The effect of short duration NaCl exposure on the surface pore structure of concrete containing GGBFS

The purpose of this study is to evaluate the effect of chloride-binder interactions on the physical pore structure of concrete containing 0–60% ground granulated blast furnace slag (GGBFS) as cement replacement. Physical and chemical chloride binder interaction was measured based on chloride binder isotherms. Physical chloride binder interaction was estimated to be the difference between the total binding capacity and chemically bound chloride. In this study it was assumed that the chemical binding is the binding capacity of the alumina phases which was estimated, using differential scanning calorimetry, as the area under the Friedel’s salt peaks. The assumption that chemical binding capacity is controlled by alumina phases is supported by multiple linear regression analysis that was conducted to assess the relationship between Fruendlich isotherm constants and the concrete’s chemical composition. To determine the effect physical and/or chemical chloride-binder interaction on the physical microstructure, paste samples were exposed to distilled water and 0.5 M NaCl solution for 6 h, and the total porosity and pore size distribution was evaluated using mercury intrusion porosimetry. Three key findings from this study are: (i) At a chloride concentration up to 0.5 M, and for mixtures containing 0 and 40% GGBFS the contribution of physically and chemically bound chloride are similar. (ii) At chloride concentrations greater than 0.5 M and less than 1 M, the total bound chloride is predominantly chemically bound for the 40% GGBFS mixture. (iii) Concrete specimens exposed to NaCl solution for 6 h exhibited different pore characteristics in comparison to concrete exposed to distilled water. The differences in the concrete microstructure are not linearly dependant on GGBFS content, but are found to be mutually influenced by the total bound chloride content, the percentage of chemically bound chlorides, and the pore size distribution of the mixture.