Claytime is not over

One of the oldest materials can help us decarbonise one of the most polluting modern materials. In the first of three articles we explore the origins, quirks, and opportunities presented by clay.

We measure the past using materials: the stone, copper, bronze, and iron ages all signified massive shifts in technology, as each namesake material was extracted from the earth and used for tools, weapons, and whatever else we could come up with. The step from found material (stone) to manufactured (metals) coincides with the beginning of recorded history, however there is one material which is arguably missed from this order – clay.

Ceramic (fired clay) sculptures discovered in Czechia have been dated at 30,000 years old, far older than copper (5,000 years) and bronze (3,000 years) and already well understood given the mix includes charred bonemeal to improve strength. Ceramics, like metals, are processed at high temperatures, so the discovery of fired clay was a precursor to the discovery of extracting metal. But unlike stone, copper, bronze, and iron, each of which replaced its predecessor, clay was here to stay – in fact a bowl used today would be familiar to someone from millennia ago. But this article is not about ancient pottery.

From the Heyne Tillett Steel office in London, surrounded by modern architecture, it is hard to believe that a fifth of the earth’s population still lives in structures built from earth. In the eighties it was more like a third, but population growth, particularly in cities, is leaving earth construction in the dust.

Does that mean that if we go back far enough, everyone lived in earth structures? In short, no.

In long, no because people build with whatever is best available, and earth is not always the best choice. Some places have abundant timber, others have abundant workable stone, and earth varies hugely too. Under your feet could be anything from gravel, sand, clay, soil, hard granite, soft chalk, Victorian rubble, or any combination of the above. Building a house from gravel is not easy.

Clay then, isn’t everywhere. It also isn’t one thing like iron or copper – clay is a mishmash of aluminium and silicon-based minerals, very finely ground, mixed with impurities and water. The mix varies based on location, impacting the properties of the raw clay. The purest clay is pale and strong – consider a fine porcelain teacup, so thin you can shine a light through it, or the distinctive yellow London brick, coloured by the chalky impurities in the local clay.

For such an ancient material, clay solves a very modern problem – how to decarbonise concrete, and since it’s responsible for about 8% of global CO₂ emissions, concrete needs decarbonising. The lion’s share of that carbon footprint comes from the cement binding it all together. Cement is made from limestone fired by burning fossil fuels at 1,400°C, driving a chemical reaction which releases even more CO₂.

The most common way to reduce cement in a concrete mix is to swap some of it with GGBS (ground granulated blast furnace slag). As mentioned above, iron (like cement) is made by cooking rocks, and the byproduct (known as slag) can be ground up and mixed into a concrete mix, replacing cement.

From a project carbon perspective this is great – GGBS is traditionally considered waste, so low value in both carbon and cost, and a 65% replacement can cut embodied carbon of the overall structure by 30%. Unfortunately, there’s nowhere near enough to go around – the steel industry, which is vast (that’s a different story), only produces enough GGBS to meet 10% of global cement demand. So the hunt is on for another cement replacement; something naturally abundant, something in the cooked rock family. I think you’ve guessed where this is going – clay is back!

Clay is fired at 1000°C, turning it into a durable ceramic. But if you go halfway, more like 600°C, you get “calcined clay”, an effective cement replacement. As the temperature is far lower than cement making and there is no chemical reaction releasing CO₂, calcined clay has a far lower carbon footprint. Early data suggests a 90% reduction compared to Portland cement is possible.¹

Current standards allow for calcined clay to replace up to 55% of Portland cement, and early data suggests 35% is the sweet spot to achieve an optimal balance between carbon reduction and maintaining concrete properties. This is a new product, so concrete suppliers are still undertaking their own testing to refine admixtures and to ensure that the wet mix can be transported and poured correctly. This is compounded by the fact that clay varies by location, so a new source may need new testing. Additionally, there are logistics to consider, new silos to build, and staff to upskill.