Bead blasting of floor tile mastic on concrete floors leaves the cleanest surface but there are a number of considerations. Not all abatement contractors have bead blasting machines so they have to rent the equipment. Also the bead blasting machine cannot clean all the way to the edge of a wall so a floor grinder with a special scarifying blade should be employed.  The grinder must be equipped with a cowl connected to a HEPA vacuum.  A thin section of mastic will remain in the corner but the majority of that can be removed with a scraper.   The major concern with Bead Blasting and grinding is dust control. The area must be under negative air pressure and adequate wetting must be employed.   The project requires full containment and the mastic debris will be friable, requiring disposal as hazardous waste.