Effects of elevated temperature on the structure and properties of calcium-silicate-hydrate gels: the role of confined water

The binding phase of cementitious materials, calcium-silicate-hydrates, can be described as nanogranular and as an inorganic hydrogel. Similar to other hydrated "soft matter," the water confined within the nano- to microscale pores of such cementitious materials plays a crucial role in the structure and properties of cement pastes. When compared to organic hydrogels, non-stoichiometric calcium-silicate-hydrates (C-S-H) are relatively robust against changes in humidity and temperature. However, under extreme physical environments, changes in the amount, and location, and physical state of water can limit damage tolerance and sustainability of otherwise stiff and strong cementitious macrostructures. Here, we employed Grand Canonical Monte-Carlo and Molecular Dynamics simulations to investigate the effect of temperature on the water content within and between C-S-H grains constituting the cement microstructure, and on the associated physical and mechanical properties of this material. We found water content within grains decreased with increasing relative temperature up to T/T* = 2 (where T* is the transition temperature at which the bulk liquid and gas are in equilibrium for a given pressure), and that C-S-H grains densified with attendant increases in heat capacity, stiffness, and hardness. Although intragranular cohesion increased monotonically with increasing relative temperature over this range, intergranular cohesion increased up to a relative temperature of T/T* = 1.1 and then decreased at higher relative temperatures. This finding suggests a rationale for the decreased mechanical performance of cement paste and concrete at high relative temperatures, and supports previous claims of peak hardness in C-S-H at an intermediate relative temperature between 1 and 2.61. Further, these atomistic simulations underscore the important role of confined water in modulating the structure and properties of calcium-silicate-hydrates upon exposure to extreme environments.