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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 is solidified 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 cementitious compounds.

Fly ash is most commonly used as a high-performance substitute for Portland cement or as clinker for Portland cement production. Cements blended with fly ash are becoming more common. Building material applications range from grouts and masonry products to cellular concrete and roofing tiles. Many asphaltic concrete pavements contain fly ash. Geotechnical applications include soil stabilization, road base, structural fill, embankments, and mine reclamation. Fly ash also serves as filler in wood and plastic products, paints and metal castings.

Contents 1 Uses 2 Problems 3 References 4 External links

Owing to its pozzolan properties, fly ash is used as a replacement of Portland cement in concrete.^[1] It can replace up to 50% by mass of Portland cement, which can add to the final strength of the concrete and increase chemical resistance and durability. The replacement of Portland cement with fly ash also reduces the greenhouse gas signature of concrete, as the production of one tonne of Portland cement produces one tonne of CO₂.

The American Coal Ash Association's annual Coal Combustion Products Production and Use Survey details the amount of fly ash produced and used in applications, including: cement, flowable fill, raw feed as clinker, soil modification/stabilization, and agriculture. American Coal Ash Association Survey 2004]. 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, called the loss on ignition (LOI), of less than 4%. In the U.S., LOI needs to be under 6% based on ASTM-C618.

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. Flowable fill includes mixtures of Portland cement and filler material and can contain mineral admixtures, such as fly ash. Filler material usually consists of fine aggregate (in most cases, sand), but some flowable fill mixes may contain approximately equal portions of coarse and fine aggregates.

Fly ash has been used as filler material in lower-strength applications. In higher-strength applications, 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.

More recently, fly ash has been used as a component in geopolymer mixtures. Another new application is using fly ash in roller compacted concrete dams. This has been demonstrated in the Ghatghar Dam Project in India.

Fly ash like soil contains trace concentrations of the following heavy metals: nickel, vanadium, arsenic, beryllium, cadmium, barium, chromium, copper, molybdenum, zinc, lead, selenium and radium. Though these elements are found in extremely low concentrations, their mere presence has prompted some to sound the alarm.

The Environmental Protection Agency (EPA) confirms that coal fly ash need not be regulated as a hazardous waste (see Federal Register Vol. 65, No. 99, Monday, May 22, 2000, page 32214). The EPA's headquarters building in Washington, D.C. is constructed with concrete containing fly ash.

Studies by the United States Geological Survey and others conclude fly ash compares with common soils or rocks and should not be the source of alarm. More information is available in the USGS fact sheet (FS-163-97) “Radioactive Elements in Coal and Fly Ash: Abundance, Forms and Environmental Significance”.

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. These days, most power plants are required by law to reduce their fly ash emissions to less than 1 percent of ash produced. The remainder is collected using electrostatic precipitators or filter bags in a baghouse.

This collected ash either is sold for use in the cement/construction industry or disposed of in ash ponds or landfills. Recently, more fly ash is used beneficially, though more than 65% of fly ash produced from coal power stations is still disposed of. This amounts to approximately 7 million tonnes (Mt) disposed of annually in Australia, 40 Mt in the United States and hundreds of megatonnes in India and China. As a result, the disposal of fly ash is a growing concern for many countries worldwide. In India alone, fly ash landfills cover an area of 40,000 acres (160 km²).

• 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
• Pozzocrete : A manufacturer of cement made from fly ash.
• UK Quality Ash Association : A site promoting the many uses of fly ash in the UK