Decarbonizing the built environment faces a significant challenge: the high embodied carbon of concrete.
Key Takeaways
- 58% of concrete’s carbon comes from cement
- Concrete contributes 30% of steel structures’ carbon
- Fly ash and slag shortages limit SCMs
- Alternative SCMs like metakaolin reduce carbon
Overview
Around 58% of the embodied carbon in installed reinforced concrete comes from the production of portland cement, the material that that binds rocks together when mixed with water. Cement production involves cooking limestone, which emits greenhouse gases due to the energy required to operate the kiln at high temperatures and the chemical decomposition of limestone.
Concrete is the primary material used in most of our projects, even when other materials such as steel or timber are the primary structural systems. We have found through projects tracked as part of our SE2050 commitment that concrete contributes approximately 30% of the embodied carbon of a structural steel superstructure—and that does not account for concrete’s role as the primary foundation material for all types of buildings.
Traditional Supplementary Cementitious Materials
The traditional approach to reducing the embodied carbon in concrete is by replacing a portion of the cement with supplementary cementitious materials (SCMs). Two of the most common SCMs are fly ash, a byproduct of coal combustion with cementitious properties, and slag cement, which is produced during iron and steel making. As we become less reliant on coal-fired power generation and blast furnaces for iron and steel, less fly ash and slag are being produced, leading to shortages in recent years.
This reduction in supply reflects progress in sustainability in other industries, but it poses a challenge to the reliable production of high-quality, cost-effective, lower carbon concrete. The use of SCMs is a crucial tool in producing desirable characteristics in fresh and hardened concrete, and cement replacement with SCMs has been the primary method of reducing embodied carbon in concrete mixes.
Bridging the Gap
To continue to provide high-quality, lower carbon concrete, suppliers and design teams must innovate. Many startups are currently exploring alternate binders, and they have come together to form the Decarbonized Cement and Concrete Alliance (DC2). These innovative binders may be available in the future. In the meantime, there are several alternative SCMs available today that have traditionally been underutilized in practice but are immediately available. The choice of these alternatives depends on the project and is influenced by local supply.
Successful Use of Natural Pozzolans at Scale
The University of New Mexico’s (UNMH) New Hospital Tower is an example of a contemporary project that utilized less commonly used SCMs. In this case, metakaolin was utilized in all concrete mixes. The hospital is a nine-story concrete moment frame system with pan-formed joists. The top two floors were added as a vertical expansion and were designed with lightweight concrete to reduce the demand on the already-built foundation. Due to its location and risk category, this is a Seismic Design Category D structure.
The New Mexico Department of Transportation introduced metakaolin in its standard specifications for highway and bridge construction as an acceptable SCM, leading to its common use in the state for roads and bridges. Faced with local supply issues for fly ash, Duke City Ready Mix, the concrete supplier for UNMH turned to metakaolin sourced from Chihuahua, Mexico. This decision ensured a reliable supply of high-quality concrete. Metakaolin is a highly reactive natural pozzolan produced through a low-temperature calcination of high purity kaolin clay. Metakaolin can replace portland cement at a lower dosage than other SCMs but provides a variety of benefits to the quality of fresh and hardened concrete.
To support this project, the supplier provided environmental product declarations for all concrete mixes, each containing metakaolin. The global warming potential (GWP) (kgCO2e/cy) of the primary mixes aligns closely with the National Ready Mixed Concrete Association regional averages, which assume approximately 18% SCM content. This level of GWP would not have been feasible without the use of metakaolin, given the unavailability of fly ash. The primary structure was completed without any quality issues or delays stemming from material availability. By adopting a less commonly used SCM, UNMH effectively addressed the risk of material shortages posed by relying solely on fly ash and mitigated the potential for poorer concrete quality resulting from no cement replacement. Through this approach, UNMH provides a tangible illustration of the benefits and potential of natural pozzolans.
