33.3% by weight bentonite clay from ground cat litter, sifted through window screen
66.6% quikcrete washed fine white silica sand
I rammed it up like a sand castle and let it dry. It has basically no insulating properties but I've had it over yellow heat multiple times and it hasn't even sintered together yet! Unfortunately this means that I keep damaging it because it's essentially a dry sand castle - little green strength.
Pros: Might take 3000F to melt it, cheap, homemade.
Cons: You have to actually get to 3000F to sinter it or it has no strength, poor insulator