Influence of Activator Solution Formulation on Fresh and Hardened Properties of Low-Calcium Fly

I S S N 1 9 4 6 - 0 1 9 8

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Influence of Activator Solution Formulation on Fresh and Hardened Properties ofLow-Calcium Fly Ash Geopolymer Concrete

Carlos Montes1, Erez N. Allouche2,*1 Post Doctoral Fellow, Alternative Binder Research Laboratory, Louisiana Tech University, Ruston, LA 71272, USA2 Associate Professor of Civil Engineering and Research Director, Alternative Binder Research Laboratory, Louisiana Tech University, Ruston, LA 71272, USA

A B S T R A C T

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.

A R T I C L E I N F O

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 silico-

aluminate minerals in the source material with the alkali metal

hydroxide/silicate activator solution, resulting in the formation of

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 poly-

condensation 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.* Corresponding author. Tel.: 1-257-2852. E-mail: [email protected]

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.

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 proper-

ties 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 (Fernandez-Jimenez 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,

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

Table 1

Chemical composition of fly ash stockpile

Oxide Class F fly ash, wt%

SiO2 50.25

Al2O3 22.56

Fe2O3 20.0

CaO 2.1

MgO 0.00

SO3 0.50

LOI 2.48

Na2O 0.00

K2O 0.00

Total 97.89

SiO2/Al2O3 2.23

SiO2 + Al2O3 72.81

Table 2

Mineralogical composition of fly ash

Minerals Weight, %

Quartz 10.33

Mullite 25.27

Amorphous 64.4

Fig. 1. Particle size distribution of fly ash stockpile.

2 Montes and Allouche / Coal Combustion and Gasification Products 4 (2012)

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 composi-

tion 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.

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 solu-

tion. The precursor (fly ash) and sand were mixed in a 1:1 ratio

Fig. 2. Main effects plot for compressive strength.

Table 3

Characteristics of liquid sodium silicates

Type of

NaOH

Na2O

(% by weight)

SiO2

(% by weight) SiO2/Na2O

Viscosity

(cPoise)

D 14.7 29.4 2.00 400

N 8.9 29.7 3.22 180

Star 10.6 26.5 2.5 60

Table 4

Experimental setup in Minitab

Montes and Allouche / Coal Combustion and Gasification Products 4 (2012) 3

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.

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.

Fig. 3. Interaction plot for compressive strength.

Fig. 4. Main effects 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

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

Fig. 5. Interaction plot for remaining compressive strength.

Fig. 6. Main effects plot for mass loss.


Fig. 7. Interaction plot for mass loss.

Fig. 8. Main effects 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 geopolymer-

ization 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 concentra-

tions 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

Fig. 9. Interaction plot for flow.


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) com-

pared 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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Influence of Activator Solution Formulation on Fresh and Hardened Properties of Low-Calcium Fly

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