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Author: Subject: castable refractory mixtures
Rosco Bodine
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[*] posted on 29-11-2004 at 22:38
castable refractory mixtures


There have been other topics about the
construction of high temperature furnaces
and I also have a need to build a furnace .
One of the things which can facilitate the construction is castable refractory materials . There are a few materials
which I am considering as candidates for
a refractory mixture which can be dampened and then tamped into place .
Some of these may have been used in some combination before with good results by others and I am asking for any
proven formulas or suggestions or links
that others may offer .

Diatomaceous earth is one material I plan to use as a component of the mixture . It has a very high silica content and is a refractory material , and it is available at reasonable cost as a swimming pool filter material .

Perlite is another material I plan to use in the mixture , and this has already been used by others .

A fusible clay material like fireclay will be an ingredient . I am considering using
porcelin slip , or some other ceramic slip
for this ingredient .

A portland type cement will be used . I have some type I portland cement , but
I noticed the white sanded grout used for tile setting , and also the white mortar mix
for setting glass blocks , and I wonder if these more highly refined low iron and light colored mortars would be better .
I also spotted a surface bonding mortar mix which is light colored and contains chopped glass reenforcing fiber .

Sodium silicate solution , magnesium oxide , hydrated lime , chopped fiberglass
"kitty whisker" , cabosil , and glass microballoons are also available materials .

Any suggestions for a mix containing DE and perlite and perhaps any of these other materials ?
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[*] posted on 30-11-2004 at 02:47


Usual Portland-cement cannot withstand high temperatures for a prolonged time.
There are special furnace cements ("Trass-Zement" in german) available for cheap for this though.

Phosphor bonded cement was suggested by Tacho and used successfully by Axehandle IIRC.




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Rosco Bodine
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[*] posted on 30-11-2004 at 06:38


Hmmm...about the portland cement then ,
I wonder if the tile grout or the glass block mortar would be better , or if it contains a phosphate and is different from regular portland based mortar .
Portland is a dark gray material , but the
others are absolutely white in color .

Also I have a half gallon of 80% phosphoric acid and I wonder if perhaps
that may be used to create a phosphate
bonding component in situ , perhaps with
dolomite or calcium carbonate or zinc carbonate . The byproduct CO2 would act as a foaming agent and give the material a cellular structure and insulating properties .



[Edited on 30-11-2004 by Rosco Bodine]
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[*] posted on 30-11-2004 at 08:24


There has been extensive discussion on this subject, mostly in furnace & foundry threads. A search will help you.

Phosphate bonded cement is sold in dentistry supply shops as "dental cast investment" or something like that. Very available.

I could never get any bonding using 85% phosphoric acid and CaO. This also was discussed elsewhere.

Axehandle has discussed this topic a lot in other threads.
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[*] posted on 30-11-2004 at 09:12


There is an aluminum silicate cement called "cement fondu" made by Fondag that is used in foundrys. I think it is good for up to 1600 deg C. A little phosphoric acid in the mix should react with the alumina in the clay to make a good bond. Use maybe
0.5% of the total mix weight It might need firing to develop the bond.
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Rosco Bodine
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[*] posted on 30-11-2004 at 09:47


Regarding the phosphoric acid , I was thinking perhaps some ammonium salt ,
or some partial neutralization with ammonium hydroxide may facilitate the
process . If there is also used some magnesium oxide or perhaps dolomite ,
along with some chloride containing material , there is the potential for forming
a magnesia type cement component , such as the oxychloride . A similar effect might occur for a related sulfate magnesia
type cement . Anyway the chemistry of what happens to the cast refractory at firing temperatures is complex and unpredictable to an extent . So the mixtures are something of a trial and error "witches brew" .

The use of diatomaceous earth with perlite , perhaps with some scheme for foaming the mixture , and use of the white
mortars like grout or glass block cement ,
and ceramic slip is not anything I saw in
the other threads about furnace construction . The topic I started here is
an effort to focus on the refractory material mixture itself , in particular what
method of bonding cement would be best for a filler material of diatomaceous earth and perlite , and hopefully to find readily available OTC materials that will serve the purpose . I could make a six hour drive to an industrial supplier and buy any of the standard refractories and pay high prices if I wanted to build a professional grade furnace . What I seek is a method
which uses the most mundane materials
from the local building supply and hardware store , not medical or industrial
specialty suppliers . If a satisfactory mixture can't be made without specialty
cements , then it would be easier to simply build a stacked box furnace from firebrick and seal the joints with that fireplace grout which comes in tubes for a caulking gun . I have on hand some angle iron and stainless steel strapping
to build a permanent frame for a stacked firebrick furnace , if I want to build a long term use and very sturdy forge type furnace . More the interest of my topic
begun here is a refractory mix which would suffice as something relatively easy
to use as an insulating liner for a more improvised furnace like would be made inside a metal five gallon bucket or a large clay flower pot .

The operative principle here is *improvised* refractory mixture . I am not looking for recomendations for commercial refractory products , but for
common OTC products which may be pressed into use in formulation of a good refractory mixture .
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[*] posted on 30-11-2004 at 11:03
Refractory mixes and chemical compositions


Hmm, I am not sure whether to merge this with Cyrus's thread on metal casting molds. Let's see how this thread evolves I guess.

Anyway, I am (like most others) also quite interested in refractory mixes.
I have various oxides (i.e. CaO/SiO2/ZrO2/TiO2/Al2O3/MgO) and am thinking of using one of the commercially know mixtures.
For instance, tables on this, this, this , or this page I find very useful - although the first one in particular uses too much ZrO2 for my liking.

I wondered 1) whether it would make sense to mix all these oxides according to the published amounts, and fire it up with a bit of water? and 2) whether anyone knows more tables such as the above, with various properties & firing temperatures, thermal expansion and so on?

[Edited on 30-11-2004 by chemoleo]




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Rosco Bodine
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[*] posted on 30-11-2004 at 12:37


As a bubble producing agent for creating
a cellular structure , perhaps a bit of hydrogen peroxide or one of the persulfate or percarbonate chlorine free bleaching materials would be useful added
to the mix .
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[*] posted on 30-11-2004 at 13:11


Voids in the cement are usually created by adding a combustible material like sawdust, then burning out once the cement is hard. Gaseous voids in wet cement cannot be 'rammed', and if the cement is pourable then all your bubbles will rise to the top.
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Rosco Bodine
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[*] posted on 30-11-2004 at 13:34


I have read about the use of sawdust but I don't favor that method because it would not produce a microcellular structure like a gas entrainment methid would provide . There are "air entrained"
concretes which are common . So long as the mix is thick , the bubbles do not migrate . And the mix I plan to make is
going to use minimal water , just wet enough to hold itself together like a damp and sticky mixture as would be used for a sand casting mold , maybe just a little wetter but not much . About like cookie dough , if I threw a ball of it against a wall it would stick there and dry where it hangs without falling off .
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[*] posted on 30-11-2004 at 21:28


Well this is very interesting . See the attached pdf file . Sodium metasilicate powder is cheaply available as a cleaning detergent , used for degreasing surfaces to be painted , a cheaper substitute for trisodium phosphate . It sits on the shelf in the paint and wallpaper department in building supply stores .

Attachment: soluble silicates in refractory mixtures.pdf (112kB)
This file has been downloaded 4223 times

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Rosco Bodine
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[*] posted on 30-11-2004 at 22:21


Here's more info on soluble silicates as binders and coatings from the same source

Attachment: soluble silicates as binders.pdf (121kB)
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Rosco Bodine
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[*] posted on 30-11-2004 at 23:24


Here's the page where I have been getting this interesting data on silicates .
From what I have read it seems the higher silica to sodium ratio silicates would be preferable for use as a binder to the more basic metasilicate commonly used as a detergent . The higher ratio product is likely the OTC product sold as an engine block sealer at automotive stores , or sold as "water glass" in some hardware stores . Possibly some concrete sealers are also the higher ratio
sodium silicate preferable for use in refractories .

http://www.pqcorp.com/technicalservice/literaturelist.asp?do...

Here is an additional file describing some of the history and general chemistry of soluble silicates .

Attachment: soluble silicates general chemistry.pdf (108kB)
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[*] posted on 1-12-2004 at 01:05


I think water glass is sodium hydroxide and HCL and water. I think its a gel.
Is it a gel?
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[*] posted on 1-12-2004 at 04:45


Water glass is sodium silicate.

IIRC, if you add NaOH or HCl to it's solution, you end up with silica GEL, which, btw, never looked like a gel to me. Curious...
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[*] posted on 5-12-2004 at 07:03


I don't know if this has been mentioned but i was just at home depot and noticed that they sold refractory cement, it runs 14 bucks for a gallon (about 4 liters). A little expensive for my blood
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[*] posted on 5-12-2004 at 08:26


Sounds interesting , if only for use as part of a blend or as a bonding agent . What department and was there any brand name , was it dry or liquid ? Refractory materials are a specialty item and many of them have limited shelf life so as a rule they are all expensive , which is a fact that motivates the desire to find an improvised mixture which would substitute
for the manufactured products .

I have been reading some interesting patents about refractory mixes which are
perhaps satisfactory . I will post the numbers here for some of those patents which I have been reading , in trying to understand the strategies others have used , and adapt or combine such information for experimental compositions .

There is an amorphous fibrous Aluminum Silicate product which is used as a spill cleanup and sweeping compound , and may be a valid substitute for "rock wool"
or loose ceramic fiber as a reenforcing material . The product is called Abzorbit .

An interesting fact which I did discover is that portland cement can be used as a component in refractory mixtures , but only if the chemistry of its setting up is modified to produce different intermediates and/or high temperature products of the reactions than would be obtained from the ordinary reactions which are produced in concrete or mortar .
So , to a limited extent portland can be used as as component in true refractory mixtures , but its usage in that regard is highly qualified by the conditions and the other ingredients also used with the portland .

GB1449484 Refractory Compositions

Some of my thoughts about foaming agents have also been confirmed valid ,
for example use of hydrogen peroxide has been used successfully .

In no particular order here are some patents which I have been reviewing in connection with this topic .

US2698251
US4303450
US5888292
US3203813
GB877025
GB657078
US6284688
US4440865
US3923534
US6740299
GB218127
GB499172
GB1478904
GB1493016

Have fun reading !

Especially interesting to me are the compostions involving the reaction between hydrated lime and diatomaceous earth , along with sodium silicates which
could be produced in situ as a preliminary
reaction with a caustic alkali . The aluminum phosphate compositions are also very interesting .

[Edited on 5-12-2004 by Rosco Bodine]
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[*] posted on 5-12-2004 at 14:44


Obviously, the choice of refractory depends on the temperature range you want. Glass, of course, softens at relatively low temperatures. I wouldn't recommend it for anything hotter than aluminum casting. You mentioned porcelain slip, so I assume you ave access to a pottery supply. The folks there probably can give you good info.

My furnace actually used regenerative cooling instead of any refractory. This was enough to melt copper easily, though the steel wall oxidized somewhat badly. The device is now in disrepair, having self-destructed in an attempt to melt iron.
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[*] posted on 7-12-2004 at 15:17
Alumina based refractory....


Hallo to all,

I would suggest that alumina (Al2O3)based refractories are good.
The problem may be get alumina.
It can be made in a simple method:

-make a soluble Al salt
(ex. 2Al + 6HCl ---> 2AlCl3 + 3H2)

- add aqueous NH3 to this solution
AlCl3(aq) + NH3(aq) -----> Al compound
the addition of aqueous NH3 to Al salt will yield a gel salt of a formula that in this moment I don't remember.

-calcination of gel
the hot heating of this gel will yield powder alumina

then you can made refractories in what shape do you want.

Have fun ;)
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[*] posted on 7-12-2004 at 16:53


I’d guess that the following happens:

AlCl<sub>3(aq)</sub> + 3NH<sub>4</sub>OH<sub>(aq)</sub> -> Al(OH) <sub>3(s)</sub> + 3NH<sub>4</sub>Cl<sub>(aq)</sub>

Upon drying and heating:

2Al(OH) <sub>3</sub> -> Al<sub>2</sub>O<sub>3</sub> + 3H<sub>2</sub>O
NH<sub>4</sub>Cl<sub>(s)</sub> -> NH<sub>4</sub>Cl<sub>(g)</sub>
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[*] posted on 7-12-2004 at 17:25


Alumina could be had quite easily if one wanted it, take a large container, make a trip to a scrap yard and buy 15 kg of aluminum scrap, take a small amount of mercury (3g) and spread this over aluminum in different places, give it a few weeks and you should have converted most of your aluminum to the oxide.

I remember reading about using aluminum powder as a foaming agent in Portland cement, it increases the volume of the cement 2x and the cellular structure adds the necessary strength to accommodate that expansion. Fumed silica also adds more strength then even it's most powdered of brethren.




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[*] posted on 7-12-2004 at 19:23


Your local ceramic supply shop is once again your friend! Alumina, and a lot of other refractories are sold in pure form (relatively speaking), as fine powders for use primarily in glazes. There's really no need to go to the effort of making the stuff yourself.
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Rosco Bodine
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[*] posted on 7-12-2004 at 19:53


A major ceramics supply distributor would likely have such components available to the minority who formulate their own special mixtures . But most of your local ceramics shops do not have and do not use pure component ingedients , but use proprietary mixtures and products that have already been mixed for some particular use .

The use of mercury is something I actually have considered as a way of producing alumina in a finely divided form for use in refractory mixtures . There would probably be some percentage of the hydroxide from the effect of moisture in the air mixed in with the oxide , which is evolved fairly rapidly from the surface of amalgamated aluminum exposed to the air . Actually amalgamated aluminum is one of the best dessicants known for this reason . But anyway , if a chunk of structural aluminum was amalgamated and placed upon a grate supported over a tray to catch the loose effluent oxide falling from the aluminum as it steadily disintegrates , that should work well ,
and would yield a more pure product in terms of the oxide , if the process was done during dry weather . The best way of doing this would be to first degrease and etch the aluminum with some oven cleaner spray of the sodium hydroxide type , then rinse away the cleaner after
a few minutes and dry the aluminum .
The aluminum is then dipped into a solution of mercuric chloride which is made acidic to a slight degree with HCl .
The aluminum will rapidly , almost instantly take on a mirrored appearance as it amalgamates . A fairly heavy layer of mercury will be desirable for good reactivity with air . The chunk of aluminum
will be evolving hydrogen at a steady rate and the HCl will be converting the reaction product to a soluble chloride to
keep the surface clean and facilitate an even amalgamation . The chunk is removed and placed upon the grate .
If the chunk is removed before it is reduced to a size small enough to fall through the grate , nearly all of the mercury can be recovered by dropping the
remaining piece in dilute HCl . When all the aluminum remaining has been converted to soluble aluminum chloride ,
the mercury metal will be found on the bottom of the beaker where it may be saved for recycling . I wonder what the actual composition of the product would be in terms of oxide to hydroxide . This could indeed be a good quick and dirty method for aluminum oxide . A low intensity vibrator acting on the support grate would perhaps be necessary too ,
since the oxide comes off the amalgam
like a white snake , and it is very lightweight . The reaction is very interesting to watch , as it produces an
unusally large volume of oxide from a small volume of metal .

The amalgam might be useful also for producing aluminum phosphate gel which could have value . Perhaps an aluminum silicate gel could be also made by way of aluminum amalgam . These materials could have great usefulness as a binder component in refractory mixtures . The aluminum alcoholates like aluminum isopropoxide which forms rapidly and easily might also have use in refractory mixtures as a reactive anhydrous intermediate .

[Edited on 8-12-2004 by Rosco Bodine]
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[*] posted on 8-12-2004 at 09:02


Yes, I ordered some Al2O3 lately... it came at 15 £ (25$) per 5 kg - not that cheap. Got some SiO2 powder as well, for that very purpose too. This, if mixed & heated at proportions given in the links from my last post might hopefully make a decent refractory material.
In fact, some of the mixtures contain SiC (silicon carbide), which can be also obtained at pottery supplies, at about the same price as above. This is used for high temp refractories I believe.
ZrO2 (zirconium oxide) is also used for extremely high temp applications, I managed to obtain it for 15 £ for 500 g - definitely not cheap, so for most this isn't an option. Zircosil (ZrSiO4) is presumably anothe good material, and it is available at much cheaper prices.

One thing that I yet have to find is perlite.

I am thinking of adding sodium silicate solution, to make the binding better, and to possibly calcinate it a little with CaO.

Pottery supplies rock - wouldn't know what I'd do without them!

[Edited on 8-12-2004 by chemoleo]




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[*] posted on 8-12-2004 at 11:36


There are likely some very good refractories which can be made from relatively common OTC materials without even having to go to the commercial suppliers , and of course that is my primary interest , improvising a decent quality product from mundane materials .
Once you have a working furnace , you can actually use that furnace to process by calcination the raw materials available into refractories for higher temperature refractories , to use in building a better "second furnace" . So a small furnace even having a lining of a limited useful life , allows you to process materials for building a much better and higher temperature furnace as a subsequent project . There are hundreds and more hundreds of patents concerning refractories , and I am about halfway through the task of picking out a few of the more interesting ones , in terms of their adaptability to use of OTC materials for experimental "home made"
refractories . Diatomaceous earth is one of the raw materials which interests me greatly , but it may present problems due to expansion and crystalline transitions . I haven't tried any actual mixtures yet , so it may be workable or not , using the uncalcined , pool filter grade of D.E. as a component for refractory mixtures . A patent for a D.E. and perlite aggregate refractory bonded with aluminum silicate is

US3793042

Another improved silicate binder which may work for similar mixtures is a trisodium phosphate / water glass mixture

GB464967

Something as simple as pure quartz sand containing very fine quartz flour and perhaps some colloidal silica can be bonded by phosphoric acid alone , which upon heating forms a binder matrix of silicon phosphate . Fumed silica , cabosil , may perhaps work in such a scheme .

GB825792

The technical dificulty to be overcome with refractory mixtures is to address the structural instability which appears in the
temperature range which is intermediate ,
as the material is being heated sufficiently high for the slight melting and sintering which accomplishes the permanent and stable "ceramic bond" by vitrification of the matrix which cements all the particles together firmly . It has to be a carefully chosen mixture which will hold together as it it is heated through the temperature range upwards , so that a smooth transition occurs from the chemical bonding of the "cold bond" which holds the unfired material together , to the ceramic bond produced at much higher temperature . The trick to be accomplished is that the zone of transition is a stable formation which doesn't result in crumbling or separation between the cold bonded material and the ceramic bonded material . Ideally there should be no discontinuity in the bonding through the transition zone from the "hot face" through the thickness of the refractory all the way to its cooler outside surface , which never really gets hot enough to form a ceramic bond . If this technical difficulty cannot be addressed , then a furnace must be built from separately made and fully fired brick ,
as opposed to using a "fire in situ" refractory composition serving as a furnace liner . The intended use of the refractory has much to do with whether a particular composition will be suitable or not , not just the upper temperature limit ,
but the manner it will be subjected to that temperature , as applied to one surface or as a surrounding environment .
Refractory compositions are indeed a science all to themselves . A patent which describes the problem and a solution for the intermediate temperature zone difficulty is

GB1014446

Update : With regards to the use of aluminum amalgam , there is another useful material which may be obtained by treating the metal first with hydrochloric acid to form aluminum chloride solution , and then subsequently to slowly add phosphoric acid to the stirred solution to
form aluminum chloro-phosphate hydrate ( ACPH ) , which is a highly water soluble and effective refractory binder . ACPH is described in several patents ,

GB1322724
US4046581
GB1429555

The aluminum chloride starting material could probably also be obtained more easily from aluminum sulfate and calcium chloride , or conversion of the sulfate to a carbonate and then treating with hydrochloric acid . Anyway these are some alternative routes for producing an ACPH refractory binder from either the aluminum scrap metal , or the commonly available soil acidifier aluminum sulfate .

[Edited on 9-12-2004 by Rosco Bodine]
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