Stone washed

An 'air-purifying concrete' that uses titanium dioxide to remove harmful pollutants from the air is being tested in paving stones in

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Researchers from the University of Twente are paving a test road section in the town with air-purifying stones, the top layer of which will convert nitrogen oxide (NOx) from car exhaust fumes into harmless nitrates.

NOx is the pollutant that can cause acid rain and smog, and the researchers hope the problem can be partly solved by using the special stones.

'Our laboratory tests showed that typically 30 to 40 per cent of the NOx that flowed over the stones was degraded,' said Jos Brouwers, Twente's lead researcher.

Brouwers explained that titanium dioxide is a photocatalytic material that uses sunlight to convert the NOx in the air into harmless nitrates.

'Basically the photons from the UV light activate the titanium dioxide,' he said. 'OH ions are released and these react with the NOx which leaves nitrate salts in the stone.'

The salts can then be washed from the streets when it rains.

Brouwers said his research group believes the idea for air-purifying concrete was first developed by the Japanese about 10 years ago. But the problem with their 'recipe', he said, was that the concentration of titanium dioxide was too high and therefore the stones were too expensive.

According to Brouwers, the group at UT's concrete research laboratory set out to find a more optimum concentration of titanium dioxide that would bring down the cost significantly.

'The Japanese stones, based on our cost calculation, were double the price of a normal pavement stone,' he said. 'and we've reduced that to 30 per cent.'

The researchers tested 10cm x 20cm pavement stones in the laboratory by flowing air enriched with NO over the stone and implementing an artificial UV source. For the trial in Hengelo the streets are being divided into two sections - one half paved with conventional stones and the other with air-purifying ones. The air quality will then be measured in each section to test the effectiveness of the stones.

Those who have grown accustomed to the tidy city streets of the Netherlands will also consider it a bonus that the stones additionally repel dirt and therefore always stay clean.

Hengelo was chosen because of the volume of cars and the fact that the test road is being reconstructed. The local air quality is currently well within the norm.

The trial is being carried out with stone producer Struyk Verwo Infra. As part of its 'Effective Sustainability' programme the province of Overijssel, in which the town of Twente lies, is helping to subsidise the project.

The authorities see the stones as a good opportunity for improving the air quality at places where the norms are not met.

The road reconstruction is expected to be completed by the end of the year. Measurements will then start early next year, with the first test results expected around the summer of 2009.

'Hopefully these stones will confirm our laboratory results and prove that they actually have an effect on inner city air quality,' said Brouwers.