How old is the floor??? Some adhesives from the good old days had
asbestos in it. (I think almost EVERYTHING has asbestos in it,
milk, vitamins etc) (specifically alot of black cutback has it)
Anyway sanding it would not be a good option as that point
because you would release it into the air. Heat might be ok but
at that point its going to take a little time to heat the glue AND
cement. Freezing it is a GREAT idea when you have questionable
tile and want the glue to delaminate from the concete. When you take
the tile off if you are lucky the glue will stick mostly to the tile.
I personally used a adhesive remover from Home Depot.
You slop it on (windows open) the floor, let it sit. Its
desolves the glue a bit (enough for scraping)
Do it again and it becomes translucent. Once you are
there you are good to go. If you wanted to tile over it
you can buy a latex primer (found in the tiling isle) for the floor
and then use thinset to tile it over.