Why FireBricks / Refractories Fail: Silica+Flux, Not Heat

[masonryrocketstove]

Apr 7, 2023 6:31:06 GMT -8 solobird said:

Yes, using ceramic fiber mat, 1260C(2300F), 128kg/m3.

Planning 6" J-tube, feed tube and burn tunnel out of 1.2" vermiculite that I already have, maybe insulating firebrick on the outside. 2" ceramic fiber mat riser inside metal pipe for support. Have used CF in the past, just on the outside of some hard firebrick elements. This stove would work as an outside cookstove and oven, so no use in heating up mass, that's the main reason for trying to build directly out of some insulation material.

Sounds like a workable plan.

Just be sure to pre-fire the vermiculite board until it's hard / brittle (vitrified) if you plan on coating it with Blakite as well. Vermiculite will continue to expand and contract a lot with moisture until its fired hot enough to begin vitrification, and that can make coatings peel or crack away. A coating made from something heat resistant like Blakite would probably also prevent vermiculite from fully firing hard if it's applied directly to the raw, not yet pre-fired board.

Temps above the 900º/950ºC range should do it for pre-firing, at least enough to make it non-absorbent / non-expanding. Trev and Martyn both report having the vermiculite afterburners in their Vortex stoves vitrify to a ceramic-like hardness, but not the vermiculite boards in the firebox. I think they're both running 6" system size Vortex stoves.

Your explanations and re-reading the info on this topic makes things clear. Thanks!
Also came across this forge insulation info while doing research.

That's a really good reference point. Helps explain the concepts behind different types of refractories, their applications, and different types of durability. That guy clearly knows what he's talking about. All the same basic principles apply between forges with their metal slag and woodburners with their ash slag, and heat sink vs heat insulator vs radiant heat reflector, erosion vs corrosion, etc.

Blakite it is then.
Should I apply it at a consistency of paint or keep it the way it is now, creamy ? how thick ?

You might need to experiment with a couple very small pieces before trying to rigidize and coat the riser.. But what I'd do is thin some of the Blakite enough to get it to soak in to the fiber. If you can make it thin enough to draw in with capillary action, and without having curing problems during the drying stage, that would be ideal.

Blakite should take well to being thinned, since the manufacturer already advises thinning with water for dipping bricks:
www.morganthermalceramics.com/media/4087/jm2600-blakite-blakite-v-jm3300-_english.pdf

"It is supplied in a consistency suitable for shallow patching or trowelling but requires the addition of approximately 5% water for dipping."

After getting the riser soaked in thinned Blakite and then thoroughly dried, I'd apply a parge coat type layer of un-thinned Blakite on the inside. That should (in theory) give it the best result for interior fiber rigidizing and surface slag protection.

Given that the CF will be coated, should I start the riser from the base of the burn tunnel or start from above ?

If you're using vermiculite for the main structure of the burn tunnel, I think I'd make the riser from the base of the burn tunnel. Only reason is that I don't know how much weight load the vermiculite board might be capable of supporting. I'd guess it's not much, and might be even less while hot. Maybe a lot less after being pre-fired to vitrification.. but that's just me guessing again.

Insulating fire bricks from tore down stove based on Matt Walker's riserless, used for a few years in the summer time.
950C rated, SiO2 : 67-70 %, Al2O3 : 12-18%, Fe2O3 : < 2 %

port

This is actually a really great example pic that shows the fluxing effect of alkali slag from ash (K and Na mostly) on the aluminosilicate brick. Glad you shared these.

That "alligator skin" cracking pattern on the surface is the result of the softening and expansion of the slagged and fluxed surface. The pattern is the same type of cracking you see in clayey or loamy mud as waterlogged soil dries out. When wet, the clay/mud softens and expands, but when it begins to dry, it shrinks and hardens. The surface mud dries faster than the mud below it.. making the surface shink faster than the deeper layers underneath. That pulls the surface apart in those cracking patterns, with edges that curl upward and tear away from the subsurface.

The same basic thing happens to the insulating brick surface, except instead of water causing softening and expansion, it's the absorbed alkali ash slag. When hot, the fluxed surface softens because of the glassy lowered melting point, and it "puffs up" because of the greater coefficient of thermal expansion than the deeper layers within the brick. As the brick cools back down again, the surface cools fastest, causing it to shrink faster, pulling itself apart in the same alligator-skin pattern with upward-curling edges.

Porous insulating brick tends to have more dramatic alligator skin cracking, while dense brick with a lot of grog is more prone to crumbling.. the dense brick being less able to accommodate surface expansion the way that insulating brick does, by allowing cracks and tension forces to terminate in the brick pores. Keeps cracks from traveling all the way through and cracking the whole brick apart to crumbly bits. That plus the pores' tendency to absorb a lot more flux in the first few millimeters of the surface than dense bricks do.

[martyn]

From my own real life experience of using vermiculite as a hot face material I can tell you a few things of interest .
To start with vermiculite board is a delight to work with, especially with power tools like a power miter saw, it cuts very accurately and can be treated like a very soft wood or very tough cardboard.
You must leave room for expansion and contraction, I have considered baking the pieces in an oven before using them but it seems a little too much trouble!

The pieces can be dry screwed together but you need to make a clearance hole in the top piece and gently screw the part together then just back off the screw a tad.
The best method I have found is to use silicone on the joints then remove the screws completely, the silicone will hold everything together very well indeed but it will melt away once very hot.
So you need to consider the design to allow self supporting construction, you can leave ‘backed off screws’ in certain ares if necessary.

I have a couple of videos about coating vermiculite with Zircon but I have not made a video about the results.
Vermiculite exposed to very high temps will change its characteristics considerably by tuning into a lightweight sintered component, it will be brittle and hard however it seems to last for yeats in that state if it is not interfered with.

To cut a long story short, I would always treat vermiculite as a replaceable component as it will suffer from abrasion and it will crack if not allowed to move.
However it is a great product and allows complex and accurate designs to be built in a few hours!

[masonryrocketstove]

That's really helpful info Martyn, helps to get a clearer understanding of its working properties. Thank you for sharing that knowledge.

Vermiculite board where I am is pretty rare to come by and I normally only see it in foreign box stove imports or special orders from abroad. I guess because we don't produce it here, the import costs make it insanely expensive by comparison with other insulating inserts, liners, etc. I've seen it on Amazon but the price is very high for the size pieces you can get here.

[martyn]

Yes I know the board is not readily available in the States although it does seem to be a better supply in Canada.
Mine comes from France and a 1” x 40” x 24” sheet cost me around $55.
In England it is more expensive at around $90.
www.gratefireplaceaccessories.co.uk/vermiculite-fire-board-1000mm-x-610mm-x-25mm

Vermiculite grains are a very useful material to use when building pizza ovens, most pizza ovens in Europe sit on a base of 5-1 cement and vermiculite mix and the dome is covered in a 10-1 mix.
We just use plain Portland cement, the main issue with the wet mix is the grains soak up water so the mix takes ages to dry in the winter but, a one or two ton oven will happily sit on a 5-1 vermiculite base for many years. 

My supplier also has some interesting new forms of vermiculite like water resistant, high crush, 30mm thick and a 1250c versions however these would be special order to me at the moment….

[Vortex]

I'm in a remote area in the west of Ireland and it's readily available in the builders merchants here: €50 / $55 for a 1" x 24" x 31-1/2" sheet.

dineensales.com/Dinboard.aspx

[masonryrocketstove]

I just learned why vermiculite board is so rare in the US and why nobody wants to produce it domestically ..any more.

It's a reputation / public perception problem that's still having ripple effects throughout the construction materials market because of its historical association with asbestos in our country. Which is weird considering that you can buy bulk vermiculite granules in almost every garden supply store for horticultural soil amendment. Really popular with vegetable growers.


..But if you find vermiculite used as an insulation product, the EPA is still telling people to assume it contains asbestos, because almost all our domestic vermiculite used to make insulation and construction materials here came from a single mine in Libby, Montana ..which happened to be contaminated with asbestos. Which meant that all the vermiculite coming from that mine (and all the construction material it was used in) also contain asbestos:

www.epa.gov/asbestos/protect-your-family-asbestos-contaminated-vermiculite-insulation

"Protect Your Family from Asbestos-Contaminated Vermiculite Insulation
This page provides important information on how to protect yourself and your family if you suspect that you might have vermiculite insulation from Libby, Montana.

What is Vermiculite
Vermiculite is a naturally-occurring mineral composed of shiny flakes, resembling mica. When heated to a high temperature, flakes of vermiculite expand as much as 8-30 times their original size. The expanded vermiculite is a light-weight, fire-resistant, and odorless material and has been used in numerous products, including insulation for attics and walls. Sizes of vermiculite products range from very fine particles to large (coarse) pieces nearly an inch long.

Concerns about Asbestos-contaminated Vermiculite Insulation
A mine near Libby, Montana, was the source of over 70 percent of all vermiculite sold in the United States from 1919 to 1990. There was also a deposit of asbestos at that mine, so the vermiculite from Libby was contaminated with asbestos. Vermiculite from Libby was used in the majority of vermiculite insulation in the United States and was often sold under the brand name Zonolite. If you have vermiculite insulation in your home, you should assume this material may be contaminated with asbestos and be aware of steps you can take to protect yourself and your family from exposure to asbestos.

Identifying Insulation That is Made From Vermiculite
Look at the photos on this website and then look at the insulation without disturbing it. Vermiculite insulation is a pebble-like, pour-in product and is usually gray-brown or silver-gold in color.

You should assume that vermiculite insulation is from Libby and treat the material as if it contained asbestos by not disturbing it or by using a trained professional if it needs to be removed. Since the Libby mine was estimated to be the source of over 70 percent of all vermiculite sold in the United States from 1919 to 1990 and vermiculite from Libby was contaminated with asbestos, further testing is not necessary to take the appropriate precautions. While you can hire a trained professional to test your attic for asbestos, this may be expensive and, depending on the methods used, might give you erroneous results. We do not recommend that you open your walls to check for vermiculite.

If You Have Vermiculite Insulation
YOU SHOULD ASSUME THE VERMICULITE CONTAINS ASBESTOS AND DO NOT DISTURB IT! Any disturbance could potentially release asbestos fibers into the air. If you absolutely have to go in your attic and it contains vermiculite insulation, you should limit the number of trips you make and shorten the length of those trips in order to help limit your potential exposure."

It's apparently also related to the more recent scare about talc and baby powder:
www.asbestos.com/asbestos/types/

"Asbestos Types by AHERA Classification
Technically speaking, “asbestos” is a commercial and legal term encompassing multiple types of minerals. The Asbestos Hazard Emergency Response Act (AHERA) classifies the asbestiform varieties of the following minerals as asbestos [...] Anthophyllite was used in limited quantities for insulation products and construction materials. It also occurs as a contaminant in chrysotile asbestos, vermiculite and talc. It may have a grey, dull green or white color."

With a disastrous reputation like that, and the EPA still issuing that blanket warning about ALL vermiculite insulation.. it's pretty obvious why nobody in the States wants to try to produce insulating boards and building products from it domestically any more. That's why it's always imported, rare, and expensive here.

But yet nobody batts an eye about growing food in vermiculite granules, potting indoor plants, or using it for animal feed additive.. because vermiculite from most other mines is completely safe, and none of the vermiculite currently being sold contains any asbestos whatsoever.

Pretty amazing. And very unfortunate, when you think about it.. because of how many people here in the US buy ceramic fiber insulation and don't know to rigidize it to make it safe.. when they might have bought completely safe vermiculite board for insulation instead.

[seth]

After reading this entire thread, and giving greatest concern to issues of health and safety, I can’t help but wonder why the following formula wouldn’t work for a cheap, efficient DIY rocket stove core and riser:

-Insulating fire brick structure
- refractory mortar between bricks
-refractory mortar 1/4” thick on all hot faces
-coating of ITC 100 over refractory mortar

The mortar could be formed around sacrificial wood built to specific interior dimensions, bricks set on the outside, ITC 100 added after wood is burned off. The IFB would give insulation to the riser without introducing fibers into build and the refractory mortar would add structural protection that ITC 100 alone would be insufficient for, especially in burn box. I estimate this could be done for under 150$ and require very little skill or excessive time.

Apologies if I missed something in this thread that discounts this idea. Perhaps there are issues with this plan (such as insufficient insulation) that fall under RMF mechanics/thermodynamics and therefore deserve a separate thread, but my primary concerns are health issues which I am reading for the first time in this thread and rarely see mentioned elsewhere.

[martyn]

Well you could but…. hard refractory needs room to expand and contract, so the mortar between the bricks would crack, the face mortar will crack and before long fall off..
Dry stacking insulation bricks and coating with zircon might work for you.

[masonryrocketstove]

After seeing Trev’s and Martyn’s success with commercially available vermiculite board in most other countries, and seeing Martyn’s Homebrew recipe for vermiculite concrete used in pizza oven domes.. and learning that reason earlier up in the thread why vermiculite isn’t available in the US.. I’m gonna give it a shot at making some DIY insulating brick splits, trying to approximate what you guys can buy commercially there in Ireland, Britain, and EU.

Seems like with the way your vermiculite boards harden and ring after a hot stove firing, I’m guessing they’re made with a fat sticky clay like magnesium bentonite to hold the expanded vermiculite pieces in board shape. Might be a tiny bit of calcium aluminate in there too. Or maybe just some organic binder like a starch or glucose paste to hold it together before firing? Figured I’d try some various blends and see what works best.

Luckily the vermiculite we can buy in the states for horticulture soil amendment has already been heated and expanded, just like they do for the stuff used in the insulating boards abroad. And it’s fairly cheap, too. I just got 6 cubic feet of it for about $60.

The chemical makeup of vermiculite (magnesium-aluminum-silicate with some trace iron) looks like it will hold up under high heat a whole lot better than perlite, (which is basically just puffed volcanic glass with melting points near or below commercial fiberglass insulation.)

Big thanks to Martyn for this Homebrew recipe he gave earlier:

Jun 2, 2023 10:09:45 GMT -8 martyn said:

There are many hundreds or thousands of pizza ovens made from Home Brew, it is a 3:1:1:1 mix of sand, Portland cement, hydrated lime and powdered clay, measured by volume.
This mix will last for years on end in the form of a pizza oven dome which ‘is self supporting’ but can be run at 500c for as long as you like.
Interestingly this recipe does not seem to last very well without the portland cement, or vey well if the standard cement is substituted with high temprature refractory cement.

If anybody else has any Homebrew recipes for insulating castable our pour-in /setting insulation using loose, expanded vermiculite, I’d love to hear it.

Ultimately the vermiculite parts (if successful) will be used in a 5 inch J tube, super simple build, with a 5 inch stove pipe after the riser and a coil of 5/8ths copper tubing wound around the outside of that like a “flue-robber” on a box stove for a thermosyphon water heater. Pretty straightforward build for use outdoors on some backcountry forest where we’re doing wildfire mitigation, and there’s lots of dry, dead, small-diameter slash to clear and burn. Figured we might as well capture some of that heat in a tempering water tank and use it for showers out there where we’re way off grid. Hopefully the simple J and flue robber thermosyphon will be “light” enough to be dismantlable and semi-portable.

For the intended use, I just couldn’t see using a bunch of the expensive ceramic fiber and colloidal silica needed to rigidize and seal the fibers.. and using dense brick or castable would make it too heavy for any chance at mobility.

[masonryrocketstove]

Well, shoot. I just realized I copy/pasted the wrong recipe from Martyn. Lol. It’s been a hot day out, let’s blame the heat-brain. I’m almost sure martyn posted a recipe before for the vermiculite blend he’s used in pizza ovens.

Sorry for my density, y’all. I’ll get washed up and cool off and think clearer here shortly.

[martyn]

Vermiculite is used extensively in the construction of wood fired ovens, even in America but we dont use is as fire face material.
There are two common mixes 5-1 and 10-1. That means, 5 parts of granulated vermiculite (apparently comely available in America) to one part Portland cement.
So the 5-1 mix is used as an insulating base the oven will sit on and is structurally strong enough to support the base of the oven. The top of the mix, which is under a 50mm thick refractory base, will be receiving around 250- 300c but can stand 500c.
The 10-1 mix goes over the brick dome (or refractory dome) to a 100-150 mm thickness and offers good insulating properties, this is further coated with waterproof cement.
However many people have experimented with DIY vermiculite fire bricks, you can find lots of youtube videos.
Generally speaking these DIY attempts are never as effective as vermiculite board but they can work for certain applications.
So we have a few choices like by far the most popular….vermiculite graduals mixed with Portland cement. You can find thousands of youtube videos about building a pizza oven with a 4-1 mix with portland cement but is just does not work!
Then you get 4-1 refractory cement that will work in certain applications but is very prone to cracking.
Then you get vermiculite dust mixed with refectory and that seems to work quite well but there are often mixing issues and consistency issues.
In any case none are as durable as vermiculite board so far as I know.

[masonryrocketstove]

Thank you so much, Martyn! You are a wealth of good info

[martyn]

Well I am retired now so maybe not up to speed with the very latest ideas but I know some guys were experimenting with Vcrete (vermiculite and cement mix) by adding carbon fibre fibres to stop the bricks cracking.
The problem with refractory cement is the very fast setting time and the very dry mix required to correctly set the cement. Whereas vermiculite is very porous and sucks up the water resulting in a crumbly result. So the two components dont work easily together unless you really understand how to add the correct amount of moister to the vermiculite without drying the cement or making it to wet.
Anyway some folk were experimenting with damp vermiculite granules mixed 5-1 with refractory and added chopped carbon fibres.
The mix is pressed into a metal brick mold but or other non porous material, timing is important as the cements starts to set in minutes.
If you want to experiment it is best to use Fondu cement rather than a refractory casting mix, where I live a 20kg bag of Fondu is £20 and a 100lt bag of vermiculite loft insulation graduals cost around £30. (That is a lot of vermiculite)
Other ideas I have seen are adding salt or polystyrene micro balls to the mix, these additives will melt out when hot leaving air space to allow moister to escape and reduce the density.
The most successful method seems to be a combination of polypropylene and carbon fibres in equal amounts, (1%) this offers a combination of strength, vapour escape, and expansion and contraction movement.

I used to love that sort of experiment but I can just buy ready made vermiculite bricks from Amazon! www.amazon.co.uk/Stove-Wizard-230mm-114mm-25mm/dp/B0BZYVHZS1/ref=sr_1_14_sspa?crid=2GQXXUTC66WIX&keywords=Vermiculite+bricks&qid=1689411898&sprefix=vermiculite+bricks%2Caps%2C149&sr=8-14-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9tdGY&psc=1

[masonryrocketstove]

Jul 15, 2023 1:02:18 GMT -8 martyn said:

The problem with refractory cement is the very fast setting time and the very dry mix required to correctly set the cement. Whereas vermiculite is very porous and sucks up the water resulting in a crumbly result. So the two components dont work easily together unless you really understand how to add the correct amount of moister to the vermiculite without drying the cement or making it to wet.

Yeah, that and most refractory cement only has enough hydrated lime / calcium aluminate to set the aggregate that comes in the mix. Adding more aggregate like vermiculite throws off that balance with the refractory cement’s small percentage of water-activated calcium binder.

[martyn]

Yes that is why it is better to use Fondue cement.
I hope you have a go at making an insulating mix …….