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Low-carbon cement project reaches milestone in efforts to lower construction industry CO₂ emissions

A project to test calcined clay as an ingredient in low-carbon concrete has proven its sustainable potential to reduce the cement sector's environmental impact from its current level of 8% of global CO₂ emissions.

Modern buildings - Low carbon cement illustration

With innovation in low carbon concrete a key aim for Imerys, alongside seeking solutions to reduce emissions, we have been working since January 2022 as part of an innovative UK-based initiative to produce low-carbon concrete, which is now showing promise to support global sustainability goals. 

The cement industry accounts for 8% of global CO2 emissions, mainly due to the production of clinker as a main constituent of Portland cement. This can emit around 850kg of CO2 per ton due to the decarbonization of the limestone and energy usage throughout the process. 

Decarbonising the construction sector 

Low carbon cement can be used to produce low carbon concrete. The Re-C3 project has shown the potential of using calcined clay from reclaimed resources as an ingredient in concrete and mortars – replacing part of the Portland cement. If developed and used widely, this would improve the carbon footprint of the construction industry. 

What are calcined clays? These are materials made from low-grade clays produced through excavation. For example, the production of high-purity kaolins creates a waste by-product that cannot currently be valorized in other applications. 

Re-C3 identified 11 clays from different UK locations, including five from our sites, for testing. Different compositions were created by mixing Portland clinker with calcined clay, and also with calcined clay and limestone, to see how they work together. As a partner in the project, we were in charge of preparing the calcined clays in the lab and characterizing them, before selecting the samples to be used in larger-scale tests.

We're the only company in the UK with the equipment to produce this material, and our lines produce high purity metakaolin materials.

Tony Newton
Building Solutions Sales Director

Using calcined clay in low-carbon concrete innovation 

More than two-dozen L-shaped retaining wall units have been produced. These demonstrate that concretes containing calcined clays – and as little as 55% Portland cement clinker – at least match the performance of those containing more common ingredients, fly ash and blast-furnace slag. 

It is a careful balancing act. Supplementary cementitious materials (SCMs) are commonly used to reduce the clinker factor of cements, but using too much can reduce the mechanical properties. 

The manufacturing of the L shapes has gone exceptionally well, says Imerys' scientist Barbara Benevenuti, who carried out tests in Imerys' laboratories. 

"Additional tests by University of Dundee specifically looked at durability, the resistance to chloride and carbonization," says Barbara. "The usage of calcined clays can even improve some properties of the cement and concrete. The durability results show that the partial replacement of clinker by calcined clays allows us to obtain not only similar strength performances, but also much higher chloride resistance. 

"Also, the availability of fly ash and blast-furnace slag is insufficient worldwide to support long-term low-carbon cement production. Emissions aside, this is another reason the industry requires a solution." 

Tony agrees: "Alternative sources of SCMs, such as calcined clays, are of great interest. Kaolinitic clays are widely available in the earth's crust, all around the planet."

Industrial evidence for a sustainable future 

This project could be a game-changer for the industry, explains Tony. "This project demonstrates the science of clinker replacement in an industrial setting – not just in the laboratory. 

"When material is excavated to produce high-purity metakaolins, only 10 to 20% of material is used, and the rest has to find another home – it either goes to landfill or is sold at a cheap price, so being able to approve high volumes of waste material for use will be of great value to industry and to Imerys. 

"For the market, it meets concrete producers' commercial and environmental ambitions for cement by reducing CO2 embedded in their products and services."

Reclaimed clays in the coming years will become a standard offer for decarbonization of cements and concrete. Cement and concrete manufacturers are using the outputs of the Re-C3 project to ensure calcined clays are included in the standards for permitted use in concrete. The project has had a high profile in promoting awareness of this technology and Imerys as a partner for promoting sustainable solutions, such as reclaimed clays or calcined clays and other secondary cementitious materials.

We are part of the Re-C3 partnership with the Mineral Products Association, Forterra, Tarmac (CRH group), Heidelberg Materials, University College London and University of Dundee. It is part funded by Innovate UK, a government-backed funding body to support collaborative research and development projects.

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