Clay-calcite mixes: sintering and phase formation

Clay-calcite mixes containing tip to 20 wt-% calcite were fired at temperatures ranging from 800 to 1075degreesC and investigated by means of X-ray diffraction and scanning electron microscopy as well as by measuring some common ceramic properties. It was found that for <10 wt-% calcite additions the sintering process is controlled by viscous flow, whereas at greater levels of addition sintering proceeds mainly by diffusion. Moreover, a metastable phase identified as grossular developed and disappeared at the expense of anorthite. The absence of wollastonite is also discussed. Attempts to predict phase formation on the basis of the CaO-SiO2-Al2O3 phase diagram show, that predictions hold only for the mixtures richest in calcite.