New member curing/vermacrete question.

[daveyh]

Hello everyone.

I've been looking at lots of info and videos and have started to build my oven. It's a rather humble affair, using left overs from a friends build. Although it potentially looks ok, the mix, or video instruction I've used has resulted in a very spongy insulation layer. I think I may have to start again but am unsure. So, I've used a 4 part vermiculite (garden sort), 1 part cement, 1 part hydrated lime, 1 part fire clay mix. It's gone on fine but I'm 3 days in and it looks like it'll not gain any strength. The layer is quite spongly although it is hardening, the small bits at the side/floor are super crumbly and I'm afraid it'll not harden cure properly.

I was thinking of adding a ceramic blanket, then bricks. 

Do I need to start again or be patient?

Cheers all.

[downunderdave]

Apr 23, 2020 14:55:17 GMT daveyh said:

Hello everyone.

I've been looking at lots of info and videos and have started to build my oven. It's a rather humble affair, using left overs from a friends build. Although it potentially looks ok, the mix, or video instruction I've used has resulted in a very spongy insulation layer. I think I may have to start again but am unsure. So, I've used a 4 part vermiculite (garden sort), 1 part cement, 1 part hydrated lime, 1 part fire clay mix. It's gone on fine but I'm 3 days in and it looks like it'll not gain any strength. The layer is quite spongly although it is hardening, the small bits at the side/floor are super crumbly and I'm afraid it'll not harden cure properly.

I was thinking of adding a ceramic blanket, then bricks. 

Do I need to start again or be patient?

Cheers all.

The mix you have used results in very weak strength. It is what is called an insulation mix and has very little thermal mass. This means it will be subject to bumps and abrasions and unable to store much heat for retained heat cooking. any slight drop in the active fire will result in a quick temperature drop off in the dome. A far better mix which is much stronger is 3:1:1:1 (by volume) sand, hydrated lime, powdered clay, portland cement. This will result in a dense mix with much higher strength. The cast dome, generally 2" thick, after one week of damp curing, can then be insulated with or without blanket and a 10:1 perlite, cement mix. Allow one week of drying per inch and a half thickness of perlcrete. 
See attached table for vermiculite mix strengths. Vermiculite and perlite much the same.If you'd prefer to do an even better job search for dense castable refractory from a refractory supplier. It is way more expensive and somewhat harder to handle

[daveyh]

Thanks Dave

That's great advice, there's quite a few Youtube vids with people just using the insulation layer first