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In the history of human construction, there are two kinds of concrete: Roman and Portland cement. Roman concrete was so durable that it'd helped the buildings of Rome survive for 2,000 years. Portland cement, by contrast, is so poorly considered that in a Life After People, moisture will weaken it into collapse after a century.

Listed here are the ingredients for a different kind of cement:

  • Water
  • Sand
  • Mud
  • Manure
  • Clay
  • Chalk
  • Powdered limestone
  • Powdered shale
  • Powdered slate
  • Seashells
  • Volcanic ash

With this recipe, would modern concrete be better off in a Life After People, or would it add to Portland cement's already apparent problems?

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    $\begingroup$ I once worked with a guy in the concrete/cement business. He had a business card made of cement that would flex and bend like a piece of paper (it was cement with rubber pellets - I think). We can do amazing things with this substance (a composite) when we're not worried about the cost. Conventional concretes/cements have the properties they do because that's what the cheap & bulk stuff can do. $\endgroup$ – Jim2B Apr 11 '16 at 2:14
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    $\begingroup$ One, there's selection bias. You only consider the buildings that did survive for 2 000 years (with damage!), and completely ignore the ones that didn't. I'm pretty sure plenty of WWII bunkers are going to survive for hundreds of years. Two, there's the issue of economics - look how many concrete buildings there are nowadays, and compare with how many survived from the ancient times. The Colosseum took somewhere between 10-20 years to build, depending on who you ask - compare with something like the stadium made for the Chinese Olympics, which would be outright impossible in Ancient Rome. $\endgroup$ – Luaan Apr 11 '16 at 7:59
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    $\begingroup$ I am sitting right next to a German bunker from WWII that has been declared "Impossible to remove without destroying the rest of the city." I know where I would go in an apocalypse. $\endgroup$ – Stig Hemmer Apr 11 '16 at 9:33
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    $\begingroup$ Another thing to consider is how the "new" concrete would work with steel. Modern concrete has the not negligible property to react to temperatures (i.e. expands and contracts) the same way steel does. If it was otherwise the buildings would collapse as the weather changes. $\endgroup$ – algiogia Apr 11 '16 at 9:48
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    $\begingroup$ You're ignoring all the other points raised by everyone here. We've known how to make Roman cement before Portland cement - Portland displaced Roman almost entirely soon after it was introduced, because it's better in almost every way and cheaper (a quality of its own). I didn't mention damage "by human hands" - all concrete is very brittle and easy to damage, that's not what damaged the Colosseum etc.. It's not the concrete that fails - it's the steel bars that reinforce it. If you build without reinforcement, you'll get the same results as with Roman cement, or better. $\endgroup$ – Luaan Apr 11 '16 at 12:53
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Concrete is not as simple as throwing a bunch of ingredients together. The ratio of the materials is very important. Thus, just a list of chemicals cannot tell us what will and will not work.

From this research paper, portland cement does just fine, if you mix it right:

Most recently, Mehta and Langley 2000, designed an unreinforced, monolith concrete foundation consisting of two parallel slabs, to last for 1000 years. They used high - volume Class F fly ash concrete in the construction of the foundation. The slabs were built with HVFA concrete mixture containing 240 lb/yd^3 of Class F fly ash and 180 lb/yd^3 of portland cement. Reinforcement was not used for these slabs. A petrographic examination of one-year-old test slab that was cast and cured under similar conditions has shown crack - free nature of the HVFA concrete (Asselanis and Mehta 2001).

At present, this seems to be achievable for concrete without reinforcement to predict/speculate on a 1000 - year life.

I recommend reading that research paper. In addition to suggesting portland cement can last 1000 years, it also runs down a lengthy list of characteristics for long lasting concrete which may be helpful to you!

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Concrete is a composite of cement a binding agent and another material that provides the soft cement durability. For durability the key factor is maximizing the binding surface area between the cement and the additive.

The first step is selecting an additive with large surface area. Romans used volcanic ash, the example in Cort Ammons answer fly ash. The function is the same, to increase the surface area the cement has to attach to. Generally "ash" is good since the properties that make us call something "ash" imply large surface area to mass ratio. (Most other things on your list are not that good as they have grains that either are worn round, have smooth surfaces, or both.)

The second step is to optimize the ratio of cement to additive (almost certainly wrong term), cement is soft so if you have too much of it, the concrete will have bad mechanical properties. If you think about it, maximizing the cement to additive surface area means having the right ratio so that weak additive to additive contacts are avoided and that the amount of soft cement mass not in contact with additive is avoided.

But it is not really this simple in practice, obviously. You have to also think about economics and the intended purpose and the types of loads on the structure. Large rocks with low surface to volume ratio almost always weaken the structure, but they can drop the cost without significant weakening if the concrete is only exposed to compression. The concrete around the rocks will still be weaker and crack, but cracked concrete still can carry weight if it can't move.

A related issue is that if you want the concrete to have specific properties at scales much larger than the grain size of the additive, and cube-square law implies the additive should have small grain volume for optimal durability, you might want to, or even need to, use reinforcement. Reinforcement generally reduces long term durability due to stresses in the interfaces between the concrete and reinforcement and steel, for example, also corrodes. But people have been using glass or obsidian fiber reinforcement, which might be stable over long time.

And of course if you want long life you also need to consider chemical stability. Usually this isn't an issue or the actual issues are hard to predict. For example Roman concrete is still vulnerable to damage caused by pollution that did not even exist when the buildings were built. Nobody blames the Roman architects for that oversight. (Although since you have "manure" on the list, you should remember that organic components or most cheap metals degrade over time and cause the entire composite to fail when doing so.)

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