Recycled cement is "an absolute miracle" for lowering concrete emissions

In a recent development, researchers have unveiled a transformative method to produce low-emission concrete on a large scale using recycled cement.

This innovation promises to be a significant step towards achieving net zero emissions, addressing one of the world’s most pressing environmental challenges.

Chemistry of concrete and cement

Concrete is made from sand, gravel, water, and cement, which acts as a binder. Although cement comprises only a small proportion of concrete, it accounts for nearly 90% of its emissions.

Cement production involves a process called clinkering, where limestone and other raw materials are heated to about 1,450°C, releasing large amounts of carbon dioxide.

Over the past decade, scientists have explored various substitutes for cement. While alternative materials like fly ash can replace about half of the cement in concrete, these substitutes need to be chemically activated by the remaining cement to harden.

New approach to concrete production

Concrete, second only to water in terms of global usage, is responsible for approximately 7.5% of all anthropogenic CO₂ emissions. Finding a scalable, cost-effective way to reduce these emissions while meeting the high global demand for concrete is crucial.

Researchers from the University of Cambridge have pioneered a new method, which leverages the electrically-powered arc furnaces used for steel recycling.

This process not only recycles steel but also recycles cement, the key component of concrete known for its high carbon emissions. Described as “an absolute miracle” by the researchers, this method could revolutionize the construction industry.

Concrete without cement

Recent tests conducted by the Materials Processing Institute, a project partner, demonstrated that recycled cement could be produced at scale in an electric arc furnace (EAF) for the first time.

The successful experiments suggest that the method could eventually lead to zero-emission cement if the EAF is powered by renewable energy.

Professor Julian Allwood from Cambridge’s Department of Engineering, who led the research, highlighted the industry’s reluctance to envision a world without cement.

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

Overcoming material limitations

Professor Allwood noted the challenge of volume in finding alternatives to meet global demand, which is around four billion tons per year.

“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,” he explained.

“If it were possible to crush old concrete, remove the sand and stones, and heat the cement, it would remove the water and form clinker again. A bath of liquid metal would help this chemical reaction, and an electric arc furnace, used to recycle steel, felt like a strong possibility. We had to try,” said Dr. Cyrille Dunant, the study’s first author.

Concrete clinkering for reducing concrete emissions

The clinkering process requires heat and the right combination of oxides, all present in used cement but needing reactivation.

The researchers tested a range of slags from demolition waste, 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 flows well. 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,” explained Dr. Dunant.

The recycled cement contains higher levels of iron oxide than conventional cement, but this has little impact on performance.

The Cambridge Electric Cement process is scaling rapidly, with the potential to produce one billion tons per year by 2050, representing about a quarter of current annual cement production.

Turning cement waste into a concrete resource

The Cambridge team discovered that used cement can effectively replace lime flux, a material used in steel recycling to remove impurities.

Traditionally, lime flux ends up as waste known as slag. By substituting lime with used cement, the end product is recycled cement that can be used to create new concrete.

This innovative recycling method does not add significant costs to the production of concrete or steel. Moreover, it substantially reduces emissions from both industries by decreasing the need for lime flux.

Vast opportunities for innovation

While producing zero-emissions cement is a significant achievement, Professor Allwood emphasized the need to reduce the overall use of cement and concrete.

“Concrete is cheap, strong, and can be made almost anywhere, but we 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,” said Professor Allwood.

The researchers hope that Cambridge Electric Cement will inspire the government to recognize the vast opportunities for innovation on the journey to zero emissions, extending far beyond the energy sector.

The team has filed a patent on the process to support its commercialization. This research was supported by Innovate UK and the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI).

The study is published in the journal Nature.

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