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

Researchers from the University of Cambridge have developed a method to produce ultra-low-emission concrete in an innovation of scale that could be transformative in the transition to net zero.

The method, which researchers say is “an absolute miracle,” uses electric arc furnaces used to recycle steel to simultaneously recycle cement, the carbon-hungry ingredient in concrete.

Concrete is the second most used material on the planet, after water, and is responsible for about 7.5% of total anthropogenic CO2.₂ emissions. A scalable and cost-effective way to reduce concrete emissions while meeting global demand is one of the world’s greatest decarbonisation challenges.

Cambridge 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. But by replacing lime with used cement, the end product is recycled cement that can be used to make new concrete.

Cement recycling method developed by Cambridge researchers, reported in the journal Natureit does not add any significant cost to the production of concrete or steel and significantly reduces emissions from both concrete and steel due to the reduced need for lime flux.

Recent tests carried out by the Materials Processing Institute, a partner in the project, have shown that recycled cement can be produced at scale in an electric arc furnace (EAF), the first time this has been achieved. Eventually, this method could produce zero-emission cement if EAF were powered by renewable energy.

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

Concrete is made of sand, gravel, water and cement, which serves as a binder. Although it is a small part of concrete, cement is responsible for almost 90% of concrete emissions. Cement is produced through a process called clinkering, where limestone and other raw materials are crushed and heated to around 1450°C in large kilns. This process converts the materials into cement, but releases large amounts of CO₂ as lime decarbonate in lime.

Over the past decade, scientists have investigated cement substitutes and found that approximately 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 in order to harden.

“It’s also a matter of volume – we don’t physically have enough of these alternatives to keep up with global cement demand, which is roughly four billion [metric] tons per year,” Allwood said. “We’ve already identified the low-hanging fruit that helps us use less cement by carefully mixing and blending, but to get to zero emissions, we have to start thinking outside the box. “

“I had a vague idea from previous work that if it was possible to crush the old concrete, removing the sand and stones, heating the cement would drive off the water and then form clinker again,” said the first author. Dr. Cyrille Dunant. , also from the Engineering Department. “A liquid metal bath would help this chemical reaction, and an electric arc furnace, used to recycle steel, felt like a strong possibility. We had to try.”

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

“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 get the balance right and cool the slag fast enough, you end up with reactivated cement without adding any cost to the steelmaking process.”

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

Cambridge Electric Cement’s process has grown rapidly, and researchers say it could produce a billion tons a year by 2050, which represents roughly a quarter of current annual cement production.

“Producing zero-emission cement is an absolute miracle, but we also need to reduce the amount of cement and concrete we use,” Allwood said. “Concrete is cheap, strong and can be made almost anywhere, but we just use too much of it. We can dramatically reduce the amount of concrete we use without any reduction in safety, but there has to be the political will to do it this to happen.

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

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