STORY: A California startup that uses rocks to soak up carbon dioxide from the air has teamed up with a Canadian company to mineralize the greenhouse gas in concrete. "Concrete is an amazing way to store CO2 because it mineralizes the CO2, it turns CO2 into a carbonate, into a rock."The technological tie-up is a first of its kindand companies say it could provide a model for fighting climate change.U.N. scientists say that removing billions of tons of carbon dioxide from the atmosphere will be necessary to limit climate change. To do that, two things are needed. First, capturing CO2 with nature or technology, and second, locking it up for centuries.California-based Heirloom Carbon Technologies and Canada's CarbonCure are doing just that. Heirloom CEO Shashank Samala explains:“Heirloom uses something called limestone to pull carbon out of the atmosphere. // And limestone has this natural ability to pull carbon out of the atmosphere. The problem is it's just slow. So what we do here is just give it more superpowers to make it pull carbon much, much, much faster than it otherwise would. So what you're looking at here is we are taking limestone and spreading them out on trays and vertically stacking those trays and exposing it to the air. As the CO2 is brought in by the air, this limestone pulls up that carbon and sequesters it in the mineral.”Technology company CarbonCure then mixes the captured CO2 with concrete ingredients, turning it into a mineral that strengthens the building material.The process cuts the need for cement — the part of concrete with the biggest carbon footprint.Here's CarbonCure CEO Rob Niven.“So CO2 is actually a very powerful chemical when working with concrete. What you can do is you can actually react it with concrete to create a calcium carbonate nanomaterial. So we're turning CO2, this greenhouse gas, into a solid, a mineral format that can never be released again for thousands and thousands of years. But what's really key here is it also provides a performance benefit for that concrete. It gives it higher strength, which allows concrete producers to be able to optimize their production so they may require less cement. So they're able to gain these cost efficiencies by using CO2 in the right way, that allows them to eliminate that green premium.”But capturing and locking down carbon on a global scale will not be easy.Companies will have to build expensive, massive plants capable of capturing millions or billions of tons a year.The price of carbon also needs to fall. The U.S. government and industry broadly see $100-a-ton carbon dioxide as a reasonable price.Heirloom charges around $1,000 at present.Samala expects to be at $100 by the time his projects are soaking up millions of tons a year.Concrete itself is also controversial.The building material accounts for about 8% of global emissions of CO2. But for now, concrete's ubiquity is attractive, because there are currently few places to securely store carbon dioxide.“Carbon stays in the air for over a thousand years. So we need to find ways to permanently sequester it for thousands of years. There's not that many ways to do it. One is underground storage that we can safely and permanently store CO2 underground. The other is building materials like concrete."“Like many other folks, I want to wake up in the morning and know that we have an exciting future to look forward to, that my kids and grandkids and the future of humanity can look forward to. And, you know, some years ago, I realized that climate change is the biggest existential threat to humanity. And we need to find every which way to play our part in reducing emissions."