Concrete Myth Busted: Does Concrete Curing Emit CO₂?

Three Gorges Dam | Doomberg

I was recently catching up on my favorite Substacker Doomberg's insightful piece titled "Five Dams in Tibet," discussing China's colossal Medog hydroelectric project. Scrolling through the comments, I stumbled upon a statement from a reader that immediately caught my attention:

"How the hell is the Hoover Dam emitting CO₂? Mostly by CaCO₃ → CaO + CO₂. Concrete curing is one of the single largest sources of CO₂ emissions on the planet, collectively. And there is so much concrete in the Hoover Dam that it's still not finished curing. Every year, that concrete cures a little bit more and emits a little bit more CO₂ to the atmosphere."

Now, I appreciate the curiosity here, but we've got a concrete myth on our hands that needs busting.

Concrete itself doesn't release CO₂ as it cures. The curing process—known as hydration—is simply cement reacting with water, creating calcium silicate hydrate (C-S-H) and calcium hydroxide. This reaction produces heat but not CO₂.

So, why does concrete have this notorious reputation for CO₂ emissions? The culprit is actually cement, concrete's primary binding ingredient. Cement manufacturing involves heating limestone (calcium carbonate, CaCO₃), causing it to break down into lime (CaO) and CO₂. This critical step occurs at the cement plant, well before the concrete is mixed, placed, and cured on-site:

CaCO₃ → CaO (lime) + CO₂

Here's the key point: Once concrete is placed, it actually absorbs a small amount of CO₂ over its lifespan through a process called carbonation, slowly turning calcium hydroxide back into calcium carbonate.

It's also worth noting the only reason concrete is viewed as a major CO₂ emitter is the sheer amount we use around the globe—just look at massive projects like China's Medog hydroelectric dams. Those five mega-dams have a combined generating capacity of 60 GW, and if they're anything like the Three Gorges Dam (which required around 28 million cubic yards of concrete), they'll use millions of tons of cement. That’s a lot of cement—and that's where the CO₂ footprint really comes from.

Bottom line: Concrete curing itself doesn't emit CO₂—cement production does. And yes, concrete curing continues throughout its life, but this ongoing process does not release CO₂.

But let me be clear: I recognize cement is essential—without it, there is no concrete. Yes, cement production has its "climate-changing CO₂ emissions," but we need to collaborate with our cement brethren to make concrete a sustainable and durable building material. I'll defend the use of cement just as passionately as I defend concrete, but that means demanding high-quality cement because the durability of our concrete depends on it.

As concrete producers and contractors, it's our responsibility to hold cement manufacturers accountable. We need to actively voice both our concerns and approval regarding the cement we're using every day. Only by doing this can we ensure the success of our industry and maintain concrete’s position as the preferred material for building and paving.

Until next time, let's keep it concrete.

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