Used to work for a company that used coal. Too much ash for the concrete industry. We landfilled it.
As we live in “coal country” we keep up with these kinds of issues. We also have lots of “back roads” that have been impossible to keep passable. About 5 years ago they laid a road that I have traveled for years. It was an experimental road using coal ash mixed with whatever else they used. That road has not had the first crack, while roads in the same area are pot holed so awful you can hardly navigate them. So, I pray that they use the coal ash mixture to pave the rest of them. Coal ash also makes a good foundation for roads and buildings.