Influence of alternative mineralisers in traditional ceramics

usually quartz), plus something that may act as a flux (usually feldspars), aiding the melting at high temperature, so that to produce a ceramic material with a low porosity. The use of alternative fluxes (such as NaOH and KOH solutions) has been tested on quartz only, on a mixture of quartz and kaolinite, and on a more traditional mixture encompassing quartz, kaolinite and Na-feldspar. For all the mixtures, quartz with different grain sizes was used (4, 10, and 28 ?m). All the samples were mixed with a fixed amount of a 20%wt solution of NaOH or KOH, in order to produce a paste-like material, treated at 1250°C for 6 hours, and then slowly cooled. The final products were analysed by means of X-ray powder diffraction and the mineralogical composition evaluated with the Rietveld-RIR method, as a non negligible amount of amorphous material was present. The results showed that, for quartz only, the presence of sodium and potassium cations influence the formation of tridymite and cristobalite, as expected. The novelty of this work stands in the in-situ experiments, which showed that tridymite forms at the expense of cristobalite, which is always the first one to form. This is confirmed by ab initio calculations at HF/DFT level, which showed that cristobalite, in presence of sodium and/or potassium, is energetically favoured (Dapiaggi et al., 2015). In the more complex starting mixtures, the equilibrium mullite/amorphous has been carefully studied, using the results of the quantitative determinations. The results show that there is no significant influence of grain size on the equilibrium, and that Na is more effective than K in reducing the residual quartz. Na and K can effectively be used to control the amount of amorphous material present in the final product.