Effect of strong mineral fluxes on sintering of porcelain stoneware tiles

Strong fluxes are needed to fire vitrified ceramics at temperatures significantly lower than those usually reached in industrial firing cycles. This work is aimed at understanding the role of strong fluxes in the microstructural evolution during sintering. Six fluxes (colemanite, ulexite, wollastonite, diopside, spodumene and phonolite) were individually added to a porcelain stoneware batch and processed in standard conditions. Compacts and fired bodies were characterized by optical dilatometry, XRD-Rietveld, SEM and measuring technological properties. Strong fluxes change the firing behaviour with a complex interplay of sintering kinetics, microstructural features, and phase composition. Every flux has its own repercussion on the properties of the liquid phase (chemical composition, degree of polymerization, viscosity and surface tension) which are key points to explain the observed microstructure, densification rates, and stability at high temperature. Batches with phonolite, wollastonite or diopside exhibit characteristics closer to standard porcelain stoneware, while spodumene and borates suffer from unsatisfactory microstructures and lower densification efficiency.