Can we predict the sintering kinetics of porcelain stoneware? The case of glassy waste-based bodies

The production of ceramic tiles is continuously increasing worldwide, implying a growing demand for raw materials. On the other hand, the progressive depletion of the main feldspathic flux deposits is forcing the ceramic industry to search for suitable substitutes. Although the tile-making industry proved to be able to recycle its own processing residues, the use of wastes from further sources is at present quite limited. The use of glassy wastes, for instance, is usually hindered by technological reasons: a low-melting glass can significantly affect the firing behavior and particularly high temperature permanent deformations. The main goal of this study is therefore to assess the firing behavior of porcelain stoneware bodies containing glassy wastes, and particularly to verify if the sintering kinetics can be satisfactorily predicted applying the well-known models inherited by the glass densification theory, as the Frenkel's model. Five different sources (bottle, PC-TV screen, lamp, glaze manufacturing and porcelain stoneware grinding sludge) provided glassy wastes that were separately added to a reference body in amount of 20%. Each batch was characterized from the chemical point of view and its sintering kinetics were determined in isothermal conditions by optical thermo-dilatometric analysis (TA ODP868). The isothermal tests were carried out at Tgr (gresification temperature) with a heating rate of 80°C/min up to Tgr and 30 min of dwell time. The quantitative phase composition after firing at Tgr was determined by XRD-Rietveld in order to calculate the chemical and physical properties of the vitreous phase. Shear viscosity and liquid-air surface tension were used as input of the Frenkel's model to calculate the initial sintering rate of the bodies. Such calculated sintering rates were then compared with the experimental rates derived from sintering curves. The introduction of a certain amount of glass into a porcelain stoneware body determines a clear change in the sintering mechanism. The waste-free batch behaves substantially in agreement with the Frenkel's model, confirming that the initial sintering rate is fundamentally governed by the physical properties of the liquid phase. However, the addition of 20% glass reflects into a significant deviation from the behavior of the benchmark, that sees the Frenkel's model failing to predict accurately the sintering rate on the basis of melt viscosity and surface tension just in two cases.