Fly ash for HPC is classified into two classes. Class F Gy ash is normally produced from burning anthracite or bituminous coal and has pozzolanic properties, but little or no cementitious properties. Class C fly ash is normally produced from burning lignite or subbituminous coal, and in addition to having pozzolanic properties, has some autogenous cementitious properties.

Methods for sampling and testing can be found in ASTM C 311. Although this specification permits a higher loss on ignition, an ignition loss of 3% or less is desirable.

Variations in physical or chemical properties of mineral admixtures, although within the tolerances of these specifications, may cause appreciable variations in properties of HPC. Such variations can be minimized by appropriate testing of shipments and increasing the frequency of sampling. ACI 212.2R provides guidelines for the use of admixtures in concrete. It is extremely important that mineral admixtures be tested for acceptance and uniformity and carefully investigated for strength-producing properties and compatibility with the other materials in the HPC mixture before they are used in the work.

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