Enhancing photocatalytic durability of high strength pervious concrete: Micro-mechanical and microscopic mechanisms

Nano titanium dioxide (nano-TiO₂) has been widely used in cement materials to remove nitrogen monoxide (NO), yet the durability of the applied nano-TiO₂ in the cementitious matrix remains a substantial challenge. This study developed a high-strength photocatalytic pervious concrete (HSPPC), and employed a low-pressure cold spraying method to apply the nano-TiO₂ to its surface, aiming to enhance the durability of the photocatalytic coating through synergistic optimization of both the materials and spraying process. The effects of the amount of nano-TiO₂, spraying methods, and the aggregate-to-binder ratio (A/B) of the previous concrete on NO_x degradation were determined. Additionally, the interface mechanics and durability enhancement mechanisms of photocatalytic coatings on the HSPPC were revealed through microscale mechanical and microscopic mechanism analyses. The results indicated that the efficiency of NO removal was increased with the increase in the amount of nano-TiO₂, and A/B ratio. Compared to ordinary pervious concrete, the resistance to seepage scouring and vehicle tire abrasion of HSPPC was significantly improved. Moreover, the combination of HSPPC and the cold spraying method resulted in an efficient synergistic effect, considerably enhancing the durability of the nano-TiO₂ compared to traditional brushing methods. Micromechanical and microstructural analyses revealed that the mesoscopic pore structure formed within the cold-sprayed coating facilitated the formation of hydration products on the HSPPC substrate. This led to the generation of higher polymerization degree of C-S-H gels, which bonded the nano-TiO₂ particles together and enhanced the interfacial bonding with the substrate, effectively improving the cohesion, adhesion, and photocatalytic durability of the nano-TiO₂ coatings.