News Brief

Can Concrete Become Carbon Neutral?

Carbon-neutral concrete by 2050 is the industry’s new goal. But there are lots of hoops to jump through to make up for concrete’s basic chemistry.

 A mid-rise building with floor plates and structural columns made out of concrete.

Concrete is a one of the most widely used building materials, so even small improvements can make a big difference for CO2 emissions.

Photo: Concrete Forms. License: CC BY 2.0.
The Global Cement and Concrete Association (GCCA) recently released a “carbon ambition statement” announcing that its 40 members aspire to deliver carbon-neutral concrete by 2050. A laudable goal, but given concrete’s chemistry, is it even possible?

One problem is with cement. The majority of cement’s CO2 emissions (around 60%) come from the materials themselves, according to GCCA. Limestone emits CO2 when it is heated to produce Portland cement clinker. And there’s currently no alternative to limestone—just various ways of using less clinker by adding more additives like slag, fly ash, calcined clays, etc.

The remaining 40% of CO2 emissions mostly come from the fuel needed to heat the limestone. GCCA notes that conventional fossil fuels burned for this purpose could be replaced by alternative fuels—in fact, the rate of replacement is already 5.6% globally. But lest you think all these cement manufacturing facilities are running on solar and wind power, the replacements are typically highly combustible byproducts of industrial processes, the most common being petcoke, sewage sludge, and bone-meal, according to IntechOpen. Renewable electricity sources would more likely be used to replace indirect emissions.

In addition to relying on alternative fuels, GCCA says that it will achieve carbon-neutral concrete by enhancing recarbonation and deploying carbon-capture technologies. Carbon-capture technologies do currently exist (see CarbonCure-Capturing Carbon in Concrete Blocks) and there’s hope for bringing them to scale. As for recarbonation, concrete carbonizes as it dries, absorbing CO2 from the air, but once the surface hardens, the concrete becomes less permeable, limiting more re-absorption. By its own measure, GCCA says this mechanism currently sequesters 25% of the CO2 emissions released during clinker processing. More could be sequestered if the concrete is crushed, stockpiled, and “left exposed to the air before reuse,“ according to GCCA. But waiting for a concrete structure to be demolished and sit exposed to the air is a long time to wait for recarbonization given the time value of carbon.

GCCA has set a high goal, but at least its ambition statement acknowledges that innovation and ingenuity will be required. Furthermore, the statement’s policy framework and overview on technologies clearly articulates which levers the concrete industry has available to it today, offering a clear path to make progress.

Published November 9, 2020

Pearson, C. (2020, October 22). Can Concrete Become Carbon Neutral?. Retrieved from https://www.buildinggreen.com/newsbrief/can-concrete-become-carbon-neutral

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Comments

November 9, 2020 - 10:04 pm

Keep in mind clinker replacements such as fly ash often contain heavy metals that are hazardous to human health.- Tori Wickard, Perkins&Will

November 10, 2020 - 10:31 am

I agree with Tori's comment. We should be very wary of putting coal fly ash in building materials given how toxic it is.