Properties of coral waste-based mortar incorporating metakaolin: Part II. Chloride migration and binding behaviors

•Coral waste power was used to prepare marine mortar and effect of metakaolin addition was investigated.•Thermodynamic analysis was applied to uncover the mechanisms of hydrate conversion in chloride-rich environment.•Carboaluminate has the high capacity to form Friedel’s salt thermodynamically. This study investigated the mechanical properties, chloride migration and binding mechanisms of marine mortars with coral waste powder (CP) incorporating metakaolin. Due to the synergetic effect of carboaluminate formation and pozzolanic reaction, the compressive strength and chloride resistance of CP mortars are dramatically improved by metakaolin incorporation. When exposed to 0.5 M NaCl solution, more than 75% of the total bound chloride is bound by the hydrated mortars within 24 h. Friedel’s salt (Fs) is the main chloride-uptaking phase and its X-ray diffraction peak area is well consistent with bound chloride content. The thermodynamic analyses indicate that carboaluminate has great capacity to form Fs, and SO4-AFm can transform to Fs and release gypsum in temperatures below 21 °C. The released gypsum further reacts with SO4-AFm to form additional SO4-AFt after exposed to chloride solution. Besides, nearly linear relationship between bound chloride content and active alumina content in binder is found.