Hydrated calcium silicate (C-S-H) gel is the main bonding colloid in cement-based materials, and it is also the main source of physical adsorption of chloride and sulphate ions. Due to that the process of C-S-H physical adsorption and the change of the adsorption amount with time cannot be reflected directly by experimental methods. In this paper, a molecular dynamics model was used to simulate the process of C-S-H phy-sical adsorption of chloride and sulphate ions. The interaction between chloride and sulfate ions during the adsorption process was investigated. The coordination number and the isothermal adsorption method were used to calculate the adsorption amount of Cl- and SO42- in 3.5% NaCl and 3.5% NaCl + 3.5% Na2SO4 solution. The research results show that the adsorption capacity of C-S-H gel for Cl- in the two solutions is almost the same, about 0.1 mmol/g, and the adsorption capacity for SO42- is about 0.04 mmol/g in compound salts solution.The physical adsorption includes short-range Coulomb interaction between ions and long-range van der Waals (VDW) interaction. The adsorption of C-S-H to Cl- is delayed and the long-range VDW effect of C-S-H on Cl- is weakened by SO42-. However, the presence of SO42- doesn't reduce the amount of Cl- adsorption at the site, it only delays the time of adsorption. SiOCa+ is the main adsorption site for Cl- and SO42-. The SiOH sites which have no obvious adsorption effect on SO42- have ability to adsorb Cl-, but the amount is negligible. Moreover, there is also a competitive relationship between SiOCa+ and SiOH sites. The ions adsorption of C-S-H calculated by molecular dynamic is consistent with the ion adsorption of the C-S-H synthesized by experimental methods or the C-S-H in cement slurry. It can be proved that molecular dynamics simulations can be used to accurately and reliably characterize the complex interactions of aggressive ions in cement-based materials. © 2021, Materials Review Magazine. All right reserved.
