How to re-attach slate pieces to concrete walkway

I have an attractive walkway outside my house which has a concrete base or foundation, and (http://maplered.com) has large pieces of slate (1 to 2" thick) embedded in the concrete (grouted between each piece of slate).  So the pieces of slate are like stepping Stone (http://g603.net) s.  I would guess that the walkway is about 20 years old.  Over the years some of the pieces of slate have detached from the concrete and (http://maplered.com) I would like to find a good way to re-attach them.  The concrete base appears to be in good shape.  I'm looking for a product recommendation (e.g. glue or epoxy) that would be good for re-attaching these pieces of slate to the concrete walkway base.