Microscopic modelling of gas diffusivity in unsaturated cementitious materials considering multiple diffusion regimes

Abstract

Common degradations of concrete including reinforcement corrosion and carbonation are highly associated with the transport of aggressive gases. However, it is still challenging to accurately understand the gas diffusion behaviour and determine the gas diffusivity in unsaturated cementitious materials. This study proposes a pore-scale model for simulating gas diffusivity in unsaturated cement paste covering multiple gas diffusion mechanisms. The representative volume elements of colloidal calcium silicate hydrate and heterogeneous cement paste with various water saturation degrees are established, capturing the structural attributes of hierarchical cement paste and water-gas distribution with minimum surface energy. A finite difference model for diffusion considering Knudsen effect and molecular diffusion is developed to simulate the diffusion process of various gases through partially saturated cement pastes with different pore structures. Results indicate that gas diffusivity decreases with the increasing water saturation level, the evolution of which can be divided into stable, decreasing and non-diffusive stages. The gas diffusion in partially saturated cement paste is highly dependent on the pore structure and diffusive gas type. The results obtained from this study are expected to provide a deeper insight into the solution of diffusion-related durability problems and the prediction of service life of concrete.