This study investigates the degradation mechanisms and mechanical performance evolution of ultra-high-performance concrete (UHPC) exposed to sulfuric acid environments (pH≈0.9) through accelerated immersion tests. The influence of steel fiber content (0 %, 1 %, and 2 %) on UHPC’s deterioration behavior was systematically evaluated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Experimental results revealed a characteristic mass variation pattern: initial mass gain followed by gradual loss, with higher fiber content specimens exhibiting lower mass change rates due to enhanced corrosion layer spalling. The corrosion depth increased with exposure duration but showed significant retardation after 60 days, demonstrating steel fibers’ effectiveness in reducing penetration. Mechanical characterization indicated progressive deterioration of compressive strength, elastic modulus, flexural strength, and axial tensile strength, with rapid initial degradation followed by stabilization. Steel fiber incorporation significantly improved corrosion resistance coefficients, confirming their beneficial role in acid exposure conditions. The susceptibility of mechanical properties to sulfuric acid attack followed the order: tensile strength > flexural strength > compressive strength. Microstructural analyses identified gypsum as the predominant corrosion product, whose accumulation in matrix pores generated detrimental internal stresses. XRD patterns demonstrated decreasing gypsum peak intensities with sampling depth, while steel fibers effectively inhibited corrosion product formation. The strong correlation between microstructural observations and macroscopic performance degradation provides fundamental insights for durability assessment of UHPC in aggressive sulfuric acid environments.
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