Here is a basic Rule of thumb that works for me:
Portland cement is always at least 1/4 of my recipe.
The other 3 parts can be any combination of peat, perlite, vermiculite, sand.
It is only considered HYPERTUFA if it contains PEAT.
If it contains perlite or vermiculite without peat it would be considered LIGHT WEIGHT CONCRETE.

The more sand the stronger the mix.
The more peat the weaker the mix because peat rots out leaving holes.

Acrylic admix, Elmers glue, or latex paint replacing some of the water will make any recipe stronger. Less water stronger recipe.

So to sum up:
Recipe is at least 1/4 either GREY or WHITE Portland cement, for the rest experiment away.:)

Have fun in the mud!
Post pictures when you are done.:)

Thanks! that helps a lot :)

Any opinions on building more vertical (vs. horizontal forms)? Better to use a mold (someone mentioned watersealed cardboard in the other thread) or to work it around some sort of framework, etc?

Blue styrofoam sheets make great reusable forms.
Oil it a bit and the concrete won't stick at all you can use it again and again.
It's easy to cut and comes in 2x8 ft sheets and thickness to 2.5 inches for about $7.

And has a million other molding and form making uses for tufa and concrete.
It's all I ever use. Cardboard is weak and not reusable and wood unless it has a good release film will stick like it's glued to concrete.
Styrofoam leaves a beautiful finish when removed.

You can build a skeleton out of bent rebars anchored in to earth. Over this, you build a shell made of maetal lath. Finally, you lay chicken wire on top and just appy concrete or
hypertufa. If you want to minimize amount of concrete, just lay a layer of burlap (preferably with cement impregnated); this provides both strenth and a smooth surface) on top of the
metal lath. The strenth of the structure depends on the rebar skelton. Take a look at the waterfall shell I built using this technique.

Here is a link that might be useful: rocky walls

What a great idea! Where do you buy the sheets (HD, craft store?)? And (again, forgive my ignorance)- is the styrofoam strong enough that if I "plant" two pieces of it about 3 inches apart where I want the wall, I'll be able to fill it without it leaning to one side, sides falling out, etc? Or do I need to put in something to make the "sides" of the mold (and keep the sheets at the 3" distance).

BCMouli - do you have any "in progress" pictures of how you made the skeleton? and can you clarify what "metal lath" is?

Thanks both for the help! :)

For thick styrofoam sheets you want a supplier that supplies commercial builders. Home Depot has it up to 1.5 inches thick.
Blue styro is very strong.
You can bury the bottom edge and then make a few big 'U' clamps out of rebar or plywood. If your wall is 3 inches thick and your styrofoam 1.5 inches your big U clamps will be 6 inches on the inside of the U this will hold the top of the styrofoam sheets 3 inches apart. You can actually use wire right through the sheets tied to bits of wood on the outside holding the sheets 3 inches apart. this wire would run through your wall and be snipped off after curing.

If you buy the 2.5 inch sheets it is very stiff and won't need very many Us across the top to hold it straight.

Styrofoam is great. All you need is a sharp knife and imagination to form up any project with the stuff. And because it is water proof it helps cure your concrete as well if you leave it in place for a few days.