Embodied carbon in concrete

The January 2020 issue of Architect, the journal of the American Institute of Architects, quotes Julie Buffenbarger in its article on zero-net-carbon structural materials. Seeking to reduce the carbon footprint of their buildings, architects are looking beyond operating energy. Now they also consider the embodied carbon emissions of building materials. How do you quantify the embodied carbon in concrete?

…the building structure and sub-structure … constitute more than half of a commercial building’s embodied carbon footprint. Moreover, architects and developers should look for opportunities to reuse and renew existing structures, so as not to waste the energy already expended (and carbon dioxide already emitted) to create them–Gideon Fink Shapiro, Architect, January 2020

Most of the embodied carbon in concrete comes from portland cement, the “glue” that holds concrete together. Manufacture of portland cement is highly energy intensive because of the high clinkering temperatures it requires. Modern cement plants make the most of the energy they use, for example by preheating the kiln feed.

However, no matter how energy-efficient the plant, limestone is mainly calcium carbonate, or CaCO3. Pyroprocessing it into cement clinker drives off carbon dioxide (CO2), and there’s no getting around that. So how do you reduce the embodied carbon in concrete?

Reducing the embodied carbon in concrete

One of the best ways to do that is to reduce the portland cement content of the concrete. Judicious use of supplementary cementitious materials (SCMs) can reduce the embodied carbon emissions without sacrificing performance. A local example of this practice is the St. Croix Crossing. Concrete producers can add SCMs at the batch plant or use blended cements, which are mixtures of portland cement with SCMs.

North America lags behind Europe in adopting blended cements, says Julie Buffenbarger …Specifications are a good way to make concrete construction more efficient … but being too prescriptive can be counterproductive. Rather than dictate the ingredient proportions in a concrete mix, she suggests design teams specify performance criteria …