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Stiennon 06-29-2007 10:32 AM

Help! Vermiculite problems
OK. I am working on the hearth of my Pompeii oven. I called every garden center in Southeast Michigan. They either did not carry vermiculite or they were sold out. Finally, after finding a web page that tipped me off that vermiculite is used in the construction of swimming pools, I found a pool supply store that sold me six bags of a special mix of vermiculite and portland cement (called A-TOPS Pool Mix, comes in 2 cubic feet bags for $18.95 each).

I mixed it with as much water as it could hold and shoveled in to the form on top of the concrete stand. It is now the second day and I am concerned that it is not setting properly.

I can still push my finger into the top surface. The vermiculite mix is firm but not super rigid. Is this the way it is supposed to be? Will the edges crumble when I take the wood form away? Am I really supposed to build a brick structure on top of this soft base?

Any guidance would be appreciated!


wlively 06-29-2007 05:10 PM

Re: Help! Vermiculite problems

I have not used the mix you refer to. If the ratio of portland to vermiculite is the same as a post I found on a pool forum (7:1) then I would think you would be OK. The mix is very crumbly indeed, I have more than a bit missing from the edges of mine. But it does have some compresive strength and should hold OK. If memory serves, my mix was "soft" for more than 3-4 days. Give it a few more days and see how well it hold weight.

rtnaw 06-29-2007 07:20 PM

Re: Help! Vermiculite problems
I was a little nervous also but, it took about a week for the vermiculite layer to harden up. That said, it is still on the crumbly side and pieces around the edge chipped off in spots.

nissanneill 06-30-2007 03:42 AM

Re: Help! Vermiculite problems
I think that your mix is a little on the light side with cement, but it might come up in rigidity over a week or so. I mixed my hearth vermiculite insulation 5 parts : 1 portland cemrnt, see:

thread #2 for the finer details and pictures. It took 3-4 days to stiffen up and after a couple of weeks, it felt like a fairly 'firm' cork type consistency.
After the dome was built (the following week-end, I covered it with a thermal blanket and mixed 3 1" layers of the same vermiculiter cement and topped it with a 3/4" layer of water proof render.
See how it goes over the next 5 to 7 days. If it is still very crumbly still, I might be tempted to remove an outer layer (say 6 inches or so and remix a little more cement 5:1 and relay it to take the load of the bricks of the dome, which will be quite considerable. It will bond onto the existing layer but would be much stronger and harder.

Unofornaio 06-30-2007 07:11 AM

Re: Help! Vermiculite problems

Originally Posted by Stiennon (Post 12127)
I mixed it with as much water as it could hold


Hey Richard and welcome to the forum,
I have 2 concerns:
1. 7:1 is pretty lean on the cement side (as mentioned by Nissaneill) This is also what the FB guide recommends.
2. "as much water as it could hold" is the BIGGEST concern and was my first thought.
Mixing this vermiculite as most on here will agree I'm sure is tricky at best. the key is to have it mixed very well BEFORE the water and then add just enough water for it to hold together. Since you used a bag mix we can assume it was mixed well prior to water.
So this is where we are at with your mix, did you add just enough water to make it hold together like.....loose garden soil when you squeeze it it stays together other wise its kinda like the crumb topping on a muffin.
Or was it wet and soggy? too much water and the cement is greatly diluted thus the strength. also too much water will make the vermiculite softer that it should be and susceptible to compression if not placed correctly. Id say wait at least a week and see if it comes up to strength. The mixing of cement products even bagged mixes such as spec mix or your basic concrete mix form H.D. can be DRASTICALLY altered by the mixing method, water content and mixing time. Mix until moist, mix until sticks to trowel, mix until free flowing, all are "relative" in definition unless specified by weight. the good news is if it doesn't get hard you can re use the mix... MAYBE? For now patients...

maver 06-30-2007 07:57 AM

Re: Help! Vermiculite problems
Well, the recurring theme here is that everyone who mixes vemiculite cement has misgivings - it doesn't feel right the first time, but generally after a week all is well. Hang in there. Most of the comments here about this can be used if it completely fails, but usually it just works!

Balty Knowles 06-30-2007 07:59 PM

Re: Help! Vermiculite problems
4 Attachment(s)

Is the slab firm without any loose material? If there's not enough cement then the vermiculite stays loose & can almost be brushed out, If this is the case then I would scrape out the loose material & pour another slab on top of what's left. If you can't scrape it out then it's probably OK. When the pad's cured its still kind of springy, Nissaneill's analogy of cork is a good one

This is how mine looked the first time. I had added too much water & mixed in a mixer, not good, the slab ended up with some loose vermiculite where the cement had kind of washed off. I mixed another batch by hand 5:1 dry first & then less water & re poured another 3" on top of the old slab. I also took this opportunity to install a split brick sub floor

In both cases I seem to remember it setting in a day or 2, maybe I'm wrong but I think I would have freaked out if it was still wet after a week.

Pouring this slab is miniscule compared to the work you'll put in on the dome & rest of the project, not sure of the consequences of building on a soft pad but personally I pour some more.



Versachi 06-30-2007 09:10 PM

Re: Help! Vermiculite problems
I just poured my vermiculite slab this past week and it is now 4 days old and it is starting to get stiff. Was a little worried the day after I poured it that the mix was wrong because it seemed so soft. But now no worries.


Stiennon 07-02-2007 08:25 AM

Re: Help! Vermiculite problems
Thanks for all the input everyone. By the time I left for California the vermiculite was starting to get that firm, cork-like, feel that several of you described. I think I am going to be OK. If I did it again I would leave space around the periphery for some regular concrete just so the edges don't crumble away.

I am looking forward to sharing more with this excellent forum!


edschmidt 07-02-2007 07:27 PM

Re: Help! Vermiculite problems
Im in western michigan and found vermiculite availible at menards. I know it doesnt help with the hearth but it might still help for the loose fill on top of the dome.

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