Vermiculite

[mokomon]

Is there a specific size of vermiculite that I should use for making insulating concrete for under the oven floor ? Is it something that a nursery would carry ?

[david s]

As with any concrete mix, a range of aggregate size is desirable for both strength and mix workability. I've found a 50/50 mix of fine grade vermiculite and medium grade perlite works very well. The drawback is that the finer the grade of aggregate, the more water is required. Because both vermiculite and perlite absorb large quantities of water, you are left with a mix that contains a very large amount of free water, over and above that which is required for the hydration process, that needs to be eliminated. For the mix I now use I need to add 4 litres of water to every 10 litres of aggregate and for a 5:1 mix it requires 2 litres of cement. When slowly adding the water to the mixed dry materials, the correct amount is when the water just begins to pool in the bottom of the barrow.
For drying of the vermicrete slab, check out my experiment attached.

Vermicrete insulating slab PDF.pdf