Rice husk ash as a silica source in a geopolymer formulation

Geopolymer is the refractory inorganic ps formed from both aluminum and silicon source. South Korean rice husk ash (RHA) has been investigated as an alternative silica source to replace fume silica currently used in geopolymer rdfdffdfdesearch. In order to acquire proper RHA from raw rice husk, the optimal calcination temperature, which produces the highest amount of amorphous silica, was studied. Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM-EDS) and X-ray diffraction (XRD) have been used to verify the contents of RHA. Previously many researchers have studied mixtures of RHA/fly ash (FA) or RHA/red mud (RM), resulting the ratio of SiO₂/Al₂O₃ varied by the each mixture. However, in this paper, RHA is only replaced to a certain portion of fumed silica. Thus, the fundamental ratio of each ingredient in the geopolymer (M: SiO₂: AIO₂: H₂O =1:1:4:11) where M is Na, K or Cs was strictly kept. Five variations of RHA/Fumed silica mixture (0/100. 25/75, 50/50, 75/25, 100/0) have been studied. Each mixture was dissolved with potassium hydroxide (KOH) and deionized water for 24 hours on the stirring plate and then became water glass. Metakaolin was added to water glass and mixed properly. The geopolymer slurry was poured into the mold and cured at 50°C for 24 hours. After demolding, the RHA geopolymer samples were kept for a minimum of 3 days at the room temperature. The flexural stress of RHA geopolymer samples were investigated by 3-point bend tests at room temperature using the guidelines prescribed by ASTM C78/C78-M standard. The fracture surfaces from the bending tests were examined by using Scanning Electron Microscope (SEM). It was demonstrated that the RHA can clearly be the fumed-silica alternative.