Numerical Simulation of Chloride Ion Ingression in Mortar Incorporating the Effect of ITZ Using an Integrated COMSOL-IPHREEQC Framework

The prediction of chloride ingression in cement-based material has gained a great deal of interest among researchers as it causes long-term structural damage in buildings by chloride-induced reinforcement corrosion. The Cl⁻ diffusion in mortar is influenced by internal factors including pore-structure and hydrates which are determined by clinker properties, mixture recipe, and curing conditions and exposure conditions. The Cl⁻ penetration in mortar leads to the modification of the microstructure and pore-solution due to the disequilibrium of the hydrates-pore solution system. Considering the complexity of the process by incorporating all aforementioned factors and interaction of Cl⁻ with hydrates, a new model is herein proposed for predicting the microstructure of the mortar during the Cl⁻ diffusion. In this work, the microstructure of mortar is considered as a three-phase material: aggregates, interfacial transition zone (ITZ) and bulk paste, and ITZ is realistically considered as high W/C paste compared to the initial W/C. The developed COMSOL-IPHREEQC model involves hydration model for calculating the dissolution of clinker in bulk paste and ITZ, thermodynamic model including the surface complexation model to predict the hydrates and the Cl⁻ adsorption by hydrates, homogenization approach to compute the average hydrates, porosity, pore solution composition and diffusion parameters of the mortar and COMSOL Multiphysics to perform the transportation calculation. The predicted results are validated with experimental results available in the literatures to verify the reliability of the proposed model. The effect of ITZ on the penetration of Cl⁻ is also assessed in this work.