Design and performance regulation of low-alkalinity seawater sea sand concrete: A novel approach to enhancing the durability of fiber reinforced composite

In marine environment, reducing the alkalinity of seawater sea sand concrete (SWSSC) is an effective approach to prolonging the service life of basalt fiber reinforced polymer (BFRP) bars. This study analyzed the previous experimental data on the deterioration of FRP and identified the optimal environmental pH range for delaying the degradation of BFRP. A design method for low-alkalinity SWSSC was proposed using activated excavated sediment and silica fume as supplementary cementitious materials. Microscopic characterization techniques were employed to analyze the evolution of the microstructure and chemical composition of low-alkalinity SWSSC, revealing the underlying alkalinity regulation mechanism. Results indicated that in the dual-component admixture system, activated sediment and silica fume consumed Ca(OH)₂ through pozzolanic reactions, generating secondary hydrated calcium silicate gel while inhibiting the formation of the AFm phase. This process reduced the pH value of pore solution from 12.8 to 11.6 while maintaining compressive strength. Furthermore, to evaluate the feasibility of applying low-alkalinity SWSSC in BFRP-reinforced structures, the long-term mechanical performance and bond-slip behavior of BFRP bars embedded in low-alkalinity SWSSC were examined. The low OH⁻ concentration in low-alkalinity SWSSC decreased the activation energy for resin hydrolysis, significantly retarding the degradation of BFRP. This resulted in a 47.7 % increase in the long-term tensile strength of BFRP bar and a 57.5 % improvement in long-term bond strength between BFRP bar with low-alkalinity SWSSC. This study promotes the application of low-alkalinity SWSSC in marine infrastructure, providing novel insights for enhancing the durability of FRP-reinforced concrete structures.