Thermodynamics of water confined in porous calcium-silicate-hydrates

International audience; Water within pores of cementitious materials plays a crucial role in the damage processes of cement pastes, particularly in the binding material comprising calcium-silicatehydrates (C−S−H). Here, we employed Grand Canonical Monte Carlo simulations to investigate the properties of water confined at ambient temperature within and between C−S−H nanoparticles or "grains" as a function of the relative humidity (%RH). We address the effect of water on the cohesion of cement pastes by computing fluid internal pressures within and between grains as a function of %RH and intergranular separation distance, from 1 to 10 Å. We found that, within a C−S−H grain and between C−S−H grains, pores are completely filled with water for %RH larger than 20%. While the cohesion of the cement paste is mainly driven by the calcium ions in the C−S−H, water facilitates a disjoining behavior inside a C−S−H grain. Between C−S−H grains, confined water diminishes or enhances the cohesion of the material depending on the intergranular distance. At very low %RH, the loss of water increases the cohesion within a C−S−H grain and reduces the cohesion between C−S−H grains. These findings provide insights into the behavior of C−S−H in dry or high-temperature environments, with a loss of cohesion between C−S−H grains due to the loss of water content. Such quantification provides the necessary baseline to understand cement paste damaging upon extreme thermal, mechanical, and salt-rich environments.