Concrete Canvas is a really interesting material which at its simplest is a plastic cloth that has been impregnated with concrete. You can drape the canvas over any form, add water, and have a structure.
The company Concrete Canvas is based in the U.K. but they have a few US distributors. I was able to get a roll from Milliken shipped to me for 105$. See here
It looks like the company has focused on mainly construction applications. But there are a few creative uses of the product out there