A multi-scale framework for modelling effective gas diffusivity in dry cement paste: Combined effects of surface, Knudsen and molecular diffusion

Owing to the hierarchical pore structures of cement paste, varying gas diffusion mechanisms, i.e. surface, Knudsen and molecular diffusion, coexist. However, their combined effects have not been fully investigated. To this end, a multi-scale model which considers the combined effects is developed to simulate and analyse the effective gas diffusivity in dry cement paste. The multi-scale structures of cement paste using representative volume elements are reconstructed, capturing the structural characteristics at each scale. Afterwards, based on classical adsorption theory, random walk method and lattice Boltzmann method, the gas diffusivity at each scale is predicted and analysed. The results indicate that surface diffusion plays an important role in the gas diffusion in C-S-H, while the main diffusion regime in C-S-H gradually becomes Knudsen diffusion as the porosity increases. The surface and Knudsen diffusion cannot be ignored or simplified as molecular diffusion in the prediction of gas diffusivity in cement paste.