Chloride binding behavior of cement paste influenced by metakaolin dosage and chloride concentration

The chloride binding capacity of cement paste is of great significance for mitigating steel corrosion and failure of concrete structures. Metakaolin (MK), as an important mineral admixture, can improve the chloride binding capacity of cement paste effectively and strengthen its durability under chloride attack environments. This study systemically investigated the effects of MK and chloride concentration on the chloride binding capacity of cement paste and of C-S-H, and quantitatively characterized the evolution of chemically bound chloride and physically bound chloride. The results show that the presence of MK contribute to the growth of total bound chloride content, the underlying mechanism for which is the increase of Friedel's salt (Fs) and hydrated calcium silicate gel (C-S-H) caused by MK. Meanwhile, the monotonous growth in C-S-H adsorbed chloride is the major cause for the continuous increasing of total bound chloride with chloride concentration increasing, considering that Fs fixed chloride first increases, and then declines and stabilizes along with chloride concentration. Furthermore, the incorporation of MK slightly decreases the content of chloride adsorbed by unit mass C-S-H, while the increasing of chloride concentration significantly increases the chloride binding capacity of C-S-H. The last but not the least, when the concentration of salt solution is lower than 1.0 mol/L, the chloride contained in Fs dominates in total bound chloride, and when it is beyond 1.0 mol/L, the C-S-H adsorbed chloride starts taking the absolute lead.