Researchers from the University of Cambridge, UK, developed a way to produce concrete that is more environmentally friendly, according to a study published in the journal Nature. The authors believe this low-emission process, which can be done at a large scale, could be a valuable tool in our transition to net zero.
Concrete is the planet’s second-most-used material, accounting for about 7.5% of total man-made CO₂ emissions. Over 90% of these emissions come from cement. Cement is made through a process called clinkering, with limestone and other raw materials crushed and heated to about 1,450°C in large kilns. This process is efficient but releases large amounts of CO₂ as limestone decarbonates into lime. For this reason, a scalable, cost-effective way of reducing cement and concrete emissions that cover global demand is one of the world’s biggest decarbonisation challenges.
For a long time, scientists have been trying to develop ways to replace cement. Results show that roughly half of the cement in concrete can be replaced with alternative materials, such as fly ash, but the process still needs cement to ensure that all components harden.
“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 Allwood. “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.”
“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,” said first author Dr. Cyrille Dunant, also from the Department of Engineering. “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.”
The team found that used cement can be used as lime flux, often used in the steel industry to remove impurities, and usually ends up as a waste product known as slag. However, by replacing lime with used cement, the end product is now recycled cement, which can be used to make new concrete. Crucially, this new recycling method doesn’t add any major costs to concrete or steel production and significantly reduces emissions from both concrete and steel due to the reduced need for lime flux. Tests showed that this recycled cement can be produced on a large scale in an electric arc furnace (EAF). The aim is to produce zero-emission cement when an EAF is powered by renewable energy.
“We found the combination of cement clinker and iron oxide is an excellent steelmaking slag because it foams and it flows well,” said 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 team has been trying to scale up production and suggest that we could produce one billion tonnes per year by 2050, representing roughly a quarter of current annual cement production.
“Producing zero emissions cement is an absolute miracle, but we’ve also got to reduce the amount of cement and concrete we use. 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,” concluded Allwood. “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.”
Dunant, C.F., Joseph, S., Prajapati, R. et al. Electric recycling of Portland cement at scale.Nature 629, 1055–1061 (2024). https://doi.org/10.1038/s41586-024-07338-8