Blaine Brownell Former concrete abattoir in Shanghai, China was turned into a popular mixed-use space.

As the world’s most popular building material, concrete comprises a significant portion of our built environment, with some 10 cubic kilometers (about 13 trillion cubic yards) poured, placed, or cast annually. Yet concrete’s poor environmental track record is also notorious. Based on the large quantity of energy required to produce Portland cement (concrete’s active ingredient) via limestone calcination—during which the material is burned and the greenhouse gas is released—it is estimated that concrete manufacturing contributes at least 5 percent of the world’s carbon dioxide emissions.

Yet a less-known fact is that, over its lifecycle, concrete also acts as a carbon sink—something that absorbs more carbon than it emits over time. According to a 2005 series of reports by the Danish Technological Institute, concrete theoretically absorbs as much carbon dioxide throughout its lifetime as the amount emitted through calcination. Although this claim constitutes nothing short of a revelation regarding concrete’s environmental impact, measurement challenges have prohibited its verification. “It is not documented in what way and to what extent the carbonation can be taken into account in assessments of concrete carbon-dioxide emissions, e.g., in life cycle assessments,” the DTI summary states. “Models for calculating the rate of carbonation exist, but they are simple, developed for a special outdoor type of concrete, and they do not take into account that the concrete is crushed and recycled after use.”

A November article in Nature Geoscience offers some of the missing proof. The report provides a comprehensive set of calculations to determine the extent of global concrete carbonation—the process whereby carbon emissions are naturally absorbed. According to the authors, 43 percent of cement-based emissions generated between 1930 and 2013 have been sequestered by cementitious materials within that same time period. Furthermore, because the production of cement continues to increase, thereby adding to the volume of concrete structures, the global carbon uptake is similarly increasing. For example, the authors claim that cementitious materials absorbed roughly 2.5 percent of global carbon emissions in 2013—roughly half of the amount contributed via cement calcination the same year. More impressively, this quantity equates to nearly a quarter of all carbon dioxide sequestered in forests annually.

Such a disclosure represents a fundamental shift in thinking about concrete’s ecological footprint—but how accurate are its calculations? What consequences does it suggest for future carbon calculations, and what approaches does it imply for future building construction?

Assessing dependability begins with understanding motivations. The Nature Geoscience report was supported by the National Science Foundation of China and a variety of other international research and development programs—with no industry funding that might suggest biased results. The staggering growth of concrete production in China inspired the study, says one of the study’s authors, University of California, Irvine Earth-system science associate professor Steven Davis. “Scientists and engineers have known about the process of carbonation for a long time, but no one had ever tallied up the collective global quantities of the process, probably because they assumed the process was too slow to make a big difference,” he says. “But we suspected that the sheer quantity of various types of cement materials might add up, and now we’ve shown that it does.”

Flickr user Friscocali a via a Creative Commons license The Tunkhannock Creek Viaduct was considered the largest concrete structure in the world when it was completed in 1915. It spans 2,375 feet in length.

To tackle the daunting task of quantifying global cement carbonation, the research team employed an algorithm-based simulation technique called Monte Carlo analysis, which uses repeated sampling to solve complex statistical problems. The authors generated an intricate list of variables, including the thickness of concrete walls, the quantity of cement present in different materials, the effects of various surface treatments, and the sizes of demolished concrete fragments. They further evaluated the data by assessing regional and global differences in the material.

After determining a reasonable range for each factor based on previous studies as well as field surveys, the team ran its model 100,000 times—each time selecting at random a specific value from the reasonable ranges of the 26 different criteria. “When we were finished with the 100,000 model runs, we looked at the results and reported the range of carbon-dioxide estimates that came out, focusing on the median estimates,” Davis says. The reliability of the report is based on the substantiation of each detail as well as the thoroughness of the Monte Carlo analysis. “The full range of results represents our estimate of uncertainty,” he adds. The team concluded that carbonating cement materials have sequestered a total of 4.5 gigatons of carbon between 1930 and 2013, and therefore represent a significant carbon sink that has been largely ignored.

So if such a significant carbon uptake exists, how does one account for the 2.5-percent discrepancy in anthropogenic carbon emissions? Atmospheric carbon is measured in a variety of ways, and some methods are more accurate than others. For example, carbon dioxide concentrations may be directly measured from the air. Industrial carbon outputs from processes like fossil fuel production and cement manufacture are also regularly recorded. Such measurements may be calculated with a high degree of accuracy.

However, carbon sinks are more difficult to assess, and Davis believes these represent the margin of error in global carbon calculations. "So our new estimates suggest that perhaps less carbon than we thought may be getting taken up by other processes, like growing plants and absorption by the oceans," he says. “Of course, there is still quite a bit of uncertainty when it comes to estimating how much carbon the oceans and plants are absorbing, so our new cement sink probably fits without upsetting those estimates too much.”

For the construction industry, the most intriguing aspect of the report—in addition to the conclusion that concrete is not quite as ecologically deleterious as once thought—concerns the relationship between environmental performance and material degradation. According to the Nature Geoscience article: “It is well known that the weathering of carbonate and silicate materials removes carbon dioxide from the atmosphere on geologic timescales (104 years)…. Our results indicate that such enhanced weathering is already occurring on a large scale.”

Weathering is not a term that architects and builders typically welcome, yet it is a boon in this case. But can we build highly carbonating concrete structures that are also durable? “Something I learned in the process is that the carbonation itself is apparently not what weakens structures, but it changes the pH of the cement materials in a way that facilitates corrosion of the reinforcing steel," Davis says. Thus, it is important to develop concrete systems designed to encourage carbonation without structural decay of either the concrete or the reinforcement. “Carbonation awareness” suggests that architects will seek to increase the exposed surface area of concrete while minimizing surface treatments that prohibit this chemical weathering. Furthermore, increased life-cycle consciousness can further enhance carbonation. “For instance, some additional mechanical breakdown of concrete and disposal or recycling above-ground where it can react with the air could increase the completeness of its carbonation,” Davis notes.

When these strategies are coupled with approaches to reduce cement-based emissions at the front end, concrete could be a net-positive material. “Indeed, if carbon capture and storage technology were applied to cement process emissions, the produced cements might represent a source of negative carbon-dioxide emissions,” the authors write in their report. If such practices became standard—a far-fetched notion although not inconceivable—concrete could be employed and even regarded as an ecological asset rather than a detrimental agent.