Making thin brittle sand design that holds together
Fascinating project. The solution was right in your question; make it like a cracker &mdash; flour.

You're essentially making a sturdy paste instead of a glue. I mixed: 


3 parts sand
1 part all-purpose or bread flour
Water, just enough to wet the sand


I drained off any excess water, packed it into a mold and let it dry in the sun (1-2 hours?). I guessed at the proportions empirically, but it worked great!


The disc to the right is a failed experiment with cornstarch

The coin was pretty solid&hellip; yet breakable. I could carry it around and drop it from a few feet without breaking it, yet I could snap the disc with a bit of deliberate force, and crush it down into sand by pounding it or working it between my fingers. 


  &hellip;after it is crushed it should appear as close to sand as possible.


Bonus that you're not adding anything permanent or inorganic to the sand. I could not see any flour when I crushed the coins back into sand, and dropping it into a cup of water broke it down completely (which makes it easy to pour off the flour-silt, if that's important to you).