Why calcined clay could be a crucial ingredient in decarbonising UK construction

The construction industry could help the UK to make great strides towards its net zero goals by adopting calcined clay extensively as a low-carbon cementitious material.

That was the view of two leading materials experts who spoke at the autumn meeting of the ICE’s Decarbonisation Community Forum.

Dr Pete Winslow, director at Expedition Engineering, and Jon Knights, materials and durability lead at High Speed 2, stressed that calcined clay could be used to form a much less carbon-intensive cement than the industry’s most popular mixes. They also highlighted the main barriers to its widespread uptake – and set out recommendations for overcoming these.

The most widely used cement by far in the UK is Portland cement, especially as a key component of concrete. Portland cement mostly comprises clinker. Producing clinker, by heating limestone and other minerals to high temperatures in a kiln, releases large volumes of carbon dioxide.

Methods to reduce this concrete related carbon are already established, typically by replacing cement with ground granulated blast furnace slag (GGBS) or pulverised fly ash (PFA). Both GGBS and PFA are byproducts of carbon-intensive industrial processes but with a move towards greener practices — notably with the shut down of coal power stations and an increased re-use of steel — the availability of GGBS and PFA is declining. The development of new alternatives are vital to help fill this void.

Calcined clay is one promising option that has the potential to replace up to half of a cement mix's clinker content. It is “a mature technology, with over 120 years of recorded use in construction”, according to Knights. He noted that its use in cement and concrete could reduce a building project’s overall carbon footprint by as much as 50%.

Surmounting the adoption barriers

Despite the clear environmental benefits of using calcined clay as a supplementary cementitious material, there are factors hindering its extensive use in the UK.

Although there are several clay-rich parts of the country, Knights stressed that “not all clays are suitable for calcination”. He called for more investment in R&D to identify deposits suitable for extraction and use on a commercially viable scale.

Given that very few facilities for testing or processing clay have been built in the UK, Knights also pointed out that “concrete users don’t specify calcined clay because it’s not available… They aren’t familiar with it.”

A stakeholder group led by HS2 is finalising a position paper on how to stimulate the sector-wide use of Calcined London Clay to produce lower-carbon cement and concrete.

Winslow revealed four key areas of activity covered by the paper:

Develop a set of advance market commitments signalling demand for the material.
Start a series of demonstrator projects involving big clients.
Procure a clay testing facility.
Identify a suitable site for a calcining kiln.

PAS 2080: setting carbon management targets

A separate presentation at the forum meeting featured two decarbonisation experts from East West Rail, the project building a direct link between Oxford and Cambridge.

Tristan Lincoln-Gordon, East West Rail’s head of environment, and James Langstraat, its carbon technical lead, gave a detailed overview of how they used the PAS 2080 standard to set carbon reduction targets.

One of the central themes of their talk was the importance of managing carbon from the earliest possible stages of the project.

Langstraat stressed the value of “targeting significant reductions during the client-led design phase” and being clear with suppliers about East West Rail’s decarbonisation targets at the outset.

Amanda Rice is climate programme specialist at the ICE
Image credits: iStock/Iryna Melnyk; iStock/ezza116