The substance is most likely "cut back" adhesive. Be careful as this stuff can be toxic and flammable. I would use an electric floor scraper (with a wide, flat blade) to remove as much as possible. Do not use flammable solvents to remove the residue as this can be extremely dangerous. You have a very tough job ahead of you and I strongly recommend you have a professional come out and prep your floor.
Michael Prachniak - Expert Tile and Stone Consultant and Writer, Owner - Tile Productions
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