Reactivity of kaolinitic clays calcined in the 650°C-1050°C temperature range: Towards a robust assessment of overcalcination

Using calcined clays as supplementary cementitious materials is one of the main solutions available for a substantial and widespread reduction of the clinker factor in cement. Thus, understanding the phenomena governing their production process and reactivity is crucial for a successful technological deployment. In this study, the mineralogical and physical changes experienced by kaolinitic clays calcined within 650 and 1050°C were explored, with emphasis on the identification of overcalcination. It was observed that the decrease in reactivity above the optimum calcination temperature is associated with the formation of Al-Si spinel and a decrease in specific surface area, which is linked to a modification of the pore size distribution within the particles. Differential scanning calorimetry (DSC) was identified as the most robust, and repeatable technique to identify an overcalcined kaolinitic clay. In combination with thermogravimetric analysis (TGA), it can provide a complete calcination process overview, relevant for quality control purposes.