15 July 2022
by Andrea Gaini

Recycled steel slag meets cement clinker

By replacing lime-flux with used cement paste for steel recycling, the resulting slag could, in turn, create a zero-emissions cement in a closed loop process, propose scientists at the University of Cambridge, UK.

Celsa’s site in Cardiff, Wales, where researchers hope to trial the closed loop process
Celsa’s site in Cardiff, Wales, where researchers hope to trial the closed loop process © Philippa Horton

They have found that the slag has properties of Portland clinker and can be used as new cement.

‘If the electric arc furnace [for steel recycling] is powered by non-emitting electricity, the only significant emissions in today’s steel recycling come from making the lime flux. So, with our process, it looks like we can make zero-emissions steel and zero-emissions cement,’ says Professor Julian Allwood HonFIMMM from the University.

The cement is made in a recycling loop that could eliminate the emissions of cement production, save raw materials and reduce the emissions required in making lime-flux.

The process begins with concrete waste from demolished buildings. This is crushed to separate the stones and sand from the mixture of cement powder and water that bind them together.

The cement powder is then used instead of lime-flux in steel recycling. As the steel melts, the flux forms a slag that floats on the liquid steel to protect it from oxygen in the air.

After the recycled steel is tapped off, the liquid slag is cooled rapidly in air, and ground up into a powder that is virtually identical to the clinker that is the basis of new Portland cement.

The researchers are now testing its durability, but early trials suggest that, in the right conditions, the cement can be chemically identical to Portland clinker.

Allwood explains, ‘Prior to this invention, although there are lots of ideas around about how to reduce the emissions of cement-making, there were no processes offering zero emissions. The possibility of attaching a cement kiln to a carbon capture and storage process has been discussed for many years, but not yet been realised in practice.

‘We have therefore been running workshops for the construction industry anticipating what construction will look like in 28 years with zero cement – as that is the implication of the UK’s Climate Change Act. While doing this, Dr Cyrille Dunant, [one of the researchers involved in the project], had an inspiration about the chemical similarity of today’s lime-flux and used cement paste.’

Allwood explains they are forming a spin-out company that will hold the license to the patent and are working with advisors to explore various business models.

The research has been awarded a grant by the Engineering and Physical Sciences Research Council, to work along with Dr Rupert Myers at Imperial College London, UK, and Dr Zushu Li at Warwick University, UK, to investigate the science behind the process.

Allwood ends, ‘We need to find out how different sources of used cement paste affect our product, and to explore how the new flux interacts with existing steel recycling operations. We are also planning a large-scale demonstrator project to confirm that the process works at scale. Our immediate priority is to recruit some excellent post-doctoral scientists to join our team.’


Andrea Gaini