Someday we may be storing carbon in sidewalks

There is a big climate problem right under your feet.

Manufacturing the cement that goes into sidewalks, driveways and other structures accounts for about 8 percent of global carbon emissions, according to one estimate.

Now two companies want to suck carbon from the air and store it in concrete. In a test last week in the Bay Area, they demonstrated this can be done by injecting CO2 into a freshly poured batch of concrete, a first for the industry, they said.

The companies, Heirloom Carbon Technologies and CarbonCure, only used a few canisters’ worth of CO2 — about 37 kilograms. But the process, if scaled up, could help answer the vexing question of where to safely store carbon and also reduce the cement industry’s carbon footprint.

“In the broader carbon-removal ecosystem, this is meaningful,” said Anu Khan, an expert at the environmental group Carbon180. Finding permanent storage for captured carbon, she added, is “a major bottleneck.”

How to get CO2 from the air and into concrete

The firms’ process starts with a bunch of rocks in a warehouse outside San Francisco.

Stacks of trays at Heirloom’s headquarters in Brisbane, Calif., hold crushed limestone. The rocks have been treated to starve them of carbon.

As air passes over the minerals, they soak up ambient carbon dioxide like a sponge. Heating the rocks a few days later wrings out the gas, which is collected and stored in tanks. The carbon-depleted rocks can be used again to collect more CO2, repeating the process.

Last week, Heirloom trucked canisters of carbon dioxide directly captured from the air to a plant in San Jose run by Central Concrete.

Using a process developed by the Canadian company CarbonCure, concrete makers injected that CO2 into recycled water, which was used to make fresh concrete.

Rob Niven, chief executive of CarbonCure, called the amount of CO2 locked away “a drop in the bucket,” but he added that the experiment showed “the technical viability” of sequestering carbon from the air in concrete.

“The entire carbon-removal community was waiting for this day because we knew that it could happen,” said Julio Friedmann, chief scientist at Carbon Direct, which advises companies on carbon management. (Carbon Direct’s investment fund has invested in both Heirloom and CarbonCure.)

According to CarbonCure, when carbon binds with the rest of the material, it makes the concrete stronger. And without concrete acting as a vault, carbon captured from the air needs to be stored underground in highly regulated wells.

“Could you store all the carbon that we need to capture by putting it in concrete? No, you couldn’t,” said Eric Toone, a technical lead at Breakthrough

Energy Ventures, which has also invested in both Heirloom and CarbonCure.

“Could you store a significant amount of carbon that we need to store in concrete? Yes,” he continued.

Bill Gates founded Breakthrough Energy Ventures. Jeff Bezos, owner of The Washington Post, is an investor.

A long, hard road

Heating the kilns that make cement takes an enormous amount of energy. And much of it comes from fossil fuels that release greenhouse gases when burned.

The issues don’t end there. Those kilns break down calcium carbonate into two components - calcium oxide and carbon dioxide. The calcium oxide is a crucial ingredient in cement. The CO2, meanwhile, is a climate-warming byproduct released into the air.

It will take a lot more work to make concrete that locks away more carbon than it emits. “This is a multi-decade effort,” Heirloom chief executive Shashank Samala said.

New policies may help. Last month, for instance, New Jersey Gov. Phil Murphy (D) signed a law giving a financial incentive to manufacturers for delivering low-carbon concrete for statefunded construction projects. And the federal Inflation Reduction Act passed last year boosted tax breaks for directly capturing carbon from the air.

To meet the world’s goal under the Paris climate agreement, U.N. scientists say these changes won’t be enough to reduce current emissions. The ability to capture and store carbon already in the air, they say, needs to be rapidly ramped up.

For the broader public, baby steps like the recent demo also help make the idea of removing carbon from the air more, well, concrete.

“Demonstrations like this also help make carbon removal ‘real’ for people: concrete, unlike carbon, is something you can see and touch, so this can help build trust in the sector,” Carbon180’s Khan said in an email.