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Slag
Iron and steel slags are co-products of iron and steel production respectively that are used in a variety of applications including but not limited to construction, agricultural, and environmental remediation. The specific products are manufactured by controlling various parameters including cooling rates and particle size to optimize the desired properties for a particular application.
Iron / Blast Furnace Slag Overview
The American Society of Testing and Materials (ASTM C125- 21 Definition of Terms Relating to Concrete and Concrete Materials) defines blast furnace slag as “the non-metallic product consisting essentially of silicates and alumino silicates of calcium and other bases, that is developed in a molten condition simultaneously with iron in a blast furnace”. In the production of iron, the blast furnace is charged with iron ore, flux stone (limestone and/or dolomite), and coke for fuel. The furnace yields two products: molten iron and slag. The slag consists primarily of silica and alumina from the original iron ore, combined as minerals which are comprised of mainly calcium and magnesium from the flux stone. Slag emerges from the furnace in a molten state with temperatures exceeding 1480°C (2700˚F). There are four distinct methods of processing the molten slag: air cooled, expanded, pelletized, and granulated; each of these methods produces unique slag material.
The principle mineral constituents of blast furnace slag are melilite, merwinite, pseduowallastonite, and monticellite; which are primarily composed of silica, alumina, calcium, and magnesia, making up 95% of slag’s total composition. The mineral constituents are engineered by changing the cooling rate process. Analysis of most blast furnace slags falls within the ranges shown below. In the case of granulated and pelletized slag, these elements primarily exist as amorphous glass. The chemical composition of slag from a given source varies within relatively narrow limits since raw materials charged into the furnace are carefully selected and blended.
PHYSICAL PROPERTIES
The physical characteristics (weight, particle size, structural properties, etc.) vary according to the method used in processing the molten slag.
Accordingly, end use of the processed material varies, which helps to explain the unique diversity of slag products.
Types of Blast Furnace Slag Processing
Air Cooled / Atmospheric Cooling
Air-Cooled Blast Furnace Slag (ACBFS) as defined in ASTM C 125-21a is: “The material resulting from solidification of molten blast-furnace slag under atmospheric conditions; . Subsequent cooling may be accelerated by application of water to the solidified surface”.
The solidified slag characteristically has a porous structure with many non-connected cells. ACBFS crushes to angular, roughly cubical pieces with minimal flat or elongated fragments. The rough porous texture of slag gives it a greater surface area than smoother aggregates of equal volume and provides an excellent bond with portland cement and high stability in asphalt mixtures. For embankment applications, the rough surfaces improve the angle of internal friction or interlocking of the pieces.
Due to the porous nature of ACBFS, it is important that bulk specific gravity be used rather than apparent specific gravity for purposes of computing yield or estimating quantities. The bulk specific gravity (dry basis) of ACBFS coarse aggregate generally falls in the range of 2.0 to 2.5. Since large pores cannot exist in small particles, the smaller size products have higher specific gravities. Slag sand (#4 to 0 size) approaches natural sand in bulk specific gravity.
The unit weight varies with: (a) size and grading of the slag, (b) method of measuring and (c) bulk specific gravity of the slag. Typical unit weight (compacted) of crushed and screened air-cooled slag, graded as ordinarily used in concrete, is usually in the range of 1121 kg to 1281 kg per m³ (70 to 85 lb per ft³). Slag has an economic advantage in construction because it has a lower unit weight than most natural aggregates. Allowance for this differential should always be considered for design and specifications to assure equal volume irrespective of the type of aggregate used.
ACBFS is crushed and screened to conform to the grading requirements of the various state highway departments, municipalities and other specifying bodies. Gradations specified in national standards, such as ASTM D 448. Standard sizes of coarse aggregate for highway construction, are usually preferred and often the most readily available.
Water absorption of ACBFS is usually in the range of 1 to 5% by weight, as it has a greater surface area and lower specific gravity than most natural aggregates. The degree of saturation (portion of the total void space filled by water) is low. Due to its irregular surface, care must be taken when determining the surface dry condition when calculating absorption. The pores are similar to the air bubbles in air-entrained cement pastes and the resulting durability is outstanding.
Notwithstanding its toughness, the degradation of slag, as tested in the Los Angeles (LA) abrasion machine, is generally higher than for round or smooth-surfaced natural aggregates. This is due mainly to the rough edges on the surface breaking off under impact of the steel balls constituting the test charge. It has been proven that there is no correlation between the LA abrasion loss for slag in laboratory tests and degradation in field applications. For this reason ASTM exempts crushed slag from the degradation requirements in various specifications (see ASTM D 692, D1139, etc.); additionally D.O.T.’s in states where slag is available do not require LA abrasion testing for slag. LA Abrasion limits for slag, if included in specifications, should be somewhat higher than that for natural aggregates to a maximum of approximately 50% loss. This higher loss, however, does not mean that slag is softer than natural aggregates. The hardness of slag as measured by the Mohs scale is between 5 & 6. This compares favorably with the hardness reported for such materials as durable igneous rocks. Investigations have shown that slag fines produced during the LA abrasion are crystalline and non-plastic.
The small amounts of sulfur in slag are present in combined alkaline minerals, similar to those found in portland cement. These are harmless to concrete and do not cause corrosion of reinforcing steel. The corrosive properties of coal ash or cinders should not be mistakenly applied to blast furnace slag. Examination of reinforcing bars taken from slag concrete structures after 30 years of service has shown no evidence of corrosion.
Slag is highly resistant to environmental degradation. It will withstand an unusually large number of cycles of the sulfate soundness test (ASTM C 88). Freezing and thawing or wetting and drying tests, also have little or no effect. High temperatures have very little effect on slag as it is formed in the blast furnace at about 1480°C or 2700°F. ACBFS shows a slow but very uniform coefficient of expansion of approximately 0.000006 per °F, up to its melting point (1150-1426°C/2100- 2600°F). This figure is normally accepted as the coefficient of expansion for cement mortar and steel, hence, slag, when combined with these ingredients to form reinforced concrete, affords a high degree of compatibility.
Controlled Water Cooling/Expanded
Controlled quantities of water are used to accelerate the solidification process of molten blast furnace slag, resulting in a low density material. The solidified expanded slag is crushed and screened for use as a lightweight structural aggregate
Expanded blast furnace slag is angular and cubical in shape, with negligible flat or elongated particles. Due to the action of the water and resulting steam on the solidification process, the open cellular structure of the particles is even more pronounced than particles of air cooled blast furnace slag. GRADING Expanded blast furnace slag is crushed and screened to desired product sizing. Typically this is a blend of coarse and fine aggregate particles. The actual grading of products should be reviewed with the local supplier.
Water Quenching/ Granulated
The most common process for granulating blast furnace slag involves the use of high water volume, high pressure water jets in direct contact with the molten blast furnace slag at a ratio of approximately 10:1 by mass. The molten blast furnace slag is quenched almost immediately, forming a material generally smaller than a #4 sieve. When granulated blast furnace slag is formed, it must be de-watered, dried and ground, using processes similar to those used with portland cement clinker to make portland cement. Typically, granulated slag is finely ground (measured by the Blaine test), exceeding the fineness of portland cement to obtain increased hydraulic activity at early ages. As with portland cement and pozzolans, the rate of reaction increases with the particle fineness.
When GGBFS slag is mixed with water, initial hydration is much slower as compared with portland cement. Therefore, portland cement or alkali salts are used to increase the reaction rate. In the hydration process, GGBFS slag produces calcium silicate hydrate cement paste. This valuable contribution from GGBFS slag improves the paste-to-aggregate bond in concrete. GGBFS slag mixtures with portland cement typically result in greater strength and reduced permeability. ASTM C989 provides three strength grades of GGBFS slag, depending on their respective mortar strengths when blended with an equal amount of portland cement. As summarized below, the classifications are grade 80, 100 and 120, based on the slag activity index.
GGBFS slag is considerably lighter in color than most portland cements and will produce a lighter concrete end product. Occasionally, concrete containing GGBFS slag may exhibit a blue-green coloration. While this coloration effect seldom occurs, it is attributed to a
complex reaction of the sulfide sulfur in the GGBFS slag with other compounds in the cement and will diminish with age.
Steel Slags
The American Society for Testing Materials (ASTM) defines steel slag as a non-metallic product, consisting essentially of calcium silicates and ferrites, combined with fused oxides of iron, aluminum, manganese, calcium, and magnesium, which is developed simultaneously with steel in basic oxygen or electric arc furnaces.
In the steel industry, two main types of slags are produced; blast furnace slag and steel slag. Slag is a co-product of the iron and steel-making process. Iron cannot be made in a blast furnace without the production of its co-product, blast furnace slag. Similarly, steel cannot be produced in a Basic Oxygen Furnace (BOF) or in an Electric Arc Furnace (EAF) without making its co-product, steel slag. Due to their unique physical and chemical properties as well as end use applications, it is important to differentiate steel slag from blast furnace slag. This brochure will mainly discuss the steel slags currently being produced from BOF and EAF furnaces. To find out more about blast furnace slag, please contact The National Slag Association office to request the brochure entitled “Blast Furnace Slag”.
The physical characteristics (weight, particle size, structural properties, etc.) vary according to the method used in processing the molten slag. Accordingly, end use of the processed materials varies, which helps to explain the unique diversity of slag products.
Integrated Steel Making
In an Integrated Steel Mill, a blast furnace uses coke (carbon) to reduce iron to a high-carbon molten iron, called “hot metal”. This hot metal is subsequently transferred to the steel-making shop’s Basic Oxygen Furnace (BOF) where it is combined with steel scrap, various metallic elements, additional lime, or dolomitic lime fluxes and oxygen is injected to produce steel. During this process, the BOF uses oxygen to eliminate carbon and oxidizing reactions to generate heat.
Electric Arc Furnace
In an Electric Arc Furnace (EAF) steel-making operation, steel scrap is melted in an electric arc furnace along with fluxing agents to produce similar products as that of a Basic Oxygen Furnace: steel and steel slag
Steel Slag Asphalt Paving
Steel slag has evolved as an ideal aggregate in Hot Mix Asphalt (HMA) surface mixture applications. Due to its properties, steel slag qualifies as a premier surface aggregate for skid-resistant applications, particularly where friction is a crucial safety consideration within pavement design for the motoring public. With the development and implementation of Superpave technology throughout the United States as well as the further expansion of Stone Matrix Asphalt (SMA) mixes for severe traffic and axle-loading applications, steel slag has established itself as a premium surface aggregate. Furthermore, steel slag continues to be recognized as a standard material for use in both seal coating and cold patching applications.
The frictional properties of steel slag have demonstrated outstanding performance in making asphalt pavements skid resistant and maintaining that skid resistance over the life of the pavement. With safety in mind, many road agencies require steel slag when skid resistance is of prime concern. The physical properties and surface texture of steel slags provide a coefficient of friction in asphalt surface courses higher than most natural aggregates. Steel slag’s unique combination of particle structure and hardness produces a pavement wearing surface where longterm frictional properties are maintained, as aggregate polishing does not occur.
Chip and Seal also known as “Chip Seal” or “Aggregate Seal Coating”, involves applying a single or double layer of Bituminous Surface Treatment onto existing pavement surfaces. Its main purpose is to improve skid resistance on rural roads and to maximize driving safety. Chip Seal is a cost-effective way to improve the safety and integrity of a road surface in low traffic environments. A seven-year cycle is recommended to insure road reliability and safety, which helps reduce the need for other more expensive maintenance applications such as “cold patching”. Construction begins with the removal of any loose debris from the existing road surface. An asphalt emulsion binder is then sprayed on the pavement at a rate of between 0.20-0.40 gallons per square yard. Immediately after a steel slag chip or a combination with another aggregate is uniformly applied to the surface, a rubber-tired roller is driven over the surface to ensure that the aggregate adheres properly to the asphalt binder and pavement surface. Steel slag contributes a high coefficient of friction to the roads surface by providing the roughness necessary to attain a skid resistant pavement. Physically, natural aggregates are unable to provide a surface that will resist polishing; therefore, they easily become slippery when wet. Additionally, with its hard, angular, skid resistant shape and great asphalt binder affinity, steel slag is the most advantageous choice of aggregate for Chip and Seal applications.
The Hot Mix Asphalt industry has undergone significant changes due to state and federal efforts aiming to prolong the lifespan of asphalt pavements. A performance based specification system was developed to obtain a closer correlation between Hot Mix Asphalt (HMA) design objectives and actual field service life. This new design criteria, called “Superpave”, incorporates mix design procedures involving the careful selection and proportioning of materials based upon traffic volume and loading conditions. Historically, the Hot Mix Asphalt industry has been able to meet most Department of Transportation (D.O.T) highway specification requirements by utilizing a limited number of locally available aggregates. Today, with Superpave’s more extensive design criteria, many HMA producers have experienced unforeseen delays and incremental costs in their initial attempt to establish acceptable asphalt mixtures. Through the incorporation of coarse and some fine steel slag aggregates, establishing design criteria can be made easier due to steel slag’s physical characteristics.
While the effect of aggregate properties on the performance of asphalt mixtures has been well documented, Superpave is the first design method to specify consensus properties to meet specific traffic volume criteria. These consensus properties include coarse aggregate angularity, fine aggregate angularity, flat and elongated particles, and clay content.
Steel Slag Portland Cement
Portland cement is primarily composed of hydraulic calcium silicates, along with some calcium aluminates, calcium aluminoferrites and usually containing one or more form of calcium sulfate (gypsum) as an inter-ground addition. Materials used in the manufacture of portland cement must contain appropriate proportions of calcium oxide, silica, alumina and iron oxide components. While iron and steel-making slags have been utilized by some cement companies for decades as a cost-effective chemical raw feed additive, the interest in using these materials has grown dramatically. The surge is primarily due to heightened environmental regulations and market requirements for improved productivity.
Steel slag can provide a significant contribution to improving production capacity, while also reducing CO2 emissions. This beneficial feature of steel slag exists because it has previously undergone calcination, eliminating CO2 and most volatile materials. Consequently, while a ton of limestone’s yield contribution of cement clinker can be as low as 60%, steel slag’s yield is 100%. Production increases with moderate addition (8-11%) of steel slag can be almost proportionate to the amount utilized.
