Seawater-mixed alkali-activated materials: a state-of-the-art review

Seawater-mixed alkali-activated materials (Sw-AAMs) as a substitute to ordinary Portland cement (OPC) can reduce CO2 emissions and alleviate water scarcity in areas with freshwater shortages. However, compared with freshwater-mixed alkali-activated materials (Fw-AAMs), introducing seawater with complex components can affect its reaction processes, fresh performance, mechanical properties, and durability. Therefore, the main characteristics of Sw-AAMs and their comparison with Fw-AAMs are reviewed in this study. The results show that seawater directly affects the reaction process of Sw-AAMs, leading to the formation of by-products such as M-S-H gel, Cl-hydrotalcite, and gypsum, which are not observed in Fw-AAMs. The type of activator anion (particularly SiO(3)2(-)) is a key factor affecting various properties. In a system without SiO(3)2(-) ions, the addition of seawater increases the initial fluidity of the paste, reduces its setting time and increases the early strength of Sw-AAMs by 11-30%. However, when SiO(3)2(-) ions are present in the activator, the reduced formation of early primary products, coupled with the insufficient cross-linking ability of the by-products, decreases the early compressive strength of Sw-AAMs by up to 30%. Although seawater may affect the homogeneity of the cementitious material, there is a good interaction between the seawater-mixed matrix and aggregates, which helps enhance the elastic modulus and flexural toughness of Sw-AAM concrete. The research on long-term durability of Sw-AAMs and modelling of concrete at different scales is still in its infancy. Further research is required to ascertain the long-term behaviour of Sw-AAMs and to facilitate broader and more accurate applications.