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Concrete jungle acts as carbon sink over time new research suggests

According to international study published in Nature Geoscience, cement-based materials eventually reabsorb much of the carbon dioxide (CO2) released during its creation through a process called "carbonation".

An international team of researchers has found that over time, cement reabsorbs a significant portion of the CO2 emitted when the material was manufactured (photo courtesy Steven Davis / UCI).

An international team of researchers has found that over time, cement reabsorbs a significant portion of the CO2 emitted when the material was manufactured (photo courtesy Steven Davis / UCI).

Cement manufacturing is among the most carbon-intensive industrial processes, but an international team of researchers has found that over time, the widely used building material reabsorbs much of the carbon dioxide (CO2) emitted when it was made.

– It sounds counterintuitive, but it’s true. The cement poured around the world since 1930 has taken up a substantial portion of the CO2 released when it was initially produced, said Steven Davis, Associate Professor of Earth System Science at the University of California, Irvine (UCI) and one of the authors of the international study.

For a study published today in Nature Geoscience, Davis and colleagues from China, Europe and other US institutions tallied the emissions from cement manufacturing and compared them to the amount of CO2 reabsorbed by the material over its complete life cycle, which includes normal use, disposal and recycling.

They found that “cement is a large, overlooked and growing net sink” around the world – “sink” meaning a feature such as a forest or ocean that takes carbon dioxide out of the atmosphere and permanently tucks it away so that it can no longer contribute to climate change.

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.

Fossil fuels the leading culprit

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 CO2 as years pass.

More than 76 billion tons of cement was produced around the world between 1930 and 2013, according to the study; 4 billion tons were manufactured in 2013 alone, mostly in China. It’s estimated that, as a result, a total of 38.2 gigatons of CO2 was released over that period. The scientists concluded, however, that 4.5 gigatons – or 43 percent of emissions from limestone conversion – were gradually reabsorbed during that time frame.

– Cement has gotten a lot of attention for its sizeable contribution to global climate change, but this research reinforces that the leading culprit continues to be fossil fuel burning, Davis said.

 

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