Phyllite as feldspar substitute in porcelain stoneware tiles: effect on sintering behavior and phase composition

The objective of this work was to report the preliminary results of substituting feldspar by phyllite in a typical Brazilian porcelain stoneware batch (i.e., sericite instead of K-feldspar as flux). Two bodies were prepared (50% K-feldspar replaced by 50% phyllite) and their behavior during firing was compared after an industrial-like schedule and in isothermal conditions (by optical dilatometry). Pyroplastic deformation and quantitative phase composition (XRD-Rietveld) were determined, along with estimation of the chemical and physical properties of the vitreous phase at high temperature. The results show that the substitution of feldspar by phyllite has important repercussions on the technological behavior, with pros and cons, dictated by a different evolution of phase composition. The main drawback of the phyllitic batch is the higher firing temperature (+30°C) necessary to get a water absorption below 0.5%. Clear advantages in the use of phyllite are an easier grindability and a lower pyroplastic deformation. Differences are not simply justified by the viscosity of the vitreous phase, but are likely linked to the relatively large amount of skeleton (and a proper shape of crystals dispersed in the melt).