I have a problem with a ceramic tile floor. The floor is installed over a concrete slab. The floor is about 20 years old and in good condition except some tiles have just started to loosen. When I removed these tiles I found an old adhesive from the previous floor (vinyl) and suspect that this is what caused the tiles to loosen. I tried to scrape the adhesive so that I can replace the tiles (I have new replacements) but it doesn't come off. Any suggestions about how to remove this adhesive?