Investigation of the hydrate formation and mechanical performance of limestone calcined clay cement paste incorporating nano-CaCO3 and nano-SiO2 as partial limestone substitutes

This study investigates the impact of nano-CaCO3 and nano-SiO2 on limestone calcined clay cement (LC3), focusing on its hydration kinetics and mechanical properties. Nano-CaCO3 incorporation accelerated early-stage hydration and induced extensive carboaluminate formation; however, it reduced the mechanical strength at later hydration stages due to its effect on calcium-(alumino)silicate-hydrate (C-(A-)S-H) characteristics and hydrate composition. A higher CaCO3 dissolution rate increased the Ca/Si ratio in the matrix, and a large amount of carboaluminate formation consumed the Ca(OH)2 and water required for the pozzolanic reaction of metakaolin. Conversely, nano-SiO2 incorporation improved the mechanical strength across all hydration stages through the filler effect, good dispersion quality, and pozzolanic reactivity. Nano-SiO2 accelerated the early-stage hydration and produced ample C-(A-)S-H, which effectively refined the pore structure and enhanced the mechanical strength. However, the combined use of nano-CaCO3 and nano-SiO2 adversely affected the mechanical strength and pore structure of the cement pastes owing to strong agglomeration, despite accelerated hydration in the early-stages.