Effect of temperature and defects on microstructure and mechanical properties of nano-silica/C–S–H composites

Calcium silicate hydrate (C–S–H) is a primary source of concrete strength, and optimizing its properties is crucial for enhancing the durability of concrete. Nanosilica (NS) as a reinforcing phase can effectively improve the C–S–H properties; however, its actual efficacy is significantly affected by the service temperature and its defects, and the coupling mechanism between the two at the atomic scale remains unclear. In this study, molecular dynamics simulations were employed to systematically elucidate the mechanism of the synergistic effects of temperature (100 K–500 K) and three typical oxygen defects (V1, V2, and V3) on the interfacial structure and mechanical properties of NS/C–S–H composites. The results indicate that oxygen defects enhance the interaction of NS with water molecules, thereby increasing the material's hydrophilicity to a certain extent. However, this hydrophilicity gradually decreases with increasing temperature, especially at 400K and 500K. The radial distribution function (RDF) analysis reveals that an increase in temperature results in a decrease in the characteristic peaks of Oz-Ow and Si–Os, indicating that the interatomic distances have increased and the interactions have weakened. At 300 K, the tensile strength (1.590 GPa) and Young's modulus (30.872 GPa) of the NS/C–S–H composites were greater compared to those of the C–S–H gels. NS-V2 exhibits the highest tensile strength (1.773 GPa) at 300K. NS-V3, on the other hand, exhibits excellent tensile toughness and compression modulus in the 100K–200K and 400K–500K ranges. In contrast, NS-V1 has limited mechanical property enhancement. This study elucidates the mechanism of the multivariate coupling effect of temperature and oxygen defects on NS/C–S–H at the atomic scale. The findings provide a basis for optimizing the strength, modulus, and toughness of composites at different service temperatures (especially at medium and high temperatures), which is essential for the design of high-performance concretes for applications in extreme environments.