New insights into early hydration mechanisms of vaterite-metakaolin-cement ternary blended systems

While vaterite is acknowledged to enhance the early-age strength of metakaolin-calcium carbonate-cement systems, understanding of its role in hydration remains limited. This study divides the hydration kinetics of ternary blends with vaterite substituting limestone powder into two stages, including an initial acceleration (up to 7 days) and a subsequent deceleration period. The filler effect of vaterite accelerates the reaction of clinker and metakaolin during the first 1–3 days. The earlier carboaluminate precipitation is attributed to the shortened time to reach maximum ettringite content, resulting from the accelerated depletion of gypsum. However, the enhanced pore refinement reduces the acceleration of vaterite on the rate of hydration growth after 7 days, compared to limestone powder. The R 3 test demonstrates that the vaterite reactivity is reduced by the coverage of hydration products on its surface. New insights are obtained on the correlation between reactivity and morphology evolution of vaterite in low-carbon blends.