There are probably a billion or two tonnes of coal ash that is sitting somewhere.
Coal ash is one of the largest types of industrial waste generated in the United States. According to the American Coal Ash Association’s Coal Combustion Product Production & Use Survey Report, nearly 130 million tons of coal ash was generated in 2014.
https://www.epa.gov/coalash/coal-ash-basics
coal ash is not rock. So it would be completely different in performance characteristics.