Cement structures may represent a substantial but overlooked absorber of carbon emissions, offsetting some of the emissions released during cement production itself, a study says.

Conducted by the China Emission Accounts and Datasets group—an international team of researchers led by Dabo Guan, professor of climate change economics at the University of East Anglia—the study found that the natural carbonation process of cement materials represents a large and growing "sink" of CO₂.

While the Intergovernmental Panel on Climate Change guidelines for emissions inventories provide methods for quantifying CO₂ emissions during the cement production process, they do not account for carbon absorbed through cement carbonation, the researchers say.

Carbonation takes place through the life of cement-based materials. Credit: PixabayCarbonation is a process that takes place throughout the life cycle of cement-based materials. As these materials weather, CO₂ spreads into the pores and triggers a chemical reaction starting at the surface and moving inward.

Using data from field surveys in China and from studies on cement material the researchers modeled the regional and global atmospheric CO₂ uptake between 1930 and 2013. The findings indicate that existing cement stocks worldwide absorb approximately one billion tons of atmospheric CO₂ each year.

The researchers estimate that 4.5 gigatons of carbon has been reabsorbed in carbonating cement material from 1930 to 2013. They say that offsets 43% of the CO₂ emissions from cement production over the same period (not including emissions associated with fossil fuel use during cement production). An estimated 44% of cement process emissions produced each year between 1980 and 2013 has been offset by the annual cement sink, the study says.

“Existing cement is a large and overlooked carbon sink, and future emissions inventories and carbon budgets may be improved by including this,” says Guan. “Efforts to mitigate CO₂ emissions should prioritize the reduction of fossil fuel emissions over cement process emissions, given that produced cement entails creation of an associated carbon sink. We suggest that if carbon capture and storage technology were applied to cement process emissions, the produced cements might represent a source of negative CO₂ emissions.”

The research also highlights the effects of accumulating cement stocks. On average, between 2000 and 2013, 25% of the carbon captured each year was absorbed by cement materials produced more than five years earlier. Another 14% was absorbed by cement materials produced more than 10 years earlier.

Between 1990 and 2013, the annual carbon uptake grew by an average of 5.8% a year as the stock of cement buildings and infrastructure increased, aged, and was demolished and disposed. The researcher say this is faster than process cement emissions over the same period, on average 5.4% per year.