Hearth insulation layer

[GFW]

Hello all, working on foundation for an earth oven, up to the insulation layer for the hearth and had some questions,

Just wanted be sure I understood the mix. Portland cement and vermiculite mixed ratio of 5 or six to one, (I assume the measure is by volume in dry state for both) nothing else added like sand or rock. Also, I read one of the threads where someone had better success by mixing the Portland cement separately into kind of slurry and adding it to the vermiculite in a separate container than mixing into kind of an oatmeal state. Oh, how about the reinforcing rods, I am pouring a 6.5 diameter circular base, seems some say rods on 12 inch centers and others on 8 inch centers. Or can the wire screen be substituted.

Thanks for the advice!!

[Neil2]

six to one is fine. No sand or gravel. Mixing a slurry first is simpler. When mixing the vermiculite and cement - mix gently (by hand not machine) so as not do break down the vermiculite particles and reduce insulation value.

For rebar in the suspended slab, I suggest either 8 inch spacing for 1/5 inch diameter rebar or 6 inches for 3/8 inch rebar (the 3/8 is much easier to work with). Both ways, in a grid. Re-bar is cheap. Wire mesh (WWF) should not be substituted for rebar.