The titanium dioxide, a photocatalytic material, removes the NO_x from the air and converts them with the aid of sunlight into nitrate. The nitrate is then rinsed away by rain.

The tests were carried out in the municipality of Hengelo, where the busy Castorweg road was resurfaced last fall. As part of the project, around 1,000 square meters of the road’s surface were covered with concrete paving stones containing the titanium dioxide. For comparison purposes, another area of 1.000 square meters was surfaced with normal paving stones.

Researchers at TU/e 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 NO_x content was found to 25 to 45% lower than that over the area paved with normal concrete. Further measurements are planned later this year.

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 NO_x concentrations are now exceeded. The concrete stones used in the tests are made by, and co-developed with, 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.

The use of air-purifying concrete does not have a major impact on the cost of a road, Brouwers has calculated. Although the stones themselves are 50% more expensive than normal concrete stones, the total road-building costs are only 10% higher.