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Technical Brief  > Green in Practice 107 - Supplementary Cementitious Materials (SCMs)
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Fly ash, slag cement, and silica fume are industrial by-products that are used as a partial replacement for portland cement in concrete. SCMs are used in at least 60% of ready mixed concrete. These supplementary cementitious materials (SCMs) are pre-consumer materials.
Building products that have a portion of their constituent materials from recycled products reduce the need for virgin materials in new construction. Using recycled materials reduces the amount of materials that are landfilled. It also reduces the environmental impacts from extracting and processing virgin materials. The practice of using SCMs in concrete has been growing in North America since the 1970s.
Post-consumer recycled material is defined as waste generated by households or by commercial, industrial and institutional facilities as the end-users of a product. These are products that are sold and used for a specific purpose and then need to be disposed, such as newspaper, containers, computers, and batteries. Post-consumer materials include crushed concrete and masonry from demolished buildings that are reused as aggregate for concrete in new buildings. It also includes medical waste, used tires, and spent solvents used as fuel or ingredients to manufacture cement.
Pre-consumer material is defined as material from the waste stream of a manufacturing process. These include materials such as fly ash, slag cement, and silica fume which are not manufactured to be sold, but are the result of a manufacturing process for another product.


Fly ash, slag cement, and silica fume are industrial by-products; their use as a partial replacement for portland cement reduces the amount of cement needed for concrete. This reduces the energy and CO2 impacts of concrete. If not used in concrete, these materials would use valuable landfill space. Because the cementitious content of concrete is about 7 to 15%, these SCMs typically account for only 2% to 8% of the overall concrete material in buildings. However, LEED-NCv2.2 has special procedures for SCMs that allows the recycled content of concrete to be based on the recycled content of the cementitious materials. The total cementitious materials content includes the amount of portland cement plus the amount of SCMs. An example calculation is provided in the LEED-NC v2.2 Reference Guide.
Fly Ash
Fly Ash PCA No. 12190Fly ash is a by-product of the combustion of pulverized coal in electric power generating plants. Fly ash is used in about one-half of ready mixed concrete. It is commonly used as a partial substitute for 15 to 25% of the portland cement in concrete. The American Coal Ash Association estimates that use of fly ash as a cement substitute in concrete has the potential to eliminate 10 to 14 million tons of carbon dioxide emissions annually.

Fly ash is generally available throughout the United States; however, quantities are limited in some locations. About 71 million tons of fly ash were generated in 2004, of which, approximately 40% were recycled and 60% were landfilled. Not all fly ash is useable in concrete. However, most fly ash suitable for use in concrete is used and not landfilled.


Incorporating fly ash in concrete can enhance the properties of concrete. Reported improvements to the properties of fresh concrete during placement include enhanced workability, reduced bleed water, and reduced slump loss. For hardened concrete, fly ash can also increase the long-term strength, improve the permeability, increase the durability, reduce the potential for sulfate attack, reduce the heat of hydration, and reduce the potential for alkali-silica reactivity. These positive aspects of incorporating fly ash in concrete are generalized; adding the wrong type or amount of fly ash can be detrimental to concrete.

Composition and Specifications

Fly ash is primarily silicate glass containing silica, aluminum, iron, and calcium. Minor constituents are magnesium, sulfur, sodium, potassium, and carbon. Crystalline compounds are present in small amounts. The relative density (specific gravity) of fly ash generally ranges between 1.9 and 2.8 and the color is generally gray or tan.

Class F and Class C fly ashes meeting ASTM C 618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, are commonly used for general purpose concrete. Class F fly ash is a by-product of burning bituminous coal, which is generally found in the eastern portion of the United States. Class F fly ash is high in iron, silica, and alumina, but low in calcium (less than 10% CaO). Carbon contents are usually less than 5%, but some may be as high as 15%.

Class C fly ash is a by-product of burning sub-bituminous coal and lignite in western states. Class C materials are often high-calcium (10% to 30% CaO) fly ashes with carbon contents less than 2%. Concrete with Class C fly ash generally develops strength much faster than concrete with Class F fly ash. Many Class C ashes when exposed to water will hydrate and harden in less than 45 minutes. For this reason, many ready-mixed concrete suppliers prefer Class C fly ash to Class F fly ash.

Fly ash varies in composition and carbon content. Fly ash for use in concrete that will be subject to freezing and thawing should have low levels of unburned carbon in order to achieve an adequate air content. Specifications typically limit the unburned carbon content to a maximum of 6%; however, market forces currently limit this to less than 1%. Some fly ashes meet both Class F and Class C classifications.


Fly ash is often used at dosages of 15% to 25% by mass of cementitious material, that is, the total of cement, fly ash, and other supplementary cementitious materials. Dosage varies with the reactivity of the ash and the desired effects on the concrete. The amount of fly ash should be tailored to the specific constraints and requirements of individual applications. Some agencies limit the maximum fly ash content to 15% or less for all concrete. For concrete exposed to deicing chemicals, the maximum fly ash content is generally limited to 25%. Special provisions for a particular project, along with testing, may increase the fly ash content of concrete for specific applications and needs.

Fly ash will tend to delay the time of initial set, which is often a benefit in warm weather. In cooler weather, accelerators, heated materials or other precautions may be required. Fly ash, especially Type F, can cause lower early age strengths and affect the construction schedule.

High Volume Fly Ash

Concrete has been successfully placed using fly ash as up to 50% of the cementitious material. In some rare cases, concrete has been successfully placed with up to 80% fly ash. These mixes may use specific fly ashes that may not be readily available. They often contain more cementitious material (cement plus fly ash) than ordinary concrete. Strength development is generally slower so construction schedules must accommodate the extra time required before forms and shoring are removed. Deicer scaling resistance has been noted to be poor. Therefore high volume fly ash is typically limited to placements where durability is not a concern, such as indoors. Special precautions may also be needed in cold weather to ensure adequate strength development. Because this type of concrete is typically not covered by industry-standard specifications, the amount of pretesting that must be done to ensure durable concrete and satisfy building codes makes this concrete more expensive. Caution is recommended for applications using concrete with high fly ash concentrations.


Since fly ash properties vary, the project contractor and the concrete producer should exercise sufficient judgment, testing, and control procedures to ensure good concrete performance. If it is intended to encourage the use of fly ash in a particular project, the project specifications should carefully explain the required concrete properties and expected durability, and the limits on amounts of fly ash. Limits are placed based on experience and concrete performance in the field or laboratory. Contact your local ready-mixed concrete supplier to determine what fly ash is available and to verify its performance in quality concrete. ACI 232.2, Use of Fly Ash in Concrete, provides an extensive review of fly ash.

Slag Cement

Slag Cement (PCA No. 12191)Slag cement, also called ground granulated blast-furnace slag or slag, is a by-product of the steel industry. It is formed during the liquification of iron in the blast-furnace. Slag cement is commonly used as a partial substitute for portland cement in concrete at a replacement level of up to 50%.

When slag cement replaces 50% of the portland cement in a 7500 psi concrete, greenhouse gas emissions per cubic yard of concrete are reduced by 45%. The Slag Cement Association estimates that use of slag cement as a cement substitute in concrete has the potential to eliminate 3 million metric tons of carbon dioxide emissions annually. A record 3.5 million metric tons of slag cement were shipped for use in concrete and construction applications in 2004, a 16% increase over 2002.


Incorporating slag cement in concrete can enhance the properties of concrete. Slag cement generally improves workability, finishability and pumpability of concrete during placement.Slag cement in hardened concrete can improve compressive and flexural strength, improve permeability, increase resistance to chloride intrusion and corrosion, mitigate moderate to severe sulfate attack, and reduce the potential for alkali-silica reactivity. Slag cement can also reduce thermal stress in mass concrete through lower heat generation.

Composition and Specifications

Slag cement is a crystalline blend of calcium bearing silicates and alumino-silicates. The relative density (specific gravity) for slag cement is in the range of 2.85 to 2.95.

Slag cement has been used as a cementitious material in concrete since the beginning of the 1900s. Slag cement, when used in general purpose concrete in North America, commonly constitutes between 30% and 45% of the cementing material in the mix. Some concretes have a slag component of 70% or more of the cementitious material. ASTM C 989, Specification for Ground Granulated Blast-Furnace Slag for Use in Concrete and Mortars, classifies slag by its increasing level of reactivity as Grade 80, 100, or 120 (Table 1). ACI 233, Slag Cement in Concrete and Mortar provides an extensive review of slag cement.


Slag cement will tend to delay the time of initial set, which is often a benefit in warm weather. In cooler weather, accelerators, heated materials or lowering the percentage of slag cement in a mixture (as a portion of cementitious material) can be employed to decrease setting time. Early age strengths (through 7 to 14 days) of slag cement tend to be lower while later age strengths will be higher. This can affect the construction schedule.

Silica Fume

Silica Fume (PCA No. 12192)Silica fume is a by-product from the electric arc furnace used in the production of silicon or ferrosilicon alloy. Silica fume is commonly used as a partial substitute for portland cement in concrete at replacement levels of 5 to 7%.


Silica fume is used in applications where a high degree of impermeability is needed and in high-strength concrete. Using silica fume can increase the resistance of concrete to chloride penetration in bridges and parking decks.

Composition and Specifications

Silica fume, also referred to as microsilica or condensed silica fume, is essentially silicon dioxide (usually more than 85%) in noncrystalline (amorphous) form. Since it is an airborne material like fly ash, it has a spherical shape. It is extremely fine with particles less than 1 µm in diameter and with an average diameter of about 0.1 µm, about 100 times smaller than average cement particles. Condensed silica fume has a surface area of about 20,000 m2/kg. For comparison, tobacco smoke’s surface area is about 10,000 m2/ kg. Type I and Type III cements have surface areas of about 300 to 400 m2/kg and 500 to 600 m2/kg, respectively. The relative density (specific gravity) of silica fume is generally in the range of 2.2 to 2.5. Portland cement has a relative density of about 3.15.

Silica fume is sold in powder form but is more commonly available in a liquid form for ease of transport and handling. Silica fume is used in amounts between 5 and 7% by mass of the total cementitious material. Silica fume must meet ASTM C 1240, Specification for Silica Fume Used in Cementitious Mixtures. ACI 234, Guide for the Use of Silica Fume in Concrete, provides an extensive review of silica fume.

Blended Cements

Blended cements are produced by intimately and uniformly intergrinding or blending two or more types of fine materials. The primary materials are portland cement, slag cement, fly ash, silica fume, calcined clay, other pozzolans, and preblended combinations of these materials. Blended cements are used in all aspects of concrete construction in the same manner as portland cements. Blended cements can be used as the only cementitious material in concrete or they can be used in combination with other SCMs added at the concrete plant. Blended cements are often designed to be used in combination with fly ash and slag cement.

Blended cements must conform to the requirements of ASTM C 595, Specification for Blended Hydraulic Cements, or ASTM C 1157, Performance Specification for Hydraulic Cements. According to ASTM C595, the maximum fly ash content of a blended cement is 40%. If additional fly ash or other supplementary cementitious materials are to be added to concrete containing blended cements, pre-testing must be performed to ensure that the engineering properties and durability of the concrete is not compromised. However, most blended cements today are designed to be used with additional fly ash added at the concrete plant.

Blended cements meeting the requirements of ASTM C 1157 meet physical performance test requirements without prescriptive restrictions on ingredients or cement chemistry. This allows the cement manufacturer to optimize strength and durability properties through use of a variety of cementitious materials, such as portland cement, fly ash, slag, silica fume, and calcined clay.

ASTM C 595 recognizes five primary classes of blended cements as follows:

  • Type IS Portland blast-furnace slag cement
  • Type IP and Type P Portland-pozzolan cement
  • Type I(PM) Pozzolan-modified portland cement
  • Type S Slag cement
  • Type I(SM) Slag-modified portland cement

Types IS, IP, P, I(PM), and I(SM) are general purpose cements.

Table 1 lists the applicable specifications for fly ash, slag cement, and silica fume.

Table 1: Specifications and Classes of Supplementary Cementitious Materials.

Ground granulated iron blast-furnace slags – ASTM C 989

Grade 80

Slags with a low activity index

Grade 100

Slags with a moderate activity index

Grade 120

Slags with a high activity index

Fly ash and natural pozzolans – ASTM C 618

Class N

Raw or calcinated natural pozzolans including:

Diatomaceous earths

Opaline cherts and shales

Tuffs and volcanic ashes or pumicites

Calcinated clasys, including metakaolin, and shales

Class F

Fly ash with pozzolanic properties

Class C

Fly ash with pozzolanic and cementitious properties

Silica fume – ASTM C 1240

ASTM C 989, Specification for Ground Granulated Blast-Furnace Slag for Use in      Concrete and Mortars
ASTM C 618, Specification for Coal Fly Ash and Raw or Calcined Natural    Pozzolan for Use in Concrete
ASTM C 1240, Specification for Silica Fume Used in Cementitious Mixtures

When specifying cementitious materials (cements and SCMs) for a project, be sure to check the availability; specifications should allow flexibility in selection. Limiting a project to only one cementitious material, one brand, or one standard cement specification can result in project delays, increased costs, and it may not allow for the best use of local materials. Cementitious materials with special properties should not be required unless special characteristics are necessary. The project specifications should focus on the needs of the concrete structure and allow use of a variety of materials to accomplish those needs. A typical specification may call for portland cements meeting ASTM C 150 or ASTM C 1157, or for blended cements meeting ASTM C 595 or ASTM C 1157.

SCMs are often used to improve a particular concrete property such as resistance to alkali-aggregate reactivity. The optimum amount to use should be established by testing to determine (1) whether the material is indeed improving the property, and (2) the correct dosage rate, as an overdose or underdose can be harmful or not achieve the desired effect. SCMs also react differently with different brands of cements. Therefore pretesting is recommended.

Traditionally, fly ash, slag cement, calcined clay, calcined shale, and silica fume were used in concrete individually. Today, due to improved access to these materials, concrete producers can combine two or more of these materials to optimize concrete properties. Mixtures using three cementitious materials, called ternary mixtures, are becoming more common. When ternary mixtures are used, the proportioned concrete mixture using the project materials should be tested to demonstrate that it meets the required concrete properties for the project. The optimum amounts of SCMs used with portland or blended cement are determined by testing, the relative cost and availability of the materials, and the specified properties of the concrete.

The durability of products with recycled content materials should be carefully researched during the design process to ensure comparable life cycle performance. There would obviously be a net negative impact if a product offering a 30 to 60% recycled content had only half the expected service life of a product with a lower or no recycled content.