The Difference in Slag Cement and Fly Ash
Contractors and cement suppliers will each have their own opinions when it comes to the materials they prefer to use. The difference in materials can make or break a project and it’s important to understand how they work together in different situations to create the strongest and most durable concrete. Slag cement (Ground Granulated Blast Furnace Slag) and fly ash (Coal Combustion Product) are the most commonly used supplementary cementitious materials in construction today. While they are commonly used and similar in many ways, these materials produce different effects on concrete performance and this document will explore those differences. The Slag Cement Association with member company and industry input have broken down the differences in slag cement and fly ash and how they both affect concrete performance.
1: Slag cement has a more consistent chemical and physical structure than fly ash from source to source and from a single source.
What does that mean for your concrete? Because of the more consistent structure of slag cement, it produces cement paste and concrete with more consistent performance results.
While fly ash is still very efficient in certain cases, its chemical structure can vary from source to source and from season to season within a single source. This can affect how the fly ash works with other materials in concrete mix design.
“The chemical composition of fly ash depends on the source of the coal…. The performance of fly ash in concrete is strongly influenced by its chemical composition. Fly ash from different sources can behave very differently.”
-Design and Control of Concrete, PCA (r.1)
“The process of manufacturing iron in a blast furnace and producing slag is a relatively consistent process world-wide. The chemistry of the resultant slag granules and slag cement from granules is relatively consistent in chemistry within each source. Between sources, the performance of slag cement is relatively similar due to the nature of the process of iron production.
…Fly ash, on the other hand is the result of the combustion of pulverized coal and coal chemistry has a wide range of chemistry from source to source. The combustion process can also vary greatly from source to source. Since the coal source defines the chemistry of the resulting fly ash , energy requirements demand blending of coal sources, and operating conditions change, the quality of the resulting fly ash not only varies regionally but can also vary seasonally at the same plant/location/source.”
-Nick Popoff – SCA Member Company Representative
The figure on the left shows the chemical structure of slag cement with the figure on the right showing fly ash. (r.1)
2: The industry demand for slag cement and its increasing availability in the U.S. make it a more practical material.
According to the National Precast Concrete Association (NPCA), “With energy trends resulting in a move away from coal-fired plants, fly ash availability has been significantly reduced.” While the concrete industry has begun to find alternative supply options for fly ash, these options create more variations in fly ash from source to source. The NPCA goes on to say, “Regardless of the current supply status, the days of having an affordable, reliable source of fly ash are nearly over.” (r.2)
Concrete industry specifications are more commonly including provisions for slag cement use. The SCA has been actively working with state DOT’s and industry professionals who want to incorporate slag cement into their projects. This trend, combined with the increased need for consistent access to quality materials is expected to make the demand for slag cement to rise in upcoming years. Additionally, the concrete industry is more widely accepting specifications that include slag cement and slag blended cements, making the demand for the product expected to rise in upcoming years.
Once a good relationship with a supplier is established, slag cement can be used at a higher percentage than fly ash. In most cases, slag cement can be used as a 30-50% replacement to portland cement, reducing the carbon impact of the concrete. To check for suppliers in your area, use the SCA’s slag cement locator.
“To promote and maintain fair competition among concrete manufacturers, it’s imperative that specifiers and DOTs recognize the fly ash shortage and clearly allow for alternative materials in the project specifications.”
Timothy Burke, P.E. – Headwaters Resources (r.2)
3: Slag cement provides consistent ASR mitigation and increased concrete durability.
The concrete specifications of today’s construction world almost always include standards for durability and address the longevity of the concrete. Creating concrete with more consistent results helps ensure structures can withstand freeze thaw conditions, mitigate ASR and sulfate attack, and helps the concrete perform to its peak potential.
“The effectiveness of class F fly ash in controlling expansion due to ASR is well established….However, the effect of fly ash varies considerably as the composition of the ash varies.”
-Design and Control of Concrete, PCA (r.1) Pg. 163
“Used in the proper proportions, slag cement can give a Type I cement the sulfate resisting properties of a Type II cement (usually 25 to 50 percent slag cement replacement for portland), and it can give a Type I or a Type II cement the sulfate resisting properties of a Type V cement (usually 50 to 65 percent slag cement replacement for portland).”
- Slag Cement Association Website (r.4)
Ternary concrete mixtures include three different types of cementitious materials. Fly ash and slag cement work well together in certain applications and can be used to achieve higher strength gains, corrosion resistance and lowered concrete permeability. The optimum mixture proportions for ternary blends, as with other concrete, will be dependent on the final use of the concrete, construction requirements and seasonal considerations. In low W/CM applications such as paving, mixtures with 15 percent fly ash and 30 percent slag cement component have been used successfully. The spirit of St. Louis Airport in Chesterfield, MO placed airport paving with blended cement (Type I (SM)) and Class C Fly Ash for a more durable and resilient concrete. (photo credit 5)
Every project presents its own challenges, and those unique issues will decide what materials should be used. Regional availability, climate, specifications, and timing dictate the right concrete mix design for a structure or project. Slag cement and fly ash also work well together in blended cements. To learn more about blended cements and other slag cement use topics, browse the SCA’s index of technical information sheets here at SlagCement.org.
About the Slag Cement Association
The Slag Cement Association (SCA) is the leading source of knowledge on slag cement and slag blended cements. We promote the increased use and acceptance of these products by developing industry resources with the cooperation of member organizations. We educate customers, specifiers and other industry end-users on the varied attributes, benefits, and uses of slag cement.
To learn more about the SCA’s programs, resources, and events, visit the associations newly re-designed website at www.SlagCement.org
- Design and Control of Concrete Mixtures 16th Edition, Chapter 6: Supplementary Cementitious Materials; Kosmatka, Steven H. and Wilson, Michelle, L; Portland Cement Association, Skokie, IL, 2016. Pages: 147-167.
- Fly Ash Trends Downward, National Precast Concrete Association, March 27, 2017
- SCA Shipment Report Shows Strong Growth in Slag Cement Sales, Slag Cement Association Website, 2017.
- Mitigating Sulfate Attack, Slag Cement Association Website, 2017.
- News Magazine Network