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‘Miracle’ scalable recycling process for cement could be key in drive to net zero, say University of Cambridge researchers





University of Cambridge researchers have developed a method to produce very low-emission concrete at scale, which they describe as “an absolute miracle” that could prove key in the drive to net zero.

The Cambridge Electric Cement process uses electrically-powered arc furnaces used for steel recycling to simultaneously recycle cement, which is responsible for almost 90 per cent of the emissions from concrete.

Cement makes up a small proportion of concrete but is responsible for most of its carbon emissions
Cement makes up a small proportion of concrete but is responsible for most of its carbon emissions

Scaling rapidly, the method could be used to create one billion tonnes per year by 2050, which represents roughly a quarter of current annual cement production, the researchers say.

Accounting for about 7.5 per cent of total anthropogenic CO₂ emissions, concrete is the second most-used material on the planet, after water, so creating a scalable, cost-effective way of reducing emissions from it while meeting global demand is one of the world’s biggest decarbonisation challenges.

“We held a series of workshops with members of the construction industry on how we could reduce emissions from the sector,” said Prof Julian Allwood, from the University of Cambridge’s Department of Engineering, who led the research. “Lots of great ideas came out of those discussions, but one thing they couldn’t or wouldn’t consider was a world without cement.”

The researchers found that used cement is an effective substitute for lime flux, which is used in steel recycling to remove impurities and usually ends up as a waste product known as slag.

Replacing lime with used cement means the end product is recycled cement, which can be used to make new concrete.

The cement recycling method does not add significant costs to concrete or steel reduction, and significantly reduces emissions from both concrete and steel, due to the reduced need for lime flux.

The method, reported in the journal Nature, was tested by the Materials Processing Institute, a partner in the project, which showed that recycled cement can be produced at scale in an electric arc furnace (EAF).

Concrete is made from sand, gravel and water, with cement used as a binder. Cement is made through ‘clinkering’, a process in which limestone and other raw materials are crushed and heated to about 1,450°C in large kilns. This converts the materials into cement, but releases large amounts of CO₂ as limestone decarbonates into lime.

Scientists have been investigating substitutes for cement for the last decade and found that about half of it could be replaced with alternative materials, such as fly ash, but these would need to be chemically activated by the remaining cement in order to harden.

And there’s another problem.

“It’s also a question of volume – we don’t physically have enough of these alternatives to keep up with global cement demand, which is roughly four billion tonnes per year,” said Prof Allwood, a fellow of St Catharine’s College. “We’ve already identified the low-hanging fruit that helps us use less cement by careful mixing and blending, but to get all the way to zero emissions, we need to start thinking outside the box.”

First author Dr Cyrille Dunant, also from the Department of Engineering, said: “I had a vague idea from previous work that if it were possible to crush old concrete, taking out the sand and stones, heating the cement would remove the water, and then it would form clinker again.

“A bath of liquid metal would help this chemical reaction along, and an electric arc furnace, used to recycle steel, felt like a strong possibility. We had to try.”

Clinkering needs heat and the right combination of oxides, all of which are in used cement, but they need to be reactivated. The researchers tested a range of slags, made from demolition waste and added lime, alumina and silica, which were processed in the Materials Processing Institute’s EAF with molten steel and rapidly cooled.

“We found the combination of cement clinker and iron oxide is an excellent steelmaking slag because it foams and it flows well,” said Dr Dunant. “And if you get the balance right and cool the slag quickly enough, you end up with reactivated cement, without adding any cost to the steelmaking process.”

The cement recycling method could help solve one of the world’s biggest climate challenges
The cement recycling method could help solve one of the world’s biggest climate challenges

The cement made through this method contains higher levels of iron oxide than conventional cement, but that has little effect on performance.

“Producing zero emissions cement is an absolute miracle, but we’ve also got to reduce the amount of cement and concrete we use,” said Prof Allwood.

“Concrete is cheap, strong and can be made almost anywhere, but we just use far too much of it. We could dramatically reduce the amount of concrete we use without any reduction in safety, but there needs to be political will to make that happen.

“As well as being a breakthrough for the construction industry, we hope that Cambridge Electric Cement will also be a flag to help the government recognise that the opportunities for innovation on our journey to zero emissions extend far beyond the energy sector.”

The researchers have filed a patent on the process to support its commercialisation.

The research was supported in part by Innovate UK and the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI).



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