How long do I leave Vermiculite Cement?

[WoodywWun]

I'm using a 100 mm thick Vermiculite Cement mix on my concrete base for insulation. How long should it be left before I can build the cooking floor/dome?

If my Vermiculite cement isn't perfectly flat (I'll do my best!), is there anything more that I could do before laying the cooking floor?

Any experience, ideas, etc would be gratefully received.

[Hattori-Hanzo]

2-3 weeks if its warm and dry. uncover it during the day to let it air. Don't leave it covered all the time as condensation will form slowing the drying time.
Cement can take up to 28 days to fully cure.

A 50/50 mix of dry sand and powdered clay can be used to level the cooking floor.

[WoodywWun]

Thanks for your reply Hatton-Hanzi.

Wow, didn't think it would be that long! I suppose if it's not cured I may get cracking?

Can you help me with other parts of the project as well, please?

How long for my refractory mortar take to go off for the dome build? I'm going to have a go at it in one day on a sand former.

I'm going to insulate with a ceramic blanket (about 76mm (2") and then I'm proposing to cover it with Vermiculate concrete (8:1?). How long for this layer to cure, please?

I've seen comments that trying to put chicken wire over the blanket is a pain so I'm probably going to cover with concrete, just straight on.

Any other simple additions to this, such as metal foil that I've seen some folks use?

I'll top it off with a refractory/waterproof cement covering, I think. Is refractory concrete still needed on this final layer?

Many thanks for all of the advice so far. ...I may be back!

[Hattori-Hanzo]

That's for a full cure of the Vcrete. You maybe able to work on it sooner but if you start building on top it will slow the curing process considerably.

If using proper refractory cement I don't know the full cure times. As I understand it has a very short "working time" so you should mixing it in small batches. At assume it would be similar the cement so partually gone hard after 72Hrs but full cure could take up to 28 days.

2" of ceramic blanket would be 50mm. 76mm would be 3".

8:1 is correct mix ratio for vcrete insulation layer. again this can take several weeks to fully cure. I believe david s recommends making small curing fires after the vcrete insulation layer is added to help dry the vcrete this is done before adding the render layer so moisture is able to escape.

If you mean you are planning to add VCRETE straight onto the ceramic blanket without using chicken wire, I believe that is fine.
Do not use plain cement on top of the ceramic blanket.

No need for metal foil at that stage, I don't believe it would have any benefit.

The last layer should be render not cement. I don't believe you need refractory cement for this.

[david s]

Thanks for your reply Hatton-Hanzi.

Wow, didn't think it would be that long! I suppose if it's not cured I may get cracking?

Can you help me with other parts of the project as well, please?

How long for my refractory mortar take to go off for the dome build? I'm going to have a go at it in one day on a sand former.

I'm going to insulate with a ceramic blanket (about 76mm (2") and then I'm proposing to cover it with Vermiculate concrete (8:1?). How long for this layer to cure, please?

I've seen comments that trying to put chicken wire over the blanket is a pain so I'm probably going to cover with concrete, just straight on.

Any other simple additions to this, such as metal foil that I've seen some folks use?

I'll top it off with a refractory/waterproof cement covering, I think. Is refractory concrete still needed on this final layer?

Many thanks for all of the advice so far. ...I may be back!

Sorry I missed your post, there’s so much traffic on the forum atm it’s hard to keep up.
The homebrew mortar will take about the same time to harden as regular concrete. Ie, depending on temperature it will start to harden in a few hours and set pretty hard in 24 hrs. It needs to be kept damp for a week to enhance strength.You can remove the sand after 24 hrs, but keep the casting covered to keep it moist for a week before allowing it to dry.
The vermicrete will go from dark grey to white as it dries, but don’t be deceived, it will still contain lots of water that should be removed before you render over it.
I don’t bother with chicken wire to hold down the blanket as it’s a laborious process, just apply the vermicrete directly over the blanket. You will find it weird to work with. I suggest you build it up from the base about 6” high with a flat top and leave it to harden 24 hrs. This will give you a more solid base on which to build higher. It gets easier as you go because it slopes in the higher you go. Adding a foil layer creates a moisture barrier, better to leave it out as it also traps water in.
Using refractory cement on the outer layer is a waste of money. The stuff is expensive and the outer shell never gets really hot.

[david s]

I'm using a 100 mm thick Vermiculite Cement mix on my concrete base for insulation. How long should it be left before I can build the cooking floor/dome?

If my Vermiculite cement isn't perfectly flat (I'll do my best!), is there anything more that I could do before laying the cooking floor?

Any experience, ideas, etc would be gratefully received.

I did a little experiment on the drying of a vermicrete slab that you might find informative.
Vermicrete insulating slab copy.doc.zip

[WoodywWun]

Thanks to both responders! That is really useful information including the details of the experiment. OK, so I'll delay the post-lock-down pizza party by a few weeks! Hopefully, dry and warm weather will return soon. I would have been tempted to go straight into building the dome after the Vcrete and maybe light a little fire or two!

[L555BAT]

A question for David S if you don’t mind clarifying for me?

I’ve built my brick dome, I’m about to add 2 layers of blanket insulation followed by an inch or so of vermiculite concrete, then a rendered finish. in this instance, how do I best dry out the vermiculite? Small fires or just leave it?

alternatively, as I have plenty insulation, should I just do 3 or 4 layers of blanket, with a render directly onto it? I can sell the vermiculite on, or I may have an alternative use for it.

thanks

[david s]

If the vermicrete layer is only around an inch and a half and it is a lean mix, then there’ll be plenty of spaces between the grains so drying will be pretty much done in a week if the oven is exposed to sun and wind. You will need to push out moisture from the inner oven parts by fire after this though, so follow the thread on oven curing. It is better to drive out the water at this stage rather than after the outer render which would otherwise tend to keep the moisture in. It’s like trying to boil a saucepan dry that’s full of water with the lid off rather than on.
Some folk have managed to render directly on to the blanket, but because it’s springy it makes the job difficult. The blanket surface is also a bit lumpy so to compensate you have to make the layer thicker in parts. Adding a vermicrete layer over the blanket allows you to make the form perfect again as well as providing a nice firm substrate to render against.

[L555BAT]

Thank you David S, that’s great I will stick with my plan for a think vermiculite layer then cure it slowly.

Any recommendations for how to seal between render and chimney?

[david s]

If you wrap some cardboard around 4 mm thick around the flue pipe, render up to it, then when it’s hard remove it and fill the gap with high temperature silicon, it should give you a good flexible seal. But be careful decent flames up the pipe will kill the silicon. In my builds the seal is quite high where the pipe is somewhat cooler.

[L555BAT]

Perfect, thanks again. Only a few weeks away from first firing then, getting exciting!