ISSN 1946-0198
journal homepage: www.coalcgp-journal.org
Influence of Activator Solution Formulation on Fresh and Hardened Properties of
Low-Calcium Fly Ash Geopolymer Concrete
Carlos Montes1, Erez N. Allouche2,*
1
2
Post Doctoral Fellow, Alternative Binder Research Laboratory, Louisiana Tech University, Ruston, LA 71272, USA
Associate Professor of Civil Engineering and Research Director, Alternative Binder Research Laboratory, Louisiana Tech University, Ruston, LA 71272, USA
ABSTRACT
The effect of the composition of activator solutions on fresh and hardened properties of geopolymer concrete was
investigated. Research variables included liquid sodium silicate product (DH, NH, and StarH), sodium hydroxide molar
concentration (6, 10, and 14), and sodium silicate–to–sodium hydroxide ratio (1, 2, and 3). Response variables were
compressive strength, corrosion resistance expressed as remaining compressive strength and mass loss, and flowability.
Results were analyzed using Minitab statistical software. Findings suggest that activator solution formulation has a
significant effect on the properties of the resulting geopolymer binder. The experimental design used was found effective in
establishing the optimum activator solution formulation for a given fly ash stockpile to be used for an application with
specific performance requirements.
f 2012 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association
All rights reserved.
ARTICLE
INFO
Article history: Received 20 September 2011; Received in revised form 6 January 2012; Accepted 9 January 2012
Keywords: fly ash; geopolymer; activator solution; chemical resistance
1. Introduction
1.1. Background
Inorganic polymer concrete (geopolymer) is an emerging
cementitious material synthesized from pozzolanic materials of
geological origin (e.g., metakaolin) or industrial byproducts (e.g.,
metallurgical slags and fly ash). These inorganic aluminosilicate
polymers are formed via chemical reaction under highly alkaline
conditions between pozzolanic material and an activator solution,
commonly a molar mixture of sodium hydroxide and an alkaline
silicate (Davidovits, 1988).
The polymerization process involves a rapid reaction of silicoaluminate minerals in the source material with the alkali metal
hydroxide/silicate activator solution, resulting in the formation of
* Corresponding author. Tel.: 1-257-2852. E-mail: [email protected]
a 3D polymeric chain/network structure of Si-O-Al-O bonds. The
two main ingredients used in making geopolymer are alkaline
liquids and source materials rich in silica and alumina, such as
kaolinite, fly ash, and others. Commonly used alkaline liquids
include sodium hydroxide (NaOH) or potassium hydroxide (KOH)
in combination with sodium silicate. When geopolymers are
blended together with aggregates, the resulting mixture can be
handled and cast in the same manner as Portland cement–based
concretes (Diaz et al., 2010).
Because the reaction mechanism of geopolymer is polycondensation rather than hydration, as in the case of Portland
cement, it can be aided by heat. Hardjito et al. (2003) have
shown that the hardening of geopolymers can take place at
temperatures ranging between 25uC and 90uC, depending on the
raw materials used and the molar concentrations of the activator
solution. The curing rate should be carefully controlled to avoid
an accelerated loss of moisture that could lead to the
propagation of microcracks.
doi: 10.4177/CCGP-D-11-00017.1
f 2012 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association. All rights reserved.
2
Montes and Allouche / Coal Combustion and Gasification Products 4 (2012)
Table 1
Chemical composition of fly ash stockpile
Table 2
Mineralogical composition of fly ash
Oxide
Class F fly ash, wt%
Minerals
Weight, %
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
LOI
Na2O
K2O
Total
SiO2/Al2O3
SiO2 + Al2O3
50.25
22.56
20.0
2.1
0.00
0.50
2.48
0.00
0.00
97.89
2.23
72.81
Quartz
Mullite
Amorphous
10.33
25.27
64.4
1.2. Properties
Geopolymers can be utilized as a partial or total replacement for
Portland cement. By comparison to ordinary Portland concrete (OPC),
geopolymer concrete (GPC) offers high resistance to acid and sulfate
attack, high compressive strength, and rapid strength gain rate and
undergoes little shrinkage (Miranda et al., 2005). van Jaarsveld et al.
(2002) reported that calcium content and origin of the fly ash
precursor influence the properties of geopolymers. Desirable properties of the fly ash precursor include an amount of unburned material
lower than 5%, Fe2O3 content not higher than 10%, 40–50% reactive
silica content, 80–90% particles with sizes less than 45 mm, and a high
content of vitreous phase (Fernández-Jiménez and Palomo, 2003).
Diaz et al. (2010) demonstrated that the presence of calcium in fly ash
in significant quantities could interfere with the polymerization
setting rate. Chindaprasirt et al. (2007) reported that flowability of
geopolymer was dependant on the ratio by mass of sodium silicate to
sodium hydroxide and on the concentration of NaOH. Concentration
variations from 10 to 20 M were found to have little effect on strength
of the resulting hardened paste.
In terms of mechanical strength, geopolymers exhibit excellent
strength gain rate, superior even to that of rapid-setting cements,
and their significant maximum strength can be achieved in 3–
5 days depending on the curing effort. Corrosion resistance and
durability of geopolymer binders present another advantage.
Because most of their strength is not based on calcium aluminates,
Fig. 1. Particle size distribution of fly ash stockpile.
which are susceptible to sulfate attack, these materials are
practically inert to sulfate-induced corrosion (Wallah and Rangan,
2006). Song et al. (2005) suggested that class F fly ash–based
geopolymer concrete subjected to a 10% sulfuric acid solution for
8 weeks has a mass loss of only 3% and a reduction of compressive
strength of only 35%. The dense microstructure of GPCs and their
ability to neutralize chloride ions enable GPC to provide a high
level of protection to reinforcement rebar against chloride attack
and other corrosion-inducing species (Kupwade-Patil et al., 2011).
The geopolymeric net is an alkaline silicate net; therefore, these
cements are largely inert to an alkali-aggregate reaction, a
relatively common occurrence in Portland cements (at varying
degrees) (Kupwade-Patil and Allouche, in press). Montes and
Allouche (2012) demonstrated that utilization at low calcium ash
can greatly enhance the chemical resistance of the geopolymer
matrix, retaining more than 90% of the mechanical strength after
8 weeks of exposure to sulfuric acid with pH 0.6.
2. Present Work—An Overview
The present work examines the effect of three variables—type
sodium silicate product, sodium hydroxide molar concentration,
and ratio of sodium silicate solution to sodium hydroxide solution
(by mass)—on the mechanical properties and corrosion resistance
of the resulting geopolymer binder. Response variables include
compressive strength, corrosion resistance (measured in terms of
remaining compressive strength and mass loss), and flowability. A
33 design of experiments was created, and the results were
analyzed with Minitab (2011) software.
3. Materials and Methodology
Class F fly ash from a thermal power plant in Miami Fort, FL,
was used in this study. The three liquid silicate products used in
Montes and Allouche / Coal Combustion and Gasification Products 4 (2012)
Table 3
Characteristics of liquid sodium silicates
3
Table 4
Experimental setup in Minitab
Type of
NaOH
Na2O
(% by weight)
SiO2
(% by weight)
SiO2/Na2O
Viscosity
(cPoise)
D
N
Star
14.7
8.9
10.6
29.4
29.7
26.5
2.00
3.22
2.5
400
180
60
this study were obtained from PQ Corporation (Malvern, PA).
Sodium hydroxide (Na2O) 99% pure in flakes was obtained from
Baddley Chemicals (Baton Rouge, LA). The sand utilized met
specifications set by ASTM (2009) C777. The chemical composition and particle size distribution of the fly ash are given in
Table 1 and Figure 1, respectively. Mineralogical composition of
the fly ash is given in Table 2. Chemical composition and other
characteristics of the sodium silicates used in this study are
summarized in Table 3. All sodium hydroxide molar solutions
were prepared in the laboratory and allowed to cool for 1 day
before mixing.
An experimental design was created using Minitab software
for the three research variables, with three levels for each
variable (hence, a 33 design). The test variables were type of
silicate (DH, NH, and StarH), hydroxide molarity (6, 10, and 14),
and sodium silicate–to–sodium hydroxide (Na2SiO3/NaOH) ratio
(1, 2, and 3). Three cubes (replicates) were made for each
combination. The result was 27 experiments with three
repetitions each. Each cube was considered a repetition. Four
response variables were selected for this study: compressive
strength, remaining compressive strength, mass loss, and
flowability. The main effects of each research variable, as well
as the interactions among them, were evaluated. Table 4 shows
the experimental design of this study.
Fig. 2. Main effects plot for compressive strength.
NaOH solutions of three different molarities were prepared
with the use of tap water. Next, NaOH solutions were mixed with
the respective sodium silicate to prepare the alkaline solution. The precursor (fly ash) and sand were mixed in a 1:1 ratio
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Montes and Allouche / Coal Combustion and Gasification Products 4 (2012)
Fig. 3. Interaction plot for compressive strength.
before being added to the activator solution. The fresh paste
was cast into 50 3 50 3 50-mm cubical molds in two layers
following ASTM (2011) C109. After casting, the specimens
were placed in an oven and cured at 60uC for 24 hours. The
specimens to be used for the chemical tests were left at room
temperature for 6 days before immersion in the corresponding
acid solution.
Fig. 4. Main effects plot for remaining compressive strength.
Compressive strength was measured after a 24-hour curing
period, following ASTM (2011) C109. To measure corrosion
resistance, the remaining compressive strength after soaking the
specimens in a sulfuric acid solution of 0.6 pH for 8 weeks was
evaluated according to ASTM (2006) C267. Mass loss was also
evaluated for the same period of time. Flowability was evaluated for
the fresh paste immediately after mixing, as per ASTM (2007) C1437.
Montes and Allouche / Coal Combustion and Gasification Products 4 (2012)
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Fig. 5. Interaction plot for remaining compressive strength.
4. Results and Discussion
Experimental measurements were expressed graphically to
display the effect of different levels of each design factor on the
response variables (i.e., main plot), as well as the interaction among
Fig. 6. Main effects plot for mass loss.
the factors with respect to their effect on the response variables
(i.e., interaction plot). An interaction plot graphs the averages of
the output variable for each level of the factor, with the level of the
second factor held constant to reveal the presence of interactions
and interdependencies. Parallel lines in an interaction plot indicate
6
Fig. 7. Interaction plot for mass loss.
Fig. 8. Main effects plot for flow.
Montes and Allouche / Coal Combustion and Gasification Products 4 (2012)
Montes and Allouche / Coal Combustion and Gasification Products 4 (2012)
7
Fig. 9. Interaction plot for flow.
no interaction between the two variables (i.e., factors are
independent), whereas departure from the parallel state suggests
that an interaction exits between the variables under consideration.
The greater the departure from the parallel state the greater the
codependency between the variables.
4.1. Compressive strength
Figure 2 reveals that the use of silicate D results in a significant
increase in compressive strength of the hardened matrix.
Furthermore, the compressive strength tends to increase as the
molarity of NaOH increases. Additionally, it can be seen that a
lower sodium silicate–to–sodium hydroxide (SS/SH) ratio tends
to yield higher compressive strengths. This observation is in
agreement with prior observations that the extent of geopolymerization tends to decrease with increasing soluble silicon content in
the activation solution at a given Na2O/H2O ratio. This is attributed
to reduction in pH and increase in solution viscosity, leading to
reduction in mechanical strength (Duxson et al., 2007).
From Figure 3 (upper left quadrant), it can be seen that an
increase in NaOH concentration increases the compressive strength
regardless of the silicate type used. In the plot of silicate type vs.
SS/SH ratio, it can be seen that the SS/SH ratio affects the silicate N
and Star in opposite manners. Whereas silicate N exhibits
decreased compressive strength with increased ratio, compressive
strength for the Star silicate specimens increases as the ratio
increases. For silicate D, a ratio of 1:1 seems to give the best results.
In summary, all variables and interactions were found to be
significant, with the exception of the interaction between NaOH
concentration and the SS/SH ratio, which appears to have little effect
on the mechanical strength of the resulting geopolymer matrix.
4.2. Remaining compressive strength
Figure 4 reveals that activator solution formulation consisting of
silicate D, a hydroxide concentration of 14 M, and a SS/SH ratio of 3
yields the best outcome in terms of corrosion resistance. Examination of
Figure 5 suggests that when using silicate D or Star, the concentration of
NaOH is significant; however, when using silicate N, NaOH concentrations of 10 and 14 M yield similar results. Also, the SS/SH ratio affects
the three silicates in different ways. For silicate D, a slight increase in
corrosion resistance was observed as the SS/SH ratio increased. However,
for silicate N, a significant difference was observed with the use of a SS/
SH ratio of 2 or 3, compared with s SS/SH ratio of 1. The ratio affects
silicate Star in an opposite manner compared with the other silicates (e.g.,
a ratio of 3 produces lower values of remaining compressive strength).
From the plot of molar concentration vs. the SS/SH ratio, it can be seen
that for an NaOH concentration of 14 M, the SS/SH ratio plays a lesser
role compared with the use of a lower concentration.
4.3. Mass loss
Both silicate type and hydroxide concentration were found to be
significant. In Figure 6, it can be seen that Star silicate produces
the highest mass loss and D silicate the least mass loss. In term of
hydroxide molarity, the use of a 14 M hydroxide solution yielded
the least mass loss. These observations are attributed to lower
density achieved by using the Star silicate and 6 or 10 M hydroxide
solutions, with excess water evaporation forming voids in the
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Montes and Allouche / Coal Combustion and Gasification Products 4 (2012)
mass. Hence, higher porosity results in pathways via which
chemical reagents can reach and interact with greater surface area
of the geopolymer matrix, resulting in greater degradation.
From the interaction plot (Figure 7), it can be concluded that if
the silicate that produces the least mass loss (silicate D) is chosen,
an NaOH molarity of 14 and a SS/SH ratio of 3 should be selected
to provide the highest level of corrosion resistance.
4.4. Flowability
In Figure 8, it can be seen that silicate D has the largest effect on
flow, whereas N and Star exhibited more moderate effects that
were similar in magnitude. NaOH concentration seems to have a
linear effect on flowability of the fresh mix, with higher
concentrations producing lower flow values. The SS/SH ratio
seems to have the same linear effect as the NaOH concentration,
but with a shallower slope.
In Figure 9, it can be noted that a higher concentration of NaOH
results in lower flowability regardless of the type of silicate used.
With respect to the interaction between silicate type and SS/SH ratio,
the effect of the SS/SH ratio is more pronounced for silicate D
compared with silicates N or Star. Also, the effect of the SS/SH ratio
seems to be more significant for an NaOH molar concentration of 6.
5. Summary
The findings of this study suggest that silicate D produces higher
compressive strength on average. No statistical difference was
observed for compressive strength values obtained for silicates N
and Star. A tendency to obtain higher compressive strength values
with higher hydroxide concentrations was also observed. The SS/
SH ratio was found to be the least significant variable of the three,
but the lower ratio (1) had the tendency to give slightly higher
compressive strength values, especially when using silicate D. A
possible explanation for this is that a higher concentration of
soluble silicon hinders the skeletal density of the gel, leading to
lower strength (Duxson et al., 2005). For silicate type vs. SS/SH
ratio (Figure 3), it was observed that when silicates D and N were
used, lower SS/SH ratios tended to produce higher compressive
strengths. However, an opposite trend was obtained when using
Star silicate. It can be concluded that Star silicate requires
significantly more activation. The interaction between hydroxide
molarity and SS/SH ratio was found to be nonsignificant in the
context of compressive strength.
All three variables studied—silicate type, hydroxide type, and
SS/SH ratio—were found to have a significant effect on the
remaining compressive strength (see Figure 4). Star silicate was
found to produce different results compared with other silicate
types. A molar concentration of 14 produced the best results;
however, a concentration of 10 M also yielded acceptable values
for silicate D. From the interaction plots (Figure 5) it can be
concluded that NaOH molar concentration affects the three
silicates differently, with greater influence observed for the D
and Star silicates. No meaningful change in performance using a
concentration of 10 or 14 M was observed for the case of silicate N.
The optimal SS/SH ratio was 2 for silicates D and Star. For a molar
concentration of 14, the resulting compressive strength for a given
silicate was found to be nearly independent of the SS/SH ratio.
As for mass loss, two response variables were found to be
significant: silicate type and hydroxide concentration. Silicate D
and 14 M NaOH produced the least mass loss with an SS/SH ratio
of either 2 or 3. In general, the water content in the solution,
defined as the mass of water contained in the sodium silicate and
the sodium hydroxide solutions, is directly related to the mass loss
of the specimens. However, it was shown that water content within
the range used in this work does not have a significant effect on the
remaining compressive strength of the specimens. From this, it can
be inferred that corrosion resistance of the geopolymer binder is
controlled, at least partially, by variables not considered in the
study (i.e., CaO content).
All variables were found to have a significant effect on
workability (see Figure 8). From the contrasts, we could see that
silicate D produces the lowest flowability values, whereas no
significant difference was observed between silicates N and Star.
Higher hydroxide concentrations and higher SS/SH ratios resulted
in lower flow values. Figure 9 suggests that the SS/SH ratio has a
larger influence on flowability in the case of silicate D compared
with the other silicates. The hydroxide concentration vs. SS/SH
ratio plot suggests that the effect of the SS/SH ratio is more
pronounced when using a lower NaOH concentration (6 M) compared with 10 or 14 M.
Overall, optimal characteristics of the activator solution for a
given fly ash depends on the nature of the application at hand
because it offers a trade-off among mechanical strength, corrosion
resistance, flowability, and, of course, cost. Thus, the activator
solution needs to be optimized for each application on the basis of
pre-established governing performance requirements. In this
article, we presented an approach for establishing such an
optimization.
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