800 mm homebrew cast

[sergetania]

Wait, why are you torturing yourself? Just curious. I thought you have 50mm of cal sil board on top of the concrete table? Throw away the perlite mess and just use the board. I have 50mm board under the hearth and no other insulation. The underneath of the oven never ever got even hot. They say 3" is recommended but 2" is enough. And I have a wooden base. I have had the hearth at above 1000F and maintained 800-900F for 3-4 hours straight.

[Linuc]

Wait, why are you torturing yourself? Just curious. I thought you have 50mm of cal sil board on top of the concrete table? Throw away the perlite mess and just use the board. I have 50mm board under the hearth and no other insulation. The underneath of the oven never ever got even hot. They say 3" is recommended but 2" is enough. And I have a wooden base. I have had the hearth at above 1000F and maintained 800-900F for 3-4 hours straight.

Haha. Yes. 50 mm of CalSil was my initial plan, but I got convinced to use the extra 50 mm of perlcrete. Since I probably only going to build one oven in my live, I thought I would do it the right way.

And Petter , thanks. I will let it dry for one week and then we'll see.

[sergetania]

Since I probably only going to build one oven in my live, I thought I would do it the right way.

But it could be fun, not torture! Our ovens are not commercial. The most pizzas(neapolitan) I have ever baked in a night was 10 (+2 flatbreads to use up remaining dough). My oven heats up quickly and holds the heat well with outer surface just getting warm. I think I have enough insulation with only 50mm calsil board underneath. Keep in mind pretty much everything I have done I have learned here, mostly from David. Anyway, just talking, your choice, always!

[david s]

Hi, I'm very new to building a pizza oven myself, but here are my 2 cents of thought:

• 32mm firebricks: even though 32mm is not really thin, I think 5 to 6cm thick bricks are more the 'norm'. I guess thicker bricks mean more heat retaining mass, which could be usefull to keep the oven hot longer. a thinner brick might heat up faster and could be ideal if you're mainly interested in pizza's?
• 25 mm or maybe 50 mm ceramic blanket: from what I've read you can't overdo insulation, so go for the 50mm
• 50 mm perlite/portland: I'm not sure but I think 50mm is a bit much for a render layer, 2-3cm is more normal.

I could be wrong on all three . Waiting for the experienced posters to chime in.

looking forward to your build.

I like to do a 10:1 vermicrete layer over the blanket so it's a decent insulating layer as well as creating a firm substrate for a 12mm or so render layer. A vermicrete render layer lacks strength as well as being rather weather absorbent.

[david s]

Cover it for a week, remove the mold and let it dry. Don't lift it wet. It is fragile and any additional weight (from the exess water) is a risk. This big and only 50 mm thick is really pushing limits of what is manageble.

Slide it onto a plywood sheet when moving.

Lycka till!

There is so much free water in a vermicrete mix that there is plenty there for the hydration process. For this reason I don't hold the water in by covering for a week after placement. The drying is another matter and if covered over by cal oil or the oven floor too soon will result in water being locked under the floor which makes its removal very slow. A number of years ago I did an experiment of the drying of a vermicrete slab (see attachment) and was surprised at how long it took. Weep holes in the supporting slab go some way to assist the exit of water vapour, but allowing the weather to do much of the removal for you is far better, although (weather permitting) takes time.

Vermicrete insulating slab PDF.pdf

[Petter]

Thanks David, it surely takes long time and if it possible to save some, that's good. Building takes long time anyway.

I did a few tests with perlite concrete this winter and it took 2 months for a 50x400x400 slab to dry out in the garage (5-10 C and no sun..). Thats why I did cast in milk packages and dried them in the oven after a week instead.

[david s]

"Thats why I did cast in milk packages and dried them in the oven after a week instead."

That is a really good idea.

[Petter]

Thanks :-)

Sorry Linuc for borrowing your thread.

Short story is that at and beyond a 1:12 ratio by volume, the cement does not contribute to the compressive strength. Above 1:25, the cast begins to be too crumbly when dry. 1:25 was as lean as I could go with maintained workability, yielding a density of 0.22 kg/dm3. I.e. Equal to CaSi board.

I used 1:25 as dome insulation for the small garage-oven this spring. 10 cm thick. No problem to get it to stand vertical but these lean mixes are very very picky on water addition. More cement mixes are a lot easier to mix to good-enough water ratio. I would therefore not recommend it to other unless they are really devoted.

[Linuc]

Thanks :-)

Sorry Linuc for borrowing your thread.

No problem, It's interesting!

What do you guys think about puttning an dehumidifier in the same room as the perlcrete slab? Will it speed up the curing process?

[Petter]

Sure, and after a week you can lift it up some distance for the air to circulate around.

[david s]

No problem, It's interesting!

What do you guys think about puttning an dehumidifier in the same room as the perlcrete slab? Will it speed up the curing process?

No, it won't speed up the curing process. There is so much water in vermicrete for the hydration curing to be unaffected. It will aid the drying process to remove excess water, but it will cost you plenty in excess electricity, as the dehumidifier will also be removing moisture from the surrounding air in the room, just like the difference between drying clothes in a dryer as opposed to hanging them on a line outside in the sun and wind.

[Linuc]

I've done hours and hours of research about the lime for the homebrew. I know it's supposed to be builders lime, not agricultural and I've read about S-type and N-type. In Sweden however, we don't have S or N-type. The builders lime is called type E and is sold as a binder. It contains calcium hydroxide and portland cement and have a melting point at 580 C. I'm a bit unsure of the builders lime since it contains portland cement, wouldn't that screw up the homebrew recipe?

The other one, Nordkalk SL, is referred to as "suitable for many uses, but the main use is for neutralization." This is pure calcium hydroxide and is found at agricultural product stores. As I said, I know it's not supposed to be agricultural lime but from what I can see, most things seems right about it. I do know a lot of people in Sweden have been using it with good results, but the question is how sustaineable it is... Here is the safety data sheet if someone could have a look and see if it's the right stuff: https://www.nordkalk.com/document/3/..._25_2_2019.pdf

And sorry David and Petter. We've already discussed this in PM. Just trying to get some more input

[sergetania]

Considered using a commercial castable? More money but it was easier for me personally because of my lack of experience. Also, you don't have to measure different parts and mix it together. Just water (and maybe SS needles). It's an alternative.

[Linuc]

Considered using a commercial castable? More money but it was easier for me personally because of my lack of experience. Also, you don't have to measure different parts and mix it together. Just water (and maybe SS needles). It's an alternative.

I've already bought all the other ingridients for the homebrew, so there's no turning back now. We'll see. I'll probably go with the swedish version om "builders lime". Hopefully it's the right stuff.


If I have understood it correctly, the lime should kick in and strengthen the mortar when the temperature rises so high that the cement begins to break down. Nordkalk have a melting point at 450 C, so that doesn't seem right. However, the builders lime have cement in it so that doesn't sound right either.

[Linuc]

I read in another thread that you could made samples to see if you've got the right lime.

So I did two samples. One with one part lime, one part sand and water and one with only lime and water. I'd let it dry for 24 hours and both became hard. It does break apart when I press them, so they are not rock hard. See pictures.

Can you tell from the photos if I got the right kind of lime?