Steal Furnace Slag 

 

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Materials Characterization Paper 

In Support of the 

Final Rulemaking: 

Identification of Nonhazardous Secondary Materials That Are Solid Waste 

Steel Furnace Slag (Used as an ingredient in clinker manufacture and bituminous concrete) 

February 3, 2011 

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1. Definition of Steel Furnace Slag (SFS) 

Steel furnace slag (SFS), commonly referred to as steel slag, is a nonmetallic by-product from 

either the conversion of iron to steel in a basic oxygen furnace (BOF) or the melting of scrap to 

make steel in an electric arc furnace (EAF). Similar to iron blast-furnace slag, steel slag is 

produced when slagging agents and/or fluxing materials are added to molten metal to remove 

impurities. The liquid silicate slag floats on the molten metal and is separated and cooled. Steel 

furnaces typically use lime as the slagging agent, in contrast to iron blast furnaces, which 

normally use limestone and/or dolomite. (EPA 2008, Kiggins 2009) 

2. Annual Quantities Generated and Used 

(1) Sectors that generate steel slag: 

• Steel furnace slag is generated by NAICS industry sector 3311111, Iron and Steel 

Mills. 

(2) Quantities and prices of steel slag generated: 

• The United States Geological Survey (USGS) notes in the 2008 Minerals 

Yearbook on iron and steel slag that the annual production of steel slag is 

uncertain because not all slag is tapped1 during steel production and the amount 

tapped is not routinely measured. However, U.S. slag production can be broadly 

estimated based on typical slag to metal production ratios. The amount of 

marketable steel slag is usually equivalent to approximately 10 to 15 percent of 

the crude steel output. Based on these figures, U.S. output of steel slag (after 

metal removal) in 2009 was estimated to be 8 to 12 million metric tons (Mt). (van 

Oss 2008, 2010) 

• In 2008, average sale prices of SFS were $5.02 per metric ton, with a range of 

$0.20 to $15.59 per metric ton. Exhibit 1 presents the historical average and 

range of sales prices information for the U.S. from 2004-2008. (van Oss 2004-

2008) 

 

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 “Tapping” refers to the process of tilting a furnace to pour off molten steel slag into another vessel, or any other 

method that may be used to extract slag from a furnace. 
Steal Furnace Slag 

 

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Exhibit 1: Historical Sales Price Information for the U.S. 

Selling Prices for Steel Slag in the United States 

 2004 2005 2006 2007 2008 

Range ($/Mt) 0.22-7.89 0.22-12.85 0.49-13.16 0.47-11.90 0.20-15.59 

Average ($/Mt) 4.32 4.45 4.58 5.48 5.02 

Sources: van Oss (2004-2008, 2010). Values expressed in nominal dollars (not adjusted for inflation). 

(3) Trends in generation of SFS: 

• The generation of steel slag from integrated iron and steel works is in significant 

decline, owing to the idling of several plants and an increasing trend of returning 

slag to the furnaces. In contrast, slag from electric arc steel furnaces (largely fed 

with steel scrap) remains abundant. Across these two facility types, generation of 

steel slag has been relatively consistent over the past several years. Exhibit 2 

presents historical steel slag generation data for the U.S. from 2004-2009. (van 

Oss 2004-2008, 2010) 

• In 2008, approximately 8.7 million metric tons of steel slag were sold in the U.S. 

Exhibit 2 presents the historical steel slag sales and valuation data for the U.S. 

from 2004-2008. (van Oss 2004-2008) 

• Estimated generation of slag may differ from the recorded sales of slag because of 

undocumented returns of slag to furnaces and stockpiling of slag by processors. 

(van Oss 2008) 

Exhibit 2: Historical Generation and Sales Information for the U.S 

Steel Slag Produced and Sold/Used in the United States, 2004-2009 

 2004 2005 2006 2007 2008 2009 

Quantity Generated (Mt) 11-16 10-14 10-15 10-15 9-14 8-12 

Quantity Sold/Used (Mt)1 9 8.7 8.7 7.8 8.7 -2

Value Sold/Used ($ MM) 39 39 40 43 43 -2

Note: 

1.The quantity sold does not reflect undocumented returns of slag to furnaces. 

2.Blank cells indicate figure is unknown 

Sources: van Oss (2004-2008, 2010). Values expressed in nominal dollars (not adjusted for inflation). 

3. Uses of Material/Commodity 

(1) Ingredient uses of SFS:

• SFS is often used as a raw material substitute in cement clinker manufacturing 

and concrete. SFS can be added to the raw material feed in clinker and bituminous 

concrete manufacturing to contribute specific required elements, such as calcium, 

silicon, and iron in the final cement or concrete product (EPA 2008, Kiggins 

2009). 

(2) Non-combustion uses of SFS

• Steel slag is useful as an aggregate in granular base applications, and can be 

processed into coarse or fine aggregate material for use in hot mix asphalt 

concrete pavements and in cold mix or surface treatment applications. Because of 
Steal Furnace Slag 

 

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its expansive characteristics, steel slag is not typically used as an aggregate in 

concrete for fixed-volume applications. (EPA 2008) 

• In addition to granular base and hot mix asphalt applications, steel slag is also 

used as an aggregate in embankments, engineered fill, and highway shoulders 

(TFHRC 2010). Exhibit 3 presents the composition of U.S. steel slag sales across 

different applications from 2004-2008. 

• SFS can also be used for remineralization in the agricultural industry and as a 

water filter media for acid mine drainage and water pollution control. Basic 

oxygen furnace steel slag is an effective material for use in permeable reactive 

barrier systems for the removal of phosphorus and pathogens such as E.coli from 

groundwater and wastewater. (Smyth, et al. 2002) 

Exhibit 3: U.S. SFS Sales Composition 

Sales of Steel Slag in the United State, By Application, 2004-2008 

Percentage of total tons sold 

Use 2004 2005 2006 2007 2008 

Asphaltic concrete 12.9 15.6 12.1 14.4 10.9 

Road bases and surfaces 63.5 53.0 51.0 51.5 60.3 

Fill 9.6 10.5 17.9 13.3 10.8 

Clinker raw material 5.5 6.9 6.9 6.7 5.0 

Miscellaneous1 2.9 2.3 0.8 2.3 0.5 

Other or unspecified 5.6 11.7 11.3 12.0 12.7 

Note: 

1 Reported as used for railroad ballast, roofing, mineral wool, or soil conditioner. 

Sources: van Oss (2004-2008) 

(3) Quantities of SFS landfilled:

• No data are readily available on the disposal or landfilling of SFS, but it is likely 

that the utilization of steel furnace slag is nearly 100 percent of U.S. production, 

in which case disposal is minimal. (van Oss 2008, TFHRC 2010) 

(4) Quantities of SFS stockpiled/stored

• Some steel slag sales are from legacy slag piles (slag banks) produced by iron and 

steel plants that are now closed. Most steel slag currently sold, however, is 

generated by iron and steel facilities that are still in operation. As mentioned 

above, stockpiles of SFS account for some of the difference between annual 

production and sales. (Kiggins 2009) 

4. Management and Combustion Processes for SFS 

(1) Types of combustion units using SFS:

• Steel furnace slag is added to cement kilns during the clinker manufacturing 

process, and is also utilized in bituminous concrete facilities during the making of 

asphalt (Kiggins 2009). Substitution of slags for natural raw materials to make 

clinker can reduce the unit consumption of fuel and limestone in the kiln, which 

then reduces the overall and unit emissions of certain pollutants, most notably 

carbon dioxide. (van Oss 2008) 
Steal Furnace Slag 

 

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(2) Sourcing of SFS:

• Cement and bituminous concrete manufacturing facilities purchase steel furnace 

slag directly from iron and steel mills or from a slag processing/grinding facility. 

(Kiggins 2009) 

(3) Processing of SFS: 

• Steel slags form as a silicate melt that floats on top of the molten crude steel and 

is tapped from the furnace separately from the liquid metal. After cooling to solid 

form, the slag is magnetically separated, crushed, further magnetically separated, 

and sized by screening for use as a raw material in clinker manufacturing and 

concrete. (van Oss 2008, Kiggins 2009) 

• Prior to its use as a construction aggregate material, steel slag must meet the 

specified gradation requirements for the particular application. In addition to 

crushing and screening, 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. (TFHRC 2010) 

(4) State status of SFS use as ingredient: 

• According to state responses to a 2006 survey by the Association of State and 

Territorial Solid Waste Management Officials (ASTSWMO), several states have 

granted beneficial use designations for steel slag. Two states, Iowa and New 

York, have approved the use of SFS in cement on at least one occasion. Iowa, 

New Jersey, Kentucky, Tennessee, and Virginia have approved the use of SFS in 

fill on at least one occasion. Four states – Iowa, Maryland, Ohio, and Wisconsin 

– have approved the use of SFS in roads on at least one occasion.2

• In addition, four states have granted beneficial use designation for other 

applications on at least one occasion: North Carolina for use in land applied 

fertilizer; Pennsylvania for use in construction material and as a liming agent; 

Tennessee for use as an aggregate; and Wisconsin for use in railroad ballast, 

concrete, and asphalt. (ASTSWMO 2007) 

5. SFS Composition and Impacts

(1) Composition of SFS:

• SFS consists primarily of calcium silicates and ferrites combined with fused 

oxides of iron, aluminum, manganese, calcium and magnesium. Steel slag 

aggregates are highly angular in shape and have a rough surface texture. The 

cooling rate of steel slag is sufficiently low so that crystalline compounds are 

generally formed. The predominant compounds are dicalcium silicate, tricalcium 

 

2

 In its listing of “other” approved beneficial use applications, the ASTWWMO report also includes specific 

applications in Iowa and Wisconsin related to the use of steel slag in roads. Iowa has approved the use of steel slag 

in base, surface treatments, seal coats, slur coats, cold patch, and aggregate in pavement surface, and Wisconsin has 

approved steel slag for use in surface course, base course, and sub-base course. 
Steal Furnace Slag 

 

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silicate, dicalcium ferrite, merwinite, calcium aluminate, calcium-magnesium iron 

oxide, and some free lime and free magnesia. The relative proportions of these 

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

Exhibit 4 presents the approximate oxide concentrations of SFS. 

• Exhibit 5 presents the elemental breakdown of SFS in basic oxygen furnaces 

(BOF) and electric arc furnaces (EAF). 

• When heated to high temperature (1,450 degrees Celsius) in a cement kiln, the 

slag becomes molten and combines with other material in the kiln. 

Exhibit 4: Composition of SFS

Oxide Wt. % 

Aluminum oxide (Al2O3) 1.00-5.80 

Calcium oxide (CaO) 30.70-52.00 

Chromium oxide (Cr2O3) 0.00-0.28 

Iron oxide (FeO) 10.00-40.00 

Magnesium oxide (MgO) 5.00-9.95 

Manganese oxide (MnO) 3.99-5.00 

Phosphorus oxide (P2O5) 0.50-1.00 

Potassium oxide (K2O) 0.00-0.05 

Silicon oxide (SiO2) 10.00-19.00 

Sodium oxide (Na2O) 0.00-0.09 

Titanium oxide (TiO2) 0.00-0.65 

Vanadium oxide (V2O5) 0.00-0.21 

Sources: 

The range of values for SFS encompasses the concentration estimates 

from the following sources: TFHRC (2010), Kiggins (2009), and Patel 

(2008) 

(2) Impacts of SFS use as ingredient:

• In clinker and bituminous concrete manufacture, steel furnace slag partially 

offsets the need for virgin materials such as limestone, sand, clay, and shale. 

Substitution of steel slag for natural raw materials in clinker manufacturing helps 

to reduce energy use, decrease CO2 and NOX emissions, increase production 

capacity, and reduce virgin limestone extraction (EPA 2008, Kiggins 2009, van 

Oss 2008). As an aggregate, steel slag reduces virgin aggregate extraction. The 

benefits of avoided limestone or other types of aggregate extraction include 

reduced energy use and associated GHG emissions, reduced water use and 

reduced air pollution. (EPA 2008) 

• The specific lifecycle impacts of SFS use as a raw material in clinker and 

bituminous concrete production are not evaluated here because of uncertainties in 

lifecycle scenario development. For example, it is difficult to determine the 

replacement ratio between SFS and other raw feed materials in clinker and 

bituminous concrete production. Thus, the correct quantity of material to be 

modeled is unclear. In addition, SFS may substitute for a variety of virgin raw 

materials as well as other secondary materials (e.g., cement kiln dust, CCPs, 

foundry sand, etc.); the choice of material often depends on location-specific 
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factors such as the proximity of material sources to the cement kiln and relative 

availability of different materials. Avoided upstream impacts depend heavily on 

the specific material being displaced in the lifecycle scenario. (Kiggins 2009) 

 

Exhibit 5: Elemental Composition of SFS (parts per million) 

 Element Basic Oxygen Furnace (BOF) Electric Arc Furnace (EAF) 

Aluminum (Al) 23,841 35,009 

Antimony (Sb) 3.3 4.0 

Arsenic (As) - 1.9 

Barium (Ba) 75.0 557 

Beryllium (Be) 0.5 1.1 

Cadmium (Cd) 2.5 7.6 

Calcium (Ca) 180,135 250,653 

Carbon (C) 2,600 2,936 

Chromium (Cr) (total) 1,271 3,046 

Chromium (Cr) (VI) - 1.2 

Cobalt (Co) 3.8 4.8 

Copper (Cu) 30.0 178.0 

Iron (Fe) 184,300 190,211 

Lead (Pb) 50.0 27.5 

Magnesium (Mg) 55,318 54,460 

Manganese (Mn) 32,853 39,400 

Mercury (Hg) 0.1 0.0 

Molybdenum (Mo) 11.0 30.0 

Nickel (Ni) 4.6 30.0 

Phosphorus (P) 3,197 1,781 

Selenium (Se) 15.0 18.0 

Silicon (Si) 59,653 74,524 

Silver (Ag) 9.1 8.4 

Sulfur (S) 1,112 1,891 

Thallium (Tl) 7.2 11.0 

Tin (Sn) 6.5 10.0 

Vanadium (V) 992.0 513.0 

Zinc (Zn) 46.0 165.0 

Note: Blank cells indicate figure is unknown 

Source: Proctor, et al. (2000) 
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References

Association of State and Territorial Solid Waste Management Officials (ASTSWMO). November 2007, 

2006 Beneficial Use Study Report. 

Kiggins, Karen, public comment in response to EPA’s Advanced Notice of Proposed Rulemaking for 

Identification of Non-Hazardous Materials That Are Solid Waste, EPA docket EPA-HQ-RCRA2008-0329, submitted on behalf of the National Slag Association, January 30, 2009. 

Patel, Jigar P., “Broader Use of Steel Slag Aggregates in Concrete.” Cleveland State University 

Department of Civil Engineering. December, 2008. 

Proctor, D.M., Fehling, K.A., Shay, E.C., Wittenborn, J.L., Green, J.J., Avent, C., Bigham, R.D., 

Connolly, M., Lee, B., Shepker, T.O., Zak, M.A., "Physical and Chemical Characteristics of Blast 

Furnace, Basic Oxygen Furnace, and Electric Arc Furnace Steel Industry Slags." Environmental 

Science Technology, 2000, 34, 1576-1582. 

Smyth, D.J.A., Blowes, D.W., Ptacek, C.J., Baker, M.J., Ford, G., Foss, S., Bernstene, E., “Removal of 

Phosphate and Waterborne Pathogens from Wastewater Effluent Using Permeable Reactive 

Materials.” Canadian Geotechnical Groundwater Specialty Conference, Niagara Falls, Ontario, 

October 20-23, 2002. 

Turner-Fairbank Highway Research Center. “Steel Slag Material Description.” Accessed July 15, 2010. 

Available at: http://www.tfhrc.gov/hnr20/recycle/waste/ssa1.htm 

United States Environmental Protection Agency (EPA). June 3, 2008, Study on Increasing the Usage of 

Recovered Mineral Components in Federally Funded Projects Involving Procurement of Cement 

or Concrete to Address the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A 

Legacy for Users. 

van Oss, Hendrick G. “Iron and Steel Slag.” United State Geological Survey: 2004 Minerals Yearbook. 

van Oss, Hendrick G. “Iron and Steel Slag.” United State Geological Survey: 2005 Minerals Yearbook. 

van Oss, Hendrick G. “Iron and Steel Slag.” United State Geological Survey: 2006 Minerals Yearbook. 

van Oss, Hendrick G. “Iron and Steel Slag.” United State Geological Survey: 2007 Minerals Yearbook. 

van Oss, Hendrick G. “Iron and Steel Slag.” United State Geological Survey: 2008 Minerals Yearbook. 

van Oss, Hendrick G. “Iron and Steel Slag.” United State Geological Survey: 2010 Mineral Commodity 

Summaries.