Every buyer new to fly ash asks the same question within the first five minutes: Class F or Class C? The
honest answer is that they are different materials that happen to share a name, and choosing between them is a
durability decision long before it is a price decision.
Where the difference comes from
Fly ash chemistry follows the coal. Bituminous and sub-bituminous coals burn to a low-calcium, siliceous ash
that ASTM C618 calls Class F. Lignite and some sub-bituminous coals produce an ash carrying 20 percent or more
calcium oxide, which the same standard calls Class C. That single difference in calcium changes everything
downstream.
How each one behaves in concrete
| Property | Class F | Class C |
|---|
| CaO content | Below 10 percent | 20 percent and above |
| Reaction | Pozzolanic only | Pozzolanic and self-cementing |
| Early strength | Slower than OPC | Comparable to OPC |
| Sulphate resistance | Excellent | Poor to moderate |
| Alkali-silica mitigation | Very effective | Limited |
| Typical replacement | 15 to 35 percent | 15 to 25 percent |
Choose Class F when
- The structure is marine, coastal or founded in sulphate-bearing ground
- The aggregate is potentially alkali-reactive
- You are pouring thick sections and need the heat of hydration down
- Long-term durability matters more than 7-day strength
Choose Class C when
- Early strength governs the programme - precast, slipform, fast formwork cycles
- You are stabilising soil or placing flowable fill and want the ash to act as a binder
- Sulphate exposure is genuinely not a risk on the project
The mistake that actually costs money
Specifying Class C for a marine structure because it was cheaper or closer to hand. High-calcium ash forms
more calcium aluminate hydrate, and that is exactly what sulphate attacks. The damage does not show up on the
28-day cube. It shows up in year six.
If the specification does not say which class, ask what the exposure condition is. The exposure
condition answers the question.
Not sure which way to go? Send us the mix design and the exposure class and we will send back a sample of the
grade that fits, with the test report attached.