It is safe to say we are in a climate crisis. Of the last 17 years, 16 have been the hottest on record. Sea level is expected to rise by as much as eight feet by the end of the century. And by 2050, as many as 140 million people will have been displaced by climate change. The time to act is now, and a major area of impact is buildings, which account for 40% of carbon emissions in the United States. Better envelopes, lighting, and mechanical systems are helping buildings become more efficient, which means an increasing proportion of carbon—up to 68% of a building’s lifetime emissions—is locked up in materials. This “embodied” carbon gets released during a material’s extraction, manufacture, transport, maintenance, and, eventually, disposal.
If our industry is to meet the 2030 Challenge of carbon neutrality by the close of the decade, we will need to reevaluate building materials and select low-carbon alternatives.
Figure 1: Courtesy of Faithful+Gould
Concrete is a highly desirable building material with an outsized share of embodied emissions. In 2005, production of cement, the principal ingredient in concrete, accounted for approximately 6% of global carbon dioxide emissions. Cement is unique as a material in that it emits carbon both indirectly and directly; fossil fuels are combusted to extract and heat the material, but cement also emits carbon dioxide in the chemical reaction that converts limestone (calcium carbonate) to lime (calcium oxide). Architects are increasingly specifying concrete alternatives such as cross-laminated timber and rammed earth, but these materials require specialized construction techniques and have yet to achieve widespread adoption. Other industry actors, acknowledging concrete’s utility and ease of construction, have instead attempted to alter the composition and design of the material. Fly ash, a by-product of burning coal, has been used as a partial cement substitute for decades, and high levels of fly ash in concrete (30-40% by mass) can reduce annual embodied carbon consumption by nearly 40%.
One of the most promising solutions today was pioneered by a Nova Scotian startup, known as CarbonCure. CarbonCure licenses a technology that converts captured carbon dioxide into concrete, mineralizing and effectively encapsulating the greenhouse gas. Through this process, concrete transforms from a carbon emitter to a partial carbon sink. As an added bonus, injected carbon dioxide improves concrete’s compressive strength, which leads to additional embodied energy savings because less cement is needed. And, CarbonCure retrofits existing concrete plants, which allows building developers to utilize these specialty concrete products without increasing transportation-related embodied carbon consumption.
Recently, Steven Winter Associates, Inc. (SWA) gained firsthand experience with early-age-carbonized concrete. The MGM Springfield, a one-million-square-foot entertainment complex in Massachusetts, contracted SWA through AECOM Tishman to help the project earn LEED Neighborhood Development Platinum certification. In pursuit of lowering lifetime emissions, Todd Megrath, MGM’s director of sustainable development, decided to utilize approximately 120,000 CarbonCure concrete masonry units (CMUs) for the resort’s back-of-house firewall. MGM then partnered with a nearby concrete producer, A. Jandris & Sons, which had previously retrofitted the CarbonCure technology.
In total, MGM estimates that installing carbonized CMUs reduced the project’s embodied carbon by 2000 pounds, the amount of carbon dioxide sequestered annually by one acre of U.S. forestland. Although it was not feasible for Springfield, MGM is investigating using CarbonCure poured concrete in future projects as the high cement content of mixes allows greater substitution and impressive embodied carbon savings.
SWA conducts whole-building life cycle assessments, and many of the programs we assist, including LEED New Construction, Green Globes, and Living Building Challenge, incentivize decreasing embodied carbon emissions. In the coming months, stay tuned for more blogposts highlighting additional measures you can take to decrease your home or building’s embodied carbon.
 Justin Gillis, “Earth Sets a Temperature Record for the Third Straight Year,” New York Times, January 18, 2017, https://www.nytimes.com/2017/01/18/science/earth-highest-temperature-record.html.
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 Jieru Zhang, Gengyuan Liu, Bin Chen, Dan Song, Jing Qi, and Xinyu Liu, “Analysis of CO2 Emission for the Cement Manufacturing with Alternative Raw Materials: A LCA-based Framework,” Energy Procedia 61 (2014), https://www.sciencedirect.com/science/article/pii/S1876610214030707, 2541.
 Pradip Nath, Prabir K. Sarker, and Wahidul K. Biswas, “Effect of Fly Ash on the Service Life, Carbon Footprint, and Embodied Energy of High Strength Concrete in the Marine Environment,” Energy and Buildings 158 (2018), https://www.sciencedirect.com/science/article/pii/S0378778817325276, 1694.
By Adam Yarnell, Sustainability Consultant