If you’re making a building, you’re probably going to use cement—and that means contributing to global warming. But new research suggests it may not be so bad in the long run.
Creating cement is essentially a matter of superheating limestone and clays in a kiln until the mixture transforms and coughs out carbon dioxide. That actually makes it a double-barreled greenhouse gas emitter: both the process itself and the fuel burned to heat the kiln emit loads of carbon dioxide.
And humanity loves its cement—last year we made about 4.1 billion tons of it for use in concrete and mortar. The emissions that went with that production were good for about 5 percent of all the man-made carbon pumped into the atmosphere.
But a new study in Nature Geoscience says that once a building is built (and even after it’s been torn down) the mortar, concrete, or rubble soaks back up a fair amount of carbon dioxide through chemical reactions with air and water. In all, the study suggests that this carbon sponge effect may account for as much as 43 percent of what was emitted in the first place.
That’s not going to get cement off the hook for contributing to global warming, of course. But a write-up about the study in Science says that the finding could inform how we go about reducing the cement industry’s carbon footprint. We don’t need to worry quite so much about the materials that go into cement, it suggests, since it turns out that they gather back up much of the carbon they emit. Instead, it would be better to focus on figuring out how to heat cement kilns with something other than fossil fuels.