Molecular dynamics study of water and ions transport in nano-pore of layered structure: A case study of tobermorite

Abstract

Water and ions transport in the cement hydrate determines the durability of cementitious material. Due to the structural similarity, tobermorite, an important mineral analogue of the main phase of cement hydrate, was used to investigate transport behaviour at the molecular level. In this study, the structural and dynamical properties of the water/ions and the tobermorite interface were studied by the molecular dynamics (MD) simulation method. On the (001) surface of tobermorite, water molecules diffusing in the channel between the silicate chains demonstrate a number of structural water features: large density, good orientation preference, ordered interfacial organization and low diffusion rate. The dipole vector of type 1 water molecules point upwards due to the attraction from the calcium sheet, while type 2 water molecules turn downwards due to the restrained H-bonds donated from ONB atoms in the silicate chains. The stable H-bonds connected with ONB in the silicate chains restrict the mobility of the channel water molecules. The significant reduction of the diffusion coefficient matches well with the experimental results obtained by NMR, QENS and PCFR techniques. With increasing distance from the channel, the structural and dynamical behavior of the water molecules vary and gradually translate into bulk water properties at distances of 10–15Å from the liquid–solid interface. In the respect of the ions and tobermorite interaction, there is Cl− repulsion and Ca2+ adsorption from the surface. In addition, in simulation time longer than 1ns, Cl− can diffuse to the surficial Ca2+ and forms the unstable Ca-Cl cluster, which accelerates the desorption of the surface Ca atoms.