Heat capacity, isothermal compressibility, isosteric heat of adsorption and thermal expansion of water confined in C-S-H

Abstract

Nanoconfinement is known to affect the property of fluids. The changes in some thermo-mechanical properties of water confined in C-S-H are still to be quantified. Here, we perform molecular simulations to obtain the adsorption isotherms in C-S-H as a function of the pore size (spanning interlayer up to large gel pores). Then, fluctuations formula in the grand canonical ensemble are used to compute the isothermal compressibility (and its reciprocal, the bulk modulus), the heat capacity, the coefficient of thermal expansion and thermal pressure, and the isosteric heat of adsorption of confined water as a function of the (nano)pore size. All these properties exhibit a pore size dependence, retrieving the bulk values for basal spacing above 2 nm. To understand why property changes with confinement, we compute structural descriptors including the radial distribution function, apparent density, hydrogen bonds counting, and excess pair entropy of water as a function of the confinement. These descriptors reveal significant structural changes in confined water. The heat capacity shows a good linear correlation with the apparent density, entropy, and hydrogen bond number. The values of water property as a function of the basal spacing are a valuable input for multiscale modeling of cement-based materials.