It’s A Wash

How precast concrete buildings can clean themselves and our air.

By Claude Goguen, P.E., LEED AP

No kidding. Self-cleaning buildings are a reality. Better yet, architectural precast concrete with new material technology can remove pollutants from the air while actually rinsing itself clean in the rain. Self-cleaning buildings may sound like a futuristic concept, but they do exist today, providing aesthetic, environmental and no-cost maintenance service.

Case in point: Jubilee Church in Rome, Italy

Like white sails in the wind, three large, curved walls of precast concrete adorn the south side of the Jubilee Church in Rome. One of the primary purposes of these walls is to minimize thermal peak loads inside. The large thermal mass of the concrete walls controls internal heat gain; the result is less inside temperature variation and a more efficient use of energy.

As an added benefit to the owners, these “billowing” precast concrete walls contain titanium dioxide (TiO₂) to keep the appearance of the church clean, white and beautiful. The TiO₂ incorporated in the concrete absorbs ultraviolet light from the sun and becomes powerfully reactive, breaking down pollutants that come in contact with the concrete surface. There you have it: a self-cleaning building!

New formulations of cement, introduced in Europe in the ’90s, can neutralize pollution. These altered cements are used in the same way as portland cement, and therefore any precast concrete structure can potentially function as a pollution fighter. But how does it work?

How photocatalytic materials work

Strong sunlight or ultraviolet light decomposes many organic materials in a slow, natural process. You may have witnessed this in the way the plastic dashboard of a car fades and becomes brittle over time. Photocatalysts are used to accelerate this process and, like other types of catalysts, stimulate a chemical transformation without being consumed or worn out by the reaction.

Photocatalysis is a reaction that uses light to activate a substance (the catalyst) that modifies the rate of a chemical reaction without the catalyst itself being affected. An analogy for a catalyst would be an instigator who starts an argument between two people and then walks away unscathed while a fight breaks out. When used on (as a topping) or in a concrete structure (incorporated into the concrete mixture), photocatalysts decompose some rather nasty organic materials.

Some of the targets of photocatalysts on structures are dirt, soot, oil, mold, algae, bacteria and allergens, and airborne pollutants. Organic pollutants in the air are formaldehyde, benzene, tobacco smoke, nitrous oxides (N₂O), and sulfuric oxides (sulfuric oxides are found in smog). Photocatalysts break down these airborne pollutants into oxygen, carbon dioxide, water and other environmentally healthy substances. A remarkable aspect of this material technology is that dirt, mold and other polluting substances actually wash off the surfaces of photocatalytic concrete when it rains. TiO₂ is the
linchpin in self-cleaning concrete.

Titanium dioxide: white-hot cleanup agent

Proprietary material technology, based on particles of TiO₂, is what creates self-cleaning concrete. This technology can be applied to white or gray cement. TiO₂ 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 pigments.

At the nano scale, TiO₂ undergoes a quantum transformation and becomes a semiconductor. Activated by the energy in sunlight, TiO₂ creates what is technically termed an “electron-hole charge separation.” This means that electrons disperse on the surface of the photocatalyst and react with external substances (airborne pollutants), causing chemical reductions and oxidations, and forming hydroxyl radicals that act as powerful oxidants to decompose organic compounds. Simply put, TiO₂ breaks down unwanted and unhealthy organic compounds in the air and transforms pollutants into oxygen, water, carbon dioxide, nitrate and sulfate. The result is cleaner air and building surfaces.

The cost of adding TiO₂ to concrete translates into about a 50% increase versus the cost of ordinary cement. In a cost/benefit analysis, however, increased material costs are balanced by less building maintenance over the life of the structure. Besides buildings like the Jubilee Church, photocatalytic cement is used or being proposed for highway pavement, concrete pavers, traffic barriers, precast roofing tiles, concrete building entrances and other architectural precast concrete products.¹

Jubilee Church in Rome

Bonus points besides beauty

Self-cleaning concrete has big benefits in addition to its good looks. By keeping the building’s surface whiter, photocatalytic concrete maximizes its ability to reflect the sun’s heat and thereby reduce the associated heat gain. In this way, precast buildings reduce the heat island effect in urban areas.²

Photocatalytic concrete offers building professionals a unique opportunity to reach their sustainable development goals while potentially improving the value of their investment. Research on photocatalytic technology has been progressing for more than three decades, and we can expect this material technology to improve with time.

Self-cleaning buildings and roads are not science fiction. A clean building is not only aesthetically pleasing, but with the help of photocatalytic agents, precast concrete structures will provide us with a brighter future.

¹ For more photocatalytic precast concrete applications, see “Five Ways Precast Helps Save the Earth,” Precast Solutions, Summer 2012.

² See Green Piece: “Heat Islands are No Tropical Paradise,” Precast Inc., March-April 2012.

For more information on this topic or any other precast concrete related topic, contact Claude Goguen, NPCA director of Technical Services, at [email protected] or at (317) 571-9500.