Study on Photocatalytic Degradation of NO Gas by Cement Paste Incorporated with Nano-TiO₂

Nitrogen oxides (NOx) emitted from anthropogenic sources pose severe threats to urban environments. While incorporating nano-TiO₂ into cementitious materials offers a promising photocatalytic remediation strategy, the coupled effects of pore-structure modification and curing age on the degradation efficiency remain underexplored. This study systematically investigated the NO gas degradation performance of nano-TiO₂-modified cement paste by controlling the TiO₂ content (3-12%), air-entraining agent content, water-cement ratio, and curing age. The results indicate that an optimal nano-TiO₂ content of 6% yields the highest compressive strength. Meanwhile, higher nano-TiO₂ content (up to 12%) significantly enhances NO removal (reaching 9.73%), but excessive air-entraining agents trap the photocatalyst, thereby reducing efficiency. Furthermore, increasing the water-cement ratio promotes gas diffusion, whereas prolonged curing densifies the matrix and obstructs UV-catalyst contact, decreasing the removal ratio by 20.3% at 28 days. Microstructural analyses elucidate that NO is photocatalytically oxidized into nitrate ions (NO₃^-) without degrading the essential C-S-H gel. This study provides critical insights into the microstructural optimization of photocatalytic cements, offering a practical design framework for photocatalytic cements with potential for long-term environmental application, pending durability validation.