## 3 Results and Discussion
### Photocatalytic Performance  
The photodecomposition process by PCSs is summarized in Figure 7. Comparing with cementitious materials, the porous structure of PCS could provide more gas diffusion and light transmittance channels which improved the reaction areas and utilization rate of photocatalysts. The photogenerated charges can generate \"\(\bullet\)OH\" radicals and \"O\({}_{2}\)\({}^{-}\)\" intermediate species by reacting with absorbed H\({}_{2}\)O and O\({}_{2}\) molecules for the photodecomposition of benzene. The mechanism of photocatalytic reaction is shown as follows [4]:  
\[\mathrm{TiO}_{2}+\mathrm{h}\mathrm{v}\rightarrow\mathrm{h}_{\mathrm{vb}}^{+}+ \mathrm{e}_{\mathrm{cb}}^{-} \tag{1}\]  
\[\mathrm{e}_{\mathrm{cb}}^{-}+\mathrm{O}_{2}\rightarrow\ \bullet\mathrm{O}_{2}^{-} \tag{2}\]  
\[\mathrm{h}_{\mathrm{vb}}^{+}+\mathrm{H}_{2}\mathrm{O}\rightarrow\ \bullet\mathrm{OH}+ \mathrm{H}^{+} \tag{3}\]  
\[\mathrm{C}_{6}\mathrm{H}_{6}+\ \bullet\mathrm{O}_{2}^{-}+\ \bullet\mathrm{OH} \rightarrow\mathrm{CO}_{2}+\mathrm{H}_{2}\mathrm{O} \tag{4}\]  
Figure 8a showed the photocatalytic degradation of benzene on the surface of PCS and control groups. 1 PCS-1.25-2.35-UA presented more superior photocatalysis performance (degraded 2 \(\mathrm{\mu L}\) benzene completely in 350 min) compared with other PCSs, proved to be the most efficient one. It was because CS-1.25-2.35 provided more superior pore structure to absorption of gas pollutants, which led to a larger gas-solid contact area between TiO\({}_{2}\) and benzene under the same condition. After 1 month of atmospheric environment exposure, 1 PCS-1.25-2.35-UA-1 month could still degrade 96% of 2 \(\mathrm{\mu L}\) benzene in 400 min. The control group (Surface coating) showed even better photocatalytic activity (degraded 2 \(\mathrm{\mu L}\) benzene completely in 250 min) than 1 PCS-1.25-2.35-UA with same amount of photocatalysts because of full exposure of TiO\({}_{2}\) particles. However, only after 1 month of atmospheric environment exposure, the photocatalytic activity of \"Surface coating\" decreased significantly by degrading 67% of 2 \(\mathrm{\mu L}\) benzene in 400 min. The reason is that TiO\({}_{2}\) is not bound tightly with the surface of cement, which is easily washed off, abraded or enveloped by the cement hydration products. Another control group (Incorporation) showed very little photocatalytic effects because the TiO\({}_{2}\) incorporated into cement materials could barely touch the lights. The photocatalytic activity of TiO\({}_{2}\) (P25) is also shown in Figure 8a, which is lower than \"Surface coating\". Despite its higher photocatalytic activity, the agglomeration of TiO\({}_{2}\) nanoparticles and the lower adsorption abilities are prevailing in our photocatalytic tests. Besides, the degradation of benzene by 1 PCS-1.25-2.35-AC was much slower than 1 PCS-1.25-2.35-UA, which indicated more active sites were provided by ultrasonic activation than alkali activation. Thus, ultrasonic activation was selected in subsequent experiments. As indicated in Figure 8b, the photocatalytic activity of PCS increased gradually when TiO\({}_{2}\) concentrations rose from 1 g/L to 4 g/L. However, when TiO\({}_{2}\) concentration increased to 5 g/L, the photocatalytic activity decreased compared with 4 PCS-1.25-2.35. Higher concentration of TiO\({}_{2}\) solution led to more TiO\({}_{2}\) deposition on the surface of CS and higher photocatalytic activity. Nevertheless, when TiO\({}_{2}\) concentration came to 5 g/L, agglomeration problem of TiO\({}_{2}\) was serious, which led to more recombination of electron-hole and difficulties for TiO\({}_{2}\) particles to be exposed to the photocatalytic reaction. Hence, 4 PCS-1.25-2.35 was chosen as the best material to prepare PCM.