The aggressive marine environment threatens the sustainability of coastal concrete infrastructure. This study evaluates the long-term durability and mechanical performance of High Volume Fly Ash Self-Compacting Concrete (HVFA-SCC) through one-year direct exposure of specimens in the tidal zone, an in-situ approach rarely applied. The research focuses on a novel mix design incorporating high proportions of fly ash (FA), bottom ash (BA), NaOH, and CaCO3. Durability was assessed by measuring chloride and sulfate ion concentrations at 0, 2.5, and 4 cm depths, while mechanical strength was evaluated via flexural tests on unreinforced beams. Five mixture variations were examined, revealing that all HVFA-SCC mixes significantly reduced chloride and sulfate penetration compared to control concrete. The FA-60 (4) mix exhibited the best performance at 4 cm depth, with chloride and sulfate concentrations of 0.025% and 2.979% respectively. Optimal FA content effectively reduced ion penetration across depths, enhancing durability against aggressive marine conditions. The combination of HVFA with BA, NaOH, and CaCO3 further improved resistance to ion ingress, though overall performance remained lower than pure FA mixes. All HVFA-SCC variations demonstrated competitive mechanical properties, with the FA-60 formulation achieving the highest flexural strength of 7.94 MPa after one year. These findings highlight HVFA-SCC potential as a durable and environmentally friendly alternative for coastal infrastructure.