OICC Press International Journal of Nano Dimension 2228-5059 17 2 (April 2026) 2026 08 22 Mechanical Behavior and Durability of Nano TiO2–modified UHPC Reinforced with Recycled Steel Fibers under Corrosive Wastewater Environment 10.57647/ijnd.2026.1704.04 EN Pouya Khoshkanabi Department of Environmental Engineering, Graduate Faculty of Environment, University of Tehran, Tehran, Iran Naser Mehrdadi Department of Environmental Engineering, Graduate Faculty of Environment, University of Tehran, Tehran, Iran Gholamreza Nabi-Bidhendi Department of Environmental Engineering, Faculty of Environment, University of Tehran, Tehran, Iran https://orcid.org/0000-0001-9930-9080 Mohammad Javad Amiri Department of Environmental Education and Systems, Faculty of Environment, University of Tehran, Iran Journal Article 2026 08 22  This study investigates the mechanical performance and sulfate durability of ultra-high-performance concrete (UHPC) modified with nano-TiO2 and reinforced with industrial steel fibers (I-SF) or tire-derived recycled steel fibers (R-SF). UHPC specimens were exposed for 28 days to water, 5% MgSO₄, and 10% MgSO₄ solutions, followed by compressive and direct tensile testing. Under water curing, fiber incorporation significantly enhanced compressive strength from 84.0 MPa (plain UHPC) to 135.6 MPa with 0.6% I-SF and 126.8 MPa with 0.6% R-SF. Tensile strength similarly increased from 6.5 MPa to 10.53 MPa and 8.84 MPa for 0.6% I-SF and R-SF, respectively. Severe sulfate exposure (10% MgSO₄) reduced compressive strength of the control mixture to 74.0 MPa, whereas fiber-reinforced mixtures retained substantially higher strengths, with lower relative degradation observed for R-SF (≈5–7%) compared to I-SF (≈10–12%). Nano- TiO2 improved matrix compactness and durability, with 0.4 wt.% identified as the optimal dosage for maximizing strength retention. Scanning electron microscopy confirmed enhanced C–S–H densification and improved interfacial stability, particularly in TiO2/R-SF composites. The findings demonstrate that integrating nano-modification with recycled steel fibers provides a durable and sustainable UHPC solution for sulfate-rich environments.