The word photocatalysis is a compound word made up of (1) photo or light and (2) catalysis or acceleration of a chemical reaction rate by addition of a catalyst. A catalyst is a substance which although combines with the reactants participating in a chemical reaction but remains unaltered. Hence photocatalysis is a reaction which uses light to accelerate the rate of a chemical reaction without being involved itself.
When a photocatalyst is incorporated in a concrete, it decomposes organic materials like dirt, mold, algae, bacteria, allergens, air pollutants, smoke, tobacco etc in the presence of light. The catalyzed compounds break down into oxygen, carbon dioxide, water, sulfates, nitrates, and other substances that are either beneficial or have a relatively minor effect on the environment. This type of concrete having a photocatlyst in it is known as photocatalytic concrete or self-cleaning concrete, which is a green construction material.
A photcatalytic concrete along with its structural qualities like an ordinary concrete is a strong depollutant. Titanium dioxide (TiO2) or titania is the photocatalyst material mixed with the concrete. In the presence of light, titania creates a charge that disperses on the surface of the photcatalyst, and reacts with external substances to decompose organic compounds. The products of the reaction can easily be washed with water. Rain or simple rinsing can easily remove the reaction products and the building looks clean and beautiful like a new one. Titanium dioxide can be employed in all types of concrete, including plaster and mortar.
Along with TiO2, other photocatalysts like ZnO, WO3, FeTiO3 and SrTiO3 can also be utilized in concrete for the same purpose. As the sunlight strikes the concrete surface, most organic pollutants are neutralized; this would otherwise discolor the concrete surfaces. As already mentioned, the photo catalyst is not spent as it breaks down the pollutants, rather it continues to work indefinitely.
The new Jubilee Church in Italy, The Air France headquarters at Roisy Charles de Gaulle airport at Paris and the Hotel de Police in Bordeaux France have been constructed of photocatalytic concrete. In Japan, photocatalytic precast concrete pavements are constructed to depollute car exhausts.
With photcatalytic concrete:
1. Sun heat can be reflected and the energy bills can be substantially reduced.
2. Inside hospitals and laboratories, the spread of germs can be minimized or even eliminated.
3. The urban air quality can substantially be improved.
4. Structures looking new for decades can be constructed.
Figure 1 New Jubilee Church constructed in Rome Italy
Figure 2 Hotel de Police in Bordeaux, France