Fly ash is one of the residues generated in the combustion of coal. Fly ash is generally captured from the chimneys of power generation facilities, whereas bottom ash is, as the name suggests, removed from the bottom of the furnace. Bottom ash refers to the non- Combustible constituents of Coal with traces of combustibles embedded in forming clinkers and sticking to hot side walls of furnace during In the past, fly ash was generally released into the atmosphere via the smoke stack, but pollution control equipment mandated in recent decades now require that it be captured prior to release. It is generally stored on site at most US electric power generation facilities. The United States of America —commonly referred to as the Depending upon the source and makeup of the coal being burned, the components of the fly ash produced vary considerably, but all fly ash includes substantial amounts of silica (silicon dioxide, SiO₂) (both amorphous and crystalline) and lime (calcium oxide, CaO). Fly ash is commonly used to supplement Portland cement in concrete production, where it can bring both technological and economic benefits, and is increasingly finding use in synthesis of geopolymers and zeolites. Portland cement is the most common type of Cement in general usage in many parts of the world as it is a basic ingredient of Concrete, mortar, Stucco Geopolymer is a term covering a class of synthetic Aluminosilicate materials with potential use in a number of areas but predominantly as a replacement for Portland cement Zeolites (Greek zein, "to boil" lithos, "a stone" are hydrated Aluminosilicate Minerals and have a micro-porous structure

Contents 1 Chemical composition and classification 1.1 Class F fly ash 1.2 Class C fly ash 2 Disposal and market sources 3 Fly ash reuse 3.1 Portland cement 3.2 Embankment 3.3 Soil stabilization 3.4 Flowable fill 3.5 Asphalt concrete 3.6 Geopolymers 3.7 Roller compacted concrete 3.8 Bricks 3.9 Waste management 4 Environmental problems 5 Exposure concerns 6 References 7 External links

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

Fly ash material solidifies while suspended in the exhaust gases and is collected by electrostatic precipitators or filter bags. Bituminous coal is a relatively soft Coal containing a tarlike substance called Bitumen. Sub-bituminous coal is a type of Coal whose properties range from those of Lignite to those of Bituminous coal and are used primarily as fuel for steam-electric Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad, is a soft brown fuel with characteristics that put it somewhere Calcium oxide ( CaO) commonly known as burnt lime, lime or quicklime, is a widely used Chemical compound. Loss on Ignition is a test used in inorganic Analytical chemistry, particularly in the analysis of Minerals It consists of strongly heating ( "igniting" An electrostatic precipitator (ESP or electrostatic air cleaner is a Particulate collection device that removes particles from a flowing gas (such as air using Since the particles solidify while suspended in the exhaust gases, fly ash particles are generally spherical in shape and range in size from 0. "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe 5 µm to 100 µm. They consist mostly of silicon dioxide (SiO₂), which is present in two forms: amorphous, which is rounded and smooth, and crystalline, which is sharp, pointed and hazardous; aluminium oxide (Al₂O₃) and iron oxide (Fe₂O₃). The Chemical compound silicon dioxide, also known as silica or silox (from the Latin " Silex " is an Oxide Altogether there are sixteen known Iron Oxides and oxyhydroxides Fly ashes are generally highly heterogeneous, consisting of a mixture of glassy particles with various identifiable crystalline phases such as quartz, mullite, and various iron oxides. Heterogeneous is an adjective used to describe an object or system consisting of multiple items having a large number of structural variations Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Mullite, or porcelainite, is a rare Clay Mineral with the chemical composition Al 6 Si 2 O 13 Altogether there are sixteen known Iron Oxides and oxyhydroxides

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. The chemical properties of the fly ash are largely influenced by the chemical content of the coal burned (i. e. , anthracite, bituminous, and lignite). Anthracite ( Greek Ανθρακίτης literally "a type of coal" from Anthrax, coal is a hard compact variety of mineral Coal that has a high Bituminous coal is a relatively soft Coal containing a tarlike substance called Bitumen. Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad, is a soft brown fuel with characteristics that put it somewhere

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. 75% 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%. The fineness of a Precious metal refers to the Ratio of the primary Metal to any Additives or impurities. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 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.

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). A pozzolan is a material which when combined with Calcium hydroxide, exhibits Cementitious properties Lime is a general term for various naturally occurring Minerals and materials derived from them in which Carbonates Oxides and Hydroxides of 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. Alternatively, the addition of a chemical activator such as sodium silicate (water glass) to a Class F ash can lead to the formation of a geopolymer. E550 redirects here For the Italian locomotive see FS Class E550 Sodium silicate, also known as water glass or liquid glass, available Geopolymer is a term covering a class of synthetic Aluminosilicate materials with potential use in a number of areas but predominantly as a replacement for Portland cement

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 entrained in flue gases and dispersed into the atmosphere. Flue gas is Gas that exits to the atmosphere via a Flue, which is a pipe or channel for conveying exhaust gases from a fireplace oven Furnace, Boiler 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. For other uses see Water treatment and Land reclamation. A landfill, also known as a dump (and historically as 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. Sustainable development is a pattern of resource use that aims to meet human needs while preserving the environment so that these needs can be met not only in the present 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. ^[1] 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 (33,900,000 m³) of landfill space. ^[1][2] 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). A quarry is a type of open-pit mine from which rock or Minerals are extracted Portland cement is the most common type of Cement in general usage in many parts of the world as it is a basic ingredient of Concrete, mortar, Stucco

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, geopolymers, roofing tiles, paints, metal castings, and filler in wood and plastic products. Grout is a Construction material used to embed Rebars in Masonry walls connect sections of pre-cast Concrete, fill voids and seal joints (like To keep a Road or rail line straight and/or flat and where the comparative cost or practicality of alternate solutions (such as diversion is too prohibitive the land over In the manufacture of Portland cement, clinker is the solid material produced by the Cement kiln stage that has sintered into lumps or nodules typically Mine reclamation is the process of creating useful Landscapes that meet a variety of goals typically creating productive Ecosystems (or sometimes industrial or municipal Clay is a naturally occurring material composed primarily of fine-grained Minerals which show plasticity through a variable range of Water content, and Base Course in pavements refers to the sub-layer material of an Asphalt roadway and is placed directly on top of the undisturbed soil so as to provide a foundation Aggregate is the component of a Composite material used to resist compressive stress The use of flowable fill as a highway construction Material is becoming more widespread throughout the United States. Asphalt concrete, normally known simply as Asphalt, is a Composite material commonly used for construction of pavement, Highways and Geopolymer is a term covering a class of synthetic Aluminosilicate materials with potential use in a number of areas but predominantly as a replacement for Portland cement Paint is any Liquid, liquifiable or mastic composition which after application to a substrate in a thin layer is converted to an opaque Solid Casting is a manufacturing process by which a liquid material is (usually poured into a mold which ^[2][3]

Portland cement

Owing to its pozzolanic properties, fly ash is used as a replacement for some of the Portland cement content of concrete. A pozzolan is a material which when combined with Calcium hydroxide, exhibits Cementitious properties Portland cement is the most common type of Cement in general usage in many parts of the world as it is a basic ingredient of Concrete, mortar, Stucco Concrete is a construction material composed of Cement (commonly Portland cement) as well as other cementitious materials such as Fly ash and Slag ^[4] 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. ^[5] Before its use was lost to the Dark Ages, Roman structures such as aqueducts or the Pantheon in Rome used volcanic ash (which possesses similar properties to fly ash) as pozzolan in their concrete. An aqueduct is an artificial channel that is constructed to convey water from one location to another The Pantheon ( Latin Pantheon, from Greek Πάνθειον Pantheon, meaning "Temple of all the gods" is a building in Rome ^[6] As pozzolan greatly improves the strength and durability of concrete, the use of ash is a key factor in their preservation.

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 the spherical shape of fly ash particles, it can also increase workability of cement while reducing water demand. ^[7] The replacement of Portland cement with fly ash also reduces the greenhouse gas foot print of concrete, as the production of one ton of Portland cement produces approximately one ton of CO₂. Greenhouse gases are gaseous constituents of the atmosphere bothnatural and anthropogenic that absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single Since the worldwide production of Portland cement is expected to reach nearly 2 billion tons by 2010, replacement of 30% of this amount by fly ash could dramatically reduce global carbon emissions.

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. Silt is Soil or rock derived Granular material of a Grain size between sand and clay Engineering properties that will affect fly ash’s use in embankments include grain size distribution, compaction characteristics, shear strength, compressibility, permeability, and frost susceptibility. The Proctor compaction test and the related modified Proctor compaction test, named for engineer Ralph R Shear strength in Engineering is a term used to describe the strength of a material or component against the type of yield or Structural failure where the In Thermodynamics and Fluid mechanics, compressibility is a measure of the relative volume change of a Fluid or Solid as a response Permeability in the Earth sciences (commonly symbolized as κ, or k) is a measure of the ability of a material (typically a rock or unconsolidated Frost heaving (or frost heave) occurs when Soil expands and contracts due to freezing and thawing ^[7] 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 use of flowable fill as a highway construction Material is becoming more widespread throughout the United States. The strength of flowable fill mixes can range from 200 to 1,200 lbf/in² (1. The pound per square inch or more accurately pound-force per square inch (symbol psi or lbf/in² or lbf/in²) is a unit of 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. ^[7][8]

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. ^[7][9]

Geopolymers

More recently, fly ash has been used as a component in geopolymers, where the reactivity of the fly ash glasses is used to generate a binder comparable to a hydrated Portland cement in appearance and properties, but with dramatically reduced CO₂ emissions. Geopolymer is a term covering a class of synthetic Aluminosilicate materials with potential use in a number of areas but predominantly as a replacement for Portland cement Portland cement is the most common type of Cement in general usage in many parts of the world as it is a basic ingredient of Concrete, mortar, Stucco ^[10]

Roller compacted concrete

Another new application is using fly ash in roller compacted concrete dams. Roller-Compacted Concrete (RCC is a special blend of Concrete that has the same ingredients as conventional concrete This has been demonstrated in the Ghatghar Dam Project in India. India, officially the Republic of India (भारत गणराज्य inc-Latn Bhārat Gaṇarājya; see also other Indian languages) is a country

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. Henry Liu is a retired American Civil engineer and the president of Freight Pipeline Company (FPC A civil engineer is a person who practices Civil engineering, one of the many engineering professions A brick is a block of Ceramic material used in Masonry construction laid using mortar. 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. Air entrainment is the intentional creation of tiny Air Bubbles in Concrete. Owing to the high concentration of calcium oxide in class C fly ash, the brick can be described as "self-cementing". Calcium oxide ( CaO) commonly known as burnt lime, lime or quicklime, is a widely used Chemical compound. The manufacturing method is said to save energy, reduce mercury pollution, and costs 20% less than traditional clay brick manufacturing. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Liu intends to license his technology to manufacturers in 2008. ^[11][12]

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. Human Waste is Suffocation's debut EP released by Relapse Records. ^[13] Similarly, 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(VI) 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. Hexavalent chromium ( Cr(VI) compounds are those which contain the element Chromium in the +6 Oxidation state. These include nickel, vanadium, arsenic, beryllium, cadmium, barium, chromium, copper, molybdenum, zinc, lead, selenium, uranium, thorium, and radium. Nickel (ˈnɪkəl is a metallic Chemical element with the symbol Ni and Atomic number 28 Vanadium (vəˈneɪdiəm is a Chemical element that has the symbol V and Atomic number 23 Arsenic (ˈɑrsənɪk is a Chemical element that has the symbol As and Atomic number of 33 Beryllium (bəˈrɪliəm is a Chemical element with the symbol Be and Atomic number 4 Cadmium (ˈkædmiəm is a Chemical element with the symbol Cd and Atomic number 48 Barium (ˈbɛəriəm is a Chemical element. It has the symbol Ba, and Atomic number 56 Chromium (ˈkroʊmiəm is a Chemical element which has the symbol Cr and Atomic number 24 Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29 Molybdenum (məˈlɪbdənəm from the Greek word for the metal " Lead " is a Group 6 Chemical element with the symbol Mo Zinc (ˈzɪŋk from Zink is a Metallic Chemical element with the symbol Zn and Atomic number 30 Characteristics Lead has a dull luster and is a dense, Ductile, very soft highly Selenium (səˈliniəm is a Chemical element with the Atomic number 34 represented by the chemical symbol Se, an atomic mass of 78 Uranium (jʊˈreɪniəm is a silvery-gray Metallic Chemical element in the Thorium (ˈθɔːriəm is a Chemical element with the symbol Th and Atomic number 90 Radium (ˈreɪdiəm is a radioactive Chemical element which has the symbol Ra and Atomic number 88 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 has said in the past that coal fly ash does not need to be regulated as a hazardous waste. ^[14] However, a revised risk assessment may change the way CCW is regulated^[15] 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. The United States Geological Survey ( USGS) is a scientific agency of the United States government. ^[16]

Exposure concerns

Crystalline silica and lime are the major components of exposure concern. The Chemical compound silicon dioxide, also known as silica or silox (from the Latin " Silex " is an Oxide Lime is a general term for various naturally occurring Minerals and materials derived from them in which Carbonates Oxides and Hydroxides of In and of itself, fly ash is neither toxic or poisonous, nor is it considered hazardous except when it becomes airborne. However, the fine crystalline silica present in fly ash has been linked with lung damage, in particular silicosis. Silicosis (also known as Grinder's disease and Potter's rot) is a form of Occupational lung disease caused by inhalation of crystalline Silica OSHA allows 0. 10 mg/m³, (one ten-thousandth of a gram per cubic meter of air).

The other fly ash component of some concern is lime (CaO). This chemical reacts with water (H₂O) to form calcium hydroxide [Ca(OH)₂], giving fly ash a pH somewhere between 10 and 12, a medium to strong base. This can also cause lung damage if present in sufficient quantities.

These hazards can be minimised by controlling emissions of fly ash from bulk handling operations via closed pumping systems, and use of storage and handling equipment with approved automated spill containment equipment.

References

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)
• UK Quality Ash Association : A site promoting the many uses of fly ash in the UK
• Coal Ash Is More Radioactive than Nuclear Waste, Scientific American, 13 December 2007

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