I did a small similiar job. The important thing is to check with the manufacturer of the type mortar you intend to use. I applied a sealer to the quarry tile first so it wouldn't "wick" the moisture from the mortar. Then used Veribond mortar as my base for the new tile. I did this as per the instructions from the clerk at Home Depot who contacted the manufacturer while I was there. At least this way the "experts" had a little liability :-).