I have natural shale, most of it underground but near the surface. Some weathered and on the surface. That's very common, of course. But it's also in a location where a small kiln and truck access could be made pretty easily, so I was wondering if it is feasible to think of making my own "manufactured expanded shale" as you describe.
Before beginning construction of anything I'd test the existing shale, but I haven't done that yet.