Tiling outdoor projects we have to take into consideration that most tiles and normal grout lines allow small amounts of water to penetrate thru to the tile backer.
Therefore its important that the backer used be any backer like cement board which integrity and functionality won't be changed over time by water and or freeze thaw conditions. When tiling over concrete no additional backer is needed and setting mortar goes directly on tile and concrete.
Many ordinary backers is suitable only for indoor tiling and needs to be painted by a suitable waterproofing barrier when exposed to high moisture areas like custom shower area's or steam rooms.
It's always recommended to seal all grout lines and all natural stone tile. Porcelain and ceramic tile surfaces does not require sealing yet the grout lines still requires sealing.