Flooring Tile - Tiling over concrete

Is there any special type of morter that should be used when tiling over a concrete floor? Is one type better than the other?

Modified thinset is what you want. You also need to have the floor free of paint, sealer, cleaning chemicals and dust or nothing will bond the tiles to thre floor. Chewing gum would stand a better chance. :)

Also, how big is the area dimensionally? You will need a soft joint no further than every 24' (doorwasy are a good place to locate these expansion joints in multiple room lay outs) and if it is an area that gets lots of sunlight, every 12'. Fill the grout line with matching caulk instead of grout. Also, if there are any control joints cut into the slab, it's always best to use these cuts as grout line locations with you caulk. With a membrane over them, you may be able to bridge them but it's a gamble to do so.

I think that I will have to do a test area for adheasion to the concrete. The concrete looks as though it was painted at one time, although it is not flaky or peeling in any way. Also, luckily there are no joints in the slab, although the dimensions exceed 24' in one direction.