The recent announcement that HS2 may still extend to Euston station instead of terminating in the suburbs could be good news for a group of scientific alchemists planning to conjure concrete from London clay.
The boring for HS2 will produce more than a million tonnes of waste overall, and that clay will need to be carted away on the surface.
Meanwhile, vast quantities of cement will be shipped in by lorry to make concrete for HS2’s tunnels at great environmental cost. The manufacture of cement produces 8% of the world’s carbon emissions, and the industry would be the world’s third biggest emitter after China and the US if it were a country.
Now engineers are keen to adopt an intriguing method that could tackle the clay problem and the cement problem at the same time. They want to set up a giant oven at the boring site and bake some of the clay at 1,100C. They plan to mix that new product – known as calcined clay – into the concrete for the tunnels. They estimate this would change the makeup of the mixture so medium-strength concrete could be made with much less cement.
It would be the first large-scale use of the technique in the UK, but the properties of calcined clay were discovered long ago – it was used in 1932 for bridge construction in San Francisco, and in many of Brazil’s large dams. Since the 1970s Brazil has produced about 2 megatonnes of calcined clay a year.
It is by no means a total solution for the climate heating impact of the materials used to construct HS2, which will remain a net polluter into the next century – but it could be a potential step forward.
The engineering firm Arup, which is leading the project, says the calcined clay mix will be suitable for use in foundations, walkways and platforms, though not for the pre-cast tunnel sections that require high strength.
The baked clay will also displace another material in the concrete mix, blastfurnace slag, a waste product from the steel industry. For many years this has been repurposed as an ingredient for concrete, but as the steel sector cleans up its act less and less of it will be produced, so the construction industry must find other materials to fill the gap.
The calcined clay project earmarked for the Euston leg of the railway has been supported by HS2 and led by Arup’s Fragkoulis Kanavaris. He said: “Trying to manage the excavation and transportation of waste from a project such as HS2 is a massive logistics exercise. Using calcined clay created on the site allows us to take clay from the ground then put it back into the ground with a different purpose.”
Calcined clay is not a magic bullet: baking the clay in a kiln on site produces high CO2 emissions. But Prof Becky Lunn, from the University of Strathclyde, said that compared with portland cement made in the traditional way, the clay alternative would produce 30-40% fewer emissions, on top of the emissions savings from transporting the waste across the capital.
A spokesperson for HS2, which is still in turmoil after the prime minister announced the Manchester leg will be scrapped, said it favoured the reuse of materials. The Guardian understands HS2’s clay proposal has been warmly received so far by government officials and the rail industry.
HS2 is not alone in attempting to harness the benefits of calcined clay, as architects and engineers strive to limit the emissions that are fuelling the climate crisis. Another smaller project using the technique in the UK is reported to have been completed.
Heleni Pantelidou from Arup said: “Trying to turn clay spoil into a resource is not a new thing – the whole of London is made by bricks, of course. But there are more complications with modern tunnel-building techniques, which is why this is not the practice any more. Nowadays the principle of using resources near to the construction site is getting a lot more traction again.”
Lunn welcomed the use of calcined clay, especially as the world will soon be running short of concrete ingredients from coal industry waste. Unless alternatives can be found, she said, there may be a perverse incentive to keep coal-fired power stations open to harvest the waste they produce.
Lunn said: “Progress is being made on lowering the carbon in cement itself, but there remains a long way to go. There is still a need to produce the heat for production of cement (or cement replacements) with green energy – and to combine cement production with carbon capture technologies.”
