According to the current IPCC climate report, around half of the carbonate emissions from cement production are absorbed by the material when used in buildings and infrastructure.
The “cement carbonation sink” absorbs an estimated 200 million tons of carbon each year, according to an overlooked section of the report released earlier this month ahead of the Cop26 climate conference.
“The direct CO2 emissions from carbonates in cement production amount to around four percent of total fossil CO2 emissions,” says the full version of the IPCC’s Sixth Assessment Report.
“The uptake of CO2 in the cement infrastructure (carbonation) offsets around half of the carbonate emissions from current cement production.”
However, carbonate emissions only make up between 40 and 60 percent of the total emissions from cement production, so that around a quarter of the carbon emitted is absorbed through carbonation.
Concrete carbonation recognized for the first time
“The key chapter in [the IPCC report] concrete carbonization recognized for the first time, “said Richard Leese, director of industrial policy, energy and climate change at the Mineral Products Association, which represents the UK cement industry.
“Now international accounting needs to be updated to take into account the carbonation of concrete just so you can do the math.”
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The IPCC report sets out the latest scientific understanding of climate change, which for the first time states that the rise in atmospheric greenhouse gases “is clearly caused by human activity”.
“The global warming of 1.5 degrees Celsius and two degrees Celsius will occur in the 21st
Growth in direct CO2 emissions from cement slowed in the 2010s
The report suggests that direct CO2 emissions from carbonates in cement production account for around 4 percent of total fossil CO2 emissions and increased 5.8 percent annually in the 2000s, with a slower growth of 2.4 Percent per year in the 2010s.
Cement-making produces large amounts of carbon dioxide when limestone is crushed and burned to make clinker, which is used as a binder.
Additional emissions are caused by the large amounts of fossil fuels commonly used to heat cement kilns. These are not taken into account in the IPCC figures.
When burning, the limestone, a calcium carbonate material, is separated into its calcium and carbon components. Calcium, an important component of cement, is retained during calcination, but the carbon combines with oxygen to form CO2, which escapes.
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However, according to recent research cited in the IPCC report, around half of the carbon produced during calcination is later reabsorbed by cement and concrete through a natural process known as mineral carbonization.
The process is being studied as a way of capturing atmospheric carbon by companies like the Australian company Mineral Carbonation, who developed a way to artificially replicate the process.
Others who use mineral carbonation include Kukbong Kim, a graduate of the Royal College of Art, who recently unveiled a crushed concrete paint that absorbs CO2, and Dutch designer Teresa van Dongen, who highlighted the potential of olivine sand to bind carbon Has.
The role of cement as a carbon sink is overlooked
The cement industry is the largest single emitter of CO2. A landmark 2018 report by Chatham House found it responsible for around eight percent of global emissions.
But cement’s role as a carbon sink has been overlooked, according to Jenny Burridge, senior civil engineer at The Concrete Center in London.
“Concrete absorbs CO2,” she said. “We haven’t talked much about it. But actually that’s one of the things we’re looking at right now.”
“How much carbonation actually happens? How much carbon sink is the total mass of concrete that we have in the UK and elsewhere in the world? “
“Different types of concrete absorb CO2 at different rates,” she added. In the case of reinforced concrete, structural engineers specify concrete with a low carbonation potential, as this can lead to corrosion of the steel reinforcement.
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But other types of concrete, such as masonry, could be used to trap atmospheric carbon, she suggested. “Actually blocks carbonate pretty quickly,” Burridge said.
Ballast concrete has even greater potential because it has a larger surface area that can absorb CO2, Burridge said.
“There are big arguments for not demolishing a building,” she said. “But if you tear it down, be sure to break up the concrete, because then the carbonization happens very quickly.”
“You can use the crushed concrete for aggregates,” she added.
According to the International Energy Agency, 4.1 billion tons of cement were produced worldwide in 2019. China accounts for around 55 percent of world production, followed by India with 8 percent.
Efforts to decarbonise the cement industry include a project in Sweden to build “the world’s first climate-neutral cement plant” and Carbicrete’s efforts to produce carbon-scavenging concrete.
Main image: Concrete buildings like Atelier Deshaus’s Taizhou Contemporary Art Museum are slowly absorbing carbon over time.