I'm seeing a lot of recipes for insulating refractory mix.
To generalize, here is my impression:
Recipes for filler mix vary.
My general impression is:
- 1/2 perlite (by volume?)
- 1/2 refractory mortar/filler mix
Portland gives the mix more physical strength unfired.
It degrades when heated really hot.
Use less (or none) where exposed to extreme heat.
e.g. Increase fireclay percentage the more heat it is exposed to.
- 40-60% silica sand.
- Rest mostly fireclay, as much as you can afford
- Some portland cement. It is cheaper. Good at lower temperatures.
It was still quite dense.
Next time I might go with a little less sand, and perhaps
75% perlite, for insulation layers (not structural).
- 3 parts fireclay
- 1 part portland
- ~3.5 parts silica sand
- 9 parts perlite