Castable Refractory Products (VT600)

[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]

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]

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]

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.

[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.

[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.

[david s]

Slurry consistency = 66% solids on that data sheet. 34% water would be quite runny. It would be much more water than is required for the cement in the mix.
The conventional wisdom is that excess water compromises final strength.They must be assuming it won't require much structural strength, hence the 0.28 MPa compressive strength.
Owner's recommendation would supply enough water for the cement reactions, I'm guessing, so good strength.

If you don't want to just use it as is, which you could do given how cheap it is, I vote you mix a small portion of this about 1:1 with some 1/4 inch crushed aggregate for a trial.

I'm not entirely plucking that number from thin air, though it is all guess work.
My reading of the SDS suggests this stuff is about 1/3 "cement alumina chemicals".

(if you add up all the other materials in it, their maximum values total about 81%, so it is at least 19% cement I think, so 30% is probably a safe number.)

Let's assume the rest is "fine aggregates"

A pretty standard Portland cement based concrete in my part of the world is what we of a certain age refer to as "3:2:1 Government mix"
3 gravel, 2 sand, 1 cement. i.e. a paste of cement and sand coats and holds together an equal volume of gravel.

1:1 of this VT600 mix and gravel, assuming it is 1/3 cement and 2/3 fine aggregates, would give 3 parts gravel:2 parts fine aggregate:1 part cement.
At least it would be a starting point.

Pour it, cure it hit, it with a hammer. Then hit it with some serious heat.

When the manufacturers quote the chemical composition of their product, it is the total of all the materials contained,. They don’t like to quote the % of separate ingredients because they like to keep their recipes secret. Consequently you can’t assume a % of any material relates to the cementious content. Eg the uncalcined kaolin in the mix is around 40% alumina and 60% silica yet is not cementious. The quoted table in the data sheet is difficult to interpret, even more so as the list pops up differently on my iPad to my desktop, but 10-55% Silica and 20-25% Cement Alumina Chemicals is also an indication of the rather fuzzy data provided.
Adding an aggregate either fine or coarse, will also upset the balance of cement to aggregate as it increases the surface area for the cement to act upon, so will weaken the resulting product. Because the finer material increases the surface area more than the coarse material, the larger size is preferable, that is why the dust needs to be discarded. Extra cement should be added to compensate. It is probably safe to add some crushed firebrick because it’s fairly stable, but caution should be taken if other aggregates are selected as thermal expansion, conductivity as well as permeability need to be considered. “”"Permanent strain increases with the no. of cycles of heating and cooling indicating an increase of thermal cracking in the rock.”" ”
As wotavidone has pointed out the product is not a dense refractory castable, but an insulating one. hence may not contain the burnout fibres which will assist in safer water elimination. Try sieving some of the product to see if there are any tiny fibres contained in it. I suspect not in which case it would be wise to add some. You can use the fine polypropylene fibres as supplied in a two pack product used to reinforce concrete. Do not use the larger nylon fibres. CAC cement achieves its full strength after 24 hrs so there is no need to damp cure for an extended period, but it needs to be dry, so don't rush the water elimination by fire or you will risk steam spallling.
http://ceae.colorado.edu/~amadei/CVE...PDF/NOTES4.pdf

[wotavidone]

Slurry consistency = 66% solids on that data sheet. 34% water would be quite runny. It would be much more water than is required for the cement in the mix.
The conventional wisdom is that excess water compromises final strength.They must be assuming it won't require much structural strength, hence the 0.28 MPa compressive strength.
Owner's recommendation would supply enough water for the cement reactions, I'm guessing, so good strength.

If you don't want to just use it as is, which you could do given how cheap it is, I vote you mix a small portion of this about 1:1 with some 1/4 inch crushed aggregate for a trial.

I'm not entirely plucking that number from thin air, though it is all guess work.
My reading of the SDS suggests this stuff is about 1/3 "cement alumina chemicals".

(if you add up all the other materials in it, their maximum values total about 81%, so it is at least 19% cement I think, so 30% is probably a safe number.)

Let's assume the rest is "fine aggregates"

A pretty standard Portland cement based concrete in my part of the world is what we of a certain age refer to as "3:2:1 Government mix"
3 gravel, 2 sand, 1 cement. i.e. a paste of cement and sand coats and holds together an equal volume of gravel.

1:1 of this VT600 mix and gravel, assuming it is 1/3 cement and 2/3 fine aggregates, would give 3 parts gravel:2 parts fine aggregate:1 part cement.
At least it would be a starting point.

Pour it, cure it hit, it with a hammer. Then hit it with some serious heat.

[TARibs]

There is no aggregate in the product it is just a powder like cement hence the question about adding some to increase the density.

[david s]

Trowellable or “ball up “ consistency. Ie take a handful about the size of a cricket/tennis ball and throw it about two feet in the air. It should stay together. I would try mixing some and filling a cardboard milk container with the top opened right up(if you have those) waiting 24 hrs to cure then ripping off the container then you can weigh it. Cardboard is easier to remove than plastic, An ice cream container is another alternative. As the aggregate is already in the product the addition of more is unneccessary. But if you really want to you could, but no more than about 20% of total volume otherwise you’d need to add more CAC to balance it. Discard any dust because you risk weakening the mix. Also dampen any added firebrick aggregate before adding to the mix. It should be graded but nothing bigger than about 6 mm

[TARibs]

What about the idea of adding the firebrick to it? I was planning on making a trial casting both ways just to see what each was like? Would suggest pouring it into anything in particular to test it. Owner suggested starting with an 88:12 by wgt powder:water. He said he had made successful castings with it before but not sure for what. When you say trowelable, in what sense.

[david s]

The quoted density of 0.59 kg/ L would be the density of the resulting fluffed up product. If just mixed with water it should be far denser. But do check to find out if it’s also trowellable

[TARibs]

It looks like wotavidone may be correct, it is at least made by Morgan Ceramics but has very few marking on the bag. I went over and grabbed two bags (he gave them to me for nothing), turns out I played Soccer\Football with the owner of the place. He's going to look for more specs on it and send them over to me. He did tell me he's made castings out of it before (didn't say if they were poured) so I suspect it doesn't need the phosporic acid to bind. He gave me a few pieces of scrap firebricks (thin ones that were easy to break into pieces) as well. I figured I could crush it up (sieve out the dust) and add it to some of the mix and see what that does for the density? Now if I can't use for the dome would it work for an insulating layer (either underneath or on top), suggestions on how I can tell?

So spec sheet is attached, Looks a bit light density wise listed as 37pcf comes out to only .59 kg/cul would adding the crushed Firebrick to it improve it?? Temp rating is 1093C for continuous use though. So what would happen if I used it? Would it just not retain heat very well?

[TARibs]

So I am easily within driving distance and they've offered me a bag to try for nothing, so I'm going to see what it does, he said it is between a dense and insultating castable so not sure if will do what I need, Not even sure how I tell after I cast it? Going to see what information I can get on it as well.

[david s]

Tom, I'm not sure that your product is suitable, you may need to get further advice from the manufacturer. There are many different products available (must be hundreds) from a number of different manufacturers. The product I use and have been using for ten years is called a standard duty gunnable dense refractory castable, with aggregate size max 4mm. max service temp 1450 C, and it is also suitable to be applied by trowelling. But that does not mean the product you are being offered is also. You can, as Wotavidone has suggested, buy one bag and try it out. For most castable refractories the suggested water content as tested in the lab, is way too low for practical application. For my product it is 12.5 %, but that results in a mix too dry for trowel application. A gunning mix would have an even higher water content so just mix it to a consistency that is nicely workable. Any type of concrete mix is weakened if the water content is increased too much because the water simply increases volume and hence reduces density.