effect of synthesis parameffters on the compressive strength

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International Journal of Environmental Pollution Control & Management

Vol. 3, No. 1, July-December 2011; pp. 79-88

EFFECT OF SYNTHESIS PARAMETERS ON THE COMPRESSIVE

STRENGTH OF FLY ASH BASED GEOPOLYMER CONCRETE

Aditya Kumar Patra1, Manjeet Chowdhry1 & Basanta Kumar Prusty11 Department of Mining Engineering, Indian Institu te of Technology Kharagpur, Kharagpur-721302, India

Abstract: The paper presents the effect of different synthesis parameters, such as (a) fly ash to alkaline

solution ratio, (b) concentration of NaOH solution, (c) concentration of sodium silicate solution,

(d) geopolymer solids to water ratio and (e) sodium silicate to sodium hydroxide solution ratio, on the

compressive strength of geopolymer concrete synthesized from fly ash of Indian origin. It is observed

that compressive strength of the geopolymer concrete increases with the increase of the above parameters

and reaches at an optimum value above which, the strength of the concrete decreases. In the present

study, the optimum values for fly ash to alkaline solution ratio, concentration of NaOH solution,

concentration of sodium silicate solution, geopolymer solids to water ratio and ratio of sodium silicate

solution to NaOH solution are found to be 60:40, 12 M, 2 M, 2.15 and 2.5 respectively.

Keywords: Geopolymer concrete; Fly ash; Compressive strength; Polymerisation; Curing.

1. INTRODUCTION

Several efforts are in progress to reduce the use of Portland cement in concrete in order to

address the global warming issues. Davidovits (Davidovits, 1988a; Davidovits, 1994) proposed

that an alkaline liquid could be used to react with the silicon (Si) and aluminum (Al) in a sourcematerial of geological origin to produce binders. Because the chemical reaction that takes place

in this case is a polymerization process, he coined the term ‘geopolymer’ to represent these

binders (Davidovits, 1988b). Geopolymers are members of the family of inorganic polymers.

The chemical composition of the geopolymer material is similar to natural zeolitic materials,

but the microstructure is amorphous. The polymerization process involves a substantially fast

chemical reaction under alkaline condition on Si-Al minerals. It results in a three-dimensional

polymeric chain and ring structure consisting of Si-O-Al bonds (Davidovits, 1994; Davidovits

and Davidovics, 1988; Palomo et al., 1992). The concrete made of this geopolymer is called

‘ geopolymer concrete’ . The geopolymer technology shows considerable promise for application

in concrete industry as an alternative binder to the Portland cement (Duxson et al., 2006).

Several Al-Si containing source materials such as fly ash, blast furnace slag, building

residues, and some pure Al-Si minerals and clays (kaolinite and metakaolinite) have been studied

for their use as source materials for geopolymers (van Jaarsveld, 1997; van Jaarsveld, 1998;

van Jaarsveld, 1999). However, research on use of waste materials such as fly ash, blast furnace

slag and mine tailings as source material for geopolymer binder has gained momentum because

of the dual benefits of their gainful utility and reduced environmental consequences (Davidovits,

1988b; Palomo et al., 1992; van Jaarsveld et al., 1997; van Jaarsveld et al., 2002).


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80 / INTERNATIONAL JOURNAL OF ENVIRONMENTAL POLLUTION CONTROL & MANAGEMENT

Low-calcium fly ash (generally less than 10% CaO by mass) has been successfully used to

manufacture geopolymer binder (van Jaarsveld et al., 2002; Davidovits, 1991; van Jaarsveldand van Deventer, 1999; van Jaarsveld et al., 2003). The content of the silicon and aluminum

oxides in such fly ash constitutes about 80% by mass; with the Si-to-Al ratio of about 2 and the

content of the iron oxide usually range from 10 to 20% by mass. The carbon content of the fly

ash, as indicated by the loss on ignition by mass, is sometimes less than 2%. Studies have been

conducted on the reactivity of low-calcium fly ash in geopolymer matrix (Fernandez-Jimenez

and Palomo, 2003; Fernandez-Jimenez et al., 2006).

Coarse and fine aggregates used by the concrete industry are suitable to manufacture

geopolymer concrete (Sofi et al., 2007). A combination of sodium silicate solution and sodium

hydroxide (NaOH) solution has been used as the alkaline liquid (Chindaprasirt et al., 2007;

Rattanasak and Chindaprasirt, 2009). The sodium silicate solution is commercially available in

different grades. The sodium hydroxide with 97-98% purity, in flake or pellet form, is also

commercially available. The solids are dissolved in water to make a solution with the required

concentration. The mass of NaOH solids in a solution varies depending on the concentration of

the solution. Earlier studies indicated that solubility of fly ash depends on concentration of

NaOH and duration of mixing with NaOH (Rattanasak and Chindaprasirt, 2009). The mass of

water is the major component in both the alkaline solutions.

However, not all fly ashes produce geopolymer with the same properties, primarily because

of different chemical composition and physical characteristics of fly ash generated from

combustion of coal of different sources. Product properties are further found to be dependent

on activator composition and activator/fly ash mass ratio (van Jaarsveld and van Deventer,

1999; Rattanasak and Chindaprasirt, 2009; Xu and van Deventer, 2000). The present work

investigates the effect of different synthesis parameters, such as (a) fly ash to alkaline

solution ratio, (b) concentration of NaOH solution, (c) concentration of sodium silicate solution,

(d) geopolymer solids to water ratio and (e) sodium silicate to sodium hydroxide solution ratio,

on the compressive strength of geopolymer concrete synthesized from fly ash of Indian origin.

2. MATERIALS AND METHODS

2.1. Materials

Fly ash used in the present investigation is obtained from the Kolaghat Thermal Power Station,

East Midnapore district, West Bengal, India. The power station is located in close proximity to

the Raniganj coalfield, the oldest coalfield of India. The grade of coal fed to the plant is

sub-bituminous. The chemical composition of fly ash is summarized in Table 1. Due to the

relatively low calcium content (2.36%), this fly ash could be classified as Class F fly ashaccording to ASTM C618 definitions (ASTM C618-92a, 1994).

A combination of sodium silicate and sodium hydroxide solution is used as alkali solution.

Sodium hydroxide solution is prepared from the commercially procured sodium hydroxide

pellets with 98-99% purity. Sodium silicate solution used is neutral with specific gravity of

1.55 g/cc and it contains 7.5-8.5% Na2O and 25-28% SiO

2.


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EFFECT OF SYNTHESIS PARAMETERS ON THE COMPRESSIVE STRENGTH OF FLY ASH BASED… / 81

Table 1

Chemical Composition of fly ash of Kolaghat Thermal Power Station, India

SiO2

Al2O

3Fe

2O

3CaO TiO

2 MgO K

2O Na

2O P

2O

5

56.01% 29.8% 3.58% 2.36% 1.75% 0.30% 0.73% 0.61% 0.44%

In the case of Portland cement concrete, the coarse and fine aggregates occupy about 60 to

75% of the mass of geopolymer concrete. To maintain uniformity of different samples, a fixed

aggregate percentage of 65% by mass is used in this study. The ratio of fine sand: fine aggregate

(< 4mm): coarse aggregate (> 4mm; < 10mm) in the aggregate mixture is 50:35:15.

2.2. Mixing, Casting and Curing

A homogenous solution comprising NaOH and sodium silicate solutions was prepared by mixing

and manual stirring, at least 24 hours before casting the concrete samples. In the laboratory, flyash and the aggregates were first dry mixed manually for about 3 to 4 minutes to produce a

homogenous mixture. The alkaline liquid was then added to the dry materials and manual

mixing was continued for another 6 to 8 minutes to produce homogenous pastes. Pastes thus

produced, were then transferred into 54 x 108 mm cylindrical moulds (diameter: length = 1:2)

and demoulded after 3 days. Samples were tested for uniaxial compressive strength (UCS)

after 28 days of curing at ambient conditions (30-36 oC, > 60% RH). Similar moulding,

demoulding and curing times are reported in earlier studies (van Jaarsveld et al., 2002; van

Jaarsveld et al., 2003; Sofi et al., 2007).

2.3. Compressive Strength Testing

Before testing for compressive strength, cross-sectional ends of each of the concrete sample

were smoothened using the grinding machine. Tests of the UCS were performed using SATECTM

Series Model 3500KN Static Hydraulic Universal Testing System. All samples were tested for

UCS after 28 days of curing, as typical construction applications require a 28-day compressive

strength of 25-40 MPa (Diaz et al., 2010).

3. EXPERIMENTAL PROCEDURE

The proportions of aggregates fly ash and alkaline solution were maintained at 65:22.75:12.75.

In the alkaline solution, the ratio of NaOH solution to sodium silicate solution (by mass) was

kept constant at 1:2. To study the effect of fly ash to alkaline solution ratio on compressive

strength, concentration of NaOH solution and sodium silicate solution were maintained at 8 M

and 2 M respectively and mass of fly ash was varied from 40% to 80%. To study the effect of

concentration of NaOH solution on compressive strength, concentration of sodium silicatesolution was kept constant at 2 M, while concentration of NaOH solution was varied from 6 M

to 16 M. To study the effect of concentration of sodium silicate solution on compressive strength,

concentration of NaOH solution was kept constant at its optimum value 12 M observed during

the experiment, while concentration of sodium silicate solution was varied from 0.5 M to 3 M.

To study the effect of geopolymer solids to water (S/W) ratio on compressive strength, specimens


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were prepared with different concentrations of NaOH and sodium silicate solution as well as

different amount of fly ash, as given in Table 2.

Table 2

UCS of Geopolymer Concrete with Varying Solid to Water (S/W) Ratio

Aggregate = 65%, FA: AS = 65: 35, NaOH Sol. : Sodium Silicate Sol. = 1:2

Sample NaOH Sodium NaOH Sodium FA (g/0.8 Extra Solids/ UCS

Sol. (M) Silicate (g/0.8 kg Sillicate kg of GP) Water water (MPa)

Sol. (M) of GP) (g/0.8 kg (g/ 0.8 kg (g/g)

of GP) of GP)

SW-1 4 0.50 54.0 108 120 0 1.02 2.46

SW-2 5 0.75 48.0 97 135 15 1.24 6.39

SW-3 6 1.00 43.5 87 150 30 1.43 9.86

SW-4 7 1.25 38.5 77 165 40 1.67 15.67SW-5 8 1.50 33.5 67 180 50 1.89 20.79

SW-6 9 1.75 28.5 57 195 60 2.09 32.94

SW-7 10 2.00 23.5 47 210 75 2.15 36.57

To study the effect of sodium silicate to NaOH solution (by weight) ratio on compressive

strength, the ratio was varied from 1 to 3 keeping concentrations of sodium hydroxide and

sodium silicate solutions constant at their optimum values of 12 M and 2 M respectively, as

discussed in sections 4.2 and 4.3.

4. RESULTS AND DISCUSSION

4.1. Effect of Fly Ash to Alkaline Solution Ratio on Compressive Strength

UCS of concrete sample increases when fly ash content increases from 40 to 60% (Figure 1).

Addition of fly ash increases the concentration of Si and Al in the aqueous phase, and thus

enhances the process of Si and/or Si–Al oligomers formation and consequently, the processes

of oligomers polycondensation and the hardening of the geopolymeric system. Decreasing

alkaline solution doesn’t show adverse effect up to this point, as enough concentration of Na

and Si ions are present to carry out polycondensation to satisfactory level. When fly ash content

exceeds 60%, UCS of the concrete sample decreases because of less leaching of Si and Al from

fly ash due to less availability of alkaline solution content in the mixture (Rattanasak and

Chindaprasirt, 2009). When the fly ash content increases to 80%, and therefore alkaline solution

content decreases to 20%, UCS of the sample drops drastically, because the concentrations of

Si and Al in aqueous phase are too low to carry out geopolymerisation to a satisfactory extent.

4.2. Effect of Concentration of NaOH Solution on Compressive Strength

Compressive strength increases as NaOH concentration in the aqueous phase increases from 6

to 12 M and then decreases with further increase of NaOH concentration until 16 M, which is

the highest value of NaOH concentration examined in the present study (Figure 2). The increase


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Figure 1: Effect of Fly Ash to Alkaline Solution Ratio on Compressive Strength

Figure 2: Effect of Concentration of NaOH Solution on Compressive Strength

of NaOH concentration in the aqueous phase of the geopolymeric system results in the increased

leaching of Si and Al ions of fly ash and therefore strength of geopolymer concrete increaseswith the increase of NaOH concentration. However, under extremely high NaOH concentrations,

the oligomeric silicate species such as Si4O

8(OH)

62– and Si

4O

8(OH)

44– lose their stability

in favor of mononuclear silicate species like SiO(OH)3– and SiO2(OH)

2

2– (Bao et al., 2005).


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This minimizes the concentration of oligomeric silicate species in the aqueous phase and thus,

decelerates the process of polycondensation. It results in decrease of compressive strength of the geopolymer materials, as seen in this study for NaOH concentration beyond 12 M. Similar

observations are reported earlier when NaOH concentration was increased from 10 M to 15 M

(Bergna and Roberts, 2006).

4.3. Effect of Concentration of Sodium Silicate Solution on Compressive Strength

The compressive strength of the geopolymer concrete increases almost linearly with the increase

in concentration of sodium silicate solution from 0.5 to 2.0 M. Further increase in concentration

of sodium silicate solution resulted in the decrease of the compressive strength (Figure 3).

Sodium silicate solution provides the aqueous phase of the geopolymeric system with soluble

silicate species. Subsequent polymerization of these monomers results in formation of oligomers.

Polycondensation of silicate and/or aluminosilicate oligomers enhances substantially the

geopolymerisation efficiency (Xu and van Deventer, 2000). However, when concentration of

silicate solution becomes very high, high concentration of cyclic silicate species inhibits further

condensation and therefore the strength of the geopolymer material decreases. Observation in

this study conforms to the earlier study which suggested that at SiO2 /Na

2O > 2, the reactivity,

and therefore the strength of the geopolymer, is likely to reduce (Sindhunata et al., 2006).

Sodium silicate solution is highly viscous and therefore its increase in the composition of

geopolymer increases the viscosity of the geopolymeric paste and causes inefficient paste

workability (Chindaprasirt et al., 2007). During the present study, the paste composed of sodium

silicate solution concentration higher than 2 M lead to specimens with obvious defects and the

specimens failed at lesser loads. This could be another reason for having an optimum limit of

sodium silicate content to get desired strength of geopolymer paste.

Figure 3: Effect of Concentration of Sodium Silicate Solution on Compressive Strength


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4.4. Effect of Geopolymer Solids to Water (S/W) Ratio on Compressive Strength

The compressive strength of geopolymer increases as the solid to water (S/W) ratioincreases from 1.02 to 2.15 g/g. However, it sharply goes down for S/W ratio higher than 2.15

g/g (Figure 4). As the water content in the synthesis decreases, the NaOH concentration in the

aqueous phase increases substantially. The increase of NaOH concentration causes a substantial

acceleration to the dissolution reactions and enhances the process of Si and/or Si–Al oligomers

formation and therefore, the processes of oligomers polycondensation and the hardening of the

geopolymeric system (Xu and van Deventer, 2000, Xu et al., 2001). However, the continuous

decreasing of the water content in the synthesis may cause insufficient wetting of fly ash particles,

affecting negatively the paste workability and making it extremely difficult to mould. In the

present study, the reduced compressive strength obtained for the geopolymer concrete

synthesized with the highest S/W ratio (2.26 g/ml) is attributed exclusively to this reason.

Figure 4: Effect of Geopolymer Solids to Water Ratio on Compressive Strength

4.5. Effect of Sodium Silicate to NaOH Solution (by Weight) Ratio on Compressive

Strength

Figure 5 shows that compressive strength of geopolymer increases with increase in sodium

silicate to NaOH ratio due to higher degree of geopolymerisation and it supports the observations

of earlier studies (Chindaprasirt et al., 2007). However, beyond sodium silicate to NaOH ratio

of 2.5, the compressive strength decreases. At high silicate concentration (SiO 2 /Na2O > 2), thereactivity, and therefore the strength of the geopolymer, is likely to decrease (Bao et al., 2005).

This limit is also determined by the viscosity of the geopolymeric pastes, which increases

substantially under high SiO2 to Na

2O ratios. This affects paste workability and poses difficulties

during their moulding and thus leads to low compressive strength of the geopolymer concrete.

Further, the optimum ratio of 2.5 observed in this study is higher than 0.67 to 1.00 reported in


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earlier study (Chindaprasirt et al., 2007). The fly ash used in this study has SiO2 and Al

2O

3

content of 56.01% and 29.8% respectively, which is about 50% higher than the earlier study

that used fly ash with SiO2 and Al

2O

3 content of 38.7% and 20.8% respectively and reported

optimum values of sodium silicate to NaOH ratio between 0.67 and 1.00. In the present study

high SiO2 and Al

2O

3 content of fly ash resulted in increased concentration of Si and Al in the

aqueous phase and shifted the optimum ratio to higher values. It is also postulated that the

variations in the ratio of sodium silicate to NaOH ratio affects the pH conditions of the

mixture and therefore would have some effects on strength development of the geopolymer

(Chindaprasirt et al., 2007).

5. CONCLUSION

The aim of this study was to develop an understanding of the complex relationship between

different synthesis parameters which affects the compressive strength of geopolymer concrete

with fly ash as one of the constituents. The compressive strength of the geopolymer concrete

generally increases with the composition (concentration) and mixing proportion of different

synthesis parameters for a source material (fly ash). However, beyond an optimum

proportion of these synthesis parameters, the strength of the geopolymer concrete decreases. In

the present study, the optimum values for fly ash to alkaline solution ratio, concentration of

NaOH solution, concentration of sodium silicate solution, geopolymer solids to water ratio and

ratio of sodium silicate solution to NaOH solution are found to be 60:40, 12 M, 2 M, 2.15 and2.5 respectively. The chemistry behind these optimum combinations of ingredients of

geopolymer concrete is discussed. Repetition of similar studies for fly ash of different thermal

power plants of India will lead to establishing relationship between flay ash and other

synthesis parameters of geopolymer binder to yield geopolymer concrete with optimum

compressive strength. This is a necessary step for producing geopolymer in commercial scale

Figure 5: Effect of Sodium Silicate to NaOH Solution Ratio on Compressive Strength


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with reasonably consistent strength and transforming environmentally hazardous fly ash to a

revenue generating commercial commodity.

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effect of synthesis parameffters on the compressive strength

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