Coal fly ash: a retrospective and future look

Abstract To accommodate the many new subbituminous fly ashes, the American Society for Testing and Materials (ASTM) established two classes of fly ash, Class F from bituminous coal and Class C from subbituminous and lignite coal. However, the wide variability of the Class C ashes from location to location led to extensive X-ray diffraction studies, with the conclusion that the mineralogy was more important than the chemical analysis and that the classification by type of coal was erroneous. A need exists to produce a performance-oriented standard rather than a prescriptive one. Recently, the installation of NOx burners has increased the carbon content of fly ash and, consequently, the instability of entrained air in concrete. This paper presents a retrospective and future look on the use of fly ash for concrete production in the United States and makes suggestions for future research.