vermiculite to portland cement Tools, Tips and Techniques Discuss vermiculite to portland cement in the Pizza Oven Design and Installation forums; I used a 6:1 mix under the hearth and a 12:1 mix over the dome. Mix "gently" by hand. If ...

[Neil2]

I used a 6:1 mix under the hearth and a 12:1 mix over the dome.

Mix "gently" by hand. If you mix in a machine you will tend to break down the vermiculite particles resulting in a denser and less insulating layer.

[mrgweeto]

Layed the brick form Sun and hopefully will get the ver/port poured tomorrow. We are going to end up with a 3" layer of insulation hearth. I think that is going to be fine. Our hearth stand is not going to be used for wood but for storage of tables etc and will be closed off with a door. We may even insulated the ceiling and walls. I know this doesn't add much but it won't take away either and because I can get the insulation for nothing it is a mute issue. We are making the 42" oven. I know the door height is critical and should be 60 percent of the inner height of dome. In this case it will be 12.5" high by 20". Seems low but I can see how you want to keep the heat in.
Thanks for the info.
G

[pizza_bob]

Originally Posted by mrgweeto

Thanks for the pictures.
good to know that my brother and I are on the right track. We are going to put up the brick perimeter today and then get the vermiculite and portland and do our pour this Tues (weather permitting....CT is the wet state so far) and then the fun begins this weekend with the dome. How long did it take for your insulation hearth to cure ?
Thank you for the info,
G

Hello mrgweeto,
I have a triplet brother named Guido. I'm building a 42" Pompeii style oven.
I live in Ct too - Oxford - and just poured my vermiculite layer Saturday (July 4th). I picked-up my vermiculite from New Haven Masonry. They stock the smaller granular size stuff for filling cement block cores. If you want the larger "attic type" vermiculite, which is what I used and it looks great, call them ahead of time and a distributor from upstate CT delivers to them twice a week.
I went with a 5:1 vermiculite-Portland mix but went through 8 cubic feet of vermiculite and 1 90lb bag of Portland. The cavity I wa filling was only 5.5 cubic feet - not sure why it used that much vermiculite but it looks really good.
Where in CT are you?

[mrgweeto]

Bob,
Thanks for the reply.
So you're the person he special ordered the attic vermiculite for. I spoke to Tony at New Haven Masonry and my only concern was that the vermiculite was treated with silicone which keeps the moisture out. Our insulation hearth was framed out with brick to keep it from crumling on the sides. It is 5' x 5' x 3" ( 6.25 cuft ) so I am getting 1 bag of portland and 6 cuft of vermiculite. Am going to have to use the vermiculite down the line so any left over is that much less I will have to get later. We are from Hamden. Plan on doing the mix tomorrow after work weather permitting. Can't tell with this wonderful CT weather. Felt like fall Sunday morning. Made me want the pizza oven finished even more.
G

[pizza_bob]

G-
If you plan on going with 5:1 or 6:1 ratio I would consider getting an extra bag of vermiculite and possibly Portland. You will need it for the dome if you don't use it for the hearth. If you look closely at my pictures, I actually ran out of Portland and did not want to make the trip to Home Depot to get another 90lb bag. I decided to not completely fill the form and stopped approximately 1/4" below the top. I had to spend some extra time ensuring the top surface was level but it seems to have come out really nice.
Did you purchase the vermiculite from New Haven Masonry? Tony and Andy hvae been really good to me. I'm probably gonna' buy my bricks from them too. The vermiculite comes from The Schundler Company. I just called them to see if they silicone coat the vermiculite and they said "...absolutley not! It's pure unaltered vermiculite"...Here is the MSDS link:
VERMICULITE MATERIAL SAFETY DATA SHEET--- SCHUNDLER COMPANY

[mrgweeto]

Bob,
I actually found Griffin Greenhouse supply in Cheshire. I got it from a friend that owns a florist business. They sell fine, medium and coarse vermiculite for $22.18 per 6 cuft bag. May want to give them a call when and if you need anymore for the dome. Good to know about New Haven Masonry. Their prices for firebrick are good I thought. $1.25 each. Good thought on the extra portland. Your pour looks good.
G

[david s]

The lesser proportion of cement to vermiculite results in better insulation but weaker strength. Since strength is not really an issue in the insulating layer then the cement content is there to hold the material together. I found a 10:1:3 (by volume) of vermiculite,cement, water works well. The correct amount of water is important otherwise a mix of too much water will draw away the cement into a watery slurry and leave vermiculite uncoated with cement. Also don't compress the mixture when applying it. This will only reduce its insulating value. Tapping with the flat of a trowel works well.

[pizza_bob]

David,
I went with a 5:1 mix because under the vermicrete layer I had 2.5" of Foamglas. The insulation performance of the Foamglas should be superior to the vermicrete but this is my first oven so I can't say for sure. I'm hoping the 2.5" of Foamglas will provide nearly the same insulation performance of 4" of vermcrete. Then the 5:1 vermicrete ratio can provide me with a some thermal mass but not too much. I've been attempting to figure out this balancing act of insulation vs. thermal mass. The oven will be used for pizzas 90% of the time but I was concerned that minimal thermal mass would not allow me to do 8 or 10 or 12 pizzas without pulling the coals back over to recharge the hearth.
I start laying my cooking hearth this weekend.

As far as getting a uniform distribution of Portland throughout the cement...I used a paint mixer attached to a drill and mixed the Portland and water together in a bucket until it became a slurry. Then I dumped that slurry into moist vermiculite and then added water until the correct consistency. I thought it worked really well...

Regards,
Bob

[david s]

Sounds good Bob, but if you are wanting more thermal mass then it should be added under the insulation not over it.

[pizza_bob]

Originally Posted by david s

Sounds good Bob, but if you are wanting more thermal mass then it should be added under the insulation not over it.

David,

Wouldn't that defeat the whole purpose of the thermal mass? Insulation is to keep the heat in. If you have thermal mass under "perfect" insulation then heat would never reach the thermal mass. I understand the insulation used for pizza ovens isn't "perfect" but the concept doesn't change.
If built by FornoBravo plans... the 3.5" layer of concrete that is under the vermicrete layer is not for thermal mass. It's for structural purposes and the thermal mass in these designs are the cooking hearth bricks.
That's how I believe this works but I could be wrong.

~Bob