I presume this is an outdoor area? If so, do you know if the concrete slab has an underlay plastic? Terracotta is porous, and if the slab wicks moisture up, it will transfer into the terracotta, potentially discolouring it with efflorescence. If it doesn't have underlay plastic, you might wish to seal the concrete before glueing your tiles down. You can test to see if there is moisture coming up from the slab by taping a 30 x 30cm sheet of clear plastic down on its surface. Sweep the surface first, and then use gaffer tape to hold the plastic down. Try and do this in a shaded area and check after 72hours. If there is moisture under the plastic, you need to seal the slab.
After you've read the outdoor tiling article, please let me know any questions you might have. I'm keen to assist you during this project and can't wait to see you get started.