Slag hydration and chloride binding in slag cements exposed to a combined chloride-sulphate solution

The effects of curing temperature (20°C or 38°C), slag composition and slag content on the hydration and chloride binding characteristics of ground granulated blast-furnace slag (ggbs) - blended cements have been studied. Two slags (CaO/SiO2 = 1.05 or 0.94) were each blended with CEM I 52.5R at 30% and 70% replacement. Paste samples of constant 0.5 w/b ratio were hydrated for 56 days, dried and exposed to either sodium chloride or combined sodium chloride – sodium sulphate solutions. Sodium sulphate concentration was 3g/L, while chloride concentrations varied from 0.1 to 3.0M. Post-exposure reaction products were characterised by XRD and thermal analysis. Chloride binding was followed via Friedel’s salt formation, with thermal analysis showing a decrease in Friedel’s salt formation in the presence of sulphate ions. Increasing slag content changed the nature of the chloride binding mechanism significantly. Hydration of slag studied by SEM-BSE image analysis showed that the higher basicity slag was more reactive, with the degree of slag hydration increasing with temperature. An increase in the slag load reduced the degree of slag hydration. Apart from the increase in slag load, the other changes in hydration corresponded well with the chloride binding characteristics of the blends.