The brilliant white concrete of the new Jubilee Church in Rome is expected to stay clean for the ages thanks to a photocatalytic additive to the concrete.

Photocatalysts, Self-Cleaning Concrete
Photocatalysts can keep concrete clean and reduce air pollution.

Concrete Decor is typically filled with ideas you can use right away. Instead of stashing this article in your tool bag for on-the-job reference, however, take it to the beach this summer and read it when you have time to think about the future of our industry. For while photocatalysts have exciting potential for decorative concreting, they are still in the technology-transfer phase of moving from laboratory to commercial reality.

Strong sunlight or ultraviolet light decomposes many organic materials in a slow, natural process. You have seen this process, for example, in the way the plastic dashboard of your truck fades and gets brittle over time. Photocatalysts speed up this process and, like other types of catalysts, stimulate a chemical transformation without being consumed or worn-out by the reaction.

Photocatalytic titanium dioxide is energized by UV and accelerates the decomposition of organic particulates and airborne pollutants such as nitrous oxide (NOx).

When used on or in a concrete structure, photocatalysts decompose organic materials that foul the surface. The organic compounds affected by photocatalysts include dirt (soot, grime, oil and particulates) biological organisms (mold, algae, bacteria and allergens), air-borne pollutants (VOCs including formaldehyde and benzene; tobacco smoke; and the nitrous oxides (NOx) and sulfuric oxides (SOx) that are significant factors in smog), and even the chemicals that cause odors. The catalyzed compounds break down into oxygen, carbon dioxide, water, sulfate, nitrate and other molecules that are either beneficial to or at have a relatively benign impact on the environment. Most inorganic pollutants and stains, including rust, are not catalyzed.

The products of the catalytic reaction are easy to remove from the treated surface because the surface becomes hydrophilic — a term that means “water loving.” A hydrophilic surface prevents moisture from forming beads of water that may cause stains by attracting and holding dirt and then streaking the surface. Instead, moisture forms a thin film across a surface that interferes with the adhesion of dirt. Rain or simple rinsing can then easily remove the dirt. The result: your building or structure stays cleaner and more beautiful. One construction industry application that is already available in the United States is self-cleaning glass, offered by both Pilkington and PPG.

Beyond the visual benefits, other properties of photocatalysis may prove to be even more important. It is being used to purify water, fight disease by reducing the spread of germs, and to increase the shelf life of fruit by reducing the concentration of ethylene gas (associated with the ripening of fruit) in distribution facilities.

Photocatalysts are used in Japan to keep paving, like these concrete unit pavers, clean and to reduce the effect of air pollution.

It is also being used to reduce air pollution. In one study, photocatalytic paving decomposed 15 percent of the nitrous oxide released by cars traveling the roadway and was more effective, in this regard, than planting trees on both sides of the roadway. Other advocates of the new technology suggest that urban air quality could be improved by up to 80 percent if all streets, sidewalks, and exterior building surfaces were treated, an optimistic best-case scenario and one that does not take carbon dioxide emission into consideration.

Japan, Europe already using the technology
While the research models are still being refined, there is enough confidence in the air-purification capability of photocatalysts that they are already being used in Japan to make polluted cities like Tokyo more livable. Mitsubishi makes the Noxer brand of photocatalytic concrete unit pavers. Toto makes treated ceramic tiles and Hydrotect Color Coat, a pigmented water-based photocatalytic coating. You can even make the air in your home or office easier to breath with a photocatalytically-treated bouquet of flowers.

In addition to decomposing the chemicals that contribute to air pollution, photocatalytic treatments are alleged to have other environmental benefits. Self-cleaning concrete will not require the use of the solvents now used to clean buildings, eliminating another source of pollutants. Moreover, clean concrete will reflect more light, reducing the heat buildup associated with “urban heat islands.” This may help keep our cities cooler during hot seasons. It may also reduce the formation of smog since the chemical reaction that creates smog increases as air temperatures increase.