Numerical Method for Evaluating Relative Chloride Diffusivity of Partially Saturated Cement Paste

Practical concrete structures are often partially saturated during the service period, especially in tidal and splash zones, but relatively fewer studies focus on the diffusion characteristics of chloride ions in partially saturated cementitious materials. Therefore, it is important to investigate the relative chloride diffusivity of partially saturated cement paste to assess the service life of concrete structures and quantify the effects of key factors on the diffusion of chloride ions in cement paste. This paper aims at developing a fast and efficient numerical method for evaluating the relative chloride diffusivity of cement paste. First, an experiment is conducted on partially saturated cement paste. The electrical conductivity of cement paste is measured by electrochemical impedance spectroscopy, while the ionic concentrations of the pore solution are calculated by the Taylor model and the solubility equilibria. The relative chloride diffusivity is computed from the Nernst–Einstein equation. The microstructure of cement paste is then simulated. By introducing an equivalent hydrating cement particle model, the contribution of calcium silicate hydrate (CSH) with water-filled gel pores to the diffusion of chloride ions is quantified in an analytical manner, and the multiphase cement paste is simplified as a three-phase composite material. The random walk algorithm is adopted to calculate the chloride diffusivity of partially saturated cement paste. Finally, the accuracy of the numerical method is evaluated through comparisons with experimental data. It is demonstrated that the correlation coefficients are between 0.8988 and 0.9990 for different water/cement ratios and between 0.9425 and 0.9978 for different degrees of saturation. Therefore, the proposed numerical method is reliable and of higher accuracy.