Ways to reduce the carbon footprint of concrete

24 Jan 2022

Published online: RIBA, January 2022

What are the alternatives to cement and how easy are they to specify? Matt Massey of Derwent London, Elaine Toogood of The Concrete Centre, and Peter Laidler of Structure Workshop discuss how carbon emissions associated with concrete can be cut dramatically by using alternatives to cement.

Concrete is second only to water in the ranking of humankind’s most used resources, points out Peter Laidler, an engineer and Director of Structure Workshop.

“We consume a cubic metre of person per year,” he states. “Portland cement is responsible for 8% of global carbon emissions and its consumption is on a current trajectory to increase by 20% by 2050.”

Carbon emissions associated with concrete can be cut dramatically by using alternatives to cement. Derwent London is a major developer whose current strategy for cement reduction is to substitute it with GGBS (ground granulated blast furnace slag) as much as possible, reveals Derwent’s Senior Project Manager Matt Massey.

GGBS, PFA and other cementitious materials

GGBS and the alternative industrial waste product PFA (pulverised fly ash from coal-fired power stations) have long been used as a cementitious substitute. They are covered by the structures’ bible, British Standard BS 8500, which makes projects that use them less of a perceived risk to insurers.

Derwent’s 19-35 Baker Street development, due to go on site in June, will contain up to 70% GGBS substitution for cement in its concrete. Its multi award-winning White Collar Factory scheme in London’s ‘Tech Hub’ at Old Street substituted PFA for cement; Massey finds that PFA gives concrete a more aesthetically pleasing shade of grey than GGBS.

However, both PFA and GGBS are very hard to source, at a time when interest in finding solutions that reduce embodied carbon in construction is soaring. Derwent is investigating the new generation of cementitious materials, such as Earth Friendly Concrete, which recently arrived in the UK from Australia.

Earth Friendly Concrete is one of a class of cementitious materials known as Alkali-Activated Materials/Geopolymers. Some of these new proprietary materials are seen as having the potential for 100% substitution for Portland cement ‘clinker’ and GGBS and PFA blends.

British Land has been trialling Earth Friendly Concrete at the huge Canada Water area regeneration project in London’s Docklands, where it has been used for piled foundations with large carbon emissions savings. Studio Bark’s prefabricated U-Build system is applied to concrete free foundations at the No Building as Usual summer school.

Compatibility with British and European standards

“Insurers tend to struggle with projects not covered by British Standards,” Massey says. It is possible to use alternatives to GGBS and PFA not codified by BS 8500, but, currently, designs must be supported by testing.
While some AACM/Geopolymer materials are regarded as relatively advanced in terms of technical acceptance and are becoming more available, suitability for different applications will have to become well established before they will be covered by an expanded British Standard.

In the meantime, these novel cements can be used in the UK by following guidance in the Euro standard (BS EN 1990:2002) where designs have to be assisted by testing.

Elaine Toogood, Head of Architecture at The Concrete Centre, points out that there is also a proto-standard, PAS 8820 2016, for AACM/Geopolymer specification, although again this needs to be supported by testing.
Toogood is predicting an upturn in the availability of PFA and GGBS driven by the industry’s new focus on embodied carbon. Interest is turning to countless waste heaps dotted around the country that could be mined for cement substitutes.

Powdered limestone and calcined clay

There are two more materials on the horizon that could have a dramatic impact on reducing emissions from concrete in the UK: powdered limestone and calcined clay. Both raw materials are available in such huge quantities in the UK as to be effectively limitless.

The two materials can be used in combination to directly replace a high proportion of Portland clinker in cement, producing what is known as Limestone Calcined Clay Cement, or LC3. Again, there are already European standards for some blends.

“Calcined clay and limestone have the potential to half the CO2 emissions in concrete,” suggests Peter Laidler.
“Using lean construction to reduce concrete in structures, which in turn will reduce the amount of concrete needed for foundations, you can halve it again.”

Commercial availability is currently limited, but the simple abundance of these materials means there is great potential ahead, he adds.

Concrete free construction

Laidler points out an exemplar project which he worked on in collaboration with Studio Bark as part of the innovative No Building as Usual summer school: a small project called Nest House that was built without concrete.

The building is lightweight, using recycled timber. It used compacted stone foundations on adjustable footings and a timber subframe for its substructure. Crushed stone foundations offer huge carbon savings in comparison to traditional concrete ones. There are many different ways to use crushed stone, such as gravel-filled tyres, rubble-filled trenches, or hybrid solutions such as gravel-filled tyres with a steel frame.

In an ideal world, we would be abandoning cement altogether. Laidler says he would like to see architects taking more interest in the concrete they specify, particularly the hidden concrete foundations beneath their otherwise low carbon buildings, and actively supporting the use of these emerging cement substitutes wherever they can.

Contributions by  Elaine Toogood, Head of Architecture, The Concrete Centre