Floor tile must be installed in thinset, which is fine silica sand mixed with Portland cement. It’s what mice would use to build brick homes. Thinset allows the mosaic tile to bond well to a solid subfloor. Once the thinset is cured, it’s almost as hard as a rock, and it doesn’t move. Organic tile mastic that’s like thick cake icing is never to be used to set floor tile. It can move, and the tile will crack.