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Am trying to build a floating town with a starting resident population of about 3000 inhabitants.

The setting:
Around the end of the XXII century.
The town 'Pioneering Pinnacle' is floating in the Northern Atlantic, off the coast of Maine / Terranova. (I would avoid it going further to the North as I know that the area towards Greenland has some fierce waters).
The town is capable of propulsion at low speeds.
Technology has evolved steadily. Most notably energy is available in large amounts at a low cost (but not free, imagine around 10% of today lower costs).
The town is built by a large corporation. They have large means but must have a profit at the end of it.

The nexus size is about 200x200 meters. Average height 30 meters (partially submerged). Weight around 200k tons. Made mainly with composite materials and steel.

The plan is to keep on expanding it as it is attracting a lot of potential customers to open their facilities there (the owning corporation would gain rent fees). Expansion would mean adding connected floats and build upon them. Connections would be done with high stress resistant composite materials.

What to use as construction material without raising costs too much? A young executive has suggested Brine. Because it is right there and needs to be disposed of. The 3000 (and growing) residents need to use freshwater. For personal use, production processes, food growth and processing, etc. The town is mainly focused on research (due to it's peculiar status outside jurisdiction) not industrial production.
Our current needs are estimated at 3500 liters per person per day on Earth XXI century (western world). But that includes food production (the largest part). The town heavily relies on importing food. Food production on board is more intended for delicacies or food that may easily spoil during transportation. Or cost too much to be safely transported. My estimate is then a need of about 1000 liters of freshwater per day.
A part of it would come from collecting rain but the larger part is from desalination of water.
Given their more advanced methods (an advanced Multi-stage flash distillation preferred because of the avaliability of cheap energy) for each liter of water about 1 kg of brine is produced.
That means a production of 3 Mkg (3k tons) of brine per day. If only it could be used as construction material...

The question is: Can brine, used together with other components, be realistically used for construction?
Consider that it should stay for prolonged times (at least 20 years) in the North Atlantic environment.
It is ok if it is not strong enough to make a hull. It may be used for internal construction (although salt water would occasionally splash on it anyway). The hull / main frame may be done with stronger materials.
The additonal component(s) of the mix should be a smaller percentage, to avoid having to bring too much material to the town.
Periodic maintenance is fine.

In case you need further details on the technology available in this setting: see the Update at the end of the question.
Although I don't think it is strictly related to this question.

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    $\begingroup$ Do you mean brine or salt? Brine is just extremely salty water, $\endgroup$ – Tim B Sep 10 '20 at 13:24
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    $\begingroup$ @TimB You start with Brine. Of course from there you can evaporate, precipitate, and so on in order to get just the salts. $\endgroup$ – Duncan Drake Sep 10 '20 at 14:49
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    $\begingroup$ This is a little off topic, but I think you could make your floating town out of something like pykrete (en.wikipedia.org/wiki/Pykrete) extremely cheaply in the north Atlantic as a permanent settlement. It's an interesting material made from essentially waste products. Don't know if something similar can be made with other organics purified from sea water. $\endgroup$ – DWKraus Sep 10 '20 at 21:06
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    $\begingroup$ If you use salt as a building material... in an environment with a lot of water... won't it dissolve? (Assuming that the water in question isn't already completely saturated with salt, which the Atlantic Ocean definitely isn't.) $\endgroup$ – reirab Sep 11 '20 at 6:32
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    $\begingroup$ I wonder how such a colony would survive storms? $\endgroup$ – azerafati Sep 11 '20 at 7:20
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Yes, and it has already been proposed and tested

Seacrete, or Biorock, uses electric current to pull disolved minerals out of sea water to make solid stable "rock". It has already been proposed as a building material for seasteading. It even has advantages over concrete, and with a brine solution it would be much faster to make.

Further reading here.

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  • $\begingroup$ Very interesting! The process seem to be slow though, 5cm per year. Molluscs can do better than that. But i don't understand if such growth rate is limited by physics or the technology employed. $\endgroup$ – Duncan Drake Sep 11 '20 at 1:29
  • $\begingroup$ Depending on how hard you want the science to be in your story, you could argue better electrodes and better reaction kinetics because everything is more concentrated in the brine. $\endgroup$ – mart Sep 11 '20 at 7:40
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    $\begingroup$ @DuncanDrake keep in mind that is 5cm PER electrode, if you use a grid or 3D structure electrode, you can multiply that growth rate almost indefinitely. $\endgroup$ – John Sep 11 '20 at 9:59
  • $\begingroup$ @John So if I read the linked document correctly there could be a machine controlling a grid of electrodes, each of them may have a different current flow, thus controlling growth rate. During the process electrodes could progressively change their position, allowing extrusion. The result would be bricks, panels (also curved panels) depending on the shape of the grid. It should also be possible to form beehive cells like structure to incapsulate air in the panels to make them lighter and better insulators. There would be hundreds of machines doing this concurrently. (continued) $\endgroup$ – Duncan Drake Sep 11 '20 at 16:08
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    $\begingroup$ @DuncanDrake yes, originally the idea was proposed for a way to make floating platforms directly from seawater for building floating installations. the material is remarkably strong and if you leave a trickle of current flowing actually gets stronger and is self healing. $\endgroup$ – John Sep 11 '20 at 20:19
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Most likely not
Brine is just salty water. The salinity of sea water is about 35 parts per thousand. So, 35g/L. They'll have to process a lot of water just to make a single block of salt.

That said, structures can indeed be made from salt. If you've already got a huge block of salt, say in a salt mine, you can make wonderfully sculpted structures:

enter image description here

In a floating city, where they're making the salt from sea water, they'll have to resort to more conventional construction:

Saline,TX

They will still rely on imports for cement, framework, fittings and the like. I doubt this will be a very economical solution, when the company could simply transport prefabricated components to the floating city.

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    $\begingroup$ well, it then follows from the given numbers that from the desalination process alone they would get enogh brine to make 105 tons of salt per day. That is about 10 thousand bricks weighting 10kg. In one year they would get about 38000 tons. Is that right? $\endgroup$ – Duncan Drake Sep 10 '20 at 16:33
  • $\begingroup$ @DuncanDrake - depends on how much of the 200x200m area they want to use as desalinisation plant vs the relative inexpense of importing ready made components and materials. $\endgroup$ – elemtilas Sep 10 '20 at 19:31
  • $\begingroup$ good point. Aside from the volume dedicated to the production of fresh water they would need another to process the large volumes of brine. It could be more economically feasible to rent the volume and import the components. $\endgroup$ – Duncan Drake Sep 10 '20 at 19:57
  • $\begingroup$ @DuncanDrake -- In other words, it's not that it can't be done at all; just that the given space & monetary costs of doing it will most likely prevent it from happening. $\endgroup$ – elemtilas Sep 10 '20 at 20:00
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Yes - if we really quibble what a construction material is.

Brine is a liquid above -21 degrees. The northern Atlantic may just get this cold in the middle of winter, but in summer it'll melt. Melting isn't a good property of construction materials.

However...

What is a construction material? Insulation is a construction material, at least according to my local hardware store.

If you build your walls out of a lightweight, watertight, portable material (eg 3D printed ABS, painted with a UV protective paint), you can then pump your brine into the wall as insulation. This gives a great wall between your nice living area, and the very cold North Atlantic.

The more salt in the brine, the lower its specific heat capacity, which means the worst it is as an insulator:
enter image description here

I believe your brine is about 7.5% salt by mass. Not as good an insulator as water, but better then express posting fibreglass insulation.

You can also use brine to produce sodium hydroxide / causitic soda, which is used to pretreat seawater going into a desal plant. Before piping it into a wall, make enough sodium hydroxide to treat your seawater. That link explains that you can also make hydrochloric acid from brine, which has many uses.

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  • $\begingroup$ Sorry, but this doesn't make sense - you can simply dump the brine into the sea, pump up some new seawater instead, and you get yourself (cheaply) material with way better properties... I mean, if you're floating in a literal sea of a mediocre insulator, you're not going to force yourself to use even worse one just to use it, when instead you can just dump it without causing any pollution... $\endgroup$ – BIOStheZerg Sep 10 '20 at 22:55
  • $\begingroup$ Dumping brine from desal is definitely not pollution free, its a major concern for environmental groups. From your point, Why not pump the filtered water into the walls and get an even better one? The brine is a waste product that would need to be discarded, and we need a construction material. This is the best merging of the two I can come up with. $\endgroup$ – Ash Sep 11 '20 at 4:04
  • $\begingroup$ Well, maybe in a very localised sense, ON THE SHORE, in the normal (=currently used) setup. Not on a floating platform in the middle of an ocean! You're not putting into the ocean anything you didn't take from there before, in the grand sum you're just extracting 3 Ml of freshwater, full stop. Which is, btw, 1e-14, or 10 parts per quadrillion, or 0.0000000000001 times the volume of the Atlantic, what a disaster! $\endgroup$ – BIOStheZerg Sep 13 '20 at 2:45
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Have you considered using all the other stuff in sea water besides brine. The sodium and chlorine from the salt are the most common elements but there are many more elements dissolved in lower concentrations. Here are the top ones greater then 1 part per million but there are many more in lower concentrations.

seawater makeup

source

You can also use all the plastic waste, garbage, phytoplankton and other organisms that would get sucked up into the desalinisation intakes.

Finally you could also take core samples from the bottom of the ocean floor (possibly including some bedrock) as your town moves.

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  • $\begingroup$ This should be the answer. Ca and Mg could be used. $\endgroup$ – mart Sep 11 '20 at 7:34
  • $\begingroup$ Magnesium is flammable even underwater (it's what they make diver's flares out of), Calcium is your best bet for usable material from the sea. $\endgroup$ – Darrel Hoffman Sep 11 '20 at 15:44
  • $\begingroup$ @DarrelHoffman Not sure what you mean, calcium is more reactive than magnesium -- it reacts readily with water to produce hydrogen gas. You might be comparing magnesium metal to calcium compounds? $\endgroup$ – jeffB Sep 11 '20 at 21:55
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Not brine, but water. Pykrete is a mixture of water and sawdust (6:1 by mass), and then frozen. It is almost as strong a concrete, and can be repaired by spraying cold water onto it. Unfortunately the challenge is keeping it cold - there are normally steel/copper pipes laid through it, in which cold brine is pumped from a refrigeration unit. Project Habakkuk was an attempt to use this in WWII.

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Yes sort of

Brine is a liquid so technically no but you get rid of that water and you're left with salt which is a solid.

Now salt isn't a great building product for a floating town but you could use it as a filler with a waterproof binder. The salt just bulks out the limited binder like sand and cement.

Being ocean based, the best waterproof binder would be plastic and the ocean has tons and tons of plastic waste that can be scooped up and reprocessed (or seaweed produced depending on your story). What you have is blocks or sheets which are waterproof from the plastic and fire resistant from the salt you could build from.

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