Following almost two years of consensus building discussions, the definition of what constitutes Class C and Class F fly ashes is now officially changed in the United States.  The American Coal Ash Association during their recent Winter meeting reported the likelyhood of changes to come.  

The ASTM C618-19 Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete now uses calcium oxide instead of the sum of silica, alumina and iron oxide to classify fly ash. Henceforth, fly ash with less than 18 percent CaO will be categorized as Class F and more than 18 percent will be categorized as a Class C. There is still a requirement for a minimum of 50 percent sum of oxides for both C and F ashes.

Publication of the new standard was delayed while the change was harmonized within the related AASHTO M295 specification. As part of the C618-M295 harmonization efforts, AASHTO is now discussing deleting the available alkali and the effectiveness in mitigating ASR (C441) tests from the optional requirements in M295. Also, under discussion is harmonizing the lower LOI limit of 5 percent in AASHTO and the ASTM limit of 6 percent. A group volunteered to gather compelling data showing not only why the LOI limit of 6 percent should be kept, but also that LOI has become a rather irrelevant number in light of emission control technologies implemented in power plants across the country, new advances in measuring adsorption capacity of fly ash, and other factors.

Within Australian Standards AS 23582.1:2016 we don't classifiy our fly ashes as Class F or C. having said that Asutralian fly ashes under this criteria would be predominately Class F. AS3582.1 C4.2 offers commentary to this effect.