Water is the sole construction ‘material’ used worldwide today in greater volume than concrete. The globe’s citizens make use of concrete in incredibly vast quantities each year, for dams, roads, culverts, drainage ways, bridges, piers, buildings, plazas, sidewalks, curbs, play equipment, and so on. And, as our various concrete constructions age, often beneath urban skies that have become significantly polluted, they become stained and dirty, affecting their appearance as well as our everyday air quality.
What if we were able to reverse the situation, to have our various concrete constructions actually clean themselves and their environs? That is the promise of photocatalysis — a chemical process within concrete that is triggered by the incidence of sunlight. Self-cleaning photocatalytic concrete has recently joined the green designer’s arsenal as a powerful weapon against dirt and pollution.
Atmospheric pollution can be a severe problem in urban areas, with more than 120 known pollutants occurring as fine mist or liquid droplets, gas, and fine particulate matter. Some pollutants — such as wind-blown dust, volcanic ash, soot and smoke from fires, and the elements given off during the decay of plants and animals — are from natural occurrences. But an increasing number of harmful and toxic pollutants are man-made: produced either from industrial or manufacturing processes, or from vehicular or other engine exhaust. Among these are a number that are considered of greatest threat to continued human health by the Environmental Protection Agency: nitrogen oxides, sulfur oxides, carbon monoxide, ozone, lead, particulate matter, and volatile organic compounds (VOCs).
Such pollutants can have severe, broad effects on human health, first affecting respiratory and cardiovascular systems. Over the long term, exposure to such pollutants can also harm the liver, kidneys, and central nervous system, potentially affecting behavior and decreasing memory function, IQ and learning ability. Cancers and other ailments may also result.
These same pollutants are also harmful to the health of the planet. They are causative factors in acid rain, the proliferation of greenhouse gases, and damage to plant life resulting in slowed growth and lowered yields.
As a driver of the photocatalytic process, sunlight is available in vast abundance. It is estimated that our total global energy consumption amounts to less than 1/100th of 1% of the solar energy that strikes the Earth’s surface. For photocatalysis to ensue, sunlight must also be accompanied by water, oxygen, the offending compounds, and a catalyst, titanium dioxide. Obtained from its natural state in the minerals of brookite, rutile and anatase, titanium dioxide is the world’s most widely used white pigment. It is very bright, is highly refractive, and has the ability to absorb the ultraviolet (UV) radiation within sunlight without being consumed by the ensuing chemical reaction. Titanium dioxide is thus used not only in most sun protection products, but also in paints, coatings, inks, papers, plastics, medicine, foods, and even toothpaste.
Through the photocatalytic reactions, the titanium dioxide greatly accelerates a very slow natural ‘scrubbing’ process that converts toxic pollutants to harmless end products. Nitrogen oxides are converted to nitrate salts; sulfur dioxide is transformed to sulfate ions; ozone reverts to oxygen gas.