Community Answers (2)
We're in the Chicago area around here and I would not recommend trying to mortar a natural stone over a concrete slab. The freeze thaw will eventually separate the mortar from the concrete, no matter how much prep you do. You are better off laying the new stone in a sand bed over the concrete much like a standard paver patio. The concrete is a great base for the sand. The down side is that you have to sand it every couple of years but at least it won't crack up. Joel @ The Renovation Place.