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Comment by subsubzero | Hacker News Reader

I don't know about lost civilizations, but ancient Rome was using concrete technology unknown to medieval builders[1]. Somewhere along the line, this super concrete recipe was lost. I find this really fascinating and some believe that ancient Egypt also created its pyramids using man-made stone instead of quarried stone:

> His most remarkable claim is that the pyramids were built using re-agglomerated stone, a sort of geopolymer limestone concrete, rather than blocks of natural stone.

[1] - https://www.wired.com/2009/10/super-concrete-in-the-us-milit...

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The recipe for Roman concrete was recorded, but most places didn't have access to the materials and conditions necessary to produce it: https://www.sciencemag.org/news/2017/07/why-modern-mortar-cr...

I'll have to dig out an archaeologist but I don't thank that is quite right. I'm a Civ Eng graduate BTW.

Romans - conc - yes. Inventors - maybe but probably not (see below) Medieval period - conc - probably available and probably used

Conc is a funny old beast. Basically, it is a chemical reaction between its ingredients and not a drying out and setting thing. That's why it will set underwater or if you pour too much in one go, it will set itself on fire - exothermic reaction.

I suspect with no evidence whatsoever that someone will have noticed how mixing water with some types of earthly deposits etc etc.

Look at "cob" which is straw and mud and water and some pretty interesting admixtures - starting with wee (horse and human.) My aunt and uncle's last place had 1620 written on the wall near the entrance - so a bit modern for these parts. It is cob and thatch with a few sticks thrown in.

I suspect that conc of some sort is ubiquitous throughout history and has simply not been noticed by us modern lot. However it does generally require processing of some of the raw ingredients so perhaps I'm off course.

My understanding is that Roman concrete recipe/technique is considered uneconomical for most uses today because it uses very little water and must be mixed onsite and packed into place, which makes it much stronger even without rebar. But that doesn't work so well for mixing in a truck and driving to job-site, so modern industry prefers a much wetter mix even though it means our structures crumble in decades rather than millenia.

Where the similar recipe/technique has been applied is in the construction of some large dams, where they call it roller compacted concrete.

see romanconcrete.com

did anyone made tensile tests on a roman concrete specimen? Rebar is wildly used because reinforced concrete is not only used in compression but in beams, slabs ... and could be avoided in arched structures. Arches and domes can last a lost longuer but limit choices in what can be built. Roman concrete should be compared to modern concrete without rebar or this is comparing apples to oranges.

It's just a different tradeoff. We could absolutely make "Roman" concrete that lasts millennia, but it has a much lower strength than modern concrete so you can't make something like a harbor quay out of it. That said, "lasting a thousand years" is not as useful as it sounds, 99+++% of buildings will need to be demolished before that time anyway.

> you can't make something like a harbor quay out of it

Are you sure about that? The Romans built a lot of harbours. We don't really have any data on the compressive or tensile strength of Roman concrete so it's hard to say for sure.

You can't make a _modern_ harbor quay out of it, one that is strong enough for the super large container cranes. The main problem is of survivorship bias: all the flimsy Roman harbors have disappeared so yes, only the most sturdily constructed ones remain. They would still not last under the "modern" load of 40+ ton vehicles.

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