Clay ahead of the game

What does it take to deliver calcined clay on site? In this third and final part of our series on calcined clay, we dive into the reality of specifying compliant calcined clay structures, and share what we’ve learned from our unique exposure to the material.

HTS are pioneering the use of calcined clay concrete, both on live projects such as Broadgate Tower (the first UK commercial use of the material) and through R&D, such as our key technical role authoring specifications for the Accelerating Concrete Decarbonisation Group (ACDG), including two large scale prototypes using calcined clay.

• Part one highlighted the clear case for calcined clay, how this ancient material can rise again as a global and futureproof way to viably decarbonise the highly polluting concrete industry.
• Part two got stuck into the detail on supply chains, exploring what exactly calcined clay is, how we can make it from natural clay, and where we might get that clay from.

So, now we know why we need it, what it is and where it comes from, let’s talk about how we can make it happen.

Code compliance is essential, but bringing a novel SCM to site presents a totally different array of challenges to overcome. These can broadly be split into two overlapping categories; logistics and workability, solved in different ways.

Logistics

Calcined clay is a very new material – at present we are aware of two domestic suppliers (Forterra LKAB and Hepworth Clay) and a handful of international options. There is sufficient material but early communications here are essential to confirm lead times and avoid costly potential delays.

Batching plants use silos to store and portion out binder (aggregate can sit in the rain but cement reacts with water so must be kept dry). To supply a new binder, a supplier needs a free silo – this means an existing silo must be emptied, or a new silo must be built. Either option will take time and come at a cost so once again early engagement with the supply chain is essential to ensure this is correctly managed.

Workability

Although calcined clay has been thoroughly lab tested (a prerequisite for code compliance) there’s a lot more to placing concrete than lab results. When concrete is mixed, the chemical reactions begin and it must be placed before it solidifies. The core paradox is that it must remain sufficiently liquid to be transported, poured, pumped, and flow into all the areas within the formwork, but then must harden quickly so that the formwork can be removed and works can progress onwards and upwards.

Clearly it is a balancing act – cure too quickly and you risk excessive cracking and poor quality, cure too slowly and you face potential programme delays. Getting this right relies on various wet tests (the slump test is the most well-known example, where a filled cone of wet concrete is allowed to flow out in a kind of puddle, and the amount of slump is measured (see below)) and decades of experience. Concrete technicians notice small changes in the texture and behaviour of wet concrete and make small adjustments to ensure that the pour goes smoothly.

Introducing a new material like calcined clay can throw all these tests and judgement calls off, so many contractors are understandably cautious about using calcined clay on big projects without prior experience.

Starting with easy wins such as small plinths, non-programme critical pours, and test pours such as the Accelerating Concrete Decarbonisation Group (ACDG) prototypes (see below) is a vital stepping stone to build confidence and work up to full scale usage.

Broadgate Tower is a 36-storey tower in central London, where we have poured calcined clay in two locations; a plant plinth on the 34th floor, and a raised parapet on the 4-storey extension. This is the first on-site commercial use of calcined clay in the UK and is in line with our strategy to start with smaller pours and increase industry momentum.

So how did we make it happen?

With an engaged, ambitious client in British Land, HTS included specific reference to calcined clay in our tender specification back in 2024. The team including Buckley Gray Yeoman, BAM, and Getjar, have all worked together to identify the right opportunities to dovetail calcined clay into the tight project programme, and in April 2026 we reviewed and approved the final mix design from Heidelberg, with 30% calcined clay cement replacement.

Delivering calcined clay isn’t easy, but it is possible and necessary. Calcined clay is code compliant and available, so the biggest challenges today are logistics and experience.

We derisk the logistics by starting early and making sure everyone is on the same page. As ACDG and Broadgate Tower have shown, when everyone knows about it from the start, there should be no surprises.

There really is no substitute for experience, so we deliberately target smaller and less critical pours first. This gives everyone time to learn how calcined clay mixes behave, and allows us to demonstrate successful pours on a range of projects, working towards larger and more critical areas.

Momentum is building and we are receiving increasing requests from clients to deploy calcined clay on their project. We foresee these early steps as the beginning of a wider industry shift towards more tailored concrete mixes and are excited to be leading that charge.

If you have questions about calcined clay, or you’re thinking about specifying it on a live project, we’d like to hear from you.