Need to fill deep gap between ceramic floor tiles to be Grouted?

Couldn't level cement floor before ceramic tiling, as a result 1/4" wide 2" deep gap between floor tiles. How Do we fill it for grouting Rope? Sand? round foam insulation?
Update: tiles are down aready, mastic/mortar is hard beneath the tiles, 1/4" x 1.5" deep space between all of the tiles, all spaces that have to grouted
5 answers 5