Insights
•It’s still early clays
To decarbonise concrete on a global scale, we need to completely rethink our current approach to cement replacement and focus on materials available in large enough quantities to make a real impact.
This is the second article in a series of three spotlighting calcined clay. In the previous article, we looked at the history of the material, as well as the case for using it in modern construction. Now we delve deeper into the production and sourcing of this highly abundant material.
Image 1: global usage and reserves of clinker as well as other cement substitutes. The number at the top of each bar represents the embodied carbon, in kgCO2e, per tonne of material. Adapted from: Eco-efficient Cements - Scrivener, John, Gartner – UNEP 2016, with additional EPD data.
What is calcined clay?
Calcined clay is a term for finely-ground clay minerals, mostly silica and alumina, that have been activated to unlock their cementitious properties. The activation process shuffles up the internal structure of the clay, breaking the usual ordered layers and making it suitable as a cement replacement. There are two main methods of achieving this, cooking or crushing.
The “cooking” option is known as thermal calcination, and takes place between 600 and 800 ⁰C. This can be done either in a rotary calciner, where the material is heated as it travels along a rotating drum, or a flash calciner, which heats the clay very rapidly1. On the other hand, the “crushing” option involves either grinding or vibration and is known as mechanochemical activation. It is much more energy efficient than thermal calcination but is still being researchedi.
Image 2 (left): As clay is heated, its molecular structure is rearranged. Stopping at the right temperature gives us calcined clay.
1 There is a fundamental trade-off between these options: the flash calciner is more efficient and produces a more reactive calcined clay, but is associated with high start-up costs as it must be built from scratch. Existing rotary kilns can be converted into rotary calciners, reducing the infrastructure cost, but the quality of the product is lower, so is more suited as an interim solution.
Once the clay has been activated (plus secondary processes, such as grinding into powder), it can be added to a cement mixture as a supplementary cementitious material (SCM). When mixed, the aluminium and silicon ions in the clay react with calcium ions in cement, forming calcium silicate hydrate (CSH) gel, which is the main strength provider in cement. How readily a clay undergoes this reaction is called its “pozzolanic activity”, with a high pozzolanic activity indicating that a clay has good cementitious properties.
Image 3: A rotary kiln can be converted into a calciner
The VIP of clay minerals: kaolinite
There are a number of different clay minerals, but kaolinite is the gold standard. The calcination product of kaolinite, metakaolin, is commonly accepted to be the most active pozzolanic materialii, and a great cement substitute. Unlike many other clay minerals, it is well understood, as a result of extensive testing. The UK has significant kaolinite reserves in the Southwest, however they are in high demand from the ceramics industry, and command a high price making them uncompetitive as an SCM.
Image 4: Map of clay locations across the UK. Adapted from: Linda Bloomfield, BGS geology viewer, Dorset Geologists Association, and Kent Geologists Group
The challenger: common clay
Most natural “common clays” found across the UK have lower kaolinite contents, and more other clay minerals. So how low can we go? According to a recent study, low-kaolinite clays should contain a minimum of 40% clay minerals to be viable as an SCMiii, though it’s not clear how much of this should be kaolinite. Even for clays which pass this 40%, calcination is less effective compared with high-kaolinite clays. To fully exploit the potential of common clays, new processes to increase their pozzolanic activity must first be developed2, then production of these clays must be scaled. As indicated by our regional clay analysis below, all these materials are natural and different, so a tailored approach may be needed.
Image 5: maximum kaolin and total clay mineral content for a selection of UK clays. Adapted from: BGS Eureka Project and Major Projects Association, with additional data, see below
2 Such processes are currently being researched as part of the EUREKA project, with results expected to be published in early 2027. Proposed methods include acid washes and planetary ball milling.
Scalability
Image 6 (left): LKAB use waste bricks from Forterra in to create calcined clay
According to the British Geological Survey, the four most promising sources of common clay for calcination are also important raw materials for the production of other construction materials, most notably bricks. Despite the abundance of clay, rapid scaling of calcined clay production in these areas may disrupt supplies, as substitution of cement clinker by 30% in concrete across the UK would require around 2.8 million tonnes of clay to be extracted and calcinediv. The UK brick industry uses about 6 million tonnes of clay a year, for perspective.
However, this existing industry can be a blessing too, as LKAB have found out. By producing calcined clay from waste bricks instead of using raw clay, they became the first company to bring calcined clay to the UK market, and with an added circularity benefit over using virgin clay. Of course, this approach is limited by the availability of waste bricks, but it is an excellent stepping stone solution to demonstrate demand and dovetail with existing infrastructure, whilst the UK raw clay calcination industry finds its feet.
More exciting still is the secret third option: using waste common clay excavated from building sites. Using waste clay eliminates the cost and logistics associated with offsite waste disposal, increases circularity, and is a great materials story. As the clay composition on each site varies, it’s important to test the ground early to see what you’ve got.
A recent pilot project tested London clay, excavated from multiple HS2 sites for its potential to be used in concrete for the same project. In this case, due to its low kaolinite content it was only suitable for non-structural concrete, but this can still have a big impact on a project’s carbon footprintv. This also paves the way for more exploration into excavation waste, and hopefully processing advances will enable the use of London clay in primary structures in the not too distant future.
Image 7: HS2 London Clay sample – raw/as excavated (left) and calcined (right)
Conclusion
As more research is completed and our understanding of clay mineral calcination grows, we can expect new methods to unlock lower kaolinite clay reserves for primary structural use. Calcined clay is already included within concrete design standards, and the first generation of projects are underway, so part three of this series will look at what it takes to deliver calcined clay on site, the hurdles, and the opportunities to pioneer this emerging green technology.
Source list
i https://www.concretecentre.com/Specification/What-is-Concrete/Calcined-clay.aspx
ii https://pmc.ncbi.nlm.nih.gov/articles/PMC7660299/
iii https://nora.nerc.ac.uk/id/eprint/541733/1/EUREKA%20paper%20ICCCSC%202026%20Clive%20Mitchell%20BGS%20NORA.pdf
iv https://nora.nerc.ac.uk/id/eprint/541733/1/EUREKA%20paper%20ICCCSC%202026%20Clive%20Mitchell%20BGS%20NORA.pdf
v https://majorprojects.org/resources/transformation-of-london-clay-into-construction-resources-supplementary-cementitious-material-and-lightweight-aggregate/
Image credits:
Additional data for image 5 from:
– Low-carbon cements: Potential for low-grade calcined clays to form supplementary cementitious materials, Adyati et al., 2022,
– Clay mineral formation and transformation in non-marine environments and implications for Early Cretaceous palaeoclimatic evolution: The Weald Basin, Southeast England, Akinlotan et al., 2022,
– The story of China Clay Harrow and Hillingdon Geological Society, 2006,
– The mineralogy, geochemistry and surface area of mudrocks from the London Clay Formation of southern England, BGS Physical Hazards Programme”
Image 6: https://www.lkabminerals.com/news/calcined-clay-cement-production/
Image 7: https://learninglegacy.hs2.org.uk/document/transformation-of-london-clay-into-construction-resources-calcined-london-clay-clc-as-supplementary-cementitious-material-scm-in-concrete-mixes/
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