Production and quality control of calcined clay for cementitious applications

The growing demand for low-carbon alternatives in cement production has brought calcined clays—particularly kaolinitic clays—into focus as promising supplementary cementitious materials (SCMs). This review critically examines the production, characterisation, and quality control of calcined clays with emphasis on their reactivity and performance in cementitious systems. The paper identifies key challenges in ensuring consistent quality of calcined clays, including the inherent variability of natural clay sources, sensitivity to calcination parameters, and the lack of rapid, reliable quality control tools. A comprehensive dual-stage framework is proposed to address both pre- and post-calcination quality control. Pre-calcination methods such as X-ray diffraction (XRD), X-ray fluorescence (XRF), thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR) are recommended for assessing clay mineralogy, chemical composition, and structural suitability. Post-calcination techniques—ranging from isothermal calorimetry (R3 method) and methylene blue absorption to solid-state NMR and electrical conductivity measurements—are evaluated for their ability to distinguish among under-, optimally-, and over-calcined clays. The study consolidates these approaches into a practical quality control framework for industrial deployment. This integrated strategy ensures effective activation and utilisation of calcined clays, thereby enabling consistent performance and maximising the carbon-reduction potential of SCM-based cement systems.