Effects of alumina-excess on reactivity, microstructure and macroscopic properties of sanitary-ware ceramics.

We present and discuss the effects in sanitary-ware ceramic mixtures induced by partial replacement of raw materials with alumina, or andalusite, on i) phase composition, ii) Gibbs energy of formation (Delta G), iii) microstructures and iv) some reference technological properties. Measurements were carried out by X-ray powder diffraction, scanning electron microscopy, synchrotron X-ray computed micro-tomography, linear dilatometry, water absorption and mechanical testing. The results show that the introduction of alumina affects phase composition of the ceramic output as a function of the replacement scheme, leading to a mullite-ceramic glass quasi-equilibrium whose Delta G ranges from -32 to -14 kJ·mol-1. Although alumina modestly participates in the ceramic reactions, it yet influences surface microstructures, giving relevant whiteness, and some reference technological properties, providing low deformation and high flexural strength. The resulting ceramic material exhibits performances that lie in between those from fireclay and vitreous technologies, providing aesthetic improvements and high shape stability.