Effect of kaolinite, illite and talc on the processing properties and mullite content of porcelain stoneware tiles

In this study, the effect of talc, illite and kaolinite on some properties of porcelain stoneware tiles was investigated. Raw materials available locally in a Brazilian ceramics production plant were used. The mixtures were defined through the use of a 23 factorial design of experiments. The starting point was a composition reported in the literature as follows: 35% kaolinite, 25% quartz and 40% albite. The mixtures were prepared by wet grinding, followed by spray drying. The conformation was performed by uniaxial pressing. The firing was carried out in an electric furnace, with the temperature varying between 1160 and 1260 °C, according to the maximum densification of each composition. The residence time was 6 min. Two cooling conditions were applied: 0.8 and 4.0 °C/s. The dry bulk density, fired porosity, pyroplastic deformation and three-point flexural strength were measured. The contents of mullite, quartz and vitreous phase were quantified using the Rietveld method with fluorite as the internal standard. The results showed that there is an optimum content of mullite which maximizes the increase in the flexural strength of the pieces submitted to rapid cooling. It was possible to distinguish the effect of the mineralogical phases on the formation of the mullite and the processing properties.