Forno Bravo Forum: The Wood-Fired Oven Community

Forno Bravo Forum: The Wood-Fired Oven Community (
-   Wood-Fired Oven and Cooking Videos (
-   -   Mixing Insulating Concrete (

leemc 12-26-2008 03:30 PM

Mixing Insulating Concrete
I thought I would post a video of my mixing and installing perlite concrete. Since it is kind of oddball material most people don't have experience with I thought it might help some folks. This is the first time I ever worked with it so let me know if my technique looks OK


<object width="425" height="344"><param name="movie" value=""></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>

Here is the link - from the preview I don't know that the embedding is going to work right....

YouTube - Perlite 0002

Les 12-26-2008 04:32 PM

Re: Mixing Insulating Concrete

Good video - has a touch of "Blair Witch" to it. :) I only used the product for dome insulation (on top of the blanket) and mixed it in a 5 gallon bucket using a drill and beater. Your build is looking great!


Dutchoven 12-26-2008 04:45 PM

Re: Mixing Insulating Concrete
You did a very good job mixing and pouring...looks just like ours did...and I thank you for putting something like this on the forum...we have described what it looks like and what it is to work with and now there is something these same folks can look at for themselves...only one thing that I would add and that use the trowels and screed in somewhat of a tamping capacity on the surface....if you wet the trowel and use it like that it becomes somewhat of a "jitterbug" tamping the aggregate(the perlite) under the surface and brings the creamy portland cement to the surface....looking forward to watching your oven take shape!!!

dmun 12-26-2008 07:51 PM

Re: Mixing Insulating Concrete
Your video looks good: it's weird stuff but you seem to have it right. You report that the slurry method doesn't work? I thought that was the new method that avoided the clumping at the bottom of the pan, never tried it myself.

Embedding youtube videos is really easy: {youtube}number{/youtube}, but replace fancy parentheses with square brackets.

Wiley 12-26-2008 09:51 PM

Re: Mixing Insulating Concrete
Good looking video, I like your enclosed area for the insulation under the hearth, almost a basin to hold the under hearth insulation.

On my WFO I did the slurry mix and found that I could get a ratio of 9 or 10 to 1 vermiculite to cement ratio. Honestly after viewing your video I think the dry mixing followed by adding water requires much more physical work than the slurry method. Also in support of the slurry method, it allows for alot of self leveling of the mix (as it is quite fluid when first poured), that's good if your forms are level, not so much if they are not. Simply dip and pour the mix from a wheel barrow by a bucket.

When mine was cured it had the consistancy of cork...I had to dig out the corners and fill with concrete... the same corners you previously had cast as part of your "basin". I wasn't clever enough to have thought that part thru at the time I was building. Hopefully someone will alter the plans so that one isn't directed to cast the entire layer as insulation. I think a square basin with lopped off corners like yours or a true octagon would be best. But what do I know? I've only built one oven although at a Christmas Party yesterday I was approached by a friends wife who would like a oven and wanted me to assist her husband building one this coming summer.


leemc 12-27-2008 07:34 AM

Re: Mixing Insulating Concrete
The trouble I had with making a slurry was that I don't think I had right tool for it - which would be a mixing attachment for the drill. When I did it I just poured half a bucket of cement and started putting water in it. This resulted in the cement at the bottom of the bucket to become like gumbo clay and I had a real difficult time getting it into the mix. If I had a mixer or maybe tried to do it it two batches or maybe putting water in the bucket first and adding the dry cement to it probably would have gone much smoother.

The other method I used - pouring the dry cement into the perlite, mixing, adding water and mixing was pretty easy. Working with this stuff is child's play compared to mixing real concrete.

If you want to see some pictures of how I formed my recess for the insulating concrete you can go here....


SpringJim 12-27-2008 09:53 AM

Re: Mixing Insulating Concrete
1 Attachment(s)
Lee the video is's pretty weird stuff! I agree with the general description that it looks like oatmeal or is even like trying to mix styrofoam beads with water initially!

I did Craig's slurry method and was very happy. I only used a small amount of portland in the water which created a very light mixture for me. The portland was well mixed into the final product because it was part of the wet side of the formula. (much better in my opinion than dry mixing) I did use a paint/drywall mixer attachment on the portable drill and this worked well.

For later insulation layers I went with some fireclay too, just to make the mix a little more manageable for the dome insulation. I'd suggest that you can reduce the portland content if you use the slurry method and that will give you better insulating values.

I think one mistake I made was to use siliconized perlite for masonry insulation which made it harder to mix with the liquid...but it finally worked.


Wiley 12-27-2008 10:44 AM

Re: Mixing Insulating Concrete
SpringJim has it right, one really needs a mixer attachment for a drill in order to use the slurry method. And using the attachment one can achieve a better ratio of cement to vermiculite or perlite too.

I don't know how one could make the slurry without the mixer attachment, maybe slow pouring the dry cement into a bucket of water and stirring like crazy with some sort of paddle while mixing. Even using the mixer arrangement when mixing the cement in the bucket (before adding to the wheel barrow of dry or moistened vermiculite) one has to move fairly quickly...stop mixing, pull drill mixing attachment from bucket and pour the bucket in one smooth motion. No stopping to chat or a smoke or to scratch where it itches as the cement settles fairly quickly. Once poured the mixer attachment goes in the wheel barrow to mix the vermiculite and slurry. Then it's a dip of the bucket into the wheel barrow load of mix and pour into the form. All in all it goes quickly.

As for mixing for a vermicrete or perlcrete layer over a dome I have no experience. I used insulating blanket and then dry, loose filled vermiculite. I suspect one would have to mix like you did in order to get a stiffer material that would better stand on itself.


SpringJim 12-28-2008 10:05 AM

Re: Mixing Insulating Concrete
2 Attachment(s)

Originally Posted by Wiley (Post 47888)

As for mixing for a vermicrete or perlcrete layer over a dome I have no experience. I used insulating blanket and then dry, loose filled vermiculite. I suspect one would have to mix like you did in order to get a stiffer material that would better stand on itself.


I covered my whole dome with it and it worked fine for me. Started at the bottom and built it up in a thick layer. I did have an initial layer of used firebricks which I'm sure helped as it gave me a terraced effect. I did add a small amount of fireclay which I believe improved the workability. I cut some of the layer off with a handsaw for my entrance and it was a lot like styrofoam, fragile but it floated too! I had a very high perlite:portland ratio.

Lill Dave 12-30-2009 08:51 AM

Re: Mixing Insulating Concrete
Using this insulating concrete mix over a insulating blanket and wire mesh how do you think it would hold up on a portable oven, cracking wise.

All times are GMT -7. The time now is 04:42 AM.

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