Page 61 - Martin Marietta - 2024 Sustainability Report

ENVIRONMENTAL STEWARDSHIP

Recarbonation

Emerging research suggests that a large percentage of the CO originally emitted during cement production can be
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captured over the lifecycle of concrete through the process of “recarbonation”. According to Cembureau, the European
Cement Association, recarbonation is a slow process that occurs in concrete where lime (calcium hydroxide) in the cement
reacts with carbon dioxide from the air and forms calcium carbonate. At the end of their useful life, buildings and
infrastructure (reinforced concrete structures) are demolished. If the concrete is then crushed, its exposed surface area
increases and this further increases the recarbonation rate. The amount of recarbonation is even greater if stockpiles of
crushed concrete are left exposed to the air prior to reuse. 1

In fact, according to a 2021 report of the Intergovernmental Panel on Climate Change (IPCC), around half of the
carbonate emissions from cement production are reabsorbed by the material when used in buildings and infrastructure.
The “cement carbonation sink” absorbs an estimated 200 million tonnes of CO every year, according to section of the
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report published ahead of the COP26 climate conference.

“Direct CO emissions from carbonates in cement production are around four percent of total fossil CO 2 emissions,” says
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in cement
the Sixth Assessment Report from the Intergovernmental Panel on Climate Change. “The uptake of CO 2
infrastructure (carbonation) offsets about one half of the carbonate emissions from current cement production.” 2
Work on improving the capacity of concrete to serve as a carbon sink is on-going with the support of the Portland
Cement Association and others. As an example, researchers at MIT have been researching forced carbonation via localized
increase in the availability of CO during the early stages of cement hydration, and believe it offers great potential as an
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additional CO sequestration mechanism. According to their article, Cementing CO into C-S-H: A step toward
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concrete carbon neutrality, published in the March, 2023 PNAS Nexus Journal from Oxford Press, if this research is
successful, “the theoretical offset of CO amounts to at least 40% of carbon emissions from cement production, not
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including emissions associated with fossil fuels used in the process.” Studies like this one are why the industry is funding
the work of the MIT Concrete Sustainability Hub (CSHub).

1 https://lowcarboneconomy.cembureau.eu/5-parallel-routes/downstream/recarbonation/.
2 IPCC Report — “Climate Change 2021, the Physical Science Basis”, pg. 688.

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