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STEEL SLAG Material Description

ORIGIN

Steel slag, a by-product of steel making, is produced during the separation of the molten steel from

impurities in steel-making furnaces. The slag occurs as a molten liquid melt and is a complex solution of

silicates and oxides that solidifies upon cooling.

Virtually all steel is now made in integrated steel plants using a version of the basic oxygen process or in

specialty steel plants (mini-mills) using an electric arc furnace process. The open hearth furnace process is

no longer used.

In the basic oxygen process, hot liquid blast furnace metal, scrap, and fluxes, which consist of lime (CaO)

and dolomitic lime (CaO.MgO or “dolime”), are charged to a converter (furnace). A lance is lowered into

the converter and high-pressure oxygen is injected. The oxygen combines with and removes the impurities

in the charge. These impurities consist of carbon as gaseous carbon monoxide, and silicon, manganese,

phosphorus and some iron as liquid oxides, which combine with lime and dolime to form the steel slag. At

the end of the refining operation, the liquid steel is tapped (poured) into a ladle while the steel slag is

retained in the vessel and subsequently tapped into a separate slag pot.

There are many grades of steel that can be produced, and the properties of the steel slag can change

significantly with each grade. Grades of steel can be classified as high, medium, and low, depending on

the carbon content of the steel. High-grade steels have high carbon content. To reduce the amount of

carbon in the steel, greater oxygen levels are required in the steel-making process. This also requires the

addition of increased levels of lime and dolime (flux) for the removal of impurities from the steel and

increased slag formation.

There are several different types of steel slag produced during the steel-making process. These different

types are referred to as furnace or tap slag, raker slag, synthetic or ladle slags, and pit or cleanout slag.

Figure 18-1 presents a diagram of the general flow and production of different slags in a modern steel

plant.

The steel slag produced during the primary stage of steel production is referred to as furnace slag or tap

slag. This is the major source of steel slag aggregate. After being tapped from the furnace, the molten

steel is transferred in a ladle for further refining to remove additional impurities still contained within the

steel. This operation is called ladle refining because it is completed within the transfer ladle. During ladle

refining, additional steel slags are generated by again adding fluxes to the ladle to melt. These slags are

combined with any carryover of furnace slag and assist in absorbing deoxidation products (inclusions), heat

insulation, and protection of ladle refractories. The steel slags produced at this stage of steel making are

generally referred to as raker and ladle slags.

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Figure 18-1. Overview of slag production in modern integrated steel plant.

Pit slag and clean out slag are other types of slag commonly found in steel-making operations. They

usually consist of the steel slag that falls on the floor of the plant at various stages of operation, or slag

that is removed from the ladle after tapping.

Because the ladle refining stage usually involves comparatively high flux additions, the properties of these

synthetic slags are quite different from those of the furnace slag and are generally unsuitable for

processing as steel slag aggregates. These different slags must be segregated from furnace slag to avoid

contamination of the slag aggregate produced.

In addition to slag recovery, the liquid furnace slag and ladle slags are generally processed to recover the

ferrous metals. This metals recovery operation (using magnetic separator on conveyor and/or crane

electromagnet) is important to the steelmaker as the metals can then be reused within the steel plant as

blast furnace feed material for the production of iron.

Additional information on steel slag aggregate use in the United States can be obtained from:

National Slag Association

808 North Fairfax Street

Arlington, Virginia 22314

CURRENT MANAGEMENT OPTIONS

Recycling

It is estimated that between 7.0 and 7.5 million metric tons (7.7 to 8.3 million tons) of steel slag is used

each year in the United States. The primary applications for steel slag in the United States are its use as a

granular base or as an aggregate material in construction applications.

Disposal

While most of the furnace slag is recycled for use as an aggregate, excess steel slag from other

operations (raker, ladle, clean out, or pit slag) is usually sent to landfills for disposal.

MARKET SOURCES

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Steel slag can normally be obtained from slag processors who collect the slag from steel-making facilities.

Slag processors may handle a variety of materials such as steel slag, ladle slag, pit slag, and used

refractory material to recover steel metallics. These materials must be source separated, and well-defined

handling practices must be in place to avoid contamination of the steel slag aggregate. The slag processor

must also be aware of the general aggregate requirements of the end user.

The processing of steel slags for metals recovery is not only important to remove excess steel at the

market source for reuse at the steel plant, but is also important to facilitate the use of the nonmetallic steel

slag as construction aggregate. This nonmetallic slag material can either be crushed and screened for

aggregate use (steel slag aggregates), or sintered and recycled as flux material in the iron and steel

furnaces.

Steel slag aggregates generally exhibit a propensity to expand. This is because of the presence of free

lime and magnesium oxides that have not reacted with the silicate structures and that can hydrate and

expand in humid environments. This potentially expansive nature (volume changes of up to 10 percent or

more attributable to the hydration of calcium and magnesium oxides) could cause difficulties with products

containing steel slag, and is one reason why steel slag aggregates are not suitable for use in Portland

cement concrete or as compacted fill beneath concrete slabs.

Steel slag destined for use as an aggregate should be stockpiled outdoors for several months to expose

the material to moisture from natural precipitation and/or application of water by spraying. The purpose of

such storage (aging) is to allow potentially destructive hydration and its associated expansion to take place

prior to use of the material in aggregate applications. There is a wide variation in the amount of time

required for adequate exposure to the elements. Up to 18 months may be needed to hydrate the expansive

oxides.

HIGHWAY USES AND PROCESSING REQUIREMENTS

Asphalt Concrete Aggregate, Granular Base, and Embankment or Fill

The use of steel slag as an aggregate is considered a standard practice in many jurisdictions, with

applications that include its use in granular base, embankments, engineered fill, highway shoulders, and hot

mix asphalt pavement.

Prior to its use as a construction aggregate material, steel slag must be crushed and screened to meet the

specified gradation requirements for the particular application. The slag processor may also be required to

satisfy moisture content criteria (e.g., limit the amount of moisture in the steel slag aggregate prior to

shipment to a hot mix asphalt plant) and to adopt material handling (processing and stockpiling) practices

similar to those used in the conventional aggregates industry to avoid potential segregation. In addition, as

previously noted, expansion due to hydration reactions should be addressed prior to use.

MATERIAL PROPERTIES

Physical Properties

Steel slag aggregates are highly angular in shape and have rough surface texture. They have high bulk

specific gravity and moderate water absorption (less than 3 percent). Table 18-1 lists some typical

physical properties of steel slag.

Table 18-1. Typical physical properties of steel slag.

Property Value

Specific Gravity > 3.2 - 3.6

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Unit Weight, kg/m3

 (lb/ft3) 1600 - 1920

(100 - 120)

Absorption up to 3%

Chemical Properties

The chemical composition of slag is usually expressed in terms of simple oxides calculated from elemental

analysis determined by x-ray fluorescence. Table 18-2 lists the range of compounds present in steel slag

from a typical base oxygen furnace. Virtually all steel slags fall within these chemical ranges but not all

steel slags are suitable as aggregates. Of more importance is the mineralogical form of the slag, which is

highly dependent on the rate of slag cooling in the steel-making process.

Table 18-2. Typical steel slag chemical composition.(4)

Constituent Composition (%)

CaO 40 - 52

SiO2 10 - 19

FeO 10 - 40

(70 - 80% FeO, 20 - 30% Fe2O3)

MnO 5 - 8

MgO 5 - 10

Al2O3 1 - 3

P2O5 0.5 - 1

S < 0.1

Metallic Fe 0.5 - 10

The cooling rate of steel slag is sufficiently low so that crystalline compounds are generally formed. The

predominant compounds are dicalcium silicate, tricalcium silicate, dicalcium ferrite, merwinite, calcium

aluminate, calcium-magnesium iron oxide, and some free lime and free magnesia (periclase). The relative

proportions of these compounds depend on the steel-making practice and the steel slag cooling rate.

Free calcium and magnesium oxides are not completely consumed in the steel slag, and there is general

agreement in the technical literature that the hydration of unslaked lime and magnesia in contact with

moisture is largely responsible for the expansive nature of most steel slags.(1,2) The free lime hydrates

rapidly and can cause large volume changes over a relatively short period of time (weeks), while magnesia

hydrates much more slowly and contributes to long-term expansion that may take years to develop.

Steel slag is mildly alkaline, with a solution pH generally in the range of 8 to 10. However, the pH of

leachate from steel slag can exceed 11, a level that can be corrosive to aluminum or galvanized steel pipes

placed in direct contact with the slag.

Tufalike precipitates, resulting from the exposure of steel slag aggregates to both water and the

atmosphere, have been reported in the literature. Tufa is a white, powdery precipitate that consists

primarily of calcium carbonate (CaCO3). It occurs in nature and is usually found in water bodies. The tufa

precipitates associated with steel slags are attributed to the leachate combining with atmospheric carbon

dioxide. The free lime in steel slags can combine with water to produce calcium hydroxide (Ca(OH2))

solution. Upon exposure to atmospheric carbon dioxide, calcite (CaCO3) is precipitated in the form of

surficial tufa and powdery sediment in surface water. Tufa precipitates have been reported to clog

drainage paths in pavement systems.(5)

Mechanical Properties

Processed steel slag has favorable mechanical properties for aggregate use, including good abrasion

resistance, good soundness characteristics, and high bearing strength. Table 18-3 lists some typical

mechanical properties of steel slag.

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Table 18-3. Typical mechanical properties of steel slag.(3)

Property Value

Los Angeles Abrasion (ASTM C131), % 20 - 25

Sodium Sulfate Soundness Loss (ASTM C88), % <12

Angle of Internal Friction 40° - 50°

Hardness (measured by Moh's scale of mineral hardness)* 6 - 7

California Bearing Ratio (CBR), % top size 19 mm (3/4 inch)** up to 300

* Hardness of dolomite measured on same scale is 3 to 4.

** Typical CBR value for crushed limestone is 100%.

Thermal Properties

Due to their high heat capacity, steel slag aggregates have been observed to retain heat considerably

longer than conventional natural aggregates. The heat retention characteristics of steel slag aggregates

can be advantageous in hot mix asphalt repair work in cold weather.

REFERENCES

JEGEL. Steel Slag Aggregates Use in Hot Mix Asphalt Concrete. Final Report, prepared by John

Emery Geotechnical Engineering Limited for the Steelmaking Slag Technical Committee, April,

1993.

1.

Collins, R. J. and S. K. Ciesielski. Recycling and Use of Waste Materials and By-Products in

Highway Construction, National Cooperative Highway Research Program Synthesis of Highway

Practice 199, Transportation Research Board, Washington, DC, 1994.

2.

Noureldin, A. S., and R. S. McDaniel. “Evaluation of Steel Slag Asphalt Surface Mixtures,”

Presented at Transportation Research Board 69th Annual Meeting, Washington, DC, January, 1990.

3.

Emery, J. J. “Slag Utilization in Pavement Construction,” Extending Aggregate Resources. ASTM

Special Technical Publication 774, American Society for Testing and Materials, Washington, DC,

1982.

4.

Gupta, J. D., and W. A. Kneller. Precipitate Potential of Highway Subbase Aggregates. Report No.

FHWA/OH-94/004, Prepared for the Ohio Department of Transportation, November, 1993.

5.

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