• Introduction
• Matrix New!
• Glossary New!
• Notice

• Environmental and Engineering Evaluation New!
• State Contacts New!
• Cost Issues
• Standards/Specifications New!

• Baghouse Fines
• Blast Furnace Slag
• Coal Bottom Ash/Boiler Slag Updated!
• Coal Fly Ash Updated!
• FGD Scrubber Material Updated!
• Foundry Sand Updated!
• Kiln Dusts
• Mineral Processing Wastes
• MSW Combustor Ash
• Nonferrous Slags
• Quarry Byproducts
• Reclaimed Asphalt Pavement Updated!
• Reclaimed Concrete Material Updated!
• Roofing Shingle Scrap Updated!
• Scrap Tires
• Sewage Sludge Ash
• Steel Slag
• Sulfate Wastes
• Waste Glass

• General Descriptions
• Asphalt Concrete Pavement
• Portland Cement Concrete Pavement
• Granular Base
• Embankment or Fill
• Stabilized Base
• Flowable Fill

[ Material Description ] - [ Asphalt Concrete ] - [ Portland Cement Concrete ] - [ Embankment or Fill ] - [ Stabilized Base ] - [ Flowable Fill ]

COAL FLY ASHMaterial Description

ORIGIN

The fly ash produced from burning pulverized coal in a coal-fired boiler is a fine-grained, powdery particulate material that is carried off in the flue gas and usually collected by means of electrostatic precipitators, baghouses, or mechanical collection devices such as cyclones.

In general, there are three types of coal-fired boiler furnaces used in the electric utility industry. They are referred to as dry-bottom boilers, wet-bottom boilers, and cyclone furnaces. The most common type of coal burning furnace is the dry-bottom furnace.

When pulverized coal is combusted in a dry-ash, dry-bottom boiler, about 80 percent of all the ash leaves the furnace as fly ash entrained in the flue gas.⁽¹⁾ When pulverized coal is combusted in a wet-bottom (or slag-tap) furnace, as much as 50 percent of the ash is retained in the furnace, with the other 50 percent being entrained in the flue gas. In a cyclone furnace, where crushed coal is used as a fuel, 70 to 80 percent of the ash is retained as boiler slag and only 20 to 30 percent leaves the furnace as dry ash in the flue gas.⁽²⁾ A general flow diagram of fly ash production in a dry-bottom coal-fired utility boiler operation is presented in Figure 1.

Figure 1. Production of fly ash in a dry-bottom utility boiler with electrostatic precipitator.

In 2006 the American Coal Ash Association (ACAA) reported that 65.7 million metric tons (72.4 million tons) of coal fly ash were produced.⁽³⁾

Additional information on fly ash production and use in the United States can be obtained from:

American Coal Ash Association (ACAA)
15200 E. Girard Ave., Ste. 3050
Aurora, CO 80014-3955
http://www.acaa-usa.org/

Coal Combustion Products Partnership (C2P2)
Office of Solid Waste (5305P)
1200 Pennsylvania Avenue, NW
Washington, DC 20460
http://www.epa.gov/epaoswer/osw/conserve/c2p2/index.asp

Electric Power Research Institute (EPRI)
3412 Hillview Road
Palo Alto, California 94304
http://my.epri.com/

Edison Electric Institute (EEI)
1701 Pennsylvania Avenue, N.W.
Washington, D.C. 20004-2696
http://www.eei.org/

Green Highways Partnership
http://www.greenhighways.org/index.cfm

AASHTO Center for Environmental Excellence
444 North Capitol Street, NW Suite 249
Washington, D.C., 20001
202-624-5800
http://environment.transportation.org/

CURRENT MANAGEMENT OPTIONS

Recycling

In 2006, approximately 29.3 million metric tons (32.4 million tons) of fly ash were used.⁽³⁾ This is approximately a 5 percent increase in the use of fly ash over the previous year. The majority of this use can be attributed to the production of concrete, concrete products, and grout. Figure 2 shows the 2006 percentages of fly ash use for common construction applications.

Figure 2. Common applications of fly ash. ⁽³⁾

Fly ash is useful in many applications because it is a pozzolan, meaning it is a siliceous or alumino-siliceous material that, when in a finely divided form and in the presence of water, will combine with calcium hydroxide (from lime, Portland cement, or kiln dust) to form cementitious compounds.⁽⁴⁾

Disposal

Nation wide, approximately 45 percent of the fly ash produced in 2006 was used leaving approximately 55 percent for disposal. Thus, there is significant room for additional use. In comparison, over 90 percent of the European Union’s (EU) total coal combustion products, including fly ash, are recycled.(5, 6)

State Regulations and Specifications

The U.S. Environmental Protection Agency (EPA) has delegated responsibility to the states to ensure that coal combustion by-products are properly used. Each state, therefore, has its own specifications and environmental regulations. A map from the National Energy Technology Laboratory that links to a database of state regulations on the utilization and disposal of coal combustion by-products can be found at:
http://www.netl.doe.gov/technologies/coalpower/ewr/coal_utilization_byproducts/states/select_state.aspl

The state regulations database contains summary information on current regulations in each state and contact information for individuals with regulatory responsibility. Some states allow free use of fly ash while others allow limited application. States are generally most concerned with unencapsulated use of fly ash, such as in structural fills, mine applications, and embankments. Some states consider these applications to be waste disposal rather than reuse or recycling.⁽⁷⁾

In general, the specifications used by all states for use of fly ash are provided by the American Society of Testing and Materials (ASTM) or the American Association of State Highway and Transportation Officials (AASHTO) and are listed in Table 1.

Table 1. Specifications that apply to reuse of fly ash.^(4;7)

A site maintained by the Federal Highway Administration (FHWA) contains a searchable library for all highway specifications across the country. This can be found at: http://fhwapap04.fhwa.dot.gov/nhswp/index.jsp

MARKET SOURCES

Although coal-burning electric utility companies produce ash, most utilities make use of commercial ash vendors to sell fly ash. There are approximately 45 commercial ash marketing firms operating throughout the United States in all states except Hawaii. In addition to commercial ash marketing organizations, some coal-burning electric utility companies have formal ash marketing programs. Most coal-burning electric utility companies currently employ an ash management specialist whose responsibility is to monitor ash generation, quality, use, or disposal, and to interface with ash marketers. To identify a fly ash source, contact the local utility company or visit the American Coal Ash Association's web site at the like provided above.

Because of variations in coals from different sources, as well as differences in the design of coal-fired boilers, not all fly ash is the same. Although there may be differences in the fly ash from one plant to another, day-to-day variations in the fly ash from a given power plant are usually predictable, provided the plant operation and coal source remain constant. However, there can be a substantial variation in fly ash obtained from burning coal with other fuels (such as natural gas or wood) or with other combustible materials (such as municipal solid waste, scrap tires, etc.). As long as the basic operating parameters at a power plant do not change, fly ash from a known source that is supplied by a reputable ash marketing organization should be a consistent, quality-controlled product. If modifications to a plant have been made, for example low NO_x modifications, the subsequent ash should be re-characterized.

There are four types, or ranks, of coal, each of which varies in terms of heating value, chemical composition, ash content, and geological origin. These four types are lignite, bituminous, subbituminous, and anthracite.

Lignite, also known as brown coal, is the lowest grade coal and has no well defined composition. Burning lignite coal creates a large quantity of ash due to inefficient combustion and high water content. Subbituminous coal has properties between lignite and bituminous coal. Bituminous coal fly ash is principally composed of silica, alumina, iron oxide, and calcium, with varying amounts of carbon, as measured by the loss on ignition (LOI). Subbituminous and lignite coal fly ashes are characterized by higher concentrations of calcium and magnesium oxide and lower percentages of silica and iron oxide, as well as a lower carbon content, compared with bituminous coal fly ash.⁽⁸⁾ Anthracite, a form of coal containing few impurities, is not often burned. Thus, there are only small amounts of anthracite coal fly ash.

Fly ash to be used in Portland cement concrete (PCC) must meet the requirements of ASTM C618.⁽⁹⁾ Two classes of fly ash are defined in ASTM C618: Class F fly ash and Class C fly ash.

Fly ash that is produced from burning older, harder anthracite or bituminous coal is typically pozzolanic and is referred to as a Class F fly ash if the ash meets the chemical composition and physical requirements specified in ASTM C618.^(5;6) Class F fly ash generally contains less than 10 percent material reported as lime (CaO).^(5;10) Although possessing pozzolanic properties, the glassy silica and alumina of Class F fly ash require a cementing agent, such as Portland cement, quicklime, or hydrated lime, with the presence of water to react and produce calcium silicate hydrates (cementitious compounds).

Fly ash that is produced from the burning of younger lignite or subbituminous coal, in addition to having pozzolanic properties, also has self-cementing properties (ability to harden and gain strength in the presence of water).⁽⁶⁾ When this fly ash meets the chemical composition and physical requirements outlined in ASTM C618, it is referred to as a Class C fly ash. Class C fly ash typically contains between 15 and 35 percent material reported as lime (CaO).⁽¹⁰⁾ Most Class C fly ashes have self-cementing properties.

As a consequence of the Clean Air Act, many coal-fired power plants are being equipped with low NO_x burners.⁽¹¹⁾ The short-term effect of burning coal in a low-NOx burner appears to be an increase in the LOI of the fly ash. Changes in the form and quality of unburned carbon in low-NO_x fly ash have negatively impacted the use of low-NOx fly ash in concrete.⁽¹²⁾ Research investigating all aspects of the low-NOx burning processes is needed to ensure Clean Air regulations are met while at the same time producing coal combustion products that satisfy specifications for use in transportation applications.

HIGHWAY USES AND PROCESSING REQUIREMENTS

Portland Cement Concrete – Supplementary Cementitious Material

Fly ash has been used as a cement and mineral admixture in Portland cement concrete (PCC) for nearly 70 years. Approximately 50 percent of recycled fly ash is used in the production of concrete and concrete products, making it the largest single use of fly ash.⁽³⁾ Fly ash can also be used as a feed material for producing Portland cement and as a component of a Portland-pozzolan blended cement.

Fly ash quality must be closely monitored when used in PCC. Fineness, moisture content, LOI, and chemical content are the most important characteristics of fly ash affecting its use in concrete. Fly ash used in concrete must also have sufficient pozzolanic reactivity and must be of consistent quality.

Asphalt Concrete – Mineral Filler

Fly ash has been used as a substitute mineral filler in asphalt paving mixtures for many years. Mineral filler in asphalt paving mixtures consists of particles, less than 0.075 mm (No. 200 sieve) in size, that fill the voids in a paving mix and serve to improve the cohesion of the binder (asphalt cement) and the stability of the mixture.⁽¹³⁾ Most fly ash sources are capable of meeting the gradation (minus 0.075 mm) requirements and other pertinent physical (nonplastic) and chemical (organic content) requirements of mineral filler specifications.⁽⁴⁾

Fly ash must be in a dry form for use as a mineral filler. Fly ash that is collected dry and stored in silos requires no additional processing. Some sources of fly ash that have a high lime (CaO) content, such as Class C fly ash, may also be useful as an antistripping agent in asphalt paving mixes.⁽⁴⁾

Stabilized Base – Supplementary Cementitious Material

Stabilized bases or subbases are mixtures of aggregates and binders, such as Portland cement, which increase the strength, bearing capacity, and durability of a pavement substructure. Because fly ash may exhibit pozzolanic properties, self-cementing properties, or both, fly ash can and has been successfully used as all or part of the binder in stabilized base construction applications. Results from both laboratory and field studies have shown that fly ash can be used in stabilized bases or subbases and those stabilized layers can be included in flexible pavement designs.^{(14;15;16;17)}

When pozzolanic-type Class F fly ash is used, an activator must be added to initiate the pozzolanic reaction. The most commonly used activators or chemical binders in pozzolan-stabilized base (PSB) mixtures are lime and Portland cement, although cement kiln dusts and lime kiln dusts have been used^(14;18). Combinations of lime, Portland cement, or kiln dusts have also been used in PSB mixtures.

The successful performance of PSB mixtures depends on the development of strength within the matrix formed by the pozzolanic reaction between the fly ash and the activator. This cementitious matrix acts as a binder that holds the aggregate particles together, in a manner similar to a low-strength concrete.

Self-cementing Class C fly ash does not require an activator and thus offers more economical alternatives for a wide range of stabilization applications. In Wisconsin, fly ash use in stabilization has increased significantly due to environmental regulations describing appropriate uses (NR 538, Wisconsin Administrative Code). The use of fly ash in geotechnical applications depends on environmental and mechanical suitability. The results of various investigations have shown stabilization with Class C fly ash without any other activator have excellent performance.^{(19;20;15;16;21)}

Flowable Fill – Aggregate or Supplementary Cementitious Material

Flowable fill is a self-hardening slurry mixture consisting of sand or other fine aggregate material and a cementitious binder that is used as substitute for compacted earth backfill. Fly ash has been used in flowable fill applications as a fine aggregate and (because of its pozzolanic properties) as a supplement to or replacement for the cement. Either pozzolanic or self-cementing fly ash can be used in flowable fill. When large quantities of pozzolanic fly ash are added, the fly ash can act as both fine aggregate and part of the cementitious matrix. Self-cementing fly ash is used in smaller quantities as part of the binder in place of cement.

The quality of fly ash used in flowable fill applications need not be as strictly controlled as in other cementitious applications. Both dry and reclaimed ash from settling ponds can be used. No special processing of fly ash is required prior to use.

Embankment and Fill Material

Fly ash has been used for several decades as an embankment or structural fill material, particularly in Europe. As an embankment or fill material, fly ash is used as a substitute for natural soils. Fly ash in this application must be stockpiled and conditioned to its optimum moisture content to ensure that the material is not too dry and dusty or too wet and unmanageable. When compacted at or near optimum moisture content and evaluated at in field stress conditions, fly ash and fly ash mixtures perform in an equivalent manner to well-compacted soil.^(22;23)

MATERIAL PROPERTIES

Physical Properties

Fly ash consists of fine powdery particles that are predominantly spherical in shape, either solid or hollow, and mostly glassy (amorphous) in nature. The carbonaceous material in fly ash is composed of angular particles. Bituminous coal fly ash particle size is generally similar to that of a silt or fine sand (less than 0.075 mm). Subbituminous coal fly ashes are also silt-sized, although slightly coarser than bituminous coal fly ash.⁽²⁴⁾

The specific gravity of fly ash usually ranges from 2.1 to 3.0, while its specific surface area (measured by the Blaine air permeability method)⁽²⁵⁾ may range from 170 to 1000 m²/kg.

The color of fly ash is a loose indicator of its chemical content. Lignite or subbituminous fly ashes are usually light tan to buff in color, indicating relatively low amounts of carbon as well as the presence of lime or calcium. Bituminous fly ashes are usually some shade of gray. Lighter shades of gray generally indicate a higher quality of ash and a dark gray to black color is generally attributed to elevated unburned carbon content.⁽¹⁰⁾

Chemical Properties

The chemical properties of fly ash are influenced to a great extent by the chemical content of the coal burned, the air pollution control strategy at the power plant, and the techniques used for handling and storage.

Table 2 summarizes the normal range of chemical constituents of fly ashes from bituminous coal, lignite coal, and subbituminous coal. Lignite and subbituminous coal fly ashes have higher calcium oxide content and lower LOI than fly ashes from bituminous coals. Lignite and subbituminous coal fly ashes may have a higher amount of sulfate compounds than bituminous coal fly ashes.

The chief difference between Class F and Class C fly ash is the amount of calcium, silica, alumina, and iron percent in the ash.⁽⁹⁾ In Class F fly ash, total calcium typically ranges from 1 to 12 percent, mostly in the form of calcium hydroxide, calcium sulfate, and glassy components in combination with silica and alumina. In contrast, Class C fly ash may have reported calcium oxide contents as high as 30 to 40 percent.⁽²⁶⁾ The amount of alkalis (combined sodium and potassium) and sulfates (SO4) are more abundant in Class C fly ashes than in Class F fly ashes.

Table 2. Normal range of chemical composition for fly ash produced from different coal types (expressed as percent by weight).

Although the Class F and Class C designations strictly apply only to fly ash meeting the ASTM C618 specification, these terms are often used more generally to apply to fly ash on the basis of the original coal type or CaO content. Not all fly ashes are able to meet ASTM C618 requirements for concrete, yet many "off spec" fly ashes have cementing properties and can be used for base, subbase, and subgrade stabilization.⁽²⁰⁾

Loss on ignition (LOI), which is a measure of the amount of residual carbon remaining in fly ash, is one of the most significant chemical properties of fly ash. To be a classified fly ash, LOI can range up to 5 percent or 6 percent, per AASHTO or ASTM respectively.⁽⁴⁾ The LOI can indicate suitability for use as a cement replacement in concrete as variations in carbon content can affect concrete mix activity and air content.⁽¹⁰⁾

In addition to chemical composition and LOI, the quality of fly ash for concrete is predominately determined by fineness and consistency. Fineness establishes the reactivity of the fly ash as well as carbon content levels. Fly ash is typically finer than Portland cement and lime, and ranges between 10 and 100 micron. Consistency, or an awareness of a change, in the fly ash supply is needed in concrete production to allow for pre-testing of concrete mix design.⁽⁴⁾

ENVIRONMENTAL CONSIDERATIONS

Unencapsulated vs. Encapsulated Use

A concern with fly ash use is the possibility of groundwater contamination by trace elements that are associated with coal combustion by-products. The possibility for trace elements to dissolve in rainwater that percolates through fly ash has caused restrictions on fly ash use by state environmental regulations.

When fly ash is used in concrete, the potential for leaching of trace elements is very low. This is due to the constituents of fly ash being encapsulated in the matrix of the concrete.⁽²⁷⁾

Unencapsulated use, however, has the potential for trace element leaching. Use of fly ash in stabilized base or embankments requires good management to ensure the environment is not impacted negatively. Although studies have shown that coal fly ash is typically safe to use in unencapsulated applications, precautions must still be taken to ensure environmental impacts are acceptable.^{(28;29;30;31)} An evaluation of groundwater conditions, applicable state test procedures, water quality standards, and proper construction are all necessary considerations in ensuring a safe final product.⁽¹⁰⁾

Leachability

One of the main limitations of present leach test methods is that these methods do not consider the fly ash application. For example, use of fly ash in Wisconsin is regulated by Ch. NR 538 of the Wisconsin Administrative Code. This regulation requires water leaching tests (WLT) of fly ash in bulk form, but does not consider mixtures, such as fly ash stabilized soil. In addition, WLT does not necessarily model leachate produced in the field. The WLT indicates the potential for contaminant release from fly ash or mixtures, but does not evaluate how a fly ash or fly ash mixture will impact groundwater.⁽³²⁾

Five widely used standard leaching tests are outlined in Table 3.

Table 3. Extraction conditions for different standard leaching tests.⁽³²⁾

The high lime content of fly ash generally results in elevated pH levels (10.1 to 12.8) and sorption controlled release of metals and metalloids. WLTs on fly ash stabilized soil show that concentrations in leachate vary non-linearly with fly ash content. This non-linearity is believed to be due to the effects of increasing pH on adsorption and means that linear dilution calculations using results from WLT are incorrect.⁽³²⁾ This also means that decreases in pH over time may result in higher solubility and a decrease in the adsorption of metals.^(33;34)

A comparison of field to laboratory leachates from fly ash stabilized soil found that column leach tests compare well with field results while water leach tests typically underestimate field concentrations.^(32;33) Concentrations in leachate collected from field lysimeters below test sections of fly ash stabilized soil were similar or slightly lower than concentrations measured in column leach tests on the same material. Thus, the column leach test appears to provide a good indication of in field conditions directly below a soil-fly ash mixture provided the test conditions mimic the field conditions. In addition, initial concentration in the field can be conservatively estimated from water leach tests providing scaling factors are applied and the infiltrating water in the field is near neutral.⁽³³⁾ Example results from batch tests and column leaching tests can be found in references 32, 33, and 35.

A long-term environmental monitoring program on a 3.4 km (2.1 mi) long truck route using 60,000 m³ (2.1×106 ft³) of unstabilized Class F fly ash subbase showed that almost all trace element levels in the groundwater complied with Illinois class I and II standards. The concentration also showed a decreasing trend through time. No adverse effects on groundwater and neighboring soils were observed to have occurred.⁽³⁶⁾ In Wisconsin a 1.4 km (0.87 mi) test section of State Highway 60 incorporated 4 different industrial byproducts as subbase material. Analysis of leachate collected from the base of the test sections shows that the byproducts discharge contaminants of concern at very low levels.^(16;37;38)

Bioassay tests have been conducted on algae and Dapbnia (representing plant and animal life). A mixture of pure fly ash and deionized water was produced at a ratio of 1 g of fly ash for every 4 mL of water. This solution was filtered and the leachate used in the test. Fly ashes from power-generating facilities in Ohio and Indiana were tested and the results indicated growth inhibition in the Ohio sample, but not in the Indiana sample.⁽³⁹⁾ The tests showed that in its pure form, fly ash could be harmful to aquatic life. However, additional studies showed that the environmental risks markedly decreased or disappeared when fly ash was mixed with other materials.⁽³⁹⁾

Modeling

Models currently used to simulate leaching from pavement systems and potential impacts to groundwater include STUWMPP,⁽⁴⁰⁾ IMPACT,⁽⁴¹⁾ HYDRUS-2D,^(42;38;43) WiscLEACH,⁽⁴⁴⁾ and IWEM.⁽⁴⁵⁾ Among these models STUWMPP, IMPACT, WiscLEACH and IWEM are in the public domain. STUWMPP employs dilution–attenuation factors obtained from the seasonal soil compartment (SESOIL) model to relate leaching concentrations from soils and byproducts to concentrations in underlying groundwater. IMPACT was specifically developed to assess environmental impacts from highway construction. Two dimensional flow and solute transport are simulated by solving the advection dispersion reaction equation using the finite difference method.⁽⁴⁴⁾

WiscLEACH combines three analytical solutions to the advection–dispersion–reaction equation to assess impacts to groundwater caused by leaching of trace elements from CCPs used in highway subgrade, subbase and base layers. WiscLEACH employs a user friendly interface and readily available input data along with an analytical solution to produce conservative estimates of groundwater impact.⁽⁴⁴⁾

The U.S. EPA's Industrial Waste Management Evaluation Model (IWEM), although developed to evaluate impacts from landfills and stock piles, can help in determining whether fly ash leachate will negatively affect groundwater. IWEM inputs include site geology/hydrogeology, initial leachate concentration, metal parameters, and regional climate data. Given a length of time, the program will produce a leachate concentration at a control point (such as a pump or drinking well) that is a known distance from the source. In addition, Monte Carlo simulations can provide worst-case scenarios for situations where a parameter is unknown or unclear. In comparing IWEM to field lysimeter information, IWEM over predicted the leachate concentrations and could be considered conservative. Overall, however, IWEM performed satisfactorily in predicting groundwater and solute flow at points downstream from a source.⁽⁴⁶⁾

Other Considerations

Fly ash can cause a dust problem during storage and processing or through wind erosion during placement in unecapsulated use. Workers involved with dry ash handling can take precautions by requesting Material Safety Data Sheets (MSDS) from fly ash suppliers, by wearing safety goggles to protect their eyes from dust, and by wearing a suitable particulate respirator (i.e., approved by the National Institute for Occupational Safety and Health for particulates). Dust problems can be partially alleviated by compaction and covering of the fly ash, moistening fly ash during placement, and using mechanical ventilation or extraction in areas where dust could escape into the workplace.⁽¹⁰⁾ Special lay-down trucks exist that reduce dusting issues.

A source of information on assessing risk and protecting groundwater is U.S. EPA's "Guide for Industrial Waste Management"⁽⁴⁷⁾ which can be found at: http://www.epa.gov/industrialwaste/guide.asp

Finally, due to the variability in fly ash composition between coal plants, industry-wide generalizations about the environmental impact of fly ash cannot be made. Also, because of the variety of leachate testing methods and the variety of standards and regulations to compare these test results to, state regulations should be identified and followed when determining the environmental suitability of fly ash from a particular source.

REFERENCES

A searchable version of the references used in this section is available here.
A searchable bibliography of coal fly ash literature is available here.

1. Horiuchi S, Kawaguchi M, Yasuhara K. Effective use of fly ash slurry as fill material. Journal of Hazardous Materials 2000 9/15;76(2-3):301-37.
2. Steam, its generation and use. 39th ed. New York: Babcock & Wilcox; 1978.
3. American Coal Ash Association (ACAA). 2006 coal combustion product (CCP) production and use. Aurora, CO: American Coal Ash Association; August, 2007.
4. Federal Highway Administration (FHWA), American Coal Ash Association (ACAA). Fly ash facts for highway engineers. Federal Highway Administration (FHWA); 2003 06-13-2003. FHWA-IF-03-019.
5. Kalinski ME, Hippley BT. The effect of water content and cement content on the strength of Portland cement-stabilized compacted fly ash. Fuel 2005 10;84(14-15):1812-9.
6. Kalinski ME, Yerra PK. Hydraulic conductivity of compacted cement–stabilized fly ash. Fuel 2006 11;85(16):2330-6.
7. Dockter BA, Jagiella DM. Engineering and environmental specifications of state agencies for utilization and disposal of coal combustion products. In: 2005 world of coal ash conference, Lexington, KY. ; 2005.
8. Meyers JF, Pichumani R, Kapples BS. Fly ash: A highway construction material. Washington, DC: Federal Highway Administration (FHWA); 1976. FHWA-IP-76-16.
9. ASTM C618-05 standard specification for fly ash and raw or calcined natural pozzolan for use as mineral admixture in Portland cement concrete. In: Annual book of ASTM standards. West Conshohocken, Pennsylvania: ASTM; 2005.
10. Environmental Protection Agency (EPA), Federal Highway Administration (FHWA). Using coal ash in highway construction - A guide to benefits and impacts. ; April 2005. Report nr EPA-530-K-002:ID: 151.
11. Jobs Through Recycling [Internet]; c2007 [cited 2007 11/29]. Available from: http://www.epa.gov/jtr/comm/cfa.asp.
12. Fox JM, Constantiner D. The influence of fly ash after change to low-NOx burners on concrete strength, case study. In: 2007 world of coal ash (WOCA), may 7-10, Covington, Kentucky. ; 2007.
13. AASHTO. Standard specification for mineral filler for bituminous paving mixtures. Washington, DC 20001: American Association of State Highway and Transportation Officials; 2007. M 17-07.
14. Arora S, Aydilek AH. Class F fly-ash-amended soils as highway base materials. J Mater Civ Eng 2005;17:640-9.
15. Bin-Shafique MS, Edil TB, Benson CH, Senol A. Incorporating a fly-ash stabilized layer into pavement design. Proceedings of the Institution of Civil Engineers: Geotechnical Engineering 2004;157:239-49.
16. Edil TB, Benson CH, Bin-Shafique MS, Tanyu B, Kim W, Senol A. Field evaluation of construction alternatives for roadway over soft subgrade. Transp Res Rec 2000;1786:36-48.
17. Wen H, Tharaniyil MP, Ramme BW, Krebs S. Field performance evaluation of class C fly ash in full-depth reclamation: Case history study. Transp Res Rec 2004(186):41-6.
18. White DJ, Bergeson K. Long-term strength and durability of hydrated fly-ash road bases. Transportation Research Record 2001;1755(1):151-9.
19. Senol A, Edil TB, Bin-Shafique MS, Acosta HA, Benson CH. Soft subgrades’ stabilization by using various fly ashes. Resources, Conservation and Recycling 2006 4;46(4):365-76.
20. Edil TB, Acosta HA, Benson CH. Stabilizing soft fine-grained soils with fly ash. J Mater Civ Eng 2006;18:283-94.
21. Edil TB, Benson CH. Sustainable construction case history: Fly ash stabilization of road-surface gravel. In: Proceedings of the 2007 world of coal ash (WOCA), May 7-10, Covington, Kentucky. ; 2007.
22. Di Gioia AM, Nuzzo WL. Fly ash as structural fill. Journal of the Power Division 1972 June;98(1):77-92.
23. Kim B, Prezzi M, Salgado R. Geotechnical properties of fly and bottom ash mixtures for use in highway embankments. Journal of Geotechnical and Geoenvironmental Engineering 2005;131(7):914-24.
24. AASHTO. Soundness of aggregate by use of sodium sulfate or magnesium sulfate, part II tests. Washington, DC 20001: American Association of State Highway and Transportation Officials; 1999. Report nr T104-99.
25. ASTM C204-07 standard test methods for fineness of hydraulic cement by air-permeability apparatus. In: West Conshohocken, Pennsylvania: ASTM; 2007.
26. McKerall WC, Ledbetter WB, Teague DJ. Analysis of fly ashes produced in Texas. Texas A&M University, College Station, Texas: Texas Transportation Institute,; 1982. No. 240-1.
27. Environmental Protection Agency (EPA). Report to congress: Wastes from the combustion of fossil fuels – volume II. ; 1999. Report nr EPA 530-S-99-010.
28. Electric Power Research Institute (EPRI). High-volume fly ash utilization projects in the United States and Canada. Palo Alto, California: Electric Power Research Institute (EPRI); 1998. Report nr EPRI CS-4446.
29. Hassett DJ, Heebink LV. Release of mercury vapor from coal combustion ash. In: 2001 international ash utilization symposium, Lexington, KY. ; 2001.
30. Pflughoseft-Hassett DF, Hassett DJ, Dockter BA. High volume fly ash utilization and the effects on groundwater in North Dakota in high-volume Uses/Concrete applications. In: Proceedings of the 10th international ash use symposium, Orlando FL. 1993.
31. Environmental Protection Agency (EPA). Notice of regulatory determination on four large-volume wastes from the combustion of coal by electric utility power plants. EPA; 1993. Report nr Federal Register Notice 58 FR 42466.
32. Bin-Shafique MS, Benson CH, Edil TB. Geoenvironmental assessment of fly ash stabilized subbases. University of Wisconsin – Madison, Madison, Wisconsin: Geo Engineering Department of Civil and Environmental Engineering; 2002 March 11, 2002. Report nr Geo Engineering Report No. 02-03.
33. Bin-Shafique MS, Benson CH, Edil TB, Hwang K. Leachate concentrations from water leach and column leach tests on fly ash-stabilized soils. Environ Eng Sci 2006;23:53-67.
34. Kanungo SB, Mohapatra R. Leaching behavior of various trace metals in aqueous media from two fly ash samples. J Environ Qual 2000;29:188-96.
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[ Material Description ] - [ Asphalt Concrete ] - [ Portland Cement Concrete ] - [ Embankment or Fill ] - [ Stabilized Base ] - [ Flowable Fill ]