Influences of temperatures on the physical and chemical chloride binding of calcium silicate hydrate and Friedel salt at different chloride concentrations

The binding of chlorides to cement hydration products is a complex process and is governed by binder composition and exposure conditions. To enhance this understanding, this study aims to elucidate the influence of temperatures (25°C and 45°C) and chloride concentrations (0.5 M, 1.0 M, and 3.0 M) on the chemical and physical binding of chlorides in blended cement pastes containing fly ash and silica fume. It was found that the incorporation of alumina does not consistently enhance the overall chloride binding at elevated temperatures as it does at ambient temperature. When exposed to a low chloride concentration (0.5 M), akin to seawater conditions, the total chloride binding remains stable for Ordinary Portland Cement (OPC) and increases for lower contents of fly ash addition as the temperature rises Conversely, further replacement with fly ash and silica fume significantly reduced the total chloride binding despite increased chemical binding. This reduction is primarily linked to the less physically adsorbed chlorides in C-(A)-S-H gel, which transformed to low Ca/(Si+Al) based gels due to the low portlandite solubility. At higher chloride concentrations (3 M), the total chloride binding increases for all the mixes. The adverse impact of elevated temperatures on solubility is mitigated by the increased salt content, resulting in the formation of high Ca/(Si+Al) gels and higher physically bound chlorides. Additionaly, elevated temperature exposure also promotes the extensive transformation of the AFt phase into Friedel salt. As a result, the chloride binding of the binder is enhanced at the high chloride concentration (3 M), and this binding is proportional to the formation of AFm phases.