CaCO3 (limestone) + heat → CaO (lime) + CO2 (carbon dioxide)
Making cement in this way requires CO2 emissions – greenhouse gases are mostly baked into the process.
Decisions
One of the main approaches to reducing the climate impact of cement is to use less lime. You have to be careful when doing this because you don’t want to end up with cement that isn’t as strong and durable as it needs to be. But mixing in a small amount of filler can help reduce the lime that needs to be used without compromising performance.
One interesting approach to the mixing method comes from CarbonCure, which adds some CO2 to the concrete as it is mixed. The CO2 reacts with the ingredients in the mixture and hardens, a process called mineralization.
You’re basically doing the opposite of the cement making process I described above – adding CO2 back. Doing this in a controlled manner can help trap some of the CO2 and also reduce the amount of lime you end up using. product. (For more on CO2 mineralization, check out our story from last year about a facility that conducts a similar process underground).
Last week, CarbonCure announced that it had produced cement using CO2 that was drawn directly from the atmosphere, a process known as direct air capture. For the demonstration, CarbonCure added CO2 to some wastewater that would otherwise be too reactive to reuse.
For the demonstration, CarbonCure is partnering with a California startup called A relic that capture CO2, and the companies say it’s the first time CO2 captured from the atmosphere has been used to make cement.
Right now, the process is really small-scale, and there are many questions about whether direct air capture can be done cheaply and efficiently. But CarbonCure’s approach can help get rid of some of the climate pollution that gets into buildings, while also helping to clean up emissions that are already in the atmosphere.
