Thermal activation of low and moderate kaolinite clays: Optimising calcination for enhanced pozzolanic reactivity

This study investigates the thermal activation of two kaolinitic clays from southern Saudi Arabia and evaluates their suitability as supplementary cementitious materials (SCMs). Calcination was performed over a temperature range of 650–900 °C with dwell times of 1 and 2 h, and the resulting materials were characterised using XRD, XRF, TGA/DTG, SEM–EDS, and isothermal calorimetry, while performance was assessed through the strength activity index (SAI). Results show that complete dehydroxylation of kaolinite and formation of highly reactive amorphous metakaolin occur within the range of 750–850 °C, with optimal performance achieved at a 1 h dwell. At higher temperatures (≥900 °C) or longer dwell times (≥2 h), partial recrystallisation to spinel-type phases reduces the amorphous fraction and pozzolanic reactivity. Calcined clays significantly enhanced hydration due to the formation of reactive amorphous aluminosilicates, increasing cumulative heat from ∼110–130 J·g⁻¹ (raw) to ∼205–230 J·g⁻¹ (calcined), indicating active participation in secondary hydration reactions. Mechanical performance reflects these trends, with the yellow clay achieving a maximum SAI of ∼105% at 750 °C (7 days) and ∼100% at 28 days, while the red clay requires higher temperatures (800–850 °C) to reach comparable values (∼95%). The superior performance of the yellow clay is attributed to its higher kaolinite content and lower proportion of inert phases, resulting in a greater reactive amorphous fraction after calcination. Overall, the results demonstrate that calcination at 750–850 °C for 1 h provides an optimal activation window for producing reactive metakaolin from local clays, supporting their use as effective SCMs in low-clinker, low-carbon binder systems such as LC3.