Reactions in Silica-Alumina Mixtures

Examination of mixtures of extremely pure silica and alumina shows that the greatest reactivity is not encountered with stoichiometric ratios to form mullite with the formula 3Al₂O₃. 2SiO₂ but rather with the formula 2A1₂O₃-SiO₂, and that reactivity also depends on the crystalline modification of alumina. A sharp exothermic differential thermal peak at 980°C. is attributed to three simultaneous reactions dependent on the silica-alumina ratio of the mixture: (1) the crystallization of gamma alumina, (2) the crystallization of a hydrogen aluminum spinel (HAl₅O₅), and (3) the reaction of silica with the hydrogen aluminum spinel to form mullite.