I'm sure you'll get responses here, especially if our resident tile experts chime in; but, you may also want to post this over on the John Bridge forum... very very helpful over there too.

Here is a link that might be useful: Tile forum

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This is from Bill Vincent's super helpful tiling FAQ's over on the bathroom forum:

Q) What are the proper tile setting compounds?

A) This is one where I could write a book. It all depends on what kind fo tile you're installing, and what the underlayment is that you're going over. I'll give a generalized list:

Polymer/ latex modified thinset: For all intents and purposes, this is the "cure-all". For almost any installation the modified thinset, which is basically portland cement, silica sand, and chemical polymers added for strength, will work. There are some that are specialized, such as the lightweight non-sag thinsets (such as Laticrete's 255 or Mapei's Ultralite), or the high latex content thinsets (like Latictrete's 254 Platinum or Hydroment's Reflex), but with the exception of going over some membranes, there's a modified thinset for every installation.

Unmodified thinset: This is the same as above, but with no polymers added. It's usually used in conjunction with a liquid latex additive, but will also be used mixed with water for going over some membranes. It's also used as a bedding for all CBU's.

Medium Bed Mortars-- This is a relatively new class of setting mortars, used mainly for large format tiles, where the normal notched trowels just don't put down enough material, and with thinset, it would be too much, causing too much shrinkage as it dries, causing voids under, and poor bond to, the tile, but at the same time, there's not enoough room for a mudset installation. This mortar is usually used with either a 1/2x1/2" or 1/2x3/4" notched trowel.

Mastics and Premixed Thinsets: THESE HAVE VERY LIMITED USES!! Let me say that again-- THESE HAVE VERY LIMITED USES!! They work well for vertical installations, where the tile used is 8x8 or less, and it's not a wet area. ALL THREE of those conditions must be met!! I know just about every pail of type 1 mastic says it can be used in showers except for the floor. DON'T BELIEVE IT!! Also, both mastic and premixed thinset (which is just mastic with a fine sand mixed in to give it bulk) claim they can be used for floor installations. Unfortunately, for the amount of material needed under virtually all floor tiles to bond to the subfloor, neither of these will fully harden. I had a personal experience where I helped a sister in law across country, telling her husband exactly how to do his main floor, what to use, and how to use it. Unfortunately, he went to the big box store to get his tile and materials, and they talked him into using premixed thinset. I didn't hear about it until SIX MONTHS LATER when his tile and grout joints started showing cracks all over the floor. When he called me I asked him what he used for thinset, and sure enough, this is when he told me. I told him to pull one of the tiles, and SIX MONTHS LATER, IT WAS STILL SOFT!!! DOn't let them talk you into it!! Use the proper thinset, and don't try and shortcut your installation. You're spending alot of money for it to be "just practice"!!

I don't know if that helped your situation at all.

Here is a link that might be useful: tiling faq's