Mechanical behavior and microstructural degradation of basalt fiber reinforced seawater sea sand concrete after high-temperature exposure

Seawater sea sand concrete (SWSSC), recognized for its green and sustainable nature, has gained attention in marine engineering, yet its high-temperature behavior and fiber effects remain underexplored. This study examined SWSSC and basalt fiber reinforced SWSSC (SWSSC-B) with fiber contents of 0.1 %–0.4 % after exposure to 23–800 °C. Compressive and splitting tensile tests, combined with SEM and XRD analyses, were conducted to investigate mechanical and microstructural evolution. Results showed that compressive strength first decreased, slightly recovered, and then dropped sharply with increasing temperature, while tensile strength continuously declined due to dehydration of C-S-H and decomposition of AFt, Friedel's salt, and Ca(OH)2. Incorporating basalt fibers mitigated degradation; at 0.2 % content, compressive and tensile strengths increased by 65.5 % and 14 % at 600 °C, respectively, compared with unreinforced SWSSC. The fiber-bridging network delayed crack propagation and maintained structural integrity. This study provides experimental evidence and theoretical insight for improving the fire resistance of SWSSC structures.