Cement emits as much CO2 as India. Why is it so hard to fix?

- ADVERTISEMENT -

Let’s do a quiz: After water, what is the most-used material in the world? Is it steel? Wood? Aluminum? Plastic?

The answer, it turns out, is concrete. Concrete, a blend of water, sand, gravel and cement, has helped build everything from the Roman Pantheon to the Burj Khalifa in Dubai. We walk on concrete, drive on it, live in it. According to one estimate, more of the planet’s mass is locked up in concrete than it is all of the planet’s trees, bushes and shrubs.

And making all that concrete also emits an enormous amount of carbon dioxide.

This is thanks to cement, effectively the chemical “glue” that holds all the components of concrete together. “Concrete is the cake, and cement is the flour,” said Randolph Kirchain, the co-director of the MIT Concrete Sustainability Hub. Although cement is only about 10 percent of concrete by mass, it makes up the lion’s share of concrete’s carbon emissions.

The most common type of cement in the world is Portland cement, which is made from limestone. (It is named after the British Isle of Portland, not the city in Oregon.) Baked in a giant kiln at very high temperatures – think around 2,700 degrees Fahrenheit – limestone transforms into carbon dioxide and calcium oxide. That calcium oxide, or lime, is a key ingredient in the cement that holds our bridges, apartment buildings and roads together.

That process produces a lot of greenhouse gas emissions. Not only does the carbon dioxide from the reaction spill into the atmosphere, but cement producers also use huge amounts of coal or natural gas to heat the kiln.

As a result, the cement industry generates 8 percent of global carbon emissions – triple the emissions of the aviation industry.

“You make about a ton of CO2 for every ton of cement,” said Eric Toone, technical lead at Breakthrough Energy Ventures, which invests in emerging low-carbon cement technologies.

Some producers are slowly trying to implement solutions. One option is simply making concrete with less cement. Hessam AzariJafari, the deputy director of the MIT Concrete Sustainability Hub, says producers can substitute fly ash – a product of coal combustion – or waste from steel production instead of limestone. These materials can also help bind concrete together without the need for copious amounts of Portland cement. Today, around 11 to 23 percent of concrete’s “binder” is made from these other materials; experts estimate that in future, it could reach 50 or even 80 percent.

Other, more radical changes are also possible. Some start-ups are experimenting with a completely new chemistry for cement that doesn’t include calcium carbonate. The California-based start-up Brimstone is working on a recipe that involves calcium silicate instead; the resulting chemical reaction doesn’t release carbon dioxide and can be cooked at a lower kiln temperature. Other companies, including Canadian start-up CarbonCure, are working on injecting and storing CO2 into concrete itself.

But progress is slow. Concrete is a crucial component in modern construction; changing its ingredients poses not only an engineering problem but also a regulatory and safety problem. When it comes to innovating in the concrete industry, “everybody wants to be third,” Toone said. In the United States, many individual states have their own specifications for how concrete should be made and with what materials; changing the ingredients of concrete will mean a painful regulatory overhaul.

Those standards exist for good reasons. “Many of these things are safety-critical,” Kirchain said. But to clean up one of the most carbon-polluting industries in the world, some of the rules will have to change.

That’s partly why many previous attempts to green concrete have failed: Promising start-ups have run out of money or failed to find a market for their products.

Kirchain says there is a way to cut carbon emissions significantly and ensure safety at the same time. And that will be critical to the coming transition. “The things that concrete goes into are things that we need to last,” he said.