Rotted Subfloor under concrete
Gypsum concrete has often been poured over existing or new plywood floors to level them and to provide larger sound and fire resistance ratings. It is easier to remove than concrete. Perhaps a google search would provide some ideas.
The National Tile Contractors Association (NTCA) has compiled a list of unsuitable floor tile substrates. "The questionable list also names gypsum concrete. Although problems may occur, methods have been devised to allow for satisfactory ceramic tile installations. You may use a sheet membrane for isolating your tile, or use a water-free urethane adhesive that will not affect the gypsum concrete. Just be sure you choose products approved by the manufacturer for use over gypsum concrete."
Removal of the subfloor is a structural matter which requires professional assistance. Don't expose yourself to any unecessary liability.