Editor’s Note: This table has changed since this piece posted. We will address the significance in a future post.
Concrete is the most widely used substance on our planet after water. Manufacturing its primary component – cement – generates 8% of carbon dioxide (CO2) emissions globally. Even so, most concrete consumers are not making buying decisions solely on the basis of sustainability. However, the time is coming when they will have to.
In December 2021, President Biden signed an executive order directing federal agencies to achieve net-zero emissions by 2050. The deadline matches that of industry groups in the U.S. and worldwide, with both the Portland Cement Association (PCA) and the Global Cement and Concrete Association (GCCA) encouraging members to be net-zero by 2050.
This move by our federal government is unprecedented. While agencies such as the Federal Highway Administration (FHWA) have encouraged state and local transportation departments (DOTs) to use more-sustainable materials, construction-material emissions will now be used to prioritize federal funding. Per the Buy Clean Initiative, each federal agency must set annual emissions-reduction targets and revise specifications accordingly. Federal agencies have been appropriated funding that must be awarded by 2026 to projects that incorporate “low-embodied-carbon concrete.”
What constitutes a “low-embodied-carbon” concrete? How will a specifier know whether or not a particular mix qualifies? How do cement type and content affect whether or not a concrete mix achieves that goal?
Because the U.S. General Services Administration (GSA) directs the federal government’s procurement efforts, it plays a crucial role in shaping how the federal government achieves net-zero emissions. Early in 2022, the agency released this table detailing the maximum global warming potential (GWP) limits for a mix design to be considered a low-embodied-carbon concrete. They are being tweaked based on industry feedback, but for now we can use these numbers as a baseline.
The table lists maximum emissions per kilogram (kg) of carbon dioxide-equivalent (CO2e) per cubic meter (m3) by specified strength for standard, high-early-strength, and lightweight mixes (expressed as CO2e kg/m3) at 28 days. The limits will apply to all GSA projects that use 10 or more cubic yards of concrete.
Cement’s Role in Concrete Emissions
Collective EPD data suggests cement is responsible for roughly 85% of the GWP of a given concrete mix. There are two primary ways to reduce the GWP of concrete; and both have to do with cement, the highest-carbon-footprint ingredient.
The first is relatively simple: reduce the concrete’s cement content. The problem with this tactic is physical properties that impact performance often drop with it.
The second is not always as simple: reduce the GWP of the cement – and here is where cement type comes in.
Rapid Set®, the brand name for our calcium sulfoaluminate (CSA) cement, achieves both goals without compromising performance. Due to its unique chemistry, concrete made with Rapid Set® Cement can achieve similar or exceed the same high-early and 28-day strengths as ordinary portland cement (OPC) concrete with considerably reduced-cement content.
Rapid Set Cement’s Role in Low-Embodied-Carbon Concrete
Rapid Set® Cement uses 6 sacks (564 pounds) per cubic yard of concrete; OPC typically requires 6.5 sacks (611 pounds) per cubic yard to achieve a 6500-psi mix. Therefore, concrete made with Rapid Set® Cement is using 83 fewer pounds of lower-emissions cement than concrete made with OPC.
Rapid Set® concrete mixes achieve 3000 psi compressive strength within hours and can achieve 6500 psi at around seven days. Depending on mix design, their GWP ranges from 200 to 250.
The strictest GWP limit in the GSA table is 242 for up to 2499 psi compressive strength. That means a concrete made with Rapid Set®, even if it would be well above the specified strength, may meet or beat the GWP requirement for any strength category in the table.
At 6500 psi, Rapid Set® concrete’s GWP is roughly half the required limit for standard mixes – and much less for high-early-strength mixes. Additionally, it almost always meets the high-early-strength requirements without increasing “carbon cost.” The owner, such as a state DOT or municipal public works department, benefits by being able to reopen pavement to traffic within hours rather than days or weeks without increasing cement content or concrete GWP.
This is why we believe Rapid Set® Cement has a major role to play when it comes to low-embodied-carbon concrete. (Click here to request our Type III environmental product declaration.) Next time I will share examples of how to use carbon intensity – emissions per unit of specified strength – to calculate and compare the GWP of OPC and Rapid Set® Cement concrete mixes.