EUREKA Project

A joint research project is identifying the most promising candidates, and how best to use them.

Supplementary cementitious materials (SCMs) such as fly ash and GGBS are now widely used to decrease the amounts of cement used in concrete and so significantly reduce its carbon content. GGBS in particular is frequently used on major projects, usually replacing between 20-50% of cement, but as much as 95% for specialist applications.

Their time is limited, however, as Professor Leon Black of Leeds University explains: “Fly ash is a by-product of coal-fired electricity generation, and GGBS is from steel manufacture. With the decarbonisation of electricity, and UK steel manufacture shifting to electric steel recycling furnaces, these materials will become increasingly scarce here, so we are going to need other options.”

Working with Professor Hong Wong at Imperial College London, Clive Mitchell at the British Geological Survey (BGS) and others, Black is part of a team researching the extent to which UK clays can be processed and used as alternative SCMs. Their work is funded by the EPSRC, and known as the Eureka Project.

“Engineers have long used metakaolin clay as an SCM. It produces high-end, chloride-resistant concrete,” says Black. “But it mainly comes from china clay, which is expensive and has other uses. So we want to establish what role lower-grade clays might have.”

These naturally vary in composition, so a team led by BGS is helping to map the distribution of clays that might be suitable. “This is vital knowledge,” says Black. “A lot of clay is excavated – to build roads or railways, for example. But if we know its composition, it may be usable as an SCM rather than simply landscaping.”

BGS, with help from Leeds and Imperial, has already analysed some 60 clays from around the UK and is focusing on a dozen with promising characteristics. “Clay needs to be heated to around 800°C to become calcined and reactive enough to work as an SCM,” says Black.

“There is a carbon cost to this, but the temperatures are much lower than the 1,450°C needed to make cement from limestone. And, unlike limestone, there are hardly any direct carbon emissions from heating clay. You end up with an SCM with a carbon footprint roughly half that of Portland cement.”

If heating clay makes it reactive, why not simply use waste bricks or tiles that have already been fired? “It’s sometimes possible, but if the material is from demolition, the composition of the clays may not be consistent enough to be reliable. In addition, not all clay products will contain sufficient kaolin, and the firing process is not optimised for SCMs.”

As well as identifying where in the UK suitable clays can be found, Eureka is also fine-tuning how best to use them. “After heating, the clay is milled to a fine powder,” says Black. “It can cause flow issues in concrete so we are working with additives suppliers to understand how to overcome that. We expect that calcined clay SCMs can comfortably replace 20-30% of cement, but we are looking at how we might be able to increase that figure while maximising concrete performance – perhaps by blending with materials such as limestone.”

“Characteristics vary, and of course certain sources are much more useful,” adds postdoctoral researcher Yuvaraj Dhandapani. “But we’re trying to tackle all the resources – we’re looking at how to process the least promising sources too, in the most efficient way, to increase their usability.”

Black stresses that Eureka is about more than proving that calcined clay works as an SCM: “It’s no use this research staying in the laboratory,” he says. “We need to make it usable, and ensure newly developed materials satisfy industry requirements and are adopted as widely as possible to maximise carbon reductions in our built environment. That’s why we are working with industry partners right along the supply chain, from those who excavate clay, to cement and concrete suppliers, precast manufacturers and engineers and builders who will use the concrete in their projects.”

Laboratory analysis of concrete containing calcined clays suggests that they perform well in terms of setting times, strength and durability. “But we need practical testing to check the clays we’re looking at will work well in the real world. By the end of this year, we hope to have our first few tonnes of calcined clay ready for testing by industry.

Interview by Tony Whitehead

^{Published in CQ Summer 2024}