vermiculite is sandy

[woodward4]

Hello there;

I am in the process of building an outside pizza oven. Yesterday, I put in 2" of vermiculite, but today is not hardening, and is a sandy consistency. What went wrong? Should I remove it and start again? or is there something else that can be done to salvage it? I noticed that others had put in Portland cement. I did not do this, and mixed it to the directions on the package only (with water, the consistency of oatmeal).

thanks for any info you an give me.

[shuboyje]

Re: vermiculite is sandy

I'm not sure exactly what you bought, but I am pretty positive there are no products on the market that are ready mix just add water vermicrete suitable for brick ovens. My best guess is you bought it from a pool supplier?

You need to mix the vermiculite about 6:1 with portland cement to hold it all together. Even then it will take at least a couple days and more likely a week to really firm up, and still it will be rather crumbly. 2" is also not enough, you want at least 4" under the floor.

My best advice to you is to go to the fornobravo store section of this website and download the free oven plans and read them cover to cover before going any further.

[azpizzanut]

Re: vermiculite is sandy

Hi woodward4,

Yes, to what shuboyje said. That's good advice.

Here's a good way to mix vermiculite and Portland cement:

Mix dry Portland with dry vermiculite first, then add water and mix with a hoe or shovel. If you get too much water it will make the Portland run off the vermiculite flakes and puddle in the bottom of your mixing tub. Wear a dust mask.

Vermiculite comes in various granulations for various purposes. Small particles are often used to mix with potting soil. I've seen vermiculite as large as cereal flakes too. It shouldn't matter what size the vermiculite is. What does matter is the thickness of the insulation layer. I used 2" x 6" lumber to frame my hearth insulation and ended up with a total of 5 1/2" under the hearth bricks. Leave the forms on to protect the crumbly edges of the insulation and you could start placing the firebricks in about three days. You can remove the forms after the hearth bricks are in place.

You won't be able to trowel a smooth surface on vermicrete. Just level it off with a piece of wood. Don't tamp it to hard since doing so will compress the air spaces.

If you mix your own homebrew mortar you will need fire clay and silica sand (also Portland cement and lime). Mix up a 50/50 paste of silica sand and fire clay and spread a 1/4" layer in top of the cured vermicrete. You can level the hearth bricks with a soft mallet and torpedo level so they are all or mostly even. This mix fills the uneven top layer of vermicrete. An alternative is to buy 1/2" ceramic fiber insulation board from an insulation contractor supply house and place that on top of the vermicrete. No need to mortar it in. It is very firm and flat so leveling the hearth bricks is fast and easy. The weight of the bricks holds it in place.

Cheers,

[Lburou]

Lets define some terms here and that may clear some fuzzy possibilities for you.

• What is the brand name on your vermiculite?
• Is it the vermiculite used in making a swimming pool?
  • if yes, then you'd just add water for a pretty hard concrete -not very insulative
  • If no, then you need to add Portland cement and water to the vermiculite
• If you have standard vermiculite, then adding water won't do you any good without cement in the mix. Bob & shuboyje are spot on with their thoughts.

HTH

[woodward4]

Re: vermiculite is sandy

Thanks for all the info. I spoke with my husband, and he mixed a 5 to 1 ration of cement to vermiculite. It still seemed rather sandy ( it was very fine vermiculite) after two days, as it was sand castle consistency. We ended up removing it, and getting new vermiculite that was more the consistency of popcorn. This seems be be much better,as the next day it is starting to harden.

[david s]

Re: vermiculite is sandy

You may have added too much water to the mix. if you end up with water at the bottom it tends to take all the cement with it leaving grains of vermiculite washed free of cement.

[Johnny the oven man]

Re: vermiculite is sandy

Thanks for all the info. I spoke with my husband, and he mixed a 5 to 1 ration of cement to vermiculite. It still seemed rather sandy ( it was very fine vermiculite) after two days, as it was sand castle consistency. We ended up removing it, and getting new vermiculite that was more the consistency of popcorn. This seems be be much better,as the next day it is starting to harden.

What was the actual ratio?
Was it 5 V'lite to 1 cement or did you get the ratio mixed up and use more cement?

[bcovey]

Re: vermiculite is sandy

Bob - I had the same question re: "sandy" vermiculite - if it is sandy around the edges (in a couple of spots) i guess i can go ahead and set my hearth on top? do you think i will be able to stucco over the the "crumbly edge" or will i need to fill it? Any thoughts?

Brian Covey

[azpizzanut]

Re: vermiculite is sandy

Hi Brian,

If you are comfortable starting the hearth then go ahead. You can fill voids with 5:1 mix if you want.

If the hearth insulation is wider than the footprint of the oven then you can protect the crumbly edges with the old forms put back in place temporarily.

I don't think you want to put stucco directly over the edge of the vermicrete hearth insulation without first wrapping the area with wire mesh. Preferably you will wrap the entire oven with insulation down to the concrete hearth slab then apply stucco over the wire.

As an option you can cover the entire oven with vermicrete like I did for my oven #1. It looks like an igloo because I used a radius gauge to make sure there was at least 4" of vermicrete all around. Wrapping with chicken wire gives the first coat of stucco something to hold on to. You can also place blanket insulation over the vermicrete then wrap it with chicken wire, then stucco. Lots of good options for insulating and finishing your oven.

Cheers,

[Neil2]

Re: vermiculite is sandy

Bobs' post #2 above covers it nicely.

I would only add that when mixing vermiculite, mix gently by hand. You want to avoid breaking up the particles and thus reducing the insulating value.