I'm gonna put down an insulative layer of perlite for the cob base of my RMH tomorrow, and am trying to decide whether to use clay or cement to bind the perlite. A mason heater builder recommended perlite-cement at 6:1 as it has more compressive strength. I generally use clay over cement whenever possible, but this is a build in my home and I don't want it to fail. Any suggestions?
As your mason says, the preferred option would be to use a high temperature/high alumina/Fondu cement mixed with the perlite. It can be hard to get hold of and is fairly expensive stuff - over here in the UK anyway...
If you can't get hold of high temperature cement I'd use a mix of clay and 'normal' cement (OPC). If the 'normal' cement gets overheated it can crumble and fail but the clay should still hold things together.
Thanks for the response John. I see now that I should have been more clear - this perlite slab will go over a platform of hardie plank that sits on bricks on my wooden floor, to provide an air gap under my RMH. As the first layer it is just an insurance that my floor does not overheat. The base of my batch box will sit about 16=18" high and will be insulated with ceramic foam board, the space between the perlite slab and batch box will be cob and and fire brick, so I don't expect high temps at all in this perlite base (if so I've got bigger problems).