Cement has a bad rep in environmental circles. That’s mainly because its production creates 5 percent of the world’s industrial carbon emissions. But a new study gives this climate rogue a bit of a reprieve.
Cement-based materials such as concrete can, over their lifetime, absorb some of the carbon released during cement production, researchers have found. Almost a quarter of cement’s carbon dioxide emissions over the past 80 years have been sequestered in the ubiquitous building material, they report in a paper published in the journal Nature Geoscience.
Making cement involves heating limestone at high temperatures to convert it to lime. The process releases carbon dioxide, accounting for half of cement’s total carbon emissions. Burning fossil fuels in the kilns releases the other half.
But as cement-based materials sit around for years exposed to air, carbon dioxide enters the material’s pores, reacts with water and other chemicals there, and gets converted into other chemicals that stay buried. This process is called carbonation. And it has been the missing piece in calculations of cement’s carbon footprint.
So an international team led by Zhu Liu at the California Institute of Technology calculated the carbon uptake by four cement materials—concrete, mortar, construction cement waste, and cement kiln dust—in China, Europe, the US, and the rest of the world between 1930 and 2013.
They used data from several studies to estimate how cement is used: thickness, surface area, quality, and building lifespans. They also calculated the sequestration resulting from demolition into different particle sizes, and then whether the waste is reused or disposed of. Finally, they calculated the carbonation rate in the different materials under different conditions, say exposed or buried.
The team estimated that cement has soaked up more than 4.5 billion tons of carbon from 1930–2013. That is 43 percent of the total carbon emitted when limestone is converted to lime, not including the emissions from fossil fuel use. And this carbon uptake is increasing as cement use soars around the world, especially in China. Existing cement stocks worldwide sequester about 1 billion tons of carbon dioxide every year, the researchers say in the paper.
Policies to reduce emissions from cement production should focus on fossil fuel use rather than the limestone-to-lime process, the researchers say. If the emissions from cement factories can be captured and stored, they add, cement could become carbon negative, sucking up more carbon dioxide than it produces.