I wouldn't use vermiculite. It wouldn't be very strong.

The durability can be quite good, but it depends on wall thickness, water/cement ratio, and proper curing. You have to keep the mix as dry as possible. A pourable mix will be very weak. The peat doesn't really work well for tensile strength. You are better off using AR fibers made for glass fiber reinforced concrete (GFRC). I have had some that lasted many years and others that cracked in months because the mix was really wet. If long term durability is a major consideration, concrete is a better choice.

nil13
why would vermiculite weaken the hypertufa as opposed to perlite?

are the ARGF something i can get locally or is that something that usually has to be ordered?

thanks

vermiculite is squishy when wet and perlite isn't. I would think that would cause problems. Hypertufa is concrete with a lightweight aggregate. The aggregate gives concrete strength. You can use perlite and the resulting concrete will be very light, but not that strong. You could use pumice and gain some weight but quite a bit more strength. Scoria would be stronger and heavier still. You can add some sand in there for strength but sand is very heavy.

ARGF (alkali resistant glass fibers) might be available locally at a good concrete contractor's supply house or through a ready-mix concrete supplier. They are pretty easy to find on the web and they are light so shipping is cheap.