Fly ash from the precipitators of coal fired power stations was the raw material for the company Pozzolanic Ltd.
Some power stations were better than others as sources. Fiddlers Ferry, Ironbridge, West Burton and Longannet were regular sources. These produced a pale, low residual carbon ash with small particle size. 50% substitution of the cement content was used in the concrete of the Thames Barrier as it made a dense concrete which was slow to cure without the vast damaging heat release of straight ordinary Portland cement during curing. The majority of the ash was transported from West Burton by dedication trains.
Pozzolan was also used extensively to combat "concrete cancer" as Alkali Silcate reaction became known.
Fly ash is a fascinating subject if you research it.