August 25, 2021 – For the first time, the Intergovernmental Panel on Climate Change (IPCC) has acknowledged that concrete in the built environment is a significant storehouse of carbon reabsorbed from the atmosphere.
Roughly half of the carbon emitted during the production of cement, a key component of concrete used to construct buildings and associated infrastructure, is reabsorbed and permanently sequestered through a process known as carbonation.
Fossil CO2 emissions have grown continuously since the beginning of the industrial era, notes the latest IPCC Assessment Report, and direct CO2 emissions from carbonates in cement production are around 4% of total fossil CO2 emissions.
But the uptake of CO2 in cement infrastructure (carbonation) offsets about one-half of the carbonate emissions from current cement production, notes the report. The amount of CO2 uptake from the atmosphere was found to be robust across different fossil CO2 emission datasets, despite minor differences in levels and rates.
“The IPCC recognition that exposed concrete can reabsorb CO2 from the atmosphere is of tremendous importance to the cement and concrete industries,” said Bill Larson, Chair of the Building Resilience Coalition.
“Not only does it confirm the importance of the built environment as a major carbon sink, it also raises the bar for building designers and architects in their choices of building materials that are truly sustainable and climate-friendly.”
Earlier reports from the Building Resilience Coalition documented the importance of the built environment as a carbon sink due to carbonation, noting that if this reabsorbed CO2 were included in greenhouse gas accounting statistics it would alter the net emissions tallies of many nations.
The latest IPCC report cautiously acknowledges that several improvements have led to a more constrained carbon budget including improvements in the estimates of emissions from cement production and the carbon sink implications associated with cement carbonation.
As was noted in the Building Resilience Coalition report The process of making cement is very carbon-intensive, in large part from the fuels used to heat the cement kiln and from the CO2 released from raw carbonates used to make cement, the process is called calcination.
But the process of calcination is reversible and CO2 in the air can react with hydrated cement in the concrete and the carbonates are regenerated. This process is called carbonation.
Furthermore, the carbonated concrete is chemically stable, so there is no risk that the CO2 that has been taken up will return to the atmosphere. This means that concrete in the built environment is an important and stable carbon sink.
Initial estimates of the amount of CO2 reabsorbed from the atmosphere through carbonation varied considerably according to the methodologies used. As well, there are significant differences in the rates of adsorption based on the amount of concrete exposed to the elements and other factors relating to the chemistry of concrete.
Improvements in measuring CO2 emissions and carbon reabsorption suggest roughly half the emissions associated with cement production are being recaptured and sequestered permanently in concrete. No other widely used building product absorbs CO2 from the atmosphere.
As well, the cement production processes are being improved around the world in response to the need to reduce carbon emissions. Several lower carbon cement products are now available and their use in the construction of buildings and infrastructure such as road pavements and bridges is accelerating.
Researchers at the MIT Concrete Sustainability Hub estimate the carbon uptake of all pavements in the United States could offset 5 percent of the CO2 emissions generated from cement used in U.S. pavements.
One of the more popular lower carbon building products on the market today is Portland Limestone cement, a blended cement with a higher limestone content, which results in a product that performs the same as conventional Portland cement, but with a lower carbon footprint of 10% on average.
Other innovations that contribute to lowering the carbon footprint of the built environment involve sequestering CO2 captured from industrial processes directly into concrete building products.
Decarbonization of the economy is a major challenge underway in every industrialized nation in the world. The cement and concrete industries are at the forefront of technological innovation to lower the carbon footprint of the built environment and to promote circular economy best practices.
NASA Earth Observatory
As noted by NASA, the earth’s carbon cycle is undergoing considerable stress, and as highlighted in the IPCC report, limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net-zero CO2 emissions, along with strong reductions in other greenhouse gas emissions.