That the grit is used for asphalt is just fine for a garden purpose, too.
The granite grit is good, but you are not confined to it. If you find grit made from marble, limestone or something else, those should be fine, too. I use granite grit because that is what is available here. It is locally produced about a hundred miles away.
The stuff that is called traction sand here is on only very fine granules, not good for our use, and not what Caroline's traction sand is. Just know what your are getting.
I don't think you can get too much rock in a soil mix for alpines. That is what they grow in naturally. They will happily grow in ALL rock, but will grow much slower, due to a lack of nutrients to grow with. Sempervivums are very adaptable, as long as they get good drainage. A 50/50 mix, grit/soil, is probably a good mix to aim for alpines in genera. But again, semps can take most anything. I prefer a mix of grit sizes in the growing medium usually 4-8mm in pots, and 4mm to 2cm in troughs. When I make my cactus bed this season, I am going to do as a local cactus guru does: he uses all angular rock, 2-8cm, with just enough soil to fill the open spaces created by the rock.
For surface mulching in pots, I use a #2 grit. For a garden I would use much larger, at least 2cm. Be aware that using crushed rock (grit) or any angular rock, makes it very difficult to dig in, as opposed to rounded rocks. Perhaps that site also has rounded rocks, too. Most rock gardeners here prefer grit for pots and troughs, and rounded rocks for in the ground and raised bed applications.
You won't have to worry about weeds sprouting from seed at a 15cm or more depth. It won't happen. But you might if there are already established weeds growing there that you did not dig out or kill. They could have the energy to come up through from below. I would advise against using a fabric, if possible. It certainly wouldn't hurt any of the sempervivums, but many alpines will easily want to grow roots deeper than 20cm.
