Reactivity of industrial clays subjected to mechanical activation

Reducing clinker content in composite cements is a key strategy to decrease CO₂ emissions in the cement industry. To increase low-grade clay substitution in cement, mechanical activation provides an alternative activation method, particularly for clays with high levels of 2:1 layer silicate minerals. However, its effects on clay reactivity are not yet fully understood fundamentally. This study evaluates mechanically activated smectite (S) and kaolinite-rich (K) clays using two tests – Strength Activity Index (SAI) and R3 test – to assess both mechanical performance and chemical reactivity in cement-based systems, using their thermally activated counterparts as reference. The clays were activated in a planetary ball mill with a ball-to-powder ratio of 10:1 and rotation speed of 500 rpm for 20 minutes; their calcined counterparts were subjected to 850 °C (K) and 750 °C (S) for 1 hour. The clays were characterized by particle size distribution, X-ray fluorescence, X-ray diffraction, and thermogravimetric analysis. The mechanical activation efficacy was analyzed as a function of mineral structure changes, heat of reaction, and strength contribution in blended cement. Results show that mechanical activation is more effective in increasing SAI for S-type clay, while thermal activation is more effective for K-type clay. The R3 test correlated well with SAI for K but did not capture the enhanced performance of mechanically activated S, likely due to the slower reaction kinetics of smectite and physical effects which affect strength development. These findings highlight the importance of combining chemical and mechanical reactivity assessments and considering clay’s mineralogy when selecting activation methods.