The relationship of NaOH Molarity, Na2SiO3/NaOH Ratio, Fly Ash/Alkaline Activator Ratio, and Curing Temperature to the Strength of

Fly Ash-Based Geopolymer

M.M.A. Abdullah1, a, H. Kamarudin1, b, H. Mohammed2, c, I. Khairul Nizar3, d,

A. R.Rafiza1, e, and Y. Zarina1, f 1Green Concrete@UniMAP, School of Material Engineering,Universiti Malaysia Perlis (UniMAP),

01000, P.O. Box 77, D/A PejabatPosBesar,Kangar, Perlis, Malaysia

2King Abdul Aziz City Science & Technology (KACST), P.O. Box 6086, Riyadh 11442, Kingdom of Saudi Arabia

3School of Environmental Engineering,Universiti Malaysia Perlis (UniMAP), 01000, P.O. Box 77, D/A PejabatPosBesar,Kangar, Perlis, Malaysia

amustafa_albakri@unimap.edu.my, bvc@unimap.edu.my, cbnhusain@kacst.edu.sa, dnizar@unimap.edu.my, erafiza86@gmail.com, fzarina.yahya@gmail.com

Keywords: Geopolymer, NaOH Molarity, Fly Ash/Alkaline Activator Ratio, Na2SiO3/NaOH Ratio, Curing Temperature, Compressive Strength

Abstract. Geopolymer, produced by the reaction of fly ash with an alkaline activator (mixture of

Na2SiO3 and NaOH solutions), is an alternative to the use of ordinary Portland cement (OPC) in the

construction industry. However, there are salient parameters that affecting the compressive strength

of geopolymer. In this research, the effects of various NaOH molarities, Na2SiO3/NaOH ratios, fly

ash/alkaline activator, and curing temperature to the strength of geopolymer paste fly ash were

studied. Tests were carried out on 50 x 50 x 50 mm cube geopolymer specimens. Compression tests

were conducted on the seventh day of testing for all samples. The test results revealed that a 12 M

NaOH solution produced the highest compressive strength for the geopolymer. The combination

mass ratios of fly ash/alkaline activator and Na2SiO3/NaOH of 2.0 and 2.5, respectively, produced

the highest compressive strength after seven days. Geopolymer samples cured at 60 C produced

compressive strength as high as 70 MPa.

Introduction

Fly ash has been used to replace cement in the concrete industry for several years because it

contributes beneficial properties to concrete [1], especially with respect to its high compressive

strength compared to cement. The environmental issues associated with the production of OPC are

well known [2,3] with the main issue being the emission of carbon dioxide to the atmosphere. The

geopolymer technology developed by Davidovits offers an attractive solution regarding this issue

[4,5]. A combination of sodium silicate (Na2SiO3) or potassium silicate (K2SiO3) and sodium

hydroxide (NaOH) or potassium hydroxide (KOH) has been used extensively as the alkaline

activator to be added to fly ash to form geopolymer [2, 8-11].

The effect of NaOH molarity may also play an important role in producing high-strength

geopolymer. The molarities of KOH used in this process can range from 5 M to 10 M for the

activation of natural minerals [10]. Dali Bondar et al. [11] stated that highest compressive strength

can be achieved when the molarity of the KOH is between 5 and 7.5 M. However, the molarity of

the NaOH solution should be in the range of 8 to 16 M [8]. It should be noted that compressive

strength increases as the molarity of the NaOH used increases from 8 to 16 M [12]. Puertas et al.

[13] studied the use of equal parts of fly ash and slag activated with 10 M NaOH and showed that

the product material had a compressive strength of approximately 50 MPa. Rattanasak et al. [14]

concluded that a geopolymer-mortar strength of up to 70 MPa could be obtained when the mixture

is formulated with 10 M NaOH. Palomo et al. [7] reported that a 12-M activator concentration leads

to better results than an 18-M concentration.

Advanced Materials Research Vols. 328-330 (2011) pp 1475-1482Online available since 2011/Sep/02 at www.scientific.net (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.328-330.1475

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Another factors that influence the compressive strength of geopolymer are the fly ash/alkaline

activator ratio and Na2SiO3/NaOH ratio. Some researchers [2,7,15] have stated that geopolymer fly

ash with a fly ash/alkaline activator ratio of 3.3-4.0 can be used. However, Palomo et al. [7] stated

that the fly ash/alkaline activator ratio was not a relevant parameter that influenced compressive

strength, but his conclusion is contrary to the conclusions other researchers have reached on this

matter. Rattanasak et al. [14] concluded that the use of a Na2SiO3/NaOH ratio of 1.0 produced a

product with a compressive strength as high as 70 MPa. A study conducted by D. Hardjito [16]

showed that the use of a Na2SiO3/NaOH ratio of 2.5 gave the highest compressive strength, whereas

a ratio of 0.4 resulted in lower compressive strength. A. Sathonawaphak et al. [17] stated that

geopolymers produced with fly ash/alkaline activator ratios in the range of 1.4-2.3 showed high

compressive strengths, ranging from 42 to 52 MPa. Their study results indicated that the optimum

Na2SiO3/NaOH ratio was 1.5 which gives high compressive strength.

Curing conditions also have a significant effect on the development of mechanical strength in

most cementitious systems. J. Temuujin et al. [18] stated that using a curing temperature between

40 C and 100 C for 4-48 hours is one of the important conditions for the synthesis of geopolymer.

Curing at room temperature has been conducted successfully by using calcined source material of

pure geological origin, such as metakaolin [2]. D.Hardjito et al. [19] mentioned that a higher curing

temperature does not necessarily ensure that the compressive strength of the product will be higher.

Increasing the curing temperature beyond 60 C did not increase compressive strength substantially

[16].

The purpose of this study is to determine the effect of NaOH molarity, fly ash/alkaline activator

ratio, Na2SiO3/NaOH ratio, and curing temperature on the compressive strength of geopolymer

paste.

Materials and Experimental Details

Raw Materials

In this research, coal fly ash obtained from the Manjung power station in Lumut, Perak, Malaysia,

which was equivalent to ASTM Class F fly ash, was used as the base material to make the

geopolymer. The alkaline activator used in this study was a combination of sodium silicate

(Na2SiO3) and NaOH. The NaOH was in pellet form with 97% purity [8,20,21], and the Na2SiO3

consisted of 9.4% Na2O, 30.1% SiO2, and 60.5% H2O (with a SiO2/Na2O weight ratio of 3.20-3.30

and a specific gravity of 1.4 at 20 C.

Test Variables

The constituents used to prepare the geopolymer pastes were 6 M, 8 M, 10 M, 12 M, 14 M, and 16

M NaOH [8] with a constant fly ash/alkaline activator ratio and Na2SiO3/NaOH ratio of 2.5 [16].

After found out the best NaOH molarity which gives highest compressive strength, then, further

work was done to determine the best combination of various fly ash/alkaline activator ratio and

Na2SiO3/NaOH ratio at a constant NaOH concentration of 12 M.

Then, with the constant NaOH concentration of 12 M, a fly ash/alkaline activator ratio of 2.0,

and a Na2SiO3/NaOH ratio of 2.5, the curing temperatures used were room temperature, 40 C, 50

C, 60 C, 70 C, and 80 C [18]. In this manner, the properties of the geopolymers produced under

several different conditions were studied.

Preparation of Solution

The NaOH solutions were prepared by dissolving NaOH pellets in one liter of distilled water in a

volumetric flask for six different NaOH concentrations (6 M, 8 M, 10 M, 12 M, 14 M, and 16 M).

An alkaline activator with a combination of NaOH and Na2SiO3 was prepared just before mixing

with fly ash to ensure the reactivity of the solution.

1476 Mechatronics and Materials Processing I

Mixing Process

a) Various molarities of NaOH

The fly ash and alkaline activator were mixed until a homogeneous paste was achieved. This mixing

process was continued for approximately 10 minutes for all mixtures. The total mass of each

material used was kept constant for the geopolymer paste. The pastes were cured at 70 C for 24

hours based on the procedures used by D. Hardjito et al. [19]. Then, the sample mixtures were

maintained at room temperature until they were tested. All of the samples (with different molarities

of 6 M, 8 M, 10 M, 12 M, 14 M, and 16 M) are shown in Table 1.

Table 1: Proportions of constituents used to prepare the geopolymers

Items Design

Fly ash/alkaline activator ratio 2.5

Na2SiO3/NaOH ratio 2.5

Mass of fly ash (g) 335

Mass of NaOH (g) 40

Mass of Na2SiO3 (g) 95

b) Fly ash/alkaline activator and Na2SiO3/NaOH ratios

Based on past research [2,7,15,17], it was decided to use fly ash/alkaline activator ratios of 0.5, 1.0,

1.5, 2.0, 2.5, and 3.0 in this study. However, we were unable to use the ratios of 0.5 and 1.0 because

the geopolymer pastes had such high workability requirements that they were hard to handle. Also,

the ratio of 3.0 could not be used because the paste has such a low workability. Thus, our study

focused on ratios of 1.5, 2.0., and 2.5. Then, Na2SiO3/NaOH ratios of 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0

[14,16,17] were analysed in this study. The details of the proportions of the mixtures are shown in

Table 2. The curing temperature was kept constant at 70 C for 24 hours and left at room

temperature for seven days before testing [19].

c) Various curing temperatures

After finding the best molarity of NaOH, fly ash/alkaline activator ratio, and Na2SiO3/NaOH ratio,

further analyses were conducted to evaluate various curing temperatures with a constant NaOH

concentration of 12 M, a fly ash/alkaline activator ratio of 2.0, and a Na2SiO3/NaOH ratio of 2.5,

conditions that had resulted in the highest compressive strength before. The masses of fly ash,

Na2SiO3, and NaOH were kept constant at 565 g, 200 g, and 80 g, respectively, for each curing

temperature. The fresh geopolymer paste was cast in 50 x 50 x 50 mm cubic moulds for all cases.

After casting the cubes, they were cured (in the moulds) at room temperature, 40C-80C for 24

hours [18]. Then all cubes were removed from the moulds and maintained at room temperature for

rest of the required curing time (seven days).

Testing

Compressive-strength tests were performed on the geopolymer paste samples in accordance with

BS 1881-116:1983 using an Automatic Max mechanical testing machine (Instron 5569, USA) to

obtain the ultimate strength of the geopolymers. The samples were subjected to a load of 50.00 kN,

and the rate of loading was 5.00 mm/min. The reported compression-strength values were an

average of the results obtained for the three samples produced for each ratio.

Advanced Materials Research Vols. 328-330 1477

Table 2: Mix design details for various ratios of fly ash/alkaline activator and Na2SiO3/NaOH

Fly ash/Alkaline

Activator Ratio

Na2SiO3/NaOH

Ratio

Fly Ash

(g)

Na2SiO3

(g)

NaOH

(g)

1.5

0.5

505

115 225

1.0 170 170

1.5 205 135

2.0 225 115

2.5 240 95

3.0 255 85

2.0

0.5

565

95 190

1.0 140 140

1.5 170 115

2.0 190 95

2.5 200 80

3.0 210 70

2.5

0.5

605

80 160

1.0 120 120

1.5 145 95

2.0 160 80

2.5 170 70

3.0 180 60

Results and Discussion

a) Various molarities of NaOH

The compressive strength of geopolymer pastes prepared with various NaOH molarities are

presented in Figure 1, which shows that the 12-M NaOH solution had the highest compressive

strength when the tests were conducted seven days after the samples were prepared. The highest

compressive strength produced was 68.48 MPa. This is due to the increase of Na ions in the system,

which was important for the geopolymerization since Na ions were used to balance the charges and

formed the alumino-silicate networks as the binder in the mixture [17].

From Figure 1, after the 12-M NaOH solution, decreases in compressive strength can be

observed. This result is in agreement with the results of Palomo et al.s study [7], which also found

that a 12-M NaOH solution produced better results than the 18-M NaOH solution. However,

Hardjito et al. [16] found that increasing NaOH molarity increased the compressive strength of the

geopolymers. Alonso and Palomo [22] reported in their study that, when the activator concentration

was above 10-M NaOH solution, a lower rate of polymer formation was produced due to the high

concentration of NaOH, resulting in a decrease in the strength. This decrease might be due to the

differences in the types of source materials used, i.e., they used metakaolin, and we used fly ash.

1478 Mechatronics and Materials Processing I

Figure 1: Compressive strength of various NaOH molarities

b) Fly ash/alkaline activator and Na2SiO3/NaOH ratios

The results of compressive strength for different fly ash/alkaline activator ratios and Na2SiO3/NaOH

ratios are shown in Figure 2. There are no specific patterns in the graph, but the highest

compressive strengths (up to 70.27 MPa) were observed at a fly ash/alkaline activator ratio of 2.0

and a Na2SiO3/NaOH ratio of 2.5 on the seventh day.

D. Hardjito and R. Sathia et al. [16,23] stated that compressive strength increases as fly ash

content and concentration of the activator solution increase. This is due to the increase in the

sodium oxide content, which is mainly required for the geopolymerisation reaction. The

compressive strength of the product for a Na2SiO3/NaOH ratio of 3.0 was low, which could be due

to the excess OH- concentration in the mixtures [19]. Furthermore, the excess sodium content can

form sodium carbonate by atmospheric carbonation, and this may disrupt the polymerization

process [24]. In a study conducted by P. Chindaprasirt [12], it was concluded that the optimum

Na2SiO3/NaOH ratio was in the range of 0.67 to 1.00 for maximum compressive strength, which is

quite different from our finding that the optimum ratio is 2.5. This might be due to the variation in

the ratio of Na2SiO3/NaOH, which affects the pH conditions and thus would have an effect on the

development of the strength of the geopolymer [25].

Figure 2: Compressive strength of various proportions of reactants

c) Various curing temperatures

From Figure 3, it is obvious that the compressive strengths of the geopolymers cured at room

temperature were lower than those cured at higher temperature on day 7 when the tests were

conducted. The highest compressive strength of 71.04 MPa was observed at 60 C for the

geopolymer paste. It was surprising that the highest compressive strength occurred at a curing

temperature of 60 C rather than 80 C, since there is strong agreement among some researchers

that higher curing temperatures result in higher compressive strength [2,16]. Even so, other

researchers have recommended a curing temperature of 60 C for manufacturing fly ash and

kaolinite geopolymers [8,15].

Advanced Materials Research Vols. 328-330 1479

However, the results obtained by D. Hardjito et al. [19] indicated that geopolymer mortar cured

at temperatures above 70 C resulted in a decrease in the compressive strength for 24 hours of

curing at 28 days of testing. It was observed that the higher curing temperature does not ensure

higher compressive strength. These results show that curing temperature plays an important role in

the geopolymerisation process of fly ash-based geopolymer mortar.

In this study, it was obvious that the compressive strengths of geopolymers cured at 70 C were

significantly lower than the strengths of those cured at 60 C, as shown in Figure 3. It should be

noted that higher curing temperatures have not always been observed to increase the compressive

strengths of geopolymers. In addition, the geopolymer samples cured at 70 C and 80 C tend to be

more brittle when compressed due to the effects of high free alkali in the product [10], resulting in

lower compressive strength. In addition, the samples cured at temperatures above 60 C were

relatively dry. This might be due to substantial loss of moisture from the sample. It has been shown

that strength deteriorates when the evaporation of moisture is allowed [26]. Thus, it is suggested

that the geopolymer reaction requires the presence of moisture in order to develop good strength

[25].

Figure 3: Compressive strength for various curing temperatures

Conclusions

This paper presents the results of our study of the effect of NaOH molarity, fly ash/alkaline

activator ratio, Na2SiO3/NaOH ratio, and curing temperature on geopolymer paste. From the

experimental results, the following conclusions have been listed:

1) The 12-M NaOH solution produced the highest compressive strength of geopolymer paste, at

68.48 MPa.

2) The combination of fly ash/alkaline activator ratio and Na2SiO3/NaOH ratio of 2.0 and 2.5,

respectively, produced the highest compressive strength of 70.27 MPa.

3) Curing temperature also had a significant effect on compressive strength, with a temperature

of 60 C producing the high compressive strength of 71.04 MPa.

Acknowledgment

A grant from King Abdul Aziz City Science & Technology (KACST) to support this research

project is sincerely appreciated and gratefully acknowledged.

1480 Mechatronics and Materials Processing I

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The Relationship of NaOH Molarity, Na2SiO3/NaOH Ratio,Fly Ash/Alkaline Activator Ratio, and Curing Temperature to the Strength of Fly Ash-Based Geopolymer doi:10.4028/www.scientific.net/AMR.328-330.1475