According to a new study, cement is thought to produce 5% of all global greenhouse gas emissions from fossil fuels and factories. But this building block of modern civilization may eventually suck some of that carbon dioxide (CO2) back up—enough to cancel nearly a quarter of the gases released making cement, according to a new study.
To make cement, limestone (calcium carbonate) is turned into lime (calcium oxide) by baking it at temperatures topping 1000°C (1830°F). That conversion releases copious amounts of CO2—half of cement’s total greenhouse gases. The other half comes from fossil fuels used to heat cement kilns.
But there’s a silver lining: The mortar, concrete, and rubble from demolished buildings can gradually absorb CO2 through a process called carbonation. As CO2 from the air enters tiny pores in the cement, it encounters a variety of chemicals and water trapped there. The ensuing reactions convert the CO2 into other chemicals, including water. Still, just how much CO2 of the world’s cement soaked up had never been estimated …
So, Zhu Liu, [Now a] Physicist-California Institute of Technology in Pasadena teamed up with a group of Chinese scientists and researchers, along with Steve Davis, Earth Systems Scientist-University of California, Irvine, and other U.S. and European researchers set out to estimate just how much. The team compiled data from studies of how cement is used around the world and construction sites around China—the world’s largest producer of cement—to get more accurate estimates of a variety of factors that influence how much CO2 the cement absorbs. That included everything from the size range of concrete rubble and how long it was left in the open air, to how much cement was used in thick concrete versus thin layers of mortar spread on walls, where it’s exposed more readily to CO2.
At the laboratory, they calculated the carbonation rate in mortar and concrete in different settings:
• buried
• in the open air
• enclosed in a room
The results cast a different light on the cumulative impact cement has on the climate. As reported online in Nature Geoscience, the researchers estimate that between 1930 and 2013, cement has soaked up 4.5 gigatons of carbon or more than 16 gigatons of CO2. That’s more than 20% of the carbon soaked up by forests in recent decades!
Rob Jackson, Earth Systems Scientist, Stanford University-Palo Alto, California, and Chairman of the Global Carbon Project (a consortium of researchers that tracks the planet’s carbon), said, “The findings don’t represent a dramatic change in the overall picture of greenhouse gas emissions but, it adds another piece of information to the part of carbon models that is particularly prone to uncertainty—how much carbon is soaked up on land.”
Because cement effectively cancels part of its impact over time, the results might also help guide strategies for reducing its carbon footprint. Bigger gains could come from shifting away from fossil fuels to make the cement, Davis says. “If you have a choice—reduce fossil emissions or reduce cement emissions—you should prefer the fossil ones right now. It’s conceivable, that in the future, cement could even suck up more CO2 than it produces.”
To read the entire article, please go to: http://www.sciencemag.org/news/2016/11/cement-soaks-greenhouse-gases.
Home page photo: Cement-based materials, like the concrete used to build parts of Russia’s Ural Highway
from Moscow to the Ural Mountains, gradually absorb large amounts of carbon dioxide from the atmosphere.