Thanks to forum members
A big thank you to members who have responded to my questions re construction of the oven. With the feedback that I have had I have gone ahead and completed the hearth.
After installing 9" high forms ( 2"X4" joined with a 2"X6") around the concrete block stand I proceeded to pour 4" of standard concrete with rebar. Next I poured 2 1/2" of vermicrete for insulation. On top of this layer, and embedded 1/2" into it, I placed a circle of aluminum 170" long and 3" wide. (This circumference gave me a 54" diameter. I figured with my oven having a 42" interior diameter I would need another 9" to account for the thickness of the firebricks plus a little more.)
I then poured standard concrete inside the circle up to the level of the aluminum followed by more vermicrete on the outside of the circle extending out to the 2"X9" holding the whole thing in place. After getting the level right for the surface I took out the aluminum strip then troweled the whole surface one last time to get rid of imperfections.
Today (24hours after finishing) I checked the vermicrete and observed that it was level and smooth, but soft to the touch. I am going to leave the forms in place for weeks, or at least until I have finished the dome. I don't plan on starting the dome until a fortnight has gone by; perhaps by then the hearth will be strong enough take all the weight.
I have photos of this stage of construction and will soon attempt to add them to the forum.
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