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.


 

 
 
 
 

Transport for London’s fare zones secretly go up to 15

Some of these stations are in zones 10 to 12. Ooooh. Image: TfL.

The British capital, as every true-blooded Londoner knows, is divided into six concentric zones, from zone 1 in the centre to zone 6 in the green belt-hugging outer suburbs.

These are officially fare zones, which Transport for London (TfL) uses to determine the cost of your tube or rail journey. Unofficially, though, they’ve sort of become more than that, and like postcodes double as a sort of status symbol, a marker of how London-y a district actually is.

If you’re the sort of Londoner who’s also interested in transport nerdery, or who has spent any time studying the tube map, you’ll probably know that there are three more zones on the fringes of the capital. These, numbered 7 to 9, are used to set and collect fares at non-London stations where the Oyster card still works. But they differ from the first six, in that they aren’t concentric rings, but random patches, reflecting not distance from London but pre-existing and faintly arbitrary fares. Thus it is that at some points (on the Overground to Cheshunt, say) trains leaving zone 6 will visit zone 7. But at others they jump to 8 (on the train to Dartford) or 9 (on TfL rail to Brentwood), or skip them altogether.

Anyway: it turns out that, although they’re keeping it fairly quiet, the zones don’t stop at 9 either. They go all the way up to 15.

So I learned this week from the hero who runs the South East Rail Group Twitter feed, when they (well, let’s be honest: he) tweeted me this:

The choice of numbers is quite odd in its way. Purfleet, a small Thames-side village in Essex, is not only barely a mile from the London border, it’s actually inside the M25. Yet it’s all the way out in the notional zone 10. What gives?

TfL’s Ticketing + Revenue Update is a surprisingly jazzy internal newsletter about, well, you can probably guess. The September/October 2018 edition, published on WhatDoTheyKnow.com following a freedom of information request, contains a helpful explanation of what’s going on. The expansion of the Oyster card system

“has seen [Pay As You Go fare] acceptance extended to Grays, Hertford East, Shenfield, Dartford and Swanley. These expansions have been identified by additional zones mainly for PAYG caping and charging purposes.

“Although these additional zones appear on our staff PAYG map, they are no generally advertised to customers, as there is the risk of potentially confusing users or leading them to think that these ones function in exactly the same way as Zones 1-6.”


Fair enough: maps should make life less, not more, confusing, so labelling Shenfield et al. as “special fares apply” rather than zone whatever makes some sense. But why don’t these outer zone fares work the same way as the proper London ones?

“One of the reasons that the fare structure becomes much more complicated when you travel to stations beyond the Zone 6 boundary is that the various Train Operating Companies (TOCs) are responsible for setting the fares to and from their stations outside London. This means that they do not have to follow the standard TfL zonal fares and can mean that stations that are notionally indicated as being in the same fare zone for capping purposes may actually have very different charges for journeys to/from London."

In other words, these fares have been designed to fit in with pre-existing TOC charges. Greater Anglia would get a bit miffed if TfL unilaterally decided that Shenfield was zone 8, thus costing the TOC a whole pile of revenue. So it gets a higher, largely notional fare zone to reflect fares. It’s a mess. No wonder TfL doesn't tell us about them.

These “ghost zones”, as the South East Rail Group terms them, will actually be extending yet further. Zone 15 is reserved for some of the western-most Elizabeth line stations out to Reading, when that finally joins the system. Although whether the residents of zone 12 will one day follow in the venerable London tradition of looking down on the residents of zones 13-15 remains to be seen.

Jonn Elledge was the founding editor of CityMetric. He is on Twitter as @jonnelledge and on Facebook as JonnElledgeWrites.