I do decorative concrete for a living and run into this carpet adhesive problem all the time. I use a product called sunnyside tile adhesive remover or Bix with a buffer and a black pad. This works for me consistantly. Have you ever looked into decorative concrete such as acid staining,overlaying Stenciling pigmented cement to look like tile,etc? I fool people everyday with my stencil work. The actually think it is mexican tile! hehe Email me if you would like to see some pics of my work.