Synergistic mechanical and chemical activation of kaolin clays for enhanced reactivity in limestone calcined clay cement (LC3)

Abstract

Limestone calcined clay cement (LC³) produced from high-grade clays calcined at optimum temperatures demonstrates superior performance, while the utilisation of low-grade clays remains limited due to their reduced reactivity. To tackle this limitation, this study introduces an innovative multi-activation strategy that integrates calcination with simultaneous mechanical or chemical activation. Two clays with distinct kaolinite contents were subjected to this hybrid activation process, which were characterised using X-ray fluorescence (XRF), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and modified Chapelle and R³ tests. Subsequently, two grades of LC³ mortar were prepared from the activated clays, and their hydration kinetics and strength development were evaluated up to 90 d. Results indicated that thermomechanical activation significantly enhanced the pozzolanic reactivity of both clays, leading to higher heat release and strength development, particularly for LG-based LC³. Specifically, it showed a 35.5% increase in strength at 28-days compared to 7-day strength, while HG-based LC³ exhibited a 46.5% increase. In contrast, thermochemical activation resulted in the formation of zeolitic phases that adversely affected reactivity, and thus there was reduction in bound water content and Ca(OH)₂ consumption for both clays, 15.3% and 17.9%, respectively as compared to thermal activation. Overall, thermomechanical activation demonstrated superior potential for improving the performance of low-grade clays, Finally, correlation matrices were established to link clay reactivity with strength development. Furthermore, a schematic model illustrating reactivity mechanisms under different activation strategies was proposed and verified through XRD and TGA analyses.