Do it yourself

DH and I are putting up a wall in the back yard. We are going 2 blocks high (cinder blocks) and will have a raised flower garden in it. DH wants to cover the blocks with rocks. We haven't decided what kind of rock....maybe cobblestones. My question is this: can you use mastic outside to stick the rocks onto the blocks and then grout around them? It gets to below freezing here. Are there different kinds of mastic? Thanks, Joan