What exactly is self-cleaning concrete, and how does it work?

The Jubilee Church in Rome is coated in self-cleaning cement. Image: Psidium

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.


 

 
 
 
 

Older people need better homes – but then, so does everybody else

Colne, Lancashire. Image: Getty.

Towards the end of last year, I started as an associate director at the Centre for Ageing Better, working particularly on our goal around safe and accessible homes. Before I arrived, Ageing Better had established some ambitious goals for this work: by 2030, we want the number of homes classed as decent to increase by a million, and by the same date to ensure that at least half of all new homes are built to be fully accessible.

We’ve all heard the statistics about the huge growth in the number of households headed by someone over 65, and the exponential growth in the number of households of people over 85. Frustratingly, this is often presented as a problem to be solved rather than a major success story of post war social and health policy. Older people, like everyone else, have ambitions for the future, opportunities to make a full contribution to their communities and to continue to work in fulfilling jobs.

It is also essential that older people, again like everyone else, should live in decent and accessible homes. In the last 50 years we have made real progress in improving the quality of our homes, but we still have a lot to do. Our new research shows that over 4 million homes across England fail to meet the government’s basic standards of decency. And a higher proportion of older people live in these homes than the population more generally, with over a million people over the age of 55 living in conditions that pose a risk to their health or safety.

It shouldn’t be too difficult to ensure all our homes meet a decent standard. A small number of homes require major and expensive remedial work, but the overwhelming majority need less than £3,000 to hit the mark. We know how to do it. We now need the political will to make it a priority. Apart from the benefits to the people living in the homes, investment of this kind is great for the economy, especially when so many of our skilled tradespeople are older. Imagine if they were part of training young people to learn these skills.


At a recent staff away day, we explored where we would ideally want to live in our later lives. This was not a stretch for me, although for some of our younger colleagues it is a long way into the future.

The point at which the conversation really took off for me was when we moved away from government definitions of decency and accessibility and began to explore the principles of what great homes for older people would be like. We agreed they needed light and space (by which we meant real space – our national obsession with number of bedrooms as opposed to space has led to us building the smallest new homes in Europe).

We agreed, too, that they needed to be as flexible as possible so that the space could be used differently as our needs change. We thought access to safe outdoor space was essential and that the homes should be digitally connected and in places that maximise the potential for social connection.

Of course, it took us just a few seconds to realise that this is true for virtually everyone. As a nation we have been dismal at moving away from three-bed boxes to thinking differently about what our homes should look like. In a world of technology and factory building, and as we build the new generation of homes we desperately need, we have a real chance to be bold.

Great, flexible homes with light and space, in the places where people want to live. Surely it’s not too much to ask?

David Orr is associate director – homes at the Centre for Ageing Better.