Glassy wastes as feldspar substitutes in porcelain stoneware tiles: Thermal behaviour and effect on sintering process

In view of the transition to circular economy, the ceramic industry is searching for suitable flux substitutes. Glassy wastes are promising candidates, even though they may significantly affect the firing behaviour and particularly high temperature permanent deformations. The goal of this study is to assess the sintering behaviour of porcelain stoneware bodies containing glassy wastes. Five different sources (bottle, PC-TV screen, lamp, frit manufacturing and porcelain stoneware grinding) provided glassy wastes that were separately added to a reference batch (20-40-60%) in substitution of feldspars. Each batch was characterized from the chemical point of view and its sintering kinetics were determined in isothermal conditions by optical thermo-dilatometric analysis. The quantitative phase composition after firing was determined by XRD-Rietveld in order to calculate chemical and physical properties of the vitreous phase at high temperature. The substitution of feldspathic fluxes determines a clear change in the firing behaviour: waste glasses lower the temperature at which the densification starts, and extend the early stage occurring before a constant rate sintering takes place. In some cases, the deviation is important and entails a lower sintering efficiency. The reasons behind this different firing behaviour are discussed in terms of composition, structure, and properties of the melt.