IN SECTION: ENGINEERING
AN IMPORTANT STEP TOWARD ZERO CARBON CONCRETE
By Colin Lobo, PhD, PE and Lionel Lemay, PE, SE, LEED AP
Concrete producers are under pressure, just like other building product manufacturers, to reduce the carbon footprint of their products. Initiatives such as LEED, Architecture 2030 and more recently “Buy Clean” legislation are motivating building product manufacturers to reduce their carbon footprint through improved product formulation and manufacturing processes. Because concrete is the most widely used building product, it is often cited as the product that embodies the most carbon dioxide, primarily due to the cement manufacturing process. Carbon dioxide emissions from cement manufacture result from calcination of limestone, which represents about 60% of the CO2, emissions, and burning of fossil fuels to achieve temperature around 2400 °F in the kiln. Most design professionals understand that concrete is an essential component to nearly every building, road or bridge we build. Concrete is literally the foundation (and superstructure) to modern society. But concrete’s benefits – strength, safety, durability, resilience – comes at an environmental cost.
Fortunately, the concrete industry is at an advantage over other industries since concrete producers can change product formulation quickly without retooling the manufacturing process. In fact, through more efficient use of portland cement, better quality control and more effective use of supplementary cementitious materials (SCMs), NRMCA members have lowered their carbon footprint by 13% over the last five years, according to the recently published Version 3 of NRMCA’s Industry-Wide Environmental Product Declaration (IW-EPD) (ref Display footnote number:1) and Benchmark report (ref Display footnote number:2). This progress is impressive, especially considering that many projects don’t currently have a sustainability goal.
The question now is, how does the industry take its carbon footprint closer to zero? This will obviously not be an easy task. There are some promising new technologies such as alternative cements, geopolymer cements, carbon capture and mineralization technologies, new sources of SCMs, among others. Some are commercialized, but others are years away from wider adoption. There is, however, one solution that could be implemented today with little impact on concrete manufacturing – using Portland-Limestone Cement.
Portland-Limestone Cement (PLC), designated as Type IL, was established around 2012 as a specific type of blended cement in ASTM C595/C595M, Specification for Blended Hydraulic Cement. Simultaneously, it was incorporated in the equivalent standard AASHTO M 240 that is referenced in many specifications of state highway agencies for transportation projects. The adoption of this cement type was a result of several years of effort by the cement industry to reduce the carbon footprint of cement. The carbon footprint of cement is primarily associated with clinker, the product output from the cement kiln. In portland cement, which can contain up to 5% interground limestone, the clinker content is around 90% of the finished cement. Blended cements can include fly ash or natural pozzolans (Type IP), ground granulated blast furnace slag (Type IS), limestone between 5 and 15% (Type IL), and other additions.
Reducing the clinker content in the finished cement thereby reduces its carbon footprint. In a Type IL cement (PLC), the clinker content in the finished cement can be around 80%. With cement being the primary contributor to the carbon footprint of concrete, using a Type IL cement instead of a Type I or Type II portland cement for equivalent concrete performance will reduce its carbon footprint. Carbon footprint is characterized as a parameter referred to as Global Warming Potential (GWP) in a Life Cycle Assessment (LCA) report and/or Environmental Product Declaration (EPD) that includes other manufacturing and transportation factors.
An LCA report conducted by Athena Sustainable Materials Institute (ref Display footnote number:3) compared the GWP of a typical 5000 psi (35 MPa) concrete made with ordinary portland cement (OPC) and PLC with compositions shown in Table 1. Both concretes had identical mixture proportions, including the cementitious component. According to the Athena report, the GWP or carbon footprint of the concrete made with OPC is 299 kg CO2/m3 compared to 272 kg CO2/m3 for concrete made with PLC, a 9% reduction. PLC with 15% limestone would reduce carbon footprint further.
The cement industry has made great strides with state highway agencies toward the acceptance of Type IL cement in state specifications. ASTM C595 and the use of Type IL is also accepted in the ACI 318, Building Code for Structural Concrete, ACI 301, Specification for Structural Concrete, ASTM C94, Specification for Ready Mixed Concrete and the AIA MasterSpec that is used by design firms to develop their specifications for private projects. A restriction to the use of Type IL cement for concrete exposed to sulfates in soil or water was eliminated in the 2019 version of ACI 318, provided the cement was tested for sulfate resistance and complies with a MS (moderate sulfate) or HS (high sulfate) special property designation. The only restriction to the use of Type IL cement may be in specifications of private design firms that have not updated their specifications to current industry standards.
Limestone is softer than clinker and grinds preferentially. To effectively achieve equivalent performance, PLC is typically ground to a higher fineness than portland cement. While there is some evidence that limestone reacts chemically, the contribution to strength from chemical reactions is marginal. It is postulated that finer limestone particles improve particle packing in concrete (similar to fine SCMs) and provide reaction sites for more efficient hydration of cement. There is considerable research that compare and document the equivalent performance of concrete made with PLC to those made with portland cements (ref Display footnote number:4 and Display footnote number:5). These include the effects on fresh concrete, hardened concrete properties like strength and shrinkage and, for impacts on durability such as freeze-thaw resistance, alkali-aggregate reactions and other properties.
There is also published work that shows improved effectiveness of fly ash and slag cement when used with Type IL cements (ref Display footnote number:6) to further improve performance and reduce carbon footprint. The successful use of Type IL cements has been documented in field trials and actual projects (ref Display footnote number:7 and Display footnote number:8). The cement industry takes various steps to optimize Type IL cements for performance to to be comparable to that of portland cement and to comply with the requirements of ASTM C595. This typically includes additional grinding and optimizing the chemistry of the cement.
During manufacture of PLC, optimization of cement properties is based on cement tests for compliance with the cement specifications. A concrete producer should independently evaluate the impact of using Type IL cement on the performance of concrete mixtures with their local materials. This is a prudent step when using any new material. This may include evaluating water demand for equivalent slump, slump retention for typical delivery time, setting characteristics, strength to w/cm curves, admixture dosage, amount of supplementary cementitious materials that can be used for required performance and the impact on a wide range of durability concerns. If the goal of an owner of a new structure is to include sustainable construction, Type IL cement (PLC) is an important option available to the concrete producer.
NRMCA, through its P2P initiative, codes and standards activities, sustainability initiatives, EPD program and Concrete Design Center, has worked with its industry allies and design professionals to promote performance-based specifications. All these recommendations include the option of using blended cements, including PLC. We’ve made progress, but we still have work to do. While all industry standards permit the use of PLC, many project specifications still specify the use of ASTM C150 portland cement. NRMCA advocates for no restriction on the types of materials that can be used to produce ready mixed concrete, provided they comply with a material specification. If the use of PLC fits within the objectives of a ready mixed concrete producer for performance and sustainability, there should be no restriction to its use. Ready mixed concrete producers should work with cement suppliers to determine availability and, if available, consider proposing PLC as an alternate on project specifications where there is an obvious sustainability goal, especially those with a carbon footprint or GWP target.
Colin Lobo is executive vice president of NRMCA’s Engineering Division and can be reached at email@example.com. Lionel Lemay is executive vice president of NRMCA’s Structures and Sustainability Division and can be reached at firstname.lastname@example.org.
This article originally appeared in Concrete in Focus, Summer 2020