Much of northwest Ohio is a quiet, sparse grid of farms. Right in the middle of it, just 7 miles south of the Michigan border, is Delta Raceway. It calls itself the state’s premier motocross track.

Sean McCauley wasn’t there for dirt bikes anyway. He came for the small rocks that have been spread out to cover the surrounding campground and pathways. And he’s interested in what those rocks are doing for him.

“So far, we’ve removed probably at least 100 tons of CO2 from the atmosphere and hope to remove over 1,000 tons by the time the process is complete,” McCauley said.

McCauley’s background is in geochemistry. And he’s created his own business around these rocks. They’re a human-made byproduct of steel production called slag, which, just by lying here, can capture and store carbon. That’s where McCauley’s company, Alkali Earth, comes in. 

“What we’re doing is accelerating the natural processes that the Earth uses to remove excess carbon dioxide with minerals,” he said.

The company is a middleman, connecting the steel mills that make this slag with the places that want to use it.

Steel is created by dumping reactive ingredients in a giant pot and making it really, really hot.

“When you create steel, you can kind of think of it as like a massive soup,” said Yale University’s Ella Milliken, who studies carbon storage. “The steel slag is essentially like this fat skim on top that you pour off that’s just full of all of the reactive, really good stuff that you don’t want in a steel bar.”

Then all that steel slag goes into a pit to cool. The steel mill crushes it up into little rocks, and they sit there as inconvenient piles of byproduct, frequently destined for a landfill.

Carbon capture is a major piece of the climate change mitigation puzzle. Much of that conversation revolves around tree planting and regenerative agriculture. But rocks capture carbon too. And steel slag happens to be especially good at it.

“The steel slag has calcium in its most reactive form. That calcium is going to rapidly react with CO2 from the atmosphere and eventually convert it to calcium carbonate,” said Noah Planavsky, a geochemist at Yale University who focuses on carbon capture.

The gravel-esque slag rocks around the racetrack are covered with an ashy, brown substance. That’s the calcium carbonate, what the carbon turns into.

“The carbon is removed from the atmosphere and is irreversible on a thousand, even million, year time scale,” Planavsky said.

Natural rocks capture carbon dioxide too. Steel slag does it many times faster. And it’s really cheap. Actually, Sean McCauley of Alkali Earth said steel mills will pay people to take it away.

McCauley and his company also subsidize the steel slag for buyers like Delta Raceway by selling carbon credits to companies that climate-related carbon reduction goals. Businesses that bought carbon credits to help pay for the slag on the Delta Raceway include e-commerce platform Shopify and digital payment firm Stripe.  

So it might be cheap, but steel slag is heavy and annoying to haul.

“Over 1,200 truckloads were delivered to the site. But the good thing is, it’s very close to the steel mill, and so cost and the energy penalty wasn’t that large in this case,” said McCauley.

For now, all of Alkali Earth’s projects are in the Midwest, where the steel mills are. But McCauley plans to scale up his business. And thinks he won’t have a problem doing it.

“Right now, we’re the only ones we know of working with steel slag in this way,” he said.

Now he’s looking to China, India, Japan and Australia for his next expansion.