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

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

Concrete is made from sand, gravel, water and cement, which serves as a binding agent. Although a small part of concrete, cement is responsible for almost 90% of concrete emissions. Cement is made through a process called clinking, in which limestone and other raw materials are ground and heated in large kilns to about 1,450°C. This process converts the materials into cement, but releases large amounts of CO₂ when limestone decarbonates into lime.

Over the past decade, scientists have explored alternatives to cement and found that roughly half of the cement in concrete can be replaced with alternative materials, such as fly ash, but these alternatives must be chemically activated by the remaining cement to harden.

“It’s also a matter of volume – we don’t physically have enough of these alternatives to keep up with global demand for cement, which is roughly four billion tonnes per year,” Allwood said. “We have already identified the low-hanging fruit that will help us use less cement through careful mixing, but to get all the way to zero emissions we need to start thinking outside the box.”

“From previous work I had the vague idea that if it were possible to crush old concrete, the sand and stones would be taken out and the cement would be heated, the water would be removed and clinker would be formed again” , says first author Dr. Cyrille Dunant. also from the Engineering department. “A liquid metal bath would promote this chemical reaction, and an electric arc furnace, used to recycle steel, seemed like a good option. We had to try it.”

The clinker process requires heat and the right combination of oxides, all of which are found in used cement but need to be reactivated. The researchers tested a series of slags made from demolition waste and adding lime, aluminum oxide and silica. The slag was processed with molten steel in the Materials Processing Institute’s EAF and cooled rapidly.

“We found that the combination of cement clinker and iron oxide is an excellent slag for steelmaking because it foams and flows well,” Dunant said. “And if you find the right balance and the slag cools quickly enough, you get reactivated cement, without extra costs for the steelmaking process.”

The cement made through this recycling process contains higher levels of iron oxide than conventional cement, but the researchers say this has little effect on performance.

The Cambridge Electric Cement process has scaled up rapidly and researchers say they could produce one billion tonnes per year by 2050, representing around a quarter of current annual cement production.

“Producing cement with zero emissions is an absolute miracle, but we also need to reduce the amount of cement and concrete we use,” said Allwood. “Concrete is cheap, strong and can be made almost anywhere, but we simply use way too much of it. We could drastically reduce the amount of concrete we use without any reduction in safety, but there needs to be political will to make that happen.

“We hope that Cambridge Electric Cement will not only be a game changer for the construction industry, but also a flag to help the government recognize that the opportunities for innovation on our journey to zero emissions extend far beyond the energy sector.”

The researchers have applied for a patent on the process to support its commercialization. The research was partly supported by Innovate UK and the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI). Julian Allwood is a Fellow of St Catharine’s College, Cambridge.

Cyrille F Dunant, Shiju Joseph, Rohit Prajapati, Julian M Allwood. ‘Electrical recycling of Portland cement on a large scale.’ Nature (2024). DOI: 10.1038/s41586-024-07338-8

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