Influence of Chloride Transport Modes and Hydrated Cement Chemistry on Chloride Profile and Binding Mechanisms in Concrete

A chloride binding model consisting of physical adsorption and chemical ion exchange was proposed. Chemical binding was quantified based on the thermodynamic equilibrium of the relevant hydrated phases while physical adsorption was modeled by a Freundlich-type isotherm. Comparison of the proposed model results with experimental chloride binding data in the literature was found to be satisfactory. The implementation of the model in the Nernst-Planck-Poisson (NPP) reactive transport NPP model accurately predicted the free and total chloride concentration profiles in the analyzed cement paste and concrete specimens. The results of the traditional chloride diffusion model based on Fick's second law were compared with the NPP model and it was discovered that estimating the apparent diffusion coefficient by a well-established formula and using it in Fick's model gave a significantly erroneous estimation of the free and total chloride profiles compared to the corresponding experimental data. Finally, in the NPP model it was determined that chloride transport by diffusion was limited to the region near the exposed surface while in the concrete bulk it was dominated by electromigration.