# Photocatalytic Cementitious Material for Eco-Efficient Construction--A Systematic Literature Review
## 3 Photocatalytic Cemertitious Materials
### Air Purifying Cement-Based Materials  
The application of photocatalytic cementitious materials for air purification purposes has recently received considerable attention because of its potential for controlling pollutant in the urban areas. Photocatalytic cementitious materials have the capability of destroying airborne pollutants emitted by automobiles, such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). Murata, [PERSON] [25] have pioneered the development of photocatalytic cementitious materials for air purification. The work demonstrated that the cementitious materials containing TiO\({}_{2}\) particles can be used as a measure for NOx pollution control. In 2013, a German-based company, Elegant Embellishment, developed a customized photocatalytic building facade for Hospital Manuel Gea Gonzales in Mexico. According to the studies, the facade is capable of decomposing contaminants and neutralizing the pollution of 1000 cars per day.  
The basic advantage of using photocatalytic cementitious materials as building facades or concrete pavement is the availability of sunlight and rainwater for photoactivation and regeneration [25]. Furthermore, the systems installed close to an emission source react with the pollutants under higher concentration. As observed by [PERSON], [PERSON] [43] and [PERSON], [PERSON] [44], the photocatalytic activities were linearly dependent on the concentration of air pollutant and irradiance. Therefore, the strategy to immobilize TiO\({}_{2}\) photocatalyst into cementitious materials is attractive for these air-purification systems [45; 46; 47; 48; 49; 50].  
The mechanism of photocatalytic oxidation of NOx at steady state is illustrated in Figure 7[51]. The generated OH\({}^{\bullet}\) radicals are presumed to be the key oxidant in the reactions. At the nitrogen dioxide (NO\({}_{2}\)) stage, part of the gas may escape from the photocatalyst surface [34]. Nevertheless, cement matrix possesses an inherent ability to bind NO\({}_{2}\)[45]. Therefore, use of cementitious materials as catalyst supporting media may effectively trap the NO\({}_{2}\) gas.  
To assess the air-purification performance of photocatalytic materials, there are five published ISO methods (ISO 22917 series) nowadays. According to that, removal of nitric oxide (ISO 22197-1:2016), removal of acetaldehyde (ISO 22197-2:2019), removal of toluene (ISO 22197-3:2019), removal of formaldehyde (ISO 22197-4:2021), and removal of methyl  
Figure 7: Mechanism of photocatalytic oxidation of NOx at steady state.  
mercaptan (ISO 22197-5:2021) are the five standard test methods based on commonly used model air pollutants. These pollutants are well recognized pollutants found in indoor and outside air [52]. However, the current standard test methods do not consider the surface properties of cementitious materials, such as porosity, heterogeneity, time-dependent variability, and environment-dependent variability [53]. Since the photocatalytic reactions take place at the surface of photocatalyst, the surface properties of cementitious materials are expected to influence their photocatalytic efficiencies.