Alkali-Activated Binders as Sustainable Alternatives to Portland Cement and Their Resistance to Saline Water

Alkali-activated binders have emerged as promising alternatives to Ordinary Portland Cement (OPC) due to their sustainability features and potential advantages. This study evaluates the durability properties of heat-cured fly ash (FA) and ground granulated blast-furnace slag (GGBFS) geopolymer mortars activated with sodium hydroxide, which were subjected to wet–dry cycling in saline environments. Three series of FA, a FA/GGBFS blend, and GGBFS mortars previously optimized on a compressive strength basis were investigated and compared against two control OPC mixes. Performance indicators such as the water absorption, porosity, flexural strength, and compressive strength were analyzed. The results demonstrate that geopolymer mortars have significantly reduced water absorption and porosity with increasing wet–dry cycles. The compressive strength of the FA/GGBFS mortars also increased from 66.5 MPa (untreated) to 87.9 MPa over 45 cycles. The flexural strength remained stable or improved slightly across all geopolymer mortars. The control OPC specimens experienced significant deterioration, with compressive strength in CEM I 42.5R dropping from 51.8 to 17.1 MPa. These findings highlight the superior durability of geopolymer mortars under harsh saline conditions, demonstrating their potential as a resilient alternative for coastal and marine structures.