A Comprehensive Guide to Supplementary Cementitious Materials (SCMs)
- rjchiodo
- Aug 18
- 9 min read
Supplementary Cementitious Materials, otherwise known as SCMs for concrete, are unique additives used in tandem with or to replace a portion of traditional Portland cement in concrete. SCMs for concrete are used to help enhance and provide additional performance benefits, such as strength and durability improvements for specific applications.
SCMs are often by-products generated through separate industrial-producing processes and were once cast-off waste materials. Valuable SCMs for concrete, such as silica fume, fly ash, and ground granulated blast furnace slag, are a few examples of industrial by-products that have found significant relevance in the concrete industry over the last 50 - 100 years.
However, there are some newer SCMs for concrete, such as metakaolin and ground glass pozzolans, that are specifically produced and formulated with the intent of being used as SCMs. Read on to learn more about the available SCMs for concrete, their specific usages, and working with R-E-D Industrial Products to source these beneficial materials.
In addition to performance improvements, SCMs are known to significantly decrease the carbon footprint of the concrete they are used in when used to replace traditional Portland cement and even the newer “greener” Type 1L Portland cement (10-15% limestone fines).
Table of Contents
We have included a table of contents below to help you navigate this comprehensive guide to SCM materials. Click on any link to be taken to that section of the blog post.
Benefits of Using SCMs in Concrete
It's helpful to understand the overall advantages SCMs can bring to a concrete mix. Incorporating the right SCM can lead to significant improvements in both performance and sustainability.
Benefits include:
Enhanced Durability: Many SCMs create a denser and chemically enhanced concrete matrix, increasing resistance to wear.
Improved Workability: Certain types, like fly ash, have particle shapes that reduce water demand and make the concrete easier to place and finish.
Increased Chemical Resistance: These materials can make concrete less permeable, protecting it from sulfate attacks, chloride ingress, and alkali-silica reaction (ASR).
Enhanced Sustainability: Using SCMs, especially those that are by-products of other industrial processes or from recycled waste streams, reduces the amount of Portland cement needed, lowering the overall carbon footprint of the concrete.
Reduced Heat of Hydration / Thermal Cracking: Certain types of SCMs slow the chemical reaction that generates heat as concrete cures, a critical benefit in large concrete pours (mass pours), helping to reduce the risk of thermal stress and cracking.
Reduced Permeability and Porosity: Most SCMs, being finer than cement particles, fill the microscopic voids between cement particles and aggregates, which makes the concrete much less permeable to water and other harmful substances like salts.
Reduced Bleeding and Segregation: Some SCMs, particularly silica fume, help hold the concrete mix together through hydrophilic characteristics, reducing the tendency for water to bleed to the surface and for aggregates to segregate.
Most Common Concrete SCMs
SCMs are added to concrete mixtures for various purposes, primarily to enhance durability and modify key properties of the concrete. They work by reducing alkali reactivity with aggregates, decreasing permeability, and improving hardening through hydraulic or pozzolanic reactions. Considering the wide range of benefits provided by supplementary cementitious materials, it is important to look at the composition of each and the potential benefits offered for specific applications.
Silica Fume
Silica fume is an ultrafine powdery by-product typically created through the production of silicon metal or ferrosilicon alloys. Also known as micro silica, silica fume particles are roughly 100 to 150 times smaller than a particle of cement. Silica fume possesses incredible pozzolanic properties and reacts with calcium hydroxide to enhance the resilience and durability of poured concrete.
As an SCM for different concrete applications, silica fume is available in densified, semi-densified, and undensified variants, which simply refers to the amount of silica fume per cubic foot. Concrete enhanced with silica fume has several unique applications, including structural or construction applications, down-hole oil and gas well cementing, and refractory applications that require enhanced performance at elevated temperatures.
Properties
Primary Benefit | Higher Durability |
Origin | By-Product (Silicon metal production) |
Appearance | Ultrafine Grey Powder |
Typical Application | High-Performance Concrete, Bridges, Overlays, Geopolymers |
ASTM Standard | C1240 |
Ground Granulated Blast Furnace Slag (GGBFS)
This unique SCM for concrete is commonly referred to as slag and is a by-product of primary iron and steel production from blast furnaces (steel made from iron ore). The chemical composition of GGBFS is primarily amorphous (glass) silicate and aluminosilicate. When added to concrete, ground granulated blast furnace slag helps to improve the durability and sustainability of concrete while making meaningful improvements to thermal cracking resistance.
R-E-D Industrial Products has developed a proprietary slag replacement product called Re-Act 4 SR™ ground glass pozzolan(GGP) from recycled aluminosilicate glass, which has very similar performance properties to GGBFS. Ground granulated blast furnace slag and ground glass pozzolans can be used as SCMs for concrete in ready-mix concrete designs, precast projects, building foundations, and high-volume mortar applications and/or paving projects.
Properties
Primary Benefit | Durability, Thermal Crack Resistance, Permeability improvement |
Origin | Steel & iron production from blast furnaces |
Appearance | Amorphous (glass) silicate powder |
Typical Application | Ready-mix, pre-cast, dams, foundations |
ASTM Standard | C989/C989M (C1866 GGP) |
Metakaolin
R-E-D Industrial Products is a leading supplier of Dynapoz metakaolin, widely considered throughout the industry to be a premium SCM for concrete. Metakaolin is an anhydrous calcined clay structure of an aluminosilicate naturally occurring mineral called kaolinite. When added to concrete, metakaolin enhances the pozzolanic reactivity and improves the resistance to chemical attack, and provides increased structural integrity. Dynapoz metakaolin is an ultra-fine material and meets all technical specifications for ASTM C-618, AASHTO M295-11, and all Class N pozzolan specifications that guarantee reliable performance and consistent quality.
Properties
Primary Benefit | Chemical resistance and structural integrity |
Origin | Engineered (Calcined kaolinite clay) |
Appearance | Ultra-fine powder |
Typical Application | High-Performance Concrete, Architectural, Precast and Ready Mix Concrete, Geopolymers |
ASTM Standard | C618 (Class N) |
Fly Ash
Fly ash is a pozzolanic material harvested from the electrostatic precipitators in coal-fired thermal power plants and subsequently utilized as a beneficial SCM for concrete applications. Fly ash is spherically shaped and has a rich composition made from silica (glass) and alumina.
The shape of fly ash also helps increase the workability of the concrete and thereby reduces the volume of water required. Fly ash is a lower-cost option compared to Portland cement for use in soil stabilization and remediation applications, as it has similar binding properties with the soil.
Fly ash reacts with the free lime (calcium hydroxide) particles and performs like Portland cement to form a resilient binder. Fly ash is predominantly used as a partial replacement for Portland cement (up to 25%) and improves workability, durability, and works exceptionally well in heavily sloped or high-temperature applications.
Properties
Primary Benefit | Improved workability and reduced water requirements |
Origin | By-product (Coal-fired power plants) |
Appearance | Spherical powder |
Typical Application | General Ready-Mix, High-Temperature Applications, Soil remediation and stabilization, Geopolymers |
ASTM Standard | C618 |
Ground Glass Pozzolan (GGP)
Ground glass pozzolan is manufactured to generate a superior cementitious material for concrete with a low-carbon footprint and has a wide range of construction applications from architectural pre-cast to ready-mix concrete, as well as Ultra High Performance Concrete (UHPC).
R-E-D Industrial Products Re-Act 4™ is a fine, white-colored, amorphous glass powder manufactured from 100% recycled glass.
Ground glass pozzolan is environmentally safe (with no crystalline silica) and is a sustainable SCM for concrete and conforms to all ASTM C1866/CSA A3000-21 specifications. Ground glass pozzolan is quickly becoming a leading option throughout the concrete industry due to its high performance and sustainability features. GGP has become more readily available in recent years and has a more consistent chemistry than any other SCMs, like fly ash, GGBFS, or silica fume.
Properties
Primary Benefit | Sustainability, high reactivity, and availability |
Origin | Manufactured (100% recycled glass) |
Appearance | Fine, white amorphous glass powder |
Typical Application | Structural and Architectural Precast, Ready-Mix, UHPC, Geopolymers |
ASTM Standard | C1866 |
Materials That Do Not Classify as SCMs for Concrete
Many materials can be added to concrete to improve it, but not all of them are supplementary cementitious materials. The distinction depends on how an additive functions within the concrete mix. Concrete reinforcing fibers and cenospheres/microspheres are two examples of valuable additives that do not classify as SCMs.
These materials provide physical enhancements rather than the chemical reactions that define an SCM. They do not possess pozzolanic or hydraulic properties, so they do not react with the cement paste to form new binding compounds. Instead, fibers offer mechanical reinforcement to resist cracking, while cenospheres and microspheres act as lightweight fillers to reduce density.
The Special Case of Colloidal Silica
Colloidal silica occupies a unique space between a chemical admixture and a traditional SCM. It consists of nano-sized amorphous silica particles suspended in a liquid, providing high pozzolanic reactivity that densifies the concrete matrix. However, it is not used as a direct replacement for cement due to its liquid form and very low dosage rates. Instead, it functions as a performance-enhancing admixture that accelerates hydration and improves the durability of the final concrete.
SCM Comparison Chart
View the table below for a high-level comparison of SCM materials:
Origin | Primary Function | Key Advantage(s) | Sustainability Note | |
Silica Fume | By-Product | Pozzolanic | Low Permeability | Uses reclaimed industrial material |
GGBFS (Slag) | By-Product | Hydraulic & Pozzolanic | Durability, Lighter Color, Workability | Uses reclaimed industrial material |
Metakaolin | Manufactured | Pozzolanic | High Reactivity & Chemical Resistance. | Engineered for consistency and a lower carbon footprint |
Fly Ash | By-Product | Pozzolanic | Workability, Reduced Water Demand | Uses reclaimed industrial material |
Glass Pozzolan | Manufactured | Pozzolanic | High Reactivity, ASR Mitigation, White Color | Made from 100% recycled glass |
SCMs for Concrete & ASTM Standards
Since SCMs can drastically improve performance characteristics in concrete, SCMs are required to meet particular specifications to ensure safety. Currently, slag cement must conform to ASTM C989/C989M. Whereas supplementary cementitious materials like metakaolin, natural pozzolans, and fly ash all must meet ASTM C618, and silica fume to ASTM C1240, ground glass pozzolans must conform to ASTM 1866 M.
Depending on the chemical composition needed to achieve a specific type of concrete, a binary blend or a ternary blend of the cement mixture may be needed. A binary blend is when there is one supplementary cementitious material added with Portland cement, and a ternary blend is when two SCMs are added with Portland cement. For instance, a designer might specify a ternary blend for a bridge deck. This could involve using GGBFS to ensure long-term durability and mitigate thermal cracking, combined with a small amount of Silica Fume to achieve the low permeability needed to resist de-icing salts.
These unique blends are specifically formulated and developed to significantly improve the performance of the concrete based on the end application, exposure to certain chemicals, or sustainability metrics detailed in the project outline.
SCM Sustainability
Supplementary cementitious materials reduce environmental impact in concrete production. Performance standards and environmental factors guide material selection. Projects increasingly require transparent environmental data through Environmental Product Declarations (EPDs), which provide standardized reports on a product's environmental footprint via its global warming potential (GWP).
These materials support sustainability by reducing the Portland cement content in concrete mixes. Lower Portland cement content means reduced carbon emissions from the concrete poured. This reduction also usually creates stronger and more durable concrete that extends the life of roads and bridges, and in some instances lowers water consumption during production.
Choosing the Right SCM for Your Application
Selecting the correct SCM depends on the final performance goals for the concrete. Different projects require different material properties. The following examples connect common applications to specific additives.
Low Permeability: Some projects require low water permeability. Silica Fume, Ground Glass Pozzolan, and Metakaolin are excellent choices for these applications. They create dense concrete suitable for high-performance structures.
Improved Workability: Fly Ash improves the workability of fresh concrete. Its spherical particles help the mixture flow and finish more easily. This makes it a common choice for general ready-mix applications.
Mass Concrete and Thermal Control: Large concrete pours generate significant heat during hydration. This heat can cause thermal cracking. GGBFS, Re-Act 4 SR™, and Fly Ash all slow this chemical reaction and reduce heat buildup.
Architectural and Aesthetic Finishes: The final appearance of the concrete is sometimes a primary goal. GGBFS can be used to produce a lighter, more consistent color. Ground Glass Pozzolan provides a bright white finish for architectural designs.
Get a Quote for SCMs for Concrete from R-E-D Industrial
R-E-D Industrial Products has over forty years of combined experience in the SCMs for the concrete industry and offers the highest quality products at competitive prices and expedited delivery times. From silica fume to metakaolin and ground glass pozzolans, companies that choose to work with the professional team at R-E-D Industrial Products will immediately improve their concrete mix to meet whatever challenges the end application presents.
The benefits of SCMs for concrete can help address early or late-stage cracking, help resist chemical attacks or environmental conditions, and improve structural longevity. If you are interested in learning more about SCMs for concrete from R-E-D Industrial Products, please contact a member of our team and share more about your upcoming project so we can recommend the ideal supplementary cementitious materials for your application.