Cement, reduced-carbon ash and controlled mineral formation using sub-and supercritical high-velocity free-jet expansion into fuel-fired combustor fireballs

High-temperature and high-pressure water, preferably at or above supercritical conditions for water, is injected into a high-temperature flame of a fuel combustor such as a coal-fired furnace, or boiler, or a turbine, internal combustion engine, rocket or the like. The process enhances efficiency of the combustion process, and, when used with fuels such as coal, renders ash, particularly fly ash, cementitious, so that it can be used as a substitute for conventional cement, reduces the carbon content of the ash, so that it can be used as a cement additive, and reduces carbon dioxide emissions into the environment.