Contribution to the Study of Moisture Expansion in Ceramic Materials

Measurements were made of the expansions produced by autoclaving coprecipitated and mixed gels of silica and alumina fired over the temperature range 850° to 1200°C. The effect of adding soda to gels of kaolinite composition was investigated and the compositions of the phases were determined. The results show that amorphous silica has a limited influence and must be modified by alumina or by soda and alumina to produce expansions comparable with those of ceramic bodies. The active material is an amorphous alkali aluminosilicate, to be distinguished from permutites and glass. Formation of glass and crystalline compounds reduces moisture expansion. At low firing temperatures (below 950° C.) the hydration of γyAl₂O₃ to boehmite produces high moisture expansions, but γ-Al₂O₃ modified by silica (silicon spinel) has only a limited influence. Some observations are made on the nature of cristobalite developed during the firing of pure amorphous silica and amorphous silica into which additives were introduced.