This study investigates the microhardness and geometric degradation mechanisms of interfacial transition zones (ITZs) in recycled aggregate concrete (RAC) exposed to saline soil attack, focusing on the influence of supplementary cementitious materials (SCMs). Ten RAC mixtures incorporating fly ash (FA), granulated blast furnace slag (GBFS), silica fume (SF), and metakaolin (MK) at 10 %, 15 %, and 20 % replacement ratios were subjected to 180 dry-wet cycles in a 7.5 %MgSO4-7.5 %Na2SO4-5 %NaCl solution. Key results reveal that ITZ's microhardness and geometric degradation decreases with exposure depth but intensifies with prolonged dry-wet cycles. The FAGBFS synergistically enhances ITZ microhardness while minimizing geometric deterioration, with ITZ's width and porosity reduced to 67.6-69.0 mu m and 25.83 %, respectively. In contrast, FA-SF and FA-MK exacerbate microhardness degradation, increasing porosity and amplifying microcrack coalescence. FA-GBFS mitigates the diffusion-leaching of aggressive/original ions and suppresses the formation of corrosion products, thereby inhibiting the initiation and propagation of microcracks. In contrast, FA-SF and FA-MK promote the formation of ettringite/gypsum and crystallization bloedite/glauberite, which facilitates the formation of trunk-limb-twig cracks.

Ultra-high performance concrete (UHPC) exposed to the harsh western saline soil environments in western China experience accelerated damage due to the combined effects of dry-wet cycles, corrosive salt ions, extreme temperatures, and freeze-thaw cycles. This study developed a laboratory erosion protocol to simulate these conditions and evaluate the sulfate resistance of UHPC, investigating the degradation mechanisms associated with variations in water-binder ratio, silica fume content, and fiber type. Wiener theory was employed to predict the lifespan of various UHPC mixtures exposed to these conditions. The results indicate that UHPC demonstrates negligible degradation in performance under erosion simulation conditions when the water-to-binder ratio for the UHPC is 0.20, the silica fume content (relative to the total cementitious material content) is 26 %, and steel fibers are used. After 240 days of erosion, the compressive strength, bending strength and equivalent bending toughness of UHPC reinforced with polyvinyl alcohol (PVA) fiber decreased by 7.79%, 35.48% and 42.01 % respectively, with a decrease in the relative dynamic modulus of elasticity to 97.29%. These declines were more pronounced than in specimens with steel fibers. Phase composition and micro-structural analyses identified that the primary products of sulfate attack in UHPC as ettringite and gypsum, alongside the physical crystallization of anhydrous sodium sulfate, which induced expansion and crystallization stress, forming harmful pores and microcracks. A reliability function curve, based on compressive strength, effectively modeled the degradation process of UHPC under these conditions, predicting a potential durability lifespan exceeding 70 years in western saline soil environments.