Jan R. Prusinski, Slag Cement Association
Slag cement—commonly referred to as ground granulated blast-furnace slag or GGBFS—is a hydraulic cement that works synergistically with portland cement to improve concrete strength and durability.
What is Slag Cement?
Slag cement is a value-added material that results from a tightly controlled production process that ensures consistent physical and chemical properties. Molten slag—the non-metallic mineral constituent of iron ore—is tapped from an iron blast furnace, then rapidly quenched with water in a granulator. The resulting glassy granules are then dried and either ground to a fine powder to make slag cement or interground with portland cement to produce blended cement. Slag cement is different from slag aggregates, which are either air-cooled or expanded blast-furnace slag and possess no cementitious value. Available for many years in the United States, slag cement use has doubled over the last five years.
Cementitious Reaction
Slag cement is a hydraulic binder that reacts with water to form a cementitious material (calcium-silicate hydrate or CSH). Similar to a pozzolan, it also reacts with the calcium hydroxide formed during the hydration of portland cement to form additional CSH. The resulting cement paste is stronger and denser, thereby improving the properties of concrete.
High Strength and Modulus of Elasticity
Slag cement provides a higher compressive strength in concrete at later ages than is achieved by using portland cement alone. Concrete strength is usually optimized when slag cement replaces 40 to 50 percent of the portland cement. Additionally, concrete made with slag cement commonly exhibits higher ratios of flexural to compressive strength. The relationship between modulus of elasticity and concrete compressive strength for slag cement concrete is the same as that for portland cement concrete.
Permeability and Corrosion Resistance
HPC mixtures are often proportioned to achieve low permeability. The additional CSH created with slag cement forms a denser cement paste, reducing pore size and lowering concrete permeability, as illustrated in the figure. Lower permeability significantly enhances the corrosion protection offered by concrete to the reinforcing steel by reducing the rates of chloride ion diffusion and carbonation.
Resistance to Alkali-Silica Reaction and Sulfate Attack
The low permeability of slag cement concrete reduces available moisture necessary for alkali-silica reaction (ASR) and sulfate attack. In the case of sulfate attack, the low permeability keeps sulfates from migrating into the concrete. For ASR mitigation, slag cement consumes some of the alkalis produced from the portland cement during hydration, leaving them unavailable for reaction with the aggregates. For mitigation of sulfate attack, slag cement lowers the total amount of tricalcium aluminate available for reaction with the sulfates. Proper proportioning of slag cement can eliminate the need to use low alkali or sulfate-resistant portland cements.
Early Age Properties
Slag cement improves the workability, placeability, and consolidation of concrete. This results in easier and better finishing, and helps ensure proper consolidation of the placed material.
In mass concrete applications, it is often necessary to limit the temperature differential between the surface and center of the concrete to guard against thermal cracking. Dosage rates of 50 to 80 percent of slag cement normally enable mass concrete to meet low heat of hydration requirements. When slag cement is used, the time of initial set is generally extended by 1 to 3 hours at 73°F (23°C) but generally becomes unnoticeable above 85°F (29°C). Lower temperatures can extend time of set significantly, but conventional accelerators can offset this effect.
For the first 3 to 7 days, slag cement concrete exhibits lower strengths compared to portland cement concrete. By a concrete age of 28 days, slag cement concrete strengths are normally higher. Steam curing in precast, prestressed concrete operations can virtually eliminate early age strength differences and still maintain later age benefits.
Proportioning and Use with Admixtures and Pozzolans
The use of slag cement in a concrete mixture will necessitate minor adjustments in the mix proportions. Slag cement should conform to AASHTO M 302 (ASTM C 989) or, if used in blended cement, AASHTO M 240 (ASTM C 595) or ASTM C 1157. Slag cement is compatible with chemical admixtures in a manner similar to portland cement. Also, slag cement is frequently used in HPC ternary blends with fly ash or silica fume.
Further Information
Further information on the use of slag cement in high performance concrete can be obtained from the Slag Cement Association (SCA) at phone: 281-494-0782 or e-mail: [email protected]