# Photocatalytic Cementitious Material for Eco-Efficient Construction--A Systematic Literature Review
## 4 Discussion
### Photocatalyst Deactivation  
The photocatalyst lifespan is an obviously important point for economic reasons, as it sets optimum run times between catalyst regeneration and replacement [88]. Performance degradation over time, called photocatalyst deactivation, is among the most severe practical problem. The generation of intermediates is one of the main causes of photocatalytic deactivation. An incomplete process of photocatalytic oxidation produces intermediates that are retained on the surface. Intermediate products can occupy the active sites of the catalyst, leading to a number of problems that include catalyst deactivation. Further catalyst deactivation (and regeneration) studies in photocatalysis are necessary to obtain better understanding on these processes and establish the process economics of this technology.  
Research work has successfully demonstrated that using more active photocatalysts could retard the deactivation process [89]. Choosing the correct photocatalyst for each specific application during the manufacturing phase determines the overall durability of the final product in the usage stage. For example, visible light-activated photocatalysts will have a much wider light absorption range and radically improved performances under low light intensity conditions. Improving performance could, at the same time, delay the deactivation processes.  
In the usage phase, controlling relative humidity is of great importance for the efficiency of photocatalytic reaction. Water vapor plays a significant role in the formation of active species [90]. Water molecules adsorbed on the photocatalyst will react with the hole and generate some hydroxyl groups, which in turn oxidize pollutants. Figure 9a presents the concentration of air pollutants benzaldehyde and CO\({}_{2}\) with different relative humidity during the photocatalytic reaction process. [PERSON] and [PERSON] [86] found no conversion in the absence of water vapor, and the photocatalytic reaction rate increased linearly with increasing relative humidity over the range 0-60%, as shown in Figure 9a. However, excessive water vapor on the catalyst surface inhibits the photocatalytic reaction because the presence of water vapor competes with contaminants for adsorption sites on the photocatalyst, thus reducing the performance [91]. This is called \"competitive adsorption\" between water vapor and contaminants.  
Although the deactivation mechanism of photocatalyst is not clear, it can be supposed that it is due to the partial saturation of active sites for adsorption, on the photocatalyst surface, with intermediate products. [PERSON], [PERSON] [92] demonstrated that placing the used catalyst in an oven at 30 \({}^{\circ}\)C and a relative humidity of approximately 40% for 12 h to evaporate the adsorbed species could recover more than 90% of the catalysts, as shown in Figure 9b. Figure 9b-1 and Figure 9b-2 indicate the image of TNPs after 5 h reaction and after 12 h regeneration in the air, respectively. Figure 9b-3 and Figure 9b-4 indicate the image of Meso. TiO\({}_{2}\) after 5 h reaction and after 12 h regeneration in the air, respectively. The work found that most of the adsorbed species were only adsorbed physically.  
Previous studies have shown that environmental temperatures and the water-to-cement ratio significantly affect the leaching kinetics [93; 94; 95]. Temperature could increase ionic diffusivity, accelerate diffusion kinetic, and create significant change in cement paste microstructure to create higher porosity. The temperature elevation could result in the vibration of the crystal structure of C-S-H [94; 96; 97]. Increasing temperature could transform the cement hydrates (C-S-H, ettrinigite), resulting in an increased porosity and a higher diffusivity of aggressive ions [98; 99; 100].