John,
That video Drew posted explains all. It's really great stuff - the best scenery discovery I've made.
Also, as Don said, Structolite is really the same material. In our area, the Structolite brand name is more commonly available at HD.
The only point I might disagree with in the video is that the container shown is enough for what Dave calls a "moderate" size layout. In my case, I'm glad I have it in 50 lb bags! But, then again I like big scenery. 
Jim