A concrete cement mill in operation
What is the most destructive material known to humans? Plutonium? Cyanide? Whatever the hell they put in Marmite? No, it would have to be concrete. The Guardian recently ran a series on the environmental impact of concrete which is worth a read.
Much of the focus on climate change mitigation, or pollution in general, tends to focus on energy production. However, in truth this is merely one of several sources of carbon emissions. Agriculture and land use changes tends to be the next biggest headline at about a quarter of emissions (which is actually arguably larger than it looks given the amounts of fossil fuels used in agriculture both by farm machinery and the production of fertilisers).
After that its the acquisition of raw materials (mining, refining and processing of base metals and minerals). And concrete, as one of the mostly widely used materials in the world, tends to figure quite highly in this category. And at almost every step in its life cycle concrete has an environmental impact.
As I discussed in a prior post, the world is running out of sand for concrete production. Hence, there’s now a whole series of “sand Mafia’s” emerging in the developing world to steal sand, so the issues with concrete goes way beyond just climate change. Then you have to transport all these ingredients long distances, which consumes a lot of energy (cos they are kind of heavy!).
Then there’s the process of making concrete, which consumes rather a lot of energy. Making just the cement component consumes 1,700 KJ/kg and, due to chemical reactions, releases releases 750 kg’s of carbon dioxide for every tonne of concrete produced. And given its weight, simply pouring concrete entails yet more energy consumption and carbon emissions. Furthermore, poured concrete undergoes an exothermic reaction, which may require external cooling as it hardens.
And, at the end of the building’s life, when its demolished, you’ve got numerous environmental problems. Notably the disposal of masses of concrete rubble (at one point back during the boom in Ireland they did a survey and found that 4/5’s of all the material entering Irish landfills was builders rubble).
Of course, as an engineer I’d have to point out that there are good reasons why we use concrete. Its cheap, it can be moulded into complex shapes, its durable, easy to maintain and fire proof. Basically you can do your worst to a concrete building and it will still stay standing. Hell, there was even a concrete building close to ground zero at Hiroshima that took the full force of a nuclear blast and survived. And keep in mind, we’ve been using concrete since ancient times. So we need to move beyond the simple “concrete bad” narrative, same way plastics is a bit more of a complex issue than it seems at first glance.
The glory of the Roman Pantheon, concrete at its best
While concrete can be recycled, its more a form of downcycling. That is too say, you’ll get a lower quality of concrete afterwards, so you can use it for say roads or backfill, but not build a new skyscraper from the stuff. Another alternative is to change the composition of the concrete, using other materials such as fly ash, shredded rubber, waste glass, etc. into the mix. The downside is that this is again downcycling, not recycling and its generally not going to have the same structural properties.
The concrete recycling process, which isn’t 100% effective
Hence why other more radial measures are being proposed, for example a concrete tax. I’d point out that perhaps the problem here is the short life cycle of many modern buildings. I’ve seen concrete buildings that are maybe only 20 years old getting demolished. Sticking a carbon tax on, with the condition that some significant portion is refunded if the building stays in use for some extended period (e.g. at least a hundred years), or that its design life allows it to last that long, would create an incentive to only use concrete where necessary and make sure those buildings are built to last (as well as a financial incentive to refurbish rather than demolish).
There’s also alternatives to concrete. Wood as a construction material is something I’ve previously discussed. And while there are structural limits and issues with fire safety that need to be addressed (as well as where you source the wood from of course), these aren’t insurmountable. And there’s also the option of steel framed buildings. Now while yes steel, like most metals, is very energy intensive to manufacture, it has one unique advantage over concrete (or wood for that matter) – it can be recycled with 100% material efficiency (i.e. virtually no waste). So encouraging steel framed construction would offer several advantages.
Wooden skyscrapers have recently been proposed
But as so often is the case with climate change we are confronted with a problem whose dimensions aren’t immediately apparent. And where there is no nice and neat one size fits all solution, just lots of hard choices.