Multiscale insights into the enhancing effect of nano-metakaolin on early tensile performance of concrete: From experiment to molecular dynamics

Improving the early tensile performance of concrete composites are critical for precast construction. This study investigates the effects of nano-metakaolin (NMK) dosage on the early splitting tensile strength( f st ) and failure behavior of concrete composites. The mechanisms by which NMK improves early concrete performance were systematically discussed using a multiscale approach combining microstructural characterization and molecular dynamics simulations. The results demonstrate that NMK significantly enhances early f st and effectively restrains crack widening, with an optimal dosage of 5% NMK. Compared with ordinary concrete, the f st of NMC5 increased by 12.3%, 27.5%, 20.6%, and 27.3% at 1d, 3d, 7d, and 14d, respectively, while the maximum crack width decreased by 34.2%, 47.0%, 54.1%, and 47.9%. Microstructural analyses revealed a denser interfacial transition zone, and increased hydration product content, with NMC5 exhibiting 17.0% and 11.1% higher bound water contents at 7d and 14d, respectively. Molecular dynamics simulations further showed that NMK promotes the directed diffusion and adsorption of Ca 2 + ions toward silicate surfaces, increasing the diffusion rate by a factor of 3.5 and reducing Ca 2+ concentration in the pore solution, thereby accelerating clinker dissolution and early hydration. These combined nucleation and pozzolanic effects lead to a refined microstructure and enhanced crack resistance, ultimately explaining the superior early-age tensile performance of NMK concrete.