Nano-TiO2-Functionalized Porous Concrete for Agricultural Drainage Channels in Cold Regions: Performance, Pollutant Removal, and Freeze–Thaw Durability

To address the challenge of simultaneously maintaining structural durability and pollutant-removal functionality of concrete structures used in agricultural drainage channels in cold regions, a functional porous concrete with surface-loaded nano-TiO2 was developed. The effects of target porosity, TiO2 mass fraction and spraying dosage, and polypropylene fiber content on the mechanical properties, pollutant-removal performance, and freeze–thaw durability were systematically investigated. X-ray computed tomography (X-CT), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS), and X-ray photoelectron spectroscopy (XPS) were further employed to characterize the pore structure and near-surface characteristics. The results showed that increasing the target porosity from 6% to 10% reduced the 28-day compressive strength by approximately 24% while generally improving the removal performance for total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD), indicating the dual influence of pore structure on load-bearing capacity and purification functionality. At a TiO2 mass fraction of 10% and a spraying dosage of 300 g/m2, the material exhibited a favorable overall pollutant-removal performance, with TN, TP, and COD removal efficiencies reaching 27.5%, 59.0%, and 31.7%, respectively, after 120 min of UV irradiation. Additional control experiments further indicated that UV-activated TiO2 provided an additional contribution to overall pollutant removal. Incorporating 0.05 vol.% polypropylene fiber improved freeze–thaw durability. After 200 freeze–thaw cycles, compared with the fiber-free specimens, the mass loss rate decreased by approximately 0.5 percentage points, the relative dynamic elastic modulus increased by approximately 6 percentage points, and the compressive strength loss rate decreased by approximately 10 percentage points. Nevertheless, the pollutant-removal function was more sensitive to freeze–thaw damage, with the TN, TP, and COD removal capacities decreasing by approximately 69%, 70%, and 63%, respectively, relative to their initial values after 200 cycles. These results demonstrate that the design of functional porous concrete for cold-region applications should comprehensively balance pore structure, TiO2 loading, mechanical performance, and freeze–thaw durability to achieve both structural serviceability and long-term water purification.