Thermodynamics of Water Confined in Porous Calcium-Silicate-Hydrates

Abstract

Water within pores of cementitious materials plays a crucial role in the damage processes of cement pastes, particularly in the binding material comprising calcium-silicate-hydrates (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.

Hysteresis from Multiscale Porosity: Modeling Water Sorption and Shrinkage in Cement Paste

“…The 15% RH threshold differs slightly from the reference point of 11% described above, which is based on an estimate of the RH at which the adsorbed layer on the pore surface is one molecule thick [37,38], not specifically on a threshold between gel pore and interlayer water removal. The existence of a threshold RH for interlayer water desorption is supported by some of the present authors' molecular-scale simulations [21,22], as shown in Figs. 3(a)-3(c) and by NMR experiments [31], as shown in Fig. 3(c).…”

“…At present, connections between measured sorption isotherms, pore structure, and associated properties rely on models that link the evaporation and condensation of water at a given RH with the width and connectedness of the pores over a range of scales [18,19,21,22]. Models for mesoporous materials are typically extensions of the Kelvin-Laplace theory, which links the RH with the width of pores being emptied or filled and with the pore pressure causing volumetric strain [23,24].…”

“…This partial isotherm is then subtracted from the entire experimental isotherm for cement paste (second and further sorption cycles), providing a reduced sorption isotherm that includes only the contribution from the gel and capillary pores. The interpretation of this reduced isotherm using traditional theories provides information about the pore network, whereas molecular modeling and simulations [21] and results from nuclear magnetic resonance (NMR) experiments [31] help interpret the partial isotherm associated with the molecular-scale spaces. Based on this new combined approach, we propose a simple physical model that translates microstructural information from the sorption isotherm into a correct prediction of the (reversible) drying shrinkage of cement paste over the whole range of RH.…”

Thermodynamics, kinetics, and mechanics of cesium sorption in cement paste: A multiscale assessment

“…This potential has a Coulombic part, Van der Waals interactions, as well as harmonic bonds for hydroxyl groups and harmonic angular springs for water. The C-S-H and nanopore models built with this potential have been thoroughly validated against experiments [25,26]. All the parameters for the CSHFF are as described in Ref.…”

“…The model described in Ref. [25] is used. This is a 2.483 nm C-S-H configuration containing a gel pore at 100% humidity.…”