Today's concrete, or "Portland cement", has an aggregate (material used to glue everything together) of sand and gravel.

Ancestral, or Roman, concrete, by contrast, had an aggregate of volcanic ash. Not only does this make the concrete waterproof, it also allows the concrete to harden as it gets older. This may explain why Roman concrete had endured for two millennia.

But Portland cement has no hope of standing for that long. Sand and gravel are more porous than ash, which means water and plants can get through, and in a Life After People, it's estimated that they'd stand for only 100-150 years.

In this alternate scenario, ash is in short supply, and the cities are in need of a different kind of aggregate to keep their concrete strong. Disregarding volcanic ash, sand or gravel, which is the next best material to be used for aggregating the concrete?


closed as off-topic by JDługosz, Hohmannfan, Aify, Thucydides, bilbo_pingouin Jul 11 '16 at 7:28

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  • $\begingroup$ I would suggest anything that can be ground up would work so maybe fired clay would be a good bet. $\endgroup$ – Bellerophon Jul 10 '16 at 18:14
  • $\begingroup$ I think you are confused. Aggregate is the rocks being glued together by cement to produce concrete. Flyash is vey fine dust and as an additive it made it waterproof. Cement continues to cure with age, it does't have anything to do with the chunks eing glued together. $\endgroup$ – JDługosz Jul 10 '16 at 18:14
  • $\begingroup$ @Bellerephon I've heard of fired clay being used to build bricks. But how will they be used to build mortar? $\endgroup$ – JohnWDailey Jul 10 '16 at 18:28
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    $\begingroup$ en.wikipedia.org/wiki/Construction_aggregate is crushed stone etc. not the glue. I do see surfing wikipedia that ash does act as en.wikipedia.org/wiki/Pozzolan and that must be what you are referencing. See also concrete “composed of coarse aggregate bonded together with a fluid cement”. $\endgroup$ – JDługosz Jul 10 '16 at 18:40
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    $\begingroup$ Asking for the "best' is usually too opinion-based. And asking for all different possibilities, is too broad. Could you try to provide something to explain how we should judge what is "the best"? Like, durability (like the ashes in your example), price (sand), availability in a given region, etc. $\endgroup$ – bilbo_pingouin Jul 11 '16 at 7:26

You're confusing several different effects. Modern portland cement is made in a kiln and is essentially a synthetic production of what was found coming from a volcano. But the components are carefully assayed and the process carefully controlled, so it will be better than the wild stuff and always consistent in its properties.

What makes surviving Roman concrete particularly good is that besides large rocks, small rocks, sand, etc. as aggregate, they added super fine dust which fills the pores that would form without them. So their souce of volcanic flyash did that without being done on purpose…they just found that it worked.

So are you wanting a pozzolan that is far superior to quicklime in gluing the bits of aggregate together and being weatherproof in comparison to plain calcium hydroxide? Or do you mean finding the right mix of aggregate sizes including ultra-silt to produce a more waterproof result, so it lasts much longer as water doesn't move through its pores?

To make a super concrete, they would need to figure out both of these issues. Now cement that is immediatly superior to common stuff (portland cement vs plaster of paris in modern terms) is obvious to notice in the short term. Finding additives and subtle mixes of aggregate proportions to make the set concrete harder is hard to tell without more modern instruments, and longterm wear—how can they tell that this recipe will last 100 years vs 20 for harbor footings?

For the better cement, here are some natural materials that might work the same. For the blend of aggregate and filler, anything will do: they just need to find or produce ultra-fine silt and include the right proportions.

The question becomes why would they think to do that or how did they stumble upon it accidently? One answer might be from the making of fine porcelain where a smooth grain is desired. Look at bone china where the minerals are ground and reground to heroic ends. Maybe they got the idea to manipulate the building material in the same way, after seeing that the statues outside the temple stood weather better than the foundation.

See also: The Roman Pantheon: The Triumph of Concrete Research by David Moore, P.E.. Maybe it will give you some ideas for alternatives, as well as providing more authorative information on just why it was so long lasting.

  • $\begingroup$ Life After People said that modern skyscrapers won't last longer than 150 years because of Portland cement as opposed to the tougher Roman concrete. $\endgroup$ – JohnWDailey Jul 21 '16 at 1:54
  • $\begingroup$ toughness has a specific meaning in engineering. The property of interest is water and salt motility. $\endgroup$ – JDługosz Jul 21 '16 at 2:30

Why not ash from forest fires or the combustion of other carbon-based biological matter, like people?

One stone which is known to provide exceptional resistance to damage from its use in the walls of certain forts during sieges is Coquina, a sort of sedimentary rock composed of layers of finely crushed ancient seabed biomass (read: ancient clams and stuff), which is extremely absorbent of kinetic impact. One example of this usage is here: https://seashellsbymillhill.com/2014/08/29/castillo-de-san-marcos-history/

Another aggregate usage of this sort of biomass is Tabby concrete, though I have no sources on whether it's particularly good concrete: https://en.wikipedia.org/wiki/Tabby_concrete

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    $\begingroup$ Because the ash in question are based on silicon or aluminum, not carbon. Nothing to do with common ash as from a fire. en.wikipedia.org/wiki/Pozzolan#/search $\endgroup$ – JDługosz Jul 10 '16 at 18:46
  • $\begingroup$ Sure, but I figured since you CAN make cement out of carbon-based ag, why not mention it? Is such cement generally really poor quality? $\endgroup$ – Adam Wykes Jul 10 '16 at 22:21
  • $\begingroup$ I don't know what you are referring to or what is "ag" here. Black top roads use "carbon based" cement (asphalt), but that's not the same kind of building material at all. $\endgroup$ – JDługosz Jul 10 '16 at 22:59
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    $\begingroup$ Anyway, using shell for the filler (assuming the new stuff binds well to the shell) just saves energy in reducing the shell to quicklime, and you still get a new block of limestone. It should be the same strength as using all cement, but better in that the set lime is not as optimal as the shell or natural limestone in its crystalline structure and chemical makeup. Adding shells makes it less subject to cracking. $\endgroup$ – JDługosz Jul 10 '16 at 23:19
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    $\begingroup$ @JDługosz Steven S. Ross, author of "Construction Disasters", called volcanic ash "an aggregate". $\endgroup$ – JohnWDailey Aug 11 '16 at 4:23

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