The concrete answer to pollution

Dull, grey and boring: concrete may not be much to look at, but – along with other basic materials like paint – it is being upgraded so it can clean up pollution.

The development comes down to the introduction of photocatalysts – substances that promote chemical reactions using only the sun’s rays as input. When the photocatalysts are incorporated into a material such as concrete, they trap pollutants and convert them into clean products.

For Dr Andrea Folli – a scientist at the Danish Technological Institute – the technology offers an attractive way to improve the functionality of one of the most widely used building materials. ‘Our built-up infrastructure offers an incredibly big surface area that we could use in order to promote reactions that improve air quality,’ said Dr Folli, who is project manager of the Light2Cat project to develop pollution–degrading concrete.

Pollution-degrading concrete is not a new idea, but it has not been wholly successful. For years manufacturers have sold cement infused with the photocatalyst titanium dioxide.

When titanium dioxide is exposed to light, it converts water vapour in the air into hydroxyl and peroxyl radicals. These radicals react eagerly with airborne pollutants, such as nitrogen oxides in exhaust fumes, producing totally benign products.

The trouble is twofold, according to Dr Folli. First, titanium dioxide is expensive relative to cement, so that currently available pollution-degrading concrete costs about four times as much as normal concrete. Second, titanium dioxide is only sensitive to ultraviolet light, which is plentiful in hot regions such as the Mediterranean, but harder to come by in the north.