Global emissions of carbon dioxide from the combustion of coal, oil, and natural gas are trapping heat in the atmosphere, slowly warming the Earth’s surface temperature, scientists report. Also implicated in this process is concrete, one of the world’s most basic building materials: concrete requires major inputs of fossil fuel energy to manufacture, and the chemistry of cement making itself actually releases carbon dioxide.
But concrete is also a “carbon sink” — once it’s poured, concrete starts to re-absorb carbon dioxide and continues to pull CO2 in over decades of service life (and even after it’s demolished). This month, University of California at Irvine researchers put a number on that process (see UCI press release: “Concrete jungle functions as carbon sink, UCI and other researchers find”). “It sounds counterintuitive, but it’s true,” said Steven Davis, associate professor of Earth system science at the University of California (Irvine). “The cement poured around the world since 1930 has taken up a substantial portion of the CO2 released when it was initially produced.”
“Cement manufacturing is considered doubly carbon-intensive because emissions come from two sources. CO2 molecules are released into the air when limestone (calcium carbonate) is converted to lime (calcium oxide), the key ingredient in cement. And to generate the heat necessary to break up limestone, factories also burn large quantities of natural gas, coal and other fossil fuels,” the UCI release explains. “Davis and his fellow researchers looked at the problem from a different angle. They investigated how much of the gas is removed from the environment over time by buildings, roads and other kinds of infrastructure. Through a process called carbonation, CO2 is drawn into the pores of cement-based materials, such as concrete and mortar. This starts at the surface and moves progressively inward, pulling in more and more carbon dioxide as years pass.” Forty-three percent of the carbon emitted in cement manufacture so far has been removed from the atmosphere by concrete in place, the study authors estimate (although that figure does not include the fossil fuel emissions from the manufacturing process).
Future versions of concrete could take this whole idea several steps further. Researchers and some commercial companies are working on cement formulations that actually use carbon dioxide as a raw material, recapturing power plant emissions at the source and turning the offending gas into cement even as power is produced. One company, Calera, made headlines in 2008 with its efforts to scale up the process, as Scientific American reported (“Cement from CO2: A Concrete Cure for Global Warming?” by David Biello).
The 2008 recession hit the green-concrete sector hard, according to a 2015 report from MIT Technology Review (see: “What Happened to Green Concrete?” by Kristin Majcher). But Calera is still developing its carbon-negative alternative, according to the report (and the company’s website) — looking to bring its carbon-based cement to market not for use in poured concrete, but as cement siding boards. Other researchers are also working on the problem of trapping CO2 to make cement; in future years, as technology advances, the concrete industry could conceivably flip to a net remover of the greenhouse gas from the planet’s atmosphere.
