If you are trying to remove grey thinset mortar from your tiles and your tiles are ceramic with a porcelain surface (do not do this on marble or quarry pavers) mix a solution of 1 gallon muratic acid to 4 gallons of water in a 5 gallon bucket. Mix outside in well ventilated area and wear rubber gloves. Muratic acid has very strong fumes and will burn your skin. Soak your tiles in the solution overnight. The next morning the thinset should be soft enough to come off with relative ease. After they are cleaned and rinsed throughly with clean water lay them out to dry.