Fly ash works almost like cement, at least that from the Wyoming coal fields does. The bottom (clinker) ash works great as a base for paving roads.
Our local town, thirty years ago, decided to repave and curb and gutter all the streets in town, using bottom ash for a base.
During the process, one summer some local woman came down with a summer cold and decided the ash was the problem. She raised enough hell that the city stopped and no longer uses ash. Today the paved roads, curbs and gutters are non existent, all because of ONE WOMAN’S COMPLAINTS.
I am retired in Southeast Asia and masonry construction is predominate. The ash block is light, can be cut with a hand saw and is easy to shape. They use it extensively for non-load bearing partitions within a home. Paint or plaster over it and you have the finished product. The builders wouldn't be using it if it wasn't cost effective.
I've never seen any big piles of bottom ash around the coal fired electric generating plants on the coast so I would expect they are putting it to good use somewhere? They don't waste much around these parts.