Concrete foundations for net zero

It’s hard to imagine a world without concrete. The most used resource on Earth after drinking water, we make 14 billion cubic metres of it every year, about the same volume as 6,000 Great Pyramids of Giza. Concrete makes up our buildings, roads, bridges, and tunnels. It also has a near-unbeatable track record as a building material. The Pantheon in Rome is made from a form of concrete, and has stood for almost 2,000 years. 

But it’s a massive polluter. Cement, concrete’s key ingredient, makes up just 5 to 10% of the material’s mass, but is responsible for most of its emissions. In fact, cement accounts for about 8% of all global human-made CO₂ emissions. 

We’re already taking steps to reduce concrete’s carbon footprint, such as by making production more efficient and replacing some cement with other materials. However, these alone cannot do enough to reach our net zero goals. We’re going to need a cement that is near-zero carbon, as well as approaches that capture carbon and lock it away. That’s not where the challenge ends – widescale adoption of these new technologies will depend on the right incentives.

Portland cement: the key ingredient

Concrete is a mixture of aggregates (such as sand and gravel), cement, and water. Where aggregates make up the bulk of concrete, cement is the ‘glue’ in the mix. Cement and water form strong crystals that bind the aggregates together.

Portland cement is by far the most common kind of cement, and we’ve been making it in much the same way since the 19th century. The most polluting part of the process is transforming raw materials into lumps of clinker: the stuff that makes cement, cement. In this process, limestone and other materials are heated to about 1,450ºC in a rotating kiln. This heat drives carbon dioxide off the limestone to form the clinker. The clinker is cooled, pulverised and mixed with gypsum, a mineral also used in plaster, to make the Portland cement.

Portland cement is relatively cheap, uses abundant raw materials, and slots neatly into our construction industry. Finding a replacement for it will not be quick or straightforward. “It’s very difficult to move away from Portland cement, because we want our infrastructure to last for decades or centuries,” says Rupert Myers, a senior lecturer in sustainable engineering at Imperial College London. While it’s difficult to say whether we could move away from it completely, he adds, it is definitely possible to reduce its carbon footprint.

It has been common practice in the UK to replace some of the cement with what are known as supplementary cementitious materials (SCMs). SCMs both reduce the amount of clinker needed and strengthen the resultant concrete. Examples include industrial by-products such as blast furnace slag (from ironmaking) and fly ash (from burning coal). These approaches reduce embodied carbon, that is, the emissions released throughout its lifecycle.

However, SCMs can’t bring emissions down enough to reach our net zero goals. Also, as they’re by-products of other fossil fuel-intensive industrial processes, we’ll need to find alternative solutions for reducing embodied carbon in the long run.

Alternative binders

Alkali-activated binders cut the heat – and thus a lot of the carbon – from the cement-making process. These binders are also made from blast furnace slag (from ironmaking) and fly ash (from burning coal), or natural materials such as volcanic ash, which are combined with an alkaline medium. This chemical reaction creates a hard, cement-like binder.

One company creating cement in this way is UK startup company Material Evolution. “We don’t believe that heat’s the way to change materials,” says its Founder and Chief Executive, Liz Gilligan. Gilligan is a Royal Academy of Engineering Enterprise Hub member and part of the Hub's Shott Scale Up programme. Instead of a kiln, the company uses an alkali fusion reactor at ambient temperature, where feedstock materials become cement under high mechanical forces rather than heat.

The company, which has raised £15 million, claims that its kiln-free process produces up to 85% less carbon emissions than Portland cement. Last October its Wrexham plant produced its first batch of cement. Gilligan told The Times that, as the company scales up, it could be the same price as Portland cement in the next five years.

Sweden-based Cemvision uses industrial byproducts and waste – which may have been sitting dormant for decades – from steel and other heavy industries to produce the two cement binders in its Re-ment product. It claims that its binders and production process generate little to no CO2 emissions. 

By treating and purifying these feedstocks, Cemvision can also recover metals such as iron, nickel and chromium, which the steel industry “wants back” as additives for alloys, explains Cemvision’s Chief Technology Officer Claes Kollberg. 

The company aspires to this kind of circularity, as should all of society, Kollberg says. “We have to go to circular flows instead of linear flows,” he explains. In 2024, Cemvision struck up a partnership with green energy supplier Vattenfall. According to a press release, the new cement could be used in power distribution and foundations for wind power turbines.

Locking carbon up for good

Where these carbon reducing technologies can cut emissions in the short term, in the long term, we need strategies for eliminating carbon. “Reducing carbon emissions can only get us so far,” says Mike Cook, adjunct professor at Imperial College London. “We need to get better at carbon capture too – ideally by sequestering the CO₂ that is being emitted into new materials that have a use and a value. This gives an incentive to capture carbon by making it profitable.”

Carbon capture technologies involve trapping carbon emissions at source, such as in industrial flues. The carbon can either be used elsewhere or permanently stored. This is the difference between carbon capture and storage (CCS), which has a cost, and carbon capture and utilisation (CCU), which produces useful products. 

Carbon capture works by permanently locking carbon dioxide into rocks in a process known as CO₂ mineralisation. “CO₂ mineralisation is a really promising method to reduce the carbon footprint of cementitious materials,” Myers says. Cement paste or concrete exposed to the air will naturally recarbonate, he explains. In CO₂ mineralisation, exposing the concrete to a high concentration of CO₂ accelerates this process, Myers adds. The material is then ready to build with and acts as a carbon store.

Cook chairs one such company making products with captured carbon: Seratech. This Imperial College London spinout captures CO₂ from the atmosphere and reacts the gas with the naturally abundant mineral olivine. The end product is a cement that the company says can replace Portland cement in concrete products such as bricks, blocks and planks. Seratech claims there’s a CO₂ reduction of more than 95% compared with Portland cement, and that it is 20% cheaper.

In September 2024, Seratech partnered with a company called Xytel to design a pilot plant for making the material. The modular plant, which is set to run for about a year after opening in July, will test the feasibility of Seratech’s technology and provide a stepping stone for bringing it from the lab bench to industrial scale. 

We know carbonating concrete does change its properties, including making it more acidic, though those changes are well understood. That said, any new materials must be tested for their performance before they can replace conventional concrete. “It’s not like we could just pick a very exotic concrete with low CO₂ emissions and substitute that for conventional concrete,” Myers says. “It doesn’t work like that because the chemistry’s so different.”

With the image of crumbling schools still fresh in the memory following the reinforced autoclaved aerated concrete (RAAC) debacle, proving the safety of these low- to no-carbon concrete alternatives will be paramount in scaling their adoption. New products undergo compressive (squeezing) and tensile (stretching) tests to comply with the UK’s concrete and cement standards, explains Gilligan.

A new version of one of these standards, BSI Flex 350, provides a framework for identifying and using lower-carbon concrete to make it easier for engineers, designers and contractors to adopt these alternatives as they enter the market. The standard is based on the performance of these new binders, rather than prescribing a specific mix of ingredients.

“It’s a crucial step,” says Gilligan. “This is the standard that will allow new technologies to be adopted and scale in the cement industry.”

Getting the right mix

No single technology can fully replace the huge amount of concrete currently made with Portland cement. They simply cannot be scaled quickly enough to match the volume of material we’d require. But a combination of technologies could help us close the gap. Technologies we’d need would be many and various,” says Cook. “To really achieve full net-zero concrete we will need to develop alternative cements that don’t require the burning of chalk [a form of limestone].”

On top of that, no new technology will be adopted at the scale we need, unless it provides a solution that is least as profitable as what we’ve already got.

In a report comparing the maturity of different lower carbon concrete technologies, the Institution of Structural Engineers noted that while most technologies that can reduce carbon are more expensive than conventional concrete, they can become cheaper with economies of scale. Government and industry regulation aimed at reducing embodied carbon will also make these technologies more attractive. “The government needs to mandate carbon reductions in the production of our essential construction materials – not wait until it’s too late,” says Cook.

In the EU at least, a piece of legislation called the Emissions Trading System will gradually implement a CO₂ tax on the cement industry from 2026 to 2034. This means that in a few years’ time, it’s going to be much more expensive to make Portland cement, Kollberg says. These are the kinds of incentives that will bring the focus to lower-carbon cements. 

If this is the way the wind is blowing, businesses must take note. “You cannot be successful in the future if you are not transforming your business in the direction towards sustainable production,” says Kollberg.

It’s also important to remember that a net zero world depends on net zero construction. We discuss alternative sources of energy, and that’s very important, Cook says. “But what’s seldom recognised is the CO₂ that’s emitted when making the materials needed to make the massive concrete nuclear power stations, and the heavy foundations and superstructures for new wind turbines. Creating a decarbonised energy supply currently contributes massively to carbon emissions due to construction of new energy infrastructure. We need to change this as fast as possible.”

There’s no shortage of ingenious approaches to decarbonising construction. What we are short on, however, is time. The next few years will be crucial. Although with the UK government committing to taking time-bound steps to decarbonising the steel and cement sectors with the Green Public Procurement Pledge, it looks like we’re taking a step in the right direction.