Photocatalysts keep concrete clean and depollute the air we breathe.

By Michael Chusid, RA, FCSI

Important new concepts often require new words to convey their meanings. Just a generation ago, the word precast became part of the construction industry vernacular as it defined a new era of improved concrete quality and reduced construction time and costs. It was an important new word and changed the way we build. Today, the industry has two new words to get acquainted with, words that may be even more important since they affect not just construction but the very quality of life in our communities: depollution and photocatalysis.

Depollution is the opposite of pollution and means the removal of contaminants and impurities from the environment. The newest tool for achieving depollution is a photocatalyst, a material that uses solar energy to accelerate chemical reactions without being consumed or depleted in the process. These words can be used in a sentence such as; “We used a photocatalyst in the precast concrete to make it self-cleaning and able to contribute to the depollution of our community.”

As new as these words are, they are rapidly entering the vocabulary of designers and builders around the world. A major international research consortium – the Photocatalytic Innovative Coverings Applications for Depollution Assessment (PICADA) – has just completed a major study documenting the performance of photocatalysts. Photocatalytic concrete is being used with increased frequency in Europe and Asia for architectural and civil engineering projects, and photocatalytic portland cement is now available in North America from Essroc, a division of the Italcementi Group. The company offers two grades of photocatalytic cement: TX Active Arca for self-cleaning concrete, and the higher performance TX Active Aria for concrete that is both self-cleaning and air depolluting. Both cements are available in either gray or white and contain patented photocatalytic chemistry that has been specially formulated to produce photocatalytic concrete.

Italcementi Group was a partner in the European Commission-funded PICADA initiative and has taken the lead in applying photocatalytic research to concrete. The company was technical sponsor for the Jubilee Church (also known as the Dives in Misericordia) in Rome, a showplace for photocatalytic self-cleaning concrete. The award-winning international architecture firm Richard Meier & Partners Architects LLP, New York, designed the soaring structure in conjunction with structural engineer groups Arup and Guy Nordenson and Associates, New York. Jubilee Church is an awe-inspiring composition of 256 precast, post-tensioned concrete elements assembled into semi-sphere white shells rising 85 feet into the sky. The designers called for a high-performance concrete mixture with white portland cement, white high-reactivity metakaolin, and white carrara marble aggregate to create a brilliant white concrete. To keep it clean, despite the church’s location in a polluted industrial neighborhood, Italcementi’s photocatalytic TX Active cement was used.

To avoid the need for a steel framework, the concrete shells were subdivided into large precast blocks with double curvatures. The curved walls are cantilevered vertically from the ground, with glass placed in and around the structures, enveloping portions of the church. Rather than being cast-in-place concrete, the precast concrete made it easier to control the form of the curved sections and give them the same smooth, white surface inside and out. Each segment of the semi-spherical shells weighs about eight tons with segments shipped directly to the site. There, a crane, mobilized via track, installed the pieces, which were then post-tensioned. The panels were manufactured by Italian precaster Edilgori.

“No one’s ever erected anything of this scale in terms of a precast assembly,” Meier has said.

Since the completion of the Jubilee Church in 2003, other precast concrete structures have used photocatalytic cement:

• The new Air France headquarters at Roissy-Charles de Gaulle International Airport, Paris, used white TX Active. White concrete is typically nonexistent near an airport due to soot from jet engine exhaust and heavy vehicular traffic. Panels were precast by MSA.
• For the Hotel de Police in Bordeaux, France, built for the French Department of Interior, precaster CIR made two-layered precast concrete panels. The outer layer is white TX Active with embedded white marble aggregates and was polished to increase the luminosity of the precast concrete walls.
• Precaster ID-Bat used gray TX Active in panels for Cité de la Musique et des Beaux-Arts in Chambéry, France. Since its completion, the educational building has been checked regularly with colorimetric monitoring to measure any accumulation of dirt on the concrete. After three years, no significant soiling has occurred, even on the north façade that is not exposed to direct sunlight.
• Photocatalytic cement has also been fabricated into precast concrete pavers and used for roadways. PICADA determined this was a prime application for the new technology, because it puts the depollution in close proximity to pollutions emitted by automobile exhausts.

According to Paul Batt, Essroc’s director of marketing, his firm is in discussion with a number of architects and developers about using photocatalytic cement for North American projects. “We are working closely with the precast concrete industry,” he said. “[Precast concrete] can provide the quality control necessary to achieve the optimum appearance of self-cleaning concrete.”

Precast also offers an economy that is difficult to achieve with cast-in-place concrete. The premium-priced mixture with photocatalytic cement can be used for the surface of a panel and then backed with a conventional concrete mixture. While Batt sees great opportunities for photocatalytic concrete, the wide range of conditions and materials affecting concrete requires a case-by-case approach to ensure that the factors impacting the effectiveness of photocatalytic concrete are considered.

How it works

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 a car fades and becomes brittle over time. Photocatalysts accelerate this process and, like other types of catalysts, stimulate a chemical transformation without being consumed or worn out by the reaction.

When used on or in a concrete structure, photocatalysts decompose organic materials such as dirt, including soot, grime, oil and particulates; biological organisms, including mold, algae, bacteria and allergens; airborne pollutants, including volatile organic compounds, 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 worst, have a relatively benign impact on the environment. Most inorganic pollutants and stains, including rust, are not catalyzed.

Titanium oxide (TiO2), the primary catalytic ingredient, is widely used as a white pigment in paint, plastics, cosmetics and a host of other products. Making it capable of photocatalysis requires manipulating the material to create extremely fine nano-sized particles with a different atomic structure than that of the ordinary pigment. At the nano scale, this type of titanium undergoes a quantum transformation and becomes a semiconductor. Activated by the energy in light, the TiO2 creates a charge separation of electrons and electron holes. The electrons disperse on the surface of the photocatalyst and react with external substances, causing chemical reductions and oxidations and forming hydroxyl radicals that act as powerful oxidants to decompose organic compounds.

Sustainable buildings

In addition to decomposing the chemicals that contribute to soiling and air pollution, photocatalytic concrete has other environmental benefits. For example, white concrete – whether used for precast concrete pavers or roofing – reflects much of the sun’s heat and reduces the heat gain associated with dark construction materials like asphalt paving. This keeps cities cooler and reduces the need for air conditioning. It also reduces the formation of smog since the chemical reactions that create smog increase as air temperature rises. Using photocatalytic concrete keeps the concrete cleaner to optimize this environmental benefit and can contribute substantially toward sustainable design initiatives including the U.S. Green Building Council’s LEED certification program.

The same reflectance characteristic makes white precast concrete attractive as an interior finish that can improve the efficiency of lighting and reduce energy requirements. Photocatalytic materials can also neutralize indoor air pollutants such as VOCs from building materials, photocopier toner dust and cleaning chemicals. In Japan, photocatalysts have already become popular to remove odors and generally improve indoor air quality. And whether used indoors or out, photocatalytic concrete reduces the need for building maintenance and the VOC pollutants associated with cleaning compounds and coatings.

No one likes a dirty building or a polluted environment. Designing projects with photocatalytic precast concrete helps promote aesthetic endurance, keeping the structure looking like new over time. Clean commercial properties attract increased revenues, and clean institutional buildings are a source of civic pride. More companies and government agencies are paying attention to the environmental sustainability of their properties. So by applying a few new words like photocatalysis and depollution to your construction vocabulary, you just might hear your clients use that beautiful world: yes.

Michael Chusid, RA, FCSI, is an architect and marketing consultant specializing in the evaluation and promotion of innovative building materials. Chusid is based in Los Angeles and can be reached at www.chusid.com.