Coal based thermal power plants produce fly ash and there is a lot of it out there in many countries. We ended up using 50-50 with Portland cement on a project in Indonesia.

Where available fly ash should definitely be used, I'm UK based and the direction of travel away from coal already makes fly ash an unattractive option.
Calcinated clay is another option especially in the global south where lots of clay with a high kaolinite proportion is available. Unfortunately UK clay is not generally viable.

Understand your point - as an aside, the Indian Gov't when I was there passed a rule that if a coal fired power plant, i.e., fly ash, was within 100 km of the road construction - it had to be used for embankment construction. What was interesting is that that the contractor had to accept the "unions" which had to hand shovel it into the lorry and also out of the lorry - about keeping people employed - so the contractor paid for the workers to shovel it out but dumped it . . . funny things in some countries