use of opc to improve setting and early strength properties of low calcium fly ash geopolymer...
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Accepted Manuscript
Use of OPC to improve setting and early strength properties of low calcium fly
ash geopolymer concrete cured at room temperature
Pradip Nath, Prabir Kumar Sarker
PII: S0958-9465(14)00168-1
DOI: http://dx.doi.org/10.1016/j.cemconcomp.2014.08.008
Reference: CECO 2407
To appear in: Cement & Concrete Composites
Received Date: 17 February 2014
Revised Date: 22 July 2014
Accepted Date: 31 August 2014
Please cite this article as: Nath, P., Sarker, P.K., Use of OPC to improve setting and early strength properties of low
calcium fly ash geopolymer concrete cured at room temperature, Cement & Concrete Composites (2014), doi: http://
dx.doi.org/10.1016/j.cemconcomp.2014.08.008
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Use of OPC to improve setting and early strength properties of low calcium fly ash geopolymer concrete cured at room temperature
Pradip Natha,*, Prabir Kumar Sarkerb
aDepartment of Civil Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, Australia.
email: [email protected]
bDepartment of Civil Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, Australia.
email: [email protected]
*Corresponding author: Tel +61 8 9266 7568; Fax +61 8 9266 2681;
email: [email protected]
Abstract
Most previous works on fly ash based geopolymer concrete focused on concretes subjected to
heat curing. Development of geopolymer concrete that can set and harden at normal
temperature will widen its application beyond precast concrete. This paper has focused on a
study of fly ash based geopolymer concrete suitable for ambient curing condition. A small
proportion of ordinary Portland cement (OPC) was added with low calcium fly ash to
accelerate the curing of geopolymer concrete instead of using elevated heat. Samples were
cured in room environment (about 23oC and RH 65±10%) until tested. Inclusion of OPC as
little as 5% of total binder reduced the setting time to acceptable ranges and caused slight
decrease of workability. The early-age compressive strength improved significantly with
higher strength at the age of 28 days. Geopolymer microstructure showed considerable
portion of calcium-rich aluminosilicate gel resulting from the addition of OPC.
Keywords: Ambient curing, fly ash, geopolymer, OPC, setting time, workability.
1 Introduction
Geopolymer is an emerging alternative binder to Portland cement for making concrete.
Geopolymer concrete is principally produced by utilising industrial by-product materials such
as fly ash, blast furnace slag, and other aluminosilicate materials originated from geological
sources such as metakaolin [1, 2]; hence produces low CO2 emission in comparison to
Portland cement concrete [3]. Geopolymer is an inorganic polymer which is synthesized by
activating aluminosilicate materials with alkaline solutions. The polymerisation process
initiates a chemical reaction of aluminosilicate minerals with alkaline activators and results in
a three dimensional polymeric chain. The chemical composition of source materials and
alkaline activators govern the chemical and microstructural properties of the final products of
geopolymerisation [4, 5, 6].
Fly ash is a suitable material for making geopolymeric binder because of its pertinent silica
and alumina composition and low water demand. The amount of currently unused fly ash is
considerable which can be utilized in manufacture of geopolymer products. For example, the
annual generation of ash is estimated to be about 580 million tonnes in China with 67%
utilisation [7] and 200 million tonnes in India with 50% utilization [8]. Accumulation of the
unused ash in these countries is huge and the generation is estimated to increase in future. In
Australia, though it is not as large, the annual generation of ash is about 12 million tonnes
with the unused fly ash accumulation of more than 400 million tonnes in ponds and other
storages [9].
Low-calcium fly ash-based geopolymer concrete cured in high temperature has been reported
to have good mechanical properties in both short and long term tests [10, 11]. The structural
behaviour of heat-cured fly ash geopolymer concrete was found to be similar or superior to
that of members made of OPC concrete when tested for reinforced columns [12, 13] and
beams [14], bonding [15] and fracture properties [16].
Curing conditions have a great influence on the strength and microstructural development of
fly ash based geopolymer. Low-calcium fly ash based geopolymer pastes harden slowly at
ambient temperature and show lower strength in the early ages as compared to heat cured
samples [4]. Hence, these systems are usually subjected to heat curing at temperatures ranging
from 50oC to higher and wide ranges of relative humidity for a curing time ranging from
several hours to several days. While the heat curing process can be met in precast concrete
facilities, it is considered as a challenge for the wide application of fly ash based geopolymer
in normal cast-in-situ concrete applications. Hence, it is essential to develop geopolymer
concrete that can be cured in ambient condition in order to widen its application to the areas
beyond precast concrete members.
In the stride of improving fly ash based geopolymerisation in room temperature, some studies
aimed to enhance the reactivity of fly ash in alkaline environment by increasing the fineness
of fly ash [17] and by incorporating some additional materials such as silica fume, rice husk
ash, metakaolin, blast furnace slag, Portland cement, lime, nano-particles and waste by-
products from different industries [18]. The amount and source of calcium in the fly ash was
found to have significant effect on the properties of the resulting geopolymer both in fresh and
hardened state [19].
The rheology of fresh geopolymer mixture is an important parameter that influences its
practical applications. The rheology primarily depends on the type of precursor
aluminosilicate sources and composition of the activating medium [20, 21]. Metakaolin based
geopolymer pastes have been reported to show different rheological characteristics as
compared to that of cement paste due to the influence of calcium on the inter-particle action
and the reaction kinetics [20]. Fly ash based geopolymer concrete is thixotropic in nature.
Molar strength of the activator and the presence of waterglass modify the rheological
properties [22]. However, no study on the rheology of OPC blended fly ash geopolymers has
been found in literature.
Lohani et al. [23] replaced fly ash in recycled concrete with OPC at levels of 0% to 100%, by
weight and activated with NaOH solution. The results indicated lesser workability, but higher
strength with increasing OPC content. Palomo et al. [24] partially replaced fly ash with
Portland cement clinker at level of 30%, by weight. The composite was activated with either
NaOH solution or waterglass and NaOH solution. The compressive strength results showed
that the highest compressive strength was obtained when a combinations of waterglass and
NaOH was used as the activator. Wang et al. [25] studied the effect of curing temperature and
NaOH addition on the compressive strength of fly ash (FA) blended with cement kiln dust
(CKD) in paste samples. The results indicated that curing temperature was more effective for
50/50 FA/CKD binder on strength improvement than NaOH addition. Addition of calcium
oxide is believed to take part in forming additional binding product such as calcium silicate
hydrate (CSH), along with the aluminosilicate geopolymer gel [6, 26, 27]. However in
another study, Tailby and MacKenzie [28] found lower strengths of the composites containing
equal masses of aluminosilicate geopolymer and the synthetic cement minerals C3S, β-C2S,
C3A and commercial OPC when compared to the geopolymer and OPC paste alone. The final
product revealed no trace of CSH gel in the matrix.
The findings reported in the above studies indicate the likely effects of OPC inclusion on
some mechanical properties of the fly ash based geopolymers. Generally high curing
temperature and the methods of activation played important roles on the properties of
geopolymers. These observations were mainly based on geopolymer paste samples and the
results cannot be directly used for concrete since the inclusion of coarse aggregates has
significant effect on the fresh and hardened properties of concrete. It is also necessary to
improve the early age properties of fly ash based geopolymer concrete cured without applying
elevated heat. This will help reduce the energy and cost associated with the heat curing of
geopolymer concrete. This study aimed to produce low calcium fly ash based geopolymer
concrete suitable for curing without applying elevated heat. Controlled low dosage of ordinary
Portland cement (OPC) was added with low-calcium fly ash to study the fresh mixture
properties and early age compressive strength of geopolymer paste, mortar and concrete
samples.
2 Experimental program
2.1 Materials
A commercially available Class F Fly ash [29] was used as the main source of aluminosilicate
material for making geopolymer paste, mortar and concrete samples. General purpose
ordinary Portland cement (OPC) conforming to Australian standard AS 3972 [30] was used as
an additive to improve properties of the mixture at the early ages. The chemical compositions
and loss on ignition of fly ash and OPC are shown in Table 1. A mixture of sodium hydroxide
(NaOH) and sodium silicate (Na2SiO3) solutions was used as the activator solution. Sodium
hydroxide solution of desired concentration was prepared by mixing 98-99% pure NaOH
pellets with normal tap water. The concentration of sodium hydroxide solution was constant
as 14 Molar for all mixtures. Sodium silicate solution with SiO2 to Na2O mass ratio of 2.64
(SiO2 = 30.1%, Na2O = 11.4% and water = 58.6%) was used. Locally available natural sand
was used as fine aggregate and coarse aggregates were a combination of crushed granite with
nominal maximum sizes of 7, 10 and 20 mm. The aggregates met the Australian Standard
specifications [31].
2.2 Geopolymer paste, mortar and concrete mixtures
The concrete and mortar mixtures were proportioned to investigate the effect of OPC
inclusion in fly ash based geopolymer. Mix variables included the amount of OPC in
replacement of fly ash, the amounts of alkaline activator solution, and the ratio of sodium
silicate to sodium hydroxide solution. Change of these variables corresponds to the variation
of the chemical composition of the geopolymer mixture. The mixture proportions of mortar
and concrete are presented in Tables 2 and 3 respectively. The molar ratios of critical
chemical compounds in the mixture proportions are shown in Table 2. Since the basic mixture
proportion of binder and alkaline activator of mortar and concrete is same, the molar ratios
also remain same for the corresponding mortar and concrete mixtures.
Mortar: The mixture proportions of mortars were designed taking the final unit weight as
2200 kg/m3. The total binder content was constant and constituted one third of the total
mixture. Mixture 1 was designed with fly ash alone as the binder and serves as the control
mixture. Ordinary Portland cement was added as 5%, 8%, 10% and 12% of total binder in the
mixtures 2, 3, 4 and 5 respectively. All of these mixtures were activated with a constant
amount of the alkaline activator as 40% of the total binder. Mixtures 6 and 7 were designed
with a constant percentage of OPC (5%) replacing fly ash while varying alkaline activator
content as 35% and 45% of total binder respectively. The ratio of sodium silicate to sodium
hydroxide was constant as 2.5 in the mixtures 1 through 7. Mixtures 8 and 9 were designed
with varying ratios of alkaline solutions as 2.0 and 1.5 respectively to compare with mixture 4
having a ratio of the alkaline solutions as 2.5. All of these mixtures (4, 8 and 9) had 10% OPC
and 40% activator solutions of the total binder. All of the mixtures in this study were designed
without adding any extra water or superplasticiser.
Concrete: Concrete mixtures (Table 3) were designed for all the mortar mixtures of Table 2
containing 40% alkaline liquid. An alkaline liquid of 40% was selected based on the previous
works on fly ash based geopolymer concrete [32]. Mixture 2 was designed with 6% OPC
instead of 5% OPC in the corresponding mortar mixture. Considering average unit weight of
fly ash based geopolymer concretes reported in previous works and trial mixtures tested for
this study, the final unit weight of the concrete was taken as 2420 kg/m3. The total binder
content was constant as 400 kg/m3. The other mixture variables such as OPC content, total
alkaline solution content and ratio of alkaline solutions remained same as in the corresponding
mortar mixtures.
Paste: Setting time was tested on geopolymer paste mixes having the same proportions of
binder and alkaline activator of the corresponding mortar mixes with the fine aggregate
excluded.
To investigate the microstructure of OPC blended fly ash based geopolymers, two paste mixes
were prepared containing OPC as 10% and 50% of the total binder (OPC and fly ash). The
results were compared with those of a control fly ash geopolymer paste sample. Alkaline
solution with R2.5 was used as 40% of the total binder. Samples were cured in room
temperature (20-23oC) and investigated for general microstructural features after 28 days of
age.
Label: Mixtures have been designated in terms of the variable components of the mixture;
namely percentage of OPC in the total binder (P5, P6, P8, P10 and P12), percentage of
alkaline activator to the total binder (A35, A40 and A45) and the ratio of sodium silicate to
sodium hydroxide solution (SS/SH as R1.5, R2.0 and R2.5).
2.3 Casting and curing of test specimens
The mixing of geopolymer mixtures involves two general steps: preparation of the alkaline
activator solution and final mixing of all ingredients. The alkaline activator solution was
prepared about 20 minutes before final mixing with the other ingredients. The sodium
hydroxide solution and sodium silicate solution of desired quantity were mixed together in the
laboratory and left in a water bath at room temperature to cool down. The exothermic reaction
caused by the mixture of solutions produces heat; hence it requires cooling before adding to
other ingredients. The geopolymer mortars were mixed manually in a laboratory pan to obtain
a uniform mortar mixture. Saturated surface dry (SSD) sand and the binders (fly ash and
OPC) were mixed thoroughly before adding the activator solution. Premixed alkaline
activator solution was then added gradually and mixing was continued for another 4 to 6
minutes until a consistent mixture was obtained. The similar procedure of mortar mixing was
followed for the mixing of concrete. Coarse and fine aggregates, prepared in saturated surface
dry condition, and the binders (fly ash and OPC) were dry-mixed thoroughly in the mixing
pan for two minutes before adding the activator solution. Fresh mortar mixture was cast in
cube moulds (50×50×50 mm3) and concrete mixture was cast in cylindrical concrete moulds
of 100 mm in diameter and 200 mm in height. The moulds were filled in two layers and each
layer was compacted on a vibrating table. They were then stored in a controlled temperature
of 20-23oC and relative humidity 65±10%. Samples were removed from the mould after 24
hours of casting and left in air to cure at 20-23oC and relative humidity 65±10% until tested to
ensure a consistent environment for all samples rather than a variable ambient condition. The
control geopolymer mixture (Mix 1) was de-moulded three days after casting, as it was too
soft to remove from the mould after 24 hours due to its slow setting at this temperature.
2.4 Testing of specimens
Setting times of geopolymer pastes were tested in accordance with ASTM C191-08 [33]. The
test was conducted at a temperature of 21-23oC. The paste was prepared by mixing the
binders and the alkaline solutions manually in a bowl and tested for setting time using a Vicat
apparatus.
The workability of fresh geopolymer concrete mixtures was tested by slump test [34]. Flow of
fresh geopolymer mortars was measured in accordance with ASTM C1437-07 [35]. Slump
and flow tests were conducted immediately after mixing.
Compressive strength test was conducted at 3, 7, 28, 56 and 90 days. Cylinder specimens of
concrete and cube specimens of mortar were tested at a loading rate of 0.33 MPa/s with a
Controls MCC8 machine.
To observe the microstructure of the geopolymer products a scanning electron Microscope
(SEM) (Zeiss EVO 40XVP) was utilized. Elemental composition was observed with energy
dispersive X-ray spectrometer (EDX, Oxford Instruments) fitted with SEM. The sample was
cast and stored in a controlled temperature of 20-23oC and relative humidity 65±10%. After
28 days, the sectioned surface of the geopolymer paste specimen was carbon-coated prior to
microstructural imaging.
For XRD analysis, randomly oriented powder specimens were prepared by grinding small
portions of the dried specimens in a ring mill for 30 seconds (dry milling) and then packed
into a sample holder. XRD patterns from the samples were measured using a Bruker-AXS D8
Advance Diffractometer with copper radiation and a LynxEye position sensitive detector. The
data were collected for 2θ values of 7° to 90° with a nominal step size of 0.015° and a
collection time of 0.7 seconds per step.
3 Results and discussion
Nine mortar and seven concrete mixtures of geopolymer were designed to study the effect of
OPC percentage, activator liquid content and ratio of solutions in the alkaline activator on the
workability, setting time and compressive strength properties of concrete and mortar samples
cured in low ambient temperature. The results were compared for the variation of one
parameter at a time while other parameters remained constant.
3.1 Behaviour of fresh geopolymer mixture
The fresh mixtures of both mortar and concrete generally showed a flowing tendency towards
gravity which gradually depleted as the setting process initiated. The mixtures generally
appeared stiff and cohesive when the liquid content was relatively low. Fly ash geopolymer
pastes activated with NaOH solution shows non-Newtonian flow behaviour like cement
pastes [36]. However addition of waterglass makes the mixture more viscous and affects the
reaction kinetics. In the absence of substantial amount of additional water or superplasticiser,
the mixture of waterglass and sodium hydroxide solutions usually forms a thick and cohesive
paste with fly ash. Similar behaviour was also described by Provis et al [21]. Thus, the
mixture of aggregates with geopolymer paste becomes highly cohesive unlike the Portland
cement concrete mixtures. However the properties vary with the variation of the alkaline
liquid content as discussed later. When OPC is added, the concrete mixture appears stiffer
than that mixed with fly ash alone. Slump and flow values presented in the following sections
show the effects of various parameters on the workability of the mixtures. To improve
workability, extra water or superplasticiser can be added; however this usually affects the
mechanical properties of the hardened concrete. It should be noted that, all the mixtures of
geopolymer concrete and mortar in this study were designed with alkaline solution only as the
liquid component in the mixture. No superplasticiser or additional water was added except
that present in the alkaline solution and in the aggregate.
3.2 Effect of the inclusion of OPC
The mixtures 2, 3, 4 and 5 have increasing amount of OPC and represent the effect of
inclusion of OPC in the fly ash based geopolymer. The results were compared with the
control geopolymer mixture (Mix 1) which has fly ash alone as the binder. Fig. 1 shows the
effect of OPC inclusion on workability, setting time and compressive strength development of
geopolymer mixtures.
From Fig. 1(a), it can be seen that both slump of concrete and flow of mortar were influenced
by the inclusion of OPC in the binder. The control geopolymer mix 1 that contains no OPC in
the mixture showed the highest workability as indicated by higher flow and slump values,
although all the mixtures 1 to 5 were mixed with equal quantity of activator solution. A
general decreasing trend in the slump and flow values with the increase of OPC can be
observed though some scatters are seen in the measured values. It can be seen from the figure
that the reduction in the slump of the concrete mixtures are more pronounced than the
reduction in the flow values of the mortar mixtures.
Fig. 1(b) depicts the influence of OPC content on setting time of geopolymer pastes. The
setting time tests were carried out in a controlled temperature of 21-23oC. In this condition,
geopolymer pastes containing fly ash only as the binder took significantly long time (greater
than 24 hours) before showing any sign of setting due to slow rate of chemical reaction at low
ambient temperature. When OPC was incorporated in the mixture, setting time of geopolymer
pastes improved significantly. Both initial and final setting time decreased to values
comparable to that of OPC paste. The rate of setting accelerated significantly as indicated by
the substantial difference in the initial setting time. It also shows that the higher the OPC
content in the paste, the quicker is the rate of setting. Mixture having 5% OPC in the binder
(P5) achieved initial setting time of 309 minutes, which decreased to 110, 66 and 40 minutes
for inclusion of 8%, 10% and 12% OPC in mixtures P8, P10 and P12 respectively. The results
establish that OPC as a controlled small part of the binary blended binder is effective to
accelerate setting time of fly ash based geopolymer concrete in ambient condition. Thus, the
required setting time for any particular application can be achieved by varying the OPC
content of the binder.
The compressive strength developments of mortar and concrete samples are shown in Fig.
1(c) and 1(d) respectively. In this study, the control mixture (Mix 1) was found to be very
weak to produce a reasonable strength even after three days of casting when cured in low
temperature condition (20-23oC). When OPC was incorporated in the mixture with unaltered
alkaline activator content (40% of total binder) and SS/SH ratio of 2.5, the strength increased
significantly from the early age of 3 days. Both mortar and concrete mixtures indicated
similar compressive strength development over the age up to 90 days. Rate of strength
development slowed down after 28 days, which is similar to the strength development of OPC
concrete. Fig. 2 shows the variation of compressive strength at early age of 3 days and 28
days due to the variation of OPC content. At early age of 3 days, mixtures having OPC
gained significant strength (greater than 13 MPa) as compared to negligible strength of
control mixture (no OPC). The effect of OPC on strength development of both mortar and
concrete is similar. However at 28 days of age, the increase in strength is found to be greater
in the mortar specimens than in the corresponding concrete specimens. After 28 days, mortar
mixtures having 5-12% OPC of total binder achieved up to almost 100% higher strength than
the control mixture. Concrete mixtures having 6-12% OPC of total binder achieved more than
30% higher strength than the control mixture. Strength increased with the increase of OPC
content in both mortar and concrete mixtures. However, the improvement of strength due to
increase of OPC was insignificant as compared to the rapid reduction of setting time of
corresponding mixtures. It can be seen from the figures that addition of OPC as little as 5% of
total binder serves the purpose of achieving reasonable setting time with high early strength of
low calcium fly ash based geopolymer cured in room temperature.
3.3 Effect of the amount of alkaline activator solution
The properties of geopolymer mixture depend on the amount of activator liquid and are
greatly influenced by the addition of water. The effect of the amount of alkaline activator
liquid was investigated by the mortar mixture 6, 2 and 7 having activator liquid as 35%, 40%
and 45% of total binder (A35, A40, A45) respectively. Fig. 3 shows the variation of flow,
setting time and compressive strength due to the variation of activator liquid content in the
mixtures.
Mortar mixtures 6, 2 and 7 were prepared without any additional water or superplasticiser.
The amount of OPC (5% of total binder) and SS/SH ratio (2.5) were constant in the mixtures.
Hence the flow properties presented in Fig. 3(a) are caused principally by the action of
alkaline liquid added in the mixture. The workability measured in terms of flow of mortar
generally indicated the higher consistency of the mixture containing higher liquid content.
However, geopolymer mixture with 35% activator liquid (A35) produced a very stiff mix. On
the other hand, the mixtures with 40% and 45% liquid content (A40 and A45) produced
relatively leaner mixes due to increased liquid content as compared to that of the mixture with
35% liquid. The increase in the flow value by increasing the liquid content from 40% to 45%
is rather small.
Fig. 3(b) shows the effect of activator liquid content on setting time of the mixtures. Mixtures
A35, A40 and A45 showed an increasing trend of both initial and final setting time with an
increasing alkaline liquid content from 35 to 45% of the total binder. The initial setting time
almost doubled when the alkaline liquid content increased from 35% to 40%. No significant
increase of the initial setting time was observed by increasing the liquid content from 40% to
45%. The final setting time was found to increase when the activator liquid was increased
from 35% to 45%.
Variation of the amount of alkaline activator also affected the compressive strength of the
mixtures. Comparing the results shown in Fig. 3(c), it can be seen that increasing the activator
content does not necessarily improve the strength. The 28-day compressive strength of the
mixtures, which all had 5% OPC, gradually decreased with the increase of alkaline liquid
content from 35% up to 45% of total binder. This is because of the higher water to solids ratio
of the mixtures having higher liquid content. It can be seen from Table 2 that the water to
solids ratio of the mixtures A35, A40 and A45 were 0.180, 0.202 and 0.223 respectively.
Thus, strength of the concrete decreased with the increase of the relative amount of water with
respect to solid part of the binder materials.
3.4 Effect of variable SS/SH ratio
The alkaline activator liquid used in this study is a mixture of 14M sodium hydroxide (SH)
and sodium silicate (SS) solutions. The ratio of SS/SH (R) was varied in the mixtures 4
(R2.5), 8 (R2.0) and 9 (R1.5) to alter chemical composition of the activator solution and to
investigate their effect on the properties of concrete. All three mixtures had constant amount
of OPC and alkaline activator liquid as 10% and 40% of total binder respectively. Fig. 4
depicts the effect of different compositions on the workability, setting time and compressive
strength of the concrete and mortar mixtures.
Varying ratio of sodium silicate to sodium hydroxide primarily affected the consistency of the
geopolymer mixture. Since the sodium silicate is the more viscous of the two solutions, the
viscosity of mixture tends to increase with the increase of sodium silicate (ratio R from 1.5 to
2.5). Generally slump of the concrete and flow of the mortar mixtures increased with the
decrease of sodium silicate content as shown in Fig. 4(a).
Increasing the ratio SS/SH in the alkaline solution, while keeping other mix variables
constant, decreased setting time to some extent as shown in Fig. 4(b). Mixture 9 with the least
amount of sodium silicate (R1.5) had longer setting time as compared to those having higher
amount of sodium silicate (R2.0 and R2.5). The initial setting time decreased linearly from
112 minutes for the mixture with R1.5 to 66 minutes for the mixture with R2.5. The final
setting time decreased from 245 minutes for the mixture with R1.5 to 160 minutes for the
mixture with R2.0 with no further decrease for the mixture with R2.5.
The development of compressive strength over time by the mortar and concrete mixtures with
different SS/SH ratios are shown in Fig. 4(c) and 4(d). The mortar and concrete mixtures
showed similar trends of strength gain over time. The strength gain after 28 days was small in
both mortar and concrete mixtures. The 28-day compressive strength of the geopolymer
mortar and concrete mixtures did not vary significantly due to variation of the ratio R in the
range of 1.5 to 2.5. The difference among the compressive strengths at any other age is also
negligible for this range of the SS/SH ratio.
3.5 Microstructural observation
Two geopolymer paste mixes containing 10% and 50% OPC were prepared to study the
microstructure of OPC blended fly ash based geopolymers. Fig. 5 shows the back scattered
SEM images and Fig. 6 shows EDX results of elemental composition of the reaction products
after 28 days of age. The EDX graphs represented in Fig 6 are typical composition of the
corresponding spots marked in Fig. 5.
As seen in the SEM images, the paste having 50% OPC looks more compact and less porous
than that having 10% OPC. There are less partially reacted or unreacted fly ash particles in the
paste with 50% OPC as compared to that with 10% OPC. However it can be seen that though
OPC replaced 50% of fly ash in the mixture, a portion of fly ash particles did not dissolve
completely which is also evident in the XRD analysis. Unreacted or partially reacted OPC
particles are also seen in the matrix and are confirmed by the EDX result shown in Fig. 6(c).
Generally, the OPC particles are shown as irregular shaped particles with a brighter grey
shade. It is rarely seen in the paste with 10% OPC which indicates participation of the cement
particles either in the formation of geopolymer gel or in the introduction of a new gel. The
geopolymer gels produced by the dissolution of low-calcium fly ash generally have negligible
traces of calcium (Fig. 6a) in the reaction products which is primarily sodium alumino-silicate
hydrate (N-A-S-H) [37]. In the paste samples of this study, presence of calcium in the
geopolymeric gel is confirmed as shown in typical EDX diagrams shown in Figs. 6b and 6d.
Inclusion of OPC supplies additional calcium and thus contributes to formation of the binding
product containing calcium ion.
Fig. 7 shows the XRD patterns of the geopolymer pastes at 28 days of age. For all specimens,
diffraction of X-rays apparently resulted in a broad diffuse halo in the value of 2θ ranging
from 20o to 40o. This indicates the presence of large quantity of amorphous or non-crystalline
gels in the fly ash geopolymer [1]. SEM images also showed amorphous features in general.
Peaks due to the crystalline components of Quartz, Mullite and Hematite from the fly ash
were evident in all the pastes. Mixtures having OPC revealed peaks of Alite (tri-calcium
silicate) and CSH (calcium silicate hydrate) in addition to other common fly ash geopolymer
phases. The peaks increased with the increase of OPC content. Anhydrous calcium silicates
(Alite) dominated in the paste with 50% OPC. This can be attributed to the unreacted or
partially reacted OPC particles. However no Portlandite (calcium hydroxide) peaks were seen
in any of the OPC blended mixtures.
The results of EDX and XRD patterns indicate that, OPC blended with fly ash produced a
mixture of gel from OPC and aluminosilicate gel produced from fly ash dissolution. This is
consistent with the finding of Palomo et al [24] which shows the presence of CSH when an
OPC blended fly ash was activated with Waterglass and NaOH solution. The coexistence of
the CSH with geopolymer gel has also been reported for the blending of other calcium bearing
additive with fly ash activated with alkaline solutions [26, 38]. The SEM image shown in Fig.
5(a) also reveals that the two different binder gels blended together neatly and produced a
geopolymer more likely as calcium alumino-silicate hydrate (C-A-S-H), as suggested by
Garcia-Lodeiro et al. [37]. The precipitation of a mix of the gels contributes to the hardening
and early strength gain of this OPC-blended fly ash geopolymer binder.
3.6 Factors influencing the properties of fresh and hardened concrete subjected to curing
at room temperature
Water content is an important parameter that affects workability. Although no extra water was
used, every concrete mix with 40% or higher activator solution had a slump of more than 200
mm and the flow value of mortars varied in the range of 67% to 110%. The increase of OPC
content resulted in stiffer mixtures with reduced mobility and workability. The handling time
also reduced considerably because of the reduced workability. The reduction in the slump and
the flow values are because of the increased resistance of the angular shaped cement particles
as compared to the round shaped fly ash particles as described by Provis et al [21]. Presence
of additional calcium in the OPC can change reaction kinetics. The divalent calcium induces
attractive colloidal interaction forces between particles which increase shear resistance of the
mixture [20, 39].
The increase of activator liquid resulted in an increase of the water to solids ratio (w/s) as
shown in Table 2, which also contributes to the increased flow of the mixture. However the
increased activator caused increase of setting time and reduction of compressive strength.
Besides w/s ratio, another parameter which could be accounted for sufficient workability is
molar ratio of H2O to Na2O. All the mixtures in this study had the H2O/Na2O ratio within the
range of 10-14 as suggested by Hardjito [40] for workable fly ash geopolymer concrete.
However, workability of geopolymer mixtures is also dependent on some other factors such
as moisture content of aggregates, variation of ambient temperature, mixing time and degree
of condensation reaction between binder and alkaline solution.
Inclusion of OPC caused accelerated setting and increased strength in early ages. The
accelerated strength development of OPC blended mixtures is attributed to the reaction of
additional calcium present in the OPC. When mixed with water, Portland cement generates
heat that can help initiate condensation reaction in the fly ash geopolymer mixture at room
temperature [24]. Hence the setting time and hardening process are accelerated considerably
as compared to control fly ash-only geopolymer paste. As seen in the microstructural
observations, the reaction of OPC contributed formation of calcium silicate hydrate (CSH) gel
in addition to the geopolymer gel formed by fly ash. The EDX spectra presented in Figs. 6b
and 6d are indicative of the presence of geopolymer product containing calcium rich alumino-
silicate gel. Hence, the early improvement of strength due to OPC inclusion can be attributed
to the increase of dissoluted binder which produced reaction product from both alkali
activated fly ash and OPC. However it should be noted that, the hydration of OPC alone with
highly alkaline solutions having no waterglass has been shown to modify its normal hydration
process with delay in the formation of the main reaction products [41, 42]. The CSH gel may
decompose in such media. Hence the existence and form of CSH in the geopolymer matrix
may vary with variation of the alkaline media.
The ratio of Si/Al, which determines the relative amount of AlO4 and SiO4 formed in the
geopolymer gel, influences the strength of geopolymer. The molar ratio of Si/Al increases
with the increase of OPC content when other parameters remain constant (Table 2). Hence it
caused an increase of geopolymer precursors to produce greater compressive strength. The
Si/Al ratio can also be altered by varying the composition of alkaline solution. The presence
of soluble silica modifies the reaction kinetics and rate of crystallization by enhancing the
condensation process of the dissoluted geopolymer precursor [43]. As the amount of Si
decreases in the paste, polymerisation process slows down to some extent. Hence, when Si/Al
ratio decreased with the change of solutions’ SS/SH ratio (mixtures R2.5, R2.0 and R1.5) and
cured in room temperature, the setting time increased slightly due to the decrease of soluble
silica in the mixture (Fig. 4b).
Water to solids ratio (w/s) of the mixtures governs the effectiveness of geopolymeric reaction.
Since geopolymerisation reaction gradually release water, any excess water adds in the system
to make the matrix more porous and weaker. Increasing alkaline liquid content caused
reduction of strength while increasing setting time and workability. This is because of higher
water to solids ratio of the mixture having higher liquid content (Table 2). Increasing alkaline
liquid content caused abundance of liquid in the mixture which eventually slowed the
condensation process for geopolymer formation and affected the setting time and associated
compressive strength. Though the ratio of Si/Al increases for higher alkali activator content
with respect to constant total binder, it is the ratio of water to solids (w/s) that affects the
strength. Thus, the total alkaline liquid content in the geopolymer mixture is an important
parameter to ensure sufficient workability and setting time without affecting the design
compressive strength. The results indicate the presence of an optimum level of activator to
achieve desired compressive strength with reasonable workability and setting time. As long as
the replacement of fly ash with OPC remains around 5%, the amount of alkaline activator
solution provided best results in this study when added as 40% of the total binder.
3.7 Optimum blend of OPC in fly ash based geopolymer concrete for ambient curing
While the inclusion of OPC accelerated the setting with significant improvement of early
strength, setting time reduced rapidly with the increase of OPC content. Fig. 8 represents a
relationship of the compressive strength of concrete and mortar with the initial setting time of
corresponding paste samples. It was observed from laboratory trials that the setting time of
concrete is relatively shorter than that of the corresponding paste and handling of the fresh
mixture became increasingly difficult when the OPC content was increased. Hence, a cut-off
line is drawn at 200 minutes assuming this time as an acceptable setting time of paste sample.
The mixes having more than 5% OPC, although achieve relatively higher strengths tend to set
at a faster rate than the mix having 5% OPC. As described in the results, the workability could
be improved by increasing the liquid content; however it would affect the compressive
strength. Further investigation is necessary to invent appropriate superplasticiser to increase
workability without greatly affecting strength of the mixtures blended with higher percentage
of OPC. Therefore, a mixture with 5% OPC content is considered suitable to produce
reasonable early-age strength for ambient cured fly based geopolymer concrete with
reasonable setting time and workability.
4 Conclusions
This study intended to eliminate the necessity of heat curing for producing fly ash based
geopolymer concrete. Fly ash based geopolymer paste, mortar and concrete mixtures were
designed with the addition of ordinary Portland cement up to 12% of the total binder in order
to improve the early age properties in ambient curing condition. The activating liquid was a
mixture of sodium silicate and sodium hydroxide solutions. No extra water or superplasticizer
was added to the mixtures. Test specimens were cured in controlled room temperature and
tested for setting time, workability, compressive strength and microstructural development.
The results of the study are summarised below:
• Presence of OPC accelerated the reaction of geopolymerisation and helped achieve
setting time comparable to that of traditional cement concrete. Increase of OPC in the
fly ash based geopolymer mixture reduces the workability and setting time. However
the workability and setting time increased when alkaline liquid content was increased.
Mixtures having alkaline activator solution with sodium silicate to sodium hydroxide
ratio of 2.5 resulted in reduced setting time and less slump than those with 1.5 and 2.0.
• Inclusion of OPC as little as 5% in the total binder achieved compressive strength more
than 50 MPa for mortar samples and about 40 MPa for concrete samples at 28 days.
Compressive strength decreased when alkaline solution content was increased from
35% to 45% of total binder. Variation of sodium silicate to sodium hydroxide ratio
from 1.5 to 2.5 caused negligible change in compressive strength. Concrete and mortar
samples cured in room temperature developed strength gradually over the age up to 28
days. The strength development after this age was small.
• Microscopic images of OPC blended fly ash based geopolymer revealed mostly
amorphous and calcium-rich hydration product contributed by both the activated fly
ash and OPC. The compactness of the gel increased when OPC content was higher in
the paste.
Thus, fly ash based geopolymer modified with little amount of OPC can be a suitable material
for normal strength concrete production at ambient curing condition. This is a cost and energy
saving alternative as it eliminates the necessity of heat curing. The mixture with fly ash
replacement by OPC as little as 5% of the total binder containing 40% alkaline activator is
considered as the optimum mixture for ambient curing condition with a setting time
comparable to that of OPC concrete.
5 Acknowledgement
The encouragement of Professor Vijay Rangan to conduct studies on geopolymer concrete for
ambient curing is gratefully acknowledged. The authors wish to gratefully acknowledge the
support of Coogee Chemicals regarding supply of the chemicals. The authors also
acknowledge the use of equipment, scientific and technical assistance of the Curtin University
Electron Microscope Facility, which has been partially funded by the University, State and
Commonwealth Governments.
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List of figures
Fig. 1. Effect of different percentages of OPC on the workability of mortars and concretes
(a), setting time of pastes (b), compressive strength of mortars (c) and compressive strength of
concretes (d).
Fig. 2. Variation of compressive strength at 3 days and 28 days due to variation of OPC
content.
Fig. 3. Effect of the amount of alkaline activator solution on the workability of mortars (a),
setting time of paste (b) and compressive strength development of the geopolymer mortars (c)
Fig. 4. Effect of sodium silicate to sodium hydroxide ratio in the alkaline solution on the
workability of mortars and concrete (a), setting time of pastes (b) and compressive strength
development of the geopolymer mortars (c) and concretes (d).
Fig. 5. Back scattered scanning electron microscope (SEM) image of paste having (a) 10%
OPC, (b) 50% OPC and (c) magnified view of section indicated in Fig. 5b; where A = un-
reacted or partially reacted fly ash particles, B = un-reacted or partially reacted OPC particles,
C = calcium-rich aluminosilicate geopolymer gel, D = pure aluminosilicate geopolymer gel
and E = geopolymer gel showing traces of calcium in the paste with 10% OPC.
Fig. 6. Magnified view of paste having 50% OPC (section indicated in Fig. 5b); where A =
un-reacted or partially reacted fly ash particles, B = un-reacted or partially reacted OPC
particles (EDX spectrum b), C = Calcium rich aluminosilicate geopolymer gel (EDX
spectrum c).
Fig. 7. X-ray diffraction patterns of paste specimens having 0% OPC (P00), 10% OPC (P10)
and 50% OPC (P50) in the geopolymer matrix.
Fig. 8. Correlation of setting time of paste and 28-day compressive strength of concrete and
mortar.
(a) (b)
(c) (d)
Fig. 1. Effect of different percentages of OPC on the workability of mortars and concretes (a), setting time of pastes (b), compressive strength of mortars (c) and compressive strength of concretes (d).
190
200
210
220
230
240
250
260
0
20
40
60
80
100
120
P0 P5 P6 P8 P10 P12
Slump (m
m)Fl
ow (
%)
Flow
Slump
0
100
200
300
400
500
600
0 2 4 6 8 10 12 14
Tim
e (m
in)
OPC content (%)
Initial setting timeFinal setting time
0
10
20
30
40
50
60
70
80
0 20 40 60 80 100
Com
pres
sive
str
engt
h (M
Pa)
Age (day)
Mortar
P0 P5P8 P10P12
0
10
20
30
40
50
0 20 40 60 80 100
Com
pres
sive
str
engt
h (M
Pa)
Age (day)
Concrete
P0 P6P8 P10P12
Fig. 2. Variation of compressive strength at 3 days and 28 days due to variation of OPC content.
(a) (b)
(c)
Fig. 3. Effect of the amount of alkaline activator solution on the workability of mortars
(a), setting time of paste (b) and compressive strength development of the geopolymer
mortars (c)
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12 14
Com
pres
sive
str
engt
h (M
Pa)
OPC content (%)
Mortar-3 day Mortar-28 dayConcrete-3 day Concrete-28 day
0
20
40
60
80
100
120
A35 A40 A45
Flow
(%
)
0
100
200
300
400
500
600
30 35 40 45 50
Tim
e (m
in)
Alkaline activator content (%)
Initial setting timeFinal setting time
0
10
20
30
40
50
60
70
80
0 20 40 60 80 100
Com
pres
sive
str
engt
h (M
Pa)
Age (day)
A35
A40
A45
(a) (b)
(c) (d)
Fig. 4. Effect of sodium silicate to sodium hydroxide ratio in the alkaline solution on the
workability of mortars and concrete (a), setting time of pastes (b) and compressive
strength development of the geopolymer mortars (c) and concretes (d).
205
210
215
220
225
0
20
40
60
80
100
120
R2.5 R2.0 R1.5
Slump (m
m)Fl
ow (
%)
Flow
Slump
0
50
100
150
200
250
300
1 1.5 2 2.5 3
Tim
e (m
in)
R = Na2SiO3/NaOH
Initial setting timeFinal setting time
0
10
20
30
40
50
60
70
0 20 40 60 80 100
Com
pres
sive
str
engt
h (M
Pa)
Age (day)
Mortar
R2.5
R2.0
R1.5
0
10
20
30
40
50
60
70
0 20 40 60 80 100
Com
pres
sive
str
engt
h (M
Pa)
Age (day)
Concrete
R2.5
R2.0
R1.5
(a)
(b)
(c)
Fig. 5. Back scattered scanning electron microscope (SEM) image of paste having (a) 10% OPC, (b) 50% OPC and (c) magnified view of section indicated in Fig. 5b; where A
= un-reacted or partially reacted fly ash particles, B = un-reacted or partially reacted OPC particles, C = calcium-rich aluminosilicate geopolymer gel, D = pure
aluminosilicate geopolymer gel and E = geopolymer gel showing traces of calcium in the paste with 10% OPC.
(a) (b)
(c) (d)
Fig. 6. Typical EDX spectrum of (a) geopolymer gel having negligible traces of calcium
in the matrix (at point D in Fig 5), (b) geopolymer gel having traces of calcium in the
10% OPC paste (at point E in Fig 5), (c) un-reacted or partially reacted OPC particles
(at point B in Fig 5) and (d) calcium rich aluminosilicate geopolymer gel in 50% OPC
paste (at point C in Fig 5).
Fig. 7. X-ray diffraction patterns of paste specimens having 0% OPC (P00), 10% OPC
(P10) and 50% OPC (P50) in the geopolymer matrix.
Fig. 8. Correlation of setting time of paste and 28-day compressive strength of concrete
and mortar.
0
100
200
300
400
500
600
0
10
20
30
40
50
60
70
80
P0 P5 P8 P10 P12 A35 A45 R2.0 R1.5
Setting tim
e (min)
28-d
ay c
ompr
essi
ve s
tren
gth
(MPa
) MortarConcreteSetting time
List of tables
Table 1: Chemical compositions of fly ash and OPC
Table 2: Details of geopolymer mortar mix proportions
Table 3: Mix proportions of geopolymer concrete
Table 1: Chemical compositions of fly ash and OPC
Sample SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 P2O5 TiO2 LOIa
Fly ash (%) 53.71 27.20 11.17 1.90 - 0.36 0.54 0.30 0.71 1.62 0.68
OPC (typical composition, %)
21.10 4.70 2.70 63.60 2.60 0.50 - 2.50 - - 2.00 aLoss on ignition
Table 2: Details of geopolymer mortar mix proportions
Mortar mixture quantity, kg/m3 Molar ratio water/ solid Mix
no. Designation Sand Fly ash OPC SSa SHb Na2O/
SiO2 H2O/ Na2O Si/Al
1 P0 1178 730 0 208.6 83.4 0.118 11.730 1.765 0.2
2 A40 / P5 1178 693.5 36.5 208.6 83.4 0.115 11.737 1.975 0.202
3 P8 1178 671.6 58.4 208.6 83.4 0.117 11.730 1.994 0.202
4 P10 / R2.5 1178 657 73 208.6 83.4 0.118 11.725 2.008 0.202
5 P12 1178 642.4 78.6 208.6 83.4 0.120 11.731 2.018 0.204
6 A35 1214.5 693.5 36.5 182.5 73.0 0.103 11.651 1.940 0.180
7 A45 1141.5 693.5 36.5 234.6 93.9 0.126 11.801 2.009 0.223
8 R2.0 1178 657 73 194.7 97.3 0.126 11.228 1.989 0.202
9 R1.5 1178 657 73 175.2 116.8 0.136 10.608 1.961 0.203 a Sodium silicate solution; b Sodium hydroxide solution.
Label: A = percent of alkaline activator solution, P = percent of OPC, and R = the ratio of Na2SiO3 to NaOH solution (SS/SH).
Table 3: Mix proportions of geopolymer concrete
Mix no.
Designation Concrete mixture quantity, kg/m3
CAa Sand Fly ash OPC SSb SHc
1 P0 1209 651 400 0 114.3 45.7
2* P6 1209 651 376 24 114.3 45.7
3 P8 1209 651 368 32 114.3 45.7
4 P10 / R2.5 1209 651 360 40 114.3 45.7
5 P12 1209 651 352 48 114.3 45.7
8 R2.0 1209 651 360 40 106.7 53.3
9 R1.5 1209 651 360 40 96 64 a Coarse aggregate; b Sodium silicate solution; c Sodium hydroxide solution.
* Concrete mixture has OPC content (6%) different from mortar mixture 2. Label: A = percent of alkaline activator solution, P = percent of OPC, and R = the ratio
of Na2SiO3 to NaOH solution (SS/SH).