Physically and chemically bound chlorides in hydrated cement pastes: a comparison study of the effects of silica fume and metakaolin

Increasing the chloride binding capacity of hydration products is an effective countermeasure to improve the chloride resistance of cement-based materials. The alumina content of binder is usually adopted to evaluate the chloride binding capacity, in terms of Friedel’s salt. However, the influences of aluminum on the characteristics of C–S–H and finally on physically bound chloride are not taken into account. In the present study, the characteristics of hydration products were widened by introducing silica fume and metakaolin into Portland cement pastes, respectively; then, the consequent chloride binding capacity was followed using Freundlich binding isotherms; chemically and physically bound chlorides were further distinguished. The results show that the chloride binding capacity of cement pastes reduced with the increase of SF addition, but proportionally increased with the increasing MK addition. The amount of chemically bound chloride was doubled by adding 16% MK (15.07 mg/g), which was about 6 times as high as that of cement paste with 16% SF. The chemically bound chloride through formation of Friedel’s salt by ion-exchange mainly related to the monocarboaluminate content, whereas the physically bound chloride was largely depended on the specific surface area of cement pastes. Moreover, MK promoted the substitution of Al³⁺ for Si⁴⁺ in C–S–H, resulting in more sites for chloride binding and then a higher amount of physically bound chloride. In contrast, SF increased the protonation degree of C–S–H, leading to a lower positive charge density on the surface of C–S–H and finally a lower amount of physically bound chloride due to poor electrostatic adsorption.