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25 May 2022

Cambridge academics unveil zero-emission cement

Academics from the University of Cambridge claim to have invented the world’s first ever process to produce zero-emission cement. Having secured a patent for their new approach, researchers are hopeful that the innovation will limit the need for green hydrogen to be used in the sector, allowing it to be prioritized in other ‘hard-to-abate’ areas.

The new process uses waste concrete from the demolition of old buildings. This concrete is crushed, allowing the stones and sand constituents to be separated from the mixture of cement powder and water that bind them together.

This recycled cement powder can then be used in the place of lime-flux in secondary steelmaking. With this used instead, a slag is formed on top of the liquid steel after it melts. Not only does this protect the steel from the oxygen in the adjacent air, but – once the steel is tapped off – the liquid slag can be cooled and ground into a powder which is almost identical to the clinker used as the primary ingredient in Portland cement.

In traditional steelmaking, the production of one ton of clinker requires nearly 1,000 kWh of energy and temperatures of up to 1,450 degrees Celsius. Made from the calcination of limestone, the chemical reaction to make clinker also releases around 63.5% of the CO2 from the cement making process.

As such, the amount of new clinker used is almost directly proportional to the CO2 emissions in cement making. In total around 0.7 tons of CO2 emissions can be attributed to each ton of cement made by today’s standards. With over 4.1 billion tons being produced each year, cement making accounts for around 8% of all global emissions.

The Cambridge academics – Dr Cyrille Dunant, Dr Pippa Horton and Professor Julian Allwood – were inspired for the innovation when noticing that the chemistry of used cement is virtually identical to that of the lime-flux used in conventional steel recycling processes. The new cement can therefore be made in a recycling loop that eliminates the emissions of cement production, saves raw materials, and reduces the emissions required in making lime-flux.

“Combining steel and cement recycling in a single process powered by renewable electricity, could secure the supply of the basic materials of construction to support the infrastructure of a zero emissions world and to enable economic development where it is most needed,” said Professor Julian Allwood – one of the three engineers responsible for the innovation.

Invention of the cement has been rewarded with a new research grant of £1.7 million from EPSRC, with further collaboration to come with Warwick University and Imperial College London. Research will now focus on the range of concrete wastes that can be processed into Cambridge Electric Cement, and how the process interacts with steel making on a system level.

At the elevated temperatures required, low carbon options to generate heat are limited; electrification of heat can only really be used for low temperature applications, and once you get to around 400 degrees Celsius both cost and technical barriers start to arise, as circuit components need to handle higher operating temperatures.

There are several approaches most often used to lower its carbon intensity: First is the option of using less clinker, or by mixing the clinker with other supplementary materials. Until now, however, it has not been possible to make the reactive component of cement without emissions, according to Cambridge. The new invention achieves this for the first time within the parameters of established industrial processes.

Other approaches to decarbonization include replacing the use of fossil fuels with hydrogen, using electrolysis to produce calcium oxide and carbon dioxide, which can be more easily captured, and using lower temperatures to create clinker of marginally lower quality.

Which route we end up using is really primarily down to China, which uses around 57.4% of all cement, and produces around 53.7%. The solution will be a complex series of partial outcomes, with areas with low cloud cover and high radiance trying out CSP and thermal storage for overnight 24/7 running of kilns, and in other areas, the use of hydrogen.

This will likely, however, be inspired by early pilot projects in Europe and work being done by the likes of Cambridge University. In pilot-scale trials of the new process, the Cambridge team have demonstrated the potential success of its combined recycling process, with encouraging results that it can replicate the chemical composition of clinker without the calcination of limestone.

Other initiatives include those from HeidelbergCement, which is already in operation in the UK – before hydrogen-ready facilities start being installed from 2024 onwards. By 2035, Rethink Energy has previously forecast that clean cement produced using hydrogen will account for 32.1% of global supply, rising to 78.2% in 2050 when the sector’s total hydrogen demand will be close to 91 million tons per year – resulting in an overall electricity demand from the sector rising from 574 TWh in 2020 to 5,923 TWh per year in 2050. China will account for nearly half of this.