Development of Thai fly ash blended with rice husk ash geopolymers

Fly ash (FA) presented in the research is one of the by-products from combustion processes of lignite in electricity plants in Thailand. Generally, FA is exploited to mix with Portland cement to improve the mechanical properties of cement paste or mortar. In addition, FA is also claimed as an active pozzolanic material to synthesize geopolymers. The rice husk is used as fuel in rice mills and converted to ash called rice husk ash (RHA). The RHA in the research contains approximately 85%-90% amorphous silica. RHA was chosen to blend with FA producing geopolymers to retard hardening time and develop good results of compressive strength of geopolymers. The mixed proportion of FA to RHA was prepared by variation as five ratios of 0:100, 20:80, 50:50, 80:20 and 100:0 by weight. Sodium hydroxide of 8 Molar (8M NaOH) and sodium silicate (Na2SiO3) solution was used as alkali activators by mass ratio of Na2SiO3: NaOH at 3:2. The suitable Si/Al molar ratio, the Na/Al molar ratio, pH and conductivity of experiment were studied and characterized. Finally, the specimens were investigated for compressive strength at 7, 14, 21 and 28 day ages and flexural strength and microstructures at 7 days of age.