Geopolymer mortar offers an eco-friendly way to protect concrete in harsh environments. Affordable materials such as fly ash (FA) and rice husk ash (RHA) improve sustainability, with RHA providing silica. The long-term effects of geopolymer and Portland cement in marine environments, especially under severe sulfate attack, remain largely unexplored. Understanding interface mineral formations is key to evaluating concrete's resistance to chemical infiltration. This study evaluated geopolymer mortar containing 0–10% RHA, replacing FA, as a protective layer for Portland concrete. Samples were exposed to chloride- and sulfate-rich seawater for 5 months to evaluate mechanical strength, porosity, sulfate resistance, chloride penetration, and interface behavior. A 5% RHA replacement (Si/Al = 3.26) optimized performance, enhancing chemical bonding at the interface through C-A-S-H formation. The geopolymer layer significantly improved sulfate resistance, outperforming Portland mortar with 20% FA (FPC). Marine exposure accelerated geopolymerization, increasing density and compressive strength while reducing chloride ingress. The protective N-A-S-H gel, stabilized by Fe, Al, and Mg, remained resistant to sulfate and chloride ions. Additionally, dissolved silica from RHA mitigated degradation and reduced CO₂ emissions by 62.1%. The study showed that geopolymer mortar is promising for jacketing, repairing, or patching concrete structures in marine environments.

fly ash, rice husk ash, geopolymer, wrapping material, seawater