A trio of companies says they’ve taken carbon dioxide out of the atmosphere and used it to make concrete for construction projects across California’s Bay Area.

This is the first time this has been done anywhere in the world, according to the announcement Friday. And if the effort takes off, there’s potential for it to become a useful strategy for slashing the tremendous amount of greenhouse gas emissions that come from concrete.

Cleaning up concrete’s mess has become one of the trickiest climate challenges and a magnet for funding from Big Tech. Concrete is responsible for about three times as much carbon dioxide pollution as aviation. And it’s no wonder the stuff has a huge impact on climate change, given that it’s the most consumed material on Earth aside from water.

The announcement marks the success of a very small demonstration project, so it’s not something that’s likely to have much impact yet. The hope is that eventually, taking carbon dioxide out of the air and putting it into concrete will help slow climate change in two ways. First, it draws down CO2 emissions humans have already released and traps greenhouse gas where it can no longer heat up the planet. The second goal is to make concrete in a way that doesn’t produce vastly more CO2 pollution.

California-based startup Heirloom was responsible for the first step in that process. Its technology uses calcium oxide from limestone, which acts sort of like a sponge to draw carbon dioxide out of the ambient air. So far, Heirloom is only able to do this at a small scale with a demonstration plant in Brisbane, California. But it already counts Microsoft, Stripe, and Shopify as its customers — companies that have been early enthusiasts for this prospective climate fix called “direct air capture.” Bill Gates’ investment fund, Breakthrough Energy Ventures, has also funded Heirloom and its partner on this project, CarbonCure Technologies.

Founded in 2012, CarbonCure has already been using CO2 leftover from industrial processes to make concrete. Some of the concrete has even gone into building Amazon’s second headquarters in Arlington, Virginia. Most of the carbon dioxide the company uses comes from ethanol and ammonia production; this is the first time CarbonCure has been able to use CO2 taken out of the air. That brings the added benefit of canceling out some of the pollution human activity has created in the past. And once the carbon dioxide is trapped in the concrete, CarbonCure expects it to stay there for potentially thousands of years — even if a building it goes into is demolished one day.

The process might also reduce future emissions. For the demonstration project with Heirloom, CarbonCure injected the CO2 into wastewater so that it could be reused to make new concrete. Normally, the wastewater would be thrown out because it’s too reactive to be used again. But that wastewater has some leftover cement in it — the main ingredient in concrete. Cement is particularly polluting because it’s typically heated at very high temperatures, which uses up a lot of energy. On top of that, a chemical reaction in the manufacturing process for cement generates additional CO2 emissions.

So the key to making concrete in a way that’s less damaging to the climate is to use less cement in the mix. Injecting CO2 into the wastewater makes it less reactive and rescues the leftover cement in it. Then, when you reuse that water, you also reuse that cement. That way, you don’t need to put as much fresh cement into the concrete mixture. That’s exactly what the third company in this demonstration project, Central Concrete Supply Company, did. It used the treated wastewater to make concrete at its plant in San Jose, California.

It’s probably still too soon to declare a climate victory with this project, though. It was a very small test run using just under 38 kilograms of captured CO2, roughly equivalent to the emissions from driving a car about 94 miles. And there are no exact measurements yet of how many more emissions might have been avoided by reusing the CO2-treated wastewater.

“There should be more analysis [to better assess the sustainability claims] — and for that, we need data that’s publicly available,” says Dwarak Ravikumar, an assistant professor of civil and environmental engineering at the University of Waterloo. That’s important to make sure that there aren’t any steps in the process that negate some of the potential climate benefits. Capturing carbon dioxide can be energy intensive, for instance, although Heirloom says its technology runs on renewable energy.

The companies say they hope to scale up. And new climate policies have given these kinds of technologies a boost with tax credits and federal funding. The Inflation Reduction Act is pumping billions of federal dollars into developing direct air capture. And all that captured carbon is going to need to be stored somewhere. Underground wells are one option, but that route can take a lengthy permitting process.

“Concrete is ready today,” says Heirloom CEO Shashank Samala. “It is everywhere. It is the world’s most abundant commodity and it is a fundamental block of our lives.”