Concrete jungle functions as carbon sink

While cement manufacturing is one of the most carbon-intensive industrial processes, an international team of researchers has found that the widely used building material eventually reabsorbs much of the CO2 emitted when it was made.

For their study, the researchers from China, Europe and the US 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 calculated that more than 76 billion tonnes of cement was produced around the world between 1930 and 2013, releasing a total of 38.2 gigatonnes of CO2 over that period.

At the same time, they discovered that 4.5 gigatonnes were gradually reabsorbed during that time frame through a process called carbonisation. This occurs when cement-based materials such as concrete and mortar absorb CO2, starting at the surface of the material – in buildings, roads and other infrastructure – and moving progressively inward, pulling in more and more carbon dioxide as years pass.

“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 an especially carbon-intensive process because emissions come from two sources. Firstly, CO2 molecules are released into the air when limestone is converted to lime, the key ingredient in cement. Secondly, factories have to burn large quantities of natural gas, coal and other fossil fuels to generate the heat necessary to break up limestone.

The 4.5 gigatonnes that were gradually reabsorbed corresponds to 43 per cent of the emissions from limestone conversion.

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

 

Image credit: Steven Davis, UCI

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