Influence of the C/S and C/A ratios of hydration products on the chloride ion binding capacity of lime-SF and lime-MK mixtures

The mechanisms of chloride ion binding in cementitious systems when supplementary cementing materials are present are not completely understood, although it is believed to relate to the alumina content of the mixture. This relationship is investigated through the use of lime–silica fume and lime–metakaolin mixtures. It was found that while the alumina content does have an important influence, the chloride binding capacity was also controlled by the calcium-to-alumina and calcium-to-silica ratios. At a high C/A ratio, the formation of monocarboaluminate is favoured, which has a high ability to form Friedel's salt, and does so at low chloride concentrations (less than 0.1 M). With a low C/A ratio, the formation of stratlingite is favoured, with little formation of monocarboaluminate. If alumina is not present, chloride is bound by the C–S–H phase. The binding capacity of the C–S–H was found to depend on its calcium-to-silica ratio, C–S–H with a higher C/S having a greater binding capacity.