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Re: Fly Ash

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The ASTM for fly ash is C618,  it is in volume 4.02 which has all the concrete testing ASTMs.  The two types of fly ash come from different types of coal.  Class C has a higher calcium (lime) content although this is not one of the things required as a test in the ASTM.  The test is actually on the other constituents (silicon dioxide / aluminum oxide / iron oxide),.

Class F is a pozzolan,  meaning that by itself it is not cementitious but that it reacts with calcium hydroxide to become cementitious.  Class C is a pozzolan, but also has cementitious properties.

The result is that Class C will help increase early age strength,  but increases the early heat rise.  Class F will hurt early strength gain but will reduce problems with excessive heat of hydration.

The biggest difference is probably with respect to usage for ASR (alkali  silica reaction) mitigation.  Only Class F ash is effective in ASR mitigation.