Water transport in calcium silicate hydrate nanoscale pores by molecular dynamics: Unsaturated steady-state diffusion and suction

Abstract

Water transfer in calcium silicate hydrate (CSH) nanoscale pores is an important factor affecting the durability of cement-based materials. Using molecular dynamics, the transport behaviour of nanoscale water in unsaturated CSH gel pores can be simulated to explore the transport law and mechanism of unsaturated water in the gel pores. In this paper, two kinds of transport models were established: an unsaturated steady-state diffusion model and a suction model. The effects of pore size and saturation level on the steady-state density distribution, dipole orientation, dipole moment and diffusion coefficient of water in each region were studied. Then, the influence of temperature, pore diameter, initial saturation level and tortuosity on the water absorption rate in the suction process was analysed, and the CSH molecular formula for the saturation state was calculated. The results showed that the distribution of water in the CSH gel pore was mainly related to the saturation level and wall position, and the percolation effect was triggered when the saturation of the gel pore reached 53 %. Water molecules located in the interlayer zone, adsorption zone and free zone exhibit the laws of the free zone > adsorption zone > interlayer zone due to the differences in geometric constraints and bonding. With the increase in the saturation level of water in the gel pore, the overall diffusion coefficient of water presents a change law of first rising and then falling. A higher temperature will magnify the effect on the promotion of transport. The pore size of the gel pore is positively correlated with the diffusion capacity of water, while higher initial saturation level and tortuosity will both inhibit the absorption rate of the gel pore. The CSH formula in the saturated stable state is (CaO)1.65SiO2(H2O)2.1. For cement-based materials with a high proportion of large gel pores (above 2 nm), limiting the saturation level in the gel pores below the percolation threshold (preferably below 45 %) as far as possible can effectively inhibit the water transport capacity. The simulation results are in good agreement with the data in the literature.