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Fly ash

Fly ash is the residue from the combustion of coal. In the past it was generally released into the atmosphere, out of 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. 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).

Additional recommended knowledge


Fly Ash Exposure Concerns

Crystalline silica and lime are the major components of exposure concern. Understanding these concerns will help communities make informed decisions concerning the production, transport, storage and use of this material in their environment.

In and of itself, fly ash is neither toxic or poisonous, nor is it considered hazardous EXCEPT when it becomes airborne. Released into the atmosphere, fly ash can be extremely dangerous to humans as well as all other animals. To understand how and why this happens we need to more closely examine these particles of concern.

Crystalline silica

Common natural sand, or "blow sand", with which we are all familiar, is made up of amorphous silica which, if examined under a microscope, would look like a potato, rounded, smoothed and uneven. However, crystalline silica, examined under a microscope, has a regular, repeating shape that is very sharp, pointed and hard. For example, quartz crystals are also composed of silicon dioxide and they are extremely sharp and hard, just behind diamond and corundum in hardness. When you breath in crystalline silica each particle will cut the tiny air sacs in your lungs, forming scar tissue which reduces lung capacity. How small must something be to be respirable? At 7 microns, particles can be breathed into the lungs. To put this into perspective, one micron is one one-thousandth of a millimeter, which is about the size of a pencil point cut into a thousand slices. Seven of those tiny slices together (or smaller) is a respirable size piece. About 20% to 40% of fly ash is in this 7 micron range, including all of the crystalline silica.

How much is safe? OSHA and the EPA list allowable crystalline silica in the atmosphere as 0.10 mg/m3. That is one tenth of one one-thousandth of a gram (about the size of a raisin cut into ten thousand pieces) of crystalline silica in one cubic meter of air . What are the consequences of inhaling this crystalline silica dust? As mentioned above, it will cut and scar the delicate tissue of the lungs and air passages, which could lead to bronchitis, silicosis and lung cancer. How long would this take? Allowable limits are based upon healthy adults for the most part, where 5 to 20 years are required. But children, asthmatics of any age, allergy sufferers and the elderly, all of whom have reduced lung capacity, can be affected in much shorter periods of time. In addition to the lung exposure hazards, crystalline silica is irritating to the skin, causing contact dermatitis by abrasion, and it is also an extreme eye irritant.

Calcium oxide

The other fly ash ingredient of concern is Lime (calcium oxide - 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. So, when you breathe in calcium oxide (it's dust is well under 7 microns) it combines with the moisture in your lungs and air passages and causes a burning sensation as it dissolves the sensitive tissues in these regions. As with the crystalline silica, the same groups - children, asthmatics, allergy sufferers and the elderly are most at risk.

Fly ash is not your normal dust. When it is airborne, it is a significant health hazard to those who become exposed. OSHA lists safety glasses with side shields, respirators with Standards Level marked, and gloves as well as clothing that covers all skin for workers in an airborne fly ash environment. Communities cannot provide this for its residents. Therefore, if a community decides to include fly ash based industries in its economic plan, then community leadership can require that fly ash bulk handling operations be accomplished with closed pumping systems and that storage and handling equipment have approved automated spill containment equipment. In the event of a major release of fly ash into the atmosphere, warning equipment for alerting all affected residents must be in place.

As to the use of fly ash as a stiffener or strengthener for concrete products and other applications, users should be aware that concrete, concrete block, bricks etc. that contain fly ash are going to be just as hazardous in the future as the raw dust is today, when they are degraded by weathering or destroyed during demolition, and the crystalline silica is released into the atmosphere.

(From a talk by Peter Gillespie, Ph.D. a chemist (retired) with the World Health Organization (WHO) specializing in toxic materials, given to the Big Water, Utah, Town Council on June 20, 2003.)

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. 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), 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), 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.

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.[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).

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.[2][3]

Portland cement

Owing to its pozzolan properties, fly ash is used as a replacement of Portland cement in concrete.[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.[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 fly ash’s spherical shape, 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 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.[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 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.[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]


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.


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 class C 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. [10][11]

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.[12] 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. [13] 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.[14]


  1. ^ a b American Coal Ash Association. "CCP Production and Use Survey".
  2. ^ a b U.S. Environmental Protection Agency. "Using Coal Ash in Highway Construction - A Guide to Benefits and Impacts".
  3. ^ U.S. Federal Highway Administration. Fly Ash.
  4. ^ Scott, Allan N .; Thomas, Michael D. A. (Jan/Feb 2007). "Evaluation of Fly Ash From Co-Combustion of Coal and Petroleum Coke for Use in Concrete". ACI Materials Journal 104 (1): 62-70. American Concrete Institute. Retrieved on 2007-02-23.
  5. ^ Halstead, W. (October 1986), " ", National Cooperative Highway Research Project 127
  6. ^ Moore, David. The Roman Pantheon: The Triumph of Concrete. 
  7. ^ a b c d U.S. Federal Highway Administration. Fly Ash Facts for Highway Engineers.
  8. ^ Hennis, K. W. & Frishette, C. W. (1993), ,
  9. ^ Zimmer, F. V. (1970), ,
  10. ^ Popular Science Magazine, INVENTION AWARDS : A Green Brick, May 2007
  11. ^ National Science Foundation, Press Release 07-058, "Follow the 'Green' Brick Road?", May 22, 2007
  12. ^ [1] N-Viro International
  13. ^ Federal Register. “Notice of Regulatory Determination on Wastes From the Combustion of Fossil Fuels”.
  14. ^ U.S. Geological Survey. "Radioactive Elements in Coal and Fly Ash: Abundance, Forms, and Environmental Significance".
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fly_ash". A list of authors is available in Wikipedia.
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