ASTM C618
Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete
Fly ash and natural pozzolans used as supplementary cementitious materials, including the Class F and Class C distinction.
Who needs it
Batch plants, mix designers, specifiers pursuing durability or carbon goals, and contractors trying to understand why a mix finished differently than last year's.
What the test involves
It sets chemical and physical requirements for fly ash and natural pozzolans and sorts them into classes based on composition, with Class F typically from harder coals and Class C typically higher in calcium and mildly self-cementing. Qualifying materials react with the byproducts of cement hydration to build additional strength over time and to reduce permeability. The specification also covers uniformity, because a pozzolan that varies load to load makes a mix unpredictable.
Common misconceptions
- Fly ash is not filler. It is chemically reactive and contributes real long-term strength, which is why fly-ash mixes often keep gaining strength well past the age at which they are tested.
- Class C and Class F are not grades of quality. They are different materials with different behavior in sulfate exposure and different effects on set time.
- Fly ash generally slows early strength gain and can extend finishing time, especially in cool weather — a scheduling issue, not a defect.
Official source
We summarize scope and purpose only. The full text belongs to the issuing body — read it at the official source. ASTM International