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In the United States, a new generation of coal-fired power plants was built during the late 1960s and 1970s, at least par- tially in response to dramatically increased oil prices. These modern power plants, utilizing efficient coal mills and state- of-the-art pyroprocessing technology, produced finer fly ashes with a lower carbon content than those previously available. In addition, fly ash containing higher levels of cal- cium became available due to the use of new coal sources (usually subbituminous and lignitic). Concurrent with this increased availability of fly ash, extensive research in North America and elsewhere has led to better understanding of the chemical reactions involved when fly ash is used in concrete, and improved technology in the use of fly ash in the concrete industry. Fly ash is now used in concrete for many reasons, including reduced cost, improvements in workability of fresh concrete, reduction in temperature rise during initial hydration, improved resistance to sulfates, reduced expan- sion due to alkali-silica reaction, and contributions to the du- rability and strength of hardened concrete.
