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[PaulColbert]

Hi,
I am almost ready to build my oven, just finished laying the 4" vermiculite concrete insulating layer on the heath.
Certainly is odd stuff to work with. How long does it take to cure and how hard will it go?
Should I place the oven floor directly onto this or put down a small layer of concrete first?
Any advice appreciated.

Cheers,
Paul

[david s]

The strength of a vermicrete slab is determined by the proportion of cement in the mix. (see attached table)
For adequate strength most builders recommend a 5:1 mix for an underfloor oven slab. The stuff will set in 24hrs but take much longer to dry. Because there is so much free water in the mix I don't believe there is much point in holding in moisture to damp cure it in an effort to maximise strength. I did an experiment years ago to test the drying of a slab that explains how slow it dries. (see attached doc). A vermicrete slab provides good insulation at an affordable cost relative to other manufactured products like calcium silicate board or foamglas, but has the drawback of requiring the elimination of its free water.

Vermicrete insulating slab copy.doc.zip