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The steel that makes up the skeletons of modern skyscrapers consists of iron and carbon. The problem with both is that, if the entire city is untended for long enough, both metals betray their downsides. Iron corrodes easily, and though carbon is used to toughen the alloy, it, too, is vulnerable to rust.

The other piece of skyscraper anatomy is a mortar made from modern concrete. The "Portland Cement", as it is called, lacks the Romans' lime-and-volcanic-ash-mixture that made it endure for centuries.

For a posthuman metropolis to stand for centuries instead of decades, what kinds of metals could be added in the steel alloy to make it tougher and slow down corrosion? And could Roman concrete be reasonably reinforced?

Please no suggestion for stainless steel. If stainless steel weren't used for skyscraper skeletons, there has to be good reason.

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  • $\begingroup$ I suspect the reason stainless steel isn't used in modern skyscrapers is simply cost (although I could be wrong). It's simply cheaper to have regular maintenance than to build it out of materials that will last forever. $\endgroup$ Sep 29, 2015 at 14:22
  • $\begingroup$ Could you clarify something? You used the "post-apocalypse" tag but are asking for really advanced technology which doesn't usually happen in a post-apocalyptic scenario. Is this post-apocalyptic or post-modern? $\endgroup$
    – Green
    Sep 29, 2015 at 14:45
  • $\begingroup$ @Green: My assumption is that he's looking for skyscraper construction that would last a long time in a post-apocalyptic scenario. But yeah, the mix of terms with posthuman is confusing. $\endgroup$ Sep 29, 2015 at 14:50

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Use materials that don't corrode such as glass or ceramics. This being a post-modern technology, I'm going to assume that you can use whatever materials you like in an as yet unimagined manufacturing process to form tall buildings that stand essentially forever.

Glass and ceramics are ideal candidates as they don't react with oxygen and thus remain essentially unchanged for tens of thousands of years. The oldest human artifacts recovered are made of glass, stone or ceramic.

Coating steel with any non-reactive coating still leaves the reactive steel exposed should something happen to the coating. Ceramics may flake or craze under high wind or earthquake loads thus leaving the underlying steel exposed. Better to have the entire structural member made of something that doesn't corrode at all.

Materials Difficulties Glass and ceramic have amazing compressive load strength but relatively poor tensile strength (ie, they laugh at you if you try to crush them but run crying to their mommies when you try to pull them apart). This hyper-advanced civilization should have the theoretical understanding to create custom ceramics and glasses with the appropriate compression, tension and ductile properties to build long-standing megastructures with.

Building with silicon makes a lot of sense on Earth as much of the crust is silicon in the form of quartz.

Besides, glass is the future material! Everyone knows that the future is made of glass. Future City Source

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  • $\begingroup$ Concrete has exactly the same issues, so no big deal. You need another material to complement it that doesn't rust like rebar. $\endgroup$
    – JDługosz
    Sep 29, 2015 at 18:24
  • $\begingroup$ Are you saying that concrete doesn't degrade with time? I'm not sure what you mean. $\endgroup$
    – Green
    Sep 29, 2015 at 18:25
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    $\begingroup$ Concrete takes compression but can't handle being pulled apart. $\endgroup$
    – JDługosz
    Sep 29, 2015 at 18:26
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You mentioned "stainless steel". It's not just one thing, but a general label meaning a minimum amount of Chromium.

Knifes are sometimes stainless, sometimes not. Why is a good example that extends to your use. I chose the stainless for kitchen knives because I figured I might be less prompt wiping it down just once and mess it up. But alloys that don't handle the environmental conditions are better in that they are sharper and stronger. It's a trade off.

If you put an emphasis on withstanding weathering by itself, it would be a tradeoff with other properties including expense, workability/fabrication, and strength.

Now I have pocket knives that are "stainless" but tougher than traditional stainless steel: they are made with Vanadium.

I also recall when bicycle frames started using "cromolly", Chromium and Molibdinum, for better weathering while being light and strong enough.

So I'm not saying "just use stainless". I'm pointing out that the alloy or mixture can involve many ingredients besides iron and carbon, with carefully tuned proportions. You would source an alloy that has the desired properties. It costs more than using an alloy with fewer requirements.

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I'm going to get totally downvoted for suggesting this, but hear me out...

Wood.

Wood is an incredible building material but the mentality is typically to jump to "it burns," and "it rots."

Safety

Wood is largely being recognized for viability. The reason that I point this out is that it leads to your longevity requirement. People will be quick to suggest that wood will burn during an 'apocalypse' or even a lightning strike, and not be viable over time. It's understood now that wood construction is not going to 'just burn down,' (source and source- just skip to the exec. summary if you like).

Longevity

Wood in buildings and towers is pressed, creating a layer up to a foot thick. Wood has been found to be entirely the architecture of buildings 1500 years old, and has been found in elements of great former cities such as Damascus and Petra far before that. Not to mention petrified wood (which is a different situation, of course).

Alternatives

Yes, you will find glass, metals, ceramics, etc. hundreds of years from now: but don't discount wood as a viable alternative: in our minds it seems really vulnerable, but in fact, it's really strong and it does last just as long!

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  • $\begingroup$ I'd be more concerned that wood isn't structurally strong enough to support a skyscraper-sized building. $\endgroup$ Sep 29, 2015 at 20:53
  • $\begingroup$ @DanSmolinske - I know it's not popular and it sounds scary, but there's an enormous international movement for wooden skyscrapers. You can google it, or also, ted.com/talks/… $\endgroup$
    – Mikey
    Sep 29, 2015 at 21:03
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Not a metal, but I think impregnating concrete with carbon nanotubes-- basically a molecular version of steel rebar-- would be an upgrade, provided nanotubes could be made cheaply enough.

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    $\begingroup$ No. Rebar has specific properties: it expands with heat to the same degree as concrete, and it allows a tight mechanical bond. You want to add mechanical strength in tension. Dust won't do that; it needs to be spanning the length of the piece. $\endgroup$
    – JDługosz
    Sep 29, 2015 at 17:03
  • $\begingroup$ @JDługosz Not dust but fibers. Carbon nanotube fibers. $\endgroup$
    – user151841
    Jun 10, 2020 at 15:42
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Here goes:

Use electroplating to add a very thick (a couple centimeters) veneer of titanium. You get all the advantages of steel, minus the rust and erosion part.

Or ... if you are rich enough, build the skyscrapper skeleton solely out of titanium. It is going to cost you a fortune, though.

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  • $\begingroup$ Why would it need to be a few centimeters? Would a few microns work? $\endgroup$
    – Green
    Sep 29, 2015 at 14:47
  • $\begingroup$ Who knows how strong gales, storms, cyclones, earthquakes you would get. A few microns would be quickly scratched away in a single storm once the building's soft parts (concrete and glass, that is) break/rot away. $\endgroup$ Sep 29, 2015 at 14:49

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