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Robpriddy 09-12-2012 01:10 PM

Finished Hearth - Vermicrete
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As a newbe, thanks for the feedback, suggestions, and encouragement that got me through some rough spots in pouring my hearth. Great forum!

I made some mistakes and still have some issues with the vermicrete pour on top of my hearth. I will share my experiences in case it helps anyone else working with vermicrete.

I built a 7 1/2" form with the intention of pouring 3 1/2" of rebar reinforced concrete for strength with 4" of vermicrete on top of it for insulation. Instructions called for 1 90# bag of Portland and 8 cu feet of Vermiculite. Mixing procedure was dry mix Portland and Vermiculite 5:1 then add water to bring to consistancy of oatmeal.

First problem was I ran out of materials 1" short of the top of the form. Also after 4 days of drying the surface was very inconsistent with some hard spots, some spots soft to the touch and some spots very crumbly. First two photos below show the surface condition.

I got some feedback from the forum on possible next steps and then found an old post from cvdukes dated 4-12-08 on a different procedure for mixing vermicrete. Bought another bag of Portland and 4 cu feet of Vermiculite to make a second pour to finish filling the form.

Used new mixing procedure of filling bucket 2/3rd full of portland then adding water to about the top. Mixed using a drill mixer to make a slurry. Dampened the Vermiculite in a wheelbarrow then added the slurry and mixed VERY well. I felt like some of the surface crumbling may have been caused by making the mix too wet so I made my oatmeal dryer. Seemed like the excess water was washing off the cement and leaving just vermiculite on the surface.

Just finished the final pour and it "looks" good. The consistency and color of the mix looks like the cement has coated all of the vermiculite. I will know for sure in a few days when it sets up and I can judge how crumbly it is.

If I were doing this again I would probably pour concrete around the edges and just put the vermicrete under the oven.

david s 09-12-2012 01:31 PM

Re: Finished Hearth - Vermicrete
I found that for every 3 parts vermiculite you need 1part water.

Robpriddy 09-13-2012 07:51 AM

Re: Finished Hearth - Vermicrete
1 Attachment(s)
Thanks for the addition to the formula. I added water by sight and "feel", trouble was I had never made Vermicrete before so I really didn't know what to look for.

Good news...after a day of drying I took off the cover and I am very encouraged by what I found. The surface is consistent in texture and color, firm to the touch and does not seem to be crumbly.

Took off the tarp and replaced it with a popup cover to let it breath a little and so I can look at it easier. Now one more "task" before the fun to waterproof the below grade parts of my stand then I can start working on the oven!!

Thanks again to all who helped.

Gulf 09-13-2012 08:33 AM

Re: Finished Hearth - Vermicrete

Originally Posted by Robpriddy (Post 138387) to waterproof the below grade parts of my stand....

Looks great! I like the location. Building into a retainer wall is tough, though.

The pipe in the pic looks like the beginnings of a french drain. I've began one on mine. I was waiting on gravel. Now I'm waiting for a couple of days off in a row and dry weather. I bought a 5 gallon can of $Drylok$. I will have enough to waterproof the outside of the stand and should have plenty to brighten up the inside.

Looking foward to watching your project.

Robpriddy 09-13-2012 11:44 AM

Re: Finished Hearth - Vermicrete
Yeah, the pipe will be part of a french drain system. I figure the best approach is to minimize any water standing behind the wall so I plan to backfill with gravel and the pipe to channel out water as quickly as possible. Just finished mortering all the cracks between the blocks. Once that cures I will use Dryloc or possibly a Bher product that the guys at Home Depot are recommending. Thanks for the encouragement!

Heading out now to get some foam core so I can make a template for the oven floor, walls, and arch. Cannot wait to get started on the oven! Having a little trouble visualizing it all and think the patterns will help me think it all through. The old brain is not a sharp as it once was....or maybe just too much beer.

deejayoh 09-13-2012 11:59 AM

Re: Finished Hearth - Vermicrete
My recommendation for building into/on a retaining wall is to use Xypex 1000 additive in the concrete. It will make the concrete waterproof by blocking the capillary action. It is a little pricy, but well worth the cost for the peace of mind. My location required that the dome be built on top of a slab on grade. With the Xypex, the slab doesn't wick any water.

Gulf 09-13-2012 05:23 PM

Re: Finished Hearth - Vermicrete
I think that would be great to stop wicking from the slab up to the oven floor. I was conscerned about the keeping the wood storage area dry. The blocks in the stand are extremely porous. Other than pouring the stand as a monolithic concrete pour: a fench drain, sealant, and plastic seems to be the best way to accomplish this.

Robpriddy 09-14-2012 06:19 AM

Re: Finished Hearth - Vermicrete
I live on the edge of a sub-tropical rain forest so keeping everything dry is a real issue. I will check out the Xypex 1000 additive. When I backfill behind the stand the fill will be only a few inches below the hearth so anything I can do now to minimize water is a great thing.

Robpriddy 10-23-2012 04:54 PM

Re: Finished Hearth - Vermicrete
1 Attachment(s)
It has now been about 5 weeks since I have had time to work on my oven project and wanted to post a little more information on my situation since making the second pour of vermicrete.

The bad news is that the additional pour of about 1" of vermicrete did not "stick" to the previous pour. It is loose and warping in places and has cracked in a few spots. The good news is that once the oven is built and the finishing materials go on I do not think the loose surface will be a problem.

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