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  #11  
Old 08-13-2009, 12:30 AM
Serf
 
Join Date: Apr 2009
Location: UK
Posts: 8
Default Re: Advice on pouring the hearth

My technique for mixing vermicrete was messy but quick.

Lay out a big waterproof sheet of some sort on flat ground.

Pour a 4m3 bag of vermiculite onto it. Put the right amount of portland on. Iused a mix of about 8:1. Making sure to wear rubber gloves and an old long sleeved top get in there! Mix with arms and hands in a sort of sweeping method. Once thouroughly mixed add water and mix again. Add water until it's the consistensy of slightly damp porridge oats.

Although messy, the advantage of this technique is you can do very large quantities in one go, as opposed to loads and loads of smaller mixes. I also found it to give a nice consistent mix.

Just make sure you cover up exposed skin on arms and hands as Portland is highly irratative. I made the mistake of not doing this and my hands and arms went incrediblly dry and itchy! Not nice!
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  #12  
Old 08-13-2009, 11:42 AM
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Join Date: Jun 2009
Location: Sacramento
Posts: 61
Default Re: Advice on pouring the hearth

I wish I had heard that a couple of days ago. It would have saved me a lot of trouble . I got off work and bought all the materials that I needed. So I came home and thought I may as well finish the hearth, i still had about 5 hours of light and I would only have to pour about 2 inches. I guess I under-estimated how long mixing that stuff takes becuase I barely finished before I couldn't see anything. Hopefully I did the mixing and pouring correctly. It seems like the vermiculite on my hearth is already set up quite solid. I'm going to keep it damp for a few more days per everyone's advice beofre i take away the forms, but is it normal that it sets up so quickly or is that a bad sign?
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  #13  
Old 08-13-2009, 03:23 PM
Neil2's Avatar
Il Pizzaiolo
 
Join Date: Oct 2006
Location: Vancouver Island
Posts: 1,374
Default Re: Advice on pouring the hearth

"but is it normal that it sets up so quickly or is that a bad sign?"

You are OK. You can continue building around or even on it but allow water to get into it on a continuous bases for at least 6 days..

(I see you learned the first lesson about concrete pours - allow for lots of time because once started you can't stop.)
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  #14  
Old 08-20-2009, 07:32 AM
Apprentice
 
Join Date: Jun 2009
Location: kansas
Posts: 134
Default Re: Advice on pouring the hearth

You said to allow water to get into it on a continuous basis for at least 6 days -
I read somewhere else on the forum not to let water get into it, because it will get trapped, freeze in the winter, cause problems...

I laid most of the insulated hearth Monday evening - covered it because of rain...And yesterday laid the rest - covering it again because of rain.
Should I uncover and spray it with water?

Also, the first stuff we laid Monday, was with bigger grade vermiculite. (1A, I think) and then I purchased more yesterday, not knowing that I had to check grades, and it was 3A, which is so much smaller. Will it matter?

What this stuff reminds me of when it's all mixed is not damp porridge, but damp/wet sand on the beach....it doesn't absorb water like porridge does. And if you get too much water it sort of floats and rinses off the cement from the vermiculite. That was my first experience with it. If you add the water a little at a time mixing it in before adding more, it doesn't separate. Kinda like making pie dough or biscuit dough or something....

Thanks.
cecelia
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  #15  
Old 08-20-2009, 07:33 AM
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Join Date: Jun 2009
Location: kansas
Posts: 134
Default Re: Advice on pouring the hearth

And one more thing...how long should we wait before laying the oven floor after pouring the insulated hearth?

thanks
c
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  #16  
Old 08-20-2009, 10:29 AM
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Il Pizzaiolo
 
Join Date: Oct 2006
Location: Vancouver Island
Posts: 1,374
Default Re: Advice on pouring the hearth

cecelia

Portland based concrete and vermiculete needs lots of water to cure. When mixing the concrete, use as little water as possible - just enough to make it workable. Then, starting about six hours after the pour, keep it damp continually for at least six days. You can't add too much water at this time. (The people who cast structural bridge girders often submerse the entire girder in a water filled trench for three weeks.) Excess moisture will be driven out later during the firing process once the oven is built.

Most of the home project concrete failures I have seen are the result of not providing continual moisture during the initial curing.

The vermiculete will be cured enough after a couple of days to start laying the hearth and building the dome.

Last edited by Neil2; 08-20-2009 at 10:34 AM.
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  #17  
Old 08-20-2009, 12:18 PM
Laborer
 
Join Date: Jun 2009
Location: Sacramento
Posts: 61
Default Re: Advice on pouring the hearth

I have to say, everything Neil has said has been 100% correct. I followed his advice and kept wet towels on top of the vermiculite as it was curing for six days and it seems to have set up very nicely, although very soft and brittle. In one of the pictures that i posted, you can see the top layer of vermiculite-concrete. what you can't see is how it compacts, almost like a sponge, under the weight of the firebrick.
my oven floor isn't laid yet, but after about two days it seemed like it could support weight, although i opted to wait the full six days before i removed the forms and built on it.

I did have one concern with my build though. I laid down this ring of brick just to measure what 42 inches would look like on my hearth, but when i put down the oven landing, it comes right up to the edge of the hearth. Is this ok? in a lot of the pictures i've seen the landing is set back a ways from the edge of the hearth so i wonder if i'll have enough room for a 42 inch oven. I still have to cut the brick so i may be able to move the oven back a few inches. Des anyone have any advice as to what i should do next?
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  #18  
Old 08-20-2009, 03:44 PM
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Location: New Jersey USA
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Default Re: Advice on pouring the hearth

You should be fine: remember, your dome curves in as it goes up so you should have plenty of room for the flue by the six or seventh course.

My dome was quite different, but i ended up supporting one end of the flue tile directly on the dome its self: My oven was a 36 fit into a 5' square, so i was squeezed for space.

If you want a more conventional entry, you can cantilever it out over the edge of your slab, on an overhanging shelf.
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  #19  
Old 10-11-2012, 06:56 AM
Peasant
 
Join Date: Oct 2012
Location: new zealand
Posts: 33
Default Re: Advice on pouring the hearth

hi I,m building a mobile pizza oven,just finished casting the inner fire dome from a mold,its 2 inches thick in 2 sections and made from 2 parts aluminate cement, 3 parts small river pebbles, 2 part river sand and half part lime.because its on a trailer it seemed safer than using brick,less joins.i,m praying i this mix is adequate,but it was hard to find alternative info to brick work, i,m also thinking of using same mix for the hearth.any suggestions would be great,
thanks
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  #20  
Old 10-11-2012, 07:07 PM
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Join Date: Jun 2009
Location: kansas
Posts: 134
Default Re: Advice on pouring the hearth

I have no advice, but I LOVE your question. I, too, would love to know how to make a "homemade" mix for a mobile oven.

Cecelia
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