Don' t know where I went wrong. Stone place I bought my brick from gave me S type mortar. 12 hours later I could just pull have of the bricks loose. Mortar was rock hard on the back of the brick. Almost like it didn't stick that well to the Durock board. Brick box showed to only use lath on an exterior wall. So now I pulled everything back off the hearth pad, cleaned the pad and am tossing all the brick as there is no way to get that mortar off the backs. Kind of disgusted at this point. I guess maybe I should have just used thin set instead. I want to make the second time count. Mix was good, only thing I can think of is I didn't work the bricks into the mortar, enough. Any ideas as to make a second attempt good would be appreciated.