Refinement of Kelvin equation for condensation of water in CSH slits based on molecular simulation

Abstract

The adsorption and condensation of water molecules in the pores of cementitious materials play a crucial role in the mechanical performance and durability of concrete. The Kelvin equation is generally employed to describe the water condensation in porous materials. However, it requires modification for condensation behavior in nano and micro scales of pores. In the present work, various widths of slit models for several cementitious minerals were established. The adsorption isotherms in slit pores were calculated using the hybrid of Grand Canonical Monte Carlo and Molecular Dynamics simulations, obtaining the relationship between the critical pressures for condensation and pore sizes. Then, a modified Kelvin equation was proposed, with discussion on a new reasonable method for determining the real thickness of adsorbed water film. The results demonstrate that condensation will occur in the slits within the critical width range, which promotes the surface adsorption to achieve complete adsorption. Therefore, the location of the interface between the adsorbed water film and condensed water bulk is fixed in the same mineral during condensation. The modified Kelvin equation shows higher accuracy than traditional Kelvin equations at the nano and micro scale. This work promotes a deeper understanding of the interaction mechanism of water with cementitious minerals and inspires more fundamental realization in the design of concrete durability.