Several years ago I built an outdoor fireplace using river rocks and supposedly heat resistant mortar. Well, after a few years it is basically a neat stack of river rocks with mortar stuck to them. They are no longer attached to each other.
So I've been researching how to properly build an outdoor fireplace. I know there was a recent thread about fire pits, but I want an actual fireplace. I've searched online, and the best looking ones seem to use cinder blocks as a basic structure, with flat rocks then mortared onto the outside to make it appear made entirely of stone. Something like this, but probably not as fancy:
The problem is, I've seen other sites saying that you can't use concrete or it will potentially explode from the heat. As far as I know, cinder blocks are made out of cement/concrete. I can't seem to find a definite answer anywhere.
So my question is, does anyone know if it is safe to use cinder blocks for a fireplace?