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Publication Number: FHWA-RD-97-148

User Guidelines for Waste and Byproduct Materials in Pavement Construction

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BLAST FURNACE SLAG Material Description


In the production of iron, iron ore, iron scrap, and fluxes (limestone and/or dolomite) are charged into a blast furnace along with coke for fuel. The coke is combusted to produce carbon monoxide, which reduces the iron ore to a molten iron product. This molten iron product can be cast into iron products, but is most often used as a feedstock for steel production.

Blast furnace slag is a nonmetallic coproduct produced in the process. It consists primarily of silicates, aluminosilicates, and calcium-alumina-silicates. The molten slag, which absorbs much of the sulfur from the charge, comprises about 20 percent by mass of iron production. Figure 3-1 presents a general schematic, which depicts the blast furnace feedstocks and the production of blast furnace coproducts (iron and slag).

Different forms of slag product are produced depending on the method used to cool the molten slag. These products include air-cooled blast furnace slag (ACBFS), expanded or foamed slag, pelletized slag, and granulated blast furnace slag.

Figure 3-1. General schematic of blast furnace operation
and blast furnace slag production.

Air-Cooled Blast Furnace Slag

If the liquid slag is poured into beds and slowly cooled under ambient conditions, a crystalline structure is formed, and a hard, lump slag is produced, which can subsequently be crushed and screened.

Expanded or Foamed Blast Furnace Slag

If the molten slag is cooled and solidified by adding controlled quantities of water, air, or steam, the process of cooling and solidification can be accelerated, increasing the cellular nature of the slag and producing a lightweight expanded or foamed product. Foamed slag is distinguishable from air-cooled blast furnace slag by its relatively high porosity and low bulk density.

Pelletized Blast Furnace Slag

If the molten slag is cooled and solidified with water and air quenched in a spinning drum, pellets, rather than a solid mass, can be produced. By controlling the process, the pellets can be made more crystalline, which is beneficial for aggregate use, or more vitrified (glassy), which is more desirable in cementitious applications. More rapid quenching results in greater vitrification and less crystallization.

Granulated Blast Furnace Slag

If the molten slag is cooled and solidified by rapid water quenching to a glassy state, little or no crystallization occurs. This process results in the formation of sand size (or frit-like) fragments, usually with some friable clinkerlike material. The physical structure and gradation of granulated slag depend on the chemical composition of the slag, its temperature at the time of water quenching, and the method of production. When crushed or milled to very fine cement-sized particles, ground granulated blast furnace slag (GGBFS) has cementitious properties, which make a suitable partial replacement for or additive to Portland cement.

It is estimated that approximately 14 million metric tons (15.5 million tons) of blast furnace slag is produced annually in the United States.(1)

Additional information on processed blast furnace slag use in the United States can be obtained from:

National Slag Association

808 North Fairfax Street

Arlington, Virginia 22314




Almost all of the blast furnace slag produced in the United States is reportedly utilized, and approximately 90 percent of this slag is ACBFS. The proportion of ACBFS currently being produced, however, is decreasing relative to granulated and pelletized blast furnace slag production.(2) The production of expanded blast furnace slag is no longer favored and is being replaced by the pelletizing procedure.

ACBFS has been used as an aggregate in Portland cement concrete, asphalt concrete, concrete, asphalt and road bases. Pelletized blast furnace slag has been used as lightweight aggregate and for cement manufacture. Foamed slag has been used as a lightweight aggregate for Portland cement concrete. Granulated blast furnace slag has been used as a raw material for cement production and as an aggregate and insulating material. and granulated slag have also been used as sand blasting shot materials. Ground granulated blast furnace slag is used commercially as a supplementary cementitious material in Portland cement concrete (as a mineral admixture or component of blended cement).


It is estimated that a relatively small percentage (less than 10 percent) of the blast furnace slag generated is disposed of in landfills.



Blast furnace slag materials are generally available from slag processors located near iron production centers.

Cements containing ground granulated blast furnace slag are available from many producers of Portland cement or directly from ground granulated blast furnace slag cement manufacturers. AASHTO M240 describes three types of blended cements containing slag.(3) They include Portland blast furnace slag cement (type IS), slag modified Portland cement (type I (SM)), and slag cement (type S). The primary distinction among the three types is the percentage of slag they contain. Slag cement may contain Portland cement or hydrated lime (or both) while the other two are blends of Portland cement and slag only.



ACBFS - Aggregate Substitute

Many specifying agencies consider ACBFS to be a conventional aggregate. It is extensively used in granular base, hot mix asphalt, Portland cement concrete, and embankments or fill applications. The material can be crushed and screened to meet specified gradation requirements using conventional aggregate processing equipment. Special quality control procedures may be required to address the lack of consistency in some properties such as gradation, specific gravity, and absorption.

GGBFS and Vitrified Pelletized BFS — Supplementary Cementitious Materials

GGBFS is used as a mineral admixture for Portland cement concrete. Granulated blast furnace slag and vitrified pelletized blast furnace slag are also used in the manufacture of blended hydraulic cements (AASHTO M240).(3) When used in blended cements, granulated blast furnace slag or vitrified pelletized slag are milled to a fine particle size in accordance with AASHTO M302 requirements.(4) The ground slag can be introduced and milled with the cement feedstock or blended separately after the cement is ground to its required fineness.

The U.S. Environmental Protection Agency (EPA) has recommended that effective May 1, 1995, procuring agencies specifically include provision in all construction contracts for the use of GGBFS in Portland cement concrete contracts.(5)



Physical Properties

Table 3-1 lists some typical physical properties of air-cooled, expanded, and pelletized blast furnace slags.

Table 3-1. Typical physical properties of blast furnace slag.(3)

Property Slag Type and Value
Air-Cooled(6) Expanded(6,7) Pelletized(7)
Specific Gravity 2.0 - 2.5
Compacted Unit Weight,
kg/m3 (lb/ft3)
1120 - 1360
(70 - 85)
(800 - 1040)
(50 - 65)
Absorption (%) 1 - 6

Air-Cooled Blast Furnace Slag

Crushed ACBFS is angular, roughly cubical, and has textures ranging from rough, vesicular (porous) surfaces to glassy (smooth) surfaces with conchoidal fractures. There can, however, be considerable variability in the physical properties of blast furnace slag, depending on the iron production process. For example, some recently produced ACBFS was reported to have a compacted unit weight as high as 1940 kg/m3 (120 lb/ft3).(8) Higher unit weights that are reported are generally due to increased metals and iron content in the slag and tend to occur in slags that are generated from blast furnaces with higher scrap metal additions.

The water absorption of ACBFS can be as high as 6 percent. Although ACBFS can exhibit these high absorption values, ACBFS can be readily dried since little water actually enters the pores of the slag and most is held in the shallow pits on the surface.

Expanded Blast Furnace Slag

Crushed expanded slag is angular, roughly cubical in shape, and has atexture that is rougher than that of air-cooled slag. The porosity of expanded blast furnace slag aggregates is higher than ACBFS aggregates. The bulk relative density of expanded slag is difficult to determine accurately, but it is approximately 70 percent of that of air-cooled slag. Typical compacted unit weights for expanded blast furnace slag aggregates range from 800 kg/m3 (50 lb/ft3) to 1040 kg/m3 (65 lb/ft3).(6)

Pelletized Blast Furnace Slag

Unlike air-cooled and expanded blast furnace slag, pelletized blast furnace slag has a smooth texture and rounded shape. Consequently, the porosity and water absorption are much lower than those of ACBFS or expanded blast furnace slag. Pellet sizes range from 13 mm (1/2 in) to 0.1 mm (No. 140 sieve size), with the bulk of the product in the minus 9.5 mm (3/8 in) to plus 1.0 mm (No. 18 sieve size) range. Pelletized blast furnace slag has a unit weight of about 840 kg/m3 (52 lb/ft3).(7)

Granulated Blast Furnace Slag

Granulated blast furnace slag is a glassy granular material that varies, depending on the chemical composition and method of production, from a coarse, popcornlike friable structure greater than 4.75 mm (No. 4 sieve) in diameter to dense, sand-size grains passing a 4.75 mm (No. 4) sieve. Grinding reduces the particle size to cement fineness, allowing its use as a supplementary cementitious material in Portland cement concrete.

Chemical Properties

Table 3-2 depicts the typical chemical composition of blast furnace slag. The chemical compositions shown are in general applicable to all types of slag. The data presented in Table 3-2 suggest that the chemical composition of blast furnace slags produced in North America has remained relatively consistent over the years.

When ground to the proper fineness, the chemical composition and glassy (noncrystalline) nature of vitrified slags are such that when combined with water, these vitrified slags react to form cementitious hydration products. The magnitude of these cementitious reactions depends on the chemical composition, glass content, and fineness of the slag. The chemical reaction between GGBFS and water is slow, but it is greatly enhanced by the presence of calcium hydroxide, alkalies and gypsum (CaSO4).

Table 3-2. Typical composition of blast furnace slag.(9)

Constituent Percent
1949a. 1957a. 1968a. 1985a.
Mean Range Mean Range Mean Range Mean Range
Calcium Oxide (CaO) 41 34-48 41 31-47 39 32-44 39 34-43
Silicon Dioxide (SiO2) 36 31-45 36 31-44 36 32-40 36 27-38
Aluminum Oxide (Al2O3) 13 10-17 13 8-18 12 8-20 10 7-12
Magnesium Oxide (MgO) 7 1-15 7 2-16 11 2-19 12 7-15
(FeO or Fe2O3)
0.5 0.1-1.0 0.5 0.2-0.9 0.4 0.2-0.9 0.5 0.2-1.6
Manganese Oxide
0.8 0.1-1.4 0.8 0.2-2.3 0.5 0.2-2.0 0.44 0.15-0.76
1.5 0.9-2.3 1.6 0.7-2.3 1.4 0.6-2.3 1.4 1.0-1.9
a. Data source is the National Slag Association data: 1949 (22 sources); 1957 (29 sources); 1968 (30 sources) and 1985 (18 sources).

Because of these cementitious properties, GGBFS can be used as a supplementary cementitious material either by premixing the slag with Portland cement or hydrated lime to produce a blended cement (during the cement production process) or by adding the slag to Portland cement concrete as a mineral admixture.

Blast furnace slag is mildly alkaline and exhibits a pH in solution in the range of 8 to 10. Although blast furnace slag contains a small component of elemental sulfur (1 to 2 percent), the leachate tends to be slightly alkaline and does not present a corrosion risk to steel in pilings(10) or to steel embedded in concrete made with blast furnace slag cement or aggregates.(11)

In certain situations, the leachate from blast furnace slag may be discolored (characteristic yellow/green color) and have a sulfurous odor. These properties appear to be associated with the presence of stagnant or slow moving water that has come in contact with the slag. The stagnant water generally exhibits high concentrations of calcium and sulfide, with a pH as high as 12.5.(12) When this yellow leachate is exposed to oxygen, the sulfides present react with oxygen to precipitate white/yellow elemental sulfur and produce calcium thiosulfate, which is a clear solution. (See references 13,14,15,16,17,18,19.) Aging of blast furnace slag can delay the formation of yellow leachate in poor drainage conditions but does not appear to be a preventative measure, since the discolored leachate can still form if stagnant water is left in contact with the slag for an extended period.(12)

Mechanical Properties

Of all the slag types generated, air-cooled blast furnace is the type that is most commonly used as an aggregate material. Processed ACBFS exhibits favorable mechanical properties for aggregate use including good abrasion resistance, good soundness characteristics, and high bearing strength. Table 3-3 provides a listing of typical mechanical properties of ACBFS aggregates.

Table 3-3. Typical mechanical properties of air-cooled blast furnace slag.(20)

Property Value
Los Angeles Abrasion (ASTM C131) 35 - 45%
Sodium Sulfate Soundness Loss (ASTM C88) 12%
Angle of Internal Friction 40 - 45
Hardness (measured by Moh's scale of mineral hardness)* 5 - 6
California Bearing Ratio (CBR), top size 19 mm (3/4 in)** up to 250%
*Hardness of dolomite measured on same scale is 3 to 4
**Typical CBR value for crushed limestone is 100%

Other Properties

Because of their more porous structure, blast furnace slag aggregates have lower thermal conductivities than conventional aggregates.(21) Their insulating value is of particular advantage in applications such as frost tapers (transition treatments in pavement subgrades between frost susceptible and nonfrost susceptible soils) or pavement base courses over frost-susceptible soils.



  1. Mineral Commodity Summaries 1993. Bureau of Mines, U.S. Department of the Interior, Washington, DC, 1993.

  2. 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.

  3. American Association of State Highway and Transportation Officials. Standard Specification for Materials, "Blended Hydraulic Cements," AASHTO Designation: M240-85, Part I Specifications, 14th Edition, 1986.

  4. American Association of State Highway and Transportation Officials. Standard Specification for Materials, "Ground Iron Blast-Furnace Slag for Use in Concrete and Mortars," AASHTO Designation: M302-86, Part I Specification, 14th Edition, 1986.

  5. Recovered Materials Advisory Notice (RMAN). Environmental Protection Agency, Federal Register, May 1, 1995.

  6. NSA 188.1. Processed Blast Furnace Slag, The All Purpose Construction Aggregate, National Slag Association, Alexandria, Virginia.

  7. Emery, J.J. "Pelletized Lightweight Slag Aggregate," Proceedings of Concrete International 1980, Concrete Society, April, 1980.

  8. Fax memorandum, S. Szoke, Ontario Ministry of Transportation to D. Horvat, National Slag Limited, April 7, 1995.

  9. Mineral Aggregate Conservation Reuse and Recycling. Report prepared by John Emery Geotechnical Engineering Limited for Aggregate and Petroleum Resources Section, Ontario Ministry of Natural Resources. Ontario, 1992.

  10. NSA 166.2. "Blast Furnace Slag, Ideal Backfill Material for Steel Sheet Piling," National Slag Association, Alexandria, Virginia.

  11. Short, A. "The Use of Lightweight Concrete in Reinforced Concrete Construction," The Reinforced Concrete Review, British Reinforced Concrete Association, Vol.5, No.3, September 1959.

  12. National Slag Limited. Letter, April 4, 1995, D. Horvat, National Slag Limited to P. Verok, MTO Construction Office, with Attachment, Overview Report, Leachate Mechanism, Blast Furnace Slag Technical Committee, Hamilton, Ontario, March 17, 1995.

  13. BQSF. Report of the Visit of the Slag Study Group of the Japan Iron and Steel Federation of BQSF, British Quarrying and Slag Federation Limited, June 29, 1978.

  14. BSI. British Standard Specification for Air-Cooled Blastfurnace Slag Aggregate for Use in Construction, BS 1047: 1983, British Standards Institution, 1983.

  15. NSL. The Leaching of Sulphur from Blast Furnace Slag, National Slag Limited, November, 1985.

  16. Nordrhein-Westfalen. Requirements for the Use of Processed Recycled Construction Materials and Industrial By-Products for Excavation and Road Construction from the Standpoint of Water Management, (Dofasco translation), Ministerial Publication 45, State of Nordrhein-Westfalen, July 18, 1991.

  17. JIS. Iron and Steel Slag for Road Construction, JIS A 5015, Japanese Industrial Standards, 1992.

  18. ASA. Guide to the Use of Slag in Roads, Australian Slag Association, 1994.

  19. Ohio EPA. "Interim Policy: Use of Blast Furnace and Steel Slag," State of Ohio Environmental Protection Agency, June 6, 1994.

  20. Noureldin, A. S. and R. S. McDaniel. "Evaluation of Steel Slag Asphalt Surface Mixtures", Presented at the 69th Annual Meeting, Transportation Research Board, Washington, January, 1990.

  21. Smith, M. A., Resources Policy, Vol. 1, No. 3, pp. 154-170, 1975.


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