Cleaning up concrete

The manufacture of this ubiquitous material takes a heavy toll on the environment. Dave Waller looks at the construction industry’s efforts to minimise its impact.

Photo / EyeEm

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Since concrete began taking over the built landscape in the early twentieth century, its bridges, motorways and concert halls have brought billions of people together the world over. But the material has always divided people too.

These days, there’s an extra dimension to the debate. As world leaders convened in Glasgow in November for COP26, the focus was on the global net zero transition required to meet ever tightening climate targets. And concrete was getting some very bad press.

Under the Paris Agreement, governments agreed that annual carbon emissions from the cement industry have to fall by at least 16% by 2030 for the world to stay within the target 1.5°C to 2°C of warming.

Why? If the cement industry were a country, it would be the third-largest carbon dioxide emitter in the world, behind China and the US. In 2020, a UN Environment Programme report stated that concrete accounts for 9% of global carbon emissions, emitting up to 2.8 billion tons of carbon dioxide every year.

The most used man-made material

But, it seems, concrete isn’t actually an especially polluting material. The issue is that it’s currently the most consumed man-made material in the world – with more than four billion tons produced every year.

“Over 50% of all products in the world are made of reinforced concrete,” explains Phil Purnell, professor of materials and structures in the School of Civil Engineering, University of Leeds. “Suddenly, with only 8% of CO2 emissions, concrete is actually doing quite well.

“That’s the story that doesn’t get told so much: you emit roughly 100 kilogrammes of CO2 to make a ton of concrete. You emit probably 10 times that for a ton of steel. And probably the same amount for a ton of wood. So this is really a reflection of the huge volumes of these materials that we actually use.”

And those huge volumes aren’t decreasing. Chatham House predicts that global cement production is set to soar from four billion to five billion tons a year. China has used more concrete every two years since 2003 than the US did throughout the whole 20th century.

It’s easy to see why concrete is so popular. It is durable, affordable, and highly adaptable. Architectural historian Barnabas Calder described it as “a superpower for architects”. Calder’s first book, 2016’s Raw Concrete, the Beauty of Brutalism, was a celebration of the material. But even he has always warned against its over-use. “The reason to celebrate existing modernist concrete buildings is because demolishing them is a crazy waste of their embodied carbon,” he says. “That’s going to be up there for another 1,000 or 2,000 years, and we’re demolishing buildings after 50.”

The bulk of concrete’s carbon footprint comes from cement, specifically the chemical reaction that produces clinker, the binding component, which has to be heated to around 1,500°C. There’s also the deep environmental degradation caused by mining the sand from riverbeds, and the steel used for reinforcement. But the truth is there isn’t a simple alternative.

“If anyone’s claiming they’ve solved the concrete problem with a silver bullet, you should be very suspicious,” says Rupert Myers, senior lecturer in sustainable materials engineering at Imperial College London. “It’s a good way to get venture capitalist money, but it’s BS. The reality is one of systemic complexity. There needs to be a systemic transition.”

Cutting concrete’s carbon footprint

Researchers are now exploring ways to reduce carbon along the concrete value chain. As well as trying to improve the energy efficiency of cement plants, it is possible to produce cement without clinker. One option is to use coal fly ash or blast furnace slag instead. Australian research group Zeobond created an alternative binder using geopolymers.

We can also ensure concrete buildings are better maintained and conserved. And we can also cut parts of concrete buildings for reuse, thanks to developments in diamond-wire saws.

“There’s lots of investment looking for fancy cement, because it would allow the industry to continue producing billions of tons of material a year,” says Purnell. “In fact, we have to optimise the shape of our structures, and to look at ways of reusing beams and columns. But that’s a completely different business model.”

A long way still to go

Despite the decarbonisation strategies that are beginning to emerge, construction is not on track to reduce its climate change impact in line with the Paris Agreement. Figures from the Buildings Performance Institute Europe’s Building Climate Tracker suggest that, in order to hit net-zero by 2050, the annual decarbonisation of buildings needs to increase by a factor of five.

Whatever the debates around concrete’s aesthetic and practical value, it’s time to face up to the costs and consider alternatives – which may mean making some very difficult changes. Calder, for example, stresses the need for regulation and taxation to influence building habits.

“The less we build, the better for sustainability,” he says. “Our building industry is built around these superb materials. But if we keep going the way we are, they are going to wipe us out.”

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“The less we build, the better for sustainability. Our building industry is built around these superb materials. But if we keep going the way we are, they are going to wipe us out.”