The foundation of modern civilization, cement is also one of the biggest culprits of global warming, producing five percent of the world's carbon dioxide emissions – or so we've been told. Now, scientists from the US, UK and China are saying it's not quite the climate change villain it's been made out to be. In fact, it's actually a large – and largely overlooked – carbon sink.
The reason for its bad climate rep lies in its production process. Extremely high heat – some 2,800 degrees Fahrenheit – and vast amounts of fossil fuels are needed to turn limestone into cement. Not only that, carbon dioxide is also generated when limestone is converted to lime, the main ingredient in cement, making cement manufacturing a doubly carbon-intensive process.
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Between 1930 and 2013, it is estimated that 76 billion tons of cement was produced, which resulted in the release of 38.2 gigatons of carbon dioxide. While this is a staggering number, what happens when you factor in the amount of gas absorbed through cement carbonation during the same period? Cement carbonation is the process whereby carbon dioxide soaks into and spreads through the pores of cement and kickstarts a chemical reaction as the materials age.
This is what researchers, led by the University of East Anglia's Professor Dabo Guan, wanted to find out. Using data on cement materials during their service life, demolition and secondary use as waste, as well as new data from field surveys in China, the researchers created a model of the regional and global atmospheric CO2 uptake by cement between 1930 and 2013. What they discovered was that 4.5 gigatons – or 43 percent of emissions from limestone conversion – were reabsorbed during this period.
In addition, accumulating cement stocks as well as demolished structures also proved to be effective carbon sinks. On average, between 2000 and 2013, cement materials produced more than five and 10 years earlier absorbed around 25 and 14 per cent of carbon emissions respectively. Demolished buildings, owing to their large exposed and fresh surfaces, also result in an increase in carbonation rates. In fact, according to the study, the average annual carbon uptake (5.8 percent) actually showed a slight increase over cement emission rates (5.4 percent) between 1990 and 2013.
Based on the findings of the study, one can conclude that existing cement is "a large and overlooked" carbon sink, says Guan. Indeed, as the report notes, while official guidelines provided by the Intergovernmental Panel on Climate Change focus on curbing carbon dioxide emissions during the cement-production process, hardly any attention is given to concrete's role as a carbon sink, which could help governments and organizations improve their future emissions inventories and carbon budgets. In addition, when compared to fossil fuel emissions, the numbers produced by the cement manufacturing process is paltry in comparison. Indeed, while the latter makes up 90 percent of global carbon dioxide emissions from all industrial processes, it falls to five percent when fossil fuels are included in the mix.
"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," says Steven Davis, associate professor of Earth system science at the University of California, Irvine, who was also part of the study.
Cement materials could even become a net or negative carbon sink with improved carbon capture and storage technology, suggests Guan. "Policymakers might also investigate ways to increase the completeness and rate of carbonation of cement waste, for example as part of an enhanced weathering scheme, to further reduce the climate impacts of cement emissions."
The team's results were published in Nature Geoscience.