Reducing Embodied Carbon in Concrete

Specifying Portland-limestone cement makes a significant difference
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Sponsored by LafargeHolcim
By Peter J. Arsenault, FAIA, NCARB, LEED-AP

Current Concrete Industry

According to numerous sources, concrete has become the most widely used construction material in the world. In addition to buildings, it is used for roads, bridges, underground piping, railways, and other types of structures. Given these diverse uses, it is not surprising that there are a lot of different ways to formulate a concrete mixture to suit different conditions. Professional engineers and others in the industry have come together to share their expertise in two significant organizations. The American Concrete Institute (ACI) has become the leading authority and resource for the development and distribution of not only consensus-based standards but also technical resources, education, training programs, and certification programs. The Portland Cement Association (PCA) was founded as a policy, research, education, and market intelligence organization serving U.S. cement manufacturers. The stated purpose of the PCA is to promote safety, sustainability, and innovation in all aspects of construction, foster continuous improvement in cement manufacturing and distribution, and generally promote economic growth and sound infrastructure investment. Today, PCA members represent a significant majority of U.S. cement production capacity and have facilities in all 50 states.

Based on standards and programs developed by ACI and the PCA, as well as ASTM, different formulations of cement and concrete have been developed and classified. The most common formulation for the cement used in concrete for buildings and other structures is Type I Portland cement (sometimes called ordinary Portland cement or OPC). In a typical concrete mixture Type I OPC makes up about 7 to 15 percent of the total mix, aggregate makes up about 60 to 75 percent, and water is the remaining 15 to 20 percent. In many concrete mixes, intentionally entrained air may also take up another 5 to 8 percent.

Note that once the cement clinker is formed, it is ground and often blended with other approved compounds or elements. For example, gypsum (calcium sulfate) is commonly added to improve characteristics for strength gaining and setting, while processing additions and grinding aids are also allowed by the cement specifications.

Other standards exist for blended hydraulic cements of other types which allows some variation in the amount of lime or the addition of supplemental cementitious materials (SCMs) which has also become common. SCMs are utilized in the cement mix as a constituent of blended cement in or the total concrete mix as a separate component. The most common are slag cement (a by-product of iron manufacturing), Class C or F fly ash (a coal combustion by-product from power plants), silica fume (a by-product of manufacturing silicon metals), and natural pozzolans (e.g., calcined clay, calcined shale, pumice, perlite and metakaolin). These SCMs can reduce the amount of cement clinker needed and consume calcium hydroxide, a by-product of the cement hydration process, to enhance concrete durability.

Photo courtesy of LafargeHolcim

The current process of making cement produces CO2 as an unwanted by-product which can be reduced by changes to the make-up of the cement.

Embodied Carbon In Concrete

As noted already, the process of manufacturing cement produces CO2 emissions both from the burning of fossil fuels and the calcination of the limestone. When mixed with other ingredients to form concrete, the total mix can be looked at in terms of the amount of CO2 generated or “embodied” in the final finished concrete. The common definition of this embodied carbon is the total amount of CO2 that was emitted or generated during the creation, transportation, and installation of the concrete. In other words, it looks at the “carbon footprint” of the materials in the building that exists before the building is ever occupied and operating—i.e., before the lights, HVAC, and other energy consuming systems are ever turned on.

Architecture 2030 Initiatives

The not-for-profit organization Architecture 2030 was founded by AIA Gold Medal honoree Edward Mazria, FAIA. Since 2003, this independent organization has provided well-researched and documented information related to the impacts that the building and construction sector have had on energy consumption and the resulting greenhouse gas/CO2 emissions. The well-known Architecture 2030 Challenge, adopted by the American Institute of Architects (AIA) and many other organizations worldwide, calls for the operation of new and existing buildings to reach net-zero carbon emissions by the year 2030. They report that the architecture and construction community has responded, and, in fact, good progress has been made on reducing emissions from operations (i.e., more efficient, or reduced use of electricity and other energy sources when a building is occupied). (https://architecture2030.org/unprecedented-a-way-forward/) There is, of course, still more to be done.

Architecture 2030 has also researched embodied carbon in building materials. The organizations reports that on an annual basis, “the embodied carbon of building structure, substructure, and enclosures are responsible for 11 percent of global greenhouse gas (GHG) emissions and 28 percent of global building sector emissions. Eliminating these emissions is key to addressing climate change and meeting Paris Climate Agreement targets.” (https://architecture2030.org/2030_challenges/embodied/) This emphasizes the point that an important part of reducing or eliminating emissions is proper attention to embodied carbon in building products (i.e., not just building operations) since their double-digit contributions are currently quite significant. In fact, Architecture 2030 notes that it will typically take 30 years for the operation of a building to equal the emissions found in the embodied carbon of the construction of that building.

In light of the above, Architecture 2030 issued the 2030 Challenge for Embodied Carbon. This challenge asks the global architecture and building community to address the embodied carbon emissions from all buildings, infrastructure, and associated materials. The immediate target is a maximum global warming potential (GWP) of 40 percent below the industry average today. The GWP reduction shall be increased to:

  • 45 percent or better in 2025
  • 65 percent or better in 2030
  • Zero GWP by 2040

Architecture 2030 has recently updated its call to action on this topic. The organization points out that following the latest energy codes and standards plus designing buildings to use all-electric and/or renewable energy is the best current strategy to produce zero-carbon building operations. It reinforces the fact that all design professionals “must also confront the embodied carbon of building construction and materials if we hope to phase out CO2 emissions by 2040.” Architecture 2030 goes on to point out specific tactics that can help architects, engineers, and planners to minimize the embodied carbon emissions from all new buildings, major renovations, infrastructure, and construction on its website https://architecture2030.org/.

Photo courtesy of LafargeHolcim

The 2030 Challenge for embodied carbon calls for immediate action now and continued progress to net zero by the year 2040.

Assessing all of the building materials in a project toward this goal would be daunting at best. Therefore, it has been observed that addressing the products that can have the biggest immediate impact is the best approach. Those products include concrete, steel, and aluminum. Since Portland cement is the primary ingredient of concern in concrete and is responsible for the majority of concrete’s carbon emissions (i.e., approximately 95 percent of all CO2 associated with concrete), it clearly represents an immediate opportunity to act.

The place to look the most closely at is the clinker production, where most of the energy related emissions occur. On average, the “clinker factor” is about .90 to .92 ton of CO2 associated with the production of 1.0 ton of clinker in the U.S. showing a fairly strong connection. Increasing kiln efficiency helps reduce CO2 slightly by using dry kilns instead of wet kilns, and many cement manufacturers are working in this direction. Emissions related to the chemical reaction during processing is unchanged by kiln efficiency, of course. Instead, the processing and chemical formulations of the cement and/or the proportion of ingredients in a concrete mixture need to be looked at to influence its carbon impact.

Portland-Limestone Cement: Reduce Carbon, Keep Performance

In the quest to help concrete continue to evolve and improve its ability to reduce carbon emissions, those in the cement industry have pursued industry acceptable alternatives to ordinary Portland cement. One of the most promising options that can be used immediately is to switch from OPC, classified as Type I cement, to Portland-limestone cement (PLC) classified as Type IL cement. PLC is a slightly modified version of Portland cement that has been an accepted alternative since the year 2012. The primary difference between blend of ingredients to create PLC is that it contains more ground limestone, thus replacing the amount of clinker required to be used in the cement. Ordinary Portland cement is limited by definition and standards to a maximum of 5 percent limestone in its makeup. By contrast, Portland-limestone cement is allowed to contain between 5 to 15 percent limestone.

Carbon Reduction Potential

Currently, approximately 100 million tons of cement are produced annually in the U.S. However only about 2 to 3 percent is currently specified and sold as PLC. Architects and other design professionals can help increase this percentage immediately by changing their concrete specifications and switching from Type I OPC to Type IL PLC. Switching helps to move the entire construction industry forward in terms of environmentally responsible action since the embodied carbon of cement used in projects is directly reduced due to the higher percentage of limestone in the PLC. Industry wide, it is estimated that the switch to PLC could reduce energy consumption by 11.8 trillion Btus and carbon dioxide emissions by more than 2.5 million tons per year.

 

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Originally published in Architectural Record
Originally published in September 2021

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