Lava rock is used in KBS growing. Grow rocks do not hold the plant well enough and they tip over in the KBS style. In KBS you use one 5 gallon bucket per plant and fill it with rocks. Lava rock holds great but it is a bitch to clean. Yes the cement mixer does help a lot and a mixture of 5% bleach is easy to mix and use.

Look for a local garden landscape company you can get a whole truck load for about 50 bucks of what they call crushed lava rock.