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Sustainability

Concrete absorbs large amounts of CO2

Like trees, concrete buildings absorb CO2. New calculations show that concrete absorbs roughly 30 percent of the amount of CO2 that cement production emits.

By Birgitte Dalgaard, , 9/21/2020

Cement production is the industry that emits the most CO2, but now new research shows that it looks less bleak for the cement industry.

For the first time, a team of researchers has mapped the global CO2 accounts of cement's total life cycle. From the cement being born in large kilns at 1500 degrees, till it is mixed with sand, stone and water to become large concrete buildings to finally being crushed and possibly recycled in our roads.

The interesting thing is that the researchers in the overall CO2 balance also take into account that concrete during its lifetime, like trees, absorbs a large amount of CO2. The researchers call this effect for the sponge effect, and their calculations have been published in the acclaimed scientific journal Nature Communications.

The sponge effect

– Approximately 30 percent of the total CO2 emissions from cement production is being absorbed by concrete structures. Our calculations show that the global future absorption of CO2 will be significant, says Professor Gang Liu from Department of Green Technology, SDU, pointing out that it is especially buildings and large road structures such as tunnels and bridges that absorb by far the most CO2.

The sponge effect is a result of a slow process called carbonatation that takes place in connection with the lime in the cement reacting with CO2 from the air over the years.

– To make a true projection of the effect of the duality that cement has by emitting CO2 in production and then absorbing it over time, it is essential to predict cement demand and have an overview of the total amount of concrete structures in use, says Gang Liu.

Cheap and popular

Concrete is the world's most popular building material. It is cheap and easy to work with. But what concrete doesn’t cost on the wallet, it does in turn cost on the CO2 balance. Concrete consists of sand, stone, water and cement, and it is the cement that is the great environmental sinner.

– Our results clearly show that any policy or initiative aimed at combating the CO2 emissions of cement production must take into account the sponge effect, because all calculations show that there is a considerable amount of CO2 absorbed by concrete over the years.

At the same time, Gang Liu points out that even if the sponge effect is included in the cement industry's overall climate accounts, it does not change the fact that radical technology advancements such as CO2 capture and storage and widespread deployment of material efficiency measures are needed before the industry can hand in a CO2 account that goes to zero.

CO2-neutral in 2090

– Our calculations based on the most optimistic scenarios show that the cement industry can succeed in being CO2-neutral in 2090.

– It requires massive efforts on several different fronts; both in terms of streamlining the cement kilns, ensuring that the energy in production is sustainable and utilizing the future opportunities to capture CO2 in production and store it.

In addition, it is necessary for cement manufacturers to make every effort to develop a more sustainable cement. For instance, by reducing the clinker content of cement. Cement clinker is a dark gray granulate, which is produced by heating limestone and clay at a temperature of approximately 1400-1500 degrees Celsius

Among other things, Aalborg Portland has developed a cement type Futurecem that enables 40% clinker replacement in cement. The researchers point out that if cement's CO2 balance is to reach zero in 2090, 73 percent of the clinker content must be replaced with a more sustainable solution by 2050.

Meet the researcher

Gang Liu is a Professor at SDU Life Cycle Engineering, Department of Green Technology. Gang Liu describes himself as an enthusiastic industrial ecologist. The idea behind industrial ecology is that waste and recyclable material are used for new production in controlled networks that in principle leave a harmless ecological footprint.

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