Untreated cement tiles are not resistant to outdoor freeze/thaw cycles and should be protected with a penetrating sealer, such as Miracle 511 Porous Plus. Sealer should be regularly reapplied according to the sealant instructions. 
A roof or cover is recommended where cement tiles will be installed to avoid color fading caused by direct sunlight. Tiles might also be polished with wax for extra protection against direct sunlight.