Concrete’s large carbon footprint—that is, the amount of carbon dioxide emitted during the cement manufacturing process—is estimated to be 5% of industrial CO2 emissions, a source of concern in the battle against human-caused climate change. But last month, an international research team reported that substantial quantities of CO2 are reabsorbed, or sequestered, by cement-based products over their life cycle. Concrete, in fact, acts as a carbon sink. And that makes it more attractive as a material choice when it comes to battling global warming and limiting greenhouse gasses.
The study, published in Nature Geoscience, a monthly, peer-reviewed scientific journal, was led by Fengming Xi of the Chinese Academy of Sciences, with researchers from the California Institute of Technology, the University of California Irvine and the United Kingdom’s University of East Anglia. “We are not the first to observe that cement absorbs CO2,” says Steven J. Davis, UCI associate professor of earth system science, who participated in the study. But “no one has ever bothered to tally up how much carbon is being absorbed in all of the concrete in the world. That’s what we’ve added.”
To make cement, the manufacturer bakes limestone (calcium carbonate) at temperatures above 1,500° C. The process releases CO2, leaving behind calcium oxide, or lime, the basic material of cement. These are called “process emissions.” Another source of CO2 is the combustion emissions from fossil fuels—coal or natural gas, mostly—used to generate the process heat.
The most recent survey of Portland Cement Association members found an average of 927 kilograms of CO2 are emitted for every 1,000 kg of portland cement produced in the U.S. The study Davis participated in estimated that 76.2 billion tonnes of cement were produced globally between 1930 and 2013, as well as the material’s exposure and its treatment during demolition and disposal. Next, the researchers developed an estimate of total carbon uptake, Davis says.
Davis says the process-to-combustion emissions ratio is about 50:50. He adds, “At least since 1930, of all the process emissions, about 43% of it has been reabsorbed”—that is, 43% of the emissions from the limestone process, excluding the combustion emissions. “Over the lifetime of that material, just naturally it absorbs again from the air some of the CO2. The reaction essentially re-forms limestone.”
Structural engineers have known about this reaction “for a very long time, mostly because the process of the cement absorbing the CO2 affects the structural integrity of cement structures,” Davis says. “It can change the pH of the concrete and allow the reinforcing steel to oxidize more rapidly and corrode.”
Computer models of climate change are not likely to change because of this research, Davis says. The models are based on what is actually known about atmospheric concentrations of CO2. At most, this research may lead to a reassessment of the sources of CO2, instead of leading to a revision of the projections of climate change’s development.
“No one has ever looked at this cement sink before,” Davis says. “Mostly, the sinks that are looked at are trees and the oceans that absorb a lot of CO2. What this finding does suggest is that we may need to revisit that carbon-budget science to figure out where we were … overestimating [carbon] uptake because we weren’t considering this sink that apparently exists.”