Cyrus
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Colloidal ceramic bonding
Colloidal silica and alumina are used as ceramic fiber rigidizers, but to rigidize they have to go from a liquid sol to a hard coating. How exactly
does this work?
Sodium silicate hardens because when the strong base is neutralized with an acid, the silica polymerizes. (I think)
Is the reaction here similar?
Maybe sodium silicate would work
Also, what is the best way to get colloidal silica or alumina? I could acidify sodium silicate, but from Tacho's thread on H2O2 decomp.
catalysts, I think that if the conditions are right for other things, I'll get a big glob of SiO2.
I can get Na[ Al(OH)4] from NaOH, H2O, and Al, and I think that if I heat the products, 2 Na[ Al(OH)4] -> Al2O3 + Na2O + 4 H2O. Maybe I could just
use Na[ Al(OH)4]
Then I can't just dissolve away the Na2O because the reaction will reverse, leaving me with Na[ Al(OH)4] 
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Tacho
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I have no idea how to MAKE them, but I find colloidal silica in dentist's supply shop, as a liquid (suspension I gess) additive to improve
phosphate cement cast investment qualities.
I'm happy to have an opportunity to mention dentists' phosphate cement cast investment again without sounding like I sell the stuff. If you
are interested in easy-to-find refractary material, do a search. Right here at sciencemadness there are a few posts that mention it. Most posted by
me.
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Cyrus
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Thanks!
I just called a dentist I know, and he has plenty (about 15 lbs) of extra cement he was going to TOSS because they are nearing their 1 year exp. date.
Well, I'm gonna save those little guys! (they come in small packets) He also had some extra liquid that went along with the packets, probably
colloidal silica! 
I though the liquid might be phosphoric acid, and the packets calcium oxide, as in the method mentioned by BromicAcid, but he said the liquid
wasn't caustic.
Now we'll see if this stuff is as good as you say! I am going to make a new furnace, so I might use the cement there, or maybe for crucibles! Or
both!
( My happiness is directly proportional to the # of exclamation points I use!!!)
Back on topic, I made some sodium silicate yesterday with NaOH and 400 mesh SiO2 powder.
4NaOH + SiO2 --> Na4SiO4 + 2H20.
11.26 g SiO2 added, then about 90 ml dH2O, then 30 g NaOH slowly added with much scientific sloshing of container for mixing.
This was done in a cheap glass vinegar jar, and was heated by the addition of NaOH, and then by heating in a boiling soln. of nearly sat. NaCl water
for about 40 min.
All of the SiO2 OUGHT to have dissapeared, but NOPE! The bottom is covered in a thick sludgy layer of fine white powder (I bet it's SiO2)
Maybe the powder is Na4SiO4. I don't think so. I'm pretty sure some
sodium silicate was made, because when a drop of the soln. was acidified with HCl, a coarse clear powder formed on the bottom.
I also made some sodium aluminate from NaOH, H2O, and Al, but no measurements were done, I just added stuff.
That's probably why there are black chunks, white chunks, a coarse black powder, and fine white powder, and not just a fine white powder like I
wanted.
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Cyrus
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How might I get calcium aluminate from sodium aluminate, remembering that this cement hardens in contact with water, and maybe with heat? I think
it's hopeless.
SOMEONE out there has to know how colloidal ceramics bond. I mean, this is ScienceMadness.org! We have the answer, right?
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Cyrus
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Ok. I guess nobody knows how they harden.
But if I use sodium silicate to harden the fibers,
2Na2O.SiO2 + 2CO2 -> 2Na2CO3 + SiO2,
Would the sodium carbonate be soluble in water, or would it be too intimately mixed with the silica to wash off?
I'm trying it anyways.
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Tacho
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I like sodium silicate very much, but I could never make it insoluble after drying, no matter how exposed to CO2 it was.
Besides, it also swells like popcorn when heated with a blowtorch. The dry thing that is.
It is, however the only "glue" that withstands very high temperatures. Once I made a mix with zinc powder that resulted (after dry) in a
resistant material, quite thermically conductive and more resistant to water than my average cocotions.
Sodium silicate makes me think that I could do something great with it. I just haven't figured out what yet.
Wish you better luck.
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Cyrus
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I did some tests with sodium silicate, colloidal silica, zirconium silicate, and alumina hydrate soln./suspension soaked cottonballs.
The sodium silicate cottonball hardened and dried quickly, the colloidal silica cottonball hardened but did not dry as quickly, the rest did nothing
interesting.
When heated to a yellow/white heat , the sodium silicate fused into a very brittle glass coating, and the colloidal silica stayed hard, but turned brittle, crunchy, and useless.
If found some other recipes, and I did get some better results with 7g zirconium silicate, 2 g sodium silicate solution, .5 g kyanite, about .5 g
magnesium silicate, and .5 g alumina hydrate. Water was added to thin this out, when painted on a cottonball it formed a nice hard coating, but
cracked easily from pressure.
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chloric1
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Annealing
This is where some ceramic theory may come in handy. Maybe you can mix some of your silicate with a little Kaolin and fire at yellow heat and anneal
for a few hours at red heat. I believe the alkalinity may cause the brittleness. Another curiousity would to take alum and precipitate Al(OH)3 and
calcine this with your silicate to make alumisilicates? Portland Cement is an alumisilicate of calcium. Just a thought.
Fellow molecular manipulator
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tokat
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i did alittle seach on cermics and found some mixtures mainly from these.
Al203 --39%
SiO2 -- 44
Fe2O3 -- 0.4
TiO2 -- 1.5
CaO -- 15
MgO --0.1
Alkalis --0.3
mixed with water and some hydration happens i think. hope it helps
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