Densification behavior, phase transformations, microstructure and mechanical properties of fired Egyptian kaolins

The effect of firing temperature and soaking time on the densification behavior of two Egyptian kaolins was studied. Phase composition, microstructure, mechanical properties and color changes of the fired bodies were investigated. Firing at 1200 °C/1 h yielded a moderate porosity (about 13%) for the first kaolin sample and a relatively dense body for the other (about 0.6% apparent porosity). This great discrepancy of densification parameters between the two types of kaolins decreased rapidly with increasing firing temperature. When fired at 1500 °C/1 h, both samples reached almost zero apparent porosity. The finer particles and low amount of free quartz led to a more dense structure, fine isolated rounded pores, mullite grain growth, earlier densification, higher fracture toughness, and wider vitrification range. The coarser particles and the presence of larger amount of free quartz contradicted the expected effect of fluxing oxides.