The production of cement is a major source of carbon dioxide, but new research suggests the material that makes up our concrete jungles also plays an important role in reabsorbing carbon emissions.

Key points: Production of concrete contributes to 90 per cent of global CO2 emissions from industrial processes, or 5 per cent from industrial processes and fossil fuels combined

Production of concrete contributes to 90 per cent of global CO2 emissions from industrial processes, or 5 per cent from industrial processes and fossil fuels combined Estimated that 38.2 gigatonnes of CO2 released by cement production between 1930 - 2013

Estimated that 38.2 gigatonnes of CO2 released by cement production between 1930 - 2013 Over same time, modelling indicates 4.5 gigatonnes were reabsorbed

During the manufacture of cement, carbon dioxide is emitted when limestone is converted to lime under heat in a process called calcination.

But as cement ages and weathers over time, it also absorbs carbon dioxide in a process called carbonation.

In a paper published today in Nature Geoscience, a team of researchers calculated that the carbonation process has offset as much as 43 per cent of the emissions associated with cement production, not including the emissions associated with fossil fuel use during cement production, over the past 70 years.

"We think maybe there is something within human systems that can act as a carbon sink, in addition to the traditional thinking about natural carbon sinks," Dr Zhu Liu, from the Resnick Sustainability Institute at the California Institute of Technology, said.

Between 1930 and 2013, more than 69 billion tonnes of cement was poured into buildings and infrastructure around the world, with 3.6 billion tonnes in 2013 alone.

In this time, it is estimated that cement manufacturing released 38.2 gigatonnes of carbon dioxide. Globally, cement production accounts for around 5 per cent of total carbon dioxide emissions.

But modelling of the global CO2 uptake of four cement materials — concrete, mortar, construction cement waste and cement kiln waste — by Dr Liu and colleagues indicated that cement products had absorbed a total of 4.5 gigatonnes of carbon over that period.

What is more, this rate of absorption may actually be increasing, as an unexpected side-effect of the construction boom in nations such as China.

Dr Liu said the typical life expectancy of a building in China is much less than it might be in areas such as Europe or the US.

Where is all our carbon? Carbon is stored in all living things, the ocean, the atmosphere, soil and a lot of rock.

Carbon is stored in all living things, the ocean, the atmosphere, soil and a lot of rock. All carbon eventually passes through the atmosphere.



All carbon eventually passes through the atmosphere. 99.9 per cent of carbon is stored in rock, mostly as limestone.

99.9 per cent of carbon is stored in rock, mostly as limestone. After rock, the ocean is the next biggest storage site with 38,000 billion tonnes of dissolved CO2.

After rock, the ocean is the next biggest storage site with 38,000 billion tonnes of dissolved CO2. Soil stores three times as much carbon as all the world's plants.

Soil stores three times as much carbon as all the world's plants. Plants and the ocean absorb slightly more CO2 than they release each year, making up for half the excess carbon we release.

"The average is just 30 years, then those buildings have been crushed down and all of those cement materials are crushed into very small pieces," says Dr Liu.

"They have a much larger exposed surface that is exposed to the air, which will dramatically increase the carbonation process."

Because this carbon sink is not currently included in carbon accounting, Dr Liu said it may explain some of mismatch that has long been noted between what we know of existing natural carbon sinks, and the concentration of carbon dioxide in the atmosphere.

While the carbon dioxide emissions from cement manufacture — not including the fossil fuel emissions — are included in the Intergovernmental Panel on Climate Change (IPCC) Guidelines for National Greenhouse Gas Inventories, the guidelines do not factor in the carbon that is absorbed over the cement's lifetime.

Life cycle of materials and carbon budget

Commenting on the study, geophysicist Dr Richard Matear from CSIRO said it showed the importance of looking at the entire life cycle of materials.

"We're looking at all these sinks and it's quite interesting that this one, cement, is as big as reforestation of Australia over the last decade," Dr Matear said.

"It's a huge contribution that you would want to consider if you were doing a budget for carbon dioxide."

Dr Matear said the paper also reinforced the need for better understanding of the production and absorption of carbon.

"That's important if you're trying to do a budget and trying to really get at what is a country's CO2 emissions," he said.

"We're going to have to rely on people coming up with good process understandings to try to quantify these potential sinks, whether that's cement or forestry or other land uses."

Dr Liu and co-authors also suggested that the findings could inform how cement is reused and recycled.

"Finally, policymakers might productively investigate ways to increase the completeness and rate of carbonation of cement waste (for example, as a part of an enhanced weathering scheme) to further reduce the climate impacts of cement emissions."