Photocatalytic activity of semiconductor-modified cement—influence of semiconductor type and cement ageing

Photocatalyst-modified cements are discussed for various applications in building and environmental technology ranging from self-cleaning surfaces to solar-powered remediation devices for polluted waters. In this work, the photocatalytic atrazine degradation by white Portland cement samples modified with different semiconducting oxides (TiO2 in form of Degussa P25, Hombikat UV 100, coarse-grained anatase, and ZnO) was used as a model reaction to investigate a possible application of modified cements for the degradation of pollutants on building surfaces. The cement mixtures were also characterised with respect to hydration kinetics and compressive strength which are relevant for possible construction applications. Particle suspensions containing 1μg/l atrazine were irradiated with a sun simulator, and the atrazine degradation was monitored by ELISA (enzyme-linked immunosorbent assay). The atrazine degradation followed first-order kinetic and the obtained rate coefficients were compared with the degradation rate by unmodified cement samples. A significant enhancement of the photocatalytic activity due to the addition of 10wt.% TiO2 in the form of Degussa P25 and Hombikat UV 100 to Portland cement was observed, whereas addition of ZnO and coarse-grained anatase showed no effect. The primary crystal particle size of the semiconductor seems to have a significant influence on the photocatalytic efficiency of the semiconductor-modified cement samples. Additionally, modified cements were subjected to defined ageing conditions and carbonation was followed directly by laser Raman spectroscopy. Photocatalytic activity decreases during ageing of the hardened cement pastes. However, aged cement modified with Degussa P25 or Hombikat UV 100 exhibited at least threefold degradation efficiency compared to unmodified cement. Taking into account the sufficient radiation over a long time in the environment, degradation of harmful substances by modified cement samples could contribute considerably to the abiotic degradation of pollutants in the environment.