Understanding the Influence of Thermal Treatment on Phase Changes and Reactivity of Smectite for Use in Limestone Calcined Clay Cement

The partial replacement of cement by calcined clays is seen as one of the most important avenues for producing low-carbon cements. However, the limited availability of kaolinite may become a major barrier for the industrial adoption of this technology. This paper demonstrates the potential use of iron-rich smectite clays, which are more easily available in many regions, as supplementary cementitious materials by comparing the physical and chemical changes that occur in smectite with those in kaolinite upon thermal treatment. The clays were calcined between 400°C and 1,000°C and were characterized using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Fourier transformation infrared spectroscopy (FTIR) techniques. The physical and chemical properties of calcined clays like specific surface area, density, morphology, and cation exchange capacity were also examined. The dehydroxylation process of the clays was found to be distinct: smectite undergoes a two-step dehydroxylation, transforming into a highly amorphous phase only after the removal of hydroxyl groups in the second step unlike kaolinites. The effect of clay mineral type and calcination temperature on the pozzolanic reactivity and mortar compressive strength was also assessed. It was observed that properly calcined smectite clay exhibits pozzolanic reactivity similar to or even higher than that of kaolinite. The type of clay mineral influences the rate of strength development because smectite showed slower but longer hydration and ultimately matched the compressive strength of kaolinite by 28 days. These findings present the potential of producing sustainable calcined clay cements more widely around the world.