I have no experience with this stuff but i did scan the weblink you sent. I saw absolutely nothing about the thermal properties of this material.
The perlite is used as an insulator, very small amount is used and it is inexpensive. It is non reactive so I really do not see the advantage of substituting a material which apparently is reactive in many circumstances, personally, i could not name all the possible byproducts of burning wood or whatever else you might burn (say an envelope to start the stove).
If you would like to experiment with this product, I would choose to use it in some place where it is easier to change your mind about it than the core of a rmh. It sounds like there is many useful applications. Please use natural product.
Briefly read over material, yes this needs much study, when mixed with slacked lime makes a water tight plaster. Can be used to replace large amounts of portland cement and still make a denser Concrete with smaller pore structure, due to its hollow structure should be capable of being formed into a lite weight insulative product, but i worry about its re-activity and ability to hold lots of water ! Pyro Al
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