Forno Bravo Forum: The Wood-Fired Oven Community

Forno Bravo Forum: The Wood-Fired Oven Community (
-   Tools, Tips and Techniques (
-   -   Vermiculite:Cement ratio (

lwalper 05-01-2010 05:09 AM

Vermiculite:Cement ratio
I know there has already been LOTS of discussion on this, but so far I fail to see any real concensus.

I'm ready to pour the insulating slab and see numbers anywhere from 5:1 to 10:1 for the vermicrete mix. Doing a little math I come up with something like 1 PSI that will be actually built on the slab. Sure, the whole thing is heavy, but all that mass is distributed over a fairly large area. I understand that the vermicrete will always remain soft (like cork), but the heat sink slab on top of the vermicrete (I'm building a modified AS design baking oven so am planning on a bit of mass in the hearth) will be solid concrete.

I guess what I'm asking is, does the 10:1 mix actually have enough cement in it to set? or should there really be a little more cement in the mix?

egalecki 05-01-2010 07:32 AM

Re: Vermiculite:Cement ratio
I used about 10 to 1, using buckets to measure. You do have to be careful about mixing, and it's even weirder to handle at that ratio, but it works fine.

You have to mix the portland with the water seperately and then pour it over the vermiculite and mix it.

You might want to use the 5 to 1 on your base and use 10 to 1 on the insulating on the top of the oven. That way you can be sure that you have enough cement mixed in to keep it firm (if you're worried about that) for the base, but you don't have enough to interfere with insulating the top...

Neil2 05-01-2010 09:41 AM

Re: Vermiculite:Cement ratio
I used 5:1 for the layer between the suspended slab and the hearth bricks and a 12:1 mix for the dome insulation.

A 5:1 or even 10:1 mix is plenty strong enough to support your oven. You can expect at least 40 -50 psi from a 5:1 mix.

lwalper 05-01-2010 12:40 PM

Re: Vermiculite:Cement ratio
OK, thanks. That helps calm my worries. I think I'll go with the 10:1.

Actually, as I recall, the pressure on the insulation slab actually only came out to something less than 0.3 PSI. I've got 3+ inches of 8# ceramic blanket for the top, and then maybe 6" of fiberglass batt (minus the backing) on top of that. That should keep things toasty inside.

I was planning on framing the vermicrete with 2x8 lumber; filling the form to about 4-5" with vermicrete, then fill the rest of the form with concrete. The dome will actually rest directly on the support slab and not add weight to the perimeter of the hearth. I had thought I would let the dome walls float on the slab (no mortar on the first course) so that, if things move a bit, they can do just that. There will actually be 3 courses of wall laid up before things get hot.

egalecki 05-01-2010 01:49 PM

Re: Vermiculite:Cement ratio
I used newspaper to keep my first row of bricks from sticking to the hearth. You do NOT mortar the first course to the hearth.

Since you want to make yours around the hearth, I'd put newspaper in between the floor and the bricks to keep the mortar off. It will burn out in your first fire and the crack will eventually fill with ash.

david s 05-03-2010 11:46 PM

Re: Vermiculite:Cement ratio
Get the water to vermiculite ratio right otherwise you can end up with a watery mix of cement at the bottom and vermiculite with very little cement at the top. I found the right ratio is about 10 parts vermiculite to 3 parts water by volume. You might think it looks a bit dry, but it makes a good even mix.

Neil2 05-04-2010 01:35 PM

Re: Vermiculite:Cement ratio
"I was planning on framing the vermicrete with 2x8 lumber; filling the form to about 4-5" with vermicrete, then fill the rest of the form with concrete."

The suspended structural slab should be on the bottom, the vermicrete on top of that then the hearth bricks.

lwalper 05-05-2010 02:47 AM

Re: Vermiculite:Cement ratio

The suspended structural slab should be on the bottom, the vermicrete on top of that then the hearth bricks.
Right. That's the plan. I just finished "pouring" the vermicrete on top of the support slab. Decided to go with a 9:1 mix (chickened out on the 10:1). We'll see how it sets up.

I first moistened the vermiculite using just enough water to keep it from being so fly-away hard to handle. Then mixed the cement with just enough water to make it about like thick tomato soup. Poured the mixture in the moist vermiculite and gently mixed until everything was a uniform grey color. I did need to add just a bit more water along the way, but not much, then scooped it out into my forms. There was just a little bit of free water (maybe a 1/2 cup) in the bottom of the wheelbarrow.

I mixed two batches like that, then the third batch I didn't moisten the vermiculite quite as much before mixing in the cement — wrong move. The cement tended to make clumps (like little kitty surprises in the sand pile) that were difficult to mix out. Take a tip: moisten the vermiculite BEFORE you pour in the cement.

I now have the job fairly closely covered with a tarp to keep it moist. I'll probably mist and re-cover it again tomorrow, then pour the next layer of concrete after about 48 hours.

This ain't like no concrete I ever poured — it's wet, but not at all soupy. I remember doing some carving in vermicrete when I was a kid. Don't know what the mix ratio was, but from what I see looking around now it looks like a 2:1 mix makes a good carving medium. It was pretty hard, but we carved on it using a kitchen spoon.

lwalper 05-05-2010 09:16 AM

Re: Vermiculite:Cement ratio
Couldn't wait! It was nearly 4AM when I finished the vermicrete and thought I was finished for a couple of days. Came inside to take a little break, got comfy in my recliner and before ya know it, ZZzzzzz.

Next thing I knew it was after 8AM and I'm good ta go. Went out to check the job and, lo and behold, the stuff was actually getting hard (no, not like a rock, but firming up nicely). So, I'm thinking, "Why not go ahead and pour the heat sink slab? That wet concrete will be a better cover than a tarp."

Presto, chango there's now 2 inches of concrete on top of the vermicrete. I also embedded a piece of 3/8 copper tubing at the base of the slab (between the vermicrete and the heat sink slab) for a serviceable thermocouple. Don't know that I'll ever need to change that out, but thought, "Hey, why not make a way to do that?"

Royal Ranger f16 11-18-2010 06:08 PM

Re: Vermiculite:Cement ratio
I like the idea of a copper tube for the themocouple. Where do you want the themocouple to be located in the oven?

All times are GMT -7. The time now is 05:16 AM.

Powered by vBulletin® Copyright ©2000 - 2015, Jelsoft Enterprises Ltd.
Search Engine Friendly URLs by vBSEO 3.6.0
2006/10 Forno Bravo, LLC