Use Prudent Specifications with Titanium Dioxide

I read with interest your article on photoactive titanium dioxide (TiO₂) in concrete [see

EBN

Vol. 16, No. 5]. I have been following the product launch for several years, have published articles on the technology, and have spoken about the material at an American Concrete Institute’s nanotechnology conference. My research has satisfied me that the material performs as the manufacturer claims with regards to the removal of organic grime (but not inorganic stains) from the surface of concrete and the depollution of nearby air.

I note one inaccuracy in your article. Due to the small size of the particles and their low density on the surface of concrete, TiO₂ does not affect the visual properties of the concrete. The whiteness of the Jubilee Church results not from TiO₂ but from white portland cement, white marble aggregate, and metakaolin—a bright white pozzolanic concrete additive.

Despite my enthusiasm, I have some environmental concerns with the current generation of photoactive TiO₂. First, does photoactive TiO₂ pose a threat to microorganisms when it enters into surface water or groundwater? The class of TiO₂ used as a pigment—rutile—has a record of safe use. The class of TiO₂ used in photoactive products, however—anatase—is comparatively rare in nature, and the behavior of its nano-sized particles in ecosystems has not been fully studied.

The catalytic characteristics of the material do not diminish with time. What could happen, therefore, if a photoactive structure is demolished and concrete from the structure is used as riprap on the shore of a shallow sea? As the concrete erodes, what damage could be done when particles settle onto coral polyps or other vulnerable species?

I know this risk seems out of proportion in light of the product’s benefits and given the small use of photoactive compounds today. But the compounds are already finding inroads into many products worldwide—including disposable consumer products. Must we wait for another

Silent Spring before establishing guidelines?

Prudence suggests that concrete producers and the project team should minimize the release of photoactive material. At the very least, the designer should discuss the risks with the building owner. Specifications should require the concrete supplier and the contractor to dispose of waste TiO₂ in a manner that will not contaminate the environment. A plaque should be mounted notifying future generations that the concrete may require special handling upon demolition.

We also need to study the byproducts of depollution. As photoactive TiO₂ removes pollutants, it forms new chemical compounds. While these chemical byproducts are considerably less hazardous than the pollutants they replace, there is no such thing as a free lunch in a closed ecosystem, and the potential runoff from a photoactive surface should be studied for impact.

The photocatalytic reaction can also draw calcium out of the calcium silicate hydrate (CSH) that is the cementitious component of concrete, accelerating erosion. This degradation is estimated to be “just a fraction of a millimeter per decade,” a rate that can amount to a fraction of a centimeter per century and could be visible at corners or surface relief. Adding metakaolin to the concrete mixture enriches the CSH content of the concrete and may retard erosion.

I remain optimistic that photoactive titanium dioxide can make the environment cleaner and healthier. This goal, however, will be achieved only if we make informed tradeoffs between benefits and risks.

Michael Chusid, Senior Consultant

Chusid Associates

Tarzana, California