Quantifying anisotropic chloride diffusion coefficients of interfacial transition zone in concrete

Experimental measurement of chloride ion diffusion coefficient and pore structure of the interfacial transition zone (ITZ) between cement paste and aggregate in concrete is challenging because of its microscopic scale. The quantitative relationship between pore connectivity and the chloride diffusion coefficient has yet to be established and experimentally validated. In this work, the chloride ion diffusion coefficient in the ITZ was directly determined in experiments using artificial aggregates and the natural diffusion method, which closely simulates the realistic service conditions of concrete. The pore connectivity was analyzed using a three-dimensional pore structure model based on cement hydration. The results indicate that the magnitude of diffusion coefficient in the ITZ is approximately five to ten times that of the cement paste. In addition, the chloride ion diffusion coefficient in the ITZ exhibits anisotropy: in the direction parallel to the aggregate interface, there is high porosity connectivity and a large diffusion coefficient. This is the primary reason behind the distinctive diffusion behavior of chloride ions inside ITZ from that in bulk cement paste, as evidenced by the linear relationship between the diffusion coefficient and porosity.