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BLAST FURNACE SLAG Material Description

ORIGIN

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.

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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

CURRENT MANAGEMENT OPTIONS

Recycling

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).

Disposal

It is estimated that a relatively small percentage (less than 10 percent) of the blast furnace slag

generated is disposed of in landfills.

MARKET SOURCES

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

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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.

HIGHWAY USES AND PROCESSING REQUIREMENTS

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)

MATERIAL PROPERTIES

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)

840

(52)

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

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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

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Iron 

(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

(MnO) 0.8 0.1-1.4 0.8 0.2-2.3 0.5 0.2-2.0 0.44 0.15-0.76

Sulfur

(S) 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

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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.

REFERENCES

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.

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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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