Building has started - base built and load-bearing hearth poured today!
Will be looking to do the vermicrete tomorrow - weather permitting. I think I'm going to follow the Pompeii instructions to a tee and and go for a 5:1 mix. Or are there 'better' mixtures?
I remember reading somewhere that the vermicrete layer should NOT be poured all the way to the ends of the load-bearing layer because of some potential issues with moisture... Does that ring a bell with anyone out there?
Also, any idea how long this weird stuff takes to dry before building can start?
The vermicrete base should be wide enough to capture the full width of the oven, plus the insulating layers. It needs to be protected underneath your final render layer in order to protect it from moisture.
Your diameter will need to be - internal diameter of the oven + (2 times the width of your bricks or half bricks) + (2 times the total depth of your insulating layer, including both blanket and vermicrete).
I think I left mine around a week before I built on top of it.
I'm not sure about the limits of the vermicrete slab. Vermicrete gets its insulation properties from air voids in the vermiculite, which can be filled with water which reduces the insulation value. I'm not sure whether vermicrete allows the transport of water across its bulk via an open end say but if you encase on 5 sides (3D) then if water does leak in from the top its likely to become saturated.
Quite a bit of speculation but no answers I'm afraid
With regards to setting, I think i started laying firebricks on it the day after pouring. It may not be fully cured but I found it solid enough to lay bricks on though I mostly worked from the side or used a plank of wood to spread my weight.