Thesis (M.Sc.)--Chulalongkorn University, 2007

Rice husk ashes (RHA) from two local sources, Mahaphant Fibre-Cement Public Co., Ltd. (crystalline SiO2 as cristobalite) and A.T. Bio Power Co., Ltd. (amorphous SiO2), were characterized for chemical composition and mineral phases by XRF and XRD, respectively. Then 10-25 wt% of RHA were mixed with Portland cement (Type I), sand, water and plasticizer to form into 2-in cubes of mortar. After wet curing (3-28 d) the specimens were tested for compressive strength and morphologies of the hydrates were observed by SEM. Although the water demand to attain normal consistency increased with the content of RHA, it was found that the strengths of A.T. Bio Power RHA added cement mortars were respectively higher than those of reference cement mortars (Portland cement type I : sand : water) and Mahaphant RHA added cement mortars at all curing ages. The highest compressive strength obtained at 28 d was from 20 wt% A.T. Bio RHA added cement mortars. This suggested the optimal content of RHA be 15-20 wt%.