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Posted: Feb 23, 2012

Self-cleaning buildings

(Nanowerk News) The phenomenon of soiled and stained facades is probably as old as the cities themselves. Industrial pollution then has increased these problems. Especially in densely populated urban areas, building facades are covered by inorganic pollutants like nitrogen oxides and organic pollutants like benzene. All this is on the one site unhealthy for people and on the other destructive for buildings.
The European project PICADA is closely within the scope of the European programme called "Competitive and Sustainable Growth" and stands for Photocatalytic Innovative Covering Applications for De-pollution Assessment. The aim was to develop a range of self-cleaning and de-polluting materials and to evaluate their effect at a large scale.Basically to develop buildings and coatings that could have some characteristics of a tree.
In the last ten years researchers have worked on a method which makes cement or concrete self-cleaning and de-polluting. Therefore scientists use the white pigment Titianium Dioxide (Ti0˛), which natuarilly exists in a mineral called rutile. It is a high photocatalyic metal oxide. The principle of photocatalysis by Titanium Dioxide is that the white pigment acts as a semiconducter. When it is irradiated with sufficient photon energy a chemical reaction is starting.
The electrons of Titanium Dioxide particles become supercharged and interact with the water molecules in the air. This interaction releases free radicals that break down organic material on the building facade and pollutants such as nitrogen oxide in the surrounding atmosphere.
So Titanium Dioxide in building materials and coatings like cement or concrete is able to "catch" and "break down" the air pollutants. After the attack the "consumed" pollutants are harmless compounds which can be washed away by the rain.
Arnaud Plassais, an engineer of the PICADA project believes in the future of self-cleaning buildings. "The Picada project proved that the photo catalytic cement based products are useful for depollution and self-cleaning. So it helps us to finish the formulation of industrial products. And also to develop laboratory and onsite tests to assess the efficiency of these products."
The new building materials could help to reduce the concentration of nitric oxides and other toxic substances like benzene which can provoke respiratory problems and increase smog formation.
But are these nano-particles used in the new cement safe for people´s health? The independent expert Robert Copé from the French Scientific and Technical Centre for Building has tested these particular nano-particles. "Nowadays the proportion of active nano-particles in cement or in concrete is currently very low. Also once the material is exposed to the air the particles have a tendency to clump together or to stick to particles of cement - and as a result they are no longer nano-scaled particles."
First field tests with photocatalytic materials have shown that it is possible to improve the air quality. In 2002 scientists have coated a surface of 7000 square meter in Milan with a photoctalytic material. The result: the nitrogen oxide concentration was reduced by 60 percent.
Although it does not look like a tree the method proved that it has in particular functions of a tree and is equally harmless.
Source: By Ute de Groot, Youris
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