Setting 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.