Molecular dynamics simulation of coupled water and ion adsorption in the nano-pores of a realistic calcium-silicate-hydrate gel

The adsorption behavior of water and ions at the surfaces of porous calcium-silicate-hydrate (C-S-H) gels affects the durability of concrete. This paper presents the results of molecular dynamics simulations performed using the Clay Force Field that were conducted to investigate the local structure, adsorption behavior, and dynamic properties of water and ions in the pores of C-S-H gels. A realistic C-S-H gel channel model was constructed and simulated with three mixed salt solutions (NaCl + Na2SO4,NaCl + Na2CO3, and Na2SO4 + Na2CO3) in the pores. The realistic C-S-H gel surface was found to be hydrophilic, causing water molecules to adopt an orderly arrangement near the substrate surface. This hydrophilicity is due to defective silicon chains in the substrate that provide large numbers of hydrogen bonding sites for surface water molecules. Additionally, calcium atoms near the surface attract water molecules to form hydration layers. The adsorption of cations near the substrate correlated negatively with the strengths of the ionic clusters formed by the dissolved anions and cations. As a result, the adsorption of sodium ion was strongest for the NaCl + Na2SO4 solution and weakest for the Na2SO4 + Na2CO3 solution. There were also clear differences in the adsorption behaviors of the anions. Chloride adsorption was mainly driven by ion pairing with calcium atoms near the surface, while sulfate adsorption was mainly due to ion pairing with surface-adsorbed sodium ions. Conversely, carbonate ions exhibited weaker surface adsorption because of their tendency to form ionic clusters in solution. These insights into the adsorption behaviors of common ions near a realistic C-S-H gel surface will be useful in future efforts to develop modified cement-based materials with improved properties. (C) 2021 Elsevier Ltd. All rights reserved.