Hi wraunvt, PVC would work, you just cannot have too long a span without additional support. It's easy enough to do that though. Use tee's for another leg to the ground to keep long spans to a minimum. If your plans are to be able to move this, you'll need as strong a armature as possible to prevent cracking when moving.

Making a base for this to sit on would be my main concern. Make that as rigid as possible with a way to lift and move the whole structure. This way most of the stress is taken off the waterfall itself and would help reduce the chance of cracking.

The weight of the 'tufa all depends on your mix. How much sand or lightweight aggregates you use.

Have you considered doing a foam (polystyrene) armature and doing your molding on top of that? Or as they do for movie props do an expandable foam base (Great Stuff used for insulating cracks and voids) and shape it like rocks, then cover with your mix of choice? Good luck.

Here is a link that might be useful: Fake rock recipie's