Perlite for rmh base

So I'm about to start building a 6inch rmh with a bell bench in south west France. I just received a huge pallet of perlite for the base and also I thought I would insulate between my brick bell and the wall with around 4" of it. however my eagle eye didn't notice it's graded to between 0 and 1.5 mm so it's like a fine powdery dust. My thought is this isn't going to work as clay stabilised perlite? I'm sure for insulating the voids it will be fine but not sure of its structural strength.. anybody already had experience with this?

Mike,

I have made 1:10 cement to perlite ratio of insulating concrete for my outside oven/smoker.
1 part of cement by weight and 10 parts of perlite by weight. My perlite has large grains (1-5 mm) and it's specific gravity is 0.1 kg/l. You add water slowly to make slushy mix, but not too watery - it's quite different in feel than regular concrete or mortar.
Allow it bind for a week. Water stays in perlite granules and makes the whole mix to set and dry slower than cement mixed with regular aggregates.
I have also made a stronger mix 1:5 for the insulation of my small barn/coop roof. I wanted it stronger, because I was walking on it while laying rooftiles on clay mortar.
1:5 has 0,22 W/mK like K26 insulating firebrick and 0.6 MPa compressive strength.

Thanks for the reply cristobal. The grain size you have got is what I was expecting and what I have used before. The stuff I had delivered today is much more like a powder and it's that specifically that I am wondering if it will be possible to use for my rmh base.

I would expect it to make a stronger perlite concrete, but with less insulative value. In this case I would go with 1:10 ratio. Smaller particles will make it stronger than larger ones and will provide enough insulation.

Hmmm I would have thought it would be weaker? As in to make concrete strong you want more biggerer bits of aggregate no? Also this is going to be clay stabilised rather than concrete if that makes a difference..

Yes, but we are still talking about weight ratios, so the amount of aggregate would be the same and because it's smaller the compressiveness of perlite will affect it less and also cement will be covering larger surface area than with the bigger particles.
It's easier to crush with fingers a 5 mm piece of perlite than 1 mm.
In case of mixing it with clay you will have to experiment - clay is a weaker binder than cement.
I would make 1:3, 1:5 and 1:10 mixes, all by weight, pour them into some 20x20x10 cm forms and after a week (or more, depending what weather you are experiencing) would try to take them out, scratch them, compress them, break them, etc to see which one fits your needs. You could put some steel plates on them and run with a car to calculate compressive strength. Cement based perlite concrete is quite well tested and even bricks/boards are made from them commercially. For clay based you have to experiment and share the results on permies ;)

Thanks that's good advice I'll knock up a few forms tomorrow 👍