Castable Refractory Products (VT600)

[wotavidone]

I've been involved in writing a few SDS's and yes you always give a range rather than the actual amount.
This is to give yourself some wriggle room if your formulation changes and also to avoid giving away your recipe.
In this case they give a range for the cement. Could be as low as 10%, could be as high as 50%, but even if all other components are at the maximum, there is 19% cement in it.
I'd mix it like it is and see what that comes up like. If the OP must put some aggregate in it, he should go no more than 1:1
It will be interesting to see what happens.
The CAS number assigned to the "cement alumina chemicals" in the SDS is actually the CAS number for silica. HMMM.

The CAS number assigned to the Kaolin is actually the CAS number for calcium aluminate cement.
If you go by CAS numbers instead of words, then there is 20-25% calcium aluminate cement in it.
In that case it should not be mixed with anything else, there isn't enough cement for safety in my opinion.
Just mix her up, leave out the phosphoric acid foaming agent, hope the phosphate bonding isn't required and see how it casts and behaves.

[wotavidone]

I reckon I've figured the SDS out now.
Note that the list of ingredients is "off".
The material that is off set at the top, "clay, aluminium silicate" I assumed was from the bottom of the list. If you put it at the top of the list and all the names move down one position, then the names line up with the CAS numbers exactly.
This makes the material, based on the list of CAS numbers 35-45% mullite (highly desirable, very refractory), 20-25 calcium aluminate cement and the rest various silicas and kaolin.
In other words an eminently suitable castable refractory just like it is.

[TARibs]

OK so I'm pretty sure this stuff is going to work as a refractory castable as is. I made two bricks with it last Fri. 1st one was mixed at about 23% water by weight, at first that was as low a water/cement mix as I thought I could get it but after putting it in my orange juice carton mold and vibrating it a bit I was able to pour a bit of water off the top (Add less water!!). I tried a second batch and added a small handful of firebrick to the mix. I mixed this batch a bit drier, came out to 20% once I did my calculations on water to VT600 + the firebrick. Cut them out of the molds measured and weighed them today and density comes out to 2.028kg/l3 for the mix of just VT600 and 2.059 kg/l3 for the one with the firebrick. I figure if I keep the water around 20% (or even a tad lower) I should be good using just the VT600 itself.

Any suggestions on the what I do to test the strength and heat retention of this stuff? Hit it with a hammer? After how many days of curing? Heat with a what all I've got is a torch for soldering? At the price for this stuff right now it looks pretty ideal.

I've been looking at threads on casting a mold but don't see a lot of images. What is usually done? From what I gather the dome is made of Sand but then what, is the mix troweled and smoothed over the sand or would it be good to build some type of wood form around the base section perhaps a third of the way up, I thought I'd seen a thread on that but can't find it now.

[david s]

If you try to reduce the water too much you will get voids in the cast unless you vibrate it tons which is not possible when forming over a sand mould. It is better to just make the mix up to ball up consistency. You can always just build up around 6-8" and let that set for a few hours before proceeding higher.The CAC castable achieves full strength in 24 hrs, it does not need extended damp curing like normal cement castings. Here is a good well documented thread on a cast build that you should find informative.
#1

[TARibs]

Thanks David,
Yes I had seen that thread and it is one I have bookmarked. So here is my general plan so far maybe those with more experience can comment or suggest improvements

Start with the base, going to put 10 inches of crushed stone covered with a 4" reinforced concrete pad. Pad will be 48"x48" (built in the firepit area I made last year, (I've tried attaching a picture.)
Base is going to build out of Granite block (I had two, what I called barbecue pits on the property when I moved in 20 years ago, they were a mess so I tore them down but saved all the Granite blocks (see pics). They are just dry stacked in pics right now
Going to pour a 4 inch reinforced concrete slab for the on top of the Granite blocks. I question if this actually needs rebar or just steel mess in this slab (I was going to run a calculation and see if I could figure it out, I used to be a civil engineer)

32 Inch Cast Dome over sand (still looking at threads of the construction process), planning to use the refractory material I have access to that is the VT600 (will add SS needles if I can find them and need to figure out how many I'd need still, haven't seen a good calculation anywhere as to how much I'd need).
Going to cover domewith 3 inches of Vericrete (I already had bought the Vermiculite) and then a thin stucco layer over that.
Beneath the dome I was going to go with 3 inches of Vericrete, with Firebrick on top of that. I was going to build the dome on an outside ring of Firebrick that was independent of the inner floor (I've heard mention one may need to replace the floor at some time? (True?)

My biggest questions come in regarding the Entryway and inner and outer doors. Reading threads I hear mention of a firebreak but am not sure how this is achived. I understand it is to stop heat from the cast portion of the oven going through the the entryway (if there is no break) and out the chimney but not sure how the actual break is created

I've also seen mention of the magic ratio of 63% for door height or size to dome height but again haven't figure out what is being talked about there.

Not sure what kind of door I'm building yet? I'm curious how the placement of the door on the fire side of the chimney doesn't cut off the air access to the fire and put it out. That seems to be the correct design but don't see how the fire gets any oxygen to burn.

Then the chimney, I have yet to find a reasonable priced SS chimney pipe. So am looking for other alternatives at this time.

Any help is greatly appreciated.

Thanks
Tom

[david s]

Hi Tom,

You have obviously been doing some research judging from your questions.The oven will be pretty heavy, but with decent foundationa as you propose, those big granite blocks should be fine on their own without any rebar reinforcement.

The SS needles are not mandatory. I use them because it makes a stronger product, but I don’’t think most manufacturers do. They are expensive and are a bitch to work with. They don’t call them needles for nothing. If you do decide to use them 2% by weight is the min recommendation. Up around 4% andthey make the mix far less workable.

What is probably more important is the burnout fibres. If your mix does not contain them then you should add them to reduce the possibility of steam spalling.

3”” of insulation both under and over the oven is considered minimum. You would be far better off making it four. For over the dome there is a problem using vermicrete directly against the dense inner oven casting and that is because the vermicrete mix ends up with so much free water after its hydration and that water is likely to be deep in the layer against the inner dome, it can suddenly turn to steam and make you vermicrete layer swell and crack. It is far better to have a layer of ceramic blanket over the inner dome casting and then vermicrete over it. The ceramic blanket acts as an expansion layer and reduces the sudden heat against the vermicrete layer. Typically the blanket is 25 mm(1”) thick.

Your idea of a firebrick ring for the dome to sit on is a good solution, but still involves a lot of time consuming brick cuts. If any bricks need to be replaced they will be the ones in the floor centre, not the ones on the periphery. This means that you can be pretty rough cutting the bricks as they will be covered by your insulation, thereby saving you hours of work.

The thermal break is probably better referred to as an expansion joint as that is its primary function. The idea is to allow the inner oven parts to expand without placing stress on the cooler outer parts of the oven. If you are casting your oven then the flue gallery can be made quite thin and light compared to a brick oven so the heat sink effect is far reduced. So in this case it is probably better to place the expansion joint between it and the outer decorative arch which in turn can be part of the outer oven shell. To fill the gap in the thermal break/expansion joint a low conductive material is required. I use a 5:1 vermicrete mix in conjunction with ceramic fibre blanket for the floor joint and simply an air gap at the top and sides.

The magic 63% (which interestingly also happens to be the same ratio as the Golden Rectangle) is less important as many would have you believe. To capture the heat the whole length of the flame needs to be maintained in the oven to extract its energy, so it needs to be drawn down before exiting out the flue. Too low and the chamber won’t burn efficiently. 4 or 5% over or under 63% and you would notice no difference in the way the oven fires or its performance.(I’ve learnt this from practical experience building ovens that don’t conform to the ideal ratio.

The door will certainly extinguish the fire’ at least it does in my oven. If doing a roast I don’t bother raking out the coals (more work) as they go out with the door in place and lumps of unburnt charcoal are left behind when the roast is removed. You can of course cook in the oven with an active fire with the dnoor open or partially closed, there are many options.

For your oven size you need a 6” diam or equivalent cross sectional area flue or chimney. An alternative is a clay pipe but it needs to be insulated on the outside or it will crack.

Keep asking questions and reading threads. Hope this is understandable and helps
Vermicrete experiment attached

Dave
Vermicrete insulating slab copy.doc.zip

[TARibs]

So after a long layoff I'm back at it again. Finally have the base built now ready for the actual oven to get done. I was getting ready to pour the Vermiculite slab and noticed 4 inches was recommended? Would 2 be of any help or should I do the 4? Right now I have it formed up for two so I guess I need new forms. Anyway here is look at where I'm at with the base which took forever to get where I wanted it.

[TARibs]

And here it is with the top slab. Oh David you sent me a recipe with Vermiculite and Perlite. Could I just use Vermiculite? I have a 4 CuFt bag but no Perlite

[david s]

Vermicrete does not have sufficient strength to support a heavy oven. You must have a strong reinforced concrete slab on top of which you cast another vermicrete slab. The recommenced ratio is 5:1 vermiculite to cement and 4” thick. Vermiculite and perlite are interchangeable. I find that a mix of the two creates a more workable mix than either of them alone.You will need 8 cu ft of vermiculite for a 48x 48” x 4” vermicrete slab ( There is around 20% loss of volume when mixed with water and cement so account for this.

[TARibs]

Thanks David I was going from this chart I found posted here on the site https://community.fornobravo.com/for...ratio-question 4x4x.33 =5.33 CF based on that chart I'd need maybe 6CF of vermiculite unless I'm reading it wrong or it doesn't take into consideration the loss in volume but the last column seems to give the yield? The top slab in the pictures I posted is well reinforced to support the oven.

[david s]

Sorry, what I should have said was get another 4 cu ft bag as one won’t be enough. The chart does not take into consideration the loss of volume on mixing. It is always better to have a little more than you require as it’s a pain not having quite enough.

[TARibs]

Hi Again,

So the second 4 CF bag came today, How exactly does one measure out a CF of Portland cement? 5 gallon bucket? Also I've been trying to find a source for SS needles and Burnout fibres They are two different things correct? I found these Needles and from what I can see they are relatively inexpensive Are they the correct thing? http://www.newhorizonstore.com/Produ...l-needles.aspx As for the fibres could someone show me an example if not a link to a source? Thanks Again

[david s]

The 5:1 vermicrete ratio is by volume, so for 5 buckets, cups or drums, or whatever, add one of cement.
Those needles are the correct material. Also the price is very good. I pay, after freight around 3x that. The burnout fibres are actually designed for another purpose. They are the very fine polypropylene fibres used in concrete reinforcing and are sold as a two part pack system, but they melt at 160C, so work quite well. They do need extended mixing to disperse properly though.I use Novomesh 950 but there are other brands. Just use the fine ones not the larger ones.

[TARibs]

So this is the basic design I'm trying to follow as this post seems to have the best information on a complete build https://community.fornobravo.com/for...sted-over-sand, if anyone has another couple to suggest that would be helpful.

Right now I'm thinking 32-36 inch oven, 2 inches thick cast, with either a Thermal blanket and vermicrete cover or just the Vermicrete ( still trying to find references for the suggested thickness or these. If I go 32 (+2 inch cast x2) +(4 inches Vermicrete x2) that equals 42" and that leaves me about 8 inches for entryway in the front. If I eliminate the Vermicrete and instead use say 2" a blanket (I assume I then need a stucco layer over this?) I might be able to increase the size of the oven a tad. 36 + (2x2) + 2x2) = 44 so 6 inch entryway
I still have a few questions

Regarding the dome vs the entryway/door entrance and flue. There is discussion of a Thermal break, What is the purpose of this, I assume its between the two but how does one actually make it, Is some material placed between the dome and the entryway?
Are there suggested dimensions anywhere for the entryway as in how far should it extend (with a 6 inch flue it obviously has to be at least 6 inches, and how tall it should be?
For the Flue I see Formo bravo sells a transition to a stainless flue pipe, what would be a cheaper alternative?

Other questions: Is the Vermicrete under and perhaps over the cast dome waterproof or does it have to be protected from the elements in some way after the build (ie a stucco layer?)

Need to get me some Portland Cement now so I can get that Vermicrete mixed up.
thanks again

Tom

[david s]

Tom,

Here's another build, using home-brew instead of castable refractory though.
#1
You can use just vermicrete for the insulation, but at least one layer of blanket is better as it also acts as an expansion joint. There is a problem of wet vermicrete swelling when it gets hot. A layer of blanket underneath it reduces this problem. You can stucco straight over the blanket, but it is more difficult because it is not a firm surface to work against. A vermicrete layer over the blanket results in an insulate layer that is also hard and rough enough to provide a good substrate to work against. In addition it allows you to get the form back to a nice hemisphere rather than the lumpiness of the blanket.
Here's an explanation of how I managed to reduce the depth of the entryway
#99
The thermal break is an effort to reduce conductivity losses from the dome to the flue gallery, but a small gap does little to prevent loss of heat by radiation. You wouldn't expect 1/2" of insulation over theme or under the floor to be of much use in reducing heat loss. It's more important function IMO is to act as an expansion joint, preventing stress on the cooler outer parts of the oven so they don't get cracked by the expanding inner over pieces. In my design I insulate both the oven chamber and flue gallery and have the expansion joint between the flue gallery and the outer decorative arch.
The vermicrete either under or over the dome needs to be sealed from the elements or it can absorb moisture.

Dave