Durability of reinforced geopolymer concrete using coral aggregates and seawater under accelerated corrosion

This study evaluates the corrosion resistance of geopolymer concrete made with coral sand, coral rock, and seawater, materials abundant on offshore islands. Three mixtures were designed using varying proportions of fly ash and blast furnace slag as binders. Compressive strengths after 28 days were 20.11 MPa, 25.32 MPa, and 30.15 MPa for GPS-1, GPS-2, and GPS-3, respectively. Accelerated corrosion tests (NT Build 356) revealed that higher strength samples exhibited lower average current intensity before cracking (5.49 mA for GPS-1 vs. 4.68 mA for GPS-3) and longer times to cracking (204 h vs. 324 h). Chloride ion concentrations near the steel at the onset of cracking were similar across mixtures (0.53 % – 0.58 % by dry concrete weight), but initial chloride contents were lower in high-strength samples (0.08 % in GPS-3 vs. 0.22 % in GPS-1), indicating better resistance to ion penetration. Microstructural analyses (SEM, FTIR, XRD) confirmed that GPS-3 had a denser matrix, more developed geopolymer gel, and reduced presence of unreacted crystalline phases. These results demonstrate that optimizing binder content and microstructure enhances both mechanical performance and corrosion resistance, making geopolymer concrete with marine-derived materials a viable solution for durable coastal infrastructure.