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Types of Fly Ash


Fly Ash

Fly Ash is a byproduct of coal-fired electric generating plants. The coal is pulverized and blown into burning chambers for immediate combustion. Heavier ash particles (bottom ash or slag) fall to the bottom of the burning chamber and the lighter ash particles (Fly Ash) fly out with the exhaust gas, thus the term Fly Ash. Before leaving the stack, these Fly Ash particles are removed and collected by electrostatic precipitators, bag houses or other methods.

Fly Ash is a pozzolan, meaning it is a siliceous and aluminous material that, in the presence of moisture, will combine with the lime liberated during the hydration of cement to form cementitious materials.

Nearly fifty million tons of Fly Ash are produced annually in the United States with over ten million tons used annually in engineering applications. This tonnage is used primarily for ready-mix concrete.

Fly Ash is used in the following applications, in addition to:

  • Ready-mix concrete
  • Concrete block & pipe
  • Cement manufacture
  • Mineral filler for asphalt roads
  • Soil stabilization
  • Structural fill
  • Waste stabilization/treatment
  • Specialty applications

Use of Fly Ash is approved by material specifications under AASHTO and ASTM and State Department of Transportation Agencies for its intended engineering application. In addition, the use of coal fly ash is approved and recommended by the U.S. EPA.

Bottom Ash

Bottom Ash is formed in the firing chambers of boilers at electric utility power plants in the U.S. and around the world. Bottom ash is particles of ash that have been fused together at high temperature to form slag like material for use as lightweight and conventional aggregate.

The bottom ash aggregate may be graded to meet the requirements of ASTM C 331 Specification for lightweight aggregate for use in concrete Masonry units. This ash aggregate usually has a unit weight of 60 - 65 pounds per cubic foot. This aggregate may also be blended with other lightweight aggregates to achieve greater quantities.

Ash aggregate may also be used as conventional aggregates for use in asphalt, concrete, and road construction.

Bottom ash may also be used as raw material in the manufacture of portland cement and as fill material.

Power plant bottom ash is very versatile material and may be produced from either class F or C ash.

Synthetic Gypsum

Synthetic gypsum, also called flue-gas-desulfurization (FGD) gypsum, is produced through a chemical reaction in the scrubbers that use pulverized natural limestone to remove sulfur from the flue gases of coal-fired power plants.

Chemical scrubbers remove sulfur dioxide (SO2) from power plant emissions by passing the flue gases through a slurry of limestone or calcium carbonate. The SO2 in the flue gases reacts with the calcium carbonate to produce calcium sulfite and this calcium sulfite is converted into gypsum by further oxidizing.

The synthetic gypsum produced from power plant scrubbers is nearly identical to virgin natural gypsum that is extracted from gypsum mines.

Synthetic Gypsum is used in the following applications:

  • Wallboard
  • Plasters
  • Cement Production
    • Grinding aid for cement clinker
    • Slows setting time for concrete mix
  • Agriculture
    • More extensive roots for water access
    • Improves water infiltration
    • Prevents crusting and soil erosion
    • Nutrients (calcium, sulfur) for deficient soils
    • Binds phosphorus from poultry waste

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