Vermiculite cement ratio question

[WoodchuckDad]

I have seen 5/1 and 6/1 suggested on this site. is that by volume? should I take a small pail and do 5 pails vermiculite to 1 pail portland cement?

[Tscarborough]

8/1 can also be used. I had the chart showing the properties of the various mixtures linked on a thread here a while ago. Here is another one:

[lwood]

Great info, thanks TS.

[dmun]

The higher proportion mixes, (I've heard as much as 10:1) require power mixing of the slurry, and adding it to the lightweight aggregate.

Thanks, Tscarborough, for that chart, which gives exact water amounts. It also confirms the 1:1 ratio between aggregate and finished volume.

[kebwi]

I used 5:1, 8:1, and 10:1 and didn't power mix any of it. Of course, I might have done it wrong. I put the dry materials in a garbage bag and "shake-n-baked" it lightly with a lot of rolling and tumbling. Then I poured the water in and mixed it up in a wheel barrow with a trowel and my bare gloved hands (working up to the elbows), gently mixing it all up, scraping the sludge off the bottom of the wheel barrow, and slowly infusing the mixture.

Despite a lack of power mixing, it seemed to work fine for me.

[Tscarborough]

Note also that unless your oven is 20 feet tall and built from solid gold bricks, your slab loading will probably not exceed 20 PSI.

[WoodchuckDad]

That chart threw me for a bit. I had to go back and reread a lot of things. So to clarify, I don't see sand as part of the hearth insluation layer recipe, just portland cement and vermiculite. am I on the right track?

[Tscarborough]

Yes, no sand needed. Perlite is an aggregate.

[WoodchuckDad]

how long do you wait afterpouring for it to cure before moving to the oven dome?

[dmun]

One week, more or less. If you pack in vermiculite on Sunday, you're probably fine to level and lay your floor the next Saturday. As Tscarborough says, even the weakest mix has lots of compressive strength for your application, so even if it isn't at absolutely full hardness it shouldn't be a problem.