Although much of the focus of pollution from automobiles centers on carbon emissions, there are other airborne nasties spewing from the tailpipes of fossil fuel-powered vehicles. These include nitrogen oxides (NOx). In the form of nitrogen dioxide it reacts with chemicals produced by sunlight to form nitric acid – a major constituent of acid rain – and also reacts with sunlight, leading to the formation of ozone and smog. Everyone is exposed to small amounts of nitrogen oxides in ambient air, but exposure to higher amounts, in areas of heavy traffic for example, can damage respiratory airways. Testing has shown that surfacing roads with air purifying concrete could make a big contribution to local air purity by reducing the concentration of nitrogen oxides by 25 to 45 percent.

Last fall in the municipality of Hengelo, the Netherlands, researchers at the Eindhoven University of Technology (TU/e) resurfaced around 1,000 square meters of the busy Castorweg Road with air-purifying concrete paving stones, while another area of 1,000 square meters was surfaced with normal paving stones. The air-purifying concrete contains titanium dioxide, a photocatalytic material that removes the nitrogen oxides from the air and converts them into harmless nitrate with the aid of sunlight. The nitrate is then rinsed away by rain.

The researchers carried out three air-purity measurements on the Castorweg last spring, at heights of between a half and one-and-a-half meters. Over the area paved with air-purifying concrete the NOx content was found to 25 to 45 per cent lower than that over the area paved with normal concrete. Further measurements are planned for later this year.

“The air-purifying properties of the new paving stones had already been shown in the laboratory, but these results now show that they also work outdoors”, said prof. Jos Brouwers.

Brouwers, who has been professor of building materials in the TU/e Department of Architecture, Building and Planning since September 2009, sees numerous potential applications, especially at locations where the maximum permitted NOx concentrations are now exceeded. Aside from the reduction in nitrogen oxides, the stones also have another advantage: they break down algae and dirt, so that they always stay clean.

The concrete stones used in the tests are made by paving stone manufacturer Struyk Verwo Infra, and are already available for use. For roads where an asphalt surface is preferred the air-purifying concrete can be mixed with open asphalt, according to Brouwers. It can also be used in self-cleaning and air-purifying building walls.

Brouwers says the use of air-purifying concrete does not have a major impact on the cost of a road. Although the stones themselves are 50 per cent more expensive than normal concrete stones, the total road-building costs are only ten percent higher.