It has long been known that manufacturing cement generates large amounts of carbon emissions.
The calcination of carbonate rocks during cement manufacture was estimated in 2013 as the source of up to 5% of global CO2 emissions from all industrial processes and fossil-fuel combustion.
Since then, considerable attention has been paid to quantifying the industrial process emissions from cement production. However, the natural reversal of the process — carbonation — has received little attention in carbon cycle studies.
Carbonation is a natural process of carbon uptake from the atmosphere by exposed concrete.
Researchers at the MIT Concrete Sustainability Hub have used new and existing data on carbon emissions from cement materials during their entire service life, during demolition, and from the secondary use of concrete waste to estimate regional and global CO2 uptake between 1930 and 2013.
Using an analytical model describing carbonation chemistry, it was found that the carbonation of cement materials over their life cycle represents a large and growing net sink of CO2, increasing from 0.10 GtC yr−1 in 1998 to 0.25 GtC yr−1 in 2013.
In 2016 it was estimated that a cumulative amount of 4.5 GtC had been sequestered in carbonating cement materials from 1930 to 2013, equalling 43% of the CO2 emissions from cement production over the same period. This estimate did not include emissions associated with fossil use during cement production.
The MIT CSHub researchers have since concluded that the carbonation of cement products represents a substantial carbon sink that is not currently considered in the emissions inventories of many countries.
They noted that estimating carbonation at a more granular level would necessitate the integration of carbonation into carbon accounting practices. That process has not yet taken root in national or international carbon accounting practices.
In 2021 the Intergovernmental Panel on Climate Change (IPCC) did acknowledge that concrete in the built environment is a significant storehouse of carbon reabsorbed from the atmosphere. (See Building Resilience Coalition article here).
Recently, Sustainability Hub researcher Hessam AzariJafari developed a state-of-the-art, material- and facility-specific calculator for carbon uptake in concrete, including carbonation from the end-of-life stockpiling of recycled concrete aggregates (RCA).
This tool can include the details of use-phase uptake and corresponding mix design components when estimating the end-of-life uptake.
The calculator estimates carbon uptake through various accessible inputs—including concrete mixture, location and exposure characteristics, and stockpiling conditions.
The tool is easily downloaded and can be used by architects and designers to estimate the carbon emissions associated with their projects.
As noted by William Larson, Chair of the Pacific Northwest Building Resilience Coalition, design decisions and building project approvals in many jurisdictions are increasingly made based on estimates of the embodied carbon associated with the materials used in the building.
A far more accurate estimate of the carbon emissions and the associated readsorption of CO2 emissions from the atmosphere is possible when the structure’s entire lifecycle is considered. The Concrete Sustainability tool can provide these more accurate estimates before deciding on designs or project approvals.
See also ‘Concrete as a Carbon Sink‘ – Free to Download.
See this short video, ‘Buildings as Carbon Sinks.’
