Refractory Mix

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: 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.
I ran It was still quite dense. Next time I might go with a little less sand, and perhaps 75% perlite, for insulation layers (not structural).
Aaron Birenboim