In recent months, a new building material that claims to both clean itself and filter pollutants out of the air around has been popping up on new buildings and infrastructure. It's one of those advances in construction technology that does actually seem, well, really good.
As a result of its self-cleaning abilities, the concrete keeps its colour for far longer than traditional building materials, so doesn't need to be replaced so often; but it can also reduce general air pollution. Which, to be honest, sounds like a bit of a win-win. But how exactly does the magic work? And is there a catch?
How was it invented?
The technology was actually invented pretty much by accident, by Luigi Cassar, an Italian chemist at cement manufacturer Italcementi. He was trying to create a construction material which would keep a bright white colour even in polluted conditions, and hit upon a method called "photocatalysis", which uses the sun's energy to zap away dirt.
To his surprise, when the air around the treated concrete was tested, it contained up to 80 per cent less nitrous oxide: the concrete was cleaning the air as well as itself.
How does it work? (Warning: science.)
When we clean stuff, we tend to use a substance which can break down dirt so it can be washed from the object's surface, plus a bit of energy to make sure that reaction happens. When you scrub a plate, for example, you use soap and water, plus your own elbow grease, to remove dirt.
On the surface of self-cleaning cement, the cleaning happens without any scrubbing involved. The secret? The power of the sun.
When light and heat strikes the concrete's surface, catalysts (usually titanium oxides) use that energy to break down the dirt into molecules like oxygen, water, carbon dioxide, nitrates, and sulphates. Gases float away, while liquids or solids are left on surface to be washed away by rain.
Through a similar process, concrete can also break down pollutants in the air around it: if a pollutant strikes the surface, the titanium oxide reacts with it in the same way.
This diagram shows a nitrogen oxide hitting the surface and being converted into a nitrate:
So what's the catch?
Other scientists have dug into the theory behind self-cleaning cement and found a few problems.
1. Eagle-eyed readers might have already worked this one out: if those new substances left on the surface of the cement are "washed away", where exactly do they go? Unfortunately, the answer is probably "into groundwater, and then rivers and lakes". This is bad news when it comes to nitrates, which cause algae blooms and in turn deplete the body of water’s oxygen levels.
2. Researchers from Indiana University found that, while the cement does what it says on the tin in specific lab conditions, it reacts quite differently if the humidity or level of pollution is lower. In fact, they found that, in lower pollution levels, the titanium dioxide would catalyse a reaction with ammonia which actually increases nitrogen oxides in the atmosphere.
3. The kilns used to make cement actually give off large amounts of nitrogen oxides and sulphur dioxide, which means the cement would have to be pretty effective (despite the limitations outlined above) to result in a net decrease in atmospheric nitrogen.
So in summary: yes, these compounds do a good job of keeping buildings clean and white. But only time will tell whether they're 100 per cent brilliant for the environment.