Fly ash@ a by-product from the combustion of pulverized coal@ is widely used as a cementitious and pozzolanic ingredient in hydraulic cement concrete. Because it improves many desirable properties of concrete@ it is introduced either as a separately batched material (as in ASTM C 61 8@ Class For C) or as a component of blended cement (ASTM C 595 or C 1157).This report describes the use of fly ash in concrete and lists references Concerning the characterization of fly ash@ its properties@ and its effects on concrete. Guidance is provided concerning specifications@ quality assurance and quality control of fly ash@ and concrete produced using fly ash.According to ACI 116R@ fly ash is ??the finely divided residue that results from the combustion of ground or powdered coal and that is transported by flue gases from the combustion zone to the particle removal system.?? ACI 116R defines ??pozzolan?? as ??a siliceous or siliceous and aluminous material that in itself possesses little or no cementitious value but that will@ in finely divided form and in the presence of moisture@ chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties; there are both natural and artificial pozzolans.?? Fly ash possesses pozzolanic properties similar to the naturally occurring pozzolans of volcanic or sedimentary origin found in many parts of the world. About 2000 years ago@ the Romans mixed volcanic ash with lime@ aggregate@ and water to produce mortar and concrete (Vitruvius 1960). Similarly@ fly ash is mixed with portland cement (which releases lime during hydration)@ aggregate@ and water to produce mortar and concrete.All fly ashes exhibit pozzolanic properties to some extent; however@ some fly ashes display varying degrees of cementitious value without the addition of calcium hydroxide or hydraulic cement. The cementitious nature of these fly ashes is attributed to reactive constituents that reside within the fly ash@ such as crystalline@ calcium aluminate phases@ and a more highly substituted@ and therefore@ potentially reactive glass phase. Fly ash in concrete reacts with the hydrating hydraulic cement in the following ways:1. Solutions of calcium and alkali hydroxide@ which are released into the pore structure of the paste@ combine with the pozzolanic particles of fly ash@ forming a cementing medium; and2. Heat generated by hydration of hydraulic cement helps initiate the pozzolanic reaction and contributes to the rate of the reaction. When concrete containing fly ash is properly cured@ fly-ash reaction products partially fill in the spaces originally occupied by mixing water that were not filled by the hydration products of the cement@ thus lowering the concrete permeability to water and aggressive chemicals (Manmohan and Mehta 1981). The slower reaction rate of fly ash@ when compared to hydraulic cement@ limits the amount of early heat generation and the detrimental early temperature rise in massive structures. Concrete proportioned with fly ash can have properties that are not achievable through the use of hydraulic cement alone.Fly ash from coal-burning electric power plants became readily available in the 1930s. In the U.S.@ the study of fly ash for use in hydraulic cement concrete began at about that time. In 1937@ results of research on concrete containing fly ash were published (Davis et al. 1937). This work served as the foundation for early specifications@ methods of testing@ and use of fly ash.Initially@ fly ash was used as a partial mass or volume replacement of hydraulic cement@ typically the most expensive manufactured component of concrete. As fly ash usage increased@ researchers recognized that fly ash could impart beneficial properties to concrete. In subsequent research@ Davis and colleagues studied the reactivity of fly ash with calcium and alkali hydroxides in portland-cement paste and the ability of fly ash to act as a preventive measure against deleterious alkaii-aggregate reactions. Research (Dunstan 1976@ 1980; Tikalsky@ Carrasquillo@ and Snow 1992; Tikalsky and Carrasquillo 1993) has shown that fly ash often improves the concrete's resistance to deterioration from sulfates. Fly ash also increases the workability of fresh concrete and reduces the peak temperature of hydration in mass concrete. The beneficial aspects of fly ash were especially notable in the construction of large concrete dams (Mielenz 1983). Some major projects@ including the Thames Barrage in the UK and the Upper Stillwater Dam in the U.S.@ incorporated 30 to 75% mass replacement of hydraulic cement by fly ash to reduce heat generation and decrease permeability.In the U.S.@ a new generation of coal-fired power plants was built during the late 1960s and 1970s@ at least partially in response to dramatically increased oil prices. These power plants@ using efficient coal mills and state-of-the-art pyroprocessing technology@ produce finer fly ashes with a lower carbon content. In addition@ fly ash containing high levels of calcium oxide became available due to the use of western U.S. coal sources (typically sub-bituminous and lignitic). Concurrent with this increased availability of fly ash@ extensive research has led to a better understanding of the chemical reactions involved when fly ash is used in concrete. Enhanced economics and improved technologies (material- and mechanical-based) have led to a greater use of fly ash@ principally in the ready-mix concrete industry. Fly ash is now used in concrete for many reasons@ including improvements in workability of fresh concrete@ reduction in temperature rise during initial hydration@ improved resistance to sulfates@ reduced expansion due to alkali-silica reaction@ and contnbutions to the durability and strength of hardened concrete.