The climate solution under your feet

CaCO3 (limestone) + heat → CaO (lime) + CO2 (carbon dioxide)

Making cement this way requires releasing CO2 – greenhouse gases are essentially baked into the process.

Solutions

A major approach to reducing the climate impact of cement is to use less lime. You have to be careful doing this because you don’t want to end up with cement that isn’t as strong or durable as you need it to be. But mixing a small amount of filler can help reduce the lime you need without compromising performance.

An interesting take on the mixing method comes from CarbonCure, which adds CO2 to concrete as it is mixed. The CO2 reacts with the ingredients in the mixture and hardens, a process called mineralization.

You are essentially doing the opposite of the cement making process I described above – adding CO2 back. Doing this in a controlled way can help trap CO2 while reducing the amount of lime you end up using in the end. product. (For more on CO2 mineralization, see our story from last year about a facility that performs a similar process underground).

Last week, CarbonCure announced that it has made cement from CO2 extracted directly from the atmosphere, a process known as direct air capture. For the demonstration, CarbonCure added CO2 to some wastewater that otherwise would have been too reactive to reuse.

For the demonstration, CarbonCure partnered with a Californian startup called Legacy which captured the CO2, and the companies say this is 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 economically and efficiently. But CarbonCure’s approach could help remove some of the climate pollution that goes into construction while helping to clean up emissions already in the atmosphere.