Effect of talc and chlorite on sintering and technological behaviour of porcelain stoneware tiles

The use of magnesium silicates as sintering promoters in porcelain stoneware tiles was investigated in order to understand their effect on technological properties and firing behaviour. Three formulation were designed, replacing sodium feldspar with 3wt% talk or chlorite in a reference porcelain stoneware body and experimented at a laboratory scale simulating the industrial tilemaking cycle. The occurrence of magnesium silicates does not affect the technological properties of the semi-finished products, but it influences remarkably the firing behaviour. Bodies containing talc or chlorite exhibit a faster densification kinetic, with slightly larger firing shrinkage, due to their larger amount of glassy phase, which appears to be less viscous than the reference one. On the other hand, the addition of magnesium silicate brings about a slightly lower bulk density, basically due to a larger amount of closed porosity, being water absorption negligible over 1160°C. Notwithstanding the different porosity, porcelain stoneware bodies containing talc or chlorite present just a moderate decrease of mechanical strenght.