This concrete could eat carbon emissions

solid: on this Together like rock we have built our church – and our homes, roads, bridges, skyscrapers and factories. As a species we consume over 4.1 billion tons of material each year, more than any other material except water. (You’re almost certainly sitting or standing on it at this point.) This is a problem, because concrete – and especially cement, the major component of concrete – destructive to the environment, The cement industry alone generates 2.8 billion tonnes of CO2 every year, more than any country except China and the US – andsomewhere in the middle Between four and eight percent of all global man-made carbon emissions.

According to the Paris Agreement, the world needs at least a 16 percent decline in carbon emissions from cement production by 2030 to reach its goal of keeping global warming within the 1.5C limit and well below 2C. (Currently, those emissions are actually increasing, driven largely by mega-construction projects in China.) Now, the concrete industry is in a race against time to solve a very difficult, very gray problem.

The recipe for concrete has remained largely unchanged since the 19th century: You just need a mixture of larger aggregate (stone), smaller aggregate (such as sand), cement – ​​which binds it together – and water. “The main issue with concrete is the production of cement, because if you want to get cement, you have to have clinker,” explains Ashraf Ashour, professor of structural engineering at the University of Bradford. Clinker, usually a mixture of calcium carbonate, clay and gypsum (although many other ingredients can be added) is mixed and heated in a kiln. “You need to heat the clinker to a very high temperature, maybe 1500 degrees, and by doing that, you’re creating a lot of CO2 emissions,” says Ashour. Inside the kiln, the clinker undergoes calcination: calcium carbonate breaks down into calcium oxide, releasing more CO2.

One way to decarbonize concrete is to replace cement with other materials, such as fly and bottom ash created by coal power stations, or blast-furnace slag created in iron production. Cement manufacturers have been blending in waste aggregate for years, but with coal plant closures disrupting supplies, many companies are now looking for alternatives. Canada-based Carbicrete replaces cement with steel slag, a byproduct of steel manufacturing. “250 million tons of it is produced every year,” explains Chris Stern, CEO of Carbicrete. “For years, steel slag has been used basically for road filling. Some goes into the streets, small pieces go into landfills, it is sometimes used in fertilizer, but the rate of use is not very high.

Once the concrete is mixed, it has to harden, or “cure.” Traditional concrete is cured with water, a process that takes 28 days. (When you see workers engaged in work like watering a freshly laid foundation, it is curing.) However, Carbicrete’s concrete cures with carbon dioxide. CO2 from industrial processes is injected into concrete, which reacts to form calcium carbonate or limestone. “Right now, we are a carbon negative company,” Stern says. “In fact, the true marginal cost of possession is zero, because we can sell our product. “That’s what makes[concrete]such an interesting product.”

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

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