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Fly Ash

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Since there have been questions in the past about fly ash,  I thought some people must find this interesting.  Then again, some might not.  

For those looking for more information on fly ash,  the Coal Ash Association has a booklet called 'Fly Ash Facts for Highway Engineers".  Since  they got $150,000 from FHWA to write the booklet, they only charge five dollars for it, so it might be a good investment.   

In reading that publication,  however,  one might come to the conclusion that either Highway Engineers have an IQ of  50,  or the person who wrote it has an IQ of 50.


Fly ash, the material most commonly used mineral admixture, is a by-product of burning coal to generate electricity.  As the organic material burns off, the mineral impurities in the coal fuse and are carried away from the combustion chamber by the exhaust gases.  The fused materials solidify into spherical particles that are then filtered out from the exhaust stack as a fine powder.  The particles are essentially a silicate glass, consisting of silica, alumina, iron, and calcium.

Fly ash is sometimes used as a replacement for part of the fine aggregate, but it is typically used as a partial replacement for portland cement.  Fly ash can be processed into a blended cement in accordance with ASTM C 595, "Standard Specification for Blended Hydraulic Cements" but is usually added to the concrete at the batch plant as a separate material.  Fly ash is typically used as 15 to 25% by weight of the total cementitious materials.  Concrete has been successfully placed with up to 80% fly ash, but these mixtures require specific fly ashes that may not be readily available.

Fly ash varies in composition and carbon content, depending on the type of coal it came from, and the way it was processed. In general, all fly ashes will increase the long-term strength and decrease the permeability of concrete.  In addition, all fly ashes will tend to reduce the amount of water required for a given slump.  This results in concrete with improved workability and pumpability and can reduce the amount of drying shrinkage.  Properties such as strength development depend on the composition of the fly ash, however.  

ASTM C 618, "Standard Specification for Coal Fly Ash and Raw Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete," defines fly ash as either Class F, Class C, or Class N, according to its composition and method of production. Class F fly ash is a by-product of burning bituminous coal, which is generally found in the eastern portion of the United States. Per ASTM C 618, it must be at least 70% silica, aluminum oxide, or iron oxide.  It typically has less than 10% calcium oxide. Class F fly ash is a pozzolan, which means that it has little or no cementitious value by itself, but it will react with calcium hydroxide to form compounds having cementitious properties. When used as a concrete additive, fly ash reacts with calcium hydroxide produced during cement hydration. By tieing up a certain percentage of the calcium hydroxide (an alkali), Class F fly ash generally reduces the potential for alkali-silica reactions.  Class F fly ash also increases the sulfate resistance of the concrete.  If Class F fly ash is used as a replacement for the cement rather than the aggregate, the concrete will usually develop strength more slowly.  In hot weather, this can be advantageous as it results in a lower heat of hydration. In cold weather, however, it can significantly delay finishing.

Class C fly ash is a by-product of burning sub-bituminous coal and lignite. It is lower in silica, alumina, and iron than Class F fly ash, but higher in calcium. Class C fly ash is cementitious by itself as well as pozzolanic.  Concrete with Class C fly ash tends to develop strength faster than concrete with Class F fly ash but as a result, does not have a significantly lower heat of hydration than concrete with straight cement.  Class C fly ash is generally not effective in reducing the potential for alkali-silica reactions.

In recent years, Environmental Protection Agency regulations intended to reduce air pollutants such as nitrogen oxides and sulfur dioxide have resulted in an increase in the unburned carbon content of fly ash.  Carbon content is measured by Loss on Ignition (LOI).   ASTM 618 limits the LOI of both Class C and Class F fly ashes to 6%; however, specifications for concrete that will be air-entrained often require that the fly ash have a LOI of less than 1%.  Most fly ashes must be pretreated to reduce the carbon content to an acceptable level if the concrete needs to be air-entrained.  This is usually not a concern with slab on ground construction, because the concrete is typically not air-entrained.

Class N pozzolans are natural materials such as diatomaceous earth, clays, shales, and volcanic tuffs.  Most must be calcined (heated to 1200 to 1800 F) to activate the aluminosilicate component.  Some materials are further processed to enhance certain properties.  The most commonly used materials are calcined shale and calcined kaolinite clay (metakaolin.)  Class N pozzolans are similar to Class F fly ashes in composition but due to the processing, some are very reactive.

Gail Kelley