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cvdukes 04-12-2008 10:00 PM

TIP: Mixing Vermiculite Concrete
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I know a lot of us end up making vermicrete, either for isulation under the hearth or for a layer around the fire dome. I've even used it as finish roofing (not my best idea... see thread ...), and as an inner layer on the wooden doors I use on the other ovens to keep them from burning up.

Problem I've had in the past always came in getting an uniform distribution of concrete mixed throughout the vermiculite. General method has always been to get the vermiculite soaking wet, then add in the dry portland cement powder. I've used a drill mounted mixer, mixed it by hand, and have even pulled out the garden tiller, but It still ends a mish-mash of concrete consistencies. As a result, I think I end up using a lot more cement than is really needed.

The solution is to make a slurry of the Portland cement in water before mixing into the vermiculite. I fill a 5 gallon bucket about 2/3 full of the cement powder, then almost fill to the top with water. Then take a drill-mixer (pictured), and get all the cement suspended into the water. Then pour it into the moist vermiculite. At that point, I’m using a little electric garden tiller (picked up off a trash heap…took all of 5 minutes to find the loose wire that got it discarded)…but it can be mixed by hand with not much more effort. Doing it this way, I’m getting a uniform mix…

Since I started doing it this way, 90 pounds of Portland will bind up 12 cubic feet. That works out to about a 12:1 ratio by volume, which is a lot less cement than most are using. Less cement = higher insulation value.

Even though I have a more dilute cement mix, it seems to sets up fine… probably because of the more uniform distribution. Today I was walking on sections I poured yesterday and it supported my 225 pounds without leaving depressions.

Background story: Reason I use so much vermiculite is that I got it cheap (~$1.30 per cubic foot). Reason it was cheap is because I was able to buy a large amount (168 cubic feet) straight the mine where they make the stuff. Reason I got so much is that I wanted to get enough to insulate the bottom of the swimming pool this spring when I had to replace the liner. The pool project is now underway. After mixing up nearly 4 & 1/2 cubic yards of the vermicrete, I've finally stumbled on this better way to mix up the vermicrete.

For the pool vermicrete, I could have borrowed a neighbor’s concrete mixer, but the hassle of moving it down to the pool, and then having to go up and down the pool ladder with buckets of wet vermicrete didn’t seem worth it. Instead, put a wheelbarrow down in the pool and did all the mixing using the tiller. Today’s rain messed up my schedule but I should finish it tomorrow. Plan to have pizza party nest weekend and see if I can get some the guys to help put the new liner in exchange for food…

bad use of vermicrete roof thread

Xabia Jim 04-12-2008 10:35 PM

Re: TIP: Mixing Vermiculite Concrete
Craig, I've never used it so don't have experience...

....does it flow well enough that you could use a tremie pipe to get it down to the pool? something like a 6 inch pvc pipe with a funnel on top....might save a lot of steps....

david s 04-13-2008 03:42 AM

Re: TIP: Mixing Vermiculite Concrete
I agree about the lesser amount of cement to vermiculite. The more cement you add the less insulation value as all the spaces become filled with a denser material. Trouble is the less cement you use the harder it is to handle. I've been using 10:1 and mixing cement and vermiculite dry in a bucket before adding water and mixing again. Shall try your method next time.

Frances 04-13-2008 04:57 AM

Re: TIP: Mixing Vermiculite Concrete
How did isulating your wood doors work? How do you attach the vermiculcrete to the back of them and have you got any pics?

Good tip with the mixing btw. Makes good sense, sounds a lot easier... but you have to think of it first :)

cvdukes 04-13-2008 06:11 AM

Re: TIP: Mixing Vermiculite Concrete
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Originally Posted by Xabia Jim (Post 29504)
Craig, I've never used it so don't have experience...

....does it flow well enough that you could use a tremie pipe to get it down to the pool? something like a 6 inch pvc pipe with a funnel on top....might save a lot of steps....

XJ Actally you can make it pretty wet so I guess it could be tremied, but then I would need a hopper up high to dump it into and someone else to guide the other end of the pipe. As for the steps...I know people that join gyms,but I've wonder why 'cause I get enough exercise here!

Most of the wood doors were made by rivetting together a light metal frame that would fit into the door openings. The metal came from ripping ripping those lightweight metal 2x4 studs lenghtwise so it was basically like just a flimsy peice of angle iron at that point. Then I attached a peice of wood crosswise on the outside at the top & bottom of the frame...these were held on screws from the inside of the frame. Flipping them over, i glued and nailed the outside boards on the doors. The doors were then flipped back over, and wet vermicrete poured into the forms. Gave it two days to dry and hung them. The metal frame protects the edge of the vermicrete where it hits on the door openings.
All of the doors ended up with about 3 inches of vermicrete. On the attech picture, the doors on the lower portions of firboxes have a vent opening in them that is covered with expanded mesh gutter guard. The pizza oven is under the roof peak...still using a peice of steel to cover it as I haven't vermiculited ior made the wood cover for it yet.
The little wood door in the very center is a vermicrete filled wood-framed box plug that fits inside a 8" flue tile set horizontally. Just perfect for warming up some french bread while the oyster cooker (left sideof the complex) is roaring. Heat and smoke flow around bread warmer enroute to the chimeny or smoker oven (under the pizza oven) depending on how I set the dampers. The plug was the exception to using metal framing to hold the vermicrete because I figure the temps would only be in 3-500 degree range. Last week, the wood that inserted into the flue tile started smoldering, so will have to replace it.

Jed 04-17-2008 10:28 PM

Re: TIP: Mixing Vermiculite Concrete

Thanks for the tip on mixing the Portland Cement and water together in a slurry before mixing with the vermiculite (perlite in my case..).

I used this technique yesterday to form up the insulation layer on my hearth, and it all went smoothly. I did not take the time to look for a repairable garden mixer and used my wife's garden hoe and did the mixing by hand and it seams to have all come out OK. I did increase the volume of perlite up from the 5:1 mix to 6 or 7:1 and it all seams to have set hard. It was simple to get complete mixation (?) on the materials, just a bunch of work, and that is part of the fun!


cvdukes 04-18-2008 04:30 AM

Re: TIP: Mixing Vermiculite Concrete
Glad it worked. Always worry that things like that work for me because of water qality or sheer luck , and not repeatable by others.

I can tell a huge differenced in the quality of the vemricrete on the bottom of the pool based on wheter it was done before or after I started doing it this way..

Frances 04-18-2008 11:50 PM

Re: TIP: Mixing Vermiculite Concrete
Dunno, but maybe this would be worth turning into a sticky post or adding to the plans? James?

I know I would have liked to have this info before mixing my vermiculite!

SpringJim 06-10-2008 05:32 AM

Re: TIP: Mixing Vermiculite Concrete

Originally Posted by Frances (Post 30028)
Dunno, but maybe this would be worth turning into a sticky post or adding to the plans? James?

I know I would have liked to have this info before mixing my vermiculite!

James, how often are the plans updated? Are you keeping a list of updates to do? I will keep a list of comments as I go through the plans.


Wiley 06-28-2008 07:21 PM

Re: TIP: Mixing Vermiculite Concrete
I poured the vermicrete insulating layer in my WFO hearth today. One very interesting thing happened that no one has mentioned and so I am wondering if I did something wrong or missed the warning/advisory somewhere.

A four cu ft bag of vermiculite doesn't make even 2 1/2 cu ft of vermicrete.

My calculations put the needed quanity of mixed vermicrete at just over 5 cu ft. I had two bags each claiming to contain 4 cu ft. I thought, no problem I should be able to do that and have vermiculite left over. Not so, I mixed an 11 to 1 mixture using cvdukes slurry method. But came up shy by about 1/4 inch on my forms. I made a quick dash for more vermiculite but by the time I got back it had set such that it wanted to tear rather than blend new with old. Tomorrow I will top the form up and lay on the high tech insulation.

Has anyone else encountered this problem?


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