Upcycling coal- and soft-series metakaolin in blended cement with limestone

Abstract

In a ternary blended cement system, metakaolin together with limestone are often applied as replacement of cement to mitigate the carbon emissions of concrete production. This research examined and compared the performance when replacing cement with coal or soft-series metakaolin (CMK and SMK) and limestone at different levels of substitution (15–60%). The synergistic effect between hydration reaction, pozzolanic reaction, and limestone reaction were studied to understand the mechanism. At a lower cement substitution level (15–30%), the pozzolanic reactivity dominated the strength development, hence SMK samples are found to have higher compressive properties than that of corresponding CMK samples. However, with further increasing of cement substitution (>45%), the synergy effect of CMK with limestone can compensate for the strength loss due to the formation of a higher amount of reactant carboaluminate for pore refinement. The overall performance shows that it is feasible to upcycle CMK with limestone at high substitution levels (45–60%) for the production of sustainable blended cement paste.