An assessment of the nature of the components of pulverized-fuel ash (pfa) has shown a dominance of hollow, glassy spheres with a composite shell. Total of partial coating by magnetite can be present. The form, size and physical nature of the particles are shown to be the main influences affecting the workability and water demand of fresh pfa concretes. The relationship between progressive pfa particle reaction and strength gain with time has been observed for standard cured concretes up to two years old. The early pfa contribution to strength development is largely a beneficial interaction with the normal hydration of Portland cement while the pozzolanic reaction becomes significant after 28 days. The potential performance of pfa can be predicted from a consideration of its fineness, as measured by 45 μm sieve residue.