[EGeb]

Hi guys,

First off, I don't have a chemistry background so please forgive me for any mistakes and feel free to move this if it is in the wrong section. I have an architecture background and a question related to it.

I'm doing a research on building with salt in desert environments. I'd like to know if it's possible to melt/sinter salt together and what happens if you try it. Can you melt the crystals together to form one big crystal? Would you theoretically be able to pour in a mold to get any shape you want? Will it be brittle or strong?

One idea i'm playing with is to use a large lens or parabolic mirror to focus the sun's energy on the salt. With this I believe you could even 3D-print the salt. But somehow I can't find anything on the internet related to sintering/melting salt. Any thoughts?

Thanks in advance!

[Arkcon]

You topic is akin to powder metallurgy, even though your source material isn't a metal.  Yes, you can sinter NaCl together, I have done it.  And if you melt it completely, and let it cool slowly, you'll get a single crystal. If you cool quickly, you'll get a mass of smaller crystals stuck together. NaCl crystals aren't particularly strong, and sintered together, or as a mass of smaller crystals, even less so.  Some sintered materials can be pretty tough -- for example, blocks of PTFE are made by powder metalurgy, then machined into small Teflon parts, such as burette stopcocks.  But I'm not expecting dimensional strength from sintered powders.

[Corribus]

Yes, you can sinter NaCl together, I have done it.

Cool, can you elaborate?

[Arkcon]

Yes, you can sinter NaCl together, I have done it.

Cool, can you elaborate?

Oh, that was a sad, crazy time of my life.  Like lots of people (particularly on these boards) I wanted to electrolyze NaCl to get free sodium.   Completely melting NaCl in a crucible isn't possible with twin propane torches, but I could barely sinter it.  We have to leavie aside that I didn't have electrode material that would survive, and wasn't ready to handle sodium metal.  The block of sintered NaCl was impressive on its own, but not a solidified melt.

As I understand it, silver chloride can be melted, sintered and even machined, as if were a metal.

[EGeb]

Thank you for the quick reply Arkcon!

So if I understand correctly, melting a large amount of salt and pouring it in mold and then letting it cool slowly would result in 1 crystal? Say the mould was brick-shaped, would it end up looking like this: http://www.saltnews.com/blog/wp-content/uploads/2007/10/saltblock11.jpg (but white of course) ?

[Arkcon]

I would guess so, but I can only guess.  Sodium chloride melts at 800 °C, so very few people routinely melt a large amount, cast it, and keep it hot for a long while.

[Enthalpy]

Single crystals are seriously difficult to obtain, but it depends on the good will of NaCl. The more probable result is a single block, polycrystalline.

[Corribus]

but it depends on the good will of NaCl.

Relying on the good will of a chemical? Never a good idea. Chemicals will always stab you in the back, first chance they get. They are like Nature's equivalent of Robert the Bruce's father in Braveheart, and I have never forgiven that bastard for what he did to poor William Wallace.

[EGeb]

Single crystals are seriously difficult to obtain, but it depends on the good will of NaCl. The more probable result is a single block, polycrystalline.

Could you say anything about the material properties of such a block of polycrystalline? Is it very brittle? I understand that it will be weak at the grain?

Would you have any other suggestions on how to fuse a large amount of small salt grains together? Add water and pressure maybe?

This is all for the graduation project of my master's, the idea is to either pour salt building blocks or to 3D print structures using a very large 3D printer. For the 3D printing researching powder printing with salt mixed with flour/maltodextrin and water as a binder. Not sure if that's really chemistry but it's yielding interesting results so far. The material I get is quite beautiful and similar to gypsum.
I'm still very much in an experimental phase so really exploring what I can do with this material so yeah.. any input is greatly appreciated.

[curiouscat]

That NaCl block you posted a pic of: Was that mined? Curious.

As another option, would it work to just put a lot of pressure on your mould & just a little heat? I wonder. Press the powder into brick shapes.

[EGeb]

Yes it's mined, I believe from Pakistan. They call it Himalayan rock salt. It's slightly translucent which could make it a fantastic building material, which is part of why I'm so interested in doing something with salt!

[Borek]

House that gets dissolved during a first rain... not something I would like to live in.

[Enthalpy]

Polycrystalline doesn't imply brittle. Metals use to be polycrystalline.

Single crystal of good size use to take very long, like months - though a few materials are known to grow more quickly and are used in experiment kits for kids.

I suppose a brick of salt woudl take time to dissolve in rain. But is salt healthy? I heard that salt miners suffered from it. Would the atmosphere be very dry in such a house? Or could you cover the bricks with clay or similar, once the wall is built?

Make compact: it needs to experiment. To some extent, pressure can replace heat; the process is then called sintering and is common with ceramics. Made a few 100K below the melting point, and at a pressure that the best metals, or sometimes ceramics, can withstand. Under the proper conditions, ceramics can become airtight then.

Would salt outperform sand for this process and use?

[EGeb]

Polycrystalline doesn't imply brittle. Metals use to be polycrystalline.

Single crystal of good size use to take very long, like months - though a few materials are known to grow more quickly and are used in experiment kits for kids.

I suppose a brick of salt woudl take time to dissolve in rain. But is salt healthy? I heard that salt miners suffered from it. Would the atmosphere be very dry in such a house? Or could you cover the bricks with clay or similar, once the wall is built?

Make compact: it needs to experiment. To some extent, pressure can replace heat; the process is then called sintering and is common with ceramics. Made a few 100K below the melting point, and at a pressure that the best metals, or sometimes ceramics, can withstand. Under the proper conditions, ceramics can become airtight then.

Would salt outperform sand for this process and use?

Yeah I'm not too worried about the dissolving part. I'm designing for desert climates where the amount of rain is very low in the first place, plus I could treat the outdoor surface with a coating. I'm assuming proper ventilation will take care of the dry atmosphere so i'm not too worried about that either.

Sand might outperform salt in this instance yeah. The whole goal of this project though is to first separate seawater into fresh water and salt using solar energy. Then the fresh water can be used in greenhouses and for outdoor vegetation to grow crops and the salt is a waste product. I can't chuck it back in the sea because it'll be an ecological disaster so I want to use it as a building material and create a closed loop of (free) materials. There are also a lot of desalination plants in desert climates such as Qatar, Israel and the UAE. After extracting fresh water they deposit the brine back in the sea which isn't very eco friendly. If you could use their brine you'd have another large source of salt.

So the main question for me is now: what is the best way to solidify fairly large salt elements. Melting it together doesn't seem like such a great idea because the energy input would be massive. Sintering seems like an option. 3D printing seems also an option. Still have no idea on what the structural properties will be though..

[Arkcon]

Well, the polycrystalline NaCl block, made by sintering with a bit less heat a with as much pressure as possible, will not likely be stronger than the mined Himalayan salt block pictured above.  I'd bet its quite brittle, and I know engineers can measure the breaking strength of dimensional materials, I don't know if anyone's ever done that with one of these blocks, but you will probably have to address that when you suggest the feasibility of the project.  I don't think you want to buy that Himalyan salt, and try to test it's breaking point.

I don't really get the 3-priniting angle, that's either melted or low pressure sintered, as I understand the technology.  I don't think the equipment is up to molten salt capability just yet.  If you can build something like that, and you want to try, you may want to find a lower melting point salt as a test..