Thermal behaviour of clays for traditional ceramics with soda–lime–silica waste glass admixture

The thermal behaviour of clay body mixtures with soda–lime–silica waste glass for the production of traditional ceramics was assessed. The effect of calcite content in the body mixture, of the particle size distribution of glass and of the firing temperature was investigated. In the case of calcite-rich mixtures with glass, increased expansion may take place during firing, at temperatures slightly higher than 700 °C. This effect is attributed to the entrapment of released gases, mainly CO2. Sintering starts at lower temperatures for the samples with glass, whereas, a second shrinkage zone was observed for temperatures approaching 1000 °C. The content of calcite is one of the main factors determining the mineralogy of the sintered body and the extent of glass devitrification. Devitrite, cristobalite and wollastonite are the products of devitrification within the glass grains, whereas, sodium aluminum silicate, most probably nepheline, has been formed at the inter-granular rim, between glass and ceramic matrix. Increase in the firing temperature and/or decrease in the particle size distribution of glass promotes the densification of the body.