The role of chloride binding mechanism in the interpretation of chloride profiles in concrete containing limestone powder

Abstract

The effect of limestone powder addition on the chloride binding capacity of concrete was evaluated. The results indicate that the chloride binding capacity becomes weaker as the limestone content increases due to decreased physical adsorption and chemical binding. XRD patterns indicate that adding limestone restrains the formation of monosulfate (AFm), thereby undermining the chemical binding. Thermogravimetric analyses (TGA) validate that despite limited participation of calcite in the hydration reaction, it is significant enough to change the stoichiometry of the cementitious systems and restrain the formation of Friedel's salt. The ratio between the apparent and effective chloride diffusion coefficients increases with decreasing chloride binding. A modified diffusion model was proposed based on Fick’s second law coupled with Freundlich adsorption isotherm on water-accessible pore surfaces to predict the chloride profile. The model exhibits a consistent trend that an enhanced chloride binding, higher tortuosity and lower porosity contribute to a lower chloride content.