what to mix with sand? (for casting)

I am in the early stages of a project right now and I am wondering if anyone knows of any interesting methods of creating sand mixtures. I am going to be casting some large (yet simple) forms from beach sand and have considered the obvious concrete additive, however, I am interested to see if anyone knew of any other combonations that could create a durable and rigid structure. Thanks in advance, and if you have any info to share post it on up!

Resin might be cool.

I’ve mixed sand and polyester resin, but it was very brittle, epoxy is much better.
Epoxy, (water thin) for dry rot and pressure resin infusion should fill between the grains with out too much mixing. Mixing traps air bubbles.
Epoxy can be colored with transparent or opaque dyes.
Also, epoxy would be stronger than cement and less friable, however most epoxies are UV sensitive and should be kept out of direct sunlight unless coated with paint or varnish.

System Three epoxy is incredible stuff, I find more and more uses for it.
West System makes a very good product also.

I do various composite layups, using all sorts of materials, wood, fiberglas, carbon fiber, paper, cloth, and a lot of epoxy.