Mullite grown from fired andalusite grains: the role of impurities and of the high temperature liquid phase on the kinetics of mullitization and consequences on thermal shocks resistance

Mullitization and microstructure of mullite grown from fired andalusite grains has been studied in the literature. Mechanisms of mullite formation are well known: during heating, andalusite single crystal grains are transformed into a composite made of a 3:2 mullite single crystal with a capillary network filled with silica rich liquid. But very few works studied the influence of the impurities on the kinetics of mullitization. This paper shows that the presence of impurities (mainly iron and alkalis) in the andalusite increases the amount of the liquid phase and decreases the liquid viscosity. The mullitization kinetics depends on the andalusite grain size, the chemical composition, the origin of the andalusite deposit and the amount and the properties of the liquid phase. Because of the specific microstructure and the presence of the liquid phase, during thermal shocks in air, the mullitized andalusite crystals exhibit a behaviour typical of composite materials: microcracks are deflected by mullite/glass interfaces and stopped in glass zones. During a further heating, cracks generated by a thermal shock are healed.