Enhancing Concrete with Alternative SCMs

Supplementary cementitious materials (SCMs) have been used for many years to make durable concrete, but recent changes in the quality and availability of some of those SCMs is having a detrimental effect on the industry. In this article, we will explore the impact that changes in SCMs have had on concrete durability and the emergence of alternative supplementary cementitious materials (ASCMs) to make concrete that is stronger and will last longer.

Current SCMs
The American Concrete Institute (ACI) recognizes durable concrete as concrete capable of retaining its original form when placed and exposed to its field environment. The objective is for concrete to maintain its quality over a service

life of several decades. To achieve this durability, concrete is mixed with byproducts of coal combustion from electrical power plants, which are called coal combustion products (CCPs).

Of the many materials described in ACI 201.2R, Guide to Durable Concrete, Class F fly ash is one of the more popular CCP materials and is specified under ASTM C618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. The concrete industry uses Class F fly ash to increase the strength and durability of concrete infrastructure.

The significance of Class F fly ash for the concrete industry is its ability to chemically alter the microstructure of concrete, known as a pozzolanic reaction, to resist the destructive nature of physical and chemical attack. Today, however, a reduction in coal combustion throughout the United States and the world has led to a reduction in the availability and quality of the Class F fly ash that the concrete industry has relied on for decades. Interviews conducted by the National Precast Concrete Association in 2017 found that three out of five precasters had experienced a fly ash shortage.

As a result of this shortage, the concrete industry needs to adopt new SCMs. The alternatives include mature technologies like silica fume produced from silicon metal manufacturing and blast furnace slag produced from the reduction of iron ore to iron. Both alternatives have been shown to increase concrete strength and durability when used as a cement replacement.

Several emerging additives have also been found to increase concrete strength and durability. These ASCMs can act as a replacement or an enhancement for fly ash or as an additive to help further densify the hydrated cementitious matrix leading to decreased porosity, increased strength, and increased long-term durability.

Evaluating New SCMs
In order to validate the performance of ASCMs and to provide industry leaders the ability to use them in concrete, a guide was developed and published by ASTM. ASTM C1709, Standard Guide for Evaluation of Alternative Supplementary Cementitious Materials (ASCM) for Use in Concrete, is “intended to provide a technical approach to the evaluation of alternative supplementary cementitious materials.”

ASTM C1709 provides a step-by-step approach to evaluate an ASCM for its intended use. In the guide, ASCMs are defined as pozzolans and hydraulic cementitious materials that fall outside the specifications for fly ash, natural pozzolans, slag cement, and silica fume (ASTM C618, C989, and C1240).“Falling outside” these specifications can mean anything from non-specified and innovative particle-size-distributions, mined or reclaimed materials, and physical and chemical compositions that are unique but beneficial to concrete mixtures.

The C1709 evaluation includes tests for ASCMs in concrete to evaluate strength and durability. The guide not only defines how to make a physical and chemical evaluation of a new ASCM, but it also requires the manufacturer to conduct a laboratory analysis of grout to reveal its pozzolanic or strength activity index. C1709 also requires the manufacturer to conduct large-scale concrete strength and durability testing similar to that performed under ASTM C494, Specification for Chemical Admixtures for Concrete. But what makes C1709 important is that is goes a step further and requires three field evaluations using real concrete that is monitored for up to one year. This evaluation gives concrete producers, engineers, and architects a window into what the ASCM will do not only in the lab but also on the jobsite. In that way, ASTM C1709 connects theoretical concrete (lab-crete) to real concrete to help save the world’s concrete.

A New ACSM

One new alternative supplementary cementitious material on the market is Juno XP, a two-component material. One part is a traditional pozzolan that consumes a byproduct of cementitious hydration (calcium hydroxide) to produce more of the strength-bearing component of concrete (calcium-silicate-hydrate or C-S-H). Not only does Juno XP contribute to producing more of the backbone of concrete strength, but it also effectively manipulates the molecular kinetics of cement hydration to densify the concrete matrix.

Juno XP also has a secondary component: a mineral that increases the toughness of concrete. That mineral component has a toughness greater than para-aramid fibers (Kevlar) and also has the same chemical composition of C-S-H, the backbone of concrete strength.

What was ultimately seen from this combination of pozzolanic and mineral addition is not only a densification through particle-to-particle packing of the hydrated cement matrix, but also a toughening of the concrete matrix which increases its compressive and flexural strength. With other pozzolanic SCMs at high dosages there is an increase in compressive strength, but this is often accompanied by an increase in brittleness and a reduction in toughness (flexural strength).

The greatest value added to the concrete through the use of Juno XP is the replacement of a lot of cementitious material with only a small amount of the new ASCM. Ready-mix providers, precasters, and end users see a cost savings that allows construction dollars to be used for other tasks. And because of the reduction in cementitious material, there is also a benefit to the carbon footprint since less cement is needed to meet the demands of the structure.

The next value-added proposition of this ASCM is the enhanced performance of concrete. As shown in the bar charts based on preliminary lab work, Juno XP increases concrete’s compressive and flexural strength, reduces shrinkage, and increases abrasion resistance. It also creates a material that is resistant to damage from alkali-silica reactivity.

The final value-added proposition is the ease of adoption. Many new and emerging technologies never become widely implemented due to difficulty of use, but since Juno XP can be added at the front end or tail end of concrete mixing, there are no changes to the critical path of concrete sequencing and manufacturing.

Real-Life Applications
As previously mentioned, for a new ASCM like Juno XP, the ASTM C1709 guide requires the manufacturer to demonstrate its effectiveness. “If an ASCM does not yet have a significant record of performance in concrete, a comprehensive evaluation based on this Guide should be undertaken, and it should be recognized that this ASCM might be introduced for a specific project or into a limited marketplace to initially demonstrate its performance.”

To satisfy that requirement, we conducted initial laboratory tests using Juno XP in two different classes of concrete mixes: a typical residential driveway mix and a fast-track high-performance pavement mix.

Laboratory work for a future field study of residential driveway mixes was conducted on various dosages of Juno XP to determine the effect on concrete strength and maturity. This reference mix, with 432 pounds per cubic yard of portland cement and 108 pounds of Class F fly ash, was designed as a 3,000-psi mix. This mix is a workhorse for concrete producers and versions have been used for interior and exterior work that range from slabs to foundations. Using 10 pounds of Juno XP allowed us to remove about 100 pounds of cement and achieve the same strength without affecting workability or finishability. A higher replacement of 132 pounds of cement with 17 pounds of Juno XP (RPL 35%), didn’t achieve the same strength as the reference mix but still met the strength specification and allowed for cost savings of more than $7 per cubic yard. (Go to the bottom of this page for complete mix designs.)

A second study was performed with Juno XP on a high-performance mix to be used for fast-track pavements. Since 3,000-psi concrete compressive strength is typically required before the pavement can be reopened to traffic, a high cementitious mix is used to attain the early strength and workability needed for pavement placement. The Juno XP was used in two ways in this research. The first was to reduce binder content without sacrificing slump and strength, and the second was to allow the use of a subpar Class F fly ash (mix labeled “Juno XP, 3” in the bar chart above) as part of the cementitious material while still maintaining the specified slump and strength.

All of these mixes were able to reduce the binder content without sacrificing the slump and slump-life. The concrete compressive strength of the reference mixture attained 3,000 psi at 8 hours while the Juno XP mixes also exceeded the required compressive strength at 8 hours even when using a subpar Class F fly ash, which resulted in significant cost savings.

Conclusion
Using the evaluation techniques described in ASTM C1709, Juno XP has demonstrated the capability to enhance both standard and high-performance concrete. With the trend of seeing the diminishing availability and quality—as well as the higher cost—of conventional SCMs, concrete quality is being sacrificed. By adopting new technologies, such as alternative supplementary cementitious materials, the concrete industry can overcome these deficits and achieve concrete that is stronger and lasts longer.

Concrete Residential Driveway Mixtures (3000 psi) with and without Juno XP

Concrete Fast Track Pavement Mixtures with and without Juno XP