There is Old Works in Montana where slag was used in all bunkers and waste areas. Don’t know if was used as part of capping material. This was a superfund site. As I understand it, the entire site was sealed under some material on which was placed a drainage system to recycle water, then capped with the root zone mix, irrigation, etc. Construction budget supposed to be around $25M, a fraction of the original clean up estimates.
The slag is black, resembles sand, and they have a back up supply that is about three miles long and several hundred feet high. It is quite dense and plays differently than anything I ever encountered. They even sold OW specialty sand wedges in the pro shop which I found amusing because all you have to do is play the shot with a square face. First timers that open the clubface and play a normal bunker shot usually end up blading the shot over the green. Bounce is not your friend.