Green concrete: ‘Miracle’ breakthrough in cement recycling could slash emissions

Process could revolutionise production of concrete, the world's most used material and huge contributor to climate change

A 700 tonne ‘bridge-building machine’ moves one of 1,000 huge concrete deck segments, each the size of a double-decker bus, for the viaduct for the HS2 Project. Getty Images

A method which could revolutionise the production of concrete and slash emissions in the global building industry has been developed.

Concrete is the world’s most-used material, with 500kg produced for every person on the planet, a year.

But it is also hugely energy intensive, and responsible for about 7.5 per cent of emissions.

The process was discovered by the University of Cambridge and used electrically-powered arc furnaces for steel recycling to simultaneously recycle cement, the carbon-hungry component of concrete, which is responsible for almost 90 per cent of its emissions.

The researchers behind the discovery describe it as “an absolute miracle”.

They proved how used cement can serve as a replacement for a material called lime flux during the steel recycling process. Lime flux, which removes impurities in the steel, would normally end up as a waste product. But in replacing it with used cement, the cement is also recycled and can be used to make new concrete.

Concrete is a mixture of four things – cement, sand, stones and water.

Professor Julian Allwood from Cambridge’s Department of Engineering, who led the research, told The National: “Cement is the active ingredient and the one that’s associated with all the emissions.”

That is because it is made through a process called clinkering, where limestone and other raw materials are crushed and heated to about 1,450°C in large kilns.

“That’s such a high temperature that you essentially have to reach it by burning fossil fuels. So combustion emissions is about half of the emissions,” said Prof Allwood.

“But the other is the chemistry of converting limestone into cement automatically releases carbon dioxide into the atmosphere. That is the other half of the emissions.

“So the cement has already given up its Co2. There is no new chemical reaction required. In effect what we are doing is reactivating the cement.”

He said his team were motivated to seek a solution to the problem because although there are some options for making cement with less emissions, there are none for making the material with zero emissions.

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“And yet the UK, and many other countries, rightly, are committed to zero emissions in the relatively near future.

“Without it means no cement and building with other very limited options,” he added.

Tests carried out by the Materials Processing Institute, a partner in the project, showed that recycled cement can be produced at scale using an electric arc furnace.

But the researchers say it could produce zero emission cement, if the electric arc furnace is powered by renewable energy.

“The important thing is that for the first time there is a route to getting to zero emissions cement. Our estimate of the maximum volume we could make is based on the fact that we know the world’s steel recycling industry will double in size, at least, in the next 30 years,” said Prof Allwood.

“Because the supply of scrap steel has gone up. If we assume that all of them use our process, we could maybe get up to a quarter of today’s supply of cement.”

You could look at that in two ways, he said.

“One is to say oh where is the other three quarters going to come from? But the better way to look at it is all the rest of it is going to have to stop because of our commitment to getting to zero emissions. At least we now have an option for having some cement,” added Prof Allwood.

Earlier this month, it was revealed that researchers aim to harness heat generated by the sun to manufacture building materials such as glass, steel, ceramics and cement, showing that generating heat of 1,000°C heat is possible without using fossil fuels.

They have found a way to improve the efficiency of solar receivers, which concentrate and build heat with thousands of sun-tracking mirrors.

Researchers led by a team in Switzerland used quartz to trap sunlight, in a process known as the thermal-trap effect, proving a receiver with a normal efficiency of 40 per cent at 1,200°C that uses a concentration of light from 500 suns can hit 70 per cent efficiency when shielded with 300mm of quartz. To obtain that performance would normally require the light from 1,000 suns.