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It will never be like plain cement. It will be sort of spongy- sort of like cork. It will also crumble around the edges if you mess with it- I know it seems as though it will never be suitable, but it does work. I have several inches of it under my dome, and it's never compressed at all- but it was a major leap of faith to go ahead and trust it!
I decided to minimize the amoutn of space covered to 2" beyond the hearth foot print and am using a round form to create the round shape---I am thinking that I never want to remove it for fear over time this 5:1 mix may crumble..to dust. PS my vermiculite is not the 3/8in size but the 1/4 to 1/8 size.....see any problem permanently leaving the frame or the 1/4 size verm.?
What about the really fine perlite, about the consistency of laundry detergent? I posted a question about it this morning but haven't received any responses (my post's verbosity may have scared off potential readers/repliers). Is that too fine to make good insulation? I already wrote this in the other thread but my concern is that it will pack too tightly and won't leave any voids for air...which is the critical function of any insulation of course. Would the answer differ depending on loose-fill vs. permcrete or do you want the same grain size (whatever "right grain size" means, large or small) regardless of whether you are loose-filling or making 'crete?
i need some info on heat bond refractory mortar. does it mix with portland cement and set well for building a pompeii oven? i use air bond mortar now but i was told it will crack easily. please advise.
To answer two posts in one shot Neil2 is right, wait about a week. Even though everything my eyes told me the perlite and portland mixture was wrong after a week is was hard enough to stand on whilst I laid the bricks. Yes the edges to flak off at the corners but once I put brick around the base and skimmed mortar atop the perlite it was all fine. And Neil@ is right about grain size, I bought the detergent looking stuff and ended up giving it away. Here in Austin I went to a Hydroponics store and got the coarseness that I really needed at a good price. With shipping charges alot of the mail order stuff is, how do they say it? Too dear.