Good point. In the United States there are two pozzolanic materials frequently used as "admixtures" in concrete: fly ash and silica fume. Fly ash is far more co

Good point. In the United States there are two pozzolanic materials frequently used as "admixtures" in concrete: fly ash and silica fume. Fly ash is far more common. It is a beneficial byproduct of coal combustion. Being a byproduct of another industry, it is less expensive than Portland cement. There are several reasons for adding it to mix designs. It can replace a portion of the Portland cement without sacrificing long-term strength, so it is added to reduce cost. It adds to the strength by reacting with some hydration products that do not contribute to the strength of the hardened concrete, converting those hydration products to those that do increase strength (calcium-silica hydrate for example). It reduces the permeability of the hardened concrete which improves resistance to corrosion in marine environments. It reduces the heat of hydration (which can be damaging), because it reacts less vigorously than Portland cement. There are probably other benefits, but those are the ones that come to mind.