This concrete can eat carbon emissions

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.