PPotty1 came up with this mix and has had no signs of failure.
1 gallon of perlite
2 lbs of alu oxide powder
350 ml of sodium silicate
350 ml water
mix all that together you can increase the amounts just keep the ratios the same, takes 6 days to set hard enough to put heat on it then it bakes really hard its very light too.
you can make the sodium silicate by boiling 200 grams caustic,300 grams of silica gel, 500 ml water boil until silica dissolves.
The oxide powder is used for sandblasting quite cheap get the small size grit.