Cambridge Electric Cement

A Cambridge university spin-out has developed a way to produce new, potentially emissions-free cement from recycled concrete

A process that can produce cement from recycled concrete is to be trialled by a Cambridge firm this summer. If successful the technology, from Cambridge Electric Cement (CEC), could prove revolutionary – enabling waste concrete from demolition to be transformed into new, potentially zero-carbon cement.

“The process involves replacing the lime flux used in steel recycling with recovered cement paste (RCP),” explains CEC’s senior project manager Patricio Burdiles. “Most steel is recycled in electric arc furnaces (EAFs), but the lime traditionally used to remove impurities from the scrap steel results in a slag that is non cementitious and of little value.” But, if RCP is used instead of lime, the cement is reactivated by the heat in the EAF. “So the slag that comes out is very similar to the clinker for Portland cement (PC). Add gypsum and you have PC.”

CEC’s product is distinct from ground granulated blast-furnace slag (GGBS), which results from slag processed in coke-fired steel blast furnaces. “We are not producing a cement supplement or substitute,” stresses Burdiles. “We are taking deactivated cement and reactivating it to produce actual cement.”

The CEC team is led by Dr Cyrille Dunant of the University of Cambridge. He says that the idea arose during Covid, when the Use Less Research group led by Professor Julian Allwood was kicking around ideas for how we might build in a zero-emissions world. “At some point, out of slight frustration, I said ‘we’ve known how to recycle cement since we invented it. It’s not difficult – if you reheat cement, it reclinkers. In fact, I bet you could even recycle it electrically in an EAF because the chemistry is about right.’ Julian said, ‘That’s perfect – let’s try it!’”

Until recently, concrete recycling has involved simply crushing old concrete and using it as rubble in lower value applications such as road-base or backfill. Now, though, techniques are being developed to “unmake” concrete, restoring it to its constituent parts of sand, aggregate and deactivated cement powder (see CQ Spring 2023).

“Only 18 months ago, there was no interest in obtaining cement paste from recycled concrete, as it was considered waste,” says Burdiles. “But now we are aware of a number of companies developing technology to produce high-quality aggregates and sand, with high-quality RCP as a by-product. As more becomes available, techniques like ours are set to make a big difference to the carbon footprint of cement and concrete.”

RCP can already be reactivated in a cement kiln, but this leaves sulphur impurities that can spoil the quality of the cement, so it can only be used in small quantities. This method also requires considerable amounts of energy from fuel to heat the kiln. “With our process, the sulphur problem is avoided and the heat has to be generated anyway as part of the steel recycling process,” he says. “Furthermore, because steel is recycled in EAFs, the electricity to power these could potentially come from green, renewable sources. If that’s the case, then our reactivated PC is carbon-neutral since there are no process emissions, none from the burning of fuel and, potentially, not even any from the running of the EAF, beyond those already involved in recycling steel.”

CEC’s technology has produced PC with very similar properties to that made in a normal cement kiln: “The strength, and strength gain, are much the same – as we would expect from a material whose chemical composition is essentially the same as PC.” So far, it has only been produced in a laboratory, however. The team is now in the middle of a two-year industrial trial, the Cement2Zero project. This will first involve a small test EAF, able to produce about seven tonnes of steel. “This is the exciting part,” says Burdiles. “If that’s successful, we will scale up the testing to a commercial-sized EAF.”

Cambridge Electric Cement is a collaboration between the Universities of Cambridge, Warwick and Imperial College London and is funded by EPSRC. Cement2Zero is a collaboration between the Materials Processing Institute, the University of Cambridge, Atkins, Balfour Beatty, Celsa, Day Group and Tarmac, and is supported by Innovate UK funding.

Interview by Tony Whitehead
Photos Paul Burroughs
Published in CQ Summer 2023