This is exterior?
Are you in a freezing environment? If so, winter water will get between the tiles, and when it freezes and expands it could very well ruin your tiles. From cracking them to popping them right off.
Use a modified thinset to lay the tiles...not a mastic...and then grout. To a certain extent you can rake the grout joints afterwards to get depth between the tiles, just don't go overboard.
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