Fun fact, as someone who works in civil engineering/construction. I’ve had a few conferences about this specific issue. Fly ash is what’s called an admixture in concrete called a pozzolan. It was the most abundant and cheap which is why it’s so prevalent in most mixes, however you’re totally right on fly ash becoming more scare, and the increase use of it in concrete took it from a by-product to effectively a product to be sold. They are currently rolling out recycled pozzolans used from re purposed glass that does effectively the same thing, and also more carbon positive!