Porcelain stoneware versus porcelain: phase evolution, non-crystalline matrix properties and sintering kinetics.

During the various stages of ceramic tile production, sintering kinetics, phase transformations and variation of the main properties of non-crystalline matrix are considered the major parameters to be kept under control. Particularly, during the sintering process a complex evolution of both phase composition and chemistry of the liquid phase occurs, according to the dynamic equilibrium established between the residual minerals and the new crystalline phases formed during firing. This contribution aimed at comparing the evolution of phase composition and of non-crystalline matrix properties during the vitrification path of four representative industrial ceramic formulations (soft porcelain, vitreous china; two different batches of porcelain stoneware, including a glass-bearing one). These batches were designed and prepared at the laboratory scale, simulating the industrial ceramic process. The sintering kinetics of each sample was determined under isothermal conditions through an industrial-like firing schedule by optical thermo-dilatometric analysis. Samples were investigated between the temperature at which the viscous flow sintering starts (around 1000°C) up to the onset of deformation (up to 1400°C for porcelain), upon increasing dwell time. The phase composition was assessed by the Rietveld refinement and the chemical composition of the vitreous phase was obtained by subtracting the contribution of each mineralogical phase, considering its stoichiometric ideal formula. The melt properties were estimated by predictive models based on the chemical composition of the liquid phase. An increasingly faster sintering kinetics was observed in the order: soft porcelain, vitreous china, porcelain stoneware, glass-bearing stoneware. Different vitrification paths were observed with a correlation between the dissolution kinetics of feldspar and quartz. Remarkable differences were observed in those samples where mullite occurred as primary or secondary mullite. Those differences clearly reflected a distinctive evolution of chemical features and glass network connectivity parameters of the non-crystalline matrix. The porcelain stoneware sintered by fast cycles thanks to a sort of buffering effect played by quartz and primary mullite melting rates. In contrast, vitreous china and soft porcelain needed higher temperature and/or prolonged time to activate both the growth of secondary mullite and the contemporaneous quartz dissolution, and the variation of properties of the noncrystalline matrix.