EFFECT OF ALKALINE ACTIVATOR ON STRENGTH OF FLY ASH …

EFFECT OF ALKALINE ACTIVATOR ON STRENGTH OF FLY ASH

BASED GEOPOLYMER CONCRETE

Mohammed Karimul Absar Chowdhury1*, Mohammad Mohibul Hasan1 & Md Moinul Islam2

1Graduate, Department of Civil Engineering, CUET, Chittagong, Bangladesh, 2Professor, Department of Civil Engineering, CUET, Chittagong, Bangladesh,

*Corresponding Author <[email protected]>

ABSTRACT

The use of the Portland cement in concrete construction has become a global concern due to high

amount of CO2 emission to environment during its production. Geopolymer concrete technology has

become an area of increasing interest as an innovative and alternative cement technology against

environmental pollution. It has the potential to reduce globally the CO2 emission and lead to a

sustainable development of the concrete industry. Geopolymer concrete is an alternative concrete in

which the traditional binder based on cement is replaced by an alkali activated aluminosilicate material.

Fly ash was chosen as the binder material to partially replace the use of OPC. It is activated by the

geopolymerization process of an alkaline solution of NaOH and Na2SiO3. In this study, fly ash was

added as 0%, 20%, 30%, 40%, 50% of the binder with a variable activator content (35% and 40%).

Compressive strength test of the geopolymer concrete has been performed for both air and water curing

for the curing periods of 7, 28, 56, 90 days. The significant influences of the parameters like ratio of

alkali to fly ash, variable contents of fly ash, curing methods and curing periods on strength properties

of geopolymer concrete have been investigated. It is observed that a geopolymer concrete with NaOH

of molarity 15, Na2SiO3/NaOH ratio of 2.0 and alkali to fly ash ratio of 0.40 showed maximum

compressive strength which indicates that there is potential for the concrete industry to utilize alkaline

activated fly ash as an alternative to Portland cement in structural building applications.

Keywords: Activator; Cement; Compressive strength; Fly ash; Geopolymer concrete.

INTRODUCTION

Concrete is one of the widely used manmade construction materials and its consumption is second only

to water. Portland cement is the primary cementitious ingredient in concrete. It is widely known that the

production of Portland cement consumes considerable energy and at the same time contributes a large

volume of CO2 to the atmosphere. However Portland cement is still the main binder in concrete

construction prompting a search for more environmentally friendly materials. The quantity of CO2

produced due to cement manufacturing contributes to about 5% of the total release of CO2 to the

atmosphere (Malhotra, 1999). If an alternate material other than OPC is used in concrete, the

corresponding CO2 release to the atmosphere can be reduced. In Bangladesh, one of the major sources

of material for power generation is coal and it’s by product fly ash is an environmental threat to the

public, if not disposed off properly. Deposition of the fly ash in storage places can have a negative

influence on water and soil because of its granulose and mineral composition along with morphology

and filtration properties. Therefore the safe disposal of fly ash is still a major concern.

There are various methods to reduce the consumption of cement in concrete, like the partial

replacement of cement with cementitious materials, however a complete replacement is always

preferable. Geopolymer concrete is one such building material, wherein it is formed by the process of

alkali activation of alumino-silicate materials. The most commonly available alumino-silicate material

is fly ash. So, the use of geopolymer concrete with fly ash as alumino-silicate material not only helps to

reduce the release of CO2 emission, but also effectively disposes off fly ash, an industrial waste

produced in large quantities. Fly ash in itself does not possess the binding properties. However in the

presence of water as well as in ambient temperature, fly ash reacts with the calcium hydroxide during

the hydration process of OPC to form the calcium silicate hydrate (C-S-H) gel. This pozzolanic action

happens when fly ash is added to OPC as a partial replacement or as an admixture. The silicon and the

1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7

aluminium in fly ash react with an alkaline liquid that is a combination of sodium silicate and sodium

hydroxide solutions to form the geopolymer paste that binds the aggregates and other un-reacted

materials. The chemical reaction and the rate of strength development of geopolymer concrete are

influenced by several factors based on chemical compositions of the source materials, alkaline

activators and curing condition.

The aim of the research is to evaluate the performance and suitability of fly ash based geopolymer as an

alternative to the use of ordinary Portland cement (OPC) in the production of concrete. As far as

possible, the technology and the equipment currently used to manufacture OPC concrete were used to

make the geopolymer concrete. The effect of salient parameters like variable contents of fly ash along

with alkaline activator on variable curing periods that affects the properties of geopolymer concrete has

been identified in this study.

METHODOLOGY

Materials:

Fly ash was obtained from Premier Cement Mills limited, Chittagong, Bangladesh. The chemical

composition of fly ash as provided by the supplier is presented in the Table 1. In this experiment 0%,

20%, 30%, 40%, 50% of fly ashes are used as partial replacement of binder. These variable contents of

fly ash cause the variations of compressive strength in these different mixtures of concrete.

Table 1: Chemical composition of Fly ash

Parameter Content (% mass)

SiO2 59.7

Al2O3 28.36

Fe2O3 + Fe2O4 4.57

CaO 2.1

Na2O 0.04

MgO 0.83

Mn2O3 0.04

TiO2 1.82

SiO3 0.4

Loss of ignition 1.06

To activate the fly ash, a combination of sodium hydroxide solution and sodium silicate solution was

chosen as the alkaline activator. Sodium-based solutions were chosen because they were cheaper than

Potassium-based solutions. NaOH pellets of 98% purity were purchased from the local market. The

sodium hydroxide (NaOH) solution was prepared by dissolving either the flakes or the pellets in water.

Sodium silicate / sodium hydroxide ratio was taken as 2.0. NaOH solution with a concentration of 15M

consisted 600 grams of NaOH solids (in flake or pellet form) per litre of the solution. The chemical

composition of the sodium silicate solution that provided by the supplier was Na2O=14.7%,

SiO2=29.4% and water 55.9% by mass. Sylhet sand was used as fine aggregate. Aggregates were

provided from local resource, stored outside of the laboratory in storage divisions. Coarse aggregates

with nominal sizes of 12.5 mm were used. Aggregates were prepared in saturated-surface-dry (SSD)

condition and then were sealed in plastic bags about one month before the mixing. For this purpose,

coarse aggregates and sand were soaked separately in clean water, and then distributed on a plastic sheet

until their surface become dry. SSD condition for geopolymer concrete must be prepared to avoid the

absorption of the alkaline solution by the aggregates which reduce the polymerization of the fly ash.

Physical properties of aggregates are given in Table 2.


Table 2: Physical properties of aggregates

Commercially available super plasticizer (Master polyheed) has been used by 2% of the binder by mass

to improve the workability of the mixture

Preparation and casting of test specimens:

The alkali solution was first prepared by thoroughly mixing the NaOH and Na2SiO3 solutions 30 min

prior to its use. Concrete ingredients were mixed in a laboratory pan mixture. Aggregates, prepared in

saturated surface dry condition, cement and different percentage of fly ash were dry mixed thoroughly

in the mixer. Premixed alkaline activator solution was then added gradually in the mixer. Mixing was

continued for further 4-6 minutes. Super plasticizer was separately added to the dry mix and the whole

mixture was mixed together to make geopolymer concrete. All geopolymer concrete specimens were

cast using standard moulds of 100mm X 100mm X 100mm. The geopolymer concrete was compacted

with the help of a tamping rod. The fresh fly ash-based geopolymer concrete was dark in colour and

shiny in appearance. The mixtures were usually very cohesive. Strength development over time can be

achieved with geopolymer concrete when curing time is extended (Hardjito et al., 2008). Here ambient

temperature curing and water curing of the geopolymer concrete moulds were done. The average

temperature experienced ranging from 28°C to 35°C. Different Mixtures were made to study the effect

of various parameters. The mix proportion of the materials designed for the compressive strength of 40

MPa is represented in Table 3.

Table 3: Mix proportions of concrete

Mixture A35

F20

A35

F30

A35

F40

A35

F50

A40

F20

A40

F30

A40

F40

A40

F50

A00

F00

Cement (kg) 9.48 8.3 7.12 5.92 9.48 8.3 7.12 5.92 11.84

Fly Ash (kg) 2.36 3.56 4.74 5.92 2.36 3.56 4.74 5.92 -

SS (kg) 2.78 2.78 2.78 2.78 3.16 3.16 3.16 3.16 -

SH (kg) 1.38 1.38 1.38 1.38 1.58 1.58 1.58 1.58 -

CA (kg) 33.23 33.23 33.23 33.23 33.23 33.23 33.23 33.23 33.23

FA (kg) 23.4 23.4 23.4 23.4 23.4 23.4 23.4 23.4 23.4

Master

Polyheed (L) 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

Water (L) 6 6 6 6 6 6 6 6 6

The compressive tests were carried out of the specimens designed for various mixtures. In total 216 no.

of cubes were tested for 7 days, 28 days, 56 days and 90days of curing periods.

RESULTS AND DISCUSSIONS

Compressive strength is the most common property used to describe a concrete. The effects of various

salient parameters like ratio of activator liquid to fly ash, curing methods and curing periods on the

compressive strength of fly ash-based geopolymer concrete has been discussed in this study. 9 different

mixtures were made to this study. The compressive strength of the specimens cured in both ambient

temperature and water are showed in Table 4. Mixtures are named according to the fly ash content and

percentages of alkaline activators.

Properties Coarse Aggregate Fine Aggregate

Specific Gravity 2.59 2.55

Unit Weight 1560 kg/m3 1580 kg/m3

Fineness Modulus 6.77 2.57

Absorption Capacity 0.60% 1.45%

Moisture Content 0.57% 1.12%


Table 4: Compressive strength of concrete

Mixture

Fly

Ash

(%)

Activator

(%)

Ambient temperature Curing Water Curing

7

days

(MPa)

28

days

(MPa)

56

days

(MPa)

90

days

(MPa)

7

days

(MPa)

28

days

(MPa)

56

days

(MPa)

90

days

(MPa)

A35F20 20

35

27.6 43.5 44.3 47.4 25.7 36.3 37.2 38.4

A35F30 30 26.9 44.9 46.8 49.5 24.2 37.4 38.1 39.1

A35F40 40 25.4 45.6 45.5 48.2 22.8 38.2 38.9 39.9

A35F50 50 24.6 43.5 44.2 46.6 21.5 35.1 36.5 38.7

A40F20 20

40

31.4 45.2 47.1 48.5 27.2 37.2 38.5 39.2

A40F30 30 30.5 47.4 48.9 50.7 25.4 38.5 39.7 40.3

A40F40 40 28.5 46.9 47.9 49.6 24.8 39.8 40.6 41.2

A40F50 50 26.9 44.5 46.2 48.3 23.7 36.9 37.6 40.8

A00F00 0 - 29.5 41.8 42.1 43.7 26.2 34.6 36.1 37.8

Comparison of the compressive strength results among the mixtures containing 35% and 40%

activators cured for 28 days in ambient temperature is given on [Fig. 1]. The effect of the ratios of

activator liquid to binder on strength development of concrete can be investigated by this comparison.

Fig. 1: Compressive strength variation of geopolymer concrete with different activator content

This Figure represents that the compressive strength of the samples are higher for the mixtures

containing 40% alkaline activators than the mixtures containing 30% alkaline activator.

Variation of compressive strength of the samples cured in both ambient temperature and water after 28

days of casting with variable content of fly ash is represented in [Fig. 2] and [Fig. 3].

Fig. 2: Compressive strength variation for Fig. 3: Compressive strength variation for

different curing method with 35% activator. different curing method with 40% activator.

41

42

43

44

45

46

47

48

F20 F30 F40 F50

Co

mp

ress

ive S

tren

gth

(M

Pa

)

Fly Ash %

A35

A40

30

32

34

36

38

40

42

44

46

48

A35F20 A35F30 A35F40 A35F50

Com

press

ive

Str

en

gth

(M

Pa)

Fly Ash %

Air Curing

Water Curing

0

5

10

15

20

25

30

35

40

45

50

A40F20 A40F30 A40F40 A40F50

Com

press

ive

Str

en

gth

(M

Pa)

Fly Ash %

Air Curing

Water Curing


These Figures showed that the compressive strength of the samples with variable activator liquid

contents after curing for 28 days in ambient temperature are greater than of the samples curing under

water. Air curing significantly shows better strength for all types of the mixture.

The compressive strength tests were conducted for 7, 28, 56 and 90 days of curing periods. Comparison

of compressive strength with variable fly ash content for different curing periods has been represented

by [Fig. 4] and [Fig. 5]. Compressive strength of the samples has been increased gradually with the

increase of curing periods. Longer curing time improved the polymerization process resulting in higher

compressive strength. These figures suggest that the mixture cured in ambient temperature containing

30% of fly ash shows better compressive strength gradually up to 90 days than the other mixtures.

Fig. 4: Compressive strength variation for Fig. 5: Compressive strength variation for

different curing periods with 35% activator. different curing periods with 40% activator.

CONCLUSIONS

A systematic experimental study has been conducted to understand the behaviour of geopolymer

concrete. The influence of parameters like variable fly ash content, ratio of alkali to fly ash, ratio of

sodium silicate to sodium hydroxide, curing methods and curing periods on geopolymer concrete has

been investigated. The main aim of this study was to investigate the effect of activator liquid with

variable fly ash content on geopolymer concrete. The following conclusions are drawn from the study:

It is observed that a geopolymer concrete with 15M NaOH, Na2SiO3/NaOH ratio of 2.0 and

alkali to fly ash ratio of 0.40 gives maximum compressive strength.

Higher the ratio of alkaline activator to binders by mass, higher is the compressive strength of

the geopolymer concrete.

Concrete with 40% alkaline activator significantly shows greater strength than that with 35%

alkaline activator.

Ambient temperature curing drastically yields higher compressive strength than water curing

for the geopolymer concrete.

Mixture containing 30% fly ash possesses greater strength than other mixtures.

Longer curing period produces greater compressive strength of fly ash-based geopolymer

concrete.

ACKNOWLEDGMENTS

The authors wish to gratefully acknowledge the help of the laboratory staff of Chittagong University of

Engineering & Technology (CUET). The valuable suggestions and inspiring guidance of honorable

Professor Md. Saiful Islam of CUET are also gratefully acknowledged. The authors also like to show

their special gratitude to Premier Cement, Chittagong for their kind materials help.

20

25

30

35

40

45

50

55

7 days 28 days 56 days 90 days

Com

press

ive

Str

en

gth

(M

Pa)

Curing Periods

A35F20

A35F30

A35F40

A35F50

20

25

30

35

40

45

50

55

7 days 28 days 56 days 90 days

Com

press

ive

Str

en

gth

(M

Pa)

Curing Periods

A40F20

A40F30

A40F40

A40F50


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