1.2—Source of fly ash
Due to the increased use of pulverized coal as fuel for electric power generation, fly ash is now available in most areas of the United States and Canada, and in many other parts of the world. Fly ash is produced as a by product of burning coals which have been crushed and ground to a fineness of
70 to 80 percent passing a 75 m (No. 200) sieve. Approximately 45,000 Gg (50 million tons) of fly ash is produced annually in the United States (American Coal Ash Association,
1992). An estimated 10-12 percent of that total is utilized in the production of concrete and concrete products.
ASTM C 618 categorizes fly ashes by chemical composition, according to the sum of the iron, aluminum, and silica content (expressed in oxide form). Class F ashes are normally produced from coals with higher heat energy, such as bituminous and anthracite coals, although some sub- bituminous and lignite coals in the western United States also produce Class F fly ash. Bituminous and anthracite coal fly ashes rarely contain more than 15 percent calcium oxide. Subbituminous fly ashes typically contain more than 20 percent calcium oxide, and have both cementitious and pozzolanic properties. There are important differences in performance of fly ashes from different sources. As a group, Class F ashes and Class C ashes generally show different
performance characteristics; however, the performance of a fly ash is not determined solely by its classification as either Class F or Class C. In general, the sulfate resistance and ability of a fly ash to mitigate the effects of alkalisilica reaction are a function of the coal sources. Strengthening characteristics of a fly ash vary widely depending on the physical and chemical properties of the ash.
Due to the increased use of pulverized coal as fuel for electric power generation, fly ash is now available in most areas of the United States and Canada, and in many other parts of the world. Fly ash is produced as a by product of burning coals which have been crushed and ground to a fineness of
70 to 80 percent passing a 75 m (No. 200) sieve. Approximately 45,000 Gg (50 million tons) of fly ash is produced annually in the United States (American Coal Ash Association,
1992). An estimated 10-12 percent of that total is utilized in the production of concrete and concrete products.
ASTM C 618 categorizes fly ashes by chemical composition, according to the sum of the iron, aluminum, and silica content (expressed in oxide form). Class F ashes are normally produced from coals with higher heat energy, such as bituminous and anthracite coals, although some sub- bituminous and lignite coals in the western United States also produce Class F fly ash. Bituminous and anthracite coal fly ashes rarely contain more than 15 percent calcium oxide. Subbituminous fly ashes typically contain more than 20 percent calcium oxide, and have both cementitious and pozzolanic properties. There are important differences in performance of fly ashes from different sources. As a group, Class F ashes and Class C ashes generally show different
performance characteristics; however, the performance of a fly ash is not determined solely by its classification as either Class F or Class C. In general, the sulfate resistance and ability of a fly ash to mitigate the effects of alkalisilica reaction are a function of the coal sources. Strengthening characteristics of a fly ash vary widely depending on the physical and chemical properties of the ash.
