Mullite Precursor Part I. Characterization of Mullite Precursor Formed by a Reaction of Monosilicic Acid on Aluminum Hydroxide

By adsorption of monosilicic acid on aluminum hydroxide, an amorphous aluminosilicate (AAS) of Si/Al ratio of 1/3 was formed. When the AAS was heated to 1000 °C, mullite (3Al₂O₃ · 2SiO₂) crystallized directly without the formation of a spinel phase. It was clearly evidenced that correct adjustment of three factors, the Si/Al ratio, the proportion of 4- and 6-coordinated aluminums, and the single environment of a unit (no Si–O–Si bond), is important for the direct mullitization from AAS.