Microstructure development in clays upon heat treatment: Kinetics and equilibrium

Abstract Knowledge about the relationships between mineralogical and microstructural characteristics of clays and the transformations that occur during firing is of fundamental importance for various industries, enabling the manufacture of better quality products and widespread use of those raw materials. The aim of this study was to interpret phase development and morphological transformations in clays subjected to different heat treatments, with emphasis on the effect of impurities (iron and alkalis), temperature and heating rate. The experiments were carried out using conventional and microwave heating, with characterization by X–ray diffraction and scanning electron microscopy. The interpretation of the results based on the mullite–silica–leucite phase diagram showed that crystallization and grain growth are closely related to the amount and viscosity of the liquid phase present at the firing temperature, which vary with the composition and can be predicted by the phase diagram. Faster heating rates tend to slightly delay the establishment of equilibrium, which appears to have special bearing on the liquid phase viscosity, hence, on grain growth. As a consequence, the resulting microstructure contains mullite needles that are shorter and more uniform in size.