House Made by Many Hands
Case study: Cairn Architects and Structure Workshop have trialled a limestone calcined-clay cement on a low-carbon extension in east London
Designed by Cairn Architects, House, Made by Many Hands in east London is the first building structure in the UK to specify concrete containing limestone calcined-clay cement (LC3), a multicomponent blend made from 50% clinker, 30% calcined clay and 15% limestone, with 5% gypsum for workability.
LC3 was developed at the EPFL research university in Switzerland, with academic partners in Cuba and India, and has been calculated to generate 30-40% less CO2 in production than CEM I. Calcined clay is processed at a much lower temperature than Portland cement – 800°C rather than 1,400- 1,500°C – and limestone does not need to be processed at all.
Like limestone, clay is abundant in the UK, but there is also much that could be diverted from other industrial waste streams. An MPA innovation project is currently testing two secondary sources, from brick manufacturing and overlying deposits at mineral extraction sites, with the aim of enabling much wider usage of calcined clay as a supplementary cementitious material.
At House Made by Many Hands, LC3 has been used for the floor slab and to underpin the existing brick footings of a Victorian house extension. The low- carbon approach extends to the rest of the project, which has been completed in natural materials such as hempcrete, cork and lime plaster.
Structural engineer Structure Workshop took on the risk of pioneering the LC3 technology, carrying out cube tests to verify its C30 strength. “Because of the small scale of this project, hopefully it can be a test case and a piece of evidence that allows these things to be used at a much bigger scale,” says Kieran Hawkins of Cairn.
The concrete was mixed and poured in the same way as standard concrete, requiring no additional site training. “If we hadn’t told the contractor it was different, they wouldn’t have known,” says Hawkins, adding that the cost was also comparable to a conventional mix.
Structure Workshop used its own carbon calculator, which applies industry carbon factors sourced from Bath University’s ICE database. The embodied carbon of the LC3 slab was measured at 65% that of a CEM I alternative. Overall, the project’s embodied carbon was calculated to be 40% lower than a typical Victorian extension.
The LC3 for the slab was imported from Denmark, where it has already been used on several building projects, including the Bjarke Ingels Group headquarters in Copenhagen.