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.

Due to the high chloride ion concentration in marine environments, chloride ions can penetrate into concrete, along with the transportation of water molecules, and thus, cause rebar corrosion. This is an important reason for the decrease in the concrete durability. In this paper, by means of the sol–gel synthesis method, a silane emulsion, a silane compound emulsion, and silane compound gel were prepared by using isobutyl triethoxysilane and ethyl orthosilicate (TEOS) as the main raw materials. These silane materials were coated on a concrete surface and then placed in the marine exposure sites of Wheat Island, in Qingdao, for one year. The results show that these silane materials can effectively inhibit the chloride ions transporting in concrete. Upon the increase of the exposure time, the protection ability of the silane emulsion declined gradually, while the silane compound emulsion had stable protective effects. In comparison with the permeability coefficient of chloride ions in the blank test block, the permeability coefficient of chloride ions in the test block coated with the silane emulsion, the composition of silane emulsion, and silane compound gel fell off around 20%, 30%, and 50%, respectively. The test results indicate that the three kinds of protective materials have good inhibition abilities against chloride ion transportation, on which the compound gel had the best effect.

Functionalization enhances the interfacial interaction between epoxy resin and carbon nanotubes and improves the stability of interfacial bonding.

Changes in the wettability of CASH with different Al/Si ratios.

Calcium ion, as an essential component in CASH, affects the aggregation and formation process of CASH, thereby influencing its microstructure and mechanical properties. However, the mechanism by which calcium ions affect the polymerization process of CASH is not yet fully understood. In this study, the effects of calcium ions on the polymerization process, coagulation state, and microstructure of CASH are investigated via molecular dynamics simulation. The results indicate that the presence of a trace amount of Ca²⁺ attracts oligomers towards the calcium-rich region, thus speeding up the polymerization to some extent, but as the Ca²⁺ content increases, more Ca²⁺ binds to the oxygen atoms in silica–oxygen tetrahedra and aluminum–oxygen tetrahedra, forming tight ion pairs and occupying the hydroxyl binding sites required for the polycondensation reaction. This inhibits the continuous aggregation of CASH gel and slows down the rate of polymerization. Additionally, Ca²⁺ attracts oxygen atoms from free water molecules and free OH⁻, forming Ca(OH)₂ dispersed in the spatial structure, which hinders the formation of larger clusters. As a result, the higher the Ca ion content in the system, the lower the overall polymerization degree of the CASH gel, resulting in a decrease in the conversion of the Q1 dimer to Q2 and Q3 chain structures, a shorter average chain length, poorer overall connectivity, and a transition from large clusters in a better-aggregated state to dispersed small clusters. This study sheds light on the polymerization reaction mechanism of CASH gels.