Pozzolan
A siliceous material that has no cementing value alone but reacts with lime released during cement hydration to form additional binder.
What does pozzolan mean?
The category includes fly ash, silica fume, calcined clays, and natural volcanic materials, and the concept predates portland cement by two millennia. Pozzolanic reaction consumes the weakest and most soluble product of cement hydration, which is why pozzolan-bearing concrete is typically denser and more chemically resistant. The reaction is slower than cement hydration, which is the source of the delayed strength gain associated with these mixes.
In practice
When a spec calls for reduced permeability or alkali-reactivity mitigation, a pozzolan is usually the mechanism being invoked.