I've worked with that stuff. It's made from silica sand, sodium silicate ("water glass") and colorants. The sodium silicate reacts with CO2 in the atmosphere to form silicon dioxide. The end result quite similar to glass... hard and brittle.
A thick enough layer (I'm guessing 1/4 inch) could be strong enough for an extruder insulator. The thicker the layer the longer it takes for the CO2 to penetrate and "set" it.
High Temperature Furnace Cement can be found in tubs at many hardware stores. Look in the plumbing and heating section. Rutland sells several sizes under the "Chimney Sweep" brand:
8oz $2.79
16oz $4.43
32oz $6.99
64oz $11.66