Modeling the ionic diffusion coefficient of unsaturated hardened cement paste: A micromechanical approach

Most of the concrete structures in service are unsaturated, estimating the ionic diffusion coefficient of unsaturated cement-based material is of paramount significance to assessment of corrosion of reinforced concrete. Taking account of the contributions from different water configurations at multi-scale (water films, interlayer water, large gel pore (LGP) water and capillary pore water) and the corresponding diffusion mechanisms of each type of water, a micromechanical model for estimating the ionic diffusion coefficient of unsaturated hardened cement paste (UHCP), is developed in this study. Model results show that the interconnectivity of the spheroidal capillary pore water is closely related to the aspect ratio of spheroidal capillary pore, the slender the prolate pore, the higher the interconnectivity of the capillary pore water; when the capillary pore water is totally drained, water films and interlayer water within C-S-H governs the ionic diffusion, the higher the water to cement ratio (w/c), the less the contribution of interlayer water, and the greater the contribution of water film, to ionic diffusion within UHCP. Application on saturated hardened cement paste and UHCP shows the capability of the model to accurately reproduce the experimental results.