# Photocatalytic Cementitious Material for Eco-Efficient Construction--A Systematic Literature Review
## 3 Photocatalytic Cemertitious Materials
### Self-Cleaning Cementitious Materials  
Prolonged exposure to various atmospheric pollutants also leads to the discoloration of cementitious materials. [24; 41]. Due to this discoloration effect, the aesthetic of buildings, monuments, and other infrastructure exposed to outdoor environments is gradually lost with time. In addition to the visible discoloration, the contamination of cement-based surfaces by airborne particles, such as algae and fungi, can have a destructive effect on structural integrity [54; 55]. The key role of microorganisms in the deterioration of cementitious materials has been linked to the generation of biogenic acids (i.e., sulfuric acid and nitriifying acid) [56]. These substances cause dissolution of calcium hydroxide (Ca(OH)\({}_{2}\)) and other calcium containing minerals from concrete matrix. As a result, buildings should be repaired regularly to maintain a good appearance and prevent concrete deterioration. Biocides treatments are common methods for preventing biodeterioration of cementitious materials. However, it has not been well accepted due to durability issues and its toxicity. Research found that the use of photocatalytic cementitious material was more effective for controlling biodeterioration processes than the conventional biocides [57]. Early development of cementitious materials containing TiO\({}_{2}\) photocatalyst has been carried out primarily to enhance aesthetic durability of cementitious materials, especially those based on white cement [24]. Research has successfully demonstrated that the self-cleaning technique can be applied to various construction materials, such as glass and cement-based materials, i.e., concrete and mortar [13]. Apart from its function to decompose organic contaminants through the photocatalysis process, TiO\({}_{2}\) has another unique photoinduced phenomenon to alter the wettability of its surface. The latter effect is often termed'super hydrophilicity'. The first phenomenon, photocatalysis process, has been studied extensively for decades while the second one, super hydrophilicity, is more recently discovered. The ability of photocatalyst-based coating to improve the wettability upon illumination highly influences the efficiency of the self-cleaning activity [52]. In this case, after the generation of electron-hole pairs, the photoexcited electrons could react with the Ti\({}^{4+}\) cations to form the Ti\({}^{3+}\), as shown in Figure 8. Meanwhile, the holes could oxidize the O\({}_{2}\)\({}^{2-}\) anions to form the molecular oxygen. Then, the oxygen atoms are released, creating oxygen vacancies. The oxygen vacancies could be then occupied by water molecules, forming a layer of chemisorbed OH groups, which tend to make the surface hydrophilic [58]. In the super-hydrophilic surface, the close contact between surface and adsorbed contaminant could be prevented [59]. Hence, even though the number of incident photons may be insufficient to decompose the dirt and other impurities, the clean surface is maintained when water flow is supplied [34]. The efficiencies of the self-cleaning effect lie on the synergy between photocatalysis and photoinduced super hydrophilicity [58]. International standard methods already exist nowadays to determine the efficiency of self-cleaning surfaces and further can be used by manufacturers to ensure the quality and reliability of their products [60]. Measurement of water contact angle (ISO 27448:2009) and organic dye decomposition (ISO 10678:2010) are commonly used test methods to evaluate self-cleaning activity of photocatalytic surfaces. The former method is used to measure the water contact angle under UV illumination. The latter method is used to evaluate the photocatalytic processes by degradation of the dye molecule in aqueous solution under UV illumination. TiO\({}_{2}\) photocatalysis produces a more pronounced effect on smooth surfaces, such as glass and ceramic tiles, than on cementitious materials surfaces [41].