Comparative analysis of sulfate resistance between seawater sea sand concrete and freshwater desalted sea sand concrete under different exposure environments

Seawater sea sand concrete (SWSSC) is considered a potential solution to the shortage of river sand and freshwater resources in the coastal and island regions. Under the marine environment, the sulfate ions in the seawater and sea sand are detrimental to the properties of concrete. Due to the lack of systematic research on the sulfate attack characteristics of SWSSC exposed to different marine conditions, this study compare the sulfate resistance between SWSSC and freshwater desalinated sea sand concrete (FDC) under full immersion, semi-immersion, and dry-wet cycle exposure through the compressive strength, sulfate ion distribution, and microstructural changes after erosion. The findings showed that among the three different exposure conditions, the damage in SWSSC-F (fully immersion) was more severe than that in SWSSC-S (semi-immersion) and SWSSC-DW (dry-wet cycle). The SO₄^2- concentration in inner SWSSC-F was 9.3% and 21.3% higher than that in SWSSC-DW and SWSSC-S, respectively. The microstructural analysis indicated that the higher amount of sulfate attack products and the leaching of Ca²⁺/OH^- in SWSSC-F significantly damaged its microstructure. Besides, carbonation and physical crystallization in SWSSC-S and SWSSC-DW also refined their pore structure, thus enhancing their sulfate resistance. SWSSC showed better sulfate resistance than FDC under different marine exposure conditions in terms of lower erosion product content and better pore structure distribution after 12 months of erosion. The formation of Friedel's salt in SWSSC suppressed the formation of expansion products and reduced the erosion damage of concrete by SO₄^2-.