Increasing the reactivity of metakaolin-cement blends using zinc oxide

This study aimed to improve the reactivity of metakaolin-cement mixtures using ZnO additions. Kaolinite samples with 0.1–1 wt% ZnO were calcined at temperature intervals of 50 °C from 500 to 650 °C for 1 h. The resulting metakaolins were examined for structural changes after calcination and for their pozzolanic reactivity, influence on the hydration behavior of cement pastes, and impact on the compressive strength of mortar cubes. ZnO behaved as a delayed accelerator for cement paste. However, when ZnO was combined with highly amorphous metakaolin, chemical retardation was eliminated while acceleration was maintained. Such systems also had increased 28-day compressive strengths. ZnO additions did not affect the degree of dehydroxylation or the pozzolanic reactivity of the metakaolin. These results could facilitate the use of less pure calcined clays as SCM by providing a mechanism for improving reactivity and may also impact the ability to use zinc-contaminated materials in concrete.