Water and ions transport in calcium silicate hydrate: a molecular dynamics study

Transport properties of water and ions in calcium silicate hydrate (C-S-H) greatly affect the durability of cementitious materials. In this study, molecular dynamics (MD) technique is used to investigate the transport behaviors of NaCl solution in C-S-H nanopores with different sizes (from 0.5 nm to 5 nm), and the hindering effect of C-S-H on the diffusion of water molecules and Cl ions is further explored in the case of a 5 nm pore. Results show that the diffusion coefficients of water molecules and Cl ions in C-S-H nanopores increase with the expansion of nanopore. At the atomic scale, the Ca-rich C-S-H forms Ca-O and Ca-Cl clusters with water molecules and Cl ions, respectively, and the Si-O tetrahedra on silicate chains can also build hydrogen bonding interactions with water molecules, which constrain the transport behaviors of water and ions. From the molecular perspective, this study innovatively investigates the effect of C-S-H pore size on the diffusion capacity of water and ions, and reveals the chemical bonding mechanism between water molecules, Cl ions and C-S-H, which provides a theoretical basis for studying the resistance of concrete to ionic attack