Mapei Ultraflex LFT Rapid Fast-Setting Large & Heavy Tile Mortar, WhiteSetting tile? Ultraflex LFT Rapid Fast Setting Large & Heavy Tile Mortar, White is the thinset that goes between your tile and the substrate. Rapid setting formula gets you grouting same day instead of next day useful when the schedule's tight or you're working in a heated floor area where slow cure is a problem. White color so it doesn't telegraph through translucent tile or light grout joints. Use under tile from 3 to 4 thickness.