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EspañolFLY ASH
'Fly ash' (one of several coal combustion products, or CCPs) is the finely divided mineral residue resulting from the combustion of coal in electric generating plants. Fly ash consists of inorganic, incombustible matter present in the coal that has been fused during combustion into a glassy, amorphous structure.
Fly ash material solidifies while suspended in the exhaust gases and is collected by electrostatic precipitators or filter bags. Since the particles solidify while suspended in the exhaust gases, fly ash particles are generally spherical in shape and range in size from 0.5 µm to 100 µm. They consist mostly of silicon dioxide (SiO2), aluminium oxide (Al2O3) and iron oxide (Fe2O3), and are hence a suitable source of aluminum and silicon for geopolymers. They are also pozzolanic in nature and react with calcium hydroxide and alkali to form calcium silicate hydrates (cementitious compounds).
Two classes of fly ash are defined by ASTM C618: Class F fly ash and Class C fly ash. The chief difference between these classes is the amount of calcium, silica, alumina, and iron content in the ash. Engineering properties and development of strength over time are different depending on the chemical composition of the fly ash. The chemical properties of the fly ash are largely influenced by the chemical content of the coal burned (i.e., anthracite, bituminous, and lignite).
Not all fly ashes meet ASTM C618 requirements, although depending on the application, this may not be necessary. Ash used as a cement replacement must meet strict construction standards, but no standard environmental standards have been established in the United States. Three-fourths of the ash must have a fineness of 45 µm or less, and have a carbon content, measured by the loss on ignition (LOI), of less than 4%. In the U.S., LOI needs to be under 6%. The particle size distribution of raw fly ash is very often fluctuating constantly, due to changing performance of the coal mills and the boiler performance. This makes it necessary that fly ash used in concrete needs to be processed using separation equipment like mechanical air classifiers. Especially important is the ongoing quality verification. This is mainly expressed by quality control seals like the Indian ISI mark or the DCL mark of the Dubai Municipality. A typical Fly Ash processing plant with quality verification is the DIRK India plant in Nashik/Maharashtra, India.
The burning of harder, older anthracite and bituminous coal typically produces Class F fly ash. This fly ash is pozzolanic in nature, and contains less than 10% lime (CaO). Possessing pozzolanic properties, the glassy silica and alumina of Class F fly ash requires a cementing agent, such as Portland cement, quicklime, or hydrated lime, with the presence of water in order to react and produce cementitious compounds.
Fly ash produced from the burning of younger lignite or subbituminous coal, in addition to having pozzolanic properties, also has some self-cementing properties. In the presence of water, Class C fly ash will harden and gain strength over time. Class C fly ash generally contains more than 20% lime (CaO). Unlike Class F, self-cementing Class C fly ash does not require an activator. Alkali and sulfate (SO4) contents are generally higher in Class C fly ashes.
In the past, fly ash produced from coal combustion was simply taken up by flue gases and dispersed into the atmosphere. This created environmental and health concerns that prompted laws which have reduced fly ash emissions to less than 1% of ash produced. Worldwide, more than 65% of fly ash produced from coal power stations is disposed of in landfills. In India alone, fly ash landfill covers an area of 40,000 acres (160 km²).
The recycling of fly ash has become an increasing concern in recent years due to increasing landfill costs and current interest in sustainable development. In 2005, U.S. coal-fired power plants reported producing 71.1 million tons of fly ash, of which 29.1 million tons was reused in various applications. "CCP Production and Use Survey" American Coal Ash Association If the nearly 42 million tons of unused fly ash had been recycled, it would have reduced the need for approximately 27,500 acre-feet of landfill space. "Using Coal Ash in Highway Construction - A Guide to Benefits and Impacts" U.S. Environmental Protection Agency Other environmental benefits to recycling fly ash includes reducing the demand for virgin materials that would need quarrying and substituting for materials that may be energy-intensive to create (such as Portland cement).
The reuse of fly ash as an engineering material primarily stems from its pozzolanic nature, spherical shape, and relative uniformity. Fly ash recycling, in descending frequency, includes usage in:
★ Portland cement and grout;
★ Embankments and structural fill;
★ Waste stabilization and solidifaction;
★ Raw feed for cement clinkers;
★ Mine reclamation;
★ Stabilization of soft soils;
★ Road subbase;
★ Aggregate;
★ Flowable fill;
★ Mineral filler in asphaltic concrete;
★ Other applications include cellular concrete, roofing tiles, paints, metal castings, and filler in wood and plastic products.[1]
Owing to its pozzolan properties, fly ash is used as a replacement of Portland cement in concrete.[2] The use of fly ash as a pozzolanic ingredient was recognized as early as 1914, although the earliest noteworthy study of its use was in 1937.[3] Use of fly ash as a partial replacement for Portland cement is generally limited to Class F fly ashes. It can replace up to 30% by mass of Portland cement, and can add to the concrete’s final strength and increase its chemical resistance and durability. Recently concrete mix design for partial cement replacement with High Volume Fly Ash (50 % cement replacement) has been developed. For Roller Compacted Concrete (RCC)[used in dam construction] replacement values of 70% have been achieved with POZZOCRETE (processed fly ash) at the Ghatghar Dam project in Maharashtra, India. Due to fly ash’s spherical shape, it can also increase workability of cement while reducing water demand. Fly Ash Facts for Highway Engineers U.S. Federal Highway Administration The replacement of Portland cement with fly ash also reduces the greenhouse gas signature of concrete, as the production of one ton of Portland cement produces one ton of CO2. Since the worldwide production of Portland cement is expected to reach nearly 2 billion tons by 2010, its replacement by fly ash could dramatically reduce global emissions of carbon
Fly ash properties are somewhat unique as an engineering material. Unlike typical soils used for embankment construction, fly ash has a large uniformity coefficient consisting of silt-sized particles. Engineering properties that will affect fly ash’s use in embankments include grain size distribution, compaction characteristics, shear strength, compressibility, permeability, and frost susceptibility. Nearly all fly ash used in embankments are Class F fly ashes.
Soil stabilization involves the addition of fly ash to improve the engineering performance of a soil. This is typically used for a soft, clayey subgrade beneath a road that will experience many repeated loadings. Improvement can be done with both Class C and Class F fly ashes. If using a Class F fly ash, an additive (such as lime or cement) is needed whereas the self-cementing nature of Class C fly ash allows it to be used alone.
Fly ash is also used as a component in the production of flowable fill (also called controlled low strength material, or CLSM), which is used as self-leveling, self-compacting backfill material in lieu of compacted earth or granular fill. The strength of flowable fill mixes can range from 200 to 1,200 lbf/in² (1.4 to 8.3 MPa), depending on the design requirements of the project in question. Flowable fill includes mixtures of Portland cement and filler material, and can contain mineral admixtures. Fly ash can replace fine aggregate (in most cases, river sand) as a filler material. High fly ash content mixes contain nearly all fly ash, with a small percentage of Portland cement and enough water to make the mix flowable. Low fly ash content mixes contain a high percentage of filler material, and a low percentage of fly ash, Portland cement, and water. Class F fly ash is best suited for high fly ash content mixes, whereas Class C fly ash is almost always used in low fly ash content mixes.[3]
Asphalt concrete is a composite material consisting of an asphalt binder and mineral aggregate. Both Class F and Class C fly ash can typically be used as a mineral filler to fill the voids and provide contact points between larger aggregate particles in asphalt concrete mixes. This application is used in conjunction, or as a replacement for, other binders (such as Portland cement or hydrated lime). For use in apshalt pavement, the fly ash must meet mineral filler specifications outlined in ASTM D242. The hydrophobic nature of fly ash gives pavements better resistance to stripping. Fly ash has also been shown to increase the stiffness of the asphalt matrix, improving rutting resistance and increasing mix durability.[3]
More recently, fly ash has been used as a component in geopolymers mixtures.
Another new application is using fly ash in roller compacted concrete dams. This has been demonstrated in the Ghatghar Dam Project in India.
Ash bricks have been used in house construction in Windhoek, Namibia since the 1970's. There is, however, a problem with the bricks in that they tend to fail or produce unsightly pop-outs. This happens when the bricks come into contact with moisture and a chemical reaction occurs causing the bricks to expand.
In May 2007, Henry Liu, a retired 70-year old American civil engineer, announced that he had invented a new, environmentally sound building brick composed of fly ash and water. Compressed at 4,000 psi and cured for 24 hours in a 150°F (66°C) steam bath , then toughened with an air entrainment agent, the bricks last for more than 100 freeze-thaw cycles. Owing to the high concentration of calcium oxide in fly ash, the brick can be described as "self-cementing". The manufacturing method is said to save energy, reduce mercury pollution, and costs 20% less than traditional clay brick manufacturing. Liu intends to license his technology to manufacturers in 2008. [6][7]
Using a proprietary methodology, the US company N-Viro International Corporation uses the alkaline properties of fly ash to process human waste sludge into fertilizer.[8]
Similar the RHENIPAL process owned by DIRK Group utilizes fly ash mixtures for the stabilization of sewage sludge and other toxic sludges. This process was used to stabilize large amounts of Chromium 6 contaminated leather sludges in Portugal (Alcanena)
Fly ash, like soil, contains trace concentrations of many heavy metals that are known to be detrimental to health in sufficient quantities. These include nickel, vanadium, arsenic, beryllium, cadmium, barium, chromium, copper, molybdenum, zinc, lead, selenium, uranium, thorium, and radium. Though these elements are found in extremely low concentrations in fly ash, their mere presence has prompted some to sound alarm.
The U.S. EPA confirms that coal fly ash does not need to be regulated as a hazardous waste. [9] The EPA's headquarters building in Washington, D.C. is constructed with concrete containing fly ash. Studies by the U.S. Geological Survey and others conclude that fly ash compares with common soils or rocks and should not be the source of alarm.[10]
1. Fly Ash U.S. Federal Highway Administration
2. Evaluation of Fly Ash From Co-Combustion of Coal and Petroleum Coke for Use in Concrete, , Allan N ., Scott, ACI Materials Journal,
3.
4.
5.
6. Popular Science Magazine, INVENTION AWARDS : A Green Brick, May 2007
7.
National Science Foundation, Press Release 07-058, "Follow the 'Green' Brick Road?", May 22, 2007
8. [1] N-Viro International
9. “Notice of Regulatory Determination on Wastes From the Combustion of Fossil Fuels” Federal Register
10. "Radioactive Elements in Coal and Fly Ash: Abundance, Forms, and Environmental Significance" U.S. Geological Survey
★ American Coal Ash Association Established in 1969, the American Coal Ash Association is considered the "Voice of the Coal Combustion Products Industry"
★ EcoSmart Concrete : A site dedicated to the use of Fly ash and other supplementary cementing materials in concrete.
★ Fly Ash Information Center : Site explaining the history and uses of fly ash.
★ United States Geological Survey - Radioactive Elements in Coal and Fly Ash (document)
★ High Volume Fly Ash Concrete
★ UK Quality Ash Association : A site promoting the many uses of fly ash in the UK
Chemical composition and classification
| Component | Bituminous | Subbituminous | Lignite |
|---|---|---|---|
| SiO2 (%) | 20-60 | 40-60 | 15-45 |
| Al2O3 (%) | 5-35 | 20-30 | 20-25 |
| Fe2O3 (%) | 10-40 | 4-10 | 4-15 |
| CaO (%) | 1-12 | 5-30 | 15-40 |
| LOI (%) | 0-15 | 0-3 | 0-5 |
Two classes of fly ash are defined by ASTM C618: Class F fly ash and Class C fly ash. The chief difference between these classes is the amount of calcium, silica, alumina, and iron content in the ash. Engineering properties and development of strength over time are different depending on the chemical composition of the fly ash. The chemical properties of the fly ash are largely influenced by the chemical content of the coal burned (i.e., anthracite, bituminous, and lignite).
Not all fly ashes meet ASTM C618 requirements, although depending on the application, this may not be necessary. Ash used as a cement replacement must meet strict construction standards, but no standard environmental standards have been established in the United States. Three-fourths of the ash must have a fineness of 45 µm or less, and have a carbon content, measured by the loss on ignition (LOI), of less than 4%. In the U.S., LOI needs to be under 6%. The particle size distribution of raw fly ash is very often fluctuating constantly, due to changing performance of the coal mills and the boiler performance. This makes it necessary that fly ash used in concrete needs to be processed using separation equipment like mechanical air classifiers. Especially important is the ongoing quality verification. This is mainly expressed by quality control seals like the Indian ISI mark or the DCL mark of the Dubai Municipality. A typical Fly Ash processing plant with quality verification is the DIRK India plant in Nashik/Maharashtra, India.
Class F fly ash
The burning of harder, older anthracite and bituminous coal typically produces Class F fly ash. This fly ash is pozzolanic in nature, and contains less than 10% lime (CaO). Possessing pozzolanic properties, the glassy silica and alumina of Class F fly ash requires a cementing agent, such as Portland cement, quicklime, or hydrated lime, with the presence of water in order to react and produce cementitious compounds.
Class C fly ash
Fly ash produced from the burning of younger lignite or subbituminous coal, in addition to having pozzolanic properties, also has some self-cementing properties. In the presence of water, Class C fly ash will harden and gain strength over time. Class C fly ash generally contains more than 20% lime (CaO). Unlike Class F, self-cementing Class C fly ash does not require an activator. Alkali and sulfate (SO4) contents are generally higher in Class C fly ashes.
Disposal and market sources
In the past, fly ash produced from coal combustion was simply taken up by flue gases and dispersed into the atmosphere. This created environmental and health concerns that prompted laws which have reduced fly ash emissions to less than 1% of ash produced. Worldwide, more than 65% of fly ash produced from coal power stations is disposed of in landfills. In India alone, fly ash landfill covers an area of 40,000 acres (160 km²).
The recycling of fly ash has become an increasing concern in recent years due to increasing landfill costs and current interest in sustainable development. In 2005, U.S. coal-fired power plants reported producing 71.1 million tons of fly ash, of which 29.1 million tons was reused in various applications. "CCP Production and Use Survey" American Coal Ash Association If the nearly 42 million tons of unused fly ash had been recycled, it would have reduced the need for approximately 27,500 acre-feet of landfill space. "Using Coal Ash in Highway Construction - A Guide to Benefits and Impacts" U.S. Environmental Protection Agency Other environmental benefits to recycling fly ash includes reducing the demand for virgin materials that would need quarrying and substituting for materials that may be energy-intensive to create (such as Portland cement).
Fly ash reuse
The reuse of fly ash as an engineering material primarily stems from its pozzolanic nature, spherical shape, and relative uniformity. Fly ash recycling, in descending frequency, includes usage in:
★ Portland cement and grout;
★ Embankments and structural fill;
★ Waste stabilization and solidifaction;
★ Raw feed for cement clinkers;
★ Mine reclamation;
★ Stabilization of soft soils;
★ Road subbase;
★ Aggregate;
★ Flowable fill;
★ Mineral filler in asphaltic concrete;
★ Other applications include cellular concrete, roofing tiles, paints, metal castings, and filler in wood and plastic products.[1]
Portland cement
Owing to its pozzolan properties, fly ash is used as a replacement of Portland cement in concrete.[2] The use of fly ash as a pozzolanic ingredient was recognized as early as 1914, although the earliest noteworthy study of its use was in 1937.[3] Use of fly ash as a partial replacement for Portland cement is generally limited to Class F fly ashes. It can replace up to 30% by mass of Portland cement, and can add to the concrete’s final strength and increase its chemical resistance and durability. Recently concrete mix design for partial cement replacement with High Volume Fly Ash (50 % cement replacement) has been developed. For Roller Compacted Concrete (RCC)[used in dam construction] replacement values of 70% have been achieved with POZZOCRETE (processed fly ash) at the Ghatghar Dam project in Maharashtra, India. Due to fly ash’s spherical shape, it can also increase workability of cement while reducing water demand. Fly Ash Facts for Highway Engineers U.S. Federal Highway Administration The replacement of Portland cement with fly ash also reduces the greenhouse gas signature of concrete, as the production of one ton of Portland cement produces one ton of CO2. Since the worldwide production of Portland cement is expected to reach nearly 2 billion tons by 2010, its replacement by fly ash could dramatically reduce global emissions of carbon
Embankment
Fly ash properties are somewhat unique as an engineering material. Unlike typical soils used for embankment construction, fly ash has a large uniformity coefficient consisting of silt-sized particles. Engineering properties that will affect fly ash’s use in embankments include grain size distribution, compaction characteristics, shear strength, compressibility, permeability, and frost susceptibility. Nearly all fly ash used in embankments are Class F fly ashes.
Soil stabilization
Soil stabilization involves the addition of fly ash to improve the engineering performance of a soil. This is typically used for a soft, clayey subgrade beneath a road that will experience many repeated loadings. Improvement can be done with both Class C and Class F fly ashes. If using a Class F fly ash, an additive (such as lime or cement) is needed whereas the self-cementing nature of Class C fly ash allows it to be used alone.
Flowable fill
Fly ash is also used as a component in the production of flowable fill (also called controlled low strength material, or CLSM), which is used as self-leveling, self-compacting backfill material in lieu of compacted earth or granular fill. The strength of flowable fill mixes can range from 200 to 1,200 lbf/in² (1.4 to 8.3 MPa), depending on the design requirements of the project in question. Flowable fill includes mixtures of Portland cement and filler material, and can contain mineral admixtures. Fly ash can replace fine aggregate (in most cases, river sand) as a filler material. High fly ash content mixes contain nearly all fly ash, with a small percentage of Portland cement and enough water to make the mix flowable. Low fly ash content mixes contain a high percentage of filler material, and a low percentage of fly ash, Portland cement, and water. Class F fly ash is best suited for high fly ash content mixes, whereas Class C fly ash is almost always used in low fly ash content mixes.[3]
Asphalt concrete
Asphalt concrete is a composite material consisting of an asphalt binder and mineral aggregate. Both Class F and Class C fly ash can typically be used as a mineral filler to fill the voids and provide contact points between larger aggregate particles in asphalt concrete mixes. This application is used in conjunction, or as a replacement for, other binders (such as Portland cement or hydrated lime). For use in apshalt pavement, the fly ash must meet mineral filler specifications outlined in ASTM D242. The hydrophobic nature of fly ash gives pavements better resistance to stripping. Fly ash has also been shown to increase the stiffness of the asphalt matrix, improving rutting resistance and increasing mix durability.[3]
Polymers
More recently, fly ash has been used as a component in geopolymers mixtures.
Roller compacted concrete
Another new application is using fly ash in roller compacted concrete dams. This has been demonstrated in the Ghatghar Dam Project in India.
Bricks
Ash bricks have been used in house construction in Windhoek, Namibia since the 1970's. There is, however, a problem with the bricks in that they tend to fail or produce unsightly pop-outs. This happens when the bricks come into contact with moisture and a chemical reaction occurs causing the bricks to expand.
In May 2007, Henry Liu, a retired 70-year old American civil engineer, announced that he had invented a new, environmentally sound building brick composed of fly ash and water. Compressed at 4,000 psi and cured for 24 hours in a 150°F (66°C) steam bath , then toughened with an air entrainment agent, the bricks last for more than 100 freeze-thaw cycles. Owing to the high concentration of calcium oxide in fly ash, the brick can be described as "self-cementing". The manufacturing method is said to save energy, reduce mercury pollution, and costs 20% less than traditional clay brick manufacturing. Liu intends to license his technology to manufacturers in 2008. [6][7]
Waste management
Using a proprietary methodology, the US company N-Viro International Corporation uses the alkaline properties of fly ash to process human waste sludge into fertilizer.[8]
Similar the RHENIPAL process owned by DIRK Group utilizes fly ash mixtures for the stabilization of sewage sludge and other toxic sludges. This process was used to stabilize large amounts of Chromium 6 contaminated leather sludges in Portugal (Alcanena)
Environmental problems
Fly ash, like soil, contains trace concentrations of many heavy metals that are known to be detrimental to health in sufficient quantities. These include nickel, vanadium, arsenic, beryllium, cadmium, barium, chromium, copper, molybdenum, zinc, lead, selenium, uranium, thorium, and radium. Though these elements are found in extremely low concentrations in fly ash, their mere presence has prompted some to sound alarm.
The U.S. EPA confirms that coal fly ash does not need to be regulated as a hazardous waste. [9] The EPA's headquarters building in Washington, D.C. is constructed with concrete containing fly ash. Studies by the U.S. Geological Survey and others conclude that fly ash compares with common soils or rocks and should not be the source of alarm.[10]
References
1. Fly Ash U.S. Federal Highway Administration
2. Evaluation of Fly Ash From Co-Combustion of Coal and Petroleum Coke for Use in Concrete, , Allan N ., Scott, ACI Materials Journal,
3.
4.
5.
6. Popular Science Magazine, INVENTION AWARDS : A Green Brick, May 2007
7.
National Science Foundation, Press Release 07-058, "Follow the 'Green' Brick Road?", May 22, 2007
8. [1] N-Viro International
9. “Notice of Regulatory Determination on Wastes From the Combustion of Fossil Fuels” Federal Register
10. "Radioactive Elements in Coal and Fly Ash: Abundance, Forms, and Environmental Significance" U.S. Geological Survey
External links
★ American Coal Ash Association Established in 1969, the American Coal Ash Association is considered the "Voice of the Coal Combustion Products Industry"
★ EcoSmart Concrete : A site dedicated to the use of Fly ash and other supplementary cementing materials in concrete.
★ Fly Ash Information Center : Site explaining the history and uses of fly ash.
★ United States Geological Survey - Radioactive Elements in Coal and Fly Ash (document)
★ High Volume Fly Ash Concrete
★ UK Quality Ash Association : A site promoting the many uses of fly ash in the UK
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