you should use a modified thinset, no premix junk that comes in a bucket.

The bigger issue is, do you live in an area that has freezing weather? If so, a porcelain tile must be used as they do not absorb moisture and therefore wont crack in a freeze/thaw weather condition.

The cement slab must also be free of cracks and any sealers or paint that would inhibit the bond of the thinset to the slab.

this should also have a cleavage/uncoupling menbrane.

I recommend schluter ditra or troba, with the possibility of using bara depending on your exact situation.

not only is freeze/thaw stability a concern, but also there can be the different rates the different products with expand and contract with the drastic changes in temperature that occurr in exterior applications. From day to night, from hot to cold, from rainy to snow and ice. Its quite possible you can have a variance of 20-50 degrees in a frew hours transforming from cool darkness to a hot humid sunny day, especially so with a darker tile that can really pull in the heat from the sun and hold it.

Also, conrete will expand at a different rate than the tile, thinset, and grout, so that cleavage/uncoupling membrane acts as a buffer between the tile /setting-bed, and the substrate.

Lastly, if going up and fitting to, or grouting against any verticle obstructions (walls posts, etc), you will want to use a color matched grouting caulk, not grout.

here is some more information:

Schluter Ditra:

http://flooringworld.org/floorsearch.php?query=schluter+ditra

Scluter Troba:

http://flooringworld.org/floorsearch.php?query=schluter+troba

Schluter Bara:

http://flooringworld.org/floorsearch.php?query=schluter+bara

Whitepaper: Essential Water Management in Tiled Installations:

http://flooringworld.org/floorsearch.php?query=essential+water+management+in+tiled+installations

Color Matched Grouting Caulk:

http://flooringworld.org/floorsearch.php?query=grouting+caulk

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The right way and the wrong way.

Do it right everytime.

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