Typically the exterior edge of a slab on grade is made of EPS foam and can parged. Having used rigid Rockwool panels meant we needed an additional solid exterior surface. Here we installed an exterior cement board, though that left the challenge of how to attach it without using metal fasteners that would lead to thermal bridging. 
What we decided on was plastic straps that passed through the cement board and that were anchored in the concrete. It did hold as planned,  though there is more play in it than we ideally would have liked and if we were to do it again we would increase the amount of fasteners. Like many things, this did work but we found it to be labour intensive compared to finishing an EPS foam slab exterior.