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Slag Cement Frequently Asked Questions

This section contains answers to frequently asked questions (FAQ) about slag cement, and links to the Slag Cement in Concrete (SCIC) information sheet series that provides more detail. If you have questions that are not answered here, email SCA at info@slagcement.org, or call 847-977-6920.

Slag cement (also called ground granulated blast furnace slag) is a hydraulic cement produced during the reduction of iron ore to iron in a blast furnace. Molten slag is tapped from a blast furnace, rapidly quenched with water ("granulated"), dried and ground to a fine powder. The rapid quenching "freezes" the molten slag in a glassy state, which gives the product its cementitious properties. For more information, see SCIC #1, "Slag Cement. "
SCA member companies supply over 95 percent of the slag cement in the U.S. Click here to download a listing of member company sales offices, arranged by state. These sales offices will be able to let you know about the availability of slag cement in your area.
Slag cement use can be traced to the 1700’s when the material was combined with lime to make mortars. The first United States production was in 1896. Until the 1950’s, granulated slag was used in the manufacture of blended portland cements, or as raw feedstock to make cement clinker. However, in the 1950’s, slag cement became available in other countries as a separate product. The first granulation facility in the U.S. to make a separate slag cement product was Sparrows Point, Maryland, in the early 1980’s. Recent years have seen the supply and acceptance of slag cement grow dramatically throughout the U.S. The product is now widely available east of the Rockies.

Slag cement is most widely used in concrete, either as a separate cementitious component or as part of a blended cement. It works synergistically with portland cement to increase strength, reduce permeability, improve resistance to chemical attack and inhibit rebar corrosion. Slag cement is used in virtually all concrete applications:

Slag cement is also used in non-concrete applications such as soil-cement and hazardous waste solidification.

Slag cement improves many of the strength and durability properties of hardened concrete. Slag cement is a hydraulic binder that, like portland cement, reacts with water to form cementitious material (calcium-silicate hydrate or CSH). It also, similar to a pozzolan, consumes by-product calcium hydroxide from the hydration of portland cement to form additional CSH. The resulting cement paste is stronger and denser, thus improving the concrete.

A list of improvements to hardened concrete follows, with a link to individual Slag Cement in Concrete information sheets for further detail:

  • Improved compressive and flexural strength (SCIC #14)
  • Reduced permeability, and resistance to chloride intrusion and corrosion (SCIC #6)
  • Ability to mitigate moderate to severe sulfate attack (even when used in combination with Type I cements - SCIC #7)
  • Ability to mitigate alkali-silica reaction with reactive aggregates (even when used with higher-alkali cements - SCIC #8)
  • Reduced thermal stress in mass concrete through lower heat generation (SCIC #9)
Slag cement generally improves workability, finishability and pumpability of plastic concrete. It may provide a small decrease in water demand (see SCIC #5). Slag cement will tend to increase 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 times of set (see SCIC #3). Early age strengths (through 7 to 14 days) of slag cement tend to be lower while later age strengths will be higher (see SCIC #14).

Production of slag cement creates a value-added product from a material—blast furnace slag—that otherwise might be destined for disposal. Not only does the making of slag cement lessen the burden on landfills, but it also reduces air emissions at steel plants through the granulation process (as compared to the traditional air cooling process). Use of slag cement in concrete reduces the environmental impact of concrete by:

  • Reducing greenhouse gas emissions by eliminating approximately one ton of carbon dioxide for each ton of portland cement replaced.
  • Reducing energy consumption, since a ton of slag cement requires nearly 90% less energy to produce than a ton of portland cement.
  • Reducing the amount of virgin material extracted to make concrete.
  • Reducing the "urban heat island" effect by making concrete lighter in color thus reflecting more light and cooling structures and pavements with exposed concrete.

The Environmental Protection Agency recognizes the environmental benefits of using slag cement in concrete. It has classified slag cement as a "recovered" product under the Resource Conservation Recovery Act (RCRA), and has issued a procurement guideline requiring its specification on most federally-funded projects.

Slag cement, when used as a separate component in a concrete mixture, is specified through ASTM C 989 Specification for Ground Granulated Blast-Furnace Slag for Use in Concrete and Mortar. When used as a component in blended cement, one of two specifications are used: ASTM C595, Specification for Blended Hydraulic Cements or ASTM C1157, Standard Performance Specification for Blended Hydraulic Cement. For more information, see SCIC #12, "Terminology and Specifications" and SCIC #13, "Suggested Specifications for Slag Cement in Concrete."

The amount of slag cement added to a concrete mixture, as a percentage of cementitious material by weight, normally ranges from 20 to 80 percent. The amount for a specific project depends on several factors including application, early and later age strength requirements, durability requirements and ambient temperature to name a few. For information on proportioning and a table of suggested percentages for various applications and end results, see SCIC #2, Concrete Proportioning.

Most general or structural concrete applications (flatwork, paving, foundations, walls, columns, floors, etc. ) typically use between 25 and 50% slag cement. Optimum slag cement percentage for maximum strength development is generally between 40 and 50 percent. A specification based on concrete strength at 28 days may be able to use less total cementitious material (portland + slag cement) than a similar plain portland mixture is mixture strength is optimized.

If durability parameters are specified (e.g. permeability, sulfate resistance, alkali-silica reaction (ASR) resistance) up to 70 percent slag cement may be required. For instance, one combination of portland and slag cement, in combination with a highly reactive aggregate may need as much as 70 percent slag cement to mitigate ASR, while a less reactive aggregate, combined with a lower alkali cement and slag cement may only need 25 percent.

Precast and prestressed concrete is an excellent application for slag cement at levels between 20 and 50 percent. If heat curing is used (or ambient curing temperatures relatively high) higher levels within this range are common. Using higher levels of slag cement in precast, provides the additional benefits of a more flowable mixture with smoother surface finish (fewer bugholes), and a whiter concrete appearance, often favored by designers and architects.

Mass concrete utilizes the highest potential levels of slag percentage to provide effective heat mitigation and reduced thermal stress. Mass applications use between 50 and 80 percent slag cement, with the thickest, most massive placements normally in the 65 to 80 percent range. Some slag cements in massive placements may not provide sufficient heat mitigation at the 50 to 65 percent levels because slag cement is activated by heat, and may react faster than desired because of the heat generated by portland (and slag) hydration.

Slag cement is the hydraulic cement that results when molton slag from an iron blast furnace is rapidly quenched with water, dried and ground to a fine powder. The rapid quenching ("granulation") "freezes" slag cement in a glassy state and imparts cementitious properties to the product when ground finely.

Blast furnace slag aggregate comes in two forms:

  • Air cooled blast furnace slag, results when molten slag from a blast furnace slag cools slowly by ambient air (as opposed to rapid quenching), and is processed through a screening and crushing plant for use principally as a construction aggregate. Air cooled slag is not cementitious.
  • Pelletized or expanded slag results when molten slag is quickly cooled using water or steam. It produces a lightweight aggregate that can be used for concrete masonry, lightweight fill, or can be ground into a cementitious product.

For more information on slag aggregates, visit the National Slag Association's website.

Slag cement is a white-colored hydraulic cement that will lighten the color of fully hardened, cured concrete. The lighter color of slag cement concrete (compared with concrete made with other cementitous materials - see photo) is generally considered to be a positive benefit, as it not only improves the aesthetic look of concrete, but also increases concrete's reflectivity (albedo) which provides greater mitigation of the "urban heat island effect" (see SCIC #22, "Slag Cement and the Environment"). In some architectural applications, where white cement might normally be used to achieve a white look, slag cement (or a combination of slag cement and white cement) can be utilized, as was the case with the Canadian Embassy. Additionally, the improved reflectivity may reduce lighting requirements for streets or parking facilities, and certainly improves safety with brighter streetscapes at night. Finally, it is easier to achieve a desired color with colored concrete (see SCIC #19, "Slag Cement in Residential Concrete"). Slag cement is normally used in proportions from 25 to 50 percent in structural and general concrete applications; the higher the amount of slag, the lighter the concrete.

Slag cement concrete may experience some initial "greening" after placement but this temporary color change will disappear with exposure to light and air (see related FAQ on greening and SCIC #10, "Greening").

Slag cement concrete sometimes turns green for a short period of time due to the oxidation state of sulfide sulphur compounds during portland/slag cement hydration. This is normally a temporary condition and the concrete, upon exposure to air and sunlight, will ultimately become lighter than 100% portland cement concrete. The condition is prolonged with extended moist curing, or on formed surfaces, especially when the form is left on for extended periods. Often, slag cement concrete that is many years old will manifest greening when the concrete is broken up (in the non-surface interior portions of the concrete).

Greening is not harmful, and is almost always temporary. In virtually every case where concrete is exposed to air, the greening will disappear and the concrete will be lighter that concrete made with other cementitious materials (see related FAQ on concrete color and article on Canadian Embassy, where greening occured, then disappeared). However, applications that will not be exposed to air and will be constantly moist may have permanent greening, such as swimming pools; therefore SCA suggests that slag cement not be use in this type of application when aesthetic concerns are important (greening will have no effect on other concrete properties, such as strength or durability).

SCA normally suggests that concrete that displays greening be left alone, and the greening will disappear. However, if the greening period is extended and waiting is not an option, applying a solution of hydrogen peroxide can help oxidize the concrete. Another effective solution is calcium hypochlorite (usually used for pool shock). Make this into a paste, apply and leave for a few minutes, wash off thoroughly, and let it take effect.

More information can be found in SCA's SCIC #10 and in a "Problem Clinic" discussion in Concrete Producer magazine.

Low concrete permeability is essential for long-term durability, especially with regard to corrosion resistance of reinforcing steel. The additional CSH formed and denser cement paste in slag cement concrete reduce pore size and lower concrete permeability, often by several orders of magnitude. Low permeability reduces the ingress of harmful substances (such as chlorides and sulfates) and the availability of water to catalyze harmful chemical reactions within concrete. It also dramatically lowers the rate of chloride ion diffusion and carbonation, thus significantly enhancing the corrosion protection offered by the concrete to the reinforcing steel. For more information, see SCIC #6, "Reducing Permeability".
Slag cement provides higher levels of compressive strength in concrete when compared with ordinary portland cement (OPC) concrete of equal cementitious materials content. Slag cement proportions of 40 to 50% normally optimize strength. Concretes made with slag cement will generally exhibit higher flexural strength for a given level of compressive strength. Modulus of elasticity follows the same relationship as OPC concrete, when based on compressive strength. Thus with the higher compressive strengths achievable with slag cement, structural stiffness can be enhanced, and load deflections minimized. For more information, see SCIC #14, "Compressive and Flexural Strength".
Mass concrete applications require limitations on the temperature differential between the surface and center of concrete to guard against thermal cracking. Slag cement reduces the rate of heat rise in proportion to its quantity in a mixture. Generally, high replacement rates are required (50 to 80%) to meet low heat of hydration requirements for mass applications. For more information, see SCIC #9, "Reducing Thermal Stress in Mass Concrete. "
ASR occurs when the alkalis in portland cement react with certain reactive aggregates to form an expansive gel that causes the concrete to crack, swell and prematurely deteriorate. Slag cement mitigates ASR by reacting with the alkalis in portland cement and making them unavailable for reaction. It also lowers the permeability of the concrete, reducing the water available for reaction and, in some cases, lowers the total alkali content of the cement paste. For more information, see SCIC #8, Mitigating Alkali-Silica Reaction.
Sulfate attack occurs when sulfates, found in seawater and some soils, react with the tricalcium aluminate in portland cement. This causes an expansive reaction and resulting deterioration of the concrete structure. Slag cement helps in two ways: 1) reduced permeability reduces the ingress of deleterious sulfates into the concrete and 2) slag cement does not contain tricalcium aluminate, and thus lowers the total amount available for reaction. The use of slag cement with a Type I portland cement can provide equivalence to a Type II (moderate sulfate-resistant) or even a Type V (high sulfate-resistant) cement. For more information, see SCIC #7, "Mitigating Sulfate Attack".

Case Study:

Children's Hospital of Richmond

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