Questions on DIY Homebrew castable

[wolfmoonex]

1st post, Hi Y'all !

Been lurking and reading up, so much info

SO, about the classic homebrew here. I understand that the sand is used as a cheap filler because fireclay is more money. Where im at, there are a lot of furnace suppliers here and i can get silica sand and fireclay for about the same amount of money

q1 Can I skip the sand and just use Fireclay, Portland and Lime?
q2 Can I skip the fireclay and just use silica sand:Porland:Lime?
q3 Can Talcum be considered as a potential refractory addition to the homebrew? [ From what i read, it seems more suitable for an insulating refractory? Also has trace asbestos? Yikes ! ]
q3 Can stone dust like marble dust, crushed rock 1mm, quartz grain 2mm or Grog from the furnace suppliers improve upon the homebrew castable?
q4 Can I swap the Portland with refractory cement in the homebrew [ i've read here at many places the lime will make the CAC go off fast and make a weak bond, BUT then I also read at some places you can swap it out ] - has someone tried this?
q5 IF i have the aggregates sorted, can i use sodium silicate as a binder instead of CAC, OPC, lime or clay ? [ my understanding, 500 C will damage CAC, OPC, Lime & Clay ]
q6 Did some reading on pumice dust, fly ash, furnace slag and they seem like good refractory binders but the radioactive heavy metal factor scares the crap out of me, does not seem food safe, more for melting iron ore or something. Any thoughts?

I seen a few castable domes on the forum which were based around the homebrew and they all seem to be functioning really well, so I may just go ahead with the classic homebrew. BUT i have become so fascinated with this castable subject , my brain can't stop thinking about this stuff

[david s]

Q1. The extremely small particle size of clay induces shrinkage, so replacing sand with more clay leads to shrinkage cracks.
Q2. Clay introduces refractory properties that are required. It also imparts stickiness to the brew which enhances workability. Interestingly the reason we can use sand is that we don’t use temperatures high enough to test sand’S eutectic (turning to glass) qualities. Glass does not melt until around 900C and also requires the presence of some sort of flux.
Q3. Maybe but you’d need to do some testing.
Q4. You can swap the OPC for CAC but in conjunction with lime it reduces the already short working time of CAC only. Obviously those who have suggested it haven’t tried it.
Q5. I don’t know, perhaps, you’d have to test it. The beauty of the homebrew is that it has been tested and used successfully for ovens in our temperature range. It is very cheap and workable with a decent working time. It would not be suitable for the higher temperatures of a kiln, but it is adequate (just) for a WFO. The theory is that the OPC starts to give up north of 300C and the lime then takes over, which starts to fail north of 500C (about the same for CAC) north of that the clay takes over. Clay starts to sinter (becoming permanent ) at 573C and normally a kiln needs to be fired very slowly and controlled from 450-650C (a very vulnerable range) if micro cracking is to be avoided. It is then fired up to service temperature where the material becomes harder the higher the temperature goes.
Do not attempt this with a WFO. It will result in plenty of micro cracking because the temperature cannot be carefully controlled enough with a wood firing.

It is a fascinating subject. If you do some tests post your results.

[wolfmoonex]

Thank you for the detailed reply David, really filled out the missing gaps in my concepts on the homebrew and why it works so well, some of it may be obvious to you but that now i get it completely.

I made a redbrick tandoor, it puts out the tastiest bird i have ever eaten. Just stacked and tied with metal wires, and no mortar - some thing really basic to learn about fire and cooking.. that thing hits 650 700 C easy, So i was not sure how the brew is surviving for many years in so many builds.Must be just ONE cardinal rule if any with homebrew and WFO - DON'T GO ABOVE 500C

I have tested a coating I made by combining construction sand, sodium silicate and coal/wood ash on some thin carbon steel sheets. I can drop these coated sheets in a red hot tandoor for hours with the firebricks down there it glows orange red, but upon inspection when it is cooled down, there zero flame abrasion on the metal sheets.

I dont know the sience behind why S silicate works, i have read some books and google patents on metal/glass enameling & I am thinking of this idea, where you can invert a steel wok, rest it on those round circular cupola firebricks and coat the inside of the dome. I have to test this someday, cuz if the dome can work with the coating, then its a quick way to make a pizza party type mobile WFO

[wolfmoonex]

Help me choose what sand to use

So went out to look at what sand I can get and each of the places I have checked has a different stash. I can get sand like this from close my place:




Closeup of mix 1:



Closeup of mix 2



Closeup of mix 2 filtered through 2 meshes, [ don't plan to use this mix - very expensive because of labor]



Silica 2mm:



Silica sand:



Should i keep looking for better sand? I don't mind checking out a few other spots but then I will have to head out to more than a few blocks from my place. What do ya guys think?

[Petter]

I've done some testning with Sodium Silicate and sand last summer. It takes ages to dry. It never dries, only gets thicker in consistancy. You need some type accelerator for it to go off. Heat, CO2 or a chemical. The finished product is however rock solid and very very strong.

You should use the sand that gives the highest packning. It will be the strongest. Guess/suggestion: Remove the fines and coarse above 1 mm. Too fine particles causes shrinkage due to exessive water absorption.

Main issues were to get the Sodium Silicate to a reasonable price and to get it to go off. I did the cat litter crystals in Sodium Hydroxide. It was easy but pruduced a lot of hazardous gasous species I did not want to inhale. So I abanded the idea.

After doing these tests, I ended up using the standard homebrew for conveniency in a small barrel shape cast oven.


I also did some testning with CAC mortar. I did not mix in lime as to be read but I tried fire clay in different ratios. I could not get it to bond to the bricks. Only If the fire clay was removed, i.e. A standard 4:1 sand CAC mortar as was recommended by the manufacturer would adhere after curing. Wet or dry brick did not matter. I guess the shrinkage caused by the clay resulted in stresses at the brick-mortar interface or that the clay prevented the brick from sucking water/CAC into the pores for good bond adhesion.

[david s]

The use of some polypropylene fibres in the home-brew mortar is advisable IMO as the very thick mortar joints on the outside of the dome are vulnerable to steam spalling. Castable refractory products contain these fibres, but refractory mortar does not. In fact proprietary trftactory mortars have very specific instructions not to apply the mortar in thick joints, but to keep them thin. I think the reason they don't add any is that they require extra mixing time to disperse properly, they have a tendency to clump. As there are none in a homebrew mix a little added to each batch of mortar and mixed really well for good distribution, will provide a measure of insurance against steam spalling whether the homebrew is used as a castable or for thick mortar joints. The fibres also do a good job in holding the mix together making it more workable. Add less than you'd think and mix more than you'd think.

[wolfmoonex]

Thank you for the additional responses Peter and David.

I did some more testing and the finest sand particles seem to be the way to go for homebrew castable.

As for sodium silicate - I have been using it with fine sand to fill in the spaces and cracks in my mortar less tandoor and a prototype brick box with an inverted iron wok placed inside the brick box.

The sodium silicate will harden on it's own in 24 hours, but if you are impatient like me, you can take a jar > pour some vinegar and some sodium carbonate in it > place this CO2 steaming glass inside your oven and cover the whole thing in a giant plastic bag, that works as CO2 is heavier than air so it fills from bottom up and spills out at the top, leaving your structure submerged in CO2.

Heat also works but it causes the silicate to swell up like foam, may or may not be ideal depending on what you are trying to do. For now i just plug holes and gaps with it, it works a treat. I am thinking of building a larger spherical structure in 4 parts for a 31 inch Neapolitan oven - using only sand and sodium silicate, will post here if I ever get around to completing this experiment


The air flow in the tandoor is awesome, the thing hits 800 C in about 20 minutes, the brick box not so much. I think the air flow dynamics of a sphere can't be beat vs a box shape. the box smokes hella much compared to the tandoor, the smoke on the tandoor is blue and you can't see the smoke till you get real close.

If anyone is curious to know, for the brick box, right now, I use a gas burner, so it is more like a Kiln

[david s]

Help me choose what sand to use

So went out to look at what sand I can get and each of the places I have checked has a different stash. I can get sand like this from close my place:




Closeup of mix 1:



Closeup of mix 2



Closeup of mix 2 filtered through 2 meshes, [ don't plan to use this mix - very expensive because of labor]



Silica 2mm:



Silica sand:



Should i keep looking for better sand? I don't mind checking out a few other spots but then I will have to head out to more than a few blocks from my place. What do ya guys think?

A mix of grain size is actually the best, but don't use anything much bigger than around 2mm. the finer sand will produce a more workable mix than coarser stuff. With the high proportion of clay in the home-brew mix it is more than enough to make pretty much any aggregate in the mix nicely workable. Just from appearance, and it's a bit hard to tell just by looking at the pics, but I'd say the yellow bucket would be my pick.

[wolfmoonex]

those are not buckets,

those are soda bottle caps holding the samples.. maybe that gives the perspective of the pics a better point of reference. But I think i get you. Once upon a time I cracked open a firebrick and saw inside, that it is made of fine, course and some in the middle kind of grain sizes.