vermiculite mixture

[pam]

Is the 6 parts vermiculite to 1 part cement mixture based on weight or bulk? Vermiculite comes in 20 lb bags here which are quite a bit larger in bulk than the 80 lb bag of cement.

My hearth (including an extension) measures 40 square feet. The concrete I can get from Home Depot requires nine 80 lb bags to produce 2" of depth in an area that measures 4x4' ...

I think I will need 22-23 bags of cement (portland and refractory) to achieve the 5.5" depth recommended. How many 20 lb bags of vermiculite would you guys say I need for the insulating layer?

And one more question ... I used "quickcrete" to pour my slab and strengthen my oven base. We mixed it by hand (arduous) and it set up very quickly. I'm thinking I should get "un-quick" concrete for the hearth so the vermiculite layer will remain wet long enough for us to mix and pour the refractory layer.

Please give me the benefit of your collective experience and expertise.

[paulages]

i used three (i think) large bags of perlite, but don't remember the weight of the bags, and one bag of portland cement, i think. i do know i used 29 90# bags for the 3 1/2" thermal layer.

[pam]

thanks ...

that makes it sound like a 3-to-1 ratio based on bulk. how many square feet did that cover at a 2 inch depth? thanks for your help.

[james]

I don't have the volume (sorry), but can confirm that the ratios are based on volume, not weight. You can use a pail or bucket to count units, or for smaller amounts, you can count shovel loads.

James

[paulages]

sorry. i guess it's not too helpful to tell you how many bags of concrete i used if i didn't indicate the size of my hearth. it is 70" x 80.5" x 5.5", with one rounded corner, reducing the volume just slightly.