Carbon Uptake of Concrete Masonry

Concrete materials are known to absorb and sequester atmospheric carbon. Research^1,2,3,4 has shown the carbon uptake, or carbonation, of concrete occurs during its service life and after its demolition, and in its repurposing as recycled aggregate. Combined, concrete can absorb more than 50% of its carbon impact, equal to the CO₂ released by its cement's calcination process during manufacturing.

Correctly applying this attribute in life cycle accounting would show the actual impact of concrete is substantially less than has been commonly reported. The Global Carbon Project, 11 November 2022⁵, reports global cement emissions for 2021 at 1.67 GtCO₂, but also reports "cement carbonation" at -0.84 GtCO₂, reducing the net cement emissions to 0.83 GtCO₂.

So, while typical narratives repeat the "cement is 8% of global emissions" mantra, the reality is 4.5%, and accounting for carbonation reduces cement's global percentage to 2.2%. The U.S. portion is 0.11%. For more details, see Timber and Concrete Emissions: The Inequalities in GHG Reporting.

Concrete masonry units (CMU), with its various textures, has carbon absorption rates much higher than that of wet-cast concrete. CMU is installed using mortar, which has also been shown to have high carbon uptake rates.

Carbon Sequestration Rates and Practical Application

A 2022 ASTM paper, Conceptual Test Protocols for Measuring Carbon Sequestration of Manufactured Dry-Cast Concrete Products⁶, investigated the carbon sequestration of CMU in just the weeks following manufacture. Additional tests were conducted at the 2-year mark after production⁷.

Test at	Average CO₂ uptake (lb/cu ft)	LCA Module
4 weeks	1.25	A1-A3
2 years	2.93	B1

To put the 2-year value in perspective, convert the 2.93 lb/cu ft into a wall area context at square feet:

0.79 lbs.	of absorbed CO₂ per typical net volume of an 8x8x16, which equates to:
0.89 lbs.	of absorbed CO₂ per square foot of wall
100,000 sq. ft.	example wall area of a project
89,124 lbs.	*of absorbed/sequestered CO₂ in the first 2-years* after CMU manufacture**

Note that with CMU's very low cement content, relative to wet-cast concrete, the more than half-pound per unit offset of impact is considerable. Especially when you consider at 2-years, its lifespan is just beginning.

Given the known science and numerous studies of concrete in general, there is every expectation the cumulative carbon uptake amount will continue to increase through the CMU's entire lifecycle.

This methodology⁶ for measuring carbon sequestration has since become widely accepted and has had a significant impact in reducing GWP values for CMU. Visit CarbonKind™ Low Carbon CMU to see how this has impacted Angelus Block's EPDs.

Applicability to LEED

In addition to concrete masonry's contributions to building reuse, reduced GWP values in light of new data, and the effects of sequestration via carbon uptake contribute to whole building LCA assessment. See Building Life-Cycle Impact Reduction for more information.

There's More to Concrete Masonry

The inherent benefits of CMU are substantial, yet Angelus Block has lowered CMU-related CO₂ even further with CarbonKind™ Low Carbon CMU.

The tremendous improvements offered with CarbonKind CMU provide a potent element to an optimized concrete masonry wall utilizing high SCM masonry grout. They're a powerful carbon-cutting combo. See more:
Concrete Masonry: An Optimized Low Carbon Wall
Masonry Grout: Cement Replacement Studies

¹ How concrete can become a carbon sink https://www.ri.se/en/our-stories/how-concrete-can-become-a-carbon-sink

² Concrete as a Carbon Sink? The popular building material might not be as detrimental to the environment as once believed. https://www.architectmagazine.com/technology/concrete-as-a-carbon-sink_o

³ Guo, R., Wang, J., Bing, L., Tong, D., Ciais, P., Davis, S. J., Andrew, R. M., Xi, F., and Liu, Z.: Global CO₂ uptake by cement from 1930 to 2019, Earth Syst. Sci. Data, 13, 1791-1805, https://doi.org/10.5194/essd-13-1791-2021, 2021

⁴ 3 Questions: Leveraging carbon uptake to lower concrete's carbon footprint MIT News, https://news.mit.edu/2023/3-questions-hessam-azarijafari-leveraging-carbon-uptake-lower-concretes-carbon-footprint-0331

⁵ Global Carbon Budget 2022, Global Carbon Project, 11 November 2022. https://globalcarbonbudget.org/wp-content/uploads/GCP_CarbonBudget_2022_slides_v1.0.pdf, and supporting data at https://tinyurl.com/GCB22figs

⁶ C. Walloch, L. Powers, D. Broton, and J. Thompson, "Conceptual Test Protocols for Measuring Carbon Sequestration of Manufactured Dry-Cast Concrete Products," in Masonry 2022: Advancing Masonry Technology, ed. B. Trimble and J. Farny (West Conshohocken, PA: ASTM International, 2022), 59-86. http://doi.org/10.1520/STP164020210112

⁷ Supplemental Information - Concrete Masonry & Hardscapes Association - Industry Average Environmental Product Declaration