What are the benefits and considerations of using fly ash in concrete?

Fly ash (coal combustion byproduct) replaces 15-35% of cement, providing benefits: lower heat of hydration, improved workability, reduced permeability (long-term), better sulfate resistance, and economy. Class F (low calcium) is more pozzolanic; Class C (high calcium) has some cementitious properties. Considerations include: slower early strength gain, longer curing requirements, potential carbon content affecting air entrainment, and quality variability. It's widely used in mass concrete and marine structures.