News Detail

The Quest for Carbon-Neutral Concrete – Part 1

6/1/2022

At the Opening Session of the ACI Concrete Convention – Spring 2022, Past President Cary Kopczynski formally announced the founding of NEU: An ACI Center of Excellence for Carbon Neutral Concrete. ACI Executive Vice President Ron Burg subsequently provided details on NEU and its expected core functions in April’s Executive Vice President’s Memo. This month, I’ll follow up with “The Quest for Carbon-Neutral Concrete” with the common options available to design professionals as they try to meet the carbon-neutral goals of owners and the design community. While most goals for achieving carbon neutrality are earmarked for 2050, some are as early as 2030! Other options will be addressed in Part 2 of this two-part memo.

It’s universally accepted that achieving carbon neutrality must center around portland cement and, accordingly, cement organizations such as the Portland Cement Association (PCA), the Global Cement and Concrete Association (GCCA), and the Inter-American Cement Federation (FICEM) have either developed or committed to industry roadmaps for net-zero concrete—that is, to fully decarbonize the cement and concrete industry by 2050. These roadmaps involve different strategies and include savings in clinker production, savings in cement and binders, efficiency in concrete production, and efficiency in design and construction—which speaks to Past President Kopczynski’s signature objective of “construction productivity,” decarbonization of electricity, carbon capture and use/storage, and recarbonation of concrete.

Savings in cement and binders and efficiency in concrete production are not new to the industry. Supplementary cementitious materials (SCMs) such as fly ash, slag cement, calcined clay/metakaolin, silica fume, and natural pozzolans are used worldwide, depending on availability, to optimize portland cement concrete. Other SCMs such as rice husk ash and, in recent years, ground-glass pozzolans are also available regionally. Blending SCMs with portland cement during the manufacturing process is also not new, though not extensive in North America, where concrete producers typically add SCMs separately, usually as a replacement for portland cement. The main difference between blending during manufacturing and separate addition is the portland cement replacement that can be achieved by the latter. Separate addition offers flexibility with respect to the types and amounts of SCMs that can be used in a given concrete mixture. Historically, fly ash and slag cement replacement levels have ranged from about 10 to 30% and 25 to 50%, respectively, and the higher ends of the ranges have typically been used in mass concrete applications. Concrete mixtures with combinations of SCMs are currently being used to achieve significant replacement levels of portland cement in an effort to reduce the carbon footprint of the mixtures. Portland cement replacement levels of 70 to 75% are being used routinely in projects throughout the United States, particularly in high-rise buildings in New York City and California, and replacement levels of about 83% have been reported. These low cement content concrete mixtures require proportioning expertise and the right choice of chemical admixtures to meet project requirements. The projects in New York City include One World Trade Center and its companion towers, 432 Park Avenue, and 30 Park Place (Four Seasons hotel)—all constructed between 2006 and 2017 with mixtures that contained fly ash, slag cement, and silica fume (high-reactivity metakaolin in the case of 432 Park Avenue for aesthetics). The San Francisco Public Utilities Commission Headquarters building, constructed between 2009 and 2012, had a 70% replacement of portland cement using fly ash and slag cement. Not to be outdone, the concrete mixtures for the Goethals, Bayonne, and Tappan Zee Bridges in New York City, constructed between 2013 and 2017, had portland cement replacement levels of 55 to 65% using fly ash and slag cement (plus silica fume for Tappan Zee).

What is unique about these projects, and all the others not mentioned, is that the owners, through their design professionals, required the use of concrete mixtures with a low-carbon footprint. Indeed, the push for low-carbon concrete is currently being driven by owners. Greater success can be achieved if design professionals proactively take the lead and collaborate with other concrete industry stakeholders to achieve net-zero concrete by the target date of 2050! Also fascinating is the fact that common SCMs—fly ash, slag cement, silica fume, and high-reactivity metakaolin—were used. A wealth of information on these SCMs already exists in ACI documents, and one of the immediate goals of NEU will be to repackage the information for the general public and highlight and provide a list of low-carbon SCM concrete projects.

Charles K. Nmai