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Publication Number: FHWA-RD-97-148

User Guidelines for Waste and Byproduct Materials in Pavement Construction

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Material Description


Coal bottom ash and boiler slag are the coarse, granular, incombustible by-products that are collected from the bottom of furnaces that burn coal for the generation of steam, the production of electric power, or both. The majority of these coal by-products are produced at coal-fired electric utility generating stations, although considerable bottom ash and/or boiler slag are also produced from many smaller industrial or institutional coal-fired boilers and from coal-burning independent power production facilities. The type of by-product (i.e., bottom ash or boiler slag) produced depends on the type of furnace used to burn the coal.

Bottom Ash

The most common type of coal-burning furnace in the electric utility industry is the dry, bottom pulverized coal boiler. When pulverized coal is burned in a dry, bottom boiler, about 80 percent of the unburned material or ash is entrained in the flue gas and is captured and recovered as fly ash. The remaining 20 percent of the ash is dry bottom ash, a dark gray, granular, porous, predominantly sand size minus 12.7mm (½ in) material that is collected in a water-filled hopper at the bottom of the furnace.(1) When a sufficient amount of bottom ash drops into the hopper, it is removed by means of high-pressure water jets and conveyed by sluiceways either to a disposal pond or to a decant basin for dewatering, crushing, and stockpiling for disposal or use.(2) During 1996, the utility industry generated 14.5 million metric tons (16.1 million tons) of bottom ash.(3)

Boiler Slag

There are two types of wet-bottom boilers: the slag-tap boiler and the cyclone boiler. The slag-tap boiler burns pulverized coal and the cyclone boiler burns crushed coal. In each type, the bottom ash is kept in a molten state and tapped off as a liquid. Both boiler types have a solid base with an orifice that can be opened to permit the molten ash that has collected at the base to flow into the ash hopper below. The ash hopper in wet-bottom furnaces contains quenching water. When the molten slag comes in contact with the quenching water, it fractures instantly, crystallizes, and forms pellets. The resulting boiler slag, often referred to as "black beauty," is a coarse, hard, black, angular, glassy material.

When pulverized coal is burned in a slag-tap furnace, as much as 50 percent of the ash is retained in the furnace as boiler slag. In a cyclone furnace, which burns crushed coal, some 70 to 80 percent of the ash is retained as boiler slag, with only 20 to 30 percent leaving the furnace in the form of fly ash.(1)

Wet-bottom boiler slag is a term that describes the molten condition of the ash as it is drawn from the bottom of the slag-tap or cyclone furnaces. At intervals, high-pressure water jets wash the boiler slag from the hopper pit into a sluiceway which is then conveys it to a collection basin for dewatering, possible crushing or screening, and either disposal or reuse.(4) During 1995, the utility industry in the United States generated 2.3 million metric tons (2.6 million tons) of boiler slag.(1)

A general diagram depicting the production and processing operations associated with bottom ash and boiler slag management is presented in Figure 4-1.


Figure 4-1. Production and processing of bottom ash or boiler slag.

Additional information on the use of bottom ash and/or boiler slag can be obtained from:

American Coal Ash Association (ACAA)

2760 Eisenhower Avenue, Suite 304

Alexandria, Virginia 22314

Electric Power Research Institute

3412 Hillview Road

Palo Alto, California 94304




According to recent statistics on coal combustion by-product utilization, 30.3 percent of all bottom ash and 93.3 percent of all boiler slag produced in 1996 were utilized. Leading bottom ash applications are snow and ice control, as aggregate in lightweight concrete masonry units, and raw feed material for production of Portland cement. Bottom ash has also been used as a road base and subbase aggregate, structural fill material(5), and as fine aggregate in asphalt paving and flowable fill. Leading boiler slag applications are blasting grit, roofing shingle granules, and snow and ice control. Boiler slag has also been used as an aggregate in asphalt paving, as a structural fill,(5)and in road base and subbase applications.(2)

The U.S. EPA is presently undertaking a study of power plant wastes prior to disposal by the utility. It is possible that EPA’s study on mixed power plant wastes could have a regulatory impact on beneficial use or reuse of any mixed materials. It is anticipated that this investigation will be completed in 1998.


Discarded bottom ash and boiler slag are either landfilled or sluiced to storage lagoons. When sluiced to storage lagoons, the bottom ash or boiler slag is usually combined with fly ash. This blended fly ash and bottom ash or boiler slag are referred to as ponded ash. Approximately 30 percent of all coal ash is handled wet and disposed of as ponded ash.(3)

Ponded ash is potentially useable, but variable in its characteristics because of its manner of disposal. Because of differences in the unit weight of fly ash and bottom ash or boiler slag, the coarser bottom ash or boiler slag particles settle first and the finer fly ash remains in suspension longer. Ponded ash can be reclaimed and stockpiled, during which time it can be dewatered. Under favorable drying conditions, ponded ash may be dewatered into a range of moisture that will be within the vicinity of its optimum moisture content. The higher the percentage of bottom ash or boiler slag there is in ponded ash, the easier it is to dewater and the greater its potential for reuse. Reclaimed ponded ash has been used in stabilized base or subbase mixes and in embankment construction, and can also be used as fine aggregate or filler material in flowable fill.



Although electric utility companies produce ash at their coal-fired power plants, most utilities do not handle, dispose of, or sell the ash that they produce. For the most part, management of bottom ash or boiler slag is contracted out to ash marketing firms or to local hauling contractors. There are approximately 50 commercial ash marketing firms operating throughout the United States, in all states except Hawaii. In addition to commercial ash marketing organizations, certain coal-burning electric utility companies have a formal ash marketing program of their own. Most coal-burning electric utility companies currently employ an ash marketing specialist, who is responsible for monitoring ash generation, quality, use or disposal, and for interfacing with the ash marketers or brokers who are under contract to the utility companies.



Asphalt Concrete Aggregate (Bottom Ash and Boiler Slag)

Both bottom ash and boiler slag have been used as fine aggregate substitute in hot mix asphalt wearing surfaces and base courses, and emulsified asphalt cold mix wearing surfaces and base courses. Because of the "popcorn," clinkerlike low durability nature of some bottom ash particles, bottom ash has been used more frequently in base courses than wearing surfaces. Boiler slag has been used in wearing surfaces, base courses and asphalt surface treatment or seal coat applications. There are no known uses of bottom ash in asphalt surface treatment or seal coat applications.

Screening of oversized particles and blending with other aggregates will typically be required to use bottom ash and boiler slag in paving applications. Pyrites that may be present in the bottom ash should also be removed (with electromagnets) prior to use. Pyrites (iron sulfide) are volumetrically unstable, expansive, and produce a reddish stain when exposed to water over an extended time period.

Granular Base (Bottom Ash and Boiler Slag)

Both bottom ash and boiler slag have occasionally been used as unbound aggregate or granular base material for pavement construction. Bottom ash and boiler slag are considered fine aggregates in this use. To meet required specifications, the bottom ash or slag may need to be blended with other natural aggregates prior to its use as a base or subbase material. Screening or grinding may also be necessary prior to use, particularly for the bottom ash, where large particle sizes, typically greater than 19 mm (3/4 in), are present in the ash.

Stabilized Base Aggregate (Bottom Ash and Boiler Slag)

Bottom ash and boiler slag have been used in stabilized base applications. Stabilized base or subbase mixtures contain a blend of an aggregate and cementitious materials that bind the aggregates, providing the mixture with greater bearing strength. Types of cementitious materials typically used include Portland cement, cement kiln dust, or pozzolans with activators, such as lime, cement kiln dusts, and lime kiln dusts. When constructing a stabilized base using either bottom ash or boiler slag, both moisture control and proper sizing are required. Deleterious materials such as pyrites should also be removed.

Embankment or Backfill Material (Mainly Bottom Ash)

Bottom ash and ponded ash have been used as structural fill materials for the construction of highway embankments and/or the backfilling of abutments, retaining walls, or trenches. These materials may also be used as pipe bedding in lieu of sand or pea gravel. To be suitable for these applications, the bottom ash or ponded ash must be at or reasonably close to its optimum moisture content, free of pyrites and/or "popcorn" like particles, and must be non-corrosive. Reclaimed ponded ash must be stockpiled and adequately dewatered prior to use. Bottom ash may require screening or grinding to remove or reduce oversize materials (greater than 19 mm (3/4 in) in size.

Flowable Fill Aggregate (Mainly Bottom Ash)

Bottom ash has been used as an aggregate material in flowable fill mixes. Ponded ash also has the potential for being reclaimed and used in flowable fill. Since most flowable fill mixes involve the development of comparatively low compressive strength (in order to be able to be excavated at a later time, if necessary), no advance processing of bottom ash or ponded ash is needed. Neither bottom ash nor ponded ash needs to be at any particular moisture content to be used in flowable fill mixes because the amount of water in the mix can be adjusted in order to provide the desired flowability.



Physical Properties

Bottom ashes have angular particles with a very porous surface texture. Bottom ash particles range in size from a fine gravel to a fine sand with very low percentages of silt-clay sized particles. The ash is usually a well-graded material, although variations in particle size distribution may be encountered in ash samples taken from the same power plant at different times. Bottom ash is predominantly sand-sized, usually with 50 to 90 percent passing a 4.75 mm (No. 4) sieve, 10 to 60 percent passing a 0.42 mm (No. 40) sieve, 0 to 10 percent passing a 0.075 mm (No. 200) sieve, and a top size usually ranging from 19 mm (3/4 in) to 38.1 mm (1-1/2 in). Table 4-1 compares the typical gradations of bottom ash and boiler slag.

Table 4-1. Particle size distribution of bottom ash and boiler slag.(4)
(percent by weight passing)

  Bottom Ash Boiler Slag
Sieve Size Glasgow, WV New Haven, WV Moundsville, WV Willow Island, WV Rockdale, TX Moundsville, WV
38 mm (1-1/2 in) 100 99 100 100 100 100
19 mm (3/4 in) 100 95 100 100 100 100
9.5 mm (3/8 in) 100 87 73 99 100 97
4.75 mm (No. 4) 90 77 52 97 99 90
2.36 mm (No. 8) 80 57 32 85 88 62
1.18 mm (No. 16) 72 42 17 46 42 16
0.60 mm (No. 30) 65 29 10 23 10 4
0.30 mm (No. 50) 56 19 5 12 5 2
0.15 mm (No. 100) 35 15 2 6 2 1
0.075 mm (No. 200) 9 4 1 4 1 0.5

Boiler slags are predominantly single-sized and within a range of 5.0 to 0.5 mm (No. 4 to No. 40 sieve). Ordinarily, boiler slags have a smooth surface texture, but if gases are trapped in the slag as it is tapped from the furnace, the quenched slag will become somewhat vesicular or porous. Boiler slag from the burning of lignite or subbituminous coal tends to be more porous than that of the eastern bituminous coals.(6) Boiler slag is essentially a coarse to medium sand with 90 to 100 percent passing a 4.75 mm (No. 4) sieve, 40 to 60 percent passing a 2.0 mm (No. 10) sieve, 10 percent or less passing a 0.42 mm (No. 40) sieve, and 5 percent or less passing a 0.075 mm (No. 200) sieve.(4) The specific gravity of the dry bottom ash is a function of chemical composition, with higher carbon content resulting in lower specific gravity. Bottom ash with a low specific gravity has a porous or vesicular texture, a characteristic of popcorn particles that readily degrade under loading or compaction.(7) Table 4-2 lists some typical physical properties of bottom ash and boiler slags.

Table 4-2. Typical physical properties of bottom ash and boiler slag.

Property Bottom Ash Boiler Slag
Specific Gravity(6) 2.1 - 2.7 2.3 - 2.9
Dry Unit Weight(6) 720 - 1600 kg/m3
(45 - 100 lb/ft3)
960 - 1440 kg/m3
(60 - 90 lb/ft3)
Plasticity(6) None None
Absorption(4) 0.8 - 2.0% 0.3 - 1.1%

Chemical Properties

Bottom ash and boiler slag are composed principally of silica, alumina, and iron, with smaller percentages of calcium, magnesium, sulfates, and other compounds. The composition of the bottom ash or boiler slag particles is controlled primarily by the source of the coal and not by the type of furnace. Table4-3 presents a chemical analysis of selected samples of bottom ash and boiler slag from different coal types and different regions.

Table 4-3. Chemical composition of selected bottom ash and boiler slag samples
(percent by weight).(4)

Ash Type: Bottom Ash Boiler Slag
Coal Type: Bituminous Sub-bituminous Lignite Bituminous Lignite
Location West Virginia Ohio Texas West Virginia North Dakota
SiO2 53.6 45.9 47.1 45.4 70.0 48.9 53.6 40.5
Al2O3 28.3 25.1 28.3 19.3 15.9 21.9 22.7 13.8
FesO3 5.8 14.3 10.7 9.7 2.0 14.3 10.3 14.2
CaO 0.4 1.4 0.4 15.3 6.0 1.4 1.4 22.4
MgO 4.2 5.2 5.2 3.1 1.9 5.2 5.2 5.6
Na2O 1.0 0.7 0.8 1.0 0.6 0.7 1.2 1.7
K2O 0.3 0.2 0.2 - 0.1 0.1 0.1 1.1

Bottom ash or boiler slag derived from lignite or sub-bituminous coals has a higher percentage of calcium than the bottom ash or boiler slag from anthracite or bituminous coals. Although sulfate is not shown in Table 4-2, it is usually very low (less than 1.0 percent), unless pyrites have not been removed from the bottom ash or boiler slag.

Due to the salt content and, in some cases, the low pH of bottom ash and boiler slag, these materials could exhibit corrosive properties. When using bottom ash or boiler slag in an embankment, backfill, subbase, or even possibly in a base course, the potential for corrosion of metal structures that may come in contact with the material is of concern and should be investigated prior to use.

Corrosivity indicator tests normally used to evaluate bottom ash or boiler slag are pH, electrical resistivity, soluble chloride content, and soluble sulfate content. Materials are judged to be noncorrosive if the pH exceeds 5.5, the electrical resistivity is greater than 1,500 ohm-centimeters, the soluble chloride content is less than 200 parts per million (ppm), or the soluble sulfate content is less than 1,000 parts per million (ppm).(8)

Mechanical Properties

Table 4-4 lists some typical values for bottom ash and boiler slag compaction characteristics (maximum dry density and optimum moisture), durability characteristics (Los Angeles abrasion and sodium soundness), shear strength and bearing strength characteristics (friction angle and ), and permeability.

The maximum dry density values of bottom ash and boiler slag are usually from 10 to 25 percent lower than that of naturally occurring granular materials. The optimum moisture content values of bottom ash and boiler slag are both higher than those of naturally occurring granular materials, with bottom ash being considerably higher in optimum moisture content than boiler slag.

Boiler slag usually exhibits less abrasion loss and soundness loss than bottom ash because of its glassy surface texture and lower porosity.(9) In some power plants, coal pyrites are disposed of with the bottom ash or boiler slag. In such cases, some pyrite or soluble sulfate is contained in the bottom ash or boiler slag,(9) which may be reflected in higher sodium sulfate soundness loss values. Reported friction angle values are within the same range as those for sand and other conventional fine aggregate sources.(7)

Table 4-4. Typical mechanical properties of bottom ash and boiler slag.

Property Bottom Ash Boiler Slag
Maximum Dry Density
kg/m3 (lb/ft3)(7)
1210 - 1620
(75 - 100)
1330 - 1650
(82 - 102)
Optimum Moisture
Content, %(7)
Usually <20
12 - 24 range
8 - 20
Los Angeles Abrasion
Loss %(4)
30 - 50 24 - 48
Sodium Sulfate Soundness
Loss %(4)
1.5 - 10 1 - 9
Shear Strength
(Friction Angle)(6)
38 - 42°
32 - 45° (<9.5 mm size)
38 - 42°
36 - 46° (<9.5 mm size)
California Bearing Ratio
(CBR) %(6)
40 - 70 40 - 70
Permeability Coefficient
10-2 - 10-3 10-2 - 10-3

California Bearing Ratio values are comparable to those of high-quality gravel base materials. Dry bottom ash and boiler slag can both be expected to have permeability coefficients that are within approximately the same range(7) The permeability is related to the percent fines (minus 0.075 mm or No. 200 sieve) of the material.



  1. Babcock & Wilcox Company. Steam. Its Generation and Use. New York, NY, 1978.

  2. Hecht, N. L. and D. S. Duvall. Characterization and Utilization of Municipal and Utility Sludges and Ashes: Volume III -- Utility Coal Ash. National Environmental Research Center, U.S. Environmental Protection Agency, 1975.

  3. American Coal Ash Association. 1996 Coal Combustion Product-Production and Use. Alexandria, Virginia, 1997.

  4. Moulton, Lyle K. "Bottom Ash and Boiler Slag," Proceedings of the Third International Ash Utilization Symposium. U.S. Bureau of Mines, Information Circular No. 8640, Washington, DC, 1973.

  5. ASTM E1861-97. Standard Guide for Use of Coal Combustion By-Products in Structural Fills, American Society for Testing and Materials, West Conshohocken, Pennsylvania.

  6. Majizadeh, Kamran, Gary Bokowski, and Rashad El-Mitiny. "Material Characteristics of Power Plant Bottom Ashes and Their Performance in Bituminous Mixtures: A Laboratory Investigation," Proceedings of the Fifth International Ash Utilization Symposium, U.S. Department of Energy, Report No. METC/SP-79/10, Part 2, Morgantown, West Virginia, 1979.

  7. Lovell, C. W., T.-C. Ke, W.-H. Huang, and J. E. Lovell. "Bottom Ash As Highway Material," Presented at the 70th Annual Meeting of the Transportation Research Board, Washington, D.C., January, 1991.

  8. Ke, T.-C. and C. W. Lovell. "Corrosivity of Indiana Bottom Ash," Transportation Research Record No. 1345, Transportation Research Board, Washington, DC, 1992.

  9. Moulton, Lyle K., Roger K. Seals, and David A. Anderson. "Utilization of Ash from Coal Burning Power Plants in Highway Construction," Transportation Research Record No. 430, Transportation Research Board, Washington, DC, 1973.


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