Effect of Sodium Hydroxide and Sodium Silicate Solutions...

KSCE Journal of Civil Engineering (2017) 21(6):2202-2210

Copyright ⓒ2017 Korean Society of Civil Engineers

DOI 10.1007/s12205-016-0327-6

− 2202 −

pISSN 1226-7988, eISSN 1976-3808

www.springer.com/12205

Structural Engineering

Effect of Sodium Hydroxide and Sodium Silicate Solutions on Strengths of

Alkali Activated High Calcium Fly Ash Containing Portland Cement

Tanakorn Phoo-ngernkham*, Sakonwan Hanjitsuwan**, Nattapong Damrongwiriyanupap***,

and Prinya Chindaprasirt****

Received April 21, 2015/Revised February 27, 2016/Accepted April 27, 2016/Published Online October 25, 2016

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Abstract

In this paper, the mechanical performance of fly ash and Portland cement geopolymer activated with sodium hydroxide andsodium silicate solutions was studied. The Geopolymer Mortars (GM) were made from high calcium Fly Ash (FA) and ordinaryPortland Cement (PC) with FA:PC weight ratios of 100:0, 95:5, 90:10, 85:15, and 80:20. The GMs were activated with threecombinations of sodium Hydroxide Solution (SH) and sodium Silicate Solution (SS) viz., SH, SH+SS (SH:SS=2) and SS. For allmixes, 10 molar SH, alkali activator liquid/solid binder ratio of 0.60 and curing at ambient temperature of 25oC were used. The resultindicated that the compressive and shear bond strengths of GM depended on the alkali activators used and the amount of PC. The useof SH and SHSS resulted in the formation of additional Calcium Silicate Hydrate (CSH) which coexisted with sodiumaluminosilicate hydrate (NASH) gel. Whereas, the use of SS resulted in NASH gel with only a small amount of CSH. The increasingof PC content enhanced the compressive and shear bond strengths of GMs due to the formation of additional CSH. The 15% PCmixed with SHSS gave the optimum compressive and shear bond strengths.

Keywords: geopolymer, high calcium fly ash, Portland cement, compressive strength, shear bond strength

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1. Introduction

Ordinary Portland Cement (PC) is widely used as a main

building material; however, its manufacturing process results in a

high emission of carbon dioxide (CO2) to the atmosphere (Metz

et al., 2007; Sukmak et al., 2013). Currently, geopolymer binder

received a lot of attention as an alternative binder due to its low

carbon dioxide emission (McLellan et al., 2011; Pacheco-Torgal

et al., 2008), but some investigations demonstrate that low

emissions are only possible for geopolymer without sodium

silicate (Turner and Collins, 2013). Normally geopolymer is

obtained from precursor materials containing silica and alumina

such as Fly Ash (FA), calcined kaolin or metakaolin activated

with high alkali solutions (Albitar et al., 2014; Jun and Oh, 2015;

Kumar et al., 2010; Li et al., 2010; Ravikumar et al., 2010). The

main reaction products is sodium aluminosilicate hydrate (NASH)

gels. For the system with high calcium content, the main reaction

products is CASH gels (Li et al., 2010; Palomo et al., 1999).

However, both systems are considered as ‘alkali activated

materials’.

In Thailand, high calcium FA from Mae Moh power plant in

the north of Thailand has been shown to be a good precursor

materials in providing relatively high strength binder (Chindaprasirt

et al., 2011; Chindaprasirt et al., 2007). This is because of FA

with high CaO content which is still contained high SiO2 and

Al2O3 contents (Phoo-ngernkham et al., 2014; Phoo-ngernkham

et al., 2013; Phoo-ngernkham et al., 2013). The usage of FA as a

precursor material provides the greatest opportunity for commercial

utilization of this technology due to the plentiful supply of the

material worldwide (Sukmak et al., 2013; Sukmak et al., 2013).

However, the strength development of FA geopolymer at ambient

temperature condition is rather slow and relatively low strength

is obtained (Pangdaeng et al., 2014; Suwan and Fan, 2014). For

normal fly ash gepoolymer system, the setting and hardening are

slow at room temperature (Temuujin et al., 2009). With the

presence of calcium in the system, the reaction at ambient

temperature is accelerated (Temuujin et al., 2009) and the products

contains both geopolymeric and calcium silicate hydrate (CSH)

TECHNICAL NOTE

*Ph.D., Research Center for Advances in Civil Engineering and Construction Materials, Dept. of Civil Engineering, Faculty of Engineering and Archi-

tecture, Rajamangala University of Technology Isan, Nakhon Ratchasima 30000, Thailand (E-mail: [email protected])

**Ph.D., Program of Civil Technology, Faculty of Industrial Technology, Lampang Rajabhat University, Lampang 52100, Thailand (Corresponding

Author, E-mail: [email protected])

***Assistant Professor, Civil Engineering Program, School of Engineering, University of Phayao, Phayao 56000, Thailand (E-mail: natpong_chin@hot-

mail.com)

****Professor, Sustainable Infrastructure Research and Development Center, Department of Civil Engineering, Faculty of Engineering, Khon Kaen Univer-

sity, Khon Kaen 40002, Thailand (E-mail: [email protected])

Effect of Sodium Hydroxide and Sodium Silicate Solutions on Strengths of Alkali Activated High Calcium Fly Ash Containing Portland Cement

Vol. 21, No. 6 / September 2017 − 2203 −

and calcium aluminosilicate hydrate (CASH) (Somna et al.,

2011). The temperature curing at 40-70oC can improve the

geopolymerization process and hence the strength development

of FA geopolymer (Chindaprasirt et al., 2007). In order to put

this material into real use, the improvement of strength

development at ambient temperature curing is needed (Nath and

Sarker, 2015; Palomo et al., 2007; Pangdaeng et al., 2014; Phoo-

ngernkham et al., 2013; Suwan and Fan, 2014; Temuujin et al.,

2009; Williams et al., 2002). The usage of calcium-rich materials

as an additive to improve the strength development of FA

geopolymer such as calcium oxide (CaO), calcium hydroxide

(Ca(OH)2) and Portland Cement (PC). In the other words, the

increasing of calcium content enhances the mechanical properties

of FA geopolymer with additional calcium silicate hydrate

(CSH) which coexists with other geopolymer products (Guo et

al., 2010; Nazari and Ghafouri Safarnejad, 2013). In addition,

the reaction of calcium-rich materials with water generates heat

from exothermic process at ambient temperature and thus helps

enhance the geopolymerization process (Pangdaeng et al., 2014;

Suwan and Fan, 2014).

The alkali activator is one of the most important factors affecting

the strength development of geopolymer matrix. Normally,

sodium hydroxide and sodium silicate solutions are the most

used as liquid activators because they are shown to be the most

suitable activators in terms of availability and good mechanical

properties (Pimraksa et al., 2011). Sodium Hydroxide Solution

(SH) is commonly used for dissolution of Si4+ and Al3+ ions from

precursor materials to form aluminosilicate material. While

sodium Silicate Solution (SS) contains soluble silicate species

and thus is used to promote the condensation process of geopolymer

(Panias et al., 2007). A number of researchers (Kumar et al.,

2010; Rashad, 2013; Ravikumar and Neithalath, 2012) reported

that the best mechanical performance of geopolymer are obtained

with combinations of sodium silicate and sodium hydroxide

solutions. This paper, therefore, investigates the mechanical

properties viz., compressive strength and shear bond strengths,

and microstructure of high calcium fly ash geopolymer activated

with NaOH and sodium silicate solutions with Portland cement

as additive. The outcome of this study would lay a foundation for

the future use of high calcium FA as a promoter for manufacturing

geopolymer material and the use of Portland cement as additive

in this system.

2. Materials and Methods

2.1 Materials and Sample Preparation

The precursors used in this study were Fly Ash (FA) from Mae

Moh power plant in northern Thailand and ordinary Portland

Cement (PC). The use of PC as calcium-rich material is very

attractive due to its availability, low cost and additional formation

of aluminosilicate species. PC was, therefore, used as calcium-

rich material in this investigation. The FA with median particle

size of 8.5 µm, Blaine fineness of 4300 cm2/g and specific

gravity of 2.25; and PC with median particle size of 14.6 µm,

Blaine fineness of 3600 cm2/g and specific gravity of 3.15 were

used as precursors. The chemical compositions of FA and PC as

shown in Table 1 indicated that FA mainly consisted of SiO2 and

Al2O3 and some impurities. The sum of SiO2, Al2O3 and Fe2O3

was 57.9%, and the CaO content was high at 25.8%. This FA

was Class C fly ash as specified by ASTM C618-15 (2015). 10

molar Sodium Hydroxide (SH) and Sodium Silicate (SS) with

13.89% Na2O, 32.15% SiO2, and 46.04% H2O were used as

alkali activators. The SH was obtained from dissolving sodium

hydroxide pellets in distilled water and allowed to cool down at

room temperature. Local river sand with specific gravity of 2.63

and fineness modulus of 2.2 was used as fine aggregate.

The mix design was based on the previous study on the effect

of combined use of sodium hydroxide and sodium silicate

(Phoo-ngernkham et al., 2015). The Portland cement dosage was

increased to 20% to cover the wider range of addition. The

effects of the uses of Sodium Hydroxide (SH) and Sodium

Silicate (SS) alone; and the combined use of Sodium Hydroxide

Plus Sodium Silicate (SHSS) were studied. The mix proportions

of fly ash and Portland cement geopolymer are summarized in

Table 2. Mixes 6-9, therefore, had the same mix proportions as

those tested by Phoo-ngernkham et al. (2015). The abbreviations

of 0PC, 5PC, 10PC, 15PC, and 20PC with corresponding FA:PC

weight ratios of 100:0, 95:5, 90:10, 85:15, and 80:20, respectively

were the mixes used. Constant liquid/solid binder ratio of 0.60,

and sand/binder ratio of 1.0 were used in all mixtures. Previous

study on high calcium coal ash geopolymer activated with

NaOH and sodium silicate solutions (Phoo-ngernkham et al.,

2015; Sathonsaowaphak et al., 2009) shows that a relatively high

strength geopolymer could be obtained with up to liquid/solid

binder ratio of 0.60. Three combinations of alkali activators viz.,

SH, SS and SH+SS (SS/SH=2.0) were used. SH+SS (SHSS) was

prepared by mixing SH and SS prior to mixing of geopolymer.

For the mixing of fly ash and Portland cement geopolymer

mortar (GM), FA, PC and sand were dry mixed until the mixture

was homogenous which took approximately 1 minute. The alkali

activator solutions were then added and the mixing was done for

another 3 minutes. After mixing, the mortar was then placed into

a 50 × 50 × 50 mm3 cube molds for compressive strength tests as

described in ASTM C109 (2002). The samples were wrapped

with vinyl sheet to protect moisture loss for 1 day at the 25oC

controlled room and then they were again wrapped with vinyl

Table 1. Chemical Compositions of Fly Ash and Portland Cement (by weight)

Materials SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 LOI

FA 29.3 13.0 15.6 25.8 3.0 2.9 2.8 7.3 0.3

PC 20.8 4.7 3.4 65.3 1.5 0.4 0.1 2.7 0.9

Tanakorn Phoo-ngernkham, Sakonwan Hanjitsuwan, Nattapong Damrongwiriyanupap, and Prinya Chindaprasirt

− 2204 − KSCE Journal of Civil Engineering

sheet and kept in the controlled room.

The mixing, molding and demolding of pastes were the same

as those of mortar with exception of dry mixing where only FA

and PC were dry mixed for 1 minute. The paste samples were

used for the SEM and XRD analyses.

2.2 Testing and Analysis

2.2.1 Compressive Strength

The compressive strengths of fly ash and Portland cement

geopolymer mortars (GMs) were measured at the age of 28 days.

The results were the average value of three samples.

2.2.2 Scanning Electron Microscopy (SEM) and X-ray Dif-

fraction (XRD)

The geopolymer structures were studied using XRD and SEM

analyses. The analyses by XRD and SEM experiments are

widely used for preliminary observation of the microstructure of

geopolymer as shown by several researchers (Chindaprasirt et

al., 2012; Lee et al., 2014). For SEM test, the broken sample of

3-6 mm was obtained from the middle portion of geopolymer

paste sample and then placed on a brass stub sample holder with

double stick carbon tape. The sample was dried using infrared

light for 5 minutes and then coated with a layer of gold using a

blazer sputtering coater. The micrographs were recorded at 15

kV and 1000x magnification. For XRD test, the samples from

the middle portion of geopolymer paste sample was collected

and ground to fine powder and the XRD scans were performed

for 2theta between 5 and 60°.

2.2.3 Shear Bond Strength between Concrete Substrate

and Geopolymer Mortar

The adhesive strength or shear bond strength between Concrete

Substrate (CS) and fly ash and Portland cement geopolymer

mortar (GM) was evaluated using the slant shear test adapted

from ASTM C882 (2005). In this study, the preparation of CS

samples and the casting of slant shear samples were based on the

previous study (Phoo-ngernkham et al., 2014). The fresh CS

samples were cast in a 50 × 50 × 125 mm3 prism mold. The

hardened CSs were cut at the middle section with an interface

line of 45° to the vertical axis. The slant angle of 30° to the

vertical is officially recommended by ASTM C882 (2005).

However, the slant angle of 45° is also officially used for the

standard evaluation of bond (FM 3-C882 2015). In the preparation

of concrete substrate, the cutting of 45° concrete substrate was

better with less chipping and cracking than the 30° concrete

substrate. The slant angle of 45° to the vertical was, therefore,

used. The compressive strength of CS was 35.0 MPa. For the

casting of shear bond strength sample between CS and GM, the

GMs were cast into a 50 × 50 × 125 mm3 prism mold with the

other half filled with CS and then they were wrapped with vinyl

sheet to protect moisture loss and left for 1 day in the 25oC

controlled room. The samples were demoulded and covered with

vinyl sheet again and kept in the 25oC controlled room until

testing. The samples were tested at the age of 28 days using the

constant loading rate of 0.30 MPa/s. The results were the average

value of three samples.

3. Results

The 28-day compressive strengths of GMs with difference

types of alkali activator are shown in Fig. 1. The compressive

strengths of GM were depended on the types of alkali activators.

The strengths of GMs with SHSS were highest followed in turn

by mixes with SS and SH. The strengths of GMs with SH were

low compared with those of the other alkali activators. The

strengths of the control mix (without PC) with SH, SHSS, and

SS were 5.4, 29.1, and 24.1 MPa, respectively. For all PC

replacement levels, the strengths of GMs marginally increased

with increasing of PC content and activated with SH and the

strengths were between 6.1 and 6.8 MPa. For SHSS and SS

solutions, the strengths of GMs obviously increased as the

Table 2. Mix Proportions of Fly Ash and Portland Cement Geopolymer

Mix No. Mix symbol FA (g) PC (g) Sand (g) SH (g) SS (g) SiO2/Al2O3 ratios CaO/SiO2 ratios

1 SH-0PC 100 - 100 60 - 2.26 0.88

2 SH-5PC 95 5 100 60 - 2.30 0.96

3 SH-10PC 90 10 100 60 - 2.35 1.04

4 SH-15PC 85 15 100 60 - 2.39 1.13

5 SH-20PC 80 20 100 60 - 2.44 1.22

6 SHSS-0PC 100 - 100 20 40 3.25 0.61

7 SHSS-5PC 95 5 100 20 40 3.33 0.66

8 SHSS-10PC 90 10 100 20 40 3.41 0.72

9 SHSS-15PC 85 15 100 20 40 3.49 0.78

10 SHSS-20PC 80 20 100 20 40 3.58 0.83

11 SS-0PC 100 - 100 - 60 3.75 0.53

12 SS-5PC 95 5 100 - 60 3.84 0.58

13 SS-10PC 90 10 100 - 60 3.94 0.62

14 SS-15PC 85 15 100 - 60 4.04 0.67

15 SS-20PC 80 20 100 - 60 4.15 0.72


Vol. 21, No. 6 / September 2017 − 2205 −

replacement of PC increased up to an optimum level and then the

strength started to decline. The compressive strength of GMs

mixed by SHSS solutions were between 44.2 and 54.1 MPa,

whereas the compressive strength of mixes with SS solution

were between 15.9 and 40.3 MPa.

The test results of shear bond strength of CS and GMs with

different alkali activator types are illustrated in Fig. 2. The 28-

day shear bond strengths of CS and GMs of the control mix

(without PC) with SH, SHSS, and SS were 6.3, 16.4, and 13.0

MPa, respectively. The increase in shear bond strengths was

clearly observed when the mixes contained PC content. The

GMs mixed with SH solution was slightly increased from 8.0 to

10.3 MPa as the amount of PC increased. While the strengths of

GMs with SS and SHSS solutions tended to increase as the

replacement of PC content increased up to a threshold limit. The

relatively high shear bond strengths were found with SHSS

solutions between 16.4 and 24.6 MPa with the optimum at 15%

PC content. The shear bond strength for GMs with SH was

between 8.5 and 16.5 MPa with the optimum at 5%PC.

The failure patterns of the samples due to the shear bond tests

are important parameters related to the observation of bond

strength at contact zone between CS and GMs. Two failure

patterns viz., monolithic failure mode and shear bond failure

mode were observed as shown in Fig. 3. The monolithic failure

mode is observed in mixes SHSS-15PC (Fig. 3(f)), SHSS-20PC

(Fig. 3(g)), and SS-5PC (Fig. 3(i)) which corresponded to

relatively high bond strengths. The slant shear test prism acted as

a monolithic column and cracks were continuous from CS to

GM passing through the slant plane. This monolithic failure

mode is similar to that of solid CS prism (Fig. 3(a)). The second

mode was the shear bond failure mode whose cracks were found

mainly at the interface and some cracks were also found in the

weak GM with CS potion remained relatively intact. The low

bond strengths were found in mixes SH-0PC (Fig. 3(b)), SH-

15PC (Fig. 3(c)), SH-20PC (Fig. 3(c)), SHSS-0PC (Fig. 3(d)),

SS-0PC (Fig. 3(h)), and SS-20PC (Fig. 3(j)).

4. Analysis and Discussion

The alkali activator types and PC content are significant

parameters controlling the strength development of GM. The

increase in PC content can improve the strength of GMs. This is

because of free calcium ions in the system which reacts with

silica and alumina, resulting in additional CSH and/or CASH

gel. The co-existence of geopolymer gels and CSH and/or

CASH gel lead to a high compressive strength of the GMs cured

at ambient temperature. This is very attractive for the usage of

this product in the real construction. It should be noted that the

mechanical tests performed at 28 days curing do not allow for a

comprehensive understanding of the material performance over

time. The long term properties of the fly ash-Portland cement

geopolymer activated with sodium hydroxide and sodium

silicate solutions cured at ambient condition should be further

Fig. 1. Compressive Strength of Fly Ash and Portland Cement

Geopolymer Mortars

Fig. 2. Shear Bond Strength between Concrete Substrate and Fly

Ash and Portland Cement Geopolymer with Interface Line

at 45° to the Vertical Axis

Fig. 3. Specimens Failure Mode: (a) PCC, (b) SH-0PC, (c) SH-

15PC, (d) SH-20PC, (e) SHSS-0PC, (f) SHSS-15PC, (g)

SHSS-20PC, (h) SS-0PC, (i) SS-5PC, (j) SS-20PC



investigated.

The enhancement of strength development of GM is possible

by the incorporation of PC in the mixture; however, the threshold

limit of PC content depends on the types of alkali activator

solutions. For the GM activated with SH solution, the strength

development of GM is low when cured at ambient temperature

for all PC replacement levels. The SH solution is important for

dissolving of Si4+ and Al3+ ions from precursors and hence

geopolymerization process. However, the process at ambient

temperature condition is very slow and hence low strength is

normally obtained (Somna et al., 2011). High strength development

of GM is found in the mixes activated with SHSS solutions. The

use of SHSS based activators is suitable in terms of strength of

gopolymer matrix. The mix has additional silicate species in the

system and hence the geopolymerization reaction is enhanced.

This is an advantage to the strength development of GMs. For

the GM mixes activated with SS solution, its strength development

are slightly lower than those of mixes with SHSS solutions, but

still higher than those with SH solution. It is thus suggested from

this results that the accelerate reaction in FA-PC blends came

from a rapid chemical reaction between calcium in PC and alkali

activators (Garcia-Lodeiro et al., 2016). Also, the active silica

from sodium silicate solution reacts with calcium from precursors

to form the reaction products and lead to a high strength

geopolymer (Phoo-ngernkham et al., 2015). The optimum PC

contents for compressive strength of mixes with SHSS and SS

are 15%, whilst the use of PC content above 15% leads to a

reduction in the strength of GM. The increase in PC content

beyond the optimum level resulted in the acceleration of reaction

with significant reduction in setting time (Pangdaeng et al.,

2014; Phoo-ngernkham et al., 2013). This adversely affects the

strength development at the later age.

The shear bond strengths of CS and GMs are also related to the

alkali activator type and PC content. The shear bond strengths of

CS and GMs tends to increase with an increasing of PC content

up to an optimum PC content for mixes with SS and SHSS

solutions. The increase in shear bond strength is directly related

to the increase in compressive strength of GM. The high calcium

content from PC can promote the formation of additional CSH

and/or CASH gel and hence an enhancement of the bonding

strength at the interface transition zone between two materials.

The optimum PC contents for shear bond strength of mixes with

SHSS and SH are 15 and 5%, respectively. In the case of SHSS

mixes, the trend of result is the same of that reported by Phoo-

ngernkham et al. (2015). For the SS mixes, the optimum PC

content was 5%, however the shear bond strengths of the mixes

with 10 and 15%PC were only slightly less than that of 5%PC.

The strengths were not as high as the SHSS mixes as the

leaching of silica and alumina does not occur as for the mix with

SHSS. The reaction between PC and sodium silicate is rather fast

and this hinders the strength development at the later age.

Therefore, the compressive strength and shear bond strengths are

slightly lower than the case of SHSS mixes.

The SiO2/Al2O3 and CaO/SiO2 ratios are the important factors

controlling the strength development of high calcium geopolymer

matrix (Chindaprasirt et al., 2012; Phoo-ngernkham et al., 2014;

Phoo-ngernkham et al., 2013; Puligilla and Mondal, 2013). Fig.

4(a) illustrates the relationship between compressive strength and

shear bond strength with various SiO2/Al2O3 ratios. The maximum

compressive strength and shear bond strength were at SiO2/

Al2O3 ratios of 3.4-3.6. This is in line with the previous research

report of high strength geopolymer with SiO2/Al2O3 around 3.50

(Chindaprasirt et al., 2012). The relationship between compressive

strength and shear bond strength with various CaO/SiO2 ratios

are illustrated in Fig. 4(b). The optimum CaO/SiO2 ratio for

compressive strength and shear bond strength of GMs ranged

between 0.72-0.80, which was lower than those of alkali

activated slag (0.8-1.1) and Portland cement (1.3-1.8) (Lee et al.,

2014). The result is in line with the previous study (Puligilla and

Mondal 2013) with an optimum CaO/SiO2 ratio around 0.74.

In this study, the microstructures of fly ash and Portland

cement Geopolymer Pastes (GP) were investigated using SEM

and the test results are shown in Fig. 5. The microstructure of

GPs agrees well with the test results of compressive strength of

GMs. The GPs with SH solution (SH-0PC and SH-15PC) have

loose matrices with large number of non-reacted and/or partially

reacted fly ash particles embedded in poor matrices as shown in

Figs. 5(a) and 5(b). While the GP without PC content with SS

solution (SS-0PC) as shown in Fig. 5(e), the matrix is denser

Fig. 4. Compressive Strength and Shear Bond Strength of Fly Ash

and Portland Cement Geopolymer with Difference of SiO2/

Al2O3 and CaO/SiO2 Ratios: (a) SiO2/Al2O3 Ratios, (b) CaO/

SiO2 Ratios


Vol. 21, No. 6 / September 2017 − 2207 −

than those of SH-0PC and SH-15PC mixes. The dense structure

with smooth and continuous matrix is found in the mix with

15%PC (SS-15PC) as illustrated in Fig. 5(f). The increasing of

calcium content also enhances the geopolymer reaction from an

exothermal reaction with liberated heat and hence additional

formation of CSH and/or CASH. Also, the dense surface of SS-

15PC mix is due to the reaction between silica from sodium

silicate solution and calcium ions from precursors by forming

additional CSH gel within geopolymer system (Suwan and Fan

2014). The dense matrices with less number of unreacted fly ash

particles of fracture surface of GPs are also clearly noticeable in

the mixes activated by SHSS solution (Figs. 5(c) and 5(d)). The

activation with SHSS solution can accelerates the geopolymerization

process and hence high compressive and shear bond strength of

GM are obtained.

The XRD pattern is one of the methods for an observation of

the development of geopolymerization products. Fig. 6 shows

the XRD patterns for GPs without PC content with different

alkali activators viz., SH, SHSS, and SS. It is evident that the GP

with SH-0PC (Fig. 6(a)) contains the glassy phase as indicated

by a broad hump at 25-35o 2theta and the crystalline phases of

quartz (SiO2), calcium silicate hydrate (CSH), Portlandite (Ca(OH)2),

magnesioferrite (Fe2MgO4), and hydrosodalite (Na4Al3Si3O12(OH)).

While the XRD patterns of SH-15PC mix (Fig. 7(a)) show some

additional peaks of Portlandite and CSH. In this study, the

presence of high content of crystalline phases especially

hydrosodalite results in low strength development of fly ash-

Portland cement geopolymer with rather loose packing matrix.

However, previous work (Oh et al., 2012) also demonstrated that

the present of hydrosodalite could increase the strength of

geopolymer binder. The present of a large amount of crystal was

shown to adversely affect the strength of geopolymer (Lee and

Lee, 2015). For SHSS-0PC mix (Fig. 6(b)), the XRD pattern

shows also the glassy phase as indicated by the hump at 28-35o

Fig. 5. SEM of Fly Ash and Portland Cement Geopolymer Pastes: (a) SH-0PC, (b) SH-15PC, (c) SHSS-0PC, (d) SHSS-15PC, (e) SS-

0PC, (f) SS-15PC

Fig. 6. XRD of Geopolymer Without PC with Difference of Alkali

Activators

Fig. 7. XRD of Geopolymer Containing 15% PC with Difference of

Alkali Activators



2theta and the main crystalline phases of quartz, magnesioferrite

and CSH. However, the peaks of these crystalline phases are

reduced and no peaks of hydrosodalite are observed. Difference

in the XRD pattern of GP is found in the mix containing 15% PC

and activated by SHSS solutions as illustrates in Fig. 7(b). The

additional CSH and a substantial amount of amorphous phase

with reducing amount of quartz are also observed. The presence

of amorphous phases are generally corresponded to the overlap

between CSH and alumino silicate products (Escalante-García et

al., 2003). As mentioned, the formation of additional CSH is

responsible for the increase in strength development and hence a

high 28-day compressive and shear bond strengths of 54.1 and

24.6 MPa, respectively. For the XRD pattern of SS-0PC mix

(Fig. 6(c)), the amorphous gel and a small amount of CSH with

some crystalline phases of quartz and magnesioferrite are

observed. While the SS-15PC mix (Fig. 7(c)) shows a small

increase in the peak of CSH comparable to that of SS-0PC mix.

This suggests that the use of suitable alkali activator solutions

and amount of PC renders high strength development of fly ash

and Portland cement geopolymer. The outcome of this study is

thus useful and should lay a ground work for commercial

utilization of high calcium FA as a precursor material with

Portland cement as additive. This geopolymer cured at ambient

curing condition possesses high compressive and shear bond

strengths. These conditions are important for the application in

the real situation.

5. Conclusions

This paper investigates the effects of combinations of sodium

hydroxide and sodium silicate solutions (SH, SHSS, and SS) on

the strength development of fly ash and Portland cement based

Geopolymer Mortar (GM). It can be concluded that the types of

precursors and alkali activator affecting the reaction products and

strength development of geopolymer. Additional Calcium Silicate

Hydrate (CSH) is found in the GMs activated by SHSS solutions.

The CSH gel coexists with the NASH gel, leading to a high

strength GM. The active silica from sodium silicate solution

reacts with calcium from precursors by forming the reaction

products and results in a high strength geopolymer. The increasing

of PC content can help the strength development of GM due to

the formation of additional CSH and other reaction products

within the geopolymer matrix. The use of SH solution gave very

low strength development at ambient temperature condition. The

shear bond strength between CS and GM increases with the

increase in compressive strength of GM. However, for the GM

with SS solution, the optimum PC content for the compressive

strength is at 5% PC replacement solution. The SiO2/Al2O3 and

CaO/SiO2 ratios also controls the strength development of

geopolymer matrix where the maximum compressive and shear

bond strengths are found at SiO2/Al2O3 ratio of 3.4-3.6 and CaO/

SiO2 ratio of 0.72-0.80. The GM made from SHSS solution can

be used as a sustainable repair binder and its application is very

attractive.

Acknowledgements

The authors gratefully acknowledge the financial supported

from Lampang Rajabhat University 2014 budget; the Higher

Education Research Promotion and National Research University

Project of Thailand, Office of the Higher Education Commission,

through the Advanced Functional Materials Cluster of Khon

Kaen University; Khon Kaen University and the Thailand

Research Fund (TRF) under the TRF Senior Research Scholar,

Grant No. RTA5780004. The third author is grateful to the

Thailand Research Fund (TRF) for the financial support under

the TRF New Research Scholar, Grant no. MRG5580222.

The authors also would like to acknowledge the support of the

Program of Civil Technology, Faculty of Industrial Technology,

Lampang Rajabhat University.

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