These micron-sized earth elements consist primarily of silica, alumina and iron. When mixed with lime and water the fly ash forms a cementitious compound with properties very similar to that of Portland cement. Because of this similarity, fly ash can be used to replace a portion of cement in the concrete, providing some distinct quality advantages. The concrete is denser resulting in a tighter, smoother surface with less bleeding.

Fly ash closely resembles volcanic ashes used in production of the earliest known hydraulic cements about 2,300 years ago. Those cements were made near the small Italian town of Pozzuoli - which later gave its name to the term "pozzolan."

A pozzolan is a siliceous or siliceous / aluminous material that, when mixed with lime and water, forms a cementitious compound. Fly ash is the best known, and one of the most commonly used, pozzolans in the world.

Instead of volcanoes, today's fly ash comes primarily from coal-fired electricity generating power plants. These power plants grind coal to a powder fineness before it is burned. Fly ash - the mineral residue produced by burning coal - is captured from the power plant's exhaust gases and collected for use.

Fly ash is a fine, glass powder recovered from the gases of burning coal during the production of electricity. These micron-sized earth elements consist primarily of silica, alumina and iron.

The difference between fly ash and portland cement becomes apparent under a microscope. Fly ash particles are almost totally spherical in shape, allowing them to flow and blend freely in mixtures. That capability is one of the properties making fly ash a desirable admixture for concrete

• Spherical shape : Fly ash particles are almost totally spherical in shape, allowing them to flow and blend freely in mixtures.
• Ball bearing effect :The "ball-bearing" effect of fly ash particles creates a lubricating action when concrete is in its plastic state.
• Higher Strength : Fly ash continues to combine with free lime, increasing structural strength over time.
• Decreased Permeability : Increased density and long term pozzolanic action of fly ash, which ties up free lime, results in fewer bleed channels and decreases permeability
  Increased Durability. Dense fly ash concrete helps keep aggressive compounds on the surface, where destructive action is lessened. Fly ash concrete is also more resistant to attack by sulfate, mild acid, soft (lime hungry) water, and seawater.
• Reduced Sulfate Attack : Fly ash ties up free lime that can combine with sulfate to create destructive expansion.
• Reduced Efflorescence : Fly ash chemically binds free lime and salts that can create efflorescence and dense concrete holds efflorescence producing compounds on the inside.
• Reduced Shrinkage : The largest contributor to drying shrinkage is water content. The lubricating action of fly ash reduces water content and drying shrinkage.
  Reduced Heat of Hydration :The pozzolanic reaction between fly ash and lime generates less heat, resulting in reduced thermal cracking when fly ash is used to replace portland cement.
  Reduced Alkali Silica Reactivity : Fly ash combines with alkalis from cement that might otherwise combine with silica from aggregates, causing destructive expansion.
• Workability: Concrete is easier to place with less effort, responding better to vibration to fill forms more completely.
  Ease of Pumping. Pumping requires less energy and longer pumping distances are possible.
• Improved Finishing : Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity.
• Reduced Bleeding : Fewer bleed channels decreases porosity and chemical attack. Bleed streaking is reduced for architectural finishes. Improved paste to aggregate contact results in enhanced bond strengths.
• Reduced Segregation : Improved cohesiveness of fly ash concrete reduces segregation that can lead to rock pockets and blemishes.
• Reduced Slump Loss : More dependable concrete allows for greater working time, especially in hot weather.