By Kayla Hanson, P.E.
The world is ever-evolving as success depends on improved processes, optimized outcomes and making the most with what is available. To not do so invites stagnation and even decay.
Generation after generation of eager and capable minds throughout the precast concrete industry bring passion and dedication to all the challenges that need solving.
One challenge for current precast producers and cement suppliers to tackle is carbon emissions. As nature pushes back on excess CO2 in the atmosphere and governments set stricter standards, some parts of North America already are moving toward materials that satisfy these requirements in anticipation of new laws.
One approach gaining popularity is a concerted move toward limestone cement, also called portland-limestone cement (PLC). Some owners, cities and counties already are updating their specifications to require limestone cement in precast and cast-in-place concrete products. Some cement manufacturers have ceased all production of Type I cement in favor of limestone cement.
This material isn’t a new solution, so what is causing the shift?
To appreciate the motivation behind the growing prevalence of limestone cement in specification requirements and the increase in limestone cement production, it is important to examine what limestone cement is and why it is available in the first place.
BLENDING FOR OPTIMIZATION
ASTM C595, “Standard Specification for Blended Hydraulic Cements,” describes limestone cement as a hydraulic, binary blended cement that contains more than 5% but no more than 15% by mass finely ground limestone. Limestone use in cement is not new. ASTM C150, “Standard Specification for Portland Cement,” allows up to 5% by mass limestone in typical Type I cement. Therefore, switching from ASTM C150 Type I cement to ASTM C595
Type IL cement could result in a limestone content increase of less than 1% in a given concrete mix design.
Material blends such as this are common throughout daily life. Ethanol added to gasoline helps fuel burn more completely and produce cleaner emissions. Polyester added to many fabrics provides flexibility and added comfort. And precasters carefully tailor concrete mix designs with blends of different raw materials to create the most advantageous fresh and hardened concrete characteristics possible given the selected materials and proportions.
Combining or blending materials allows precasters to take advantage of both materials’ benefits in one convenient package. In some cases, the blended materials work together to unlock new advantages.
COMBINING AND BLENDING CEMENTIOUS MATERIALS
Combining different cementitious materials in specific proportions allows precasters to optimize concrete mix designs and bring out the best that each raw material has to offer. Some specifications call for a combination of two or more cementitious materials in a concrete mix – such as portland cement and fly ash – which are batched from separate silos into a concrete mixer.
However, some precasters use cements that are preblended in specified proportions and can be batched into the concrete mixer from a single silo. Using blended cements eliminates the need for separate silos for each constituent cementitious material and simplifies the batching process.
WHAT BENEFITS CAN LIMESTONE CEMENT OFFER?
Limestone cement offers more than the convenience of batching multiple cementitious materials into a mixer from a single silo.
Reduced Environmental Impacts
Cement clinker production is the most energy-intensive process involved in manufacturing concrete. The bulk of the energy consumption is attributed to limestone calcination, which is a primary ingredient in cement clinker, and burning coal, oil and natural gas to heat cement kilns. Advancements in renewable energy production have opened the door to less energy-intensive cement manufacturing processes – such as using wind energy to heat cement kilns – but these approaches have yet to go mainstream.
Using less energy-intensive raw materials in concrete mixes can reduce concrete’s CO2 emissions. Targeting the most energy-intensive aspect of concrete – cement – and reducing reliance on it, reduces the overall energy demand of the product, structure or system and reduces the overall CO2 output.
“The primary sustainability effect of using limestone as an ingredient in blended cements at levels of 5% to 15% by mass is that less clinker has to be produced for an equivalent amount of cement, and therefore less energy is consumed, and CO2 emissions (and other greenhouse gases) are reduced,” according to Tennis, Thomas and Wiess’ 2011 “State-of-the-Art Report on Use of Limestone in Cements at Levels of up to 15%.” The Portland Cement Association (PCA) reports that use of PLC typically can reduce the CO2 footprint of concrete by 10%.
Enhanced life cycle performance
Life cycle assessments (LCAs) analyze the environmental impact of a given product, structure or system. LCAs consider all phases of the product or system in question – starting before construction with the selection, production and procurement of raw materials to construction, service and beyond to reuse, recycling and disposal. Precast concrete structures boast exceptional service lives, especially in comparison to service lives of alternative materials in the same applications. Therefore, despite cement production’s significant CO2 emissions, precast concrete’s LCA is favorable over that of alternative materials.
Still, like precasters, project owners continually look for strategies to improve the environmental footprint of products, structures and systems over which they have discretion. One approach is to select raw materials that require less energy-intensive manufacturing processes. As a result, many specifications call for use of supplementary cementitious materials (SCMs) in concrete mix designs, and more specifications are calling for use of blended cements.
Local availability for some
Local availability of limestone sources for limestone cement production provides additional cost savings, time savings and reduction in CO2 emissions because of shorter transportation distances. Limestone is abundant in many – but not all – regions of North America.
Cement suppliers located in proximity to limestone will be more likely to shift toward producing Type IL cements than those located a great distance from the resource.
Chemical property consistency
All concrete raw materials must conform with specific industry standards that set forth strict requirements for quality, purity, chemical composition, fineness and size, among other characteristics, to ensure consistent properties and predictable behavior in concrete mixes.
Material standards outline rigid ranges of acceptable values for certain characteristics. Slight variations in material characteristics can occur naturally because of regional geology and the source and process by which some materials, such as fly ash, are produced. Any slight variation in material properties – even within the acceptable ranges outlined in the applicable standard – could lead to minor variations in the material’s behavior. Reducing material variability and variation in sources and processing, as with limestone cement, can help improve material consistency and predictability.
ASTM C595-compliant Type IL cement is considered general purpose cement, meaning it can be used in most applications where special properties and behaviors are not desired or required.
Additionally, limestone cement typically performs similarly to ASTM C150-compliant Type I cement, largely because the majority of the limestone cement makeup (85-95%) is ordinary portland cement while the remainder of the composition is limestone. Despite their similarities, Type IL and Type I cements are not necessarily interchangeable.
Limestone cement particle size
Limestone is softer and easier to grind than cement clinker. Therefore, when limestone is interground with portland cement, the limestone particles tend to constitute the majority of the smaller particles in the resulting blended limestone cement. Incorporation of smaller particle sizes can improve the limestone cement’s particle packing and overall particle size distribution by providing a more well-graded assortment of particle sizes.
“When limestone is interground with portland cement clinker, it is important to recognize that the Blaine fineness of the finished cement will generally be higher than the portland cement since the limestone is softer and more easily ground,” Tennis, Thomas and Weiss wrote in 2011.
Water demand, workability and bleeding
The greater the proportion of limestone in the limestone cement blend, the greater the Blaine fineness tends to be. A higher Blaine fineness corresponds to greater total particle surface area by weight of material, which can cause an increase in water demand and a reduction in workability.
However, there is conflicting data on this subject. A 1993 study Schmidt et al – “Blended Cement According to ENV 197 and Experiences in Germany” – showed that portland-limestone cement concretes displayed increased workability when the cement blends contain 13% to 17% limestone. A 1994 study by Matthews – “Performance of Limestone Filler Cement Concrete – reported that portland-limestone cement concretes made with” cement blends containing less than 25% limestone required increased water-to-cement ratios (w/c) of about 0.02 to maintain workability.
Bleeding in concrete is attributed to the water content, mix design proportions, particle size distributions of both aggregate and cementitious materials and the depth of the concrete section, among other factors. According to Tennis, Thomas and Wiess, similar to concretes made with only ordinary portland cement, bleeding rates in limestone cement concrete decrease with increased limestone cement fineness.
Concrete set time, heat of hydration and strength development
Raw material particle sizes affect numerous fresh and hardened concrete behaviors. The smaller overall the particle sizes, the greater the total particle surface area is. Greater total surface area relates to expedited chemical reactions since there is more surface area where products can come in contact and react with one another. As a result, cement hydration reactions can occur at a faster rate when cement particle sizes are smaller.
Similarly, set times for limestone cement concrete are strongly related to the limestone’s particle fineness. As with ordinary portland cement, as limestone cement’s particle fineness increases, set times decrease. Additionally, the lower the limestone content in the blended cement (closer to the minimum allowable 5% by mass), the less impact the limestone has on set times, while the greater the limestone content in the blended cement (closer to the maximum allowable 15% by mass), the greater impact the limestone has on set times.
The use of limestone cement also can affect hydration rates in other ways. Fine limestone particles can act as nucleation sites that can promote further silicate hydration.
For comparison, ASTM C150 outlines set times for Type I cement mortar of no less than 45 minutes and no greater than six hours, 15 minutes, while ASTM C595 outlines set times for Type IL limestone cement mortar of no less than 45 minutes and no greater than seven hours.
Heat of hydration typically is slightly reduced in limestone cement concrete. However, the reduction is not proportional to the reduction in portland cement content. According to Tennis, Thomas and Weiss, “Heats are generally increased when limestone is used as an addition to concrete but decreased slightly when the limestone is used as a replacement for cement. However, it does not decrease to a point where the limestone can be considered to be completely inert.”
All other factors held constant, limestone cement generally can produce slightly lower compressive strengths when compared to concrete made with ordinary portland cement, because the proportion of portland cement is reduced. This is referred to as the dilution effect.
However, other variations of limestone cement, including high early strength limestone cement, and mix design modifications can be incorporated to counteract some of the potential strength reduction. Additionally, limestone particle fineness plays a significant role in long-term concrete strength development. Figure 1 demonstrates how limestone cement concrete’s compressive strength can surpass that of ordinary portland cement concrete as the limestone’s Blaine fineness increases.
If your cement supplier has provided notice that production of Type I cement will be discontinued in favor of producing Type IL cement, start preparing now for the change.
Request a copy of the limestone cement mill certificate in advance, obtain a copy of ASTM C595 to review the specifics and consult the admixture supplier regarding how the change could affect mix design, production processes, curing, strength development and other factors.
The impacts of using Type IL cement are generally not significant, but the two most likely variations that could occur include slightly reduced early-age strengths and a slight increase in water demand. There are a variety of straightforward ways to counteract both.
Produce trial batches of each mix design using the Type IL cement and develop new initial mix qualification documentation for each mix design that states target fresh and hardened concrete test data along with allowable tolerances. Observe how concrete behaves in different applications, different forms, with varying reinforcement assemblies and in different ambient conditions.
Also, be sure to talk to customers proactively about what Type IL cement is, how it’s similar and different to the previous cement to which they are accustomed and what they should expect from you as the precaster.
NPCA’s technical services team can help review and revise specifications as needed to adapt to these changes and can assist with any other inquiries you may have. Contact us today: [email protected] or (800) 366-7731.
What are blended cements?
Blended hydraulic cements combine portland cement with at least one other cementitious material – most often slag, pozzolans or limestone – and are produced by uniformly intergrinding precise amounts of the specified materials.
Blended cements must conform with ASTM C595, “Standard Specification for Blended Hydraulic Cements,” which outlines four main classes of blended cements:
- Type IS – Portland blast-furnace slag cement
- Type IP – Portland-pozzolan cement
- Type IL – Portland-limestone cement
- Type IT – Ternary blended cement
Type IS, IP and IL are binary cement blends while Type IT is a ternary cement blend. Binary cement blends consist of two primary components – portland cement blended with either slag cement or granulated blast furnace slag (Type IS), a pozzolan (Type IP) or limestone (Type IL). Ternary cement blends consist of three components – portland cement blended with either a combination of two different pozzolans, slag and a pozzolan, a pozzolan and a limestone or slag and a limestone.
Additionally, ASTM C1195, “Standard Performance Specification for Hydraulic Cement” outlines performance-based requirements for blended cements.
What’s in a blend?
Both binary and ternary cement blends can have varying amounts of portland cement, slag, pozzolanic material and limestone. Portland cement does not always make up the greatest proportion of material in blended cements; however, portland cement is always the majority ingredient in limestone cement.
The naming convention for blended cements clearly states the target percentage of slag, pozzolan and limestone in the blend, shown in parentheses after the type designation:
- Type IS(X)
- Type IP(X)
- Type IL(X)
- Type IT(AX)(BY)
“X” and “Y” refer to the nominal mass percentage of the non-portland cement ingredients in the cement blend.
For example, a binary blend Type IL(10) cement contains 90% by mass portland cement and 10% by mass limestone and a binary blend Type IP(15) cement contains 85% by mass portland cement and 15% by mass pozzolan.
Because ternary blends contain two other cementitious materials other than portland cement, the letters “A” and “B” are used to indicate which other materials are in the blend. “A” refers to the material in the greater amount while “B” refers to the material in the lesser or equal amount.
As with binary cement blends, “X” and “Y” refer to the nominal mass percentages of the non-portland cement ingredients. For example, a ternary blend Type IT(S15)(L10) cement contains 75% by mass portland cement, 15% by mass slag and 10% by mass limestone. A ternary blend Type IT(L10)(P10) cement contains 80% by mass portland cement, 10% by mass pozzolan and 10% by mass limestone.
Blended cements have set proportion requirements for the constituent materials in the blends, just like ASTM C150-compliant cements have required minimum and maximum amounts of ingredients to ensure consistency in chemical composition and the cement behavior.
Type IL, IS(<70), IP and IT(S<70) are general purpose cements, meaning they can be used in most any application where special properties and behaviors are not desired or required. Additionally, Type IL cement can be produced with optional special properties, as indicated by the applicable suffixes: moderate or high sulfate resistance (MS or MH), moderate or low heat of hydration (MH or LH), high early strength capabilities (HE) or air-entraining characteristics (A).
Kayla Hanson, P.E. is the NPCA director of technical services.