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.”