Improving the pozzolanic reactivity of clay, marl and obsidian through mechanochemical or thermal activation

This research investigated the physicochemical properties and pozzolanic reactivity of mechanochemically and thermally treated clay, marl, and obsidian as supplementary cementitious materials (SCMs). The results suggest that the mechanochemical treatment of clay and marl resulted in delamination, dehydroxylation, and amorphisation of the mineral components (including calcite); while for obsidian, the main effect was particle size reduction. Among all samples prepared, the mechanochemically treated obsidian exhibited the best performance as a SCM and achieved marginally higher strength performance at 20% cement replacement compared with the CEM I cement mortar (with on SCM). The thermally activated clay and marl exhibited highest pozzolanic reactivity than the mechanochemically treated counterparts owning to the formation of free lime from calcination of calcite. However, the mechanochemically treated clay and marl were still able to achieve over 80% of the strength activity index and performed much better than the untreated materials. These results indicate that mechanochemical treatment can effectively improve the pozzolanic reactivity of clay minerals that contain calcite up to 68% without directly emitting process CO₂ to the environment (calcination of carbonates), which can be an alternative activation route to the high-temperature calcination-treatment method.