Jstuyfzand
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What makes clay hard?
I am doing some research on home brew refractorys and I saw that most
refractory mixtures consist of SiO2 and Al2O3, this is the same composition as for example fire clay.
When mixed with water, fireclay becomes soft and sticky, and when dried, hard.
But how does a mixture comprising mainly of sio2 and al2o3 stick together, and even for a solid mass together?
This is a question out of curiosity, but I also plan to do something with the answer, because I would like to make a refractory out of alumina.
But for that I need some information, that I have been unable to find and I hope you all can help me further.
Thank you!
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aga
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plural of refractory is refractories.
I saw a video or two about making a small microwave refractory using 'susceptor' materials to absorb/re-emit the microwave energy as heat.
One used some alumina material that looked like a block of polystyrene, yet extremly resistant to heat.
More suited to my liking was a Robert Murray-Smith one :
https://www.youtube.com/watch?v=ZJvCuGmRWL8
.. although the alumina material would be great IF it could be made by an amateur.
[Edited on 28-4-2016 by aga]
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careysub
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Clays consist of mineral particles smaller than about 1 micron, which is quite small. A typical clay is composed of flat plates. When wet moisture
lubricates the clay particles so that they can slide easily (the smaller the clay particle the more plastic the wet clay).
When the clay dries the flat plates all lock and wedge together and cannot move unless substantial force is applied, breaking the dry clay chunk.
Adobe clay bricks add sand to the clay, and the clay particles wedge the sand together, giving greater strength. Firing of course creates chemical
transformations, creating new structures.
There is a lot of data about how to make alumina refractories, NIST has some good old reports on this. As I recall a slurry is made up with alumina,
treated with HCl, which then ages for a day or so to get the proper consistency ("slip") for pouring into a mold.
Look for "alumina slip casting". In fact Googing "alumina slip casting NIST" (without the quotes) brings up one of the reports I remember:
http://nvlpubs.nist.gov/nistpubs/jres/23/jresv23n2p319_A1b.p...
To my reading making alumina refractories is very doable. One thing that interests me is lining steel pipes with alumina slip for a cheap crucible.
Differential expansion is an issue, but perhaps preventing bonding between the liner and the pipe, and using a thick lining might solve that problem.
[Edited on 28-4-2016 by careysub]
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Chemist_Cup_Noodles
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I'll try to answer your question in three ways: What makes clay "hard"(as in a solid), what makes fireclay heat resistant, and why it hardens.
What makes clay a solid is the strength of the intermolecular forces between the different molecules in it. Intermolecular forces, or IMF for short,
don't act between just different molecules of the same compound, but with any compound around it. Generally if a substance is a solid at room
temperature, then it has strong IMFs, and if it is liquid or gas, then it has weaker IMFs. There's a lot more that goes into and behind IMFs, and
that's sort of the basic version. So if you're interested look it up on Google to learn more. So it is because of strong IMF's that SiO2
and Al2O3 are able to "stick" to each other. But if I recall correctly, I've seen that firebricks aren't necessarily hard like
a diamond, in fact rather brittle at times, but still an extremely heat resistant solid.
What makes fireclay so heat resistant is the types of bonds in the molecules that mostly make it up, SiO2 (silica) and
Al2O3(alumina). This is because alumina has much stronger bonds inside of each molecule than silica does. If you think of a
diamond, you are actually thinking of just one big molecule, because all it's atoms are bonded together in a network. In many cases, alumina can bond
together as big molecules like a diamond does as you can see in the picture below. Diamonds do it better though, as they are extremely hard unlike
most alumina compounds.
So it is this what lends alumina it's high heat resistance. Silica is actually what makes up most sand and quartz. It is actually a very interesting
compound, despite looking so simple. Similar to alumina, silica molecules can sometimes network together in a way you can find in quartz, and this
makes them also very heat resistant. So both of these molecules can end up forming some big molecules compared to others, but the size of the
particles these molecules make are still microscopic in size.
The post above me has a good explanation of the structure of clay.
And so for why fireclay (and other clays, for that matter), can go from a kind of slurry to a solid is because of chemical reactions that happen once
the water is added. Fireclays are as you've seen not just alumina and silica, but there are some other metallic oxides and hydroxides in there as
well. These too will have very strong ionic bonds, so they do not lessen the heat resistance of the bricks. So once you add water, it can hydrate many
of the compounds in the fireclay slurry, an expand them slightly. Also, chemical reactions may happen too between some of the compounds in the mix and
the water, similar to in plaster. And so as all the excess water evaporates, the IMFs present, new bonds made, and the hydrated compounds are what
makes the brick a solid, heat resistant, brick.
Hope I was of help!
Helpful wikipedia pages:
https://en.wikipedia.org/wiki/Fire_clay
https://en.wikipedia.org/wiki/Clay_chemistry
https://en.wikipedia.org/wiki/Silicon_dioxide
https://en.wikipedia.org/wiki/Aluminium_oxide
https://en.wikipedia.org/wiki/Intermolecular_force
Note: If I've said anything wrong in my attempts to simplify someone please correct me.
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Artemus Gordon
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" Intermolecular forces, or IMF for short"
Should any of your IM forces be caught or killed, the secretary will disavow any knowledge of your actions.
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Magpie
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Quote: Originally posted by careysub  |
To my reading making alumina refractories is very doable. One thing that interests me is lining steel pipes with alumina slip for a cheap crucible.
Differential expansion is an issue, but perhaps preventing bonding between the liner and the pipe, and using a thick lining might solve that problem.
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I have made many alumina/kaolin slip cast tubes for use in my tube furnace for making phosphorus. After casting in a plaster-of-paris mold they would
shrink about 14%. So lining a pipe skin tight would not be possible, IMO, at least with typical recipes. Then again, if you want the clay to pull
away from the steel tube to avoid differential expansion problems, this might work for you, if it doesn't crack first.
The single most important condition for a successful synthesis is good mixing - Nicodem
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JJay
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I am going to attempt this again pretty soon. I believe it is easier to line the inside of the pipe than the outside of the pipe.
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Jstuyfzand
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Wow, I did not expect suck feedback, even my grammar was corrected!
All of this information was incredibly helpfull, it turns out I was just using the wrong search terms!
I love the molecular explanations, and the part about the normal clays particles was also very helpfull.
Alumina slip casting looks good, there is some reading for me to do.
Sorry for not replying the last few days, but thank you all so much!
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