Impact of temperature, pH value and multiple ions on the physisorption of chloride ion on C-S-H gel surface

Calcium silicate hydrate (C-S-H) gel, as the main hydration phase of cement, intensely influences the penetration and diffusion of chloride ions (Cl-) by interacting with multiple ions in diverse environments, affecting the durability of reinforced concrete. However, the adsorption mechanism and critical influence factor are indistinct. To reveal the micro/nano mechanism of the adsorption characteristics for Cl-, a comprehensive investigation was conducted, which included ion adsorption experiments, thermodynamic calculations, and molecular dynamic simulations. In contrast to the prevailing opinion that Ca2+ is a potential determining ion in cement paste systems. The results indicate that pH value and concentration of Ca2+ are the partner to determine the Cl- adsorption capacity and potential reversal of C-S-H gels. They are unable to function in isolation from each other. It should be mentioned that OH− is more important than Ca2+ for ions adsorption of C-S-H gels. Compared with Ca2+, the compensation of Na+ on the surface charge and the promotion of Cl- adsorption is limited. SO42- competes for adsorption sites with Cl-, and forms Na-SO4 clusters to inhibit the diffusion and adsorption of Cl-. Furthermore, elevated temperature not only promotes the thermal movement of ions but also decreases the Ca2+ concentration and pH value. Consequently, the activity of SiOH sites is lessened, and ions are subjected to weaker constraints by the gel, resulting in the decline of Cl- adsorption capacity. Moreover, theoretical models for calculating the amount of chloride adsorbed by the C-S-H gels are proposed from the perspectives of thermodynamics and dynamics, and the simulation results are in agreement with the experiment results.