Influence of calcination temperature on the physical and chemical characteristics of kaolinitic clays for use as supplementary cementitious materials

Abstract

Clays have the potential of being the most important supplementary cementitious material for the future. The quality of calcination, which has a very important influence on the suitability of clay as a partial replacement of cement, is influenced by the changes in the characteristics of clays during calcination. An improved knowledge of these changes can help to assess the suitability of clays. In this study, clays were calcined at seven temperatures ranging from 400 °C to 1000 °C and the effect of calcination temperature on physico-chemical properties like specific gravity, specific surface area, water demand, morphology and reactivity were investigated. The results showed that the transformation process of kaolinite to metakaolin, which is the main reactive phase in cements, completes by 800 °C and this conversion process governs the reactivity, without changing the physical properties. The metakaolin becomes unstable at calcination temperature of 1000 °C, and transforms into the spinel phase as confirmed by the differential scanning calorimetry. The spinel phase reduces the reactivity and also changes the physical properties of calcined clays, making the calcined clay unsuitable for use in cements. New test methods that can be employed to rapidly assess the quality of calcination are also discussed.