Role of chemical states of aluminum in chloride binding by cementitious systems

Incorporating aluminum-rich materials to enhance intrinsic chloride binding capacity is critical for corrosion protection of reinforced concrete. This study investigated the impact of four chemical states of aluminum in aluminum-rich materials (sodium aluminate, aluminum hydroxide, hydrotalcite and alum) on the chloride binding capacity of cement paste mixed with chloride salts. Results demonstrated that the chemical state of aluminum significantly modulated chloride binding capacity: sodium aluminate, aluminum hydroxide, and hydrotalcite incorporation all enhanced chloride binding capacity though with diminishing early-stage enhancement effects, while alum incorporation reduced this capacity. Sodium chloride-prepared pastes consistently exhibited higher free chloride concentrations than magnesium chloride or calcium chloride counterparts across all curing ages and aluminum-rich materials at equal initial chloride ion concentrations. Aluminum-rich material's dissolution governed its hydration kinetics and chloride binding rates, constituting the key chloride binding mechanism in cement paste. These findings are expected to advance the design of highly durable cementitious materials for use in chloride-rich environments.