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Recycling breakthrough paves the way for zero emissions cement at scale

Words:
Stephen Cousins

All-electric process uses steelmakers’ arc furnaces to reactivate cement recovered from demolition, and could deliver ‘a third of all UK cement within a decade’

Photographs of the first electric cement production in an electric arc furnace at the Materials Processing Institute in Teesside.
Photographs of the first electric cement production in an electric arc furnace at the Materials Processing Institute in Teesside. Credit: Materials Processing Institute

Researchers at the University of Cambridge have demonstrated a scalable low-carbon process for recycling waste Portland cement into new Portland cement using steelmakers’ electric arc furnaces.

The method, reported in the journal Nature, has been described by researchers as ‘an absolute miracle’. It involves substituting cement from demolition waste, for lime flux, which is used in arc furnaces to remove impurities from molten steel during recycling. Heating the cement in the furnace reactivates it, effectively returning it to its original state.

According to researchers, if arc furnaces are run on renewable electricity, zero-emissions cement is possible – and because the process exploits existing industrial equipment and standards it could scale rapidly.

Industrial trials are underway and if all goes to plan, Cambridge Electric Cement, the private company set up to commercialise the process, claims it could provide about a third of the UK's cement needs within 10 years, based on current usage.

‘The first trial has happened and there are more planned on a fairly regular schedule, things are looking promising,’ said Julian Allwood, professor of engineering and the environment at the University of Cambridge. ‘If trials go well we plan to use our cement to make demonstrator constructions of various forms in the latter part of this year. We've got the supply chain configured to do it in a project funded by Innovate UK.’

Photographs of the first electric cement production in an electric arc furnace at the Materials Processing Institute in Teesside.
Photographs of the first electric cement production in an electric arc furnace at the Materials Processing Institute in Teesside. Credit: Materials Processing Institute

The researchers’ method was designed to tackle both carbon emissions created during clinkering, when fossil fuels are burnt to power kilns used to produce cement, and the direct process emissions created by chemical reaction during that process.

Under the system, concrete demolition waste is first crushed and the cement fines extracted to create a paste that can be used in place of the lime-dolomite flux used in steel.

The cement combines with the impurities to create waste slag, which once removed and quickly cooled meets existing specifications for Portland cement. The reactivated cement contains higher levels of iron oxide than conventional cement, but researchers say this has little effect on performance.

The huge volumes of flux used in steel recycling – every 100 tonne batch of steel requires between 15 and 20 tonnes of flux – highlights the potential to scale up the process. At present most steel is recycled.

‘We're pretty confident the global steel recycling industry is going to double in size because it lasts for about 35 to 40 years in service and there was a massive boom in steel use in the ’90s,’ says Allwood. ‘That's good timing for us because we can try to make sure our approach is installed as part of new electric arc furnaces.’

A zero carbon cement will be possible, say researchers,  if emissions-free arc furnaces, powered by renewables or nuclear, are used and if certain other challenges can be overcome.

Photographs of the first electric cement production in an electric arc furnace at the Materials Processing Institute in Teesside.
Photographs of the first electric cement production in an electric arc furnace at the Materials Processing Institute in Teesside. Credit: Materials Processing Institute

One issue relates to the demolition waste, says Allwood: ‘Can we separate old cement from sand sufficiently? If we can't then we will have to blend our recovered cement paste with conventional lime flux. We're trying to avoid it but if we need to use a small amount of lime flux, it takes emissions to make that.’

Furthermore, anodes used to operate a steel recycling furnace are made from carbon and  account for “a small amount’ of emissions.

‘Saying zero carbon tends to provoke people into thinking of all the reasons why it might not be exactly zero, but to an order of magnitude, we're on our way to a zero emissions scenario,’ says Allwood.

Concrete is the second most used material on the planet, after water, responsible for around 7.5% of total anthropogenic CO₂ emissions and researchers underline that electric cement is only part of the solution to bringing down emissions.

‘Within the world of architecture, there's a switch that has not yet been flipped to say, "How do we deliver the client's requirements using much less material?" says Allwood. ‘What we're hoping to deliver is some cement into a world that requires much less cement. The skill of delivering with less material is an essential component in what we're offering with this new process,’ he concludes.

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