I need to repair a couple of square feet of a slate patio, which is Vermont slate over a concrete slab. These slates are irregularly shaped, about 5/8" thick, and two of them have become loose. However, the old cement base on which the slates sit is extremely solid. My problem is how to get the slates to stick to the old cement base without raising the height of the slates over the adjacent slates -- chiseling out the base underneath the slates is way too hard. Should I use thin set mortar, as if these were tiles? Or is there some other kind of adhesive that is very thin but will stand up to mid-Atlantic weather?