CaCO3 (limestone) + heat → CaO (lime) + CO2 (carbon dioxide)
Making cement this way requires releasing CO2—the greenhouse gases are basically baked into the process.
One major approach to cutting down on cement’s climate impact is to use less lime. You have to be careful doing this, because you don’t want to end up with cement that’s not as strong or durable as what’s needed. But mixing in a small amount of filler can help cut down on the lime you need to use without compromising performance.
One interesting approach to the mix-in method comes from CarbonCure, which adds some CO2 to concrete as it’s mixing. The CO2 reacts with ingredients in the mixture and hardens, a process called mineralization.
You’re basically doing the opposite of the cement-making process that I described above—adding the CO2 back in. Doing this in a controlled way can help trap some CO2 while also cutting down on how much lime you end up using in the final product. (For more on CO2 mineralization, check out our story from last year on a facility that’s doing a similar process underground).
Last week, CarbonCure announced that it had made cement using CO2 that had been pulled directly out of the atmosphere, a process known as direct air capture. For the demo, CarbonCure added CO2 into some wastewater that otherwise would have been too reactive to reuse.
For the demo, CarbonCure partnered with a California-based startup called Heirloom that captured the CO2, and the companies say it’s the first time CO2 captured from the atmosphere has been used to make cement.
This process is really small-scale right now, and there are a lot of questions about whether direct air capture can be done cheaply and efficiently. But CarbonCure’s approach could help shave off some of the climate pollution that goes into building while also helping to clean up emissions already in the atmosphere.