Concentration-induced wettability alteration of nanoscale NaCl solution droplets on the CSH surface

Due to the hydrophilic nature of concrete, moisture and corrosive ions adsorbed on its surface pose a severe challenge to the durability of concrete structures; therefore, investigating the wettability of a concrete surface is an indispensable prerequisite for designing durable and sustainable concrete. This paper utilizes molecular dynamics to simulate the concentration-induced wettability alteration of nanoscale NaCl droplets on a CSH surface, and verifies the feasibility and rationality of predicting the contact angle of a droplet on the CSH surface based on the surface tension. Results suggest that the wetting ability of droplets on the CSH surface is weakened with the increase of the NaCl mass fraction. Microscopic analysis reveals that water molecules clustered around Na⁺ and Cl^- ions to form an ion hydration shell (Na⁺-Ow and Cl^--Hw pairs); the binding energy barrier of these ion pairs is much larger than the dissociation energy barrier, which enhances the associative ability of the NaCl droplets. The particles on the CSH surface attract Na⁺ and Cl^- ions by forming Oh(Os)-Na⁺ and Ca(Ho)-Cl^- connections with droplets, which further weakens the spreading ability of water molecules on the CSH surface.