International Journal of Minerals, Metallurgy and Materials Volume 16, Number 6, December 2009, Page 720
Preparation of fly ash and rice husk ash geopolymer S. Detphan and P. Chindaprasirt Department of Civil Engineering, Khon Kaen University, Khon Kaen 40002, Thailand (Received 2008-12-07)
Abstract: The geopolymer of fly ash (FA) and rice husk ash (RHA) was prepared. The burning temperature of rice husk, the RHA fineness and the ratio of FA to RHA were studied. The density and strength of the geopolymer mortars with RHA/FA mass ratios of 0/100, 20/80, 40/60, and 60/40 were tested. The geopolymers were activated with sodium hydroxide (NaOH), sodium silicate, and heat. It is revealed that the optimum burning temperature of RHA for making FA-RHA geopolymer is 690ºC. The as-received FA and the ground RHA with 1%-5% retained on No.325 sieve are suitable source materials for making geopolymer, and the obtained compressive strengths are between 12.5-56.0 MPa and are dependent on the ratio of FA/RHA, the RHA fineness, and the ratio of sodium silicate to NaOH. Relatively high strength FA-RHA geopolymer mortars are obtained using a sodium silicate/NaOH mass ratio of 4.0, delay time before subjecting the samples to heat for 1 h, and heat curing at 60ºC for 48 h. Key words: geopolymer; fly ash; rice husk ash; sodium hydroxide; sodium silicate
[This work was financially supported by the National Science and Technology Development Agency (NSTDA) through the Reversed Brain Drain Program (No.01-49-005), Thailand.]
1. Introduction The manufacture of Portland cement is an energy intensive process and releases a large amount of green house gas of approximately 13500 million ton annually . The effort, therefore, focuses on how to replace the Portland cement with other environmentally friendly cementitious materials. Recently, the other form of cementitious material using silica and alumina activated with high alkali solution and low heat has been developed. This material is usually based on fly ash (FA) as a source material and is termed geopolymer or alkali-activated FA cement [2-4]. In Thailand, around 3 million tons of annual output of FA from Mae Moh Lignite Coal Power Station is produced. This FA contains high calcium content and is being used quite extensively for construction. It can also be used as a source material for making high strength geopolymer mortar . It is also well known that the Asian countries produce a large amount of paddy. Rice husk, a by-product, is used as a fuel in a boiler in the rice mill or in the Corresponding author: S. Detphan, E-mail: [email protected]
© 2009 University of Science and Technology Beijing. All rights reserved.
small electricity generating plant and other applications. Although the increased use of rice husk is evident, much of the husk is disposed of by open-field burning. Therefore, making cement from rice husk has attracted interest worldwide. Rice husk, when burnt, is found to contain a very high percentage of silica, which is one of the main constituents in producing geopolymer. With proper burning and grinding, the amorphous reactive rice husk ash (RHA) could be produced and used as pozzolan [6-7]. The SiO2 rich RHA can also be used to adjust the SiO2 content of the geopolymer. The use of high silica to alumina ratio results in geopolymers with higher elasticity . In addition, the bulk density of RHA is lower than the cement and FA. The use of RHA should, therefore, result in the lighter weight cement and hence lighter mortar and concrete, which is desirable. The study on FA-RHA geopolymer mortars focuses on the preparation of RHA, the mix proportions, and the manufacturing methodology. The knowledge would be instrumental and lay some foundations for the future research in this field. Also available online at www.sciencedirect.com
S. Detphan et al., Preparation of fly ash and rice husk ash geopolymer
2. Experimental procedure 2.1. Materials Lignite FA from Mae Moh power station in the north of Thailand was used. The FA has a mean particle size of 65 μm, a Blaine fineness of 2800 cm2/g, a specific gravity of 2.21, and a percentage retained on No.325 sieve of 34%. RHA was obtained from open-field burning in small heaps. The burning was carried out on a perforated steel plate that is lifted to a small distance above the ground. The maximum temperatures at the middle of the heaps of 580, 610, 690, 710, and 940°C were recorded for the burning of 10, 15, 20, 25, and 50 kg rice husk heaps. The obtained ashes were whitish gray and the losses on ignitions (LOI) were 1.94%-3.62%, indicating complete burning. The ashes were then ground to three finenesses of Table 1. Material RHA FA
SiO2 91.91 38.72
Al2O3 0. 12 20.76
5%, 3%, and 1% retained on No.325 sieve (5RHA, 3RHA, and 1RHA) with corresponding Blaine finenesses of 12000, 14000, and 18000 cm2/g. Chemical compositions of the FA and RHA are given in Table 1. The RHA contains 91.93wt% SiO2, and the specific gravities were between 1.95-1.99. The specific gravity slightly increases with the increase in the temperature of burning due to the collapse of the cellular structure of RHA . The FA contains 16.55wt% of CaO and 74.76wt% of SiO2+Al2O3+Fe2O3, which are common for the lignite FA . The physical properties and LOI of RHA are shown in Table 2. The scanning electron microscope (SEM) photos of the as-received FA and the ground RHA indicate that the FA particles are spherical and those of RHA are irregular with cellular porous surfaces as shown in Fig. 1.
Chemical compositions of FA and RHA
Fe2O3 0.29 15.28
CaO 0.69 16.55
Physical properties of RHA
Rice husk / kg
Max. burning temperature / ºC
10 15 20 25 50
580 610 690 710 940
3.62 3.33 3.25 2.84 1.94
1.95 1.96 1.97 1.99 1.98
Na2O 0.20 1.19
TiO2 0.01 0.44
MgO 0.08 1.49
wt% K2O 1.98 2.69
P2O5 0.54 0.16
SO3 0.78 2.63
Commercial grade sodium hydroxide (NaOH) and sodium silicate solutions with 15.32wt% Na2O, 32.87wt% SiO2, and 51.81wt% water were used. The distilled water was used for the whole experiment. Sodium hydroxide was dissolved in distilled water prior to the mixing of geopolymer. Local river sand with a specific gravity of 2.64 and a fineness modulus of 2.85 was used for making mortar.
SEM photomicrographs of as-received fly ash (a) and ground RHA (b).
2.2. Preparation of mortar The mixing was carried out in an air conditioned room at approximately 25°C. FA and RHA were thoroughly mixed for 2 min until a uniform color was obtained. Sand was then added and mixed for 3 min. The NaOH, sodium silicate, and extra water were then added and mixed for another 5 min. Extra water was needed to maintain the workable mortar. Right after
the mixing, flow of mortar was determined in accordance with ASTM C124 . All geopolymer mortars were made with a sand to solid binder (FA+RHA) mass ratio of 2.75. After the determination of flow, the fresh mortar was placed in the 50 mm×50 mm×50 mm cube moulds. The specimens were compacted in two-layer with tamping as described in ASTM C109 . Additional vibration of 10 s was applied using a
International Journal of Minerals, Metallurgy and Materials, Vol.16, No.6, Dec 2009
vibrating table. The specimens were wrapped with vinyl sheets to protect moisture loss and kept in the controlled room at 23±2°C. After a delay period, the specimens were then placed in the oven for heat curing. After the heat curing, the specimens were put in the laboratory to cool down and demoulded the next day and kept in the controlled room. The bulk density and compressive strength of mortars were determined at the age of 7 d. The compressive strengths were Table 3.
Mix proportions of geopolymer mortars
100FA-A 80FA20RHA-A 60FA40RHA-A 40FA60RHA-A
RHA 0 128 253 381
NaOH 51 51 51 51
100FA-B 80FA20RHA-B 60FA40RHA-B 40FA60RHA-B 100FA-C 80FA20RHA-C 60FA40RHA-C 40FA60RHA-C
635 507 381 253 635 507 381 253
0 128 253 381 0 128 253 381
70 70 70 70 110 110 110 110
280 280 280 280 280 280 280 280
96 128 189 253 96 128 189 253
1747 1747 1747 1747 1747 1747 1747 1747
100FA-D 80FA20RHA-D 60FA40RHA-D 40FA60RHA-D
635 507 381 253
0 128 253 381
149 149 149 149
280 280 280 280
96 128 189 253
1747 1747 1747 1747
Series A Sodium silicate/NaOH=5.5
Series D Sodium silicate/NaOH=1.9
The mortars were mixed using sufficient water to produce the flow of 110±5%. The mix proportions for making mortars are given in Table 3. In addition to the strength test, the mortars were tested for bulk densities.
FA 635 507 381 253
Series C Sodium silicate/NaOH=2.5
2.3. Series of tests
Content / (kg·m3) Sodium silicate 280 280 280 280
Series B Sodium silicate/NaOH=4.0
tested in accordance with ASTM C109. The reported results were the average of three samples.
(1) Temperature of burning of the rice husk To test RHA burnt at different temperatures, the specimens of 80FA20RHA-B and 80FA20RHA-C were used with no delay time and curing at 75°C for 48 h, and 3RHA was used in the specimens. (2) Effect of the sodium silicate to NaOH mass ratio and SiO2/Al2O3 molar ratio To test the effect of the sodium silicate to NaOH mass ratio, all the specimens shown in Table 3 were used with SiO2/Al2O3 molar ratios of 4.1, 6.1, 9.5, and 16.3, no delay time, and curing at 75°C for 48 h and 3RHA was chosen for the specimens. (3) Effect of curing The effects of delay time, temperature of heat curing and period of heat curing were investigated. Series B was used with 3RHA. (a) Delay time before heat curing. Delay time is the time duration before placing the specimens in the oven. In this test, the delay time of 1, 3, 6, 12, 24, 36, and 48 h were used. After the delay time, the specimens were placed in the oven at 75°C for 48 h.
Extra Water 96 128 189 253
Sand 1747 1747 1747 1747
(b) Duration of heat curing. In this test, the periods of heat curing were 6, 12, 18, 24, 48, 60, and 72 h. The specimens were placed in the oven at 75°C using delay time of 1 h based on the result of delay time test. (c) Temperature of heat curing. The curing temperatures of 60, 75, and 90°C were assigned in this test. The specimens were placed in the oven using a delay time of 1 h, and the duration of heat curing of 48 h in the oven based on the result of the duration of heat curing test. (4) Fineness of RHA In this test, the 3 different RHAs, namely, 1RHA, 3RHA, and 5RHA, were used in 80FA20RHA-B. The specimens were placed in the oven with a delay time of 1 h and a duration of heat curing of 48 h at 60°C based on the temperature of heat curing test.
3. Results and discussion 3.1. Burning temperature of rice husk The results of strength of mortars made from RHA burnt at different temperatures as shown in Fig. 2 in-
S. Detphan et al., Preparation of fly ash and rice husk ash geopolymer
dicate that the strength is dependent on the burning temperature of rice husk. The optimum temperature of RHA burning at 690°C provided the highest strengths of 22.5 and 37.0 MPa for the sodium silicate/NaOH mass ratios of 2.5 and 4.0, respectively. The burning temperature of less than 700°C produces amorphous silica, which is reactive . Although low burning temperature produced amorphous silica, at very low temperature, the burning is incomplete. This results in the undesirable high amount of the unburnt materials. As shown in Table 2, the LOI of RHA decreased from 3.6 to 1.9 as the temperature of burning increased. On the other hand, the higher temperature of burning produces a larger amount of crystalline silica, which is less reactive [7, 13].
contained in the geopolymer pore system and decreased the strength and performance of geopolymers . The results of Fig. 4 and Table 1 indicate that the increase in the RHA content results in the increases in the SiO2/Al2O3 molar ratios. The normal FA mortar consisted of SiO2/Al2O3 molar ratio of 4.08. For the mortar containing RHA of 60wt%, the SiO2/Al2O3 molar ratio increased to 16.32 with a slight decline in the strength of mortars owing to the increased water requirement and the increase in the silica to alumina molar ratio .
Fig. 3. Relationship between compressive strength and the mass ratios of sodium silicate to NaOH (3RHA, no delay time and curing at 75°C for 48 h). Fig. 2. Relationship between compressive strength and maximum burning temperature of rice husk (3RHA, no delay time and curing at 75°C for 48 h).
3.2. Ratio of sodium silicate to NaOH Fig. 3 shows that the strengths of mortar are greatly affected by the ratios of sodium silicate to NaOH. The optimum mass ratio is 4.0 for all mix series. The strength increases significantly with the increase of the mass ratios of sodium silicate to NaOH from 1.9 to 4.0. The increase in the ratio of sodium silicate to NaOH in this range results in a higher concentration of silicate in the solution and thus increases the strength of the geopolymer. Further increasing the mass ratio of sodium silicate to NaOH from 4.0 to 5.5 results in a very viscous mix. Extra water is needed to produce a workable mortar and hence the strength of the mortar is low in this range . 3.3. Molar ratio of SiO2/Al2O3 The results as shown in Table 3 indicate that the water requirements of fresh geopolymer mortars increased with the increase in the amount of RHA. The water contents of mortars are 96, 128, 189, and 253 kg corresponding with FA:RHA (mass ratios) of 100:0, 80:20, 60:40, and 40:60, respectively. The RHA still maintained its cellular structure, and the mix with RHA thus required a larger amount of extra water to produce mortars with the same flow. This adversely affected the strength of the mortar as extra water was
Fig. 4. Relationship between compressive strength and molar ratio of SiO2 to Al2O3 (3RHA, no delay time and curing at 75°C for 48 h).
3.4. Effect of curing (a) Delay time before heat curing The results of delay time before heat curing are shown in Fig. 5. The patterns of strength development of geopolymer mortars with different dosage of RHA are similar. A delay time of 1-6 h slightly increases the compressive strength of the mortars. An increase in delay time beyond this results in a small reduction in strength. The delay time adopted for FA geopolymer paste and mortar is generally between 1-24 h [15-16]. A period of delay time allows the dissolving of silica and alumina, which are important for the alumino silicate geopolymerization. (b) Duration of heat curing
International Journal of Minerals, Metallurgy and Materials, Vol.16, No.6, Dec 2009
The relationship of the duration of heat curing and strength is shown in Fig. 6. The strength increases with an increase in curing time from 6 to 48 h. Heat curing beyond 48 h had little effect on the strength of the geopolymer. An increase in heat curing duration is expected to improve the strength characteristics of the geopolymer. Similar strength development with high temperature curing has been reported [15, 17].
mortars. The strengths of mortars made with 5RHA, 3RHA, and 1RHA were 34.5, 37.0, and 43.0 MPa, respectively. The increase in the fineness of RHA increased its surface area and its reactivity and hence resulted in an increase in strength of geopolymer mortars. Increases in strength with the use of fine RHA in the partial replacement of Portland cement have also been reported [9-10]. RHA with 1%-5% retained on No.325 sieve were suitable source materials for making geopolymer. Although the very fine 1RHA gave high strength geopolymer mortars, it required intense grinding. For practical purpose, 3RHA was recommended as it gave reasonably high strength geopolymer mortars.
Fig. 5. Relationship between compressive strength and delay time (3RHA and curing at 75°C for 48 h).
Fig. 7. Relationship between compressive strength and curing temperature (3RHA, delay time of 1 h, and curing for 48 h).
3.6. Bulk density Fig. 6. Relationship between compressive strength and the duration of heat curing (3RHA, curing temperature of 75°C, and delay time of 1 h).
(c) Curing temperature The relationship of curing temperature and strength is given in Fig. 7. The influence of curing temperature is more significant in the mix containing a large amount of FA. For example, the strengths of 100FA-B and 80FA20RHA-B mixes with curing temperatures of 60, 75, and 90ºC vary from 39.0-56.0 and 35.0-43.0 MPa, whereas the strengths of 60FA40RHA-B and 40FA60RHA-B with curing temperatures of 60, 75, and 90°C vary within narrower ranges of 28.0-32.5 and 24.0-28.5 MPa, respectively. The optimum temperature was 60°C for all series. The use of 75 and 90ºC results in some reduction in strength of the mortar. At high curing temperature, the specimens especially a small one with high surface to volume ratio experience losses of moisture and this adversely affects the strength [5, 18]. 3.5. Effect of RHA fineness The results indicated that the increase in the fineness of RHA increased the strength of geopolymer
The bulk density of mortars was lower with an increase in RHA content as the density of RHA was lower than that of FA. The bulk densities of geopolymer mortars with RHA contents of 0, 20wt%, 40wt%, and 60wt% of total solid binder were 2320, 2270, 2135, and 2080 kg/m3, respectively. Moreover, the increase in water content as a result of the increase in the water requirement of RHA also reduced the density of the mortar. The low density of high RHA content suggested that it was suitable for the development of light weight geopolymer mortars and concretes. 3.7. Microstructures The images of SEM and energy dispersive spectroscopy analysis (EDX) of 100FA-B and 40FA60RHA-B geopolymer mortars are shown in Fig. 8. The result as shown in Fig. 8(a) indicates the continuous mass of binder and some unreacted FA particles. The sample of the matrix with a SiO2/Al2O3 molar ratio of 2.04 and a Na2O/Al2O3 molar ratio of 1.0 was observed. The strength of this geopolymer mortar was 41.5 MPa. The SiO2/Al2O3 molar ratio of 2.04 conformed with the previously reported SiO2/Al2O3 molar ratio around 2.0 of FA geopolymer . In general, physical and mechanical properties of geopolymers were a function of the SiO2/Al2O3 ratio .
S. Detphan et al., Preparation of fly ash and rice husk ash geopolymer
The result shown in Fig. 8(b) also indicates a continuous mass of binder, unreacted FA, and a minor phase of sodium silicate of the FA-RHA geopolymer. The sample of the matrix with a SiO2/Al2O3 molar ra-
tio of 3.68 and a Na2O/Al2O3 molar ratio of 2.22 was observed. The increase in RHA content increased the ratio of SiO2/Al2O3 and resulted in geopolymers with a lower strength and a higher elasticity [8, 21].
Fig. 8. SEM-EDX photomicrographs analysis of 100FA-B (a) and 40FA60RHA-B (b) (3RHA, delay time of 1 h, curing at 60°C for 48 h).
4. Conclusion Based on the obtained data, it can be concluded that RHA can be used in conjunction with FA to produce geopolymer. The workability and strength of geopolymer mortars are dependent on the properties and dosage of FA and RHA. The optimum burning temperature to produce reactive RHA for making geopolymer is 690°C. The flow of the mix is much reduced with the incorporation of RHA owing to the fine and cellular structure of RHA particles. The strength of geopolymer mortars are affected by the fineness of RHA. The increase in the fineness of RHA increases its reactivity and the strength of mortars. RHAs with 1%-5% retained on No.325 sieve are suitable for making geopolymer mortars. The bulk density of the mortar is reduced with an increase in RHA content. Relatively high strength geopolymer mortars are obtained using delay time before subjecting the samples to heat for 1 h, curing temperature in an oven at 60°C for 48 h, and a sodium silicate/NaOH mass ratio of 4.0. The strength of geopolymer mortars slightly decreases with an increase in RHA content as a result of the increase in SiO2/Al2O3 ratio and the in-
creased water requirement of the mixes. The lower strengths are compensated with a reduced bulk density of mortars with high silica content.
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