Cement production is a significant source of the more than 40 billion tons of global CO₂ emissions generated by human activity each year. Many efforts are underway to curb these emissions. A study recently published in Nature Geoscience reports that although it is often overlooked as a carbon sink, existing cement also absorbs as much as 1 billion tons of atmospheric CO₂ per year. This data should be factored into carbon emission inventories and carbon budgets, say the authors, and could inform mitigation efforts and more environmentally friendly concrete design.

The study was carried out by an international team of researchers and is the first to provide a comprehensive estimate of global atmospheric CO₂ uptake from cement. The study integrates data collected from recent field studies in China with existing data sets from around the world, covering the years 1930–2013.

In cement production, carbon dioxide is released during the process of converting calcium carbonate (limestone) into calcium oxide (lime). This process accounts for about 10% of the global CO₂ emissions that come from sources other than fossil fuel combustion. Fossil fuels are burned during cement production to heat the kiln, leading to additional CO₂ emissions.

Although the creation of cement releases CO₂ into the atmosphere, existing cement reabsorbs a significant amount of atmospheric CO₂ throughout its lifetime. This happens through carbonation, a process facilitated by water in which CO₂ diffuses into the pores of cement at the surface and leads to the formation of calcium carbonate. The carbonation process is well established, but this study is the first to quantify its impact on global CO₂ uptake.

The researchers estimated CO₂ uptake by considering the construction and carbonation of concrete, mortar, construction cement waste, and cement kiln dust over 1930–2013. They modelled the uptake for each type of material using Fick’s diffusion law and carbonation rate coefficients determined by experiments as well as from the literature. The model accounts for the time spent in each lifecycle stage (service life, demolition, and secondary use of waste), exposure conditions, exposed surface area, thickness, treatments and coatings, and several other variables.

“By conducting a global scale analysis, we found that more than 76 billion tons of cement were produced around the world from 1930 to 2013; 4.0 billion tons were produced in 2013 alone, much of it in China,” says Zhu Liu, a member of the research team from California Institute of Technology and Harvard University.

Liu continues, “Combined emissions are estimated to have released 38.2 gigatons of CO₂ over that period. But the team has concluded that 16.5 gigatons [of CO₂], or 43% of the emissions from the limestone conversion process, has been gradually reabsorbed during that time.”

As cement production rapidly increases, particularly in China, and the overall amount of existing cement worldwide increases, the rate of sequestration follows suit. The results show an average increase of 5.8% per year in carbon uptake over the span of 1990–2013.

In the quest to lower the environmental impact of cement production, the researchers suggest that the largest gain could come from a focus on reducing fossil fuels emissions rather than on process emissions. This is because the process of converting calcium carbonate into calcium oxide creates this natural carbon sink that helps to offset the initial emissions over time. In addition, the researchers encourage policymakers to consider programs that promote the carbonation of cement waste.

An expert on climate change and carbon cycle-climate interaction at the University of Maryland, Ning Zeng calls cement carbonation “one of the several carbon sink processes that have been poorly studied.” The researchers behind this work estimate that nearly half of the process emissions associated with cement production is countered by re-absorption, he says. “Such a magnitude is unexpectedly large, a novelty of this study. If further confirmed, this will have important implications for understanding the global carbon balance and other carbon sinks and emission sources.”

Read the abstract in Nature Geoscience.