"SteveB" wrote in
:

Try an air hammer with a flat chisel. Hold it as flat as possible to the
floor and move it around to knock raised parts loose. If you hold it too
vertical it will eat a large hole in a second. Maybe dial down the air
pressure to start to avoid knocking out large pieces.

Surface is not smooth when done but you are going to fill anyway.

Take about a minute and make less dust than a grinder. Pieces all get
knocked all over the place.