surface. Therefore, carbonation depth

measurements were taken from both inner and outer exposed surfaces. The carbonation

rate of the top slab part is higher than the leg part of the culvert. The maximum

carbonation depth value obtained for the core specimen from leg part of the culvert was

10 mm, whereas the core specimen form top slab of the culvert showed a maximum

value of 20 mm. It is well known that the internal humidity of the concrete is strongly

influenced on the carbonation rate [194]. The moisture content of the leg part of the

culvert structure is higher than top part due to the continued contact of seawater

compared to the top part and also the top part of the slab is highly exposed to the sun,

which would produce less moisture compared to leg part. Therefore, it can be concluded


102

that the high moisture content (greater than the optimum moisture level) is contributing

to the lower carbonation effect in the leg part of the culvert by lower CO2 diffusion.

Table 4-3 Carbonation depth measurement of OPC-S specimens

Core No Carbonation depth

measured from the

outer surface (mm)

Carbonation depth

measured from the

inner surface (mm)

OPC-S 1(Leg) 10 0

OPC-S 2(Leg) 7 0

OPC-S 3(Leg) 5 4

OPC-S 4(Leg) 5 4

OPC-S 5(Top slab) 20 10

OPC-S 6(Top slab) 20 9

On the other hand, phenolphthalein application showed that the FGPC-S concrete core

specimens from both top slab part and the leg part of the culvert structure were

completely carbonated, which indicated that the carbonation rate of fly ash based

geopolymer concrete is very much higher than OPC concrete in the saline environment

under the wet and dry conditions. As explained in the previous chapter, the soluble

Na2CO3 is producing as a primary carbonation product in fly ash based geopolymer

concrete and would be easily dissolved with any contact water in the field condition and

attributes higher porosity on the concrete surface. Therefore, this induced higher CO2

penetration through the concrete surface. This was clearly explained in Chapter 3.

Furthermore, in this study, phenolphthalein indicator has been provided clear

carbonation depth identification for fly ash based geopolymer concrete. However, the

study on atmospheric exposed fly ash based geopolymer concrete showed that the

carbonation depth measurements were not clear with the phenolphthalein indicator.

Simultaneously, past research studies also confirmed that the phenolphthalein solution

was not provided clear carbonation identification for fly ash based geopolymer concrete

[21, 25, 90]. It should be noted that the pH of the concrete is strongly influenced on the

identification of colour change by the phenolphthalein indicator. When the pH of

concrete is less than 9.0, the indicator is, colourless and when the pH level is greater than

9.0, the indicator shows a pink or purple colour. Therefore, all identification by the


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phenolphthalein solution depends on the pH of the concrete after carbonation. This will

be discussed in section 4.4.8.

4.4.1.2 Carbonation resistance of slag-fly ash blended geopolymer concrete

Fig. 4-6 illustrates the carbonation depth measurements on the core specimens from slag-

fly ash blended geopolymer (SFGPC-M) and OPC (OPC-M) concrete exposed to the

marine environment for four years and six years period, respectively. The carbonation

depth values also determined for SFGPC-M and OPC-M after two years and four years

of exposed time, respectively. Fig. 4-7 shows the carbonation depth values of SFGPC-M

and OPC-M concrete in the marine environment. Although OPC concrete exposed more

time compared to geopolymer concrete, it shows lower carbonation rate compared to

geopolymer concrete. As shown in Fig. 4-7, SFGPC-M specimen had 6 mm carbonation

after two years of time, while OPC-M concrete shown only 1 mm depth value after a

four year of the exposure period. In addition, the carbonation depth of SFGPC-M

measured after four years was 11 mm, and OPC-M concrete had only 4 mm depth after

six years of exposure time. The past research studies revealed that the carbonation of

OPC concrete was 4 mm when it exposed to the marine environment for six-year [195],

which is similar to the value obtained for OPC-M concrete. However, slag-fly ash

blended geopolymer concrete displayed a higher rate of carbonation compared to OPC

concrete.


104

Fig. 4-6 Carbonation depth measurements of core specimens by phenolphthalein

solution, (a) SFGPC-M specimen before applying phenolphthalein, (b) SFGPC-M

specimen after applying phenolphthalein, (c) OPC-M specimen before applying

phenolphthalein, (d) OPC-M specimen after applying phenolphthalein

Fig. 4-7 Carbonation depth values after 02 years and 04 years of exposure.

4.4.2 Chloride penetration

4.4.2.1 Chloride penetration of fly ash based geopolymer concrete

Chloride depth measurements

The depth of the chloride penetration was visually determined by spraying the AgNO3

solution on freshly split core samples from the leg part of the culverts. Fig.4-8 shows the

depth measurements of chloride ion penetration of FGPC-S and OPC-S concrete core

specimens. As shown that white colour AgCl precipitation was observed on the full

length of FGPC-S concrete core specimens, which indicates, the chloride ion was

completely penetrated the entire depth of the FGPC-S culvert after six years period. As

opposed to that chloride depth values were observed in the OPC-S concrete specimens

were 10 mm and 20 mm from the outer and inner exposed surface, respectively. This

indicates the diffusion of chloride in OPC-S concrete is less compared to FGPC-S

concrete. However, the chloride penetration was quantitatively evaluated according to

the free chloride and total chloride content measurements.


105

Fig.4-8 Chloride penetration depth measurements of core specimens using an AgNO3

solution (a) FGPC-S specimen before applying AgNO3, (b) FGPC-S specimen after

applying AgNO3, (c) OPC-S specimen before applying AgNO3, (d) OPC-S specimen after

applying AgNO3

Free chloride measurements

Fig.4-9 represents the free chloride profiles of the FGPC-S and OPC-S concrete core

specimens from the leg part of the culverts after exposed for six years. Since the leg part

of culvert is exposed to saline water in both inner and outer direction, the measurements

have been taken from both exposure surfaces of the core specimens. In Fig.4-9, ‘FGPC-S

T’ and ‘FGPC-S B’ are indicated the free chloride profiles of FGPC-S samples from inner

and outer exposed surfaces, respectively. Similarly, the free chloride profiles ‘OPC-S T’

and ‘OPC-S B’ are associated with the samples collected from the inner and outer surface

of OPC-S concrete core specimens, respectively. Free chloride profiles were plotted

according to Fick’s second law mathematical equation, provided below [196].

𝐶𝑥 = 𝐶𝑠(1 − 𝑒𝑟𝑓 (𝑥

4 𝐷𝑎×𝑡)) (8)

Where Cx is chloride concentration at depth x, Cs is surface chloride content at the

surface, Da is apparent diffusion coefficient, x is depth, t is a time of exposure and erf is


106

an error function. As shown in Fig.4-9, free chloride content in FGPC-S was found to be

higher than the OPC-S concrete at the same depth levels. It should be noted here that the

amount of free chloride in the concrete is more influenced to induce the corrosion

activity of reinforcement compared to total chloride content [197]. According to the test

results, free chloride contents at the 25 mm depth level (reinforcement level) from the

inner and outer exposed surfaces were exceeded or closed to the value of 0.4%. On the

other hand, OPC-S concrete displayed lower contents of free chlorides from the both

exposed surfaces at that level, which is approximately 0.05%. Therefore, this indicated

that the risk for the corrosion of reinforcement bar was greater than the risk of corrosion

in OPC-S concrete. The corrosion activity in reinforcement bar is explained further in the

section of 4.4.13.

Fig.4-9 Free chloride variation with depth values from the saline environment

Total chloride content and Chloride diffusion coefficients (Da)

The total chloride profiles of the core specimens were plotted according to Fick’s second

law mathematical equation (Eqn (8)) by using best-fit curve method. The apparent

diffusion coefficient (Da) and the surface chloride content (Cs) were calculated by using

the total chloride profiles. Although the free chloride is generally considered as a

responsible to initiate the corrosion in reinforcement, the chloride threshold value is also

necessary to depassivate the reinforcement bar, which is determined from the total

chloride measurement [198, 199]. The threshold limit of the chloride is generally


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considered as 0.06% by wt. of concrete [200]. The plots of total chloride content of FGPC-

S and OPC-S concrete are shown in Fig.4-10. As previously mentioned, ‘FGPC-S T’,

‘FGPC-S B’, ‘OPC-S T’ and ‘OPC-S B’ are associated with the total chloride profile of

powder specimens collected from the outer and inner surface of FGPC-S and OPC-S

concrete, respectively. Table 4-4 shows the calculated Da and Cs values for the FGPC-S

and OPC-S concrete specimens after six years of exposure. It should be noted that that

the resistance of chloride penetration through the concrete can be evaluated by using

chloride diffusion coefficient values of concrete [201]. According to Fig.4-10 and Table

4-4, the chloride content in FGPC-S concrete was much higher than the chloride contents

in OPC-S concrete. FGPC-S specimens displayed higher chloride diffusion compared to

OPC-S concrete. The total chloride profiles in Fig.4-10 shown that the amount of total

chloride content at the reinforcement bar level (25 mm depth) of FGPC-S concrete was

much higher than the assumed threshold Cl- limit values (0.06% by wt. of concrete),

whereas the total chloride content at the reinforcement bar in OPC-S concrete is also

higher the level of the threshold limit. However, compared to FGPC-S specimens,

chloride content in OPC-S concrete specimens are lower at 25 mm depth level. This

indicated that the corrosion activity in FGPC-S concrete should be greater than the

corrosion of reinforcement bar in OPC-S concrete.

Fig.4-10 Total chloride variation with depth values from the saline environment.


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Table 4-4 Apparent diffusion coefficient and surface chloride content values

Apparent diffusion

coefficient (Da)×10-12 m2/s

Surface chloride content

(Cs) (% by wt. of concrete)

FGPC-S T 2.50 1.38

OPC-S T 1.0 0.58

FGPC-S B 2.20 1.38

OPC-S B 0.95 0.48

It is well-known that the chloride diffusion and the surface chloride content of concrete

depend on the mix proportions, especially the binder type and curing conditions [202].

In this study, two different curing processes were conducted, a heat curing method for

FGPC-S concrete and an ambient temperature curing for OPC-S concrete. The high-

temperature curing method accelerates the curing process and produces better chloride

resistance than ambient cured concrete at the younger stage, whereas ambient cured

concrete may contain lower chloride diffusion at the latter stage due to the continuous

curing process with age [203]. The previous research studies have shown that the fly ash

based geopolymer concrete had lower chloride diffusion compared to OPC concrete at

an early age [25, 121, 122]. In contrast, this investigation revealed that the FGPC-S had

higher chloride diffusion than OPC-S concrete after six years of exposure. It can,

therefore, be concluded that the heat curing process in fly ash based geopolymer

concrete produces higher chloride diffusion after long-term exposure in the saline

environment due to the early reaction process.

Moreover, higher surface chloride content in FGPC-S specimens is possibly also related

to the availability of carbonation reaction components in the concrete surface. In OPC

concrete, the pore structure of the concrete surface is filled with CaCO3 components,

which reduces the surface chloride concentration, while in fly ash based geopolymer

concrete, porosity increment due to the formation of soluble carbonation components

encouraged more sorption on the surface, which produced more surface chloride

concentration. In overall, the effect of chloride and chloride diffusion of fly ash-based

GPC concrete was severe in saline environments with long-term exposure.


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Chloride binding capacity of fly ash based geopolymer concrete

The chloride binding capacity of concrete can be evaluated by using the values of free

chloride and total chloride content. The following equation is used to calculate the

chloride binding capacity of concrete after exposure to 6 years in saline aggressive

condition [190, 204].

𝑃𝑏𝑐 =

𝐶𝑡 − 𝐶𝑓

𝐶𝑡∗ 100%

(9)

Where Pbc is the chloride binding capacity, Ct is the total chloride content, and Cf is the

free chloride content in the concrete.

The calculated chloride binding capacity of the FGPC-S and OPC-S concrete specimens

are 20% and 31%, respectively. In general, there are two mechanisms influenced on the

chloride binding capacity of Ordinary Portland cement concrete: such as physical

adsorption and chemical reactions [205, 206]. The chloride binding capacity of the

concrete increases with the tricalcium aluminate (C3A) content and tetra calcium

alumina ferrite (C4AF) phases in concrete. The C3A component in the cement binder

reacted with bound chloride and produced calcium chloroaluminate hydrate

(3CaO·Al2O3·CaCl2·10H2O), which is known as Friedel’s salt [190]. This enhances the

chemical binding of chloride. On the other hand, chloride ions are physically bounded

with the C-S-H gel in the hydration products and the ettringite components [124, 205].

Nevertheless, higher chloride diffusion in fly ash based geopolymer concrete is due to

no C3A, C4AF phases or C–S–H gel compared to OPC concrete [207]. Moreover, chloride

ions in OPC concrete are bound by Friedel’s salt and calcium chloride phases, from the

reaction between hydration phases of cement and the chloride ions. In contrast, the

formation of soluble metal salt components, as a reaction between fly ash based

geopolymer concrete phases and chloride ions, would not be sufficient to provide

enough chloride binding capacity [208].

4.4.2.2 Chloride penetration of slag- fly ash blended geopolymer concrete

Chloride penetration depth measurements

Fig.4-11 showed the measurement of chloride penetration depth of SFGPC-M and OPC-

M concrete core specimens by using AgNO3 solutions after the exposure of 4 years and

6 years in the marine environment, respectively. Although lesser exposure in the marine


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environment compared to OPC-M concrete, the chloride penetration depth identified in

SFGPC-M was high, which is about 10 mm, whereas OPC-M concrete displayed 8 mm

depth value only. This indicates SFGPC-M showed lower penetration resistance than

that of OPC-M concrete. However, compared to fly ash based geopolymer concrete, slag-

fly ash blended geopolymer concrete displayed greater resistance against chloride

penetration in the aggressive condition.

Fig.4-11 Chloride penetration depth measurements of core specimens using an AgNO3

solution (a) SFGPC-M specimen before applying AgNO3, (b) SFGPC-M specimen after

applying AgNO3, (c) OPC-M specimen before applying AgNO3, (d) OPC-M specimen

after applying AgNO3

Free chloride measurements

Fig. 4-12 depicts the free chloride profiles of SFGPC-M and OPC-M concrete after

exposed to the marine environment for 04 years and 06 years, respectively. These profiles

were plotted by using the Fick’s second law mathematical equation (Eqn (8)) by using

best-fit curve method. As observed from the AgNO3 solution, the free chloride profile

also confirmed the higher chloride ingress in SFGPC-M concrete compared to OPC-M

concrete. According to Fig. 4-12, the amount of free chlorides in both types of concrete


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are less than 0.4% throughout the depth (30 mm), and this indicated that there would be

less effect on the reinforcement bar in both SFGPC-M and OPC-M concrete after four

years and six years of exposure. However, the value of free chloride at the surface of the

SFGPC-M specimen is closed to 0.4% and therefore, this revealed that the reinforcement

bar in SFGPC-M is more susceptible to corrosion compared to the reinforcement bar in

OPC-M concrete.

Fig. 4-12 Free chloride profiles of both SFGPC-M after 4 year of exposure and OPC-M

concrete after 04 years of exposed time in marine environment.

Total chloride measurements and Chloride diffusion coefficients (Dc)

The total chloride profiles of SFGPC-M and OPC-M concrete specimens are shown in

Fig. 4-13. As shown that the SFGPC-M concrete consists higher chloride content

compared to OPC-M concrete throughout the depth. The total chloride measurements

indicated that the predicted total chloride of OPC-M concrete is intersected with a

threshold value of 22.5 mm depth, whereas the predicted chloride content in SFGPC-M

was greater than 0.06% at 28.5 mm depth. Therefore, this indicated that the required

cover to protect the reinforcement bar in slag- fly ash blended geopolymer concrete

should be greater than the cover value usually provides for the reinforcement bar in OPC

concrete in the marine environment. Unfortunately, no reinforcement bar has been

identified in the core specimens, which provides the limitation for determining the

corrosion behaviour of reinforcement in the concrete structures.


112

Fig. 4-13 Total chloride profiles of SFGPC-M and OPC-M concrete after 04 years and 06

years of exposure in the marine environment

Table 4-5 illustrates the Dc values and the surface chloride concentration values (Cs) of

SFGPC-M concrete specimens after two years and four years of exposure in the marine

environment. Dc and Cs values of OPC-M concrete specimens after six years of exposure

in marine environment also present in Table 4-5. As can be observed from that, these

results also displayed that the SFGPC-M specimen had lower resistance to chloride

transportation compared to OPC-M concrete. Despite that, the previous studies have

shown that the chloride resistance of slag-based geopolymer concrete is greater than

OPC concrete [22, 209]. The reason provided for the high chloride resistance is the slag-

based geopolymer concrete produces cross-linked C-A-S-H (tobermorite) phase, which

is high density, compared to OPC concrete. In OPC concrete, the formation of non-cross

linked C-S-H (tobermorite) [17] attributes higher porosity and high pore volume [24],

which represents the higher chloride penetration compared to slag based geopolymer

concrete. This conclusion has been taken according to the accelerated chloride testing

methods with freshly prepared geopolymer concrete specimens. In contrast with that

conclusion, this current investigation shows higher chloride diffusion in 4-year-old

SFGPC-M compared to 6-year-old OPC-M concrete after exposure to the real marine

conditions. As mentioned earlier, diffusion of chloride in OPC concrete is reduced with

the age of the concrete due to continuous hydration reaction of cement produced a

denser structure, which is reducing the ingress of chloride ions with age. Conversely,

geopolymerisation reaction is quicker than the hydration of OPC concrete [21] and


113

therefore, the pore structure of SFGPC-M would not be changed much with the age of

the concrete. Therefore, the chloride diffusion of SFGPC-M is higher compared to OPC

concrete in the marine environment.

Furthermore, Table 4-5 showed that the Dc and Cs values of the geopolymer concrete are

decreased with the time of exposure. The diffusion coefficient of SFGPC-M was

1.92 × 10−12 and 1.15 × 10−12 m2/s at 02 and 04 years periods, respectively. These values

are higher than the value observed by Ismail et al. [22] for high slag based geopolymer

concrete with accelerated testing methods. On the other hand, for OPC-M concrete, the

comparison could not be conducted with the time of exposure due to no Dc and Cs

values were calculated for OPC-M concrete after two years and four-year exposed

period.

Table 4-5 Chloride diffusion coefficient and surface chloride content of concrete.

Chloride Diffusion Coefficient, Dc (1 x 10-12 m2s-1)

Surface Chloride Concentration, Cs (% by weight concrete)

02 years 04 years

06 years

02 years 04 years 06 years

SFGPC-M concrete

1.92 1.15 N/A 0.651 0.68 N/A

OPC-M concrete

N/A N/A 0.45 N/A N/A 0.45

Chloride binding capacity of slag-fly ash blended geopolymer concrete

The calculated chloride binding capacity of the SFGPC-M and OPC-M concrete

specimens are 34% and 46% after four years and six years of the exposed period,

respectively. This revealed that in OPC-M concrete, more chloride ions bound with the

reaction phases compared to the binding of chloride ion with SFGPC-M reaction

components.

4.4.3 Sulphate attack in an aggressive environment

4.4.3.1 Sulphate resistance of fly ash based geopolymer concrete

The test results of the sulphate measurements of core specimens extracted from the leg

parts of the box culvert are shown in Fig.4-14. As similar to CO2 and chloride penetration,

FGPC-S showed higher sulphate ingress compared OPC-S concrete. The sulphate


114

contents of the FGPC-S are greater than OPC-S concrete throughout the depth. In the

sulphate environment, aluminosilicate compounds from the geopolymer matrix are

degraded easier than OPC concrete reaction phases and induce higher sulphate ingress

to the concrete surface. In addition to that, the porosity of the concrete also influenced

on the sulphate penetration of the concrete. The porosity and the pore size distributions

of the samples are further discussed in the below sections.

Fig.4-14 Sulphate concentration versus depth for FGPC-S and OPC-S concrete

4.4.3.2 Sulphate resistance of slag-fly ash blended geopolymer concrete

Fig.4-15 illustrates the sulphate measurements of slag-fly ash blended geopolymer and

OPC concrete core specimens from the marine environment after four years and six years

of exposure, respectively. The ingress of sulphate ions to the geopolymer concrete

surface was higher than the penetration through the OPC concrete at the less exposure

period. It should be noted that the sulphate attack in geopolymer concrete is varied from

OPC binder due to the variation of reaction phases. Therefore, further investigation

required to determine the degradation mechanism of sulphate attack in geopolymer

concrete.


115

Fig.4-15 Sulphate concentration versus depth for FGPC-S and OPC-S concrete

4.4.4 Scaling effect of geopolymer concrete

4.4.4.1 Scaling effect in fly ash based geopolymer concrete

Fig.4-16 shows the visual appearance of the FGPC-S and OPC-S concrete culvert

structures after exposed six years in the saline environment. The figure revealed that the

OPC-S concrete structures have not any significant changes in the visual appearance

after six-year period, while the mortar from the FGPC-S surface, has been lost and the

aggregates are clearly exposed on the surface. This is called a scaling effect in concrete

structures. Compared to top slab part of FGPC-S culvert, leg part of the structure has

more effect due to the frequent contact with saline lake water. Salt scaling occurs when

the concrete surface is subjected to wet and dry cycles. This causes salt crystallisation on

the concrete surface, which results in severe surface damage. Here, in addition to salt

crystallisation, the presence of a high concentration of MgSO4 in the exposed soil and

lake water also attributes a higher scaling effect on the concrete structure. Soil rich in

sulphate can also cause the deterioration of the concrete structure and soften and

spalling of the concrete surface due to the reaction between the sulphate ions and the

hardened concrete surface [210] and the reaction with Mg2+ ions also degraded the

binder in the concrete.


116

Fig.4-16 Visual appearance of concrete structure (a) FGPC-S concrete culvert, (b) the

outer surface of FGPC-S showing exposed aggregate, (c) OPC-S concrete culvert, (d) the

outer surface of OPC-S with no visual evidence of deterioration

Even though both culverts are exposed in the same environmental condition, the scaling

effect in FGPC-S was high, whereas no significant effect was identified in OPC-S

structure. This is due to the higher diffusion of chloride and sulphate ions into FGPC-S

structure compared to OPC-S concrete at the sample exposure period. In addition to that,

the reaction between geopolymer concrete and the sulphate or the chloride ions from

saline water is significantly different to the reaction between OPC concrete and those

aggressive agents. This is due to the nature of the aluminosilicate gel in geopolymer

materials compared to hydration reaction components in OPC binder. Geopolymer

concrete is rich in Na in the pore solution, and this produces thenardite (Na2SO4), from

the reaction with sulphate ions. In the previous investigation, Hime et al. [211]

determined that the thenardite turns to Mirabilite (Na2SO4. 10H2O) components during

the wetting periods and this causes an expansion of the structure. This can, therefore,

explain the higher scaling activity in FGPC-S compared to OPC-S concrete, particularly

if the GPC contained excessive Na+. Therefore, fly ash based geopolymer concrete

showed higher scaling effect compared to OPC concrete in the saline environment.


117

4.4.4.2 Scaling effect in slag- fly ash blended geopolymer concrete

Fig. 4-17( a) appearance of the SFGPC-M concrete surface after four-year, (b) appearance

of the OPC-M concrete surface after six-year.

Fig. 4-17 illustrates the visual appearance of the SFGPC-M and OPC-M concrete

structures in the atmospheric zone of the marine environment after 04 years and 06 years

of exposed period. It can be seen that the SFGPC-M surface was undergone erosion in

the aggressive exposure condition. In SFGPC-M surface, mortar particles were removed,

and the coarse aggregates were exposed to the outer surface, whereas, there were no

such things observed in OPC-M concrete surface. However, the small size of cavities was

observed in the OPC-M concrete surface. As similar to fly ash based geopolymer

concrete, higher scaling activity in slag-fly ash blended geopolymer concrete also due to

the Na in the pore solution due to the activator solutions and higher chloride and

sulphate penetration compared to OPC concrete in the same exposure condition.


4.4.5.1 pH measurement in fly ash based geopolymer concrete

Fig.4-18 shows the pH profiles of FGPC-S and OPC-S concrete specimens from both

inner and outer exposed surfaces. By considering the penetration of CO2 and chloride

ions through both surfaces (outer and inner surface) of the culverts, the pH measurement

was conducted on the powder samples collected from both surfaced with 5 mm depth

intervals up to 30 mm depth level. As shown in Fig.4-18, pH of FGPC-S specimens was

not much varied with the depth, which is around 7.0-7.5 from the exposed surface to 30

mm depth level. This range is lower to the pH value obtained for the carbonated fly ash

based geopolymer concrete [11, 21]. Moreover, the previous chapter showed that fly ash


118

based geopolymer concrete prepared with same mix composition (FGPC-A) exposed to

an atmospheric environment for eight years displayed a pH range 9.92 to 10.41 from the

exposure surface to 30 mm depth level. This indicates compared to the normal

atmospheric condition, pH of fly ash based geopolymer concrete is reduced highly in an

aggressive environment. In a wet saline environment, contact with water, ingress of salt

and carbonation of the concrete surface induces a greater pH reduction compared to the

concrete structures exposed to the atmospheric environments.

Furthermore, compared to FGPC-S, OPC-S concrete had higher pH values, which is in

the range of 8.5-10. These pH values are comparable with the carbonation depth values

determined in FGPC-S and OPC-S concrete core samples. According to the carbonation

test, FGPC-S concrete core specimens were fully carbonated, and the pH values

confirmed the carbonation up to 30 mm depth level. Carbonation depth of the OPC-S

concrete also consistence with the pH test results.

Fig.4-18 pH variation with depth for FGPC-S and OPC-S concrete

Furthermore, in this investigation, phenolphthalein indicator was suited to determine

the carbonation depth of fly ash based geopolymer concrete. However, it was difficult

to identify the carbonation depth of fly ash based geopolymer concrete in atmosphere

exposure conditions and laboratory carbonation tests. These all related to the pH

reduction due to carbonation of concrete. In this study, pH of fly ash based geopolymer

concrete is reduced to 7.0-7.5 and the phenolphthalein indicator should turn to colourless


119

at this pH range. However, pH range of fly ash based geopolymer concrete in

atmospheric exposed condition was 9.92 to 10.41, which is difficult to determine the

colour change with the phenolphthalein application.

4.4.5.2 pH measurement in slag-fly ash blended geopolymer concrete

Fig. 4-19 illustrates the pH variation with the depth of the SFGPC-M and OPC-M

concrete specimens after exposed to 4 years and 6 years in the marine environment,

respectively. The carbonation depth values of SFGPC-M and OPC-M are also marked in

the figure. As shown in Fig. 4-19, pH of carbonated SFGPC-M concrete was in the range

of 8.5- 10.0, whereas the pH of OPC-M concrete was reduced to 9.0 from 12.3 after the

carbonation. In OPC-M concrete, pH reduction is due to the formation of CaCO3, which

is corresponding to the pH reduction is less than 9.0. On the other hand, in slag-fly ash

blended geopolymer concrete, carbonation reaction created CaCO3 and Na2CO3 as

carbonation reaction components. As explained previously, the amount of CaCO3 in

slag-fly ash based geopolymer concrete is lower than OPC concrete [26]. In addition to

that, the pH of Na2CO3 is higher than the pH of CaCO3. Therefore, due to these reasons,

pH of the carbonated blended geopolymer concrete was maintained at a high level

compared to OPC concrete. As similar to previous pH analysis, this pH test results also

consistent with carbonation depth values obtained with phenolphthalein application.

Fig. 4-19 pH variation with depth of the concrete from the exposed surface


120

4.4.6 Test results from the sorptivity analysis

4.4.6.1 Sorptivity test results for fly ash based geopolymer concrete

Fig.4-20 depicts the sorptivity curves of the concrete core specimens to compare the

sorption of the concrete surface with chloride diffusion values. In a tidal environment,

capillary absorption is an important mechanism for the ingress of chloride to the

concrete structures. Capillary absorption parameters represent the ability of water

absorption of the concrete surface by the capillary suction, and it is related to the pore

structure and interconnectivity of pores [22]. Therefore, higher absorption characteristic

represents the lower resistance to chloride diffusion to the concrete surface. Although

the rate of water absorption can be calculated by two stages such as initial water

absorption rate and secondary water absorption rate, initial water absorption rate is

associated with the capillary pores in the concrete surface, and that is related to chloride

transportation to the concrete. Table 4-6 shows the initial sorptivity coefficient values for

both FGPC-S and OPC-S specimens. As illustrated in Fig.4-20 and Table 4-6, the

reduction of sorptivity in OPC-S concrete is an agreement with well refined and lower

pore structures that are related to lower chloride penetration. By contrast, a higher

sorptivity of FGPC-S concrete accompanied the higher pore structure and porosity,

which are associated with the higher chloride penetration in the FGPC-S compared to

OPC-S concrete. Further discussion regarding the pore characteristic of the concrete

specimens are explained in the later section.

Fig.4-20 Capillary absorption of FGPC-S and OPC-S concrete


121

Table 4-6 Coefficients of sorptivity values

Specimens Initial sorptivity coefficient(mm/s1/2)

FGPC-S 0.0024

OPC-S 0.0012

4.4.6.2 Sorptivity test results for slag-fly ash blended geopolymer concrete

Fig. 4-21 shows the capillary absorption curves of the SFGPC-M and OPC-M concrete

samples. The initial rate of water absorption or the sorptivity coefficient of SFGPC-M

and OPC-M concrete samples are provided in Table 4-7. As shown in Fig. 4-21 and Table

4-7, capillary absorption of SFGPC-M specimens is high compared to OPC-M concrete

specimen. This is consistent with chloride diffusion test results and indicated that the

capillary test could be used to assess chloride diffusion performance of geopolymer

concrete. In addition, capillary sorption parameters are related to total porosity and the

tortuosity of the concrete pore structure [22]. Therefore, this test results indicates the

OPC-M concrete had a dense and lesser porous network compared to SFGPC-M.

However, compared to capillary absorption test, mercury intrusion porosimetry (MIP)

analysis provides the better understanding of the pore structure and pore size

distribution of concrete. Therefore, the concrete samples were included into MIP test to

determine the pore size distributions. The detailed of the MIP test results are explained

in the later section.

Fig. 4-21 Test results from the sorptivity analysis


122

Table 4-7 Coefficients of sorptivity values

Specimens Initial sorptivity coefficient(mm/s1/2)

SFGPC-M 0.0101

OPC-M 0.0016

4.4.7 FT-IR analysis

4.4.7.1 FT-IR analysis of fly ash based geopolymer concrete

The FT-IR spectrum of FGPC-S and OPC-S concrete specimens are shown in Fig.4-22 and

Fig.4-23, respectively. ‘FGPC-S -T’ series reflect the powder samples collected from the

outer surface of the structure with 5 mm depth intervals, whereas ‘FGPC-S-B’ series

indicate the test results from the powder sample collected from the inner surface of the

geopolymer concrete structure. Similarly, in Fig.4-23, ‘OPC-S- T’ and ‘OPC-S- B’ are

associated with the test results of powder collected from the outer and inner surface of

OPC-S concrete specimens, with 5 mm depth variation, respectively. As we can see from

Fig.4-22, there are no peaks has been observed at the wave number of 1620 cm-1 for all

FGPC-S samples. This indicates carbonation components are not presented in FGPC-S

after exposed to the saline environment. Similar results have been observed in the

samples collected from fly ash based geopolymer concrete (FGPC-A) after exposure to

the atmospheric environment, that was explained in the previous chapter. Carbonation

depth results and the pH test results are confirmed the severe carbonation in FGPC-S at

the saline environment. Therefore, as explained in the previous chapter, Na and K based

carbonation reaction components in fly ash based geopolymer concrete is removed from

the concrete by dissolving of water under the aggressive field conditions.


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Fig.4-22 FTIR spectrum of FGPC-S concrete samples with various depth intervals

On the other hand, FT-IR spectrum of OPC-S concrete in Fig.4-23 showed the peak at

1620 cm-1 for the samples collected up to 30 mm and 15 mm depth level from the outer

and inner exposed surfaces, respectively. This confirmed the presence of C-O bond in

OPC-S concrete samples up to 30 mm and 15 mm depth level from both outer and inner

exposed surfaces. However, the maximum carbonation depth values of leg part of the

OPC-S concrete were 10 mm and 4 mm from the outer and inner exposed surfaces.

Therefore, this clearly indicated that the FT-IR analysis showed higher carbonation than

the phenolphthalein indicator and it is known that the carbonation front is ahead of that

shown by phenolphthalein [212]. As mentioned previously, the phenolphthalein

indicator can only determine the complete carbonation zone when the pH range is less

than 9.0 (presence of more CaCO3 components), whereas the FT-IR test is an accurate

method to identify the partially carbonated zone, which contains the combination of


124

Ca(OH)2 and CaCO3 components. The pH range of the partially carbonated zone is about

9.0-11.5 [180], and the carbonation depth with this pH range cannot be determined

accurately by phenolphthalein. A higher carbonation depth was therefore obtained from

FT-IR spectrum due to the presence of a partially carbonated zone. However, comparing

to other depth intervals, a higher intensity peak was observed for the powdered samples

collected from the surface to 5 mm depth intervals at the band 1410-1420 cm-1 and the

stretching vibration of CO32- was also recognised at the band 873 cm-1 [182]. The peak

obtained at 1410-1420 cm-1 is due to the existence of calcite, and the carbonation

components, such as vaterite and aragonite were identified at the band of 873 cm-1 [213].

This indicates that the effect of carbonation is high in the first layer (up to 5 mm) compare

to other layers. A further, peak obtained at 1100 cm-1 in Fig.4-22 and Fig.4-23 is

corresponding to the S-O bond [214], which confirmed the penetration of sulphate ion

in the saline environment.

Fig.4-23 FTIR spectrum of OPC-S concrete samples with various depth intervals


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4.4.7.2 FT-IR analysis of fly ash-slag blended geopolymer concrete

Fig.4-24 illustrates the FT-IR spectrum of SFGPC-M and OPC-M concrete specimens. The

tests were conducted on the powder samples collected from the exposed surface of

concrete core specimen with 3 mm depth intervals. As opposed to fly ash based

geopolymer concrete, the C-O bond has been observed at the wave number of 1620 cm-1

from slag-fly ash blended geopolymer concrete. This indicates carbonation reaction

components are retained in the slag-fly ash blended geopolymer concrete surface after

exposure to the marine environment. Similar results have been observed in the samples

collected from atmospherically exposed fly ash- slag based geopolymer concrete as well

(Chapter 3). Therefore, as explained earlier, the formation of Ca-based carbonation

components due to the slag content in SFGPC-M type specimen remains on the surface,

and this produces the peak at 1620 cm-1 in FT-IR analysis. However, the soluble

carbonation components in SFGPC-M should be removed after the field exposure.

Fig.4-24 FTIR spectrum of (a) SFGPC-M concrete samples, (b) OPC-M concrete with

various depth intervals


126

The FT-IR spectrum of OPC-M concrete in Fig.4-24 also showed the peak at 1620 cm-1 for

the samples due to the C-O bond. Furthermore, FT-IR spectra of both types of samples

showed the peaks at the band 873 cm-1, which is due to the stretching vibration of CO32-

[182]. The peak obtained at 1410-1420 cm-1 is due to the existence of calcite, and the

carbonation components, such as vaterite and aragonite were identified at the band of

873 cm-1 [213].

4.4.8 Pore size distribution analysis with MIP test

4.4.8.1 MIP test on fly ash based geopolymer concrete

Fig. 4-25 illustrates the cumulative intrusion of FGPC-S and OPC-S concrete specimens

from the saline environment. According to that the main increment in the cumulative

intrusion of FGPC-S concrete occurred in the pore diameter interval of 10 -100 nm,

whereas the intrusion curve of the OPC-S concrete is gradually increased between the

full diameter range. However, the total intrusion of FGPC-S concrete samples was higher

than the total intrusion of OPC-S concrete. This revealed that the total porosity of the

FGPC-S samples was greater than OPC-S concrete porosity values.

Fig. 4-25 Cumulative intrusions of aggressive exposed FGPC-S and OPC-S concrete

specimens.

Fig. 4-26 shows the differential pore size distribution obtained for both samples and the

Table 4-8 depicts the pore distributions of the concrete specimens, categorised based on

the IUPAC classification system. According to that, the FGPC-S contains fines pores, and


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the majority of pores have a diameter of 1.25 nm-25 nm. By contrast, OPC-S concrete

displayed a range of pore diameters with a substantial percentage of pores detected in

the range of macropores and air voids/cracks range. This indicates that the average pore

size in OPC-S concrete was greater than the average pore size of FGPC-S. This would be

due to the ingress of salt in the saline environment. Higher ingress of salt in FGPC-S fill

more pores and reduce the pore size compared to the pore structure changes in OPC-S

concrete. This has been confirmed by the SEM/EDX test results, indicates the chloride

ions deposited as a film layer on the FGPC-S concrete surface. However, the MIP results

show the total porosity of the FGPC-S specimens is slightly higher than the porosity of

the OPC-S concrete specimens.

Fig. 4-26 Differential pore size distribution obtained for aggressive exposed concrete

specimens

Table 4-8 Pore size percentages (based on IUPAC classification)

Specimens Porosity Pore size distribution (%)

Micropores

(<1.25×10-3µm)

Mesopores

(1.25–25

×10-3µ m)

Macropores (25–

5000 ×10-3µ m)

Air

voids/cracks

(5000–50,000

×10-3µ m)

FGPC-S 12.6% - 62% 19.3% 18.9%

OPC-S 9.5% - 12.3% 48.4% 39.2%


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In fly ash based geopolymer concrete, Sodium Alumino Silicate Hydrate gel (N-A-S-H)

structure is the main reaction components during the geopolymerisation. This is a three-

dimensional network product, and therefore this promotes a higher porosity in concrete

structures. In addition, the presence of un-reacted fly ash particles in geopolymer

concrete also caused the higher porosity and a higher transfer of chloride ions into the

concrete surface [22]. However, the inclusion of slag into fly ash based system produces

a dense C–S–H phase, in addition to N-A-S-H gel, establishes a highly refined pore

network by filling the pore volume of aluminosilicate geopolymer gel and reducing

the chloride ion penetration to the concrete surface [215]. Here, only fly ash was used

as a precursor for the geopolymer concrete. Therefore, FGPC-S concrete had porous

structure, and this attribute the higher diffusion of CO2 and chloride ions into the

concrete.

4.4.8.2 MIP test on slag-fly ash blended geopolymer concrete

Fig. 4-27 shows the cumulative pore size distribution of both types of concrete samples

from 0-3 mm (top-level) and 25-30 (mid-level) mm depth levels. The results indicated

that the total volume of intruded mercury for SFGPC-M specimens is higher than that of

the OPC-M concrete at all depth level. The SFGPC-M specimens had 0.085 ml/g for 0-3

mm depth level and 0.081 ml/g of cumulative pore volumes for the sample from 25-30

mm, whereas OPC-M concrete showed the of cumulative intrusion volumes are 0.048

and 0.055 ml/g for the samples from 0-3 mm and 25-30 mm depth levels, respectively.

This revealed that the porosity of the SFGPC-M is greater than OPC-M concrete. The

differential pore size distribution and overall pore characteristic details of both types of

concrete are provided in Fig. 4-28 the Table 4-9, respectively. According to that, the total

porosity of the SFGPC-M specimens from both depth levels was greater than OPC-M

concrete samples, In addition, the total porosity of top level of SFGPC-M is higher than

the sample from mid-level, whereas, the top-level OPC-M concrete specimen had lower

porosity compared to mid-level of OPC-M concrete. This is due to the carbonation effect

in concrete after exposed to the field environment. In OPC concrete, carbonation reaction

reduces the porosity of the surface due to the formation of carbonation components.

During the carbonation reaction, insoluble CaCO3 fills the pore structure of the OPC

concrete and reduce the size of pores on the surface. Therefore, samples from the top

level (0-3 mm) of OPC concrete had lower porosity compared to mid-level (25-30 mm)

sample.


129

As explained previously, amount of CaCO3 formation in slag-fly ash blended

geopolymer concrete is lower than OPC concrete [26] and sodium carbonation

components can also form in the geopolymer concrete due to the reaction between the

NaOH and the CO2. As explained previously, the sodium carbonate components are

highly soluble in water and cannot withstand the concrete surface in the outside exposed

condition. Therefore, due to these all reasons, the porosity of the slag-fly ash blended

based geopolymer concrete surface would not be reduced with carbonation reaction. As

a result, the top level of geopolymer concrete samples showed higher porosity compared

to mid-level of the sample.

Even though the SFGPC-M specimens had a higher porosity, SFGPC-M possesses

similar or lesser average pore diameter values compared to OPC-M concrete

specimens. This suggests that the both concrete contains the almost same size of pores,

while the higher the total pore area in SFGPC-M specimens confirmed the higher

porosity in SFGPC-M concrete. As per Fig. 4-28, both types of concrete specimens had a

larger proportion of the pores at the diameter less than 50 nm, which are harmless or less

harmful pores [187]. However, there are some noticeable amount of harmful pores with

the diameter range of 50-100 nm has been identified in the surface level of geopolymer

concrete (0-3 mm), which are corresponding to the higher amount of chloride diffusion

through the SFGPC-M surface.

Fig. 4-27 Cumulative pore size distribution obtained for both types of concrete at the

surface level and the mid-depth level.


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Fig. 4-28 Differential pore size distribution obtained for both types of concrete at the

surface level and the mid-depth level.

Table 4-9 The pore characteristics details of both types of concrete specimens.

SFGPC-M (0-3 mm)

SFGPC-M (25-30 mm)

OPC-M (0-3 mm)

OPC-M (25-30 mm)

Porosity (%) 14.5 14.2 8.7 9.9

Average pore diameter (nm)

28.3 21.9 22.3 34.8

Total pore area(m2/g)

12.09 14.97 8.52 6.27

4.4.9 Microstructural analysis by SEM/EDX method

4.4.9.1 SEM/EDX test results of fly ash based geopolymer concrete

To determine the potential detrimental effect of carbonation and chloride ions on the

microstructure of the concrete, SEM morphology analysis was conducted on the samples

collected from various depth levels of the core specimens. Fig.4-29 shows the test results

from SEM/EDX analysis of the samples collected from the outer, middle and inner part

of FGPC-S concrete core specimens. From the EDX test results, amount of Cl- ions were

determined in a higher proportion compared to Si and Al components, in all FGPC-S

concrete samples (outer, middle, inner). This indicates chloride was penetrated


131

completely through the culvert, which is consistent with the chloride penetration depth

results obtained by the application of the AgNO3 solution. Fig.4-29 demonstrated that

the sodium chloride salt was deposited as a film layer on the microstructure of FGPC-S

concrete. Therefore, EDX result was determined on that film layer to determine the

elements presence in that layer Fig.4-30 shows the EDX test results on the deposit layer.

According to that, only the elements Na and Cl were observed as major components in

that film layer, which confirmed the deposition of NaCl on the microstructure of FGPC-

S under saline environment.

The test results of SEM/EDX analysis of outer, middle and inner part of OPC-S concrete

samples are shown in Fig.4-31. As per that, the existence of Cl- ion was identified only

in the sample collected from outer and inner parts of samples. This specifies chloride

ions has not been penetrated throughout the structure, which is consistent with the

chloride penetration depth results. In addition, not like FGPC-S samples, chloride was

not observed as a deposition layer on the OPC-S microstructure, which indicates lower

ingress of chloride in OPC-S concrete compared to FGPC-S.

Furthermore, the presence of C in the top part of the OPC-S concrete specimens (Fig.4-31)

confirmed the carbonation components in OPC-S concrete. In contrast, there was no

evidence have been identified in FGPC-S samples (Fig.4-29) for the presence of C. These

results are correlated well with the FT-IR spectrum analysis.


132

Fig.4-29 SEM micrograph of (a) Top, (b) Middle and (c) Bottom part of FGPC-S concrete

core specimens with corresponding EDX analysis

Fig.4-30 SEM micrograph of chloride deposit on FGPC-S concrete specimens with

corresponding EDX analysis


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Fig.4-31 SEM micrograph of (a) Top, (b) Middle and (c) Bottom part of OPC-S concrete

core specimens with corresponding EDX analysis

Fig.4-32 illustrates the SEM micrograph of FGPC-S and OPC-S concrete specimens

obtained from the outer surface of core specimens. The ettringite formation was

identified in the morphology of OPC-S concrete provided in Fig.4-32 (b), and there is no

evidence for the existence of ettringite in FGPC-S specimens (Fig.4-32 (a)). The exposed

soil is rich in magnesium sulphate (MgSO4), and the test results from the sulphate

analysis confirmed the penetration of sulphate ion through the both FGPC-S and OPC-

S concrete culverts. In OPC-S concrete, the ingress of MgSO4 reacts with Ca(OH)2 and

calcium aluminate hydrate components and this produced the ettringite component.

However, in FGPC-S concrete, ettringite should not be formed due to the absence of

those components (Ca(OH)2 and calcium aluminate hydrate components). A further, the

EDX results (Fig.4-29) of FGPC-S revealed that the FGPC-S concrete had a higher level


134

of Mg ions compared to OPC-S concrete. Therefore, this indicates that the mechanism of

sulphate attack in fly ash based geopolymer concrete is different from OPC concrete, and

this requires further investigation.

Fig.4-32 SEM micrograph of (a) FGPC-S, (b) OPC-S concrete specimens

4.4.9.2 SEM/EDX test results of slag-fly ash blended geopolymer concrete

Fig.4-33 depicts the test results from SEM/EDX analysis of the samples collected from 0-

3 mm level of SFGPC-M and OPC-M concrete core specimens. EDX graph of SFGPC-M

specimen displayed that the presence of Cl-. By contrast, Cl- was not identified from the

OPC-M concrete. The previous section depicted that the sodium chloride salt was

deposited as a film layer on the microstructure of fly ash based geopolymer concrete

under saline aggressive environment. However, there are no such things have been

identified in the slag-fly ash blended geopolymer concrete.

SEM micrograph of SFGPC-M and OPC-M concrete specimens obtained from the outer

surface of core specimens are provided in Fig.4-34. OPC-M concrete morphology

indicated the formation of ettringite (needle shape) (Fig.4-34 (b), whereas no indication

for the existence of ettringite in FGPC-S specimens (Fig.4-34 (a)). As mentioned earlier,

MgSO4 reacts with Ca(OH)2 and calcium aluminate hydrate components from OPC

concrete and produced the ettringite component and the reaction mechanism is different

in slag-fly ash based geopolymer and needs to identify in the future investigation.


135

Fig.4-33 SEM micrograph of SFGPC-M and OPC-M concrete specimens (0-3 mm depth)

with corresponding EDX analysis

Fig.4-34 SEM micrograph of (a) SFGPC-M, (b) OPC-M concrete specimens

4.4.10 Corrosion of reinforcement in fly ash based geopolymer concrete

The reinforcement bar in fly ash based geopolymer and OPC concrete culverts from the

saline environment was visually inspected to determine the corrosion activity after six

years of the exposure period and the corrosion products at reinforcement/concrete

interface was examined by SEM/EDS analysis. Such inspections could not be able to

carry out in slag-fly ash blended geopolymer concrete structures due to the absence of


136

rebar in near-surface in those concrete structures, and the rebar had been embedded in

the depths higher than the core size.

4.4.10.1 Visual inspection

The corrosion activity of the reinforcement bar, which are possessed in FGPC-S and

OPC-S concrete, was visually inspected by breaking the core specimens. The condition

of the reinforcement bars and the photograph of the steel-concrete interfaces are shown

in Fig.4-35. Fig.4-35 (a) and Fig.4-35 (d) illustrate the steel/concrete interface of the leg

part of FGPC-S and OPC-S culverts with a 25 mm cover value. Fig.4-35 (b) and Fig.4-35

(e) show the conditions of the steel bars at the same location of the FGPC-S and OPC-S

specimens, respectively. According to that, the reinforcement bar in FGPC-S culvert

display more corrosion activity, whereas there is little visible sign of corrosion products

in an OPC-S concrete culvert after six years exposed in a saline environment. The

reinforcement bar in FGPC-S concrete was corroded over the entire surface, and more

corrosion products were deposited at the steel/concrete interface compared to OPC-S

concrete. The combination of higher chloride ingress and carbonation accelerated the de-

passivation of steel and resulted in more extensive corrosion in FGPC-S concrete

compared to OPC-S concrete. The microstructural behaviour of the steel/concrete

interface after the corrosion was identified with the aid of SEM/EDX analysis and

explained in the following section.


137

Fig.4-35 Rebar interface and reinforcement bar after six years of exposure, (a) typical

rebar interface of FGPC-S concrete specimen, (b) reinforcement bar in leg part of FGPC-

S culvert, (d) typical rebar interface of OPC-S concrete specimen, (e) reinforcement bar

in the leg part of OPC-S culvert

4.4.10.2 SEM analysis on steel-concrete interface area

Fig.4-36 shows the SEM micrograph of FGPC-S and OPC-S concrete specimens collected

at the steel/concrete interface area. As shown in SEM images, the form of flowery

structures indicated the presence of lepidocrocite [γ-FeO(OH)] at the interface area of

both concretes [121]. However, the amount of γ-FeO(OH) in FGPC-S interface is higher

than the amount observed at the interface of steel/OPC-S concrete interface. An EDX

analysis of FGPC-S concrete also showed strong peaks of Fe and O and confirmed the

presence of high corrosion products such as γ-FeO(OH) at the interface area, whereas

EDX test results of OPC-S concrete revealed intermediate peaks of Fe and O, which


138

represents the fewer corrosion products at OPC-S steel/concrete interface. These results

are similar to visual observations of the interfacial area.

Fig.4-36 SEM micrograph of (a) FGPC-S; (b) OPC-S at the rebar/matrix interface with

corresponding EDX analysis


139


This chapter depicts the durability of geopolymer concrete in saline and marine

environment. The combined effect of carbonation, chloride diffusion and sulphate

penetration on the durability of geopolymer concrete was studied, and the durability

parameters were compared with OPC concrete from the same exposure environments.

In this investigation, fly ash based geopolymer concrete structure from the saline

environment after six years of exposure and slag-fly ash blended geopolymer concrete

exposed in the marine environment for four years were included. Based on the

experimental investigations, following conclusions can be extracted:

The effect of carbonation in fly ash based geopolymer was much greater than the

influence in OPC concrete over the six years of exposure in saline environments.

According to the test results, the core specimens from FGPC-S culvert was

completely carbonated in the leg parts (90 mm) as well as in the top slab (135 mm

thickness), whereas a maximum of 10 mm and 20 mm carbonation depth values

were obtained in the leg parts and top slab part of the OPC-S concrete structure,

respectively. The CO2 diffusion to the slag- fly ash blended geopolymer concrete also greater

than OPC concrete in the marine environment. Even though SFGPC-M concrete

structure was exposed less period (4 years) in the marine environment, it showed

higher carbonation values (11 mm) compared to OPC-M concrete. OPC-M

concrete displayed only 4 mm carbonation after 06 years of exposed period.

The chloride penetration in the fly ash based geopolymer concrete was high in

saline environments. Besides, SEM analysis revealed that the chloride contents

were deposited as a film layer on the FGPC-S concrete. As similar to fly ash based

geopolymer concrete, chloride penetration in slag-fly ash blended geopolymer

concrete also greater than OPC concrete under the exposure in the marine

environment. However, compared to fly ash based geopolymer, the chloride

penetration in slag-fly ash based geopolymer concrete is low, and SEM analysis

was not provided with any deposition of chloride layer ion the microstructure of

the concrete.

Considering sulphate penetration, FGPC-S concrete displayed higher ingress of

sulphate compared to OPC-S concrete. This produced more scaling effect in GPC


140

structure. SEM/EDX test results also revealed the higher sulphate penetration,

and there is no formation of ettringite observed in FGPC-S specimens. Similar

behaviour has also been identified on slag-fly ash blended geopolymer concrete.

This indicates the mechanism of sulphate attack in geopolymer concrete is

different from OPC concrete.

The salt scaling effect in both types of geopolymer concrete (FGPC-S and SFGPC-

M) was higher than OPC concrete. The mortar from the FGPC-S surface,

especially from the leg part of a culvert that is frequently contacted with saline

lake water, has been lost and the aggregate is clearly exposed on the surface,

whereas no significant changes have been identified in the visual appearance of

an OPC-S concrete culvert over time. Similarly, mortar from the surface of

SFGPC-M concrete also removed after four years exposed in the marine

environment, while OPC-M concrete was not shown such observation after six

years of exposed period.

The combination of higher carbonation and chloride penetration produced

higher corrosion activity of the steel bar in fly ash based geopolymer concrete.

The reinforcement bar in FGPC-S concrete was corroded on the entire surface,

and the deposition of corrosion products at the interface area of FGPC-S concrete

was much higher than for OPC-S concrete.

Chapter 5 Study on alkali leaching, wet and dry

cyclic resistance of geopolymer

5.1 Introduction

This chapter presents the experimental investigation of alkali leaching and wetting-

drying cyclic resistances of geopolymer in different exposure conditions. The leaching

of alkali elements from the geopolymer specimens was determined by immersed in

deionised water. Accelerated wetting-drying cyclic resistance test was conducted on the

samples exposed to three different types of media such as water, chloride solution and

the combination of chloride and sulphate solution. To determine the influence of the

source materials on the leaching properties and the wetting-drying cyclic resistances,

geopolymer mortar samples were prepared with different proportion of fly ash and slag

constituents. The compressive strength and the weight of the specimens were measured

with the time interval to determine the strength and weight losses of the samples in the

wet and dry cycle analysis. Although this is an accelerated testing method, the test was

carried out due to the insufficient data determined from the field exposed geopolymer

concrete specimens. According to the field investigations, a limited number of mix

compositions of geopolymer was evaluated, and the leaching ions from geopolymer and

the compressive strength variation were not be evaluated. Therefore, this study was

conducted to evaluate the leaching effect, compressive strength losses of geopolymer

samples in different exposed condition. Furthermore, the test results of geopolymer

specimens were compared with OPC mortar specimens.

5.2 Materials and Methods

5.2.1 Materials

Class F fly ash from Gladstone power station, and Ground granulated blast furnace slag

(GGBFS) from Independent Cement Australia Pty Ltd was used as the source materials

for the preparation of geopolymer binder. The fly ash/slag ratios used in M1, M2, M3 and

Chapter 5 Study on alkali leaching, wet and dry cyclic resistance of geopolymer

142

M4 types of geopolymer mixes were 100/0, 75/25, 50/50 and 25/75, respectively. To

prepare the geopolymer binder, a combination of sodium silicate (Na2SiO3) (29.4%

SiO2 and 14.7% Na2O by weight) and sodium hydroxide (NaOH) solution was used as

an activator solution. The concentration of NaOH was 8 M. A commercially available D

grade Na2SiO3 solution was supplied by PQ Australia. The NaOH pellet was used to

prepare the NaOH solution with 8 M concentration. The ratio of the Na2SiO3 to NaOH

was 2.5, and the alkaline liquid to binder ratio was 0.35 throughout the study. The

geopolymer binder/sand ratio used in this study was 1:2. The preparation method of

geopolymer mortar is as follows. Initially, the Na2SiO3 and NaOH solutions were mixed

and kept for 24 hr. During the preparation of geopolymer mortar, the dry materials were

mixed in a small concrete mixer (Hobart mixer) until the materials had been mixed well

and then the activator solution was added into the dry mix. The mixer was kept

continuously mix for 4 minutes to get a uniform mortar mix. Fig. 5-1 displays the Hobart

mixer, which is used to prepare the mortar mix. Mortar specimens were prepared in

50 mm cubic moulds. The geopolymer mortar specimens were removed from the

moulds after 24 hrs of the casting. Thereafter, all the specimens were included in curing

procedures. It should be noted here that the geopolymer mixes M2, M3 and M4 were

cured at 23°C ambient temperature due to the slag inclusion in the mixture. However,

the M1 type geopolymer mix was prepared with 100% of fly ash binder, and therefore,

those samples were cured at 60°C for 24 hr and then kept in an ambient temperature

until the investigation started.

To compare the performance of geopolymer specimens, control samples (CT) also

prepared with OPC binder. The general-purpose cement was used in the CT specimen

preparation. Water to binder ratio used in the CT specimens was 0.35, and the cement to

sand ratio was 1:2, which is similar to binder/sand ratio used in geopolymer mixes. As

similar to geopolymer mix preparation, first, dry materials were added in Hobart mixer

and mixed for 4 minutes. Then the water was added to dry materials, and the mixing

was continued for 3 minutes. CT specimens were also cast in 50 mm cubic moulds, and

the specimens were removed from the moulds after 24 hr of casting period. Then all

specimens were cured in the water tank at 23°C temperature for 28 days. Wetting-drying

cyclic resistance tests were started after 28 days of curing period.


143

Fig. 5-1 Mortar mixer (Hobart mixer)

5.2.2 Testing methods

5.2.2.1 Leaching test

Leaching of alkali metals from the geopolymer and CT samples were determined by

continuous immersion of the samples in the deionised water. All the samples were

immersed in the deionised water in the separate container, and the pH value of the

solutions was measured for 7 days by using pH electrode. The Aqua pH meter was used

for the pH measurements. After 7 days of immersion, the water solutions were collected

from each container, and the leaching ions from the samples were measured. Leaching

ions in the solutions were determined by using Inductively Coupled Plasma (ICP)

analysis. ICP test was carried out by using Inductively Coupled Plasma (ICP)

spectrometer (Fig. 5-2).

Fig. 5-2 ICP Spectrometer


144

5.2.2.2 Accelerated wetting-drying cyclic resistance test

Accelerated wetting-drying cyclic resistance test was conducted in three different types

of solutions such as water, chloride solution (3%) and the combination of chloride (1.5%)

and sulphate (1.5%) solutions. All the samples were immersed in the water or solutions

for 24 hrs, and then the samples were allowed to dry for following 24 hrs at 23° C

temperature. This process was continued for a 6-month period. The water or solutions

were replaced in every after 15 cycles. The chloride solution was prepared with NaCl in

water, and the MgSO4 was used to produce a sulphate solution. The changes in

compressive strength, weight and the visual appearance of the samples in all three

exposed conditions were evaluated in every one-month interval for a 6-month period.

The compressive strength of the mortar samples was determined by using the Techno

test automatic compression testing machine (Techno test C030/2T) according to ASTM

C109 standard. The test machine was shown in Fig. 5-3. The compressive strength values

of the samples were measured after 28 days of curing period and then every month

interval after immersed in the solutions.

Fig. 5-3 Compressive strength testing equipment

The changes of compressive strength after subjected to accelerated wetting-drying cyclic

resistance test was determined by using the following formula:

𝑠 =

𝑆𝑛 − 𝑆𝑜

𝑆𝑜 × 100%

(10)


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Where, s= changes of compressive strength values after subjected to every wetting-

drying cyclic resistance, Sn= Compressive strength values of the specimens every month

interval after the wetting-drying cyclic test, So = 28 days compressive strength of the

specimens before the wetting-drying cyclic test.

Moreover, the changes of the weight of the samples were taken in every month intervals.

The total weight changes were calculated after every one-month time interval by using

the following formula:

𝑤 =

𝑊𝑛 − 𝑊𝑜

𝑊𝑜 × 100%

(11)

Where, the w= percentage of total weight changes, the Wn= weight of specimen at the

end of immersion of ‘n’ cycle, Wo= Saturated weight of specimen before the accelerated

test. During the weight measurement, specimens were taken out from the immersed

solutions and wiped the surface before measuring the weight of the specimens. The

weight measurements were measured by using an electronic scale with the accuracy of

0.01 g.

5.3 Results and discussions

5.3.1 Alkali leaching test

5.3.1.1 pH measurements of the alkali leaching solution

Fig. 5-4 illustrates the pH value variation of the solution after immersed the mortar

samples. As can be seen from Fig. 5-4, pH values of all types of the samples were

increased with the time of exposure. This indicates the alkali metals from the mortar

samples were leached out, and due to this leaching effect, pH value of the solution was

increased with exposed time. It should be noted that the pH of the solution with CT

mortar was greater than the solutions with geopolymer samples. The next day after

immersion, pH of the solution with OPC mortar sample increased to 10.8, and the pH of

the solution was continuously increased at a slow rate with the exposure period. After 7

days of the exposure period, pH of the solution was reached to a value of 12 and

maintained at that value. Moreover, higher slag based geopolymer displayed higher pH

values compared to other types of geopolymer specimens. The first day after the

immersion, pH of the solution with M4 type geopolymer was 10.5, while the increment


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of pH is lower than the increase in OPC concrete, which is then reached to stable

condition with the value of 11.5 after 8 days of exposure.

On the other hand, pH value of the solutions with other geopolymer mixes such as M1,

M2 and M3 types are lower, which is in the range of 8.5-9.5 after immersed one day and

the final pH of the solution were in the range of 10.5-11.5. Therefore, this indicated that

the pH value of the leaching ions from fly ash based geopolymer are low. The pH value

of the leaching ions increases when slag constituent included in the fly ash-based binder.

Fig. 5-4 pH of the solutions after continues immersion of the mortar samples

Fig. 5-5 illustrates the visual observations of the specimens after immersed in the

deionised water for a one-week period. Fig. 5-6 depicts the conditions of deionised water

after the immersion of the samples for a one-week period. As shown in Fig. 5-5 and Fig.

5-6, higher leaching effect has been identified in high slag based geopolymer (M4 type).

Compared to other geopolymer mixes, high amount of deposition and high leaching

effect was observed for M4 type mix. Compared to geopolymer samples, no depositions

were observed in the CT samples, while the immersed solution indicated the leaching of

alkali metals from the CT samples. The deposition is due to the efflorescence effect in

concrete. In OPC concrete, efflorescence occurs due to the reaction between the soluble

calcium in concrete and the water and CO2 near the surface of concrete [216, 217]. In

geopolymer concrete, efflorescence is mainly caused by the reaction between the

presence of residual soluble alkalis in geopolymer surface and the CO2 from the

atmosphere. For the efflorescence reaction, suitable humidity and water media are

important at the surface of the geopolymer concrete. Efflorescence is also caused due to


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the presence of excess alkalis or insufficient geopolymerisation reaction in geopolymer

[99]. Moreover, every traces of alkali ( NaOH and KOH) in geopolymer, increases the

pH of the leaching solution [218]. In addition to that, the curing type is also influenced

on the efflorescence and leaching rate of the geopolymer concrete. It was mentioned that

the geopolymer concrete cured at elevated temperature showed lower leaching and less

efflorescence compared to the geopolymer concrete cured at ambient temperature [97].

Therefore, in this study, M1 type geopolymer mix cured at 60° C temperature and other

types of geopolymer samples (M2, M3 and M4) mixes were cured at ambient temperature.

Therefore, 100%FA type geopolymer mix showed lower leaching and less efflorescence

behaviour compared to the ambient temperature cured geopolymer concrete mixes. This

is because the reaction rate of fly ash materials is increased at the elevated temperature,

a denser microstructure is formed. This reduced the excess alkalis in the geopolymer

system. Therefore, the pH of the leaching solution is lower than the other concrete mixes.

Fig. 5-5 visual observation of mortar sample after 1 week of immersion, (a) M1, (b) M2,

(c) M3, (d) M4, and (e) CT


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Fig. 5-6 Visual observation of solutions after 1 week of immersion, (a) M1, (b) M2, (c) M3,

(d) M4, and (e) CT

5.3.1.2 Leaching ions measurements

Table 5-1 illustrates the leaching ions from the geopolymer and OPC mortar specimens

when it is immersed in the deionised solution. Leaching of Si, Al, K, Na and Ca elements

were identified from the mortar specimens. As shown in Table 5-1, leaching of K and Na

ions were identified from all types of geopolymer specimens. The Ca ion also was

identified as leached from M2, M3 and M4 type specimens, whereas Ca ions was not

significantly leached out from M1 type specimens. Furthermore, a higher amount of Ca

is leached out from OPC specimens compared to geopolymer specimens. Moreover,

Table 5-1 displayed the leachate of Na and K from M2 type is very much higher than the

leachate of other ions and the leaching rate of Na is reduced with the substitution of slag

in the binder. M3 and M4 type geopolymer mixes showed a lower amount of leachate of

Na and K elements. However, 100 % fly ash based geopolymer mix (M1) displayed lower

leachate of Na compared to M2 type mix. This is due to the different curing method used

in between that two types. As explained earlier, elevated temperature curing reduces the

leachate of Na in M1 type, whereas ambient cured M2 type geopolymer (75% of fly ash)

shows higher leaching effect. This is correlated with the previous test results [219].


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Table 5-1 Leaching ions from the mortar specimens in de-ionised solution

Specimens Si (ppm) Al (ppm) K (ppm) Na (ppm) Ca (ppm)

M1 13.4 7.4 23.13 110.52 10.41

M2 24.23 9.4 32.71 145.71 0.12

M3 17.7 3.76 22.1 94.43 20.43

M4 16.5 3.4 19.5 85.8 25.35

CT 32.8 0.3 4.5 7.4 45.13

5.3.2 Concrete Resistance in Wetting-drying Cycles in water

5.3.2.1 Visual observation

Fig. 5-7 illustrates the visual observation of the mortar specimens after subjected to a

wetting-drying cycle test in water after 6 months of the exposure period. As shown that,

the different types of geopolymer specimens were undergone different types of surface

degradation. Compared to other types of geopolymer specimens, M1 type specimens

were more deteriorated after 6 months of the testing period. The second higher

deterioration effect was observed on the surface of M2 type specimens. As shown in the

figure, many numbers of pores were created on the surface of the M2 type specimens.

Compared to M1 and M2 specimens, M3 and M4 type specimens are depicted less

deterioration as only a few pores were identified after the 6 months of the test period.

This indicated that the geopolymer binder prepared with fly ash materials is more

susceptible to deterioration in the wetting-drying cycles exposed in a water

environment. The geopolymer specimens prepared with more slag content is more

stable when it is subject to cyclic contact with water. Moreover, the CT specimens also

showed a few pores, and this indicated the degradation in OPC specimens were less

compared to geopolymer specimens.


150

Fig. 5-7 Visual observations of the mortar specimens after subjected to wetting-drying

cycles in water for 6 months period, (a) M1, (b)M2, (c)M3, (d) M4, and (e) CT

5.3.2.2 Changes in weight

Fig. 5-8 illustrates the weight changes of the mortar specimens after subjected to wetting-

drying cycles in water for 6 months period. According to that, the weight changes is very

small for all types of the specimens after 6 months of exposure. Although the weight of

the M1 and M2 type geopolymer specimens are increased initially, the weight of the

specimens is decreased after the 4 months of exposure time. However, other geopolymer

type mixes such as M3 and M4 and CT mixes are subjected to continuous weight gain

during the 6 months of experiment period. The initial weight gain during the wetting-

drying cycles would be due to the penetration of water into the specimens. When the

specimens are continuously exposed to water in wet and dry cycle conditions, the alkali

species can be leached out from the pore structures of the concrete, and this creates the

space for the penetration of water into the specimens. Moreover, M1 and M2 type

specimens showed a weight loss after some period, and this indicates the weight loss is

due to the degradation of the specimen surfaces after wetting-drying cyclic exposure.

According to the Fig. 5-7, the mortar particles were lost from the surface of the M1 and

M2 specimens, and this causes the weight of the specimens reduced after 4-month of

exposure time. Moreover, the continued weight gain of the other specimens would be


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due to the formation of the voids and pores on the surface of the specimens. As shown

in Fig. 5-7, M3, M4 and CT specimens are exhibited a higher number of pores and void,

and this creates a path for more water penetration into the concrete surface.

Fig. 5-8 The weight changes of the mortar specimens after subjected to wetting-drying

cycles in water for 6 months period.

5.3.2.3 Changes in compressive strength

Fig. 5-9 illustrates the compressive strength values of the mortar specimens after 28 days

of curing period. As shown that the strength values of the geopolymer specimens were

increased with the incorporation of slag into fly ash mixture. According to the figure, 28

days compressive strength values of M2, M3 and M4 mixes are 38.6 MPa, 49.52 MPa and

52.78 MPa, respectively. This is because the incorporation of slag produces additional C-

S-H gel phase with alumina-silica geopolymerisation network reaction due to the

presence of Ca in the slag materials. Therefore, the strength of the geopolymer is

enhanced with the formation of such two geopolymer phases. It should be noted that

the geopolymer prepared with 100% of fly ash material (M1 type) produces higher

compressive strength compared to the geopolymer mix prepared with 75% fly ash and

25% of slag combination (M2 type). This is because the two different curing methods

have been used in between those two types of mixes. The M1 type geopolymer mix was

cured at 60 °C elevated temperature, whereas M2 type geopolymer mix was cured at

ambient temperature. It is worth to mention here that the heat curing method is

favourable for fly ash based geopolymer mix. The reaction is accelerated at high


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temperature, and due to that, the rate of reaction will be increased. Therefore, heat cured

fly ash based geopolymer produces higher compressive strength compared to ambient

cured geopolymer mix with high fly ash content.

Moreover, the 28 days compressive strength of M4 type mix and CT specimens can be

comparable. This indicates geopolymer binder depicts similar or superior strength

properties when the slag content is high in the geopolymer mix compositions.

Fig. 5-9 28 days Compressive strength of the mortar specimens

Fig. 5-10 shows the percentage of strength loss of mortar specimens after subjected to

wetting and drying resistance test for a 6-month period. As shown that all the specimens

except the M1 type displayed an initial increment in the compressive strength and then

followed by a continuous deduction in the strength values. The increase of the strength

is due to the hydration of calcium silicates and the pozzolanic reactions, which result in

internal confinement and the strength increment in the specimens [87]. Therefore, the Ca

components is available in M2, M3 and M4 types of geopolymer specimens due to the

slag inclusion, whereas 100% fly ash binders produced M1 type geopolymer mix.

Therefore, the strength increment was observed all other geopolymers mix except M1

type. Moreover, CT samples are prepared with OPC binder, and therefore, hydration

reaction is usual in OPC specimens when it contacts with water. Therefore, this

attributed the strength increment in the first few months of exposures.


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However, it should be noted that the compressive strength values of all the specimens

were started to reduce after some period of exposure. As can be seen in Fig. 5-10, the rate

of strength loss is high when the geopolymer specimens contain fly ash binders, and the

strength loss is reduced with the incorporation of slag into the geopolymer mix.

Moreover, CT specimens exhibited lower strength loss rate compared to all types of

geopolymer specimens. This indicates OPC concrete is more durable compared to

geopolymer concrete when they have partially exposed to water.

Fig. 5-10 Compressive strength loss of the mortar specimens after subjected to wetting-

drying cycles in water

5.3.3 Concrete Resistance in Wetting-drying Cycles in chloride solution


Fig. 5-11 depicts the visual conditions of the mortar specimens after subjected to wetting-

drying cycles in 3% of NaCl solutions for 6 months period. As shown that the more

deterioration was identified in M1 type geopolymer specimens compared to all other

types of the mix. The formation of cracks was observed on the surface of the M1 samples,

and some mortar particles were started to remove from the surface of the specimens.

This indicated that the degradation of fly ash-based binder in a chloride environment is

higher than the geopolymer binder produced with slag materials. In other types of

geopolymer specimens (M2, M3 and M4), some small pores and holes were identified

after the exposures. Moreover, it should be noted that there is no visual sign of


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degradation was observed on the surface of CT specimens after 6 months of exposure.

This indicates the degradation of OPC binder is less than the geopolymer binder under

chloride exposed solutions.

Fig. 5-11 Visual observations of the mortar specimens subjected to wetting-drying cycles

in 3% of NaCl solutions for 6 months period, (a) M1, (b) M2, (c) M3, (d) M4, and (e) CT


Fig. 5-12 depicts the weight changes of the mortar specimens after subjected to wetting-

drying cycles in NaCl solution for 6 months period. As shown that the weight of all

specimens is increased with the exposure time. The weight increment would be due to

the intrusion of chloride ions into the mortar specimens. The intrusion of chloride ions

also created the internal micro-cracks, and this causes more chloride ions penetrates into

the specimens. Therefore, the weight of the specimens increased with the time of

exposure. Moreover, M1 type geopolymer specimens displayed more weight gain and

this indicates the penetration of Cl ions to the fly ash based geopolymer specimens is

higher than the penetration to the geopolymer specimens prepared with fly ash- slag

blended geopolymer specimens. In addition, M4 type geopolymer and CT specimens are

showed similar weight changes when subjected to wetting-drying cycles in NaCl


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solution. The visual observation also correlated with the weight change results. As

shown in Fig. 5-11, more surface deterioration was identified on the surface of the M1

type specimens compared to other types of the mortar specimens. It should be noted that

the weight of the M1 type geopolymer mortar specimens was decreased after subjected

to wetting-drying cycles in water due to the surface deterioration. However, here, even

though the surface of the M1 type specimens was deteriorated, the weight of the

specimens was continuously increased with the time. This is due to the higher amount

of chloride ion penetration to the concrete samples.

Fig. 5-12 The weight changes of mortar specimens after subjected to wetting-drying

cycles in 3% of NaCl solutions for 6 months period.


Fig. 5-13 displays the loss of compressive strength values of geopolymer and control

specimens after subjected to wetting-drying cycles in chloride solutions. As we can see

in Fig. 5-13, the compressive strength of geopolymer specimens were decreased with the

exposed period. However, deduction rate is not same between the different types mixes.

The geopolymer mix prepared with 100% fly ash materials exhibits a higher loss in

compressive strength values compared to other types of mixes. On the other hand, CT

specimens was initially showed a strength increment and then the strength values are

started to decrease with the time of exposoure. The strength increment in CT specimesn

is due to the hydration reaction in contact with water.


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Fig. 5-13 Compressive strength loss of the mortar specimens after subjected to wetting-

drying cycles in 3% of NaCl solutions for 6 months period.

5.3.4 Concrete Resistance in wetting-drying Cycles in a chloride+ sulphate

solution


Fig. 5-14 shows the visual observation of the mortar specimens after subjected to

wetting-drying cycles in 1.5% NaCl+1.5% of MgSO4 solutions after 6 months of the

exposure period. As shown in Fig. 5-14, the white colour deposition was observed on

the surface of all specimens. It should be noted that the geopolymer specimens displayed

the higher amount of depositions compared to the deposition on the control specimens.

Moreover, in between the geopolymer mixes, a higher amount of deposition was

observed on the geopolymer specimens prepared with a higher proportion of fly ash.

The development of this white layer on the geopolymer specimen would be due to the

formation of sodium carbonate (Na2CO3) components. Similar results were observed in

previous studies as well [87, 220]. Here, the specimens were taken out, wiped with

clothes, and then it was freely allowed to dry in the room condition. After that only, this

white layer was formed on the surface of the specimens. It should be noted here that,

Bakharev [131] observed the migration of alkalis from geopolymer samples at the

sulphate exposed environment. Therefore, the formation of the Na2CO3 is due to the

reaction between the alkalis (Na+, K+) and the carbon dioxide (CO2) from the

atmospheric environment. Furthermore, this study confirms the higher leaching effect


157

was observed in fly ash based geopolymer higher white deposition on the M1 sample.

Therefore, this indicated that under aggressive conditions more alkalis are leached out

from fly ash based geopolymer compared to slag based geopolymer.

Fig. 5-14 Visual observations of the mortar specimens subjected to wetting-drying cycles

in 1.5% NaCl+1.5% of MgSO4 solutions for 6 months period, (a) M1, (b) M2, (c) M3, (d)

M4, and (e) CT.


Fig. 5-15 illustrates the weight changes of the mortar specimens after subjected to

wetting-drying cycles in chloride and sulphate solution for 6 months period. As shown

that the weight of all specimens is increased with the exposure time, which is similar to

the weight changes after exposure to wetting-drying cycles in chloride solution. The

weight increment under the exposure of chloride and sulphate solution would be due to

two possible reasons. The first reason would be due to the intrusion of the chemical

particles, and the weight of the mortar specimens is increased by the weight of the

chemical particles. The second reason would be the expansion of the mortar specimens

when it is exposed to sulphate solutions. The intrusion of sulphate ion into the concrete

attributes the expansion and causes internal micro-cracks will be formed in the concrete.

Due to these cracks, the intrusions of chemical particles will be increased, and therefore,

the weight of the specimens was increased with the time of exposure.


158

Moreover, M1 type geopolymer specimen showed a rapid weight increment compared

to other types of samples. This indicates the formation of the cracks in the M1 type would

be higher than other types of geopolymer and OPC specimens. Therefore, this revealed

that the deterioration of fly ash based geopolymer specimens is higher than slag-fly ash

based geopolymer and OPC specimens when it is exposed to the environment with

chloride and sulphate contents.

Fig. 5-15 The changes of the weight of the mortar specimens after exposed to 1.5%

NaCl+1.5% of MgSO4 solutions.


Fig. 5-16 displays the loss of compressive strength values of geopolymer and control

specimens after subjected to wetting-drying cycles in 1.5% NaCl+1.5% of MgSO4

solutions. As we can see in Fig. 5-16, the compressive strength of all types of specimens

were decreased with the exposure period. However, deduction rate is not the same

between the different types of mixes. The geopolymer mix prepared with 100% fly ash

materials exhibits a higher loss in compressive strength values compared to other types

of mixes. The percentage of strength losses of M1, M2, M3, and M4 are 27%, 20%, 14% and

13%, respectively.

On the other hand, the strength loss in CT samples was only 9% after 6 months of

exposure. This indicates the geopolymer binders showed higher degradation under the

chloride and sulphate solutions media compared to OPC binder. Moreover, this study


159

also shows, 100% fly ash based geopolymer binder is more vulnerable to chloride and

sulphate attack and the strength reduction is decreased with the increment of slag in the

geopolymer binder. This contradicts with the test results obtained in a previous research

study. It was mentioned previously that the heat cured low Ca fly ash based geopolymer

mortar consists of higher sulphate resistance under MgSO4 and NaCl media [134].

However, according to this study, higher strength degradation was obtained for heat

cured fly ash based geopolymer binder. Moreover, the visual observations of the

specimens after 6-month period also confirmed that the more alkalis leached from 100%

fly ash based geopolymer compared to another type of geopolymer mix. More leaching

of alkali ions can react with the Mg2+, Cl- and SO42- ions and this reduces the higher

amount of strength compared to other geopolymer mixes. Therefore, this study

indicated that the curing method is not affected the degradation and strength loss in the

combination of chloride and sulphate environment. The rate of deterioration and the

strength loss depends on the type of source materials used in geopolymer preparation.

Moreover, it should be noted that the strength loss under the 1.5% NaCl+1.5% MgSO4

solution was higher than the strength loss observed in chloride solutions and water. This

is due to the sulphate attack on the specimens under 1.5% NaCl+1.5% of MgSO4.

Fig. 5-16 Compressive strength loss of the mortar specimens after exposed to 1.5%

NaCl+1.5% of MgSO4 solutions.


160


This chapter investigated the alkali leaching and wetting-drying cyclic resistances of

geopolymer in different exposed condition. To determine the influence of the source

materials on the leaching properties and wetting-drying cyclic resistances, geopolymer

mortar samples were prepared with different proportion of fly ash and slag constituents.

The leaching of alkali elements from the geopolymer specimens was determined by

immersed in deionised water. Accelerated wetting-drying cyclic resistance test was

conducted on the samples exposed to three different types of media such as water,

chloride solution and the combination chloride and sulphate solution. The degradation

of the mortar specimens was evaluated with the measurements of compressive strength

loss and the weight changes after the exposed conditions with the time intervals.

According to the investigation results, the following conclusions can be drawn:

The pH measurements of the leaching solutions indicated that the pH value

leaching ions from fly ash based geopolymer are low and the pH value increases

when slag constituent included in the fly ash-based binder.

The leaching test confirmed that the curing temperature is influenced on the

leaching rate from fly ash based geopolymer binder in the deionised water.

According to the test results, ambient cured geopolymer with high fly ash

content exhibits higher leachate of Na compared to higher temperature cured fly

ash based geopolymer.

The degradation of geopolymer specimens is higher than OPC specimens when

subjected to accelerated wetting and drying resistance test under the water,

chloride and the combination of chloride and sulphate environment.

The resistance to accelerated wetting and drying environment of geopolymer

specimens is increased with the increment of slag content in the geopolymer mix.

Geopolymer specimens displayed more strength loss and the weight changes

than OPC specimens after subjected to the accelerated environment.

The degradation of the specimens in the combination of chloride and sulphate

environment is severe than the effect on chloride and water environments. In

addition, the chloride environment attributes more deterioration compared to

the mortar specimens exposed to water.

Chapter 6 Accelerated carbonation and

corrosion test on geopolymer concrete

6.1 Introduction

This chapter presents the investigation of accelerated carbonation and corrosion tests on

geopolymer concrete. According to the field exposed geopolymer concrete samples,

corrosion of reinforcement in geopolymer concrete was not completely studied. The

reinforcement bars have been identified from some of the structures and not recognised

in some structures due to the higher depth level of the bar in the structures. Therefore,

this chapter investigates the corrosion of reinforcement bar in the geopolymer concrete

prepared with different of mix compositions. The effect of source materials on the

carbonation and corrosion behaviour of geopolymer concrete has been evaluated in this

chapter. The tests were conducted on the geopolymer concrete specimens prepared with

different proportion of fly ash, ground granulated blast furnace slag (GGBFS) materials

and the test results were compared with Ordinary Portland cement (OPC) concrete in

1% of the CO2 environment.

Moreover, this chapter also depicts the carbonation depth measurements by a new type

of indicator; i.e. universal solution. Carbonation of geopolymer concrete was determined

after 6 months of exposure.

6.2 Materials and methods

6.2.1 Materials

Three different types of geopolymer concrete mixes were prepared by changing the

proportion of fly ash and slag in the binder content. To prepare the geopolymer binder,

class F fly ash and GGBFS are used as source materials with different proportions. The

proportion of fly ash and GGBFS in the mix of 100FA, 75F/25S and 50F/50S are 100:0,

75:25 and 50:50, respectively. A combination of sodium silicate (Na2SiO3) solutions and

sodium hydroxide (NaOH) solutions are used as activators to activate the geopolymer

Chapter 6 Accelerated carbonation and corrosion test on geopolymer concrete

162

materials. The NaOH solution (concentration of 8M) was prepared by dissolving NaOH

flakes (98% of purity) in water. The mix compositions details of geopolymer and OPC

concrete are provided in Table 6-1.

Table 6-1 Mix composition details of geopolymer and OPC concrete mixes

Materials(kg/m3) 100FA 75F/25S 50F/50S CT

Total binder 400 400 400 400

Fly ash 400 300 200 -

GGBFS - 100 200 -

Cement - - - 400

Fine aggregate 630 630 630 630

Coarse aggregate 1150 1150 1150 1150

NaOH solution 41 (8M) 41 (8M) 41 (8M)

Na2SiO3 solution 102 102 102

Water 22.5 22.5 22.5 Water/cement

ratio=0.5

Extra water - - 20 -

Superplasticiser (SP) 6 6 6 -

The alkaline solutions are mixed one day before the concrete casting. During the

preparation of concrete, the dry components such as coarse aggregate, sand and

geopolymer precursors (fly ash or GGBFS or combination of fly ash and GGBFS) were

placed into the concrete mixer, and the mixing was continued for 3 minutes. Thereafter,

the activator solution and the water components were added to the dry mixture and

continuously mixed for another 4 minutes. To improve the workability of geopolymer,

high range water reducing (naphthalene sulphonate- based) super plasticiser was used

in all types of geopolymer mixes [33]. The amount of superplasticiser added in the mix

was 1.5% of the total binder content. Fig. 6-1 shows the process methods for the concrete

preparation. The mixing of dry components and the fresh concrete after the mixing

process are shown in Fig. 6-1 (a) and Fig. 6-1 (b), respectively. Fig. 6-1 (c) shows the prism

type moulds with reinforcement, which are used to prepare the concrete specimens for

the accelerated corrosion test. For accelerated corrosion test, 75 mm×75 mm×400 mm

size of concrete prism specimens were prepared with 12 mm diameter of embedded


163

reinforcement bar. The cover size provided for the reinforcement bars was 15 mm. For

the accelerated carbonation test, 100 mm diameter ×200 mm height of cylindrical

specimens were prepared. Fig. 6-1(d) shows the cast specimens for both accelerated

carbonation and corrosion tests.

OPC concrete (CT) also prepared with the same method as geopolymer concrete

production. Initially, the dry mix components such as coarse aggregate, sand and cement

were placed into the concrete mixer and mixed for 3 minutes, and then the wet mix was

continued for 4 minutes after adding the water into the dry mix. The water-cement ratio

used in the OPC concrete production is 0.5.

Fig. 6-1 Concrete preparation process, (a) mixing of dry components in a concrete mixer,

(b) concrete after mixing with water, (c) mould for accelerated corrosion test, (d) concrete

specimen after casting.

The concrete specimens were removed from the moulds after 24 hrs of casting period.

Thereafter, the samples were subjected to the curing process for the next 28 days. The

100FA type geopolymer mix sample was cured at 60°C for 24 hrs and then kept at

ambient temperature. Other types of geopolymer mixes such as 75F/25S and 50F/50S

are subjected to curing at ambient temperature. On the other hand, OPC concrete


164

specimens were cured in water curing methods. The samples were kept in the water

curing tank with 23 °C control temperature for 28 days.


6.2.2.1 Carbonation and corrosion testing procedure

The accelerated carbonation and corrosion test were conducted after the 28 days of

curing period. For the carbonation assessment, cylindrical specimens were cut into 4

pieces with a 50 mm thickness of each section. The cutting process of the specimen is

provided in Fig. 6-2. Thereafter, the epoxy coating was applied on the peripheral surface

of each specimen to avoid the penetration of CO2 through the peripheral surface.

Therefore, this ensures the CO2 penetration is through the circular surfaces only.

Fig. 6-2 Cutting of concrete specimens by using a table saw

The exposure condition of the concrete influence on the rate of carbonation. In general,

the CO2 diffusion is high when the concrete specimens are contacted with water

compared the concrete specimens exposed in a dry environment. Moreover, the rat of

carbonation is high when the concrete specimens are exposed in marine/saline

environment compared to normal atmospheric environment. Therefore, the accelerated

carbonation and the corrosion assessment were carried out with the following three

different methods of exposure condition:


165

One set of specimens were continuously exposed to 1% CO2 environment.

Other sets of specimens were subjected to cyclic exposed wet and dry conditions;

04 days exposed in 1% CO2 environment and the next 03 days exposed in the

water media.

The next sets of specimens were subjected to wet and dry cyclic exposure in the

CO2 environment and chloride water solutions. The concrete specimens were

exposed to 1% CO2 environment for 04 days and then the next 03 days were kept

in the chloride solutions

The carbonation test was conducted in a controlled environment chamber. All

carbonation and corrosion test specimens were placed in an environmental chamber at

a temperature of 23°C, relative humidity of 65%, and a CO2 concentration of 1%. The

CO2 concentration was maintained at 1% throughout the testing period. Because, it was

found that the higher concentrations of CO2 create bicarbonate reaction products for

geopolymer binder under accelerated testing method. This attributes higher carbonation

depth compared to normal field exposed conditions [28]. Fig. 6-3 shows the carbonation

chamber, which is used for this investigation.

Chloride solution was prepared by dissolution of 3% of sodium chloride (NaCl) in water.

The test was conducted for a 6 month period. Carbonation depth values of the concrete

specimens and the corrosion of reinforcement bar in the concrete specimens were

evaluated after 6 month exposed period.


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Fig. 6-3 Environment chamber for Carbonation test

6.2.2.2 Carbonation depth measurements

Carbonation depth of the concrete specimens was determined by using two different

types of indicator solutions such as universal solution and phenolphthalein indicator.

Since phenolphthalein indicator was identified as a not suitable solution to determine

the carbonation depth of geopolymer concrete prepared with fly ash materials, the

universal solution was used to measure the carbonation depth of geopolymer concrete.

The universal solution provides the colour change with the pH variation between 1 to

14. Fig. 6-4 shows the colour chart of universal solution.

Both types of indicators were applied to the fresh concrete surface immediately after the

split the specimens, and the depth of carbonation was measured with the colour change

of the indicators by using the measuring tape.

Fig. 6-4 Colour chart of the universal solution with a pH value

Moreover, the carbonation depth of field exposed geopolymer concrete samples also

measured by universal solutions.

6.2.2.3 Evaluation of corrosion of reinforcement

Corrosion behaviour of the reinforcement bar was visually inspected after 06 months of

testing period. After 06 months of exposure, concrete specimens were split, and the

corrosion of the reinforcement bar is visually observed. The electric resistivity

measurements were also taken at every month interval. Electric Resistivity readings

were determined by Resipod fully integrated, four-point Wenner probe resistivity meter

(Fig. 6-5). The relationship between the electric resistance and the risk of corrosion for

OPC concrete at 20⁰C is given in Table 6-2 [221]. In addition, Table 6-3 shows the

relationship between the electric resistance and the rate of corrosion.


167

Fig. 6-5 Resipod resistivity meter

Table 6-2 The relationship between resistivity and risk of corrosion [221].

Table 6-3 The relationship between resistivity and corrosion rate

Resistivity (kΩcm) Corrosion rate

> 20 kΩ cm Low

10-20 kΩ cm Low to moderate

5-10 kΩ cm High

< 5 kΩ cm Very High

Before taking the reading, four probes of the Resipod were soaked into the water to

increase the conductivity. Then, the Resipod was pressed on the concrete surface to

Resistivity value(kΩcm) Risk of corrosion

≥ 100 kΩcm Negligible

50 to 100 kΩcm Low

10 to 50 kΩcm Moderate

≤ 10 kΩcm High


168

measure the resistivity readings. Three readings were measured from each specimen to

determine the average value.


6.3.1 Carbonation depth measurement by universal and phenolphthalein

indicators

Fig. 6-6 shows the concrete specimens after the application of universal solution on a

fresh stage of the concrete (before carbonation). As shown that the colour of the concrete

surface was varied in different types of the concrete according to the pH of the concrete

surface. The colour identified in 100 FA and 75 FA/25 S specimens are a type of green

colour, which is corresponding to the pH value in the range of 10.0-11.0. On the other

hand, the colour identified on the surface of the 50FA/50S concrete is closed to a violet

colour, and that is in the range of 12.0-14.0. This indicated that the mix compositions is

influenced on the pH of the geopolymer concrete. The geopolymer concrete prepared

with fly ash-based materials exhibits lower pH range, whereas pH of the geopolymer

concrete increases with the addition of slag materials into the fly ash based geopolymer

mix. Moreover, Fig. 6-6 also displayed that the OPC concrete surface was turned to violet

colour after the universal solution application. This confirmed that the pH range of fresh

OPC concrete was in the range of 12.0-14.0, which is the normal pH range of OPC

concrete.

Fig. 6-6 Application of universal solution on the fresh geopolymer and OPC concrete

surfaces (before carbonation)


169

Fig. 6-7 represents the carbonation depth measurements of concrete specimens after

exposed to 1% CO2 environment for 6 months of period. The carbonation depth was

measured by the application of phenolphthalein indicator and universal indicator

solution. For every specimen, the image on the left-hand side represents the concrete

surface after the phenolphthalein application, and the image on the right-hand side

indicates the concrete surface after the application of the universal solution. As shown

in Fig. 6-7, concrete surface of 100FA and 75FA/25S specimens was turned into a pink

colour and green colour after the application of phenolphthalein and universal solution,

respectively. According to the colour chart of the universal solution, the surface of both

types of concrete specimens is matched with the pH range of 9.0-10.0. It should be noted

that the type of green colour identified by the application of universal solutions on the

surface of 100FA and 75FA/25S specimens are different before and after the carbonation

(Fig. 6-6 and Fig. 6-7). The corresponding green colour before carbonation was in the

range of 10.0-11.0, whereas the green colour observed after the carbonation was matched

with the pH range of 9.0-10.0. This indicated that the pH of 100FA and 75FA/25S

concrete is reduced after the carbonation. However, the reduction is very small.

Therefore, the phenolphthalein solution was turned to a pink colour after the

carbonation. The colourless zone can be able to identify when the pH is reduced to less

than 9.0. Furthermore, the colour identified after the carbonation of both types of

geopolymer concrete was unique throughout the depth. This indicated that the

carbonation rate of 100FA and 75FA/25S is very fast and the CO2 is penetrated

completely throughout the specimen in a 6-month period.

On the other hand, carbonation depth values for 50FA/50S geopolymer mix was clearly

identified by universal solutions as well as the phenolphthalein indicator. Carbonation

depth value identified from the phenolphthalein indicator was 10 mm after 6 months of

exposure in 1% of the CO2 environment. Moreover, the universal solution application

also provides a similar pattern of carbonation. The colour observed in the carbonated

zone of 50FA/50S is clearly varied from the carbonated part colour of 100FA and

75FA/25S. The identified colour for the carbonated part of 50FA/50S concrete is

corresponding to the pH range 8.0-9.0, respectively. The pH value observed before

carbonation is in the range of 12.0-14.0. This indicated the reduction of pH after the

carbonation was greater than the reduction observed in 100FA and 75FA/25S type

mixes.


170

Moreover, OPC concrete revealed the carbonation depth was 6 mm after 6 months of

exposure in 1% of the CO2 environment. The phenolphthalein indicator showed 6 mm

colourless zone, and 6 mm greenish yellow colour zone was identified from the universal

solution application. The colour observed in the carbonated zone of OPC concrete from

the universal solution is corresponding to the pH range of 7.0-8.0. Therefore, this

indicated that the pH reduction due to the carbonation in OPC concrete was higher than

the geopolymer concrete. However, the carbonation depth of OPC concrete is lower than

the geopolymer concrete in the same exposed period.

Fig. 6-7 Carbonation depth measurements of the concrete samples after exposed to 1%

CO2 environment for 6 months period

Fig. 6-8 illustrates the carbonation depth measurements of the concrete specimens after

subjected to wet and dry cyclic exposure in 1% of CO2 + water environment for a 6-


171

month period. As similar to the normal CO2 environment, the concrete surface of 100FA

and 75FA/25S specimens was turned into pink and green colours after the application

of phenolphthalein and universal solution, respectively. This indicated that the pH

reduction in the cyclic exposure in CO2 + water environment for 100FA and 75FA/25S

types were in the range of 9.0-10.0. Furthermore, the CO2 is penetrated throughout the

samples, which is also similar to the normal 1 % CO2 exposure condition. Compared to

100FA and 75FA/25S specimen, 50FA/50S and OPC concrete are showed better

responses with phenolphthalein as well as the universal solution. Carbonation depth

was observed in the 50FA/50S, and OPC concrete was 16 mm and 8 mm, respectively.

These values are greater than the values observed in 1% of the normal CO2 environment.

It should be noted that the rate of CO2 diffusion depends on the internal moisture content

of the concrete. The moisture content of the concrete is varied under the wet and dry

cyclic environment, and due to that, the carbonation rate is greater than the normal

exposed environment.


172

Fig. 6-8 Carbonation depth measurements of the concrete samples after subjected to

wet and dry cyclic exposure in 1% of CO2 + water environment for a 6-month period

Fig. 6-9 displays the carbonation depth measurements of geopolymer and OPC concrete

specimens after subjected to wet and dry cyclic exposure in 1% of CO2 environment +

chloride solution for a 6-month period. As shown that the carbonation depth of all types

of concrete specimens higher than the carbonation depth observed in the other two

exposed conditions. The phenolphthalein application on the surface of 100FA and

75FA/25S specimens were turned to almost colourless, and the application of universal

solution was changed to light yellowish green colour (pH range of 8.0-9.0) for all over


173

the surface. This indicated that the carbonation effect on 100FA and 75FA/25S is severe

when it is subjected to wet and dry cyclic exposure in CO2 and chloride solution. The

application of phenolphthalein and universal solution indicated that the carbonation

depth obtained for 50FA/50S sample was 18 mm after exposed to 6 months in wet and

dry chloride and CO2 environment. In the same environment, OPC concrete displayed

the carbonation depth was 10 mm. This is revealed that the carbonation effect on

geopolymer concrete is higher than OPC concrete in any exposed environment.

Moreover, this investigation indicated that the geopolymer concrete prepared with more

slag content displayed higher carbonation resistance compared to fly ash based

geopolymer.

It was determined from this investigation that the universal solution is a more suitable

indicator for geopolymer concrete to determine the carbonation depth values.

Especially, for the geopolymer concrete prepared with the fly ash based material, which

is not able to determine the carbonation by phenolphthalein indicator. Therefore, due to

the wide range of colour variation in universal solutions, the pH reduction in

geopolymer concrete can be easily identified. Therefore, universal solution is suitable to

determine the carbonation measurements of geopolymer binders.


174

Fig. 6-9 Carbonation depth measurements of the concrete samples after subjected to wet

and dry cyclic exposure in 1% of CO2 environment + chloride solution for a 6-month

period


175

6.3.2 Carbonation depth measurement of field-exposed fly ash based

geopolymer concrete samples by universal solution

The carbonation depth values of the core specimens were determined by universal

solution. Fig. 6-10 shows the carbonation depth of fly ash based geopolymer concrete

exposed in field condition. It should be noted that the phenolphthalein indicator was not

provided precise carbonation measurements for fly ash based geopolymer concrete

(FGPC-A) after exposed to the atmospheric environment. Therefore, a new indicator; the

universal solution was applied on the concrete surface after splitting the core specimens.

As shown in Fig. 6-10, the concrete surface of fly ash based geopolymer concrete samples

from the aggressive environment (FGPC-S) was turned into yellow colour, which

indicates the pH of the concrete surface was in the range of 6.0 -7.0. OPC concrete

samples from the same aggressive exposed condition (OPC-S) showed yellow colour for

15 -17 mm and green colour identified below that depth level (pH range between 8.0 -

9.0). Therefore, carbonation measurements by the universal indicator are similar to the

carbonation measurements by phenolphthalein solution. During to the phenolphthalein

indicator application, geopolymer concrete (FGPC-S) was turned to colourless

throughout the surface, and OPC concrete (OPC-S) displayed a maximum 20 mm

carbonation values.

On the other hand, fly ash based geopolymer concrete core specimens in the atmospheric

environment (FGPC-A) was changed to pink colour during the phenolphthalein

application (Chapter 3). According to Fig. 6-10, the application of universal solution also

displayed green colour throughout the surface. However, by the variation of colour

change of universal solution with pH value, it can be confirmed that the pH of

geopolymer concrete was in between 9.0-10.0. This is similar to the values obtained from

pH profile measurements (Chapter 3). Furthermore, OPC concrete specimens (OPC-A)

displayed a yellow and green colour combination (pH values in the range of 6.0-9.0) for

4 mm, and similar behaviour has been identified by phenolphthalein solution.

Therefore, according to this study, the carbonation depth of fly ash based geopolymer

concrete is measured by the universal solution. The reduction of pH in fly ash based

geopolymer concrete with the carbonation reaction is very small. The pH of the fly ash

based geopolymer concrete after carbonation is in the range of 9.0-10.0. Therefore, the

phenolphthalein solution is not workable to identify the carbonation reaction in fly ash


176

based geopolymer concrete. Therefore, determine the pH value is an appropriate

method to identify the carbonation of fly ash based geopolymer concrete. However, in

practical, it is difficult to determine the pH profiles in all the time. Therefore, this study

confirmed that the universal solution is a suitable way to determine the carbonation

depth and pH measurements of fly ash based geopolymer concrete.

Fig. 6-10 carbonation measurements of core specimens from aggressive and atmospheric

exposed environment.

6.3.3 Evaluation of corrosion of reinforcement

6.3.3.1 Visual observations of the specimens after 6 months of exposure

Fig. 6-11 depicts the visual conditions of the geopolymer and OPC concrete specimens

after exposed to 1% of the CO2 environment, wet and dry cyclic exposure in 1% of CO2

+ water environment and wet and dry cyclic exposure in 1% of CO2 + chloride solution.

As shown that the surface of the concrete specimens was not deteriorated after exposed

to the 1% of the CO2 environment (Fig. 6-11 (a)). The concrete surfaces remained similar

as before conducting the experimental analysis. Fig. 6-11 (b) shows the visual conditions

of the specimens after subjected to wet and dry cyclic exposed in CO2 environment and

water. According to that, the concrete surfaces were not changed after the 6 months of

the exposed condition, which indicated that the concrete specimens were not

deteriorated in CO2 and water exposure.


177

On the other hand, Fig. 6-11 (c) displays the concrete specimens after subjected to CO2

and chloride solution. As shown that the deposition of salt was recognized on the surface

of OPC concrete and 100FA concrete specimens. In addition, the formation of cracks also

identified on the surface of 100FA concrete specimens. Compared to 100FA, the surface

deterioration of 75F/25S and 50F/50S specimens were lower in CO2 + chloride exposed

conditions. This indicated that the deterioration of geopolymer concrete prepared with

100% fly ash materials is high in the aggressive condition, and the deterioration effect is

reduced by the incorporation of slag materials in the geopolymer mix.

Fig. 6-11 Visual observation of the concrete specimens after 6 months of exposure in (a)

1% of the CO2 environment, (b) wet and dry cyclic exposure in 1% of CO2 environment

+ water, (c) wet and dry cyclic exposure in 1% of CO2 environment + chloride solution


178

6.3.3.2 Electric resistivity measurements

The electrical resistance of concrete provides the information about the corrosion risk

and the rate of the corrosion in reinforced concrete structures. Fig. 6-12 represents the

electric resistivity measurements of geopolymer and OPC concrete before exposed to the

CO2 environment. As shown that the geopolymer concrete specimens displayed lower

electric resistance values compared to OPC concrete specimen. It should be noted that

the alkali contents, which presents in the pore solution of the concrete are mainly

influenced on electrical resistance values [222]. Resistivity values are decreased with

high alkali content in the pore solution. Geopolymer concrete is produced with a high

amount of alkali components as activators. Therefore compared to OPC concrete, lower

resistance values are obtained for geopolymer concrete due to the high alkali species

(NA+, K+) in the pore solution of concrete.

Moreover, Fig. 6-12 also showed that the resistivity values are increased with the slag

substitution in the fly ash based geopolymer mix. As shown that 50FA/50S type

geopolymer mix shows higher resistivity values compared to 75FA/25S and 100FA type

geopolymer mixes. It is worth to mention here that the electric resistance depends on the

porosity of the concrete surface. The electrical resistivity values are increased with less

pore structure. In slag based geopolymer concrete, the porosity and pore size are

reduced by filling of pores with fine slag particles and produced dense geopolymer

concrete structure. Therefore, the resistivity of the geopolymer concrete increased by

higher slag content in the binder. This is correlated with the carbonation test results.

Carbonation resistances of geopolymer concrete also increased with the substitution of

slag in the geopolymer mix. Furthermore, according to the information provided by

Polder [221], the risk of corrosion in geopolymer concrete structure with the electric

resistivity values in the range of 50 -100 kΩ.cm is in the low-risk region for corrosion.


179

Fig. 6-12 Electric resistivity measurements before exposed to the CO2 environment

Fig. 6-13 illustrates the electric resistance of the concrete specimens subjected to 1 % of

the CO2 environment. As shown that the electric resistance of the concrete specimens is

much not varied with the time of exposure. The electric resistance values are almost

similar to the values obtained before exposed to the CO2 environment. This indicates the

risk of corrosion is low when it is exposed to the normal CO2 environment (without

contact water or any aggressive solutions). Fig. 6-14 illustrates the electric resistance of

the concrete specimens after subjected to 1 % of CO2 + water environment. According to

that, the resistivity values of all types of concrete specimens are lower than the exposed

in the CO2 environment. This is because the moisture content of the concrete is

influenced on the electric resistivity values of the concrete specimens. When the concrete

specimens are subjected to 1 % of CO2 + water environment, the moisture level of the

concrete is increased, and due to that, the electric resistivity values of the concrete

specimens are decreased. Moreover, as shown in Fig. 6-14, the electric resistance of the

geopolymer concrete samples are highly decreased with the exposed period, whereas

OPC concrete specimen displayed not a significant variation with the exposed time.

Electric resistance also depends on the pore structure of the concrete surface and

dissolved the salt in the pore solution [223]. Carbonation of geopolymer increases the

porosity of concrete [90] and more alkali salt in pore solution. This is because the


180

carbonation reaction in GPC increases the porosity by the formation of soluble

carbonated products and that could also be attributed high alkaline salt in pore

solutions. Therefore, the lower electric resistance of geopolymer concrete was associated

with higher porous surface and presence of dissolved salt in the pore solution.

Furthermore, 100FA type geopolymer specimen displayed greater reduction in electric

resistivity values, and the rate of decrement is reduced with the increment of slag in the

geopolymer mix. This is due to the different carbonation components in different

geopolymer concrete types. In fly ash based geopolymer concrete, the formation of

soluble carbonation products removed in the water and attributed higher porosity,

which causes higher electric resistivity reduction observed on fly ash based geopolymer

concrete. On the other hand, the porosity increment is reduced in slag-fly ash blended

geopolymer concrete due to the formation of insoluble carbonation products with

soluble carbonation components. Therefore, the electric resistivity reduction is low

compared to fly ash based geopolymer concrete. This indicates, the risk of corrosion also

reduced when the incorporation of slag into the geopolymer binder.

Fig. 6-15 displays the electric resistance of the concrete specimens subjected to 1 % of

CO2 + chloride environment. As shown, that the electric resistance of all types of

specimens was lower than the test results obtained from the samples exposed to CO2

and water environment. As shown in Fig. 6-15, the electric resistivity of OPC samples

also reduced with the time of exposure, whereas there are no such reductions have been

identified when the specimens ware subjected to the CO2 environment and CO2 +water

environment. The electric resistivity reduction in CO2 +chloride environment is due to

the penetration of chloride ion into the pore structure of the concrete surface [224]. The

penetration of chloride ion increases the conductivity and reduces the electric resistance.

It can also be observed from the test results, that the resistivity values of geopolymer

concrete samples subjected to CO2 + chloride environment is lower than the specimens

exposed to CO2 + water environment. This indicates the porosity increment and the

penetration of chloride ions reduces the resistance values in CO2 +chloride environment.

This suggested that the corrosion risk is high when the concrete subjected to CO2

+chloride environment compared to CO2 + water environment. Furthermore, according

to the Fig. 6-15, electric resistivity values for geopolymer concrete is lower than OPC

concrete. This indicates the risk of corrosion in geopolymer concrete is higher than OPC

concrete. The corrosion on reinforcement bar after 6 months of exposure was observed,


181

and the discussion regarding the corrosion activity of the reinforcement bar is described

in the following section.

Fig. 6-13 Electric resistivity measurements of concrete specimens subjected to the CO2

environment.

Fig. 6-14 Electric resistivity measurements of concrete specimens subjected to CO2 +

water environment.


182

Fig. 6-15 Electric resistivity measurements of concrete specimens subjected to CO2 +

chloride environment.

6.3.3.3 Corrosion of reinforcement bar after a 6-month period

The concrete samples were split, and reinforcement bars are visually inspected to

determine the corrosion effect in a 6-month period. Fig. 6-16 represents the

reinforcement bars from the concrete samples exposed in 1% of CO2 environment for a

6-month period. As can be seen from Fig. 6-16, there is no sign of corrosion activity was

developed in all reinforcement bars. This indicated that the reinforcement bar in the

concrete specimens was protected against the corrosion activity. According to the

carbonation depth measurements, it was found that the 100FA and 75FA/25S concrete

specimens were completely carbonated (maximum 25 mm carbonation depth. Because

the CO2 is penetrated from two circular surface direction). The cover provided for the

reinforcement bar in the prism specimens was 15 mm. This indicated that the CO2 is

already reached to the level of reinforcement bar after 6 month of exposed period.

However, according to Fig. 6-16, the reinforcement bar was not corroded. This indicated

that the even though after the carbonation, reinforcement is protected against corrosion

in the normal 1% CO2 atmospheric environment.

Fig. 6-17 displays the reinforcement bar from the concrete samples exposed to CO2 and

water environments for 6 months. The reinforcement bar from 100FA and 75FA/25S mix

showed little sign of corrosion, whereas the reinforcement bar exposed in 50FA/50S

geopolymer mix and OPC concrete mix was not showed any corrosion development


183

after 6 months of exposure. In addition, the reinforcement bar from 100FA showed

higher corrosion compared to the reinforcement bar from 75FA/25S. This indicates the

risk of corrosion is reduced with the increment of slag content in the geopolymer mix.

This is due to the reduction of pore structure in slag based geopolymer. Increasing of

slag content in the geopolymer mix reduces the pore structures due to the formation of

C-A-S-H and C-S-H gel phases with N-A-S-H gel [51].

The reinforcement bar embedded in the concrete specimens from CO2 and chloride

water environment are shown in Fig. 6-18. As can be seen in Fig. 6-18, reinforcement bar

from all types of geopolymer concrete specimens displayed a higher level of corrosion,

whereas OPC concrete reinforcement bar only showed very little sign of corrosion after

subjected to wet and dry cyclic exposure to CO2 and chloride water environment. The

amount of corrosion products observed on the reinforcement bar from 100FA

geopolymer mix was greater than other types of geopolymer mixes, and the amount of

corrosion is reduced with the increase of the slag in the geopolymer mix. This indicates,

fly ash based geopolymer concrete is not suitable for the aggressive environment, and

the durability of geopolymer concrete can be enhanced by using a higher amount of slag

content in the geopolymer mix. However, compared to geopolymer concrete, OPC

concrete showed better durability performance in the aggressive environment.

This study revealed that the risk of corrosion in geopolymer concrete is greater than the

corrosion risk in OPC concrete and the risk becomes very severe when the geopolymer

concrete is exposed to the aggressive environment. Especially, the geopolymer concrete

prepared with 100% of fly ash binder consists of higher risk compared to the geopolymer

concrete prepared with slag binders. This indicates the geopolymer concrete prepared

with fly ash binder is more suitable for indoor construction application (without contact

any water or aggressive agents). Furthermore, this investigation validates the test results

obtained from the field exposed geopolymer concrete.


184

Fig. 6-16 Reinforcement bar from the concrete exposed in 1% of CO2 environment for 6

months

Fig. 6-17 Reinforcement bar from the concrete exposed in 1% of CO2+water environment

for 6 months

100FA

OPC

50FA/50S

75FA/25S

100FA

75FA/25S

50FA/50S

OPC


185

Fig. 6-18 Reinforcement bar from the concrete exposed in 1% of CO2+chloride water

environment for 6 months


This chapter illustrates the investigations of carbonation and corrosion behaviour of

geopolymer concrete when it is exposed to the accelerated testing environment. The

different types of geopolymer concrete specimens were prepared by changing the

proportion of fly ash and slag materials, and the test result of geopolymer concrete was

compared with OPC concrete. The accelerated carbonation and the corrosion assessment

were carried out with three different methods of exposure conditions such as

continuously exposed to 1% CO2 environment, cyclic exposed wet and dry conditions in

1% CO2 and water, and cyclically exposed wet and dry conditions in 1% CO2 and

chloride solution. The assessment was conducted for a 6-month period. Based on the test

results, the following conclusions can be extracted:

Carbonation of geopolymer concrete is higher than OPC concrete in all three

environmental conditions. The carbonation effect is reduced in geopolymer

when the slag content increased in the geopolymer mix.

Universal solution is identified as a suitable indicator for geopolymer concrete.

Especially, the carbonation depth of fly ash geopolymer concrete is difficult to

determine by phenolphthalein indicator due to the minor reduction in pH after

the carbonation. Therefore, due to the wide range of colour variation with the pH

100FA

75FA/25S

50FA/50S

OPC


186

values in the universal solutions method, that is more suitable to determine the

carbonation in geopolymer concrete.

The risk of corrosion in geopolymer concrete is greater than the corrosion risk in

OPC concrete, and the risk becomes very severe when the geopolymer concrete

is exposed to the aggressive environment. Especially, the geopolymer concrete

prepared with 100% of fly ash binder showed higher corrosion compared to the

geopolymer concrete prepared with slag binders. The level of corrosion is

reduced when the slag content increases in the geopolymer mix.

The electric resistivity measurements are correlated well with the corrosion of

reinforcement. However, in real field conditions, the moisture content of the

concrete is fluctuating in various locations of the concrete structure and different

time. Therefore, it is difficult to predict the corrosion risk in the real exposed

environment with electric resistance measurements.

Chapter 7 Mathematical models for carbonation

of geopolymer concrete

7.1 Introduction

This chapter presents the mathematical models developed with the diffusion equation

based on the Fick’s law and the empirical equations for geopolymer concrete.

Accelerated carbonation test was carried out on geopolymer concrete prepared with

different proportion of the source materials, and the test was conducted with 1% of the

CO2 environment. Based on the carbonation depth values, the mathematical models

were developed. The geopolymer concrete test results also compared with OPC

concrete.

7.2 Materials and methods

7.2.1 Materials

The mix compositions details are provided in Table 7-1. Geopolymer concrete specimens

were prepared with different proportions of fly ash and slag contents. OPC concrete also

prepared to compare the carbonation behaviour with geopolymer concrete mixes. A

combination of Bayswater type fly ash and ground granulated blast furnace slag

(GGBFS) are used as precursors to prepare geopolymer binders. The proportion of fly

ash to slag in the geopolymer mix types S1, S2 and S3 are 75:25, 50:50, and 25:75,

respectively. The mix OT is produced with the Portland cement binder. The geopolymer

mixes were activated by the combination of sodium hydroxide (NaOH), potassium

hydroxide (KOH) and sodium silicate (Na2SiO3) solutions. The concentration of

hydroxide activator was 7 M (50 mol % Na cations and 50 mol % K cations). A

commercially available Na2SiO3 solution with D grade (29.4% SiO2 and 14.7% Na2O by

weight) was purchased from PQ Australia. The water to binder ratio used to prepare

activator combination was 0.3.

Chapter 7 Mathematical models for carbonation of geopolymer concrete

188

The preparation of geopolymer and OPC concrete mixes are similar procedures as

explained in the previous chapter. For compressive strength test and carbonation test,

100 mm diameter ×200 mm height of cylindrical specimens was prepared. All

geopolymer concrete specimens were cured at ambient temperature (23 °C), whereas OT

specimens were cured at the water tank at 23 °C temperature.

Table 7-1 Mix compositions of concrete (kg/m3)

Materials(kg/m3) S1 S2 S3 OT

Total binder 400 400 400 400

Fly ash 300 200 100 -

GGBFS 100 200 300 -

Fine aggregate 630 630 630 630

Coarse aggregate 1150 1150 1150 1150

NaOH pellet 14 14 14 -

KOH pellet 19.6 19.6 19.6 -

Na2SiO3 solution 34.48 34.48 34.48 -

Water used to prepare activator

81.04 81.04 81.04 Water/cement ratio=0.65

Extra water 20 20 20 -


7.2.2.1 Compressive strength test

The compressive strength test was conducted with 100×200 mm size of cylindrical

specimens according to ASTM C39 standards by using Techno test compressive strength

testing machine (Techno test C030/2T) (Fig. 7-1). The accuracy of the machine was 0.1

kN. The compressive strength of the specimens was determined after 7 days and 28 days

from the casting period. The strength values were derived from an average of three

specimens from each mix.


189

Fig. 7-1 Compressive strength test

7.2.2.2 Accelerated carbonation test

For the carbonation test, cylindrical specimens were cut into four pieces with 50 mm

thickness, after 28 days of casting time. The cutting process of the specimens is provided

in the previous chapter. Thereafter, all the concrete specimens were placed in an

environmental chamber at a temperature of 23°C, relative humidity of 65%, and a CO2

concentration of 1%. Since the higher concentrations of CO2 change the carbonation

reaction products for geopolymer, 1% of CO2 concentration was used [28]. Carbonation

depth of the concrete specimens was determined by the phenolphthalein application.

Phenolphthalein indicator was applied on the fresh concrete surface immediately after

the split the specimens, and the depth of carbonation was measured with the colour

change of the indicator by using measuring tape.

7.2.3 Mathematical approach on the carbonation of geopolymer concrete

Carbonation models for geopolymer concrete were developed in two methods by using

theoretical approach and mathematical equation. Carbonation model with the

theoretical path was created with diffusion theory. A mathematical model to predict the

carbonation depth was developed by using the model proposed by Czarnecki et al. [225].

7.2.3.1 Carbonation model with diffusion theory

Initially, the carbonation model was developed with Fick’s first law and diffusion

theory. It was assumed that the carbon dioxide content between the surface and the

moving boundary is linear [226] and the diffusion process is a constant, not changing


190

with the period [227]. According to that, the following equation is explained the

relationship between the flux of carbon dioxide, CO2 concentration, depth of carbonation

and the diffusion of CO2 [226]:

𝐽 = 𝐷

∆𝐶

𝑋𝑐𝑎

(12)

Where J = Flux of carbon dioxide into concrete g/ (m2s).

D= Diffusion coefficient with respect to carbon dioxide (m2/s).

Xca= The distance of the moving carbonation boundary from the surface of the structure

(m).

∆c = Cs- Cx (g (CO2)/m3).

Cs = CO2 content of air at the surface of concrete (g (CO2)/m3)

Cx= CO2 content of air at the moving boundary, g (CO2)/m3.

However, it is known that the carbon dioxide flux into concrete is equal to the rate of

mass growth of bound CO2. Therefore, carbon dioxide flux can be explained as follows:

𝐽 =

𝑑𝑄𝑐𝑎

𝑑𝑡

(13)

Where Qca = Mass of chemically bound CO2 in concrete

The mass of bound CO2 (Q) can be expressed as follows:

𝑄 = 𝑎 × 𝑋𝑐𝑎 (14)

Where a is the CO2 binding capacity of concrete.

𝑑𝑄

𝑑𝑡= 𝑎 ×

𝑑𝑋𝑐𝑎

𝑑𝑡

(15)

Where, t = time.

Therefore, by combining Equations 1 and 4 and integrating over time (xca = 0 when t =

0), the following solution was developed with the flux of carbon dioxide(CO2) and the

CO2 binding capacity [228].


191

X=√

2×𝐷×∆𝑐× 𝑡

𝑎

( 16)

Where, X is carbonation depth. We also can represent the above equation by following

way:

X=√

𝐷×∆𝑐 𝑡

𝐴

( 17)

Where A=a/2.

Therefore, by using the Eqn ( 17) predicted carbonation depth was determined. For the

calculations, ∆c is considered as 1%. The graph was fitted with the laboratory

carbonation depth values by using least square root error methods.

7.2.3.2 Carbonation model with a mathematical approach

The carbonation model was also developed with a mathematical approach. The

mathematical model proposed by Czarnecki et al. [225] was used to determine the

carbonation depth is explained as follows:

ℎ = 𝑎 +

𝑏

√𝑡

( 18)

The formula presented above is based on the general hyperbolic model of carbonation,

according to which change in the depth of carbonation (h) with the time of t.

Where a, b are characteristic coefficients of the function.

Therefore, according to the above equation, carbonation depth was predicted based on

the graph was fitted with the laboratory carbonation depth values by using least square

root error methods.


7.3.1 Compressive strength test results

Fig. 7-2 displays the compressive strength values of geopolymer and OPC concrete

samples after 7 and 28 days of curing period. All concrete samples confirmed the

strength increment from 7 days to 28 days of curing. This indicated that the geopolymer

reactions is continued for 28 days of curing period. It should be noted that the source of


192

materials and the curing methods are influenced on the geopolymerisation reaction. In

this study, geopolymer concrete mixes were prepared with fly ash-slag materials and

the samples were cured at ambient temperature. It was proved that the reaction rate of

Ca in slag materials is slow. During the geopolymerisation of fly ash and slag blended

mix, alumino silicate binding gel forms initially, due to the reaction of dissolute fly ash

particles with activator solution and then the Ca in the slag particles dissolute slowly

and produced high compressive strength at the later aged of concrete [52]. Therefore,

this attributes the strength increment with the curing period.

Moreover, as shown in Fig. 7-2, the compressive strength of geopolymer is increased

with the increment of slag content in the geopolymer mix. The 28 days of compressive

strength values of S1, S2 and S3 types are 27 MPa, 35 MPa and 38 MPa, respectively. As it

was mentioned previously, the pore structure of the concrete is reduced with the

incorporation of slag materials into fly ash mixture due to the formation of a C-A-S-H

binding gel with N-A-S-H gel [51]. Kumar et al. [229] determined that the compressive

strength of the geopolymer concrete is increased with the addition of slag materials due

to the formation C–S–H and C-A–S–H gel phases and compactness of microstructure.

Therefore, a less porous structure in the high slag content mix produces higher

compressive strength compared to the mix have higher fly ash content. Moreover, the

compressive strength of geopolymer concrete is increased with the addition of slag

content in the mix due to the increment of Si/Al ratio, which creates a more

geopolymerisation reaction.

It can also notice from Fig. 7-2, the 28 days of compressive strength values of S1 and S2

type mixes were lower than OT specimens, whereas, S3 type geopolymer mix exhibits

higher strength compared to OT specimens. This indicates the geopolymer concrete

prepared with higher slag content showed superior strength behaviour compared to the

concrete prepared with Portland cement binder.


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Fig. 7-2 Compressive strength values of geopolymer and OPC concrete specimens.

7.3.2 Accelerated carbonation test results in 1% CO2 environment

Fig. 7-3 presents the evolution of the carbonation depth with the time of exposure for

geopolymer and OPC concretes. The test results are correlated with the carbonation

results obtained from field investigation. According to the Fig. 7-3, geopolymer concrete

displayed higher carbonation compared to OPC concrete at same exposure conditions.

As shown that the maximum carbonation depth observed on OT specimens 9 mm after

exposed to 81 days in 1% CO2 environment. On the other hand, the carbonation depth

determined after exposed to 81 days for S1, S2 and S3 types geopolymer specimens are 20

mm, 18 mm, and 16 mm respectively. Moreover, it should be noted that the resistance to

carbonation of geopolymer concrete is increased with the increment of slag content in

the geopolymer concrete mix. As explained earlier, incorporation of slag reduces the

pore size and the porosity of the geopolymer network. Therefore, the diffusion of CO2

to the concrete surface is reduced, and due to that, lower carbonation behaviour was

identified on the geopolymer prepared with higher slag contents.


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Fig. 7-3 Carbonation depth measurements in 1% CO2 environment

7.3.3 Carbonation model with diffusion theory and mathematical approach

Fig. 7-4, Fig. 7-5, Fig. 7-6 and Fig. 7-7 are illustrating the carbonation models developed

with diffusion theory and mathematical approach for S1, S2, S3 and OT type concrete,

respectively. The experimental carbonation depth values determined with the

phenolphthalein applications also included in the figures. As shown in all the figures,

carbonation models are correlated well with the experimental laboratory results.

As can be observed from the results, the carbonation model developed with diffusion

theory is fitted very well with the laboratory experimental test results. However, the

models devolved with the mathematical approach slightly deviate from the

experimental results for geopolymer concrete. However, the mathematical model is

fitted well with the experimental results for OT concrete. This is indicating the

carbonation depth of geopolymer concrete is proportional to the square root of time

variation, and not related to 1/ square root of time variation as explained in the empirical

equation.

Moreover, the diffusion model provides the diffusion coefficient values of concrete

specimens. The coefficient of diffusion obtained for S1, S2 and S3 types of geopolymer

concrete are 7695 m2/s, 6844 m2/s and 6299 m2/s respectively. This indicates the

diffusion rate is reduced with the slag content in the geopolymer mix. On the other hand,

OPC concrete revealed lower diffusion coefficient, which is about 4085 m2/s.


195

It should be noted that the diffusion coefficient values cannot necessarily be generalised

to all geopolymer concrete and are only applicable to the specific mix and materials

studied. Therefore, for different types of geopolymer concrete mix, it should be

necessary to generate the new diffusion equation.

Fig. 7-4 Carbonation models for S1 type concrete



196


Fig. 7-7 Carbonation models for OT type concrete


This chapter illustrates the carbonation models developed with Fick’s law and diffusion

equation and the empirical equations for geopolymer concrete. Accelerated carbonation

test was carried out on geopolymer concrete prepared with different proportion of the

source materials and OPC concrete in 1% of the CO2 environment. Based on the

carbonation depth values, the mathematical models were developed. According to the

test results, the following conclusions can be extracted:

The compressive strength values of the geopolymer concrete are increased with

the increment of slag content in the geopolymer mix.


197

The geopolymer concrete prepared with fly ash material showed higher

carbonation, and the carbonation depth values were decreased with the addition

of slag content in the geopolymer mix.

The model developed with diffusion theory is correlated with the laboratory test

results for OPC and geopolymer concrete.

The CO2 diffusion coefficient values were calculated with the diffusion model.

The coefficient of carbonation diffusion of geopolymer concrete is greater than

OPC concrete. According to the diffusion models, the coefficient values are

decreased with the increment of slag content in the geopolymer mix.

Chapter 8 Conclusions and Recommendations

198


8.1 Summary

This thesis presented an extensive study on the long-term durability of geopolymer

concrete structures exposed in the field and laboratory environmental conditions.

Chapter 3 investigated the durability of geopolymer concrete exposed to the

atmospheric environment for 8 years. The durability performance was assessed by

studying the carbonation properties of concrete core specimens. For this investigation,

the core specimens were extracted from the fly ash-based geopolymer concrete structure

and two different mix compositions of fly ash- slag based geopolymer concrete

structures. The test results were compared with OPC concrete structures located in the

same exposed environmental conditions. The research found that the carbonation

resistance of geopolymer concrete is lower than that of OPC concrete in the atmospheric-

exposed environment. Among the different geopolymer compositions, fly ash based

geopolymer concrete exhibited lower carbonation resistance compared to the

geopolymer concrete prepared with fly ash-slag blended geopolymer concrete. In fly

ash-based geopolymer concrete, the formation of water-soluble carbonation products of

Na2CO3 and K2CO3 is the main reason for the lower carbonation resistance since they

can be washed out with the contact of water, leading to the increased porosity with the

carbonation. This further exacerbated the CO2 diffusion and significantly affecting the

durability of fly ash based geopolymer concrete structures. In fly ash-slag blended GPC,

in addition with the soluble carbonation components, insoluble CaCO3 products are also

formed during the carbonation as a result of the slag content. Therefore, the increase in

porosity is comparably lower than fly ash based geopolymer concrete, and this leads to

better carbonation resistance. Furthermore, this study also revealed that the choice of

activator has a significant influence on the carbonation resistance of fly ash-slag blended

geopolymer concrete. The GPC prepared with NaOH activator showed better

carbonation resistance compared to the combined activators of NaOH and Na2SiO3.

Chapter 4 of this thesis reported the durability of geopolymer concrete in the aggressive

environment by determining the durability performance in terms of carbonation,


199

chloride diffusions and sulphate attack of the GPC. The investigation was conducted on

core specimens from fly ash-based GPC structure exposed in the saline environment for

6 years, and slag-fly ash blended GPC exposed in the marine environment for 4 years,

and the durability properties were compared with OPC concrete from the same exposed

environment. The results showed that the durability of GPC concrete is lower than OPC

concrete in saline/marine environmental conditions. The GPC prepared with fly ash

binder displayed lower resistance to chloride penetration compared to OPC concrete

with the exposure period of 6 years in the saline environment. The GPC showed higher

chloride diffusion coefficient values compared to OPC concrete. Besides, the

microstructural analysis of the fly ash based GPC studied with SEM test revealed that

the chloride ions were deposited as a film layer on the surface of fly ash particles. As

similar to fly ash-based GPC, chloride penetration in slag-fly ash blended GPC was also

greater than OPC concrete under the exposure in the marine environment. However,

compared to fly ash based geopolymer, the chloride penetration in slag-fly ash based

geopolymer concrete is low, and the SEM analysis has not indicated deposition of

chloride ions on the microstructure of the slag-fly ash blended geopolymer concrete.

Furthermore, the carbonation of fly ash-based GPC was found to be much higher than

OPC concrete over the six years of exposure to aggressive saline conditions. According

to the test results, the core specimens from fly ash based GPC was completely carbonated

in the leg parts (90 mm) as well as in the top slab (135 mm thickness). On the other hand,

the corresponding carbonation depths in the OPC concrete structures were determined

as 10 mm and 20 mm, respectively. The CO2 diffusion of the slag- fly ash blended GPC

also greater than OPC concrete in the marine environment. Even though slag-fly ash

based GPC concrete was exposed for a shorter period (4 years) than OPC concrete (6

years) in the marine environment, GPC showed higher carbonation values (11 mm)

compared to OPC concrete (4 mm).

Chapter 4 also showed that the salt scaling effect in GPC was higher than OPC concrete

in aggressive exposed conditions. The mortar from the fly ash-based GPC surface,

especially the vertical surface of the culvert which was frequently in contact with saline

lake water, has been found lost and the aggregate is exposed to the surface, whereas no

significant changes have been identified in the visual appearance of an OPC concrete

culvert over time. Similarly, mortar from the surface of slag-fly ash blended GPC also

removed after four years exposed in the marine environment, while OPC concrete was


200

not shown such observation after six years of exposure in the similar environment.

Higher scaling effect in GPC concrete is due to the poor sulphate resistance of GPC. The

test results showed that the GPC concrete displayed higher ingress of sulphate

compared to OPC concrete. Furthermore, the SEM/EDX test results also revealed the

higher sulphate penetration, and there is no formation of ettringite observed in GPC

specimens. This indicates the mechanism of sulphate attack in GPC is different from

OPC concrete.

Moreover, fly ash based geopolymer concrete provides lower protection against

corrosion of reinforcement than the OPC concrete in the saline environment. The

reinforcement bar in fly ash-based GPC was corroded on the entire surface, and the

deposition of corrosion products at the interface area of fly ash-based GPC was much

higher than the OPC concrete. The combination of higher carbonation and chloride

penetration caused higher corrosion activity of the steel bar in fly ash based geopolymer

concrete.

Chapter 5 presented the accelerated wetting-drying analysis of geopolymer mortar in

different solution such as water, chloride solution and the combination of chloride and

the sulphate solutions. The test results revealed that the degradation effect of GPC

specimens is higher than OPC mortar. The loss of compressive strength was found to be

low with the increasing level of slag in the GPC. Moreover, fly ash based geopolymer

showed a higher amount of sodium leaching compared to the geopolymer concrete

prepared with the higher amount of slag content.

Chapter 6 highlights the test results of the corrosion of reinforcement in the GPC when

subjected to three different exposure conditions in the laboratory such as continuous

exposure in 1% CO2 environment, cyclic exposure to wet and dry conditions in 1% CO2

and water solution, and cyclic exposure to wet and dry conditions in 1% CO2 and

chloride water solution. The assessment was conducted for a period of 6-month. The test

results showed that the carbonation and corrosion of rebar in fly ash-based GPC was

higher than the fly ash- slag blended GPC and the carbonation and corrosion rate was

reduced with the incorporation of slag in GPC. However, OPC concrete displayed

superior durability performance against carbonation and corrosion effect compared to

GPC. Furthermore, among the different in-situ carbonation testing indicators, the

universal solution was identified as a suitable indicator for measuring the carbonation


201

depth in geopolymer concrete. Especially, GPC prepared with fly ash-based materials

did not show a clear carbonation indication with phenolphthalein indicator. Universal

solution is a more suitable indicator for fly ash-based GPC due to the wide range of

colour variation with pH values.

The mathematical models for theoretically determining the carbonation properties of

geopolymer concrete was presented in Chapter 7. The diffusion equation based on the

Fick’s law and corresponding empirical equations were used to develop the carbonation

diffusion models for geopolymer concrete. The developed carbonation profile models

were calibrated with experimental results observed in this study. It was found that the

developed carbonation models are well fit with the experimental results and hence, these

models can be used to analytically determine the carbonation depth of concrete over the

period.


Carbonation resistance of geopolymer concrete is lower than OPC concrete when

it is exposed to both atmospheric and aggressive field environment conditions.

The formation of soluble carbonation products is the main reason for the higher

carbonation rate in GPC, which attributes higher porosity, and increases the CO2

diffusion in GPC.

The salt scaling effect in GPC was higher than OPC concrete. The mortar from

the GPC surface has been removed, and the aggregate is exposed on the surface,

whereas no significant changes were identified in the visual appearance of OPC

concrete over time.

The chloride penetration in the GPC concrete was high in saline and marine

environments. According to the total chloride analysis, the surface chloride

content and the chloride diffusion coefficient of fly ash-based GPC concrete was

approximately 2.5 times greater than the values obtained for OPC concrete. The

SEM analysis revealed that the chloride contents were deposited as a film layer

on the fly ash-based GPC concrete.

GPC exhibited higher ingress of sulphate compared to OPC concrete causing

more scaling effect in GPC structure. SEM/EDX test results also revealed the

higher sulphate penetration, and there is no formation of ettringite observed in


202

GPC specimens. This indicates the mechanism of sulphate attack in GPC is

different from OPC concrete.

The fly ash-based GPC displayed lower resistance to corrosion in the saline

aggressive conditions. The reinforcement bar in GPC concrete was corroded on

the entire surface, and the deposition of corrosion products at the interface of

concrete and rebar were much greater than for OPC concrete.

The wetting and drying cycle test results revealed higher compressive strength

loss degradation in GPC. As similar to field investigation, GPC showed less

durability performance compared to OPC concrete in laboratory controlled

conditions. Moreover, strength reduction and the deterioration effect were found

to be low with the increasing level of slag in the GPC.

The accelerated carbonation test results (1 % of the CO2 environment) showed

that the corrosion of rebar in fly ash-based GPC is greater than the OPC

specimens and, the corrosion rate is reduced with the slag content in GPC.

Carbonation coefficient models were developed and calibrated for GPC

according to the diffusion equation based on the Fick’s law and the use of

empirical equations.

Finally, this investigation suggested that the geopolymer concrete prepared with

fly ash binder is more suitable for interior construction applications (less effect

from water or aggressive agents) and the GPC prepared with slag binder can be

used for exterior construction applications. However, the required cover size to

protect the reinforcement should be higher than the cover size used in OPC

concrete.

8.3 Recommendations for future work

This research study shows that the GPC has less durability in the field

environments. However, the finding cannot necessarily be generalised to all

geopolymer concrete and are only applicable to the specific mix and materials

studied. Therefore, investigation of suitable geopolymer concrete chemistry and

mix design is required to enhance durability in the field environment.

The sulphate attack in geopolymer concrete was found to be significantly higher

than OPC concrete in an aggressive environment. However, the test results

revealed that the mechanism of sulphate attack in GPC is different from OPC

concrete as no observation of ettringite in GPC microstructure. Therefore, further


203

investigation is required to determine the mechanism of sulphate attack in GPC

and the formation of sulphate reaction components in GPC.

The strength loss of GPC in real field environment condition has not been

evaluated in this investigation. Therefore, further research is recommended to

determine the strength reduction of GPC with the period of exposure in the field

environment condition.

This study determined only the corrosion of reinforcement bar in fly ash based

geopolymer concrete when it is exposed to field conditions. However,

accelerated laboratory test results indicated that the type of source materials

influence on the corrosion of the reinforcement bar. Therefore, it is recommended

to study the influence of the source materials on the corrosion behaviour of

geopolymer concrete exposed to the field environment.

Furthermore, this study found that the carbonation behaviour of fly ash-slag

blended GPC in the atmospheric environment depends on the type of activator

used for the geopolymer preparation. Therefore, further research investigations

will be recommended to conduct the detailed investigations on this conclusion.

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