Fly ash
A fine byproduct of burning coal, used to replace a portion of cement. It improves workability and long-term durability while slowing early strength gain.
What does fly ash mean?
Fly ash is sorted into classes reflecting its composition, with lower-calcium ash generally offering better sulfate and alkali-reaction resistance and higher-calcium ash contributing more early strength. Its spherical particles act like ball bearings, which is why fly ash mixes pump and finish smoothly. Supply has become unreliable as coal generation declines, and some producers now reclaim ash from landfills.
In practice
Fly ash mixes often stay open longer, so a crew used to straight cement can be caught waiting on a slab that is not ready to float.