Metakaolin-based concrete for marine structures: Performances, chloride binding, and perspectives

Metakaolin (MK) contains highly reactive alumina, which could be a proper supplementary cementitious material to design low-carbon and durable concrete in marine structures. This review critically examines the characteristics of MK and its effects on cement hydration, mechanical properties, durability, and the chloride-binding capacity. The relatively fine particle size and high volume of reactive contents of silica and alumina enable MK to provide proper filler effects and pozzolanic reactions, forming additional C-A-S-H gels to refine the pore structure of the cementitious matrix. For ternary-blended system, MK could react with calcite to form hemicarboaluminate (Hc) and monocarboaluminate (Mc), while the synergistic use of MK and calcite can enable cement replacement levels of approximately 40-50 wt% while maintaining comparable strength. Appropriate MK incorporation (5 wt% to 10 wt%) can reduce water absorption by up to 38.82% and, in some studies, the carbonation depth could be reduced by 46.1%. Furthermore, MK enhances chloride resistance through physical pore refinement and adsorption, while Hc and Mc promote chemical chloride binding and Friedel’s salt formation. These mechanisms demonstrate the potential of MK for durable and low-carbon marine concrete.