By Don Greive, PE, and David Buzzelli 

Introduction

Type IL cement, also commonly referred to as Portland Limestone Cement (PLC), is in the process of replacing Type I/II cement in the US. Type IL is a cementitious product with more than two decades of usage in Canada and parts of Europe, but the US market has only recently experienced it in the last two years. Local and regional markets have begun the transition, incrementally, based on a variety of factors. Where US markets have yet to fully commit, the timing of the transition depends on location. For example, in Texas, Type IL cement has been common in the Dallas/Fort Worth area over the last couple of years; it is being used more and more in the Austin/San Antonio area at this time; and it has not been used very much in the Houston area (the authors are currently aware of two vendors). The Portland Cement Association has a dedicated website for this product (https://www.greenercement.com) where awareness and technical information, including specifications, are available. A real-time map for the transition is maintained with vendors identified in available markets. Where availability is yet to be confirmed, vendors have indicated that it will be coming but they are not sure of the timing.

Type IL cement is considered a blended cementitious material consisting of Type I Portland cement blended with finely ground limestone. The transition to Type IL cement is primarily being marketed as a method to cut CO2 emissions, and represents the latest effort from the cement industry to meet the net zero carbon initiative set for 2035. Cement manufacturing continues to be one of the leading contributors to CO2 emissions, accounting for approximately 5% of the total global CO2 emissions each year (1.5% in the US market). CO2 emission in cement production comes from the clinker process, where limestone chunks are refined and purified by intense heat. The combination of the heat generation and the release of CO2 trapped in the limestone generates a high emission rate. Although the US market has more modern plants with cleaner operations and capture technologies, CO2 emissions remain a significant issue globally. With Type IL cement, the amount of clinker produced for cement is reduced by adding between 8% and 15% crushed limestone to produce the blended cement. Cement producers have estimated that producing Type IL cement, instead of Type I/II, will reduce CO2 emissions by 8% to 10%. Other potential benefits include a potential decrease in cost, as clinker is expensive and limestone is cheaper. In addition, cement availability may be slightly improved due to increased cement production per amount of clinker.

The transition to Type IL cement, however, has not been seamless. Some issues associated with Type IL cement have resulted in various organizations discussing the cement. The American Society of Concrete Contractors (ASCC) has conducted surveys to see what impacts the use of Type IL cement was having. About half of the survey respondents reported issues with using Type IL cement, but half also reported no issues.

Issues

The ASCC survey indicates that not everyone is seeing issues with mix designs utilizing Type IL cement. There are many reasons this may be happening. There are differences in the Type IL cements that the various manufacturers are producing, including the amount of limestone and the fineness of the limestone. Also, there are variations in aggregates across the country, and the interaction between the aggregates and cements will hence differ. Also, differences in the use of admixtures and supplementary cementitious materials (SCMs) can also impact how mixes perform. 

As Type IL cement is relatively new and just now being introduced into some markets, there is confusion about the impact of the cement and what (if anything) needs to change. One company that manufactures cement and operates ready-mix plants sent a letter noting the environmental enhancements resulting from Type IL cement as the primary reason for the switch. They also indicated that the change to Type IL, in a particular market, would happen at some unknown time in the future, and that the change would be as simple as a pound-for-pound substitution of Type IL for Type I/II cements in existing concrete mixes without a change in mix numbers. It is worth noting that the US market changed from Type I cement and a separate Type II cement (low-heat hydration) to the Type I/II variety with very little industry knowledge. There were a variety of factors for this, including changes in ASTM specifications, product evolution of the Type II performance, and decreasing product availability.

Change is common and inevitable in any industry, and cement is no different. Historically, migrations were based on pure or true cement types, so the introduction of supplementary waste material products with hydration capabilities initiated previous migrations. The latest leverages on such evolution are the desire, and now mandate, for greater sustainability and off-setting of carbon emissions. While this is yet another source of consternation among some whose work involves concrete, the migration to Type IL cement is the most significant change the industry has endured in some time. Engineers and concrete contractors generally agree that the change in cement type should result in new mixes. Mix numbers referencing Type I, II, or I/II concrete should be changed, related to the use of Type IL, so that there is no confusion regarding which cement type was used. Although batch plants may not have the option, they should not allow a switch in cement types to occur without notice to all involved. 

With different cement types, the concrete mixes are not the same. As such, historical data for mixes with Type I/II cement cannot be used for Type IL cement; trial batches should be required for mixes with Type IL cement because there is no historical data. This is no different than what would occur for a change in aggregates. For example, most engineers and concrete contractors would expect a different mix number if everything in two mixes are identical except that one uses limestone for coarse aggregate and the other uses river rock. These notifications of change should then prompt ready-mix producers and contractors to run tests with these mixes to understand behavior more fully.

Strength is not the only requirement for concrete. For example, on the typical tilt-up office/warehouse project, developers are looking for a consistent appearance in all interior slab sections. Many developers will insist on using only one mix design for all interior slabs. Therefore, a change in cement type should be considered a change.

In some geographic areas, differences in compressive strength have been observed with the delivery of Type IL mixes. This was seen in the strength gain or maturity after placement as well as the 28-day strength. In tilt-up construction, the initial strength gain rate can impact the timing of panel erection. Slower strength gain has been noted in some projects. In fact, on one project, the contractor elected to test cores from some panels to determine if they could lift panels after cylinder testing failed to give adequate strength results. On some projects, 28-day compressive test results have been lower than with mixes with Type I/II cement. If no historical data for mixes with Type IL cement exists, this reiterates the need for trial batches. To demonstrate the potential differences in compressive strength in the Houston market, one batch plant mix design had the exact same proportions of cement, water, and aggregate for two different mixes: one with Type I/II and one with Type IL. However, the compressive strength indicated for the Type I/II mix was 3,500 psi and 3,000 psi for the Type IL mix. The weight was identical, the individual weights of the constituents were identical, the admixtures were identical, and the aggregate types were identical. The only variation was the actual cement type.

Additional testing of concrete mixes with Type IL cement should be considered, especially for tilt-up concrete construction. Since most lifting and bracing engineers specify a minimum modulus of rupture (as determined by ASTM C78 standard beam test for flexural strength of concrete), in order to lift panels, flexural tests should be performed for new mixes with Type IL cement. Since there is even less data to document the relationship between compressive strength and flexural strength with Type IL cement, concrete flexural testing is encouraged to help identify the appropriate times for panel erection. Occasionally, the construction team raises concerns regarding the costs of performing adequate flexural tests. However, compared with delays or other issues associated with lifting panels because of performance differences due to the change in cement, testing costs are minimal.

At least one cement manufacturer has indicated that Type IL cement results in increased water demand, which would indicate that more cement would be required in order to obtain the same strength as concrete made with Type I/II cement. Again, this contradicts the claim that Type IL cement can be substituted pound for pound for Type I/II cement.

In addition to strength and moisture issues, some concrete contractors are having issues with placement. The ASCC survey reported respondents having projects with both faster and slower set times. Some contractors have reported evaporation rate issues, resulting in the increased use of evaporative retarders.

There have been reports of reductions in bleed water with certain Type IL cements. This may be related to the fineness of the limestone in the Type IL cement, which varies between manufacturers. The lack of bleed water can result in water being trapped just below the top surface of the concrete, which could cause blistering. It also complicates the timing of finishing operations, as finishers normally use bleed water to time their finishing operations.

In central Texas, there have been reports of increased curling stresses with the use of IL cement. The most common detail at construction joints of slabs-on-grade is the hybrid detail, which alternates deformed bars and plate dowels. As a result of the increased curling stresses, consideration has been given on some projects to reducing the spacing between dowels and/or increasing the size of the dowels.

With the various issues associated with strength and placement, mix designs utilizing IL cement may need to consider differences in admixtures and (SCMs). Current mix designs typically include a note stating that, due to supply issues, the concrete supplier may change the cement supplier at any time. With the variations in Type IL cement between suppliers, this becomes a larger issue regarding a change in supplier can affect the strength and placement of a mix, as a concrete contractor can have totally different issues with the same mix design number from a vendor if there are different cement vendors. Changes in vendors of cement should not happen without all parties being notified of the change.

Where Do We Go

Twenty-eight day compressive strength is often the one item that everyone looks at regarding the acceptance of concrete. However, there are many other factors. For example, in hot weather, concrete suppliers will provide letters that guarantee the strength of the concrete at an elevated temperature range. But this does not consider other factors associated with placement, such as the increased likelihood of shrinkage cracking with elevated temperatures. Equivalent strength does not always mean equivalent performance. Even if the hardened properties are the same, the plastic properties may not be. Success needs to consider placement concerns as well as strength.

With the introduction of Type IL cement into markets, there is a learning process at hand. Everyone recognizes the environmental benefit of using Type IL cement, and understands that it will not be going away. As such, everyone must recognize the potential differences between Type IL and Type I/II cements, and be willing to make changes. New mix designs are necessary, as a pound-for-pound substitution of Type IL cement for Type I/II cement, with no implications, is unrealistic. In addition, mix design numbers should change with different cement manufacturers. Standard details may need to be adjusted. Finishing procedures may need to be modified. Cooperation is needed between the cement manufacturers, the batch plants, general contractors, concrete contractors, and design professionals to make the transition as smooth as possible.

In Texas, concrete contractors have offered to pour test slabs with Type IL cement. This allows the concrete contractors to learn of the differences in working with this new cement. Also, this provides additional opportunities for testing the concrete at different ages to observe if there are differences in the rates of strength gain. With tilt-wall construction, the lifting of panels is a key part of the construction schedule. Since most lifting and bracing engineers require a certain modulus of rupture of concrete, flexural tests for new concrete mixes with Type IL cement should be performed. In addition, it may help concrete suppliers (and concrete contractors) understand how changes in admixtures and supplementary cementitious materials (SCMs) can benefit mix designs.

Conclusion

Type IL cement is here to stay. Therefore, to embrace the existing or inevitable change, it is paramount that everyone develops an understanding of these differences between mixes with Type IL cement versus Type I/II cement. These differences could be regional, related to different environments or standard procedures, or related to differences between Type IL cements as manufactured by the different vendors. A position that pretends everything is the same, or assumes that “concrete is concrete,” will only lead to troubleshooting the resulting issues because of these inherent and discoverable differences. Utilizing experience, and all parties working together, will lead to successful projects.

While the use of Type IL cement has brought some challenges, it also provides opportunities. The environmental impact is obviously a plus. Also, Type IL cement offers a reminder that strength is not the only quality that matters with concrete. The plastic properties of concrete are just as important as the hardened properties, as concrete formulated to reach its strength requirement may not be adequate if placement and finishing cannot be accomplished so that the concrete can serve its intended purpose. By working together, all parties can take this opportunity to use concrete, making the most of plastic and hardened properties, and resulting in successful projects while cutting CO2 emissions.

About the Authors:

Don Greive is CEO at Pinnacle Structural Engineers of Houston, Texas. He holds a BS in civil engineering (structures option) from Rice University, an MS in civil engineering (structural engineering/structural mechanics) from the University of California-Berkeley, and an MBA from the University of Houston-C.T. Bauer College of Business. Don currently serves on the board of directors for the Tilt-Up Concrete Association. Pinnacle Structural Engineers is a multidisciplined firm that provides comprehensive and constructible structural designs while meeting schedule requirements for economic structures. Their project work is designed and detailed, considering architectural and MEP constraints with full attention to each and every client, irrespective of project size and budget.

David Buzzelli is vice president at Texas A&M Concrete, LLC, in Houston, Texas, and an ACI fellow. He studied construction management at San Jacinto College and has led a talented team of construction professionals with a foundation of estimating and project management over the past three decades. He serves the industry on numerous ACI committees, including chairing the subcommittee on tilt-up for ACI 117 and the Craftsman Certification committee. Texas A&M Concrete offers full, turn-key solutions to clients as one of the largest full-service concrete construction companies in Texas, with the staff, equipment, and experience to supply, form, place, and finish all concrete requests.