Fly ash is produced as filter dust in coal-fired power plants. It is pozzolanic — meaning it reacts with calcium hydroxide from cement and additionally binds strength. Established as a supplementary material in CEM II and special cements.
With the politically mandated coal phase-out (Germany: 2030–2038), supply is becoming critically scarce. The cement industry is seeking active alternatives: calcined clay, microsilica, blast furnace slag.