Civil Engineering Dimension, Vol. 6, No.

2, 8893, September 2004

ISSN 1410-9530

FACTORS INFLUENCING THE COMPRESSIVE STRENGTH OF

FLY ASH-BASED GEOPOLYMER CONCRETE
Djwantoro Hardjito, Steenie E. Wallah, Dody M.J. Sumajouw, and B.V. Rangan
Faculty of Engineering and Computing, Curtin University of Technology
GPO Box U 1987, Perth 6845, Australia
e-mail: hardjitd@vesta.curtin.edu.au

ABSTRACT
This paper describes the effects of several factors on the properties of fly ash based geopolymer
concrete, especially the compressive strength. The test variables included were the age of concrete,
curing time, curing temperature, quantity of superplasticizer, the rest period prior to curing, and the
water content of the mix.
The test results show that the compressive strength of geopolymer concrete does not vary with age,
and curing the concrete specimens at higher temperature and longer curing period will result in
higher compressive strength. Furthermore, the commercially available Naphthalene-based
superplasticizer improves the workability of fresh geopolymer concrete. The start of curing of
geopolymer concrete at elevated temperatures can be delayed at least up to 60 minutes without
significant effect on the compressive strength. The test data also show that the water content in the
concrete mix plays an important role.
Keywords: Compressive strength; Fly ash; Geopolymer concrete.

INTRODUCTION
The global cement industry contributes around
1.35 billion tons of the greenhouse gas emissions
annually, or about 7% of the total man-made
greenhouse gas emissions to the earths
atmosphere [1,2]. Therefore, one of the most
challenging issues faced by the concrete
industries in the future is the impact of cement
production on the environment.
McCaffrey [2] suggested three alternatives to
reduce the amount of carbon dioxide (CO2)
emissions from the cement industries, i.e. to
decrease the amount of calcined material in
cement, to decrease the amount of cement in
concrete, and to decrease the number of
buildings using cement. Likewise, Mehta [3]
also proposed two stages in the production of
environmentally friendly concrete. A short-term
effort, also known as industrial ecology, is an
attempt to use fewer natural resources, utilise
less energy, and minimise the carbon dioxide
emissions. The long-term view is to reduce the
impact of unwanted industrial by-products by
lowering the rate of material consumption.
Note: Discussion is expected before November, 1st 2004.
The proper discussion will be published in Dimensi Teknik
Sipil volume 7, number 1, March 2005.

The development of geopolymer concrete is an

important step towards the production of
environmentally friendly concretes. Geopolymer
is an inorganic alumino-silicate compound,
synthesized from materials of geological origin
or from by-product materials such as fly ash,
rice husk ash, etc., that are rich in silicon and
aluminium [4]. The geopolymer concrete is
produced by totally replacing the Ordinary
Portland Cement (OPC). Therefore, the use of
geopolymer technology not only substantially
reduces the CO2 emissions by the cement
industries, but also utilises the waste materials
such as fly ash. It is to be noted that fly ash, one
of the possible sources for making geopolymer
binders, is available abundantly world wide, and
yet its usage to date is very limited [1,5,6].
Consumption of fly ash in the manufacture of
geopolymers is an important strategy in making
concrete more environmentally friendly. For this
reason, fly ash has been chosen as a base
material for this project in order to better utilise
this industrial waste.
As a relatively new material, the nature of fresh
state of geopolymer concrete and its effect on the
properties in the hardened state are yet to be
studied. The fresh geopolymer concrete has a
stiff consistency and high viscosity [7,8]. This
paper presents research data on the effect of

Civil Engineering Dimension

ISSN 1410-9530 print 2007 Thomson GaleTM
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D. Hardjito, et al. / Factors Influencing The Compressive Strength / CED, Vol. 6, No. 2, 8893, September 2004

several parameters on the compressive strength

of geopolymer concrete.

PREVIOUS RESEARCH
The chemical composition of geopolymer is
similar to zeolite, but amorphous in microstructure [4]. The silicon and the aluminium
atoms in the source materials are induced by
alkaline solutions to dissolve and form a gel.
The polymerisation process may be assisted by
applied heat, and followed by drying. The
geopolymer gel binds the loose coarse aggregates, fine aggregates and other un-reacted
materials together to form the geopolymer
concrete. The chemical reaction period is
substantially fast.
Davidovits [9,10] claims that the Egyptian
Pyramids were built by casting geopolymer on
site. He also reported that this material has
excellent mechanical properties, does not
dissolve in acidic solutions, and does not
generate any deleterious alkali-aggregate
reaction even in the presence of high alkalinity
[4]. Some of the immediate applications of
geopolymer concrete are marine structures, precast concrete products such as railway sleepers,
sewer pipes, pre-fabricated units for the housing
market etc., as well as waste containment or
encapsulation.
Only limited research data on geopolymer
concrete are available in the literature. Earlier
work by the authors [11,12] reported the
manufacturing process and the effect of various
parameters such as curing temperature, curing
time, sodium silicate-to-sodium hydroxide ratio,
sodium hydroxide-to-free water ratio and the
age of concrete on the compressive strength of
geopolymer concrete.

EXPERIMENTAL WORK
Materials
In the experimental work, class F-fly ash
obtained from Collie Power Station, Western
Australia, was used as the base material. Table
1 shows the chemical composition of the fly ash,
as determined by X-Ray Fluorescence (XRF)
analysis.

Table 1. Composition of fly ash as determined

by XRF (mass %)
SiO2 Al2O3 Fe2O3 CaO Na2O K2O TiO2 MgO P2O5 SO3 LOI*)
53.36 26.49 10.86 1.34 0.37 0.80 1.47 0.77 1.43 1.70 1.39
*) Loss on ignition

Analytical grade sodium hydroxide in flake form

(NaOH with 98% purity), and sodium silicate
solution (Na2O = 14.7%, SiO2 = 29.4% and water
= 55.9% by mass), were used as the alkaline
activators. In order to avoid the effect of
unknown contaminants in the mixing water, the
sodium hydroxide flake was dissolved in
distilled water and the activator liquid was
prepared at least one day prior to its use. To
improve the workability of fresh concrete, a
commercially
available
naphthalene-based
superplasticizer was used. Four types of locally
available aggregates, i.e. 20 mm aggregate, 14
mm aggregate, 7 mm aggregate, and fine sand,
in saturated surface dry condition were mixed
together. The grading of this combined
aggregate had a fineness modulus (FM) of 5.0.
Manufacture and Test of Specimens
The aggregates and the fly ash were mixed dry
in a pan mixer for 3 minutes. The alkaline
solutions and the superplasticizer were mixed
together, then added to the solid particles in the
mixer, and mixed for another 3 to 5 minutes.
The fresh concrete had a stiff consistency and
was glossy in appearance. The mixture was cast
in 100x200 mm cylinder steel moulds in three
layers. Each layer received 60 manual strokes
and vibrated for 10 seconds on a vibrating table.
Five cylinders were prepared for each test
variable.
Immediately after casting, the samples were
covered by a film, and left in room temperature
for 30-60 minutes. The specimens were then
cured in an oven at a specified temperature for a
period of time in accordance with the test
variables selected. The aim of covering the
samples was to reduce the loss of water due to
evaporation during curing at an elevated
temperature.
At the end of the curing period, the 100x200 mm
test cylinders were removed from the oven, and
kept in the moulds for six hours in order to
avoid drastic change of the environment. The
specimens were then removed from the moulds,
left to air dry at room temperature until loaded
in compression at the specified age in a
universal test machine. The loading rate and
other procedures used were in accordance with

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D. Hardjito, et al. / Factors Influencing The Compressive Strength / CED, Vol. 6, No. 2, 8893, September 2004

TEST RESULTS
Table 2. Detail of Solutions and Curing of
Specimens
8M
2.5
24 hours
60oC

th

Concentration of NaOH solution (Molarity)

Sodium silicate/NaOH solution by mass
Curing time
Curing temperature

The details of the solutions used in the mix, and

the curing condition are given in Table 2,
otherwise it will be stated specifically. The
activator liquids-to-fly ash ratio by mass was
kept constant approximately at 0.35. The coarse
and fine aggregates constituted about 77
percent by mass in the mixes.
Compressive Strength at Different Ages
Figure 1 shows the effect of age of concrete on
the compressive strength. Because the chemical
reaction of the geopolymer gel is due to
substantially fast polymerisation process, the
compressive strength does not vary with the age
of concrete. This observation is in contrast to the
well-known behaviour of OPC concrete, which
undergoes hydration process and hence gains
strength over the time.

80

Compressive strength (MPa)

70
60
50
40
30
20
10
0
20

40

60

80

Age (days)

Figure 1. Compressive Strength at Different Ages

90

80
60
40
20
0
0

20

40

60

80

100

120

Curing time (hrs)

In this paper, the effects of various parameters

on the compressive strength of geopolymer
concrete are reported. Each of the test data
points plotted in various graphs corresponds to
the mean value of the compressive strengths of
five test cylinders in a series. The standard
deviations were plotted on the test data points
as the error bar.

100

7 day Comp. strength (MPa)

the details specified in the relevant Australian

Standard for making and testing OPC concrete.

100

Figure 2. Influence of Curing Time on Compressive

Strength
Effect of Curing Time
Figure 2 shows the influence of curing time on
the compressive strength. Longer curing time
improves the polymerisation process resulting in
higher compressive strength. Davidovits [4]
noted that when geopolymer is made using
geological materials such as special metakaolin
called KANDOXI as the source material, curing
at a lower temperature for a shorter period of
time is sufficient to achieve satisfactory results.
The results shown in Figure 2 indicate that
longer curing time does not produce weaker
material as claimed by van Jaarsveld et al [13].
However, the increase in strength after curing
for 48 hours is not significant.
Effect of Superplasticizer
In order to study the effect of superplasticizer,
the other test parameters such as mix
composition, curing period, curing time etc. were
kept constant. The superplasticizer was added
in proportion to the fly ash in the mix by mass.
The cylinders were tested in compression on the
7th-day after casting.
In the fresh state, the geopolymer concrete has a
stiff consistency. Although adequate compaction
was achievable, an improvement in the workability was considered as desirable. Tests were
therefore performed to study the effect of adding
commercially
available
naphthalene-based
superplasticizer. The results of these tests are
shown in Figure 3. The addition of
superplasticizer improved the workability of the
fresh concrete but had very little effect on the
compressive strength up to about two percent of
this admixture to the mass of fly ash. Beyond
this value, there is some degradation of the
compressive strength.

D. Hardjito, et al. / Factors Influencing The Compressive Strength / CED, Vol. 6, No. 2, 8893, September 2004

2 NaOH Na2O + H2O

(1)

Table 3. Basic Mix Used in Water Content

Series Tests

70
60
50
40
30
20
10
0

Concentration of NaOH solution (Molarity)

Sodium silicate/NaOH solution by mass
Curing time
Curing temperature
Rest for 60'
No Rest

14 M
2.5
24 hours
30, 45, 75, 90oC

Cured at 60 C
for 24 hrs

% of Superplasticiser (by mass of fly ash)

Figure 3. Effect of superplasticizer on compressive

strength
Figure 3 shows two sets of data. In one set, the
test cylinders were allowed to rest for 60
minutes after casting and then placed in the
oven for curing at 60oC for 24 hours. In the other
set, there was no rest period and the test
cylinders were placed in the oven immediately
after casting.
The results plotted in Figure 3 show that there
is very little difference between the strengths of
the two sets of specimens. This is an important
outcome in practical applications of geopolymer
concrete. For instance, when geopolymer
concrete is used in precast concrete industry,
the results in Figure 1 indicate that there is
sufficient time available between casting of
products and sending them to the curing room.
Effect of Water Content in the Mix
Previous research by Barbosa et al. [14] on
geopolymer pastes showed that the water
content in the mix played an important role on
the properties of geopolymer binders. In order to
study the effect of water content on the
compressive strength of geopolymer concrete,
several tests were performed. The details of the
basic mix used in this series of tests are given in
Table 3. The other details of the mixes were the
same as those used in the earlier part of this
paper. The percentage of the superplasticizer to
the mass of fly ash was 1.5%, the delay time was
30 minutes, and there was no rest period. In
order to quantify the water content in the
geopolymer concrete mix, the ratio of water
(H2O)-to-sodium oxide (Na2O) was calculated in
terms of molar ratio of the oxides. Note that
both H2O and Na2O are identified in both the
activator liquids used in this study. That is, the
sodium silicate is composed of H2O and Na2O.
Also, the sodium hydroxide flake (NaOH), which
was dissolved in water, can be expressed as.

In addition, the fly ash also contained a small

trace of Na2O (see Table 1). For a given
geopolymer mixture, the moles of H2O and Na2O
from sodium silicate solution, sodium hydroxide
solution, and fly ash can therefore be summed
together and hence the molar ratio of H2O-toNa2O can be calculated. For the basic mixture
given in Table 3, this ratio was calculated as
10.0.
In order to vary the H2O-to-Na2O molar ratio,
water was added to the basic mixture (Table 3)
to yield two other values of molar ratio of H2Oto-Na2O. By adding extra water of 10.6 kg/m3,
the molar ratio of H2O-to-Na2O became 11.25,
and by adding extra water of 21.2 kg/m3, this
ratio was 12.50. The 7-day compressive
strengths of geopolymer concrete cylinders
produced from the basic mixture and the two
other mixtures as described above, are plotted in
Figure 4 for different curing temperatures.
Compressive Strength at 7 days (MPa)

Compressive Strength at 7 days

(MPa)

Effect of Rest Period Prior to Curing

80
70
60
50
40

90oC
75oC
45oC
30oC

30
20
10
0
9.00

10.00

11.00

12.00

13.00

H 2O/Na 2O

Figure 4. Effect of the molar H2O-to-Na2O ratio on

Compressive Strength
As to be expected, the addition of water
improved the workability of the mixtures. The
results shown in Figure 4 clearly demonstrate
the effect of the molar ratio of H2O-to-Na2O on
the compressive strength of geopolymer
concrete. The trends of these test results are
similar to those observed by Barbosa et al [14]
for their tests on geopolymer pastes. The results
shown in Figure 4 also confirm that an increase
in the curing temperature increases the concrete
compressive strength. However, increasing the
curing temperature from 75oC to 90oC did not

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D. Hardjito, et al. / Factors Influencing The Compressive Strength / CED, Vol. 6, No. 2, 8893, September 2004

Compressive Strength at 7 days (MPa)

show any significant gain in compressive

strength.
80
70
60
50
40

90oC
75oC
45oC
30oC

30
20
10
0
0.150

0.170

0.190

0.210

0.230

Water/Geopolymer Solids (by mass)

Figure 5. Effect of the Water-to-Geopolymer Solids ratio

on Compressive Strength
The effect of water content is also illustrated in
Figure 5 by plotting the compressive strength
versus water-to-geopolymer solids ratio by mass.
For a given geopolymer concrete, the total mass
of water in the mixture is taken as the sum of
the mass of water in the sodium silicate
solution, the mass of water in the sodium
hydroxide solution, and the mass of extra water,
if any added to the mixture. The mass of
geopolymer solids is the sum of the mass of fly
ash, the mass of sodium hydroxide flake, and
the mass of sodium silicate solids (the mass of
Na2O and SiO2 in sodium silicate solution).
The test data shown in Figure 5 demonstrate
that the compressive strength of geopolymer
concrete decreases as the ratio of water-togeopolymer solids by mass increases. The test
trends shown in Figure 5 are somewhat
analogous to the well-known effect of water-tocement ratio on the compressive strength of
OPC concrete, although the chemical processes
involved in the formation of the binders of both
these types of concretes are entirely different.

superplasticizer can be utilised to improve

the workability of the fresh geopolymer
concrete without resulting in any segregation
and degradation in the compressive strength
(Figure 3) up to 2% of this admixture by
mass of fly ash.
d. There is very little difference in compressive
strengths of specimens cured immediately
after casting and those sent to curing 60
minutes after casting (Figure 3).
e. Water content plays an important role in
determining the compressive strength of
geopolymer concrete as well as the workability of the fresh concrete (Figsure 4 & 5).
f. An increase in the curing temperature
increases the concrete compressive strength,
especially up to 75oC (Figs. 4 and 5).

ACKNOWLEDGEMENTS
The authors are grateful to Dr. Terry Gourley
and Mr. Chris Busck for introducing them to the
fascinating topic of Geopolymers and for their
advice and encouragement. The first and second
authors are recipients of the Australian
Development Scholarships. The third author is
supported by the TPSDP - Asian Development
Bank.

REFERENCES
1.

Malhotra, V.M., Introduction: Sustainable

Development and Concrete Technology, ACI
Concrete International, 2002. 24(7): pp. 22.

2.

McCaffrey, R., Climate Change and the

Cement Industry, Global Cement and Lime
Magazine, (Environmental Special Issue),
2002, pp. 15-19.

3.

Mehta, P.K., Greening of the Concrete

Industry for Sustainable Development, ACI
Concrete International, 2002. 24(7): pp. 2328.

4.

Davidovits, J., Chemistry of Geopolymeric

Systems, Terminology, Geopolymer '99
International Conference. France. 1999.

5.

Malhotra, V.M., Making Concrete Greener

With Fly Ash, ACI Concrete International,
1999. 21(5): pp. 61-66.

6.

Malhotra, V.M., High-Performance HighVolume Fly Ash Concrete, ACI Concrete

International, 2002. 24(7): pp. 1-5.

CONCLUSIONS
Several series of tests on geopolymer concrete
were performed. Based on the experimental
results reported in the paper, the following
conclusions are drawn:
a. The compressive strength of geopolymer
concrete does not vary with the age of
concrete (Figure 1).
b. Longer
curing
time
improves
the
polymerisation process resulting in higher
compressive strength (Figure 2).
c. Commercially available Naphthalene-based

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7.

Hardjito, D., et al., Properties of Geopolymer Concrete with Fly Ash as Its Source
Material, Concrete in The Third Millenium,
The 21st Biennial Conference of The
Concrete Institute of Australia, Brisbane,
Queensland, Australia. 2003.

8.

Teixeira-Pinto, A., Fernandes P., and Jalali

S.. Geopolymer Manufacture and Application - Main problems When Using Concrete Technology, Geopolymers 2002 International Conference. Melbourne, Australia:
Siloxo Pty. Ltd. 2002.

9.

Davidovits, J., Ancient and Modern Concretes: What is the real difference ?, ACI
Concrete International, 1987. 9(12): pp. 2329.

10. Davidovits, J., They Have Built the Pyramids (in French), Paris: Jean-Cyrille
Godefroy. 2002.
11. Hardjito, D., Wallah S.E., and Rangan B.V.,
Study on Engineering Properties of Fly
Ash-Based Geopolymer Concrete, Journal
of the Australasian Ceramic Society, 2002.
38(1): pp. 44-47.
12. Hardjito, D., Wallah S.E., and Rangan B.V.,
The Engineering Properties of Geopolymer
Concrete, Concrete in Australia, 2002.
28(4): pp. 24-29.
13. van Jaarsveld, J.G.S., van Deventer J.S.J.,
and Lukey G.C., The Effect of Composition
and Temperature on the Properties of Fly
Ash and Kaolinite-based Geopolymers,
Chemical Engineering Journal, 2002. 89(13): pp. 63-73.
14. Barbosa, V.F.F., MacKenzie K.J.D., and
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