By Stephen Beech via SWNS
An innovative way of recycling cement has been developed by British scientists.
They say it could help solve one of the world’s biggest climate challenges.
Cambridge University researchers came up with a method to produce very low-emission concrete at scale – an innovation they say could be “transformative” in the drive to achieve net zero.
The method – described by the team as “an absolute miracle” – uses the electrically-powered arc furnaces used for steel recycling to simultaneously recycle cement, the carbon-hungry component of concrete.
Concrete is the second-most-used material on the planet, after water, and is responsible for around 7.5 percent of total man-made carbon dioxide.
Finding a scalable, cost-effective way of reducing concrete emissions while meeting global demand is one of the world’s biggest decarbonization challenges.
The Cambridge team found that cement is an effective substitute for lime flux, which is used in steel recycling to remove impurities and normally 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.
The cement recycling method developed by the Cambridge team, described in the journal Nature, doesn’t add any significant costs to concrete or steel production and “significantly” reduces emissions from both concrete and steel, due to the reduced need for lime flux.
Tests conducted by the Materials Processing Institute showed that recycled cement can be produced at scale in an electric arc furnace (EAF), the first time that has been achieved.
The method could eventually produce zero-emission cement if the EAF was powered by renewable energy, say the researchers.
Study leader Professor Julian Allwood said: “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.”
He explained that concrete is made from sand, gravel, water, plus cement, which serves as a binder.
Although it’s a small proportion of concrete, cement is responsible for almost 90 percent of concrete emissions.
Cement is made by a process know as “clinkering” – where limestone and other raw materials are crushed and heated to about 1,450°C in large kilns.
The process converts the materials into cement but releases large amounts of CO2 as limestone decarbonates into lime.
Scientists working on substitutes for cement have found that roughly half of the cement in concrete can be replaced with alternative materials, such as fly ash, but the alternatives need to be chemically activated by the remaining cement in order to harden.
Allwood, of Cambridge’s Department of Engineering, said: “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.
“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 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.”
He explained that the clinkering process requires heat and the right combination of oxides, all of which are in used cement, but need to be reactivated.
The research team tested a variety of slags, made from demolition waste and added lime, alumina and silica.
The slags were processed in the Materials Processing Institute’s EAF with molten steel and rapidly cooled.
Dr. Dunant said: “We found the combination of cement clinker and iron oxide is an excellent steelmaking slag because it foams and it 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.”
The cement made through the recycling process contains higher levels of iron oxide than conventional cement, but the research team says that has little effect on performance.
The Cambridge Electric Cement process has been scaling rapidly, and the researchers say it could be producing one billion tonnes per year by 2050, which represents around 25per cent of current annual cement production.
Allwood said: “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.”
He added: “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 recognize that the opportunities for innovation on our journey to zero emissions extend far beyond the energy sector.”
The research team has filed a patent on the process to support its commercialization.
