Durability and microstructural evolution of seawater sea-sand coral concrete under different exposure environments of sulfate attack

Seawater sea-sand coral concrete (SSCC) shows a broad application prospect in marine engineering, while sulfate in seawater is one of the important causes of deterioration of concrete materials . In order to gain a deeper understanding of the deterioration pattern and deterioration mechanism of SSCC under different sulfate attack conditions, this study investigates the mass change, relative dynamic elastic modulus (RDEM) change, compressive strength change, and sulfate ion concentration of SSCC under different sulfate attack conditions, and reveals the deterioration mechanism by using microscopic characterization means (SEM-EDS, XRD , and MIP). The results indicated that the macroscopic performance indexes of marine concrete initially increased and then decreased under all three exposure conditions. Among the tested materials, hybrid polypropylene fiber-reinforced seawater sea-sand coral concrete (HPFSSCC), incorporating fly ash, silica fume , and multi-sized polypropylene fibers, demonstrated the highest resistance to sulfate attack. Under different exposure conditions, the erosion severity ranks as follows: semi-immersion > dry-wet cycle > full immersion. In the semi-immersion condition, the mass loss, compressive strength loss, and RDEM loss of HPFSSCC were reduced by 53.2 %, 55.1 %, and 39.7 %, respectively, compared to SSCC after 240 days of sulfate attack. After 240 days of erosion, for SSCC at a depth of 2 mm, the dry-wet cycles and semi-immersion conditions increased the sulfate ion concentration by 13.2 % and 21.7 %, respectively. For SSCC, the increase in porosity due to semi-immersion and dry-wet cycles was 24.54 % and 4.94 %, respectively, compared to full immersion. SEM-EDS and XRD analyses identified the chemical erosion products as ettringite , brucite, gypsum, and M-S-H. Physical erosion due to Na 2 SO 4 crystallization was observed under dry-wet cycles and semi-immersion conditions. The findings of this study provide valuable insights for the durability assessment of in-service coral concrete structures and offer a reference for the durability design of future coral concrete constructions in marine environments. • Different exposure conditions of SSCC under sulfate attack were studied. • The microstructure and corrosion products were researched by microscopic analysis. • The erosion solution was a complex sulfate solution (5 %Na 2 SO 4 +5 %MgSO 4 ). • Utilizing FA, SF and fibers to enhance the sulfate attack resistance of SSCC.