I am not sure I have insight but have been researching it for a bit and talked to manufactures a few times (so far). To be honest if you can find it in your location I really think it is the best media one could use . It has a very high surface area ,is PH neutral , holds air ,water and nutrients ,but not too much .
I did the numbers on hydrotron and it would cost me over 2000$ to do my beds and being a cheap ol goat I think I would make mine before I paid that much as I do have kilns ( wife is a glass artist ). Also I think the cost has a lot to do with it's an import and more and more is coming from China
Check to see if their is a firing plant anywhere close and if not check with some of the road contractors as they seem to be the biggest user and see if they have a source . I am still working on getting at least enough for one bed for testing to see if it's worth the problems getting it as I really can not afford 20 some tons right now . I even thought of buying a load bagging it and selling it but shipping would be the deal breaker for most
You are correct in that you do NOT want limestone or you will be chasing your PH all the time and that is mostly what you find down here Limestone!
Have A Kind Day
Mike