The cement industry is undergoing a major transition to lower its carbon footprint to align with stakeholder 2030 and 2050 climate-action goals. According to the World Green Building Council, concrete can represent 50% or more of the global warming potential (GWP) of a building. Additionally, it can be responsible for as much as 8% of global carbon dioxide (CO2) emissions predominately due to making the main binder used in concrete—ordinary Portland cement (OPC).
In 2021, many major U.S. cement manufacturers began to reconfigure their product portfolios and prioritize developing and marketing products aimed at lowering the amount of OPC clinker in concrete. One such scalable solution was Portland Limestone Cement (PLC), which is specified as Type IL cement. Many cement manufacturers have embraced this solution by swiftly transitioning away from producing Type I/II OPC and supplying the market exclusively with Type IL cement.
The cement industry has suggested that Type IL cement is an easy blended cement solution for meeting these demands as it can be used as a one-to-one replacement of OPC in all types of general and special applications. However, many engineers and concrete producers that work with CTLGroup have questioned whether the product will meet requirements for durability and concrete placement practices. While research from the cement and concrete industry concludes that Type IL cement offers like for like performance when compared to OPC, more research is needed to iron out the nuances needed to improve concrete predictability and quality.
What is Type IL Cement?
Whereas the U.S. standard for Portland cement (ASTM C150) allows up to 5% limestone in typical Type I/II cement, specification standards for blended cements (ASTM C595 and AASHTO M 240) allow up to 15% of clinker to be replaced by finely ground limestone, thus reducing concrete’s carbon footprint. This is significant considering that more than 400 million cubic yards of concrete were produced in the U.S. during 2022. To further reduce the concrete’s carbon intensity, similar percentages of supplementary cementitious materials (SCMs), such as fly ash and slag cement, can also be used.
PLC has a long history of use in a variety of exposure conditions and applications throughout Europe and it has been increasingly used in Canada since the adoption of the CSA A3000 standard in 2008. It was introduced into ASTM and AASHTO U.S. specifications in 2012.
With the recent acceptance of PLC by CalTrans, 44 state departments of transportation now allow the use of this low-carbon cement for their projects. Type IL cement is also widely included in building codes—such as the International Building Code, ACI 318 (Building Code Requirements for Structural Concrete), and ACI 301 (Specification for Structural Concrete).
It is recognized by the International Code Council, Federal Aviation Authority, Army Corps of Engineers, and American Institute of Architects Master Specs.
The State of Research: Performance Attributes
The cement and concrete industry have performed product-development testing and field application evaluations to measure Type IL cement’s performance attributes. According to these studies, Type IL cement has similar fresh and hardened performance properties to traditional Type I/II OPC.
The Cement Association of Canada released a technical report in May 2021 that includes studies around workability, pumpability, set time, early and ultimate strength (compressive and flexural) gain, permeability, freeze-thaw durability, resistance to deicer salt scaling and sulfate exposure, and mitigation of shrinkage and alkali silica reactivity.
Some skepticism is still being expressed by the engineering and ready-mix industries, as many project and product specific studies have not been performed. For example, CTLGroup’s experts have received recurring questions around wearability for industrial flooring applications. Numerous clients have questioned whether concrete produced from PLC will have the same wear durability as a concrete produced from OPC.
Material Behavior Nuances
The impacts of using Type IL cement are generally not significant, largely because most of the cementitious composition is OPC (85-95%), while the remainder is limestone. Despite the similarities between Type IL and Type I/II cements, variations in particle fineness, softness, and limestone levels can lead to differences in the ways the material behaves. Knowledge of these performance nuances can help improve material predictability and the quality of the cement paste and finished concrete.
When limestone is interground with clinker, the fineness of the PLC will be higher than OPC because the limestone is softer. The greater the proportion of limestone in the blended cement, the greater the fineness and total particle surface area of the material. This can cause reduced workability, decreased bleeding, increased water demand, and less desirable finishing characteristics. If the concrete is not kept moist and cured properly, durability, strength, and abrasive resistance are affected. Failures can include plastic shrinkage cracks, thermal cracks, and a weak and dusty surface.
Similarly, as Type IL cement’s particle fineness increases, set times decrease. The greater the limestone content in the blended cement, the greater the impact on set times. In addition, Type IL cement can produce slightly lower compressive strengths when compared to concrete made with Type I/II cement, because the proportion of OPC is reduced. This is referred to as the dilution effect.
Challenges of Current Testing Standards and Performance Metrics
The switch to Type IL cement offers an opportunity to significantly reduce the carbon intensity of concrete and create a more sustainable future. However, the change also brings some challenges with retesting, regulatory issues, and performance nuances that need to be addressed.
With Type I/II OPC quickly becoming more difficult to procure, there has been a heightened level of testing and performance reverification requests on Type IL cement mixes. One regulatory issue, which is impacting laboratory work, is that many of the test method (ASTM or ACI) performance metrics are based on Type I/II OPC, which are not necessarily applicable for Type IL cement. A specific application recently encountered was in the determination of modeling the heat evolution of a mass concrete structure (a “thermal control plan”). Where the chemical evaluation was not readily provided as part of the construction documentation (ASTM C595 does not require such). The problem was alleviated with communication between the engineer and cement supplier. This has prompted questions from the specifying community on whether current test methods are appropriate for verifying the performance of Type IL-based concrete mix designs in their building and infrastructure projects.
There are, as previously stated, continuing apprehension within the industry on Type IL cement behavioral nuances that may impair workability, pumpability, finishing, surface wearability and overall constructability. Specifiers are increasingly looking for scientific data and questioning whether enough research has been done to verify that Type IL will deliver the same level of durability, scaling resistance, wear, and extended service-life properties that they have come to expect with using Type I/II. After all, OPC has been used in the U.S. for a very long time, while PLC is still considered a relatively new development.
Making the Case for Additional Research
The industry is marking a major milestone on its journey to a more sustainable future with the market conversion to lower carbon-intensive Type IL cement. While the performance attributes of Type I/II and Type IL cement are similar, much work lies ahead in addressing the impact of minor differences in material behaviors—and providing the industry with the confidence to embrace it fully.
Updating performance standards and modifying test methods to support the reliable assessment of PLC-based concretes need to be expedited. Additional clarity is required about material behavior nuances and their impact on fresh and hardened concrete properties. And, finally, more in-depth investigative research studies are necessary to further assess and verify long-term performance.
Studies around workability (especially with varying cement products), flat surface wear, scaling due to freeze-thaw, test behavior with SCMs, and additional refinement test methods for alkali silica reactivity would benefit the industry and help create an even better product in the future.
CTLGroup is enthused, prepared, and well positioned to support the industry in building greener and better with blended cements, including Type IL PLC. We have worked with the cement industry over the last 25 years on the testing and development of limestone into cement, our material scientists are actively engaged in ASTM and AASHTO standard committees, and we participate in CCRL round-robin testing and qualification.