Concrete production alone accounts for around 8% of global CO₂ emissions. Most of this carbon footprint comes from clinker, which, although it represents only one component of concrete, is responsible for around 90% of the emissions associated with this material. Replacing part of the clinker with lower-carbon alternatives therefore represents a major challenge for decarbonising the construction sector.
The initiatives currently being developed in the United Kingdom, France and Australia presented in this article share a common observation: advances in materials science are progressing faster than the evolution of standards, supply chains and business models required for industrial-scale deployment. Nevertheless, significant progress has been made.
Furthermore, the transition towards more sustainable construction cannot rely solely on the decarbonisation of concrete. It requires, upstream, more efficient use of resources, a careful assessment of needs, and greater reliance on a mix of materials. However, concrete remains a key material in modern construction due to its strength, durability and reliability. Essential for foundations, bridges, tunnels, dams and certain types of buildings, it continues to play a major role in the development of large-scale infrastructure.
Retreat from GGBS and fly ash
For years, the dominant strategy for cutting the clinker content of concrete relied on ground granulated blast-furnace slag (GGBS), a by-product of steel production, and pulverised fly ash (PFA) from coal-fired power stations. Both materials are now becoming harder to source.
At VINCI Construction, whose Exegy programme develops low-carbon concrete formulations, that shift is already driving decisions. “We want to use less slag,” says Exegy director Olivia Tillier-Devauges. “It’s the same thing for fly ash, both are becoming less available, and more expensive.”
The same dynamic is playing out in the UK, where Arup senior engineer Michael Sataya is working on the Environment Agency’s Decarbonisation Technology Accelerator (DTA), a three-year programme running from March 2024 to March 2027. “GGBS historically was treated as a waste from the production of steel. PFA was from the burning of coal. But we’re not burning coal in power stations anymore, and we’ve been closing the blast furnaces, and outsourcing some of that production to other parts of the world. So GGBS is becoming scarcer and we’re having to import a lot of it now,” Sataya says.
Trialling limestone filler and calcined clay
In place of GGBS and fly ash, both programmes have converged, in part, on a pairing of limestone filler and calcined clay. On the DTA, attention has turned to ternary, or composite, cements combining Portland cement, GGBS and a larger proportion (typically 6-20%), of limestone filler. “If you can substitute about 20% of that overall binder content with the limestone filler, then that means you start to diversify your alternatives to Portland cement,” Sataya says.
Arup has also trialled a mix design developed by Ecocem that is not yet commercially available, made up of roughly half limestone filler and half GGBS and clinker. Because that proportion exceeds current standard limits, the concepts require proving. “You need to do more testing to demonstrate that it’s feasible or suitable for a particular situation,” says Sataya.
Exegy’s formulations follow the same logic. Its response to the retreat from GGBS and fly ash has been to build mixes primarily around limestone filler and metakaolin, a calcined form of kaolin clay already recognised within French concrete standards for use in structural concrete. An Exegy sprayed concrete formulation currently being tested in underground construction, replaces a significant portion of clinker with limestone filler. This sees it account for close to 50% of the mix, and approximately 20% metakaolin. A second formulation shares the same material base but has been adapted for use in cast and precast structural applications.
On the calcined clay itself, Sataya notes a benefit within the chemistry: “Because the production of cement uses a lot of limestone, which is calcium carbonate, you’re burning off the CO₂ from that. So not only are you using energy to do the burning, but the chemical reaction also releases quite a lot of CO₂,” he says. “Whereas for calcined clay, the chemical reaction is different and doesn’t release anywhere near as much CO₂, because your starting constituents are different, and you’re doing it at a lower temperature.”
Regulation as gatekeeper
Both the UK and French work is mindful of the common obstacle of standards, since these determine what can be built with, regardless of what the materials science allows. In France, regulation has pushed the other way, accelerating adoption. Tillier-Devauges says RE2020 has created both the pressure and the incentive to adopt lower-carbon formulations, and that we have reached 80% use of low carbon concrete on our job sites. But elsewhere, the picture is different. “In Canada for example, we don’t have the same regulation, or the standard which allows the use of low-carbon concrete,” she says. “It’s difficult to use new solutions to develop low-carbon concrete.”
She describes a circular problem that while the standards are developed in response to demonstrated use, demonstrated use requires the confidence that standards allow it. “Some clients don’t want to discuss new solutions, because they have guides that describe what concrete is, and it’s not possible for us to explain that we have another way to reduce the clinker but keep the durability and strength of the concrete.”
From trial to scale
Both organisations describe a similar pathway of real-conditions trials on live projects, followed by an attempt to standardise and repeat. Arup has completed one ACT trial, at a wetlands habitat creation scheme called Fishwick Bottoms near Preston, where a small slab was needed for access to a farmer’s field. A further ACT trial is planned at Hebble Brook, near Halifax, expected by autumn 2026. Separately, Arup colleagues working on the UK’s HS2 high speed rail line have produced calcined clay from waste clay generated during tunnelling, using it in walkways and staircases.
The Exegy sprayed concrete has moved significantly along this pathway, having been first tested on a shaft at the Lyon-Turin “TELT” rail tunnel project. “As the formulation worked very well, we would like to use this formulation on line 15 of the Grand Paris Express,” Tillier-Devauges says. That work begins in September 2026, involves an initial 10,000m³, and will run through to September 2029. “It’s a very important experiment for this solution,” she says. The adapted sprayed concrete formulation has been used at Nantes University Hospital, in precast staircase elements, wall panels and slabs, as well as in-situ pours. “Our goal is to standardise the use of the solution,” Tillier-Devauges says, describing widespread use as the ambition behind both formulations. “The innovation has to begin with a new solution, and then we standardise that new solution with massification,” she says.
Capturing carbon at source
A novel approach in the construction value chain that focusses on locking away site-produced carbon in concrete, is coming from Australian startup Kapture. This involves retrofitting modular carbon capture hardware to diesel generators on construction sites. It’s a category of emissions neglected by the kinds of carbon capture technologies being developed for power stations, cement kilns and steelworks.
“Most of the carbon capture that exists in the world is focused on large-scale industrial emissions, and I believed that small-scale emissions were overlooked,” says Kapture founder and CEO Raj Bagri. She explains that the unit connects to a generator’s exhaust in under an hour, with no required engine modifications, and captures up to 90% of CO₂ emissions. It then converts these into a solid, 99%-pure calcium carbonate mineral. “The carbon is permanently sequestered in that material,” Bagri says. “There’s no risk of leakage or storage. It’s just permanently embedded.”
That byproduct can be used in the filler constituent of concrete that Arup and Exegy are also pioneering with. “Ours is different to traditional calcium carbonate that construction companies are currently using,” Bagri says. “It’s very high purity, and it’s micro and nano particle size behaves very differently in concrete.” Testing in Australia has validated the byproduct as a filler at around 5% of a mix, and Kapture’s first construction trial is with Seymour Whyte, a VINCI civil engineering business in Australia, scheduled for July and August.
But interestingly, Kapture is beginning a programme with Deakin University in Victoria to see whether it can function as a supplementary cementitious material, replacing a larger share of Portland cement. “The university seems confident that we could increase the percentages,” Bagri says. “We could try and hit 20-30%.”
The economics of adoption
Unsurprisingly, all three initiatives reference cost as a critical constraint in driving adoption for low carbon concrete. “We must develop solutions where the cost is ideally less, or just a little more expensive, in the order of a few euros, otherwise we are not competitive,” says Tillier-Devauges. “It’s the team at the construction job sites who order the concrete, and they have tricky budgets.” If anything, it’s an understatement, with contractors often operating within profit margins of low single digits.
Bagri describes the same challenge from the Kapture side that relies on a leasing model, rather than capital purchase. She says this is around €1,500 a month for a 30-40 KVA generator, plus solvent costs of a few hundred euros a month. “It comes down to cost,” she says. “Right now, it seems like we’re on the mark, but it’s quite tight.”
It’s encouraging to hear that the figures may well stack up in such a price-sensitive market, not least since the longer term could see the technology moving beyond the construction industry. “I think if we can make construction work, you can make any use-case work, because it’s the toughest in terms of operational and base constraints, as well as margins,” she says.
Whether measured in mix ratios, national standards or cost per cubic metre, the constraint across all three efforts is less about whether low-carbon concrete works, and more about whether the systems around it can move at the same pace as the chemistry.
