floral foam!! Light bulb just lit up in my head!! haha, I will be looking into that, and the size and amount of what can be found.
I just said Portland C, just to let you know what I use, meaning not a premade bag. I don't use concrete mixes, usually, which is cement plus rocks in it, although sometimes as a base layer, then use sand and PC for the sculping outer layer. I will say some of the premixed bags are great, just need to match up what your using each one for.
the sand casting/molding, I have found to be, for me, the most beneficial. although, that may all change when I try thee floral foam! haha