Vermiculite concrete

[pocahontius]

Hello,

I think I have an easy to answer question: At the time, I am building my oven stand. I plan to finish the hearth this year and to build the rest in the next year. I know that I have the two possibilities: calcium silicate insulating boards or vermiculite concrete under the oven floor. Calcium silicate insulating bords don't seem to be waterproof, so I would put them on the hearth in the next year. But if I would use vermiculite concrete, can I pour that on the hearth and rain / snow will not destroy it? Or do I have to cover it, too, like the calcium silicate bords?

Thanks for your help,
Jens

[Karangi Dude]

Hi Jens,

Welcome to the forum, yes it is important to cover whatever insulation you use so that it does not absorb any moisture, the support slab will be ok uncovered as it is just concrete and when cured will repel water

Cheers Doug

[pocahontius]

Ok, thanks. Then I'll only pour the concrete top this year...

Greetings Jens

[pocahontius]

Hello again :-),

another question came up:

I have noticed, that I have mixed some concepts: In the fornobravo manual are two methods described: vermiculite concrete under the oven floor or ceramic fiber insulating board. In this forum I read something about calcium silicate boards and I thought, this would be the same as ceramic fiber insulating board...

I want to use a board material and not the vermiculite concrete, because I want a thinner oven hearth. My local supplier has:

calcium silicate board, compressive and flexural strength at 100 tons per square meter, 1000 °C classification temperature and thermal conductivity of:

200°C - 0,09 W/mK
400°C - 0,11 W/mK
600°C - 0,13 W/mK
800°C - 0,16 W/mK

cold compressive strength: 1,4 N/square millimeter = 203 psi

THEY SHOULD NOT GET WET, as my supplier stated.

1.) Are these boards I can use? All the values are in the right range I think, but I am not sure.

2.) When they should not get wet, how do I put the wet fireclay on them and lay the floor tiles? I have seen some videos where builders used some aluminium foil and wrapped the boards. Do I have to do this? I haven't found any information about this step.

Thanks
Jens

[UtahBeehiver]

Off the cuff, the material specs look okay. Why don't you attach a PDF of the cut sheet for the material. If your concrete hearth is flat the the CaSi board is uniform thickness, then you may not need a fire clay/sand mix. You should be more concerned about how to address water getting on to the hearth then soaking into the CaSi.. Things to do are drilled weep holes in hearth, raise CaSi off hearth (cheap tiles have been used lately, concrete patio stone as well. I use FoamGlas, non water absorbing insulation as my first layer. Wet floor/saturated floor insulation is one of the most common problems with poor WFO performance.

[pocahontius]

Hello,

here is the spec sheet (in german). I'm not sure if I understood you right - The shop told me the following: Onto the concrete hearth I have to put a waterproof coating. On this coating I should lay the CaSi board. So it ist protected from the bottom. When I put the fire bricks on top, where do you mean should water come in? through the fire bricks when cooking? I'm wondering because I saw people using this aluminum foil around the CaSi board...

Jens

[UtahBeehiver]

water can migrate in from the hearth as rain hits the hearth or through the door opening via the brick floor opening. Placing a water proof membrane on the hearth can also trap water between the waterproofing and CaSi with no means of egress. This is why weep holes are advocated and raising of CaSi off the hearth is the latest improvement to construction. The specs, although I cannot translate German, I can tell this product is CaSi (roughly 40% each). Compression is good at 203 psi although I did not verify the conversion. The K values are in the range of CaSi products. As far as aluminum foil, same holds true of holding in water if it gets in. At least on the forum, use of foil in not promoted. There are a lot of WFO you tube videos out there, some are good and some are down right bad.

[pocahontius]

Ok, Now I understand and I took a look at your build, Foamglas would be easily available here. So if I understood you right I put several small holes in my concrete heart right down, on top of that I place the foam glass and then the CaSi? I did not see any holes in your build? Is there a recommendable thickness of the foam glass?

(1) I am thinking about that, because I will pour my hearth in the next week or so and think about pouring one thin concrete layer and then, when the build is completely done a second layer on top of that to reach the height of my cooking hearth.

(2) Or if I pour one thick layer of concrete with a deepening for the oven in the middle. But then I have to know exactly the heights of the different isolating layers and so on.

Did version (1) work good for you or would you try it like version (2)?

Jens

[UtahBeehiver]

Is there something driving that your oven height needs to be an exact height dimension? Rule of thumb for oven height is elbow level, not too low so you have to stoop, not too high so you can see the pizzas, but give or take an inch or so. One piece hearth pour is your best and strongest hearth. Weep hole was drilled in after the fact while hearth was still green. I am not sure what you mean in #2 deepening in the middle.

[Smokamatic]

I see on another website where the CaSi is covered in aluminum foil. I assume this is a water barrier. It seems the top and bottom cold be covered in foil. Do you know pros/cons with this method?

[UtahBeehiver]

read post #7

[pocahontius]

Another short question:

Why should I use vermiculite + portland cement for the vermicrete and not vermiculite + refractory cement? Wouldn't that be better suited?

Greetings Jens