Fly ash consists of inorganicInorganic, incombustible matter present in the coal that has been fused during combustion into a glassyGlass, amorphous structure. Coal can range in ash content from 2%-30%, and of this around 85% becomes fly ash. (The remaining 15% is called bottom ash and isn’t lifted up by the flue gases.)
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.
In the past, fly ash produced from coal combustion was simply taken up by flue gases and dispersed into the atmosphere. This created significant environmental concerns and health risks. These days, most power plants are required by law to reduce their fly ash emissions to less than 1% of ash produced. The remainder is collected using electrostatic precipitators or filter bags in a baghouse.
They consist mostly of silicon dioxide (SiO2), aluminum oxide (Al2O3) and iron oxide (Fe2O3), and are hence a suitable source of aluminum and silicon for geopolymers. Fly ash also contains some heavy metals.
They are also pozzolanic in nature and react with calcium hydroxide and alkali to form cementitious compounds.
Owing to its pozzolanic properties, fly ash is currently used as a replacement of Portland cement in concrete . 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 flyash also reduces the GHG signature of concrete, as the production of one tonne of Portland cement produces one tonne of CO2.
Ash used as a cement replacement must meet strict construction standards. 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 US, LOI needs to be under 6% based on ASTM-C618
It 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.
For filler material
Fly ash has also 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 also been used as a component in geopolymer mixtures. Another new application is using fly ash in roller compacted dams. This has been proven in the Ghatgar Dam Project  in India.
Producers of fly ash contend that it is harmless and has much the same properties as soil. However, according to the EPA, fly ash contains heavy metals, including nickel, vanadium, arsenic, beryllium, cadmium, barium, chromium, copper, molybdenum, zinc, lead, selenium and radium. Fly ash disposed at Chisman Creek, near Norfolk, Virginia, created surface and groundwater contamination and it was declared a Superfund site in 1983. Although fly ash producers blame situations like Chisman Creek on improper disposal, Chrisman Creek demonstrates its hazardous nature. Additionally, traces of radioactive materials are present in fly ash. Given the large quantities of fly ash that are produced, a tremendous amount of radioactive waste is generated.  This radioactivity is due to the elements in the decay chain of uranium and thorium, the radium is of great concern as 226Ra decays to form radon (222Rn) which has a half life of days and is able to form mobile daughter radioisotopes. As a result in many parts of the world a limit has been set on the level of radioactivity permitted in building materials made using fly ash. This type of radioactive material is often known by the code name NORM.
This collected ash is either sold for use in the cement/construction industry or disposed of in ash ponds or landfill. In recent times, more and more fly ash is used beneficially, although, 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 world wide. In India alone, fly ash landfill covers an area of 40,000 acres.
- FLY-ASH MIX
- PERFORMANCE RECORD, fly ash
- MATERIAL PROCESSING REQUIREMENTS, fly ash
- ENGINEERING PROPERTIES, fly ash 
- EcoSmart Concrete : A site dedicated to the use of Fly ash and other supplementary cementing materials in concrete.
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