Coal ash from a source where calcium content of the coal is high produces a fly ash high in calcium Oxide at the electric generating stations This has a great affinity for water and will set up rapidly into a strong cementing agent. So, mixing it in with soil and with some moisture, it is similar to stabilizing with Portland cement. Many fly ashes are low in calcium oxide and thus are only a fine grained material sometimes with hollow class-like particles. thus not much good for this purpose.
The fly as associations have classified the various types, but I have not seen that they tie them to calcium oxide content. In the States, coal from the western areas are types producing high calcium oxide in the fly ash.
