Unlocking clay’s potential: a comparative analysis of activation techniques for enhanced reactivity in SCMs

Clays’ reactivity can be improved through different treatments that transform their crystalline structure into an amorphous state. This study compares thermal, mechanical, and chemical activation on three laboratory grade clays (kaolinite, halloysite and montmorillonite), investigating changes in particle morphology, composition, and evaluating their reactivity as supplementary cementitious materials (SCMs) in composite cement through compressive strength tests. Across all activation methods, harsher conditions (higher temperatures, grinding times or acid contents) resulted in larger particle sizes and lower specific surface areas, reducing the clays’ degree of mineral amorphization and pozzolanic activity. All activated clays were tested with a 20% replacement of CEM I, a water-to-binder ratio of 0.5, and compared against a baseline of 100% CEM I mortar. Our findings indicate that thermal activation, particularly at 750 °C, is most effective for kaolinites and halloysites, resulting in an average 28-day compressive strength increase of 40% and 30%, respectively. For montmorillonite, mechanical activation with a ball-to-powder ratio of 10 and a duration of 75 min works best, yielding a 23% increase in the 28-day strength. While chemical activation of montmorillonite led to a 15% 28-day strength gain, thermal activation had no significant impact on its performance compared to 100% CEM I baseline. However, challenges related to the scalability and safety of chemical activation hinder its industrial application. Overall, this study provides new insights into optimizing activation methods based on clay mineralogy to improve strength performance as SCMs in composite cement, while promoting the utilization of clays that are currently deemed unsuitable for composite cement production.