Durability of photocatalytic cement subjected to nitrogen dioxide and wet–dry cycling

Although it is well known that photocatalytic titanium dioxide nanoparticles effectively decompose atmospheric nitrogen oxides (NO_x), the long-term effects of the photocatalytic oxidation on the cementitious substrate have not been well explored. In this study, cement paste samples containing titanium dioxide nanoparticles were exposed to multiple cycles of nitrogen dioxide gas and ultraviolet radiation, along with wetting and drying, to simulate the effects of field exposure. The photocatalytic efficiency was monitored at each cycle and the surface of the samples was examined visually, chemically and mechanically between the cycles. The results indicate that the nitrogen dioxide oxidation efficiency decreased with cycling, possibly due to carbonation of the paste or loss of titanium dioxide particles. The pits found from the scanning electron microscopy images indicate there has been deterioration on the sample surfaces. Observations of surface deterioration imply possible acid attack, leaching and/or loss of titanium dioxide, suggesting that particularly the surfaces of cement-based materials can be negatively affected by photocatalysis.