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. 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 concrete offers a distinct architectural benefit with improved textural consistency and sharper detail. Fly Ash is also known as Coal Ash, Pulverized Flue Ash, Pozzolona.
What is Fly Ash?
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.
Define the features of fly ash?
â¢ Spherical shape : Fly ash particles are almost totally spherical in shape, allowing them to flow and blend freely in mixtures.
â¢ Ball bearing effect :The "ball-bearing" effect of fly ash particles creates a lubricating action when concrete is in its plastic state.
â¢ Higher Strength : Fly ash continues to combine with free lime, increasing structural strength over time.
â¢ 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.
â¢ Reduced Sulfate Attack : Fly ash ties up free lime that can combine with sulfate to create destructive expansion.
â¢ Reduced Efflorescence : Fly ash chemically binds free lime and salts that can create efflorescence and dense concrete holds efflorescence producing compounds on the inside.
â¢ 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.
â¢ 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.
â¢ Improved Finishing : Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity.
â¢ 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.
â¢ Reduced Segregation : Improved cohesiveness of fly ash concrete reduces segregation that can lead to rock pockets and blemishes.
â¢ Reduced Slump Loss : More dependable concrete allows for greater working time, especially in hot weather.
What are the specifications of fly ash?
ASTM (West Conshohocken, PA USA)
â¢ ASTM C 618: Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete
â¢ ASTM C 311: Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use as a Mineral Admixture in Portland-Cement Concrete
â¢ ASTM D 5239: Standard Practice for Characterizing Fly Ash for Use in Soil Stabilization
â¢ ASTM E 850: Standard Practice for Use of Inorganic Process Wastes as Structural Fill
â¢ ASTM E 1861: Standard Guide for Use of Coal Combustion By-products in Structural Fills
â¢ ASTM D 5370: Standard Specification for Pozzolanic Blended Materials in Construction Applications
â¢ ASTM C 1240: Standard Specification for Silica Fume for Use in Hydraulic-Cement Concrete and Mortar
IDOT (Illinois Dept. of Trans. Springfield, IL USA)
â¢ 306.01: Special Provision for Fly Ash Modified Soils
â¢ 308.01: Special Provision for Fly Ash Stabilized Soil Mixture Subbase
â¢ Special Provision for Cement-Fly Ash-Aggregate Mixture (CFAM) Base Course
â¢ Special Provision for Pozzolanic Base Course, Type A
â¢ Special Provision for Use of Fly Ash in P.C.C. Pavement, Base Course, Base Course Widening
â¢ AASHTO Standard Specifications for Transportation Materials and Methods of Sampling and Testing
â¢ BS 3892 Part 1 Fly Ash standard; Part 2 Fly Ash for Use as a Type II Addition
â¢ BS EN 450 European Standard for Fly Ash
â¢ BS EN 197 European Standard for Multiple Binders ( fly ash, cement, silica fume) Allowed in Concrete
â¢ Portland Cement -AS 3972-1991,
â¢ Fly ash -AS 3582.1-1991, and
â¢ Ground Granulated Blast Furnace Slag -AS 3582.2.
â¢ A.S. 1129 Fly Ash Specification