A test procedure based on heat of hydration was developed to assess more accurately the reactivity and behaviour of fly ash for utilization. The technique uses a Dewar flask or a modified oxygen bomb calorimeter to determine the temperature change and rate of change after the addition of water to fly ash. X-ray diffraction is performed on the hydrated material after the hydration test to follow mineralogical changes due to the hydration process. A study of coal ash samples is under way to determine whether correlations exist between the temperature change and results from empirical tests, such as those of the ASTM, used to determine properties of ash for engineering applications. A protocol has been developed for an improved classification scheme for coal ash.