Based Geopolymer

Study On The Effect Of Alkaline Activators Ratio In Preparation Of Fly Ash - Based Geopolymer A. Mohd Mustafa Al Bakri*, Omar. A.K.A.Abdul Kareem* and San Myint * School of Material Engineering University Malaysia Perlis (UniMAP) 01000, P.O Box 77, D/A Pejabat Pos besar, Kangar, Perlis, Malaysia

E-mail : [email protected]

ABSTRACT A fly ash - based geopolymer samples with different alkaline activators (sodium silicate and sodium hydroxide) weight ratio were prepared at constant curing temperature 70°C, curing time 24 hr and total additional water content approximately 17% of samples weight. The compressive strength test results of the samples for 15 M sodium hydroxide solution and 7 days aging indicated increased in compressive strength with increased in sodium silicate content but did not significantly change the workability in mixing and moulding steps in sample preparation process.

Keywords:

Fly ash, Geopolymer, Alkaline Activator, Compressive strength, Geopolymerization.

1. Introduction: The climate change due to global warming has become a major concern. The global warming is caused by the emission of greenhouse gases, such as carbon dioxide CO2, to atmosphere by human activities. Among the greenhouse gases, CO2 contributes about 65% of global warming (1).The cement industry held responsible for some of CO2 emission, because the production of one ton of Portland cement emits approximately one ton of CO2 in to the atmosphere (2). As the demand for concrete as a construction material increases, so also the demand for Portland cement. It is estimated that the production of cement will increase from about 1.5 billion tons in 1995 to 2.2 billion tons in 2010 (3). Therefore several efforts are in progress to reduce the use of Portland cement in concrete in order to address the global warming issues as well as Portland cement structures disintegration problems (4). Recently, another form of cementitious materials using silicon and aluminum activated in a high alkali solution was developed (5). Davidovits (2) (6), proposed that an alkaline liquid could be used to react with silicon Si and the aluminum Al in source material of geological origin or in by-product materials. And because the chemical reaction that takes place in this case is polymerization process, he coined the term “Geopolymer” to represent thesis binders. 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 (4). The polymerization process involves a substantially fast chemical reaction under alkaline condition on Si-Al minerals, that result in three dimensional polymeric chains and ring structure consisting of Si-O-Al-O bonds (2).

Generally, there are two main constituents of geopolymers, namely the source materials and the alkaline liquids. The similarity of some fly ashes to natural aluminosilicate materials (due to the presence of Si and Al in the ash) has encouraged the use of geopolymerization as a possible technological solution in the making of special cement (7). Most of fly ash available globally is low-calcium fly ash ASTM Class F formed as a by product of burning anthracite or bituminous coal. Although coal burning power plants are considered to be environmentally friendly, the extent of power generated by these plants is on the increase Due to the huge reserves of good quality coal available worldwide and the low cost of the power produced by from these sources. Therefore, huge quantities of fly ash will be available for many years in the future (8). The most used alkaline liquid activator is a mixture of sodium or potassium hydroxide (NaOH, KOH) with sodium waterglass (nSiO2Na2O) or potassium waterglass (nSiO2K2O) (5)(9)(10)11). The mechanical strength increase when the concentration of the activator increases (12). The concentration of NaOH solution can vary in the range between 8 to 16 M (4). Using NaOH solution with concentration out of this range leads to less mechanical strength (10). The alkaline activator of both NaOH solution and waterglass gives better strength than activator of only NaOH (10) (11). The waterglass favours the polymerization process leading to a reaction product with more (Si) and more mechanical strength (13). The mixing ratio of waterglass/ NaOH seems to play a crucial role in the strength development of the geopolymer. For the high calcium fly ash Class C the optimum ratio of waterglass/ NaOH to produce high strength geopolymer was in the range of 0.67 - 1.00 (14). The goal of this contribution is to investigate the effect of this ratio on the final strength of low-calcium fly ash Class F based- geopolymer at constant curing temperature 70°C, curing time 24hr, NaOH solution 15M and total additional water content approximately 17% of the samples weight .

2. Experimental method:

2.1. Materials The low-calcium fly ash used in this study was obtained from Sultan Abdul Aziz power station in Kapar, Selangor. Malaysia. The chemical composition of the fly ash was determined by X-ray fluorescence (XRF) and listed in Table 1. The composition analysis indicated that CaO content was less than 10% which conforming that the fly of Class F type (15). The particle size distribution was measured using Malvern Mastersizer 2000 (Malvern Instrument). The particle size in rang of (18 m). A technical grade sodium silicate (Sigma Chemicals Ltd.) with modulus 3.20, (SiO2 =30.1%, Na2O =9.4%and H2O=60.5%). The sodium hydroxide pellets 97-99 % purity supplied from (Sigma-Aldrich Co.), was used to preparing sodium hydroxide solution.

2.2. Geopolymer synthesis

The alkaline activator was prepared using different mixing ratio of waterglass/ NaOH solution. These ratios were 0.6, 0.8, 1.00and 1.2 respectively. The NaOH solution was prepared by dissolving sodium hydroxide pellets in deionised water, the concentration of the NaOH was kept constant at 15 M. It is preferable to mix the waterglass and the NaOH solution together at least one day before adding the liquid to the solid constituent (16). Geopolymer samples were synthesized by adding the alkaline activator gradually to fly ash at a certain constant (liquid/ solid) ratio. An additional water of a constant 17% of the sample weight was added to achieve a suitable workability. After 15 min of mechanical mixing, the fresh homogeneous geopolymers were poured in to 50mm x 50mm x 50mm standard steel molds. The samples aggrieved in room temperature for 24 hr prior to cure in electrical low temperature furnace (L T Furnace, L6-1200) for one day at 70°C. The water evaporation was prevented by sealing the top of the molds by a

thin plastic layer during the storage as well as the curing stage. After the end of curing, the molds took out from the furnace and left cooling to room temperature, before demolding and kept aging for 7 days at room temperature. The samples were kept sealed from ambient during aging period also. The effect of the varying the waterglass/NaOH solution ratio on the geopolymer composition is indicated in Table 2. Changing the ratio doesn’t significantly affect the workability of the fresh geopolymer. It seems to be the same for all mixtures. 2.3 Compressive strength:

The samples tested after 7 days of aging. Using (Instron, 5569 USA) mechanical testing machine with Automatic Max. Loading of 50KN, the compressive strength of the geopolymer tested with speed rate of 50 mm/min. The compression strength values were an average of testing 3 samples for each ratio.

3. Results and discussion

9 8.325

8 7

(MPa)

Compressive strength at 7 days

Figure 1, shows the influence of the Waterglass/NaOH solution ratio on the compressive strength of the fly ash-based geopolymer. It’s stated that the geopolymerization rate arise with the increase in the ratio, which gained the geopolymer samples a rapid increasing in the strength when the ratio increased from 0.6 -0.8, to reach the maximum strength value at 1.00 as in Fig. 1. The explanation of this increasing in the compressive strength depend on the nature of complex chemical geopolymerization process .One of the possible explanation of this increasing, may be connected to the increasing in the Waterglass/NaOH solution mass ratio, which leads to generate more SiO2 species, leading to increase in the ratio of SiO2/Al2O3 and increase the geopolymer strength, because with increasing SiO2/Al2O3 ratio , more SiO-Si bonds are formed , which are stronger in comparison with Si-O-Al (17).

6

5.615

5.18

5 4 3

2.353

2 1 0 0.6

0.8

1

1.2

1.4

Waterglass/NaOH ratio

Fig. 1 Effect of the Waterglass/ NaOH solution mass mixing ratio on the compressive strength of fly ash-based geopolymer. Figure 2, shows that increasing in the Waterglass/NaOH solution mixing ratio leads to increase in the ratio of SiO2/Al2O3. Samples with SiO2/Al2O3 less than 4.08, show low mechanical strength 2.353 MPa. This maybe because it provides insufficient SiO2 that could increase the geopolymerization rate. However, an increasing of Waterglass/NaOH solution ratio from 1.00 to 1.2 leads to decrease the compressive strength from 8.325 to 5.16MPa as in Fig.1. Due to the high SiO2/Al2O3 4.16 ratio provides excess SiO2 species hinders the geopolymerization subsequent processes, so the strength decreasing as indicates in Fig.2. So the waterglass content have the major effect on the compressive strength, by increasing the waterglass content in alkaline activator liquid from 9.7 to 11% of sample weight increases the mechanical strength, and by increasing it to 13% of sample weight the strength reached to 8.325 MPa as indicated in Fig 3. This trend is agreed with results obtained on the fly ash-based concrete (19).

9

1:00

7 6 5

(MPa)

compressive strength at 7 days

8

0.8 1.2

4 3 2

0.6

1 0 4.03

4.08

4.12

4.16

SiO2/Al2O3

Fig. 2 Effect of molar SiO2/Al2O3 molar ratio on the compressive strength of fly ash-based geopolymer.

This explanation is supported by the behavior of the ratio of Na2O/ SiO2, Table 2. Many previous researches indicated the effect of this ratio on the mechanical strength of geopolymer (5) (9) (10) (11) (18). They mentioned that the mechanical strength of the geopolymers increase with the decreasing in the Na2O/ SiO2 (18). It’s clear in Table 2, that the mechanical strength increasing with decreasing in Na2O/ SiO2 until reaching 0.1, the samples start to lose the gained strength. These results are agreed with a results held on the fly ash-based geopolymer (19). Another explanation on the influence of the Waterglass/NaOH solution ratio on the compressive strength is the effect of the water content in the alkaline activator liquid. The water content considered as the most important factor on the geopolymerization process (19). Generally the geopolymerization process can be approximately partitioned in to two periods: I dissolution –hydrolysis, II hydrolysis-polycondensation. But the fact, these two steps probably occur simultaneously once the solid material mixed with liquid activator (20). The period I including the dissolution of the SiO2 and Al2O3 species and hydrolysis it as indicated in the equations Eqs. (1)- (3), (21) : _

OH -Si – O- Si + H2O

2-Si –OH.

(1)

2- Al – OH.

(2)

- Si – OH + -Al – OH.

(3)

_ OH -Al – O – Al - + H2O

_ OH

-Si- O – Al - + H2O _

OH

OH

OH

- Si – OH + -Al – OH

HO - Si – O – Al – OH + H2O. OH

OH

(4)

NaOH Na+ ….(Si – O – Al – O)n +3n H2O .

n (OH)3- Si- O – Al(OH)3 OH

(5)

OH

NaOH Na+ ...( - O –Si – O – Al – O – Si – )n +

n(OH)3- Si – O – Al – O Si - (OH)3 OH

OH

3nH2O.

(6)

9

1.00

compressive strength at 7 days (MPa)

8 7 6 5

0.8 1.2

4 3

0.6

2 1 0

9.7

11

13

14

% w aterglass

Fig. 3 Effect of the %waterglass content of the alkaline activator on the compressive strength of fly ash – based geopolymer. Water is indispensable during the geopolymerization, especially for the destruction of solid particles and the hydrolysis of dissolved (Al and Si ions), water is the reactant in this period and if the OH concentration is high enough more water will accelerate the dissolution and hydrolysis. In the meantime, the water plays as product in the period II, Eqs. (4) - (6). If the water content too much, will hider the geopolymerization kinetically (21). Depend on this suggestion the water content seems to be critical factor, if less or more the suitable limit, will effect on the geopolymerization rate development resulting in low mechanical strength. Figure 4, shows the effect of increasing the molar ratio of H2O/Na2O on the geopolymer strength, this ratio increase by the increasing of the water content in the alkaline activator, due to changing the mass mixing ratio of Waterglass/NaOH solution ratio, the strength increasing with increasing this ratio up to 11 for 1.00 Waterglass/NaOH solution ratio, further increasing lead to low strength as in H2O/Na2O = 12 for 1.2 Waterglass/NaOH solution ratio. compressive strength at 7day (MPa)

9 8 8.325 7 6

5.615

5 5.18

4 3 2.353

2 1 0 9

9.5

10

10.5

11

11.5

12

12.5

H2 O/Na2 O

Fig. 4 Effect of the H2O/Na2O molar ratio on the compressive strength of fly ash –based geopolymer.

4. Conclusion: A fly ash-based geopolymer synthesized by activating fly ash Class F with alkaline activator (Waterglass and NaOH solution), at constant (liquid/solid) mixing ratio. The effect of the alkaline activator in term of mass mixing ratio of (Waterglass/NaOH solution), on the geopolymerization process was investigated. It was found that increasing of the Waterglass content in the activator significantly increasing the geopolymerization reaction rate. The waterglass provide extra Si species and more based water and both of them plays a crucial role in the geopolymerization reaction. More Si helps in producing of Si-O-Si bonds and the increase in these bonds until a cretin limit, the compressive strength of the geopolymers increase significantly. If the content of Si excess the suitable limit would effect negatively on the geopolymerization rate leading to low strength geopolymers. The water content found to be having the same trend, increasing the water content provide more dissolution of the SiO2 and Al2O3 species. Thus, the geopolymerization rate increases leading to increase the geopolymer strength. It has been found that the water content in the alkaline activator also limited, crossing the suitable limit decreasing the geopolymerization rate lead to low strength geopolymers. It can be concluded from this research work that the total Si content and the water content plays an important role in geopolymerization reaction as it a complex chemical reaction. 5. Acknowledgement: Author wish to thank Mr. (Abdul Haqi Bin Ibrahim), School of Environmental Engineering. University Malaysia Perils “ UinMAP” ,for his help in providing the fly ash of this study .

6. Appendix:

Table 1: XRF analysis data of fly ash composition

Chemical SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O P2O5 SO3 MnO Loss on Ignition .

Component (%) 52.11 23.59 7.39 0.88 2.61 0.78 0.42 0.80 1.31 0.49 0.03 5.59

Table 2: Effect of Waterglass/NaOH solution ratio on the chemical composition, %waterglass content and the water content in the alkaline solution of the geopolymers.

Waterglass / NaOH mass ratio(R).

SiO2/Al2O

0.6

H2O/ Na2O Molar ratio.

Na2O/ SiO2 Molar ratio.

% Waterglass in samples.

Water content of alkaline activator (mole).

4.03

9

0.13

9.7

5.4038

0.8

4.08

10

0.12

11

5.3902

1.00

4.12

11

0.11

13

5.38

1.2

4.16

12

0.1

14

5.372

3

Molar ratio.

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