This article originally appeared in the 2023 Q3 edition of Precast Today.
By Claude Goguen, P.E.
The invention of portland cement in the 1800s ushered in an era for concrete to become the most used construction material in the world.
Portland cement seems to have everything going for it:
- Abundance of raw materials.
- Easily manufactured at a relatively low cost.
- Can be used to make a good, quality, durable binder.
But scientists continue to strive to make an even better cement with less environmental impact and increased durability. As a result, the industry now has an array of potential supplementary cementitious materials for use in blends with portland cement or as alterntive cements for use as standalone products.
These products are manufactured in a variety of locations and have a variety of construction and curing needs across a spectrum of very similar to very different from portland cement.
The question is: Can these cements work for precast concrete?
WHY THE NEED TO CONSIDER OTHER CEMENTS?
The essence of continuous improvement is to never accept the status quo as the final destination. There always is room for improvement on materials and processes used in manufacturing precast concrete.
At first glance, many new cements, admixtures, reinforcement and plant equipment may not seem like a good fit, but they at least merit review.
Are there cementitious blends or portland cement alternatives manufacturers should consider? It comes down to project goals.
What situations justify the contemplation of different cement blends?
- Enhanced durability that could extend a product’s life cycle.
- Shorter setting times that could increase productivity.
- Lower embodied carbon that could improve a product’s sustainability and meet requirements of emerging low-carbon related regulations.
Some cements could be seamlessly incorporated into precast concrete production, requiring only mix design modifications. Some cements require a different curing system. Others require an entirely new batch plant.
The order in which they are presented below reflects the difficulty in incorporating such a cement in a precast plant, from fairly simple to complex.
BLENDED HYDRAULIC CEMENTS
The concrete used in precast manufacturing can contain just one hydraulic cement – or a blend of hydraulic cements and supplementary cementitious material (SCM). That blending can occur at the mixer by adding the binder materials individually, or the blending can be done by the cement supplier.
The latter process often results in a blended cement governed by ASTM C595 (AASHTO M240) or ASTM 1157. These preblended products are higher quality than blending at the precast plant.
Blended cements governed by ASTM C595 have made their way into precast plants by way of the cement industry’s mass introduction of type IL portland limestone cement (PLC). Other blended cements can present some viable alternatives to the current binder system.
ASTM C595 binary blends include portland cement or clinker with one slag, pozzolan or limestone. A ternary blend includes portland cement or clinker with two other materials, which could be two different pozzolans, a pozzolan and a slag, a pozzolan and a limestone or a slag and a limestone.
For binary blends, the allowable portland cement substitutions are up to 40% by mass for pozzolans and 95% by mass for slag cement (70% for structural applications). For ternary blends, the maximum pozzolan content is 40% by mass of the blended cement and for limestone it is 15% by mass of the blended cement. The maximum total content of pozzolan, limestone and slag is 70% by mass of the blended cement.
Type IL cement allows up to 15% limestone as opposed to the 5% limitation in ASTM C150. This additional limestone content helps reduce the cement’s carbon footprint by lowering the amount of portland cement.
Lowering concrete’s embodied CO2 emissions is the main reason blended cement use is increasing in the US, and that’s not just limited to type IL. Types IP, IS and IT can result in much higher portland cement replacement and, consequently, lower embodied CO2. They also often exhibit lower permeability, enhanced durability and higher longterm strengths.
However, replacing higher amounts of portland cement with one or two other materials can present other issues. Depending on the proportions and materials, the primary issue that may affect precast manufacturers is longer setting times and slower early age strength gain. This could impact the stripping or release of prestressing strand schedule and impair precast production.
However, these delays could be mitigated with the use of accelerating admixtures.
Overall, the benefits of higher SCM proportioned blended cements may outweigh the drawbacks.
“Higher SCM replacement blended cements are definitely a viable option moving forward,” said Dr. Kyle Riding, researcher and professor of civil and coastal engineering at the University of Florida.
LIMESTONE CALCINED CLAY CEMENT
One specific cementitious blend emerged out of Switzerland around 2012. It’s called LC3 and is a blend of portland cement, limestone and calcined clay. The proportions can result in portland cement replacements typically ranging from 30% to 50%. It can be classified as an IT ternary blend cement under ASTM C595 if the proportions are within this standard’s limits. It also can be considered a C1157 cement.
The specific type of calcined clay in this application mostly is metakaolin, which is obtained by lower temperature calcination of kaolin clay. Metakaolin calcining temperature are 600 to 700° C
compared to portland cement, which can require around 1,400° C.
With metakaolin, studies have shown that a proportion to limestone of 2-to-1 enables the optimum synergistic effect. 1
The limestone powder reacts with the alumina in the metakaolin to form carboaluminate hydrates. This, along with the more typical calcium silicate hydrates (C-S-H), refines the pore structure.
Calcium hydroxide is consumed by pozzolanic activity, and the refined pore structure inhibits further formation of CH.
More C-S-H means lower permeability and improves concrete’s resistance to chemical attacks.
The sustainability benefits are significant. The manufacturing of LC3 can result in 30% to 40% reduction in CO2 embodiment compared to portland cement.2
Dr. Maria Juenger, researcher and professor in the Department of Civil, Architectural and Environmental Engineering at University of Texas at Austin, said: “One of the great things LC3 has going for it is its familiarity. When batching, mixing and placing concrete with this material, there’s not a lot of differences than with conventional mixes.”
As with other higher pozzolanic material content blended cements, one of the main drawbacks is its potentially lower early age strength. At lower dosages, metakaolin typically has little effect on concrete setting time.
However, at the higher dosages consistent with this blend, the setting would be slowed.
Lowering the amount of pozzolan and increasing the portland cement content to 60% or 70% could help but at a detriment to its sustainable and potentially its durability properties.
Chemical admixtures such as calcium nitrate or sodium carbonate shorten the setting time of LC3 but have lesser impacts on early age strengths especially at lower temperatures.3 One study also looked at using C-S-H seeding and micro limestone, and the results showed limited success.4
Dr. Jason Ideker, professor and researcher in the School of Civil and Construction Engineering at Oregon State University, said when considering LC3 for low, early strength gain for precast applications, a particular mineral accelerator is gaining some interest.
“Amorphous calcium aluminate cement reacts very quickly,” he said. “It may be blended with citric acid or calcium sulphate to slightly retard that reaction, but, overall, it could be used in an LC3 blend to boost early strength.”
During the Calcined Clays for Sustainable Concrete conference in 2022, French industrial minerals manufacturer Imerys introduced their patented CAC-based accelerator, referred to as LC3-plus.5
In terms of material availability, LC3’s raw materials seem to be widely available. Lower grade clays have been studied and may meet the requirements of ASTM C618, which makes the calcined clay sources much more abundant.6
Alternative Cementitious Materials
Moving farther away from the conventional, alternative cementitious materials (ACM) come into focus. “Alternative cement” is defined by the American Concrete Institute as an inorganic cement that can be used as a complete replacement for portland or blended hydraulic cements and that is not covered by applicable specifications for portland or blended hydraulic cements.
It is important to understand that as the portland cement proportion diminishes as in blended cements and is completely absent as in alternative cements, the similarity with fresh and hardened properties of the resulting concrete, compared to portland cement concrete, diminishes significantly.
Dr. Lisa Burris, researcher and professor in the Civil, Environmental and Geodetic Engineering Department at Ohio State University, said: “When you’re working with alternative cements, things just work differently, and you can no longer apply the same assumptions as you would with a portland cement-based system.
“Users must be aware of differences in the timing of reactions, curing requirements and meaning of indirect test methods, such as electrical resistivity measurements of durability.”
The ACI ITG 10.1 R-18 Report on Alternative Cements7 categorizes these materials based on the calcining and clinkering required in manufacturing. (Table 2)
Clinkering is the chemical bond breakdown of several raw materials through exposure to high temperatures to form new compounds. In the clinkering of portland cement, another process called “calcination” transforms limestone into lime.
Clinkered alternative cements require similar production compared with ordinary portland cement but with varying precursor chemistry. That leads to formation of different clinkered phases.
For example, portland cement phases include tri-calcium silicate (C3S) and di-calcium silicate (C2S). A clinkered alternative cement such as calcium sulfoaluminate (CSA) has C4A3S (ye’elimite) and calcium aluminate cement has calcium aluminate (CA) compounds.
Calcined compounds on the other hand are all magnesia compounds combined with a cross-linking agent. They do not form clinkered cement compounds but still react hydraulically with water to form a solid binder.
Some alternative cements can be manufactured without application of heat but rather by exposing raw materials to an activating solution, which dissolves then reforms into a solid binder structure. Those are called non-clinkered alternative cements.
Obviously, the degree or absence of high temperature manufacturing will directly impact that ACM’s embodied CO2.
CALCIUM SULFOALUMINATE CEMENT
Calcium sulfoaluminate (CSA) cement is a hydraulic cement that is being used commercially, especially in China. It is produced by heating limestone, bauxite and gypsum.
One clinker compound found in CSA is 4CaO·3Al2O3·SO3 also referred to as ye’elimite (Klein’s compound). CSA also contains belite (C2S) with added anhydrite or gypsum or other materials to help regulate reaction speed and expansive properties.
CSA hydrates to form ettringite, monosulfoaluminate, amorphous aluminum hydroxide and sometimes stratlingite – when the belite reacts. As with all systems in which large quantities of ettringite form, setting times are shorter, and early strength gain is high compared to portland cement concretes.
It has been shown that CSA cements can result in 70% of the 28-day strength forming in the first 24 hours.
It also has been shown to exhibit good durability in high sulfate environments and freeze-thaw conditions.
A subset of CSA has emerged recently with a higher belite (C2S) concentration. This cement is called belitic calcium sulfoaluminate, or BCSA cement. The addition of belite extends rapid setting times and aids in later strength development while preserving other beneficial CSA properties.
In China, BCSA cements have been used to manufacture precast concrete products.
A 2019 study explored the use of BCSA cement in precast prestressed concrete beams.8 The study revealed that while typical prestressed precast concrete manufacturers release prestress at 18 to 24 hours, “BCSA cement could allow prestress release in as little as two hours.”
The study’s results indicate that “casting prestressed concrete beams at a commercial facility using BCSA cement is feasible and can result in shorter required curing times before removing beams from the prestressing bed.”
Dr. Cameron Murray, researcher and assistant professor at the University of Arkansas’ College of Engineering, is one of the study’s authors. He was enthusiastic about the use of BCSA cements in precast concrete manufacturing.
“It’s not as much of a departure from what we are using today,” Murray said. “It is more expensive than portland cement, but the potential increase in production could justify that expense.”
The ACI Report on Alternative Cements notes: “CSA cement is used for some structural applications but are especially well suited for precast and cold-weather applications, which take advantage of the rapid strength gain of these materials.”
CSA and BCSA cement manufacturing requires slightly lower clinkering temperatures and grinding energy than portland cement, which enhances its environmental sustainability. Use of CSA cements can result in 30% or higher reduction in embodied CO2 compared to PC depending on composition and raw material availability.
CALCIUM ALUMINATE CEMENT
Calcium aluminate cement (CAC) is another hydraulic cement originally developed in France in the early 1900s. Hydration products mainly are calcium aluminate and other hydrates but excludes CH, which boosts the concrete’s durability in certain environments.
Concrete made with CAC results in short setting times with fast strength gain. Some of the hydration products are not stable at first, and a conversion occurs, transforming them into more stable phases.
During this conversion, an increase in porosity and subsequent drop in compressive strength can occur. This drop in strength was blamed for structural stability issues in the United Kingdom throughout Europe in the 1900s, including the collapse of a pool house in London.
This perceived issue relegated the use of CACs to mostly repair and refractory applications. However, CACs are gaining renewed interest in the concrete industry due to their rapid strength gain and durability properties. Studies have resulted in a better understanding of the conversion process, and how designs can account for it.
For precast concrete applications, Riding said, “CAC’s rapid strength gain, which could be beneficial in some precast applications. CAC is much more expensive than portland cement, likely limiting potential use to niche precast products until the cost decreases.”
CACs could be useful in environments prone to microbiologically induced concrete corrosion (MICC). A 2014 study concluded that: “This study highlights that active biofilms will not readily colonize CAC materials surfaces and will not evolve as biofilm developed on OPC materials.”9
MAGNESIUM BASED CEMENTS
Magnesia-based cements differ from portland cement by substituting magnesium oxide (MgO) for calcium oxide (CaO). Magnesium-based cements include magnesium oxychloride cement (MOC), magnesium oxysulphate cement (MOS) and magnesium phosphate cements (MPC).
These cements are made with an acid-base reaction by combining calcined magnesite (base) and a crosslinking agent (acid) in an aqueous solution. The reaction is fast and can produce high early strength depending on the type of magnesia-based cement. MPC reactions are exothermic, generating very high temperatures. Variations have been used in the construction of the Great Wall of China and the Pantheon.
These cements can exhibit high strength, strong bonding to inorganic substrates and good durability against sulfate, freeze-thaw and acid environments.10 MPC also can offer better corrosion protection that portland cement.11
The main issue with MOC concrete is that it should not be exposed to water for prolonged periods of time. Even for MPC, extended exposure to water or salt solutions however can decrease strength by 10-20%. 12
As for environmental performance, magnesium-based cements are not ideal, at least not at first.
Dr. Hongyang Ma, researcher and professor at the Civil, Architectural and Environmental Engineering Department of Missouri S&T, said, “Calcination temperatures to produce dead-burned magnesia for making MPC can be higher than those needed in portland cement manufacturing.”
However, Ma has been involved in research focused on using light burned MgO and energy-efficient alternative feedstocks for formulations of MPC and other acid-base cements, and noted, “Substituting light-burned magnesia for dead-burned magnesia would result in much lower calcination temperatures and reduce carbon footprint reduction.”
He added, “Regardless of initial calcination temperatures and CO2 emission, these magnesia cement-based concretes do potentially possess higher carbonation rates which allows them to absorb more CO2 in service than many other concretes. A special member of the family of magnesia-based cements is reactive magnesia cement, hardening due to the reaction between light-burned magnesia and CO2 forming magnesium carbonate hydrates. In this cement, the theoretical potential of CO2 uptake of 1 kg of MgO is 1.1 kg. Again, the challenge is to find energy- and CO2-efficient pathways to produce magnesia or alternative precursors.”
Use in precast applications currently is limited, but continued research seeks to address cost and durability concerns to enable future use of magnesia-based binders in future.
lkali-activated cements (AAC) are made by mixing fly ash, blast furnace slag or metakaolin with an alkaline activating solution. The reaction product produced varies based mainly on the amount of reactive calcium.
This chemical reaction between the alkali activating solution and these lower calcium materials is a polymerization reaction, and that’s why the resulting products were formerly called “geopolymers.”
AACs made with fly ash and slag are referred to as alkali activated fly ash (AAFA) or alkali activated slag cement (AAS). AAFAs exhibit high early and late strengths compared to PC.
AACs have been shown to exhibit good resistance to sulfate and acid attack and ASR. Among alternate cements, these alkali activated cements exhibit the highest environmental sustainability since incorporating waste materials and require no clinkering or calcination. Reduction in embodied CO2 compared to PC can be exceed 80%. 13
One main challenge to wider adoption is the availability of fly ash and slag and the inherent variability of these materials.
Juenger siad: “The inconsistencies with fly ash or slag would require adjustments to the alkaline activators prior to mixing. … There also exists a significant safety component to working with alkali activated cements since exposure to the alkaline activators is dangerous. However, development of test methods to guide producers in making these adjustments are currently in progress.”
While these cements are used in very specific applications, use in the precast industry will require better guidelines for proportioning of materials and chemicals and methods to assess variability in the starting material constituents.
PLENTY MORE OUT THERE
There are many other cements in various stages of research and on the market. Much is still unknown about the implementation of many of these cements and how laboratory results will translate to viable products for precast manufacturing.
An evolution of cement use is going to require an evolution of applicable standards. Many of the cements on the market or being developed will not abide by any current prescriptive standard but may satisfy performance-based cement standards.
Dr. Kimberly Kurtis, researcher and professor at the School of Civil and Environmental Engineering at Georgia Tech, is wrapping up a study for Georgia DOT titled “Recommendations for Future Specifications to Ensure Durable Next Generation Concrete.”
She noted: “Advancements in cements relies on adoption of performance-based specifications. … We have to enhance existing and create new laboratory test methods for these cements to provide more useful data. How we’re testing cements need to change.”
Since precast represents such a small percentage of overall concrete use, most studies and material development are primarily focused on the ready-mix industry. A cement that may work in a ready-mix application may not work in a precast application.
For one thing, precast manufacturing requires shorter set times and early strength development. Also, precast structures used in infrastructure applications may have specific exposure environments that require specific hardened properties. Microbially induced concrete corrosion is one example, where bacterial activity in high H2S gas concentrations can expose concrete to biogenic sulfuric acids.
Some of the cements discussed in this article could be ideal candidates for use in this exposure, but more testing and field research needs to be done.
However, precast manufacturing may present the optimal testing conditions for these cements.
“When it comes to real-world testing these materials for use in concrete, the precast manufacturing environment is an ideal process due to its higher degree of consistency compared to those of other concrete applications,” Juenger said.
There’s a significant opportunity for the research community and the precast industry to partner in exploring the use of these cements and further enhance the value of our products.
M.Antoni et al. “Cement Substitution by a Combination of Metakaolin and Limestone”. Cement and Concrete Research, Volume 42, Issue 12, December 2012, Pages 1579-1589
- LC3 “Limestone Calcined Clay Cement – Why LC3?” https://lc3.ch/why-lc3/
- Benevenuti et al. “Improving the early age strengths of LC3 using a mineral accelerator system based in calcium aluminates.” International Conference on Calcined Clays for Sustainable Concrete, 2022.
- Ouellet-Plamondon et al. “Acceleration of Cement Blended with Calcined Clays.” Construction and Building Materials Volume 245, 10 June 2020
- Imerys “Showcasing the Real Science Behind Sustainable Concrete and Construction” https://www.imerys.com/news/showcasing-real-science-behind-sustainable-concrete-and-construction
- Jafari and Rajabipour. “Performance of Impure Calcined Clay as a Pozzolan in Concrete” National Academies of Sciences: National Transportation Research Board, Volume 2675, Issue 2, 2020
- ACI ITG 10.1 R-18 “Report on Alternative Cements” American Concrete Institute, 38800 County Club Drive, Farmington Hills, MI
- Murray et al. “Using Belitic Calcium Sulfoaluminate Cement For Precast, Prestressed Concrete Beams” PCI Journal, March- April 2019
- Herisson et al.“Behaviour of different cementitious material formulations in sewer networks” Water Science & Technology 69(7):1502-8, April 2014
- Ma et al. “Effects of water content, magnesia-to-phosphate molar ratio and age on pore structure, strength and permeability of magnesium potassium phosphate cement paste,” Materials & Design, Volume 64, December 2014, 497-502.
- Zhang et al. “Investigation of corrosion mechanism of ribbed mild steel bars coated with magnesium potassium phosphate cement paste,” Construction and Building Materials, Volume 371, March 2023, 130639.
- Yang at al. “Characteristics and Durability Test of Magnesium Phosphate Cement-Based Material for Rapid Repair of Concrete,” Materials and Structures, vol. 33, pp. 229-234, 2000.
- Juenger et al. “Advances in Alternative Cementitious Binders” Cement and Concrete Research Volume 41, Issue 12, December 2011, Pages 1232-1243
Claude Goguen, P.E., is the director of outreach and technical education at NPCA.