Insights into the thermal effect on the fracture toughness of calcium silicate hydrate grains: A reactive molecular dynamics study

Cement-based materials behave with brittle properties at the macro scale. By contrast, at the nanoscale, its main phase, the calcium silicate hydrate (C-S-H) grain, shows better ductility. However, the nature of the relationship between the fracture properties and the heating temperature remains elusive. To this end, reactive molecular dynamics simulations on the pre-cracked C-S-H grain with various Ca/Si (C/S) molar ratios (1.10-1.80) are carried out to study the thermal effect on the fracture property. For the first time, we report that heating can enhance the C-S-H grains' fracture toughness and the energy release rate. According to simulation results, we further discuss the evolution of the brittleness of C-S-H grains with temperature. Interestingly, it can be found that heating can make improvements in the brittleness of the C-S-H grain. Nevertheless, the degree of improvement heavily depends on the chemical compositions, wherein a lower C/S ratio can lead to a more thermally stable structure and a higher yielding strength and in turn, a slighter change in the dissipated energy at higher temperature levels. Moreover, we report that heating can induce crack blunting which can enhance the plastic deformation and the dissipated energy and improve ductility. Meanwhile, crack blunting can occur at a lower temperature level for a higher C/S ratio. These new insights into C-S-H grains' fracture behavior can help formulate multiscale models of designing higher fire-resistant cementitious composites.