Cast risers, adding things to the refractory

I am not enamored of the ceramic fiber risers, so I'm happy to see that cast risers are still a thing here at permies.
I was reading another thread that questioned some of the things one might add to a cast riser.
The two items in question were perlite and clay.
The perlite adds insulation but reduces strength.
The clay was said to add nothing.
The third ingredient was the castible refractory.

I want to suggest some alternatives.
The clay and perlite , not as additives , but as the main structure.
Refractory added as a way to speed the setting of the of the mix, so it solid enough to be fired.
I think this is how Matt Walker described his earliest core.

I have found that high temperature clay costs a lot where I live, but perlite and refractory still make sense.
I think perlite and cheap clay will do fine for all but the hot face.
How thin can we cast the refractory and have it stand up to the flames?
I have added rockwool to refractory to strengthen it against cracking.
Rockwool has a very high temperature rating, but not against direct flame.
Coat carbon felt is like this as well.
Can we coat carbon felt with a castable refractory to protect it from direct flames and use that as a riser?



Bulk perlite is cheap but not always available here.
Can we use shredded styrofoam and cheap clay mix , behind a hot face?
Where the heat is strong enough, the hope is  the foam particles will "burn out", leaving insulating voids behind.
Everywhere else, its just insulation.
Sawdust might be a more earth friendly choice.
Rice hulls would be great, but they are relatively expensive here in Ohio.

Just some thoughts, what do you thinK?

Ceramic fiber board isn't great, but if you coat it in a zirconium based refractory coating it seems to eliminate the cons.
https://www.youtube.com/watch?v=tOZCI1Udzk4

Your mention of clay and perlite remind me of vermiculite board. It's vermiculite bound in clay. Harder to find in the US but I ordered some from Canada. I'm yet to build with it but I see fantastic results from our European builder friends.

Castable refractory is so finnicky that I wouldn't mess with the recipe outside of small scale tests. Make some samples and stick them in an existing afterburner to see how they fare!

The vendor for Missou Castable plus (a 3000 degree refractory) told me I could cast it as thin as half an inch. Wouldn't risk that with untested additives though.

I don't think you want voids in high heat areas by burning out styrafoam in refractory. There is so much emphasis on castables needing to be vibrated to eliminate bubbles since voids lead to weakness and maybe even explodingness.

Some of these ideas of combining insulating qualities and heat resistance qualities into one layer of castable may be asking a bit too much of the materials, but I'd be happy to be proven wrong if it means cheap reliable castable recipes!