Concrete: upon this rock-like composite we have built our church – and our houses, roads, bridges, skyscrapers, and factories. As a species we consume more than 4.1 billion tonnes of the stuff every year, more than any other material except water. (You’re almost certainly sitting or standing on it right now.) That’s a problem, because concrete – and in particular cement, concrete’s key ingredient – is catastrophic for the environment. The cement industry alone generates 2.8bn tonnes of CO2 every year, more than any country other than China and the US – and somewhere between four and eight percent of all global man-made carbon emissions.

According to the Paris agreement, carbon emissions from cement production need to fall by at least 16 percent by 2030 for the world to reach its target of keeping global warming within the limit of 1.5C and well below 2C. (At present, those emissions are actually increasing, driven in large part by mega construction projects in China.) Now, the concrete industry is in a race against time to solve a very hard, very grey problem.

The recipe for concrete has been largely unchanged since the 19th Century: you just need a mixture of large aggregate (stones), small aggregate (like sand), cement – which binds it together – and water. “The main issue with concrete is the production of cement, because if you want to get a cement, you need to have clinker,” explains Ashraf Ashour, professor of structural engineering at the University of Bradford. Clinker, typically a mixture of calcium carbonate, clay, and gypsum (although many other materials can be added) is mixed and heated in a kiln. “You need to heat clinker at a very high temperature, maybe at 1500 degrees, and by doing this, you are producing lots of CO2 emissions,” Ashour says. Inside the kiln, the clinker undergoes calcination: the calcium carbonate breaks down into calcium oxide, releasing even more CO2.

One way to decarbonise concrete is to replace cement with other materials, such as the fly and bottom ash created by coal power stations, or blast-furnace slag, created in iron production. Cement makers have been mixing in waste aggregates for years, but with supplies constrained by the ongoing closure of coal plants, many companies are now exploring alternatives. Canada-based Carbicrete replaces the cement with steel slag, a byproduct of steel manufacturing. “There’s 250 million tonnes of it made every year,” explains Chris Stern, Carbicrete’s CEO. “For years, steel slag has basically been used for road fill. Some goes into roads, the smaller bits go into landfill, it's sometimes used in fertiliser, but there's not a huge usage rate.”

Once concrete is mixed, it has to be hardened, or “cured”. Traditional concrete is cured with water, a process that takes 28 days. (When you see workers engaged in what looks like watering freshly-laid foundations, that’s curing.) Carbicrete’s concrete, however, is cured with carbon dioxide. CO2 captured from industrial processes is injected into the concrete, which reacts to form calcium carbonate, or limestone. “Right now, we're a carbon negative company,” Stern says. “In fact, the actual marginal cost of capture is zero, because we can sell our product. That’s what makes [concrete] such an interesting product.”

Another company hoping to scale CO2-cured concrete is New Jersey-based Solidia. Its cement uses less lime and more clay, including wollastonite (or synthetic pseudowollastonite) which lets Solidia fire it at a lower temperature. Solidia claims its method requires 30 percent less energy and produces 30 percent lower emissions. Its curing process also uses CO2, locking carbon up inside the finished product.

Curing with CO2 also saves water. “Ordinary Portland cement-based concrete products consume around 2.6 trillion liters of water annually,” says Solidia CEO Bryan Kalbfleisch, referring to the type of cement most commonly used in construction. “Our technology enables the customer to recover greater than 90 percent of the process water. Hopefully, that frees up water in countries that are experiencing a water crisis to be used for drinking and agriculture.”

Solidia currently sources from an industrial gas supplier. But the company hopes to soon begin sequestering CO2 from other sources, including industrial facilities and eventually direct air capture plants. “We could potentially sequester 0.4 gigatonnes of CO2,” says Kalbfleisch. The company has to date sold more than 92,000 square metres of precast paving, but its target is the market for ready-mix, concrete fabricated in a plant before being delivered to construction sites by mixing trucks, which makes up around 75 percent of the global market. Solidia is soon to launch a pre-mixed cement, which is currently awaiting certification from industry bodies. “As soon as the product is approved with ASTM [the American Society for Testing and Materials, the international regulatory body] and other governing code associations, we can quickly scale within this enormous portion of the market,” Kalbfleisch says.

Ashour and his colleagues at the university of Bradford are exploring an entirely different approach, which requires neither heat nor water. In geopolymer concrete, an aluminosilicate-rich material reacts with an alkali chemical solution, hardening into a concrete-like polymer. “Geopolymer needs an alkali activator – sodium hydroxide, calcium hydroxide – and at the same time it needs what we call precursors,” explains Ashour. “We are using construction and demolition waste.” The UK alone produces 66million tonnes of that type of waste every year. Using it in building materials would reduce the need for mining raw materials – thus removing a major polluter. Geopolymer is too early to scale, and there are issues with using activators like sodium hydroxide (otherwise known as caustic soda) on construction sites.

But for Ashour, decarbonising concrete means something more fundamental: changing the way we build. Many architects and engineering firms are now working alternative materials into their designs, including more wood, metal, and glass. Ashour, together with Mustafa Sahmaran, a professor at Hacettepe University in Turkey, has been working on producing LEGO-like concrete blocks. The precast bricks would be reinforced with carbon fibre or reinforced polymer bars, and secured with bolts, rather than cement, making them re-usable at the end of life. “Rather than demolishing you can de-mount the structure and reuse the structural element again,” he explains. “By doing this, you are saving lots of raw materials.” Ashour is still working on ways to scale the approach, and to convince partners in industry.

Any solution to concrete’s problem needs to be global in scope – between them, China and India are responsible for about 63 percent of global production, with demand growing in the Global South. It also needs to be cheap and local in practice. The majority of concrete is made in the country that uses it, and that likely means there is no global silver bullet. “We're going to have to capture CO2, we're going to have to use less cement by a whole variety of means, and we're going to have to have technologies like ours,” Carbicrete’s Stern says. “There's just so much concrete being made that there's no one solution.”

Updated 26.10.2021, 21.20 GMT: This article has been amended to correct the name of the company led by Chris Stern: it is Carbicrete, not CarbonCure. CarbonCure, which also uses recycled CO2 in concrete manufacturing, is the winner of the NRG COSIA Carbon XPRIZE, as previously mentioned in a sentence that has now been removed.

Reaching net zero emissions by 2050 will require innovative solutions at a global scale. In this series, in partnership with the Rolex Perpetual Planet initiative, WIRED highlights individuals and communities working to solve some of our most pressing environmental challenges. It’s produced in partnership with Rolex but all content is editorially independent. Find out more.