Synergistic effects of nano-metakaolin and nano C–S–H on the early hydration of LC3

Limestone calcined clay cement (LC3) is a promising low-clinker binder system, but its relatively slow early-age strength development remains a practical limitation. This study investigated the effects of nano-metakaolin (nMK), used as a partial replacement of the calcined-clay fraction, and a nano C–S–H accelerating suspension on the hydration, microstructure, and mechanical performance of LC3. These additives were evaluated both individually and in combination to assess their complementary influence on early-age performance. nMK was incorporated as a partial replacement of the calcined-clay fraction at levels between 5% and 20%, with 15% identified as the optimum dosage, while the nano C–S–H suspension was added at 2 wt% by mass of binder. The results show that both nanomaterials accelerated hydration and refined the pore structure of LC3. nMK promoted CH consumption and hydrate formation, while the nano C–S–H suspension primarily accelerated early hydration through nucleation-seeding effects. Their combined use generated a complementary enhancement of hydration and microstructural development, leading to increased hydrate formation, matrix densification, and pore refinement. Consequently, the combined system achieved the highest mechanical performance, with compressive strength increasing by up to 70% at 1 day and more than 30% at 28 days compared with the LC3 reference, while maintaining comparable initial workability. These findings show that nMK and a nano C–S–H suspension can enhance the early-age performance of LC3 and provide a potential strategy for mitigating the early-age strength limitations of low-clinker cement systems.