This paper presents a three-dimensional mesoscale model of concrete with random packing of spherical aggregates, in which the mortar and interfacial transition zone (ITZ) are discretized based on the Voronoi tessellation. A non-steady state model is developed to simulate the chloride diffusion-sorption behavior, with different water diffusion coefficients for wetting and drying processes, respectively. The results confirm general suitability of Voronoi diagram in element mesh generation. Large deviations in the chloride profile predictions may be generated if a single value or unified equation is used for the water diffusion coefficient of both wetting and drying processes. The peak chloride concentration increases with the exposure time. The overall chloride concentration increases as the porosity increases and the tortuosity decreases. Sensitivity analyses suggest that, compared with porosity and constrictivity, tortuosity has a more significant effect on the overall chloride concentration.