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Utilization of Recycled Materials in Illinois Highway Construction

Fly Ash

Fly ash is a by-product produced in large quantities during the day to day operations of coal-fired power plants. In general, the coal source is pulverized and blown into a burning chamber where it ignites to heat boiler tubes. Heavier particles of ash (bottom ash or slag) fall to the bottom of the burning chamber, while the lighter particles (fly ash) remain suspended in the flue gases. Before leaving the stack, these fly ash particles are removed by electrostatic precipitators, baghouses, or other such dust collectors/air pollution control devices (4).

Fly ash is divided into two classes-Classes F and C-based upon the type of coal source. Class F fly ash is produced by burning anthracite or bituminous coal; whereas, Class C fly ash is produced from lignite or sub-bituminous coal (1).
Physical Properties:
Fly ash is a fine, powdery silt-sized amorphous residue. Varying amounts of carbon affect the color of fly ash. Gray to black represents increasing percentages of carbon, while tan coloring is indicative of lime and/or calcium content. Fly ash may exhibit pozzolanic properties and, in certain types, cementitious properties (1, 4).
Engineering Value:
In PCC, Class F fly ash has pozzolanic properties when introduced to water, whereas Class C fly ash is naturally cememtitious due to its high amount of calcium oxide. Fly ash can be added to PCC to modify pH, change the hydration process (fly ash retards hydration thus lowering heat of hydration), reduce water demand, and reduce permeability (1, 4).
Present Application:
Dry fly ash can be used as an inert fill material or supplementary cemetitious material to improve cohesion and stability of bituminous concrete binder and soil embankments. In Illinois, fly ash is used as a fine aggregate or supplementary cementitious material in PCC; however, the Department limits the use of Class F to no more than 15 percent by weight, and Class C to no more than 20 percent by weight. In combination with sand, fly ash may be a supplement or substitute for cement to make a flowable fill, or as grout for concrete pavement sub-sealing (1, 3, 4).
Quantity Used:
95,570 tons (2001 MISTIC estimate).
Economic Impact:
The use of fly ash as a supplementary cementitious material cost the Department approximately $2,630,000, aided in the reduction of landfill space need, and reduced emissions and fuel consumption required for cement production.
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John Bukowski
Office of Asset Management, Pavements, and Construction
E-mail John

Updated: 02/20/2015

United States Department of Transportation - Federal Highway Administration