Cleaning up the air with cement
Buildings that clean the air around them
By Aedan Kernan, a freelance journalist specializing in
business relationships and environmental issues.
Multinational company Italcementi has developed a new preventative method against air pollution. Aedan Kernan explores this new product and its results
Walls and road surfaces coated in special cement or paint can now help neutralize pollution from traffic and industry. In a congested city such as Milan, researchers have calculated that covering one percent of visible urban surfaces with the pollution-neutralizing coatings would enable a reduction in pollution of approximately 50 percent.
Amazingly, the coatings are self-cleaning and the surfaces—drawing energy from the sun—do not lose their pollutant-neutralizing powers over time. A simple shower of rain will wash the pollutants into the drains, leaving a pristine building.
The cement coatings are now being marketed worldwide by the Italcementi Group, under the brand name TX Active. They are being used in applications ranging from motorway sound barriers, paving blocks, roof tiles and pre-cast architectural panels, to a thin finishing coat over the façade of entire buildings.
While TX Active is comparatively expensive, only a thin surface layer is necessary, according to Italcementi spokesman Francesco Galimberti. “For a building six floors high, it adds something like 10 percent to the costs of covering the building in cement or mortar.”
Magic ingredient
The magic ingredient in both the cement and paint coatings is titanium dioxide. Titanium dioxide is widely used as a raw material in industrial products ranging from toothpaste and sunscreen to cosmetics, normally as a white pigment to enhance colors.
In the coatings, it is the photocatalytic properties of titanium dioxide that add real value. A photocatalytic material is one that, in light, accelerates the speed of a chemical reaction without being consumed during the process.
Basically, the photon energy from sunlight (or another light source) changes the state of the titanium dioxide (TiO2). With the added energy, this change in state will initiate reactions with water and oxygen that degrade mineral and organic molecules.
Once the photon energy is expended, it reverts back to titanium dioxide, but any further light energy will convert it once more into a photocatalyst. TiO2 is all the more efficient as its particles are finely divided to give a high surface contact, and the denser the pollution in the air, the better the results obtained from the photocatalytic cement.
The air pollutants are mainly converted by the chemical reaction into nitrate and sulphate salts (carbon dioxide and water vapour are a by-product). The nitrates and sulphates are then washed away into the drains by the next rain shower.
TX Active is not the first application to exploit a photocatalytic material. It has previously been used in self-cleaning lamps and car coatings, anti-fogging mirrors and in antibacterial products such as water purifiers. The Milan Fair Complex began using photocatalytic paints on its pavilions five years ago to counter the air pollution of 15,000 cars that drive through its busiest areas.
A feast of nasties
Photocatalytic materials can extract a whole range of nasties from the atmosphere. These include inorganic compounds such as SOx, carbon monoxide, NH3 and H2S. Organic compounds such as acids are equally neutralized, as are deadly dioxins and chlorobenzene. In fact, the photocatalytic coatings will also deal with pesticides such as diazinon and atrazine, and it can reduce levels of micro-organisms such as bacteria and fungi. Residents of one urban area in Italy reported that a constant and unpleasant odor disappeared after a TX Active product was installed on a surface nearby.
A major health hazard coming out of the tail pipes of our vehicles is nitric oxide (NOx). Bad enough on its own, nitric oxide also reacts with volatile organic compounds that have evaporated from chemical plants, refineries, factories, fuel stations—even the paints we put on our walls—to create ozone, the principal ingredient in smog.
High in the atmosphere, ozone acts as a protector for our environment. At city street level, ozone (O3) reacts chemically with lung tissue, causing decreased lung function, respiratory infections, lung inflammation and aggravation of respiratory illnesses.
In tests, Italcementi demonstrated that a road paved with concrete made with TX Active cement can reduce NOx levels by 20 percent to 80 percent, depending on atmospheric conditions, eliminating a key ingredient for ozone.
Testing theory at street level
One of Italcementi’s first experiments beyond the laboratory was the creation of an 8,000m2 yard of self-locking photocatalytic cement blocks at one of its own plants. Measurements showed a 45 percent reduction in NOx levels in the yard.
They also applied a thin layer of photoactive TX Active binder-based mortar to a 230-meter-long section of Via Morandi, a street in Milan lined with apartment blocks that carries more than 1,000 vehicles every hour. Measurements revealed up to 60 percent reduction in NOx levels during hot summer days.
TX Active has even proved effective in a tunnel. The 100-meter-long, seven-meter-wide tunnel at Porpora Street in Milan is a major connector, carrying up to 30,000 vehicles per day. Working closely with the lighting contractor to maximize UV light in the tunnel, Italcementi placed a layer of photocatalytic material on the tunnel road surface, while a second firm sprayed the ceiling with photocatalytic paint. They achieved more than 20 percent reduction in NOx levels where lighting conditions were worst in the tunnel.
Italcementi Group continues to develop the product for new applications, including a product for road surfaces typically found in northern Europe, and products are being manufactured and distributed in the US. It was recently used on high-profile towers at each end of a bridge across the Mississippi at Minneapolis.
Local not global solution
The results achieved by photocatalytic materials in confined and localized spaces may be spectacular, but it’s no magic bullet for the air pollution we continue to pump out. One academic expressed his skepticism about the promise of the materials back in 2005.
“Trying to clean up air pollution seems to me to be a stretch,” said Reynaldo Barreto, a chemistry professor at Purdue University in Indiana. “It doesn’t mean it can’t be done. But there’s an awful lot of air and not a whole lot of surface.”