Are all non-kaolinitic phases detrimental to LC3 performance? Effects of mineral type, particle size, and mechanochemical activation

Limestone calcined clay cement (LC 3 ) has emerged as a promising low-carbon alternative to Ordinary Portland Cement (OPC), yet its performance is often constrained by the presence of non-kaolinitic minerals in natural clays. Although traditionally regarded as detrimental, these impurities pose practical and environmental challenges due to the high cost of their removal. This study systematically evaluates four representative types (montmorillonite, quartz, chlorite, and illite), to clarify their distinct impacts on hydration and strength. The effects of particle size and morphology evolution induced by mechanochemical activation (MCA) were further decoupled using a controlled metakaolin-based system. Results reveal distinct impurity-specific behaviors: montmorillonite exhibits higher reactivity than other impurities and enhances both hydration and strength owing to its porous layered structure that facilitates pozzolanic reactions. Quartz, although chemically inert, contributes positively through micro-filler effects and morphological refinement; when replacing 30 % of metakaolin, the 90-day compressive strength remains comparable to the pristine metakaolin reference group without quartz. In contrast, illite and chlorite exhibit limited or even adverse influences on hydration and strength development. Further analysis indicates that specific surface area exerts a more decisive influence on mechanical performance than particle size. This effect becomes particularly evident under MCA, which increases the BET surface area by approximately three- to sixfold (from 1 to 3–8–11 m²/g) and enhances reactivity by four- to sixfold as confirmed by R³ testing, accompanied by a moderate (∼10 %) strength improvement. The study demonstrates that MCA can effectively mitigate the negative effects of certain impurities while amplifying the positive contributions of others. These findings highlight morphology engineering through MCA as an energy-efficient strategy for optimizing LC 3 formulations derived from low-grade clays. • Evaluated four representative non-kaolinitic minerals in LC 3 systems. • Montmorillonite improves pozzolanic reactivity and strength. • Specific surface area outweighs particle size in determining reactivity. • Mechanochemical activation enhances reactivity and overall performance.