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How Has Roman Concrete Lasted for Millennia? 1,900-Year-Old Latrine Offers Clues

270 points by divbzero

by iknownothow

9 subcomments

For those not familiar, the secret ingredients are to do with the Lime Cycle [1]:
1) Quicklime/Slakedlime (Calcium Oxide, CaO)
2) Lime (Calcium Hydroxide, CaOH2)
3) Limestone (Calcium Carbonate, CaCO3)
To keep it simple, typically you start with Quicklime (CaO) and after construction you end up with some mix of all three and after hundreds of years, the masonry transitions to mostly Limestone with microscopic traces of the other two. The slow transition of the lime cycle upon exposure of the masonry to both air and water (rain) ends up making the structure "self healing" and "stronger over time".
Fun fact! Lime putty is anti-mold even in humid conditions because upon exposure to moisture, CaOH2 + H20 becomes too basic for mold to grow on.
Cement/Concrete (based on Portland cement) is water proof but Lime by itself is not. But Roman Concrete, made from Lime and mixing with ash or broken pottery or ceramics makes it water proof [2]. I beleive Roman concrete was used whenever contact with water was expected. Both concrete and Roman concrete have the same underlying chemistry (Pozzolanic reaction) to make them water proof [3].
The fascinating thing is that Lime is everywhere in ancient masonry. Lime is more breathable, workable and sustainable. The only thing is, it requires maintainance, which is why Portland cement has taken over the world.
Modern cement/concrete is amazing for large structures. I hope Lime makes a comeback soon at least partly as putty and for building smaller homes.
[1] https://en.wikipedia.org/wiki/Lime_(material)
[2] https://en.wikipedia.org/wiki/Roman_concrete
[3] https://en.wikipedia.org/wiki/Pozzolanic_activity#Reaction

by bombela

4 subcomments

> It turns out that another chemical reaction, known as carbonation, might also contribute to Roman concrete’s longevity.
Roman concrete was made lime cement (calcium dioxide); which cures via carbonation (hardens with carbon oxide). And adding pozzolan to lime makes it hydrolic (hardens with water). Is it surprising that it can still carbonate some? Modern concrete has steel which rust and crack concrete. You can use fiberglass rebar for longevity, or build without rebar even, but that is more costly and and less efficient.

by skybrian

5 subcomments

Modern concrete has steel rebar, which is very useful, but eventually corrodes. Stainless steel rebar could be used if longevity mattered, but usually it doesn’t because the building will likely become functionally obsolete and need replacing before then.

by ajwin

1 subcomments

A lot of the reason why concrete structures have failed so quickly is that the material was still fairly new when most of the structures failing now were created. There’s nothing that says that well designed and constructed modern high performance polymer modified concretes will have the same problems. They also have additives like zypex paste that make it more waterproof and self healing. They also have permeable formwork liners for increasing the surface density of the concrete and chemical concrete hardeners etc.

by demosthanos

4 subcomments

Related, Grady Hillhouse on the myth of Roman concrete.
> The miracle of modern chemistry has given us a wide variety of admixtures like superplasticizers to improve the characteristics of concrete beyond a Roman engineer’s wildest dreams. So why does it seem that our concrete doesn’t last nearly as long as it should? It’s a complicated question, but one answer is economics. There’s a famous quote that says “Anyone can design a bridge that stands. It takes an engineer to build one that barely stands.” Just like the sculptors job is to chip away all the parts of the marble that don’t look like the subject, a structural engineer’s job is to take away all the extraneous parts of a structure that aren’t necessary to meet the design requirements. And lifespan is just one of the many criteria engineers must consider when designing concrete structures. Most infrastructure is paid for by taxes, and the cost of building to Roman standards is rarely impossible, but often beyond what the public would consider reasonable.
https://practical.engineering/blog/2019/3/9/was-roman-concre...
A large part of why Roman concrete lasted longer than ours tends to is that we suffer from a shortage of narcissistic emperors with the means to wield entire economies towards their own immortality.

by middayc

1 subcomments

Very interestin discussion below/above. I just want to say that building with hempcrete, which relies on lime and NHL (natural hydraulic lime) or Baumit Trassitplus whic is some version of roman "cement" relies on may of these principles and is a "modern building technique". It's also much more carbon neutral than many other methods, can produce good thermal dynamics and "breathable" walls. And is DIY friendly.

by aiauthoritydev

6 subcomments

The whole promise of engineering is not to build a bridge that stands but to build a bridge that barely stands. It is not a good idea to build a bridge that last 500 years. You likely destroyed valuable resources to build one. Build a bridge that lasts 100 years and save those resources. In 100 years the technology to build bridges improves so much that it is lot easier to build a new one. At least in most countries like India.

by simonebrunozzi

1 subcomments

A picture of one of these toilets would have been useful.

by cm2187

6 subcomments

I am glad the pantheon still stands. But the real question is are there really any modern structure that will be worth preserving for 2000 years. Architects have long abandoned aiming for beauty to aim for originality. Originality doesn’t last. Humans in 2000 years will only wonder why on earth our society could manufacture atom level micro ships but couldn’t come with a single structure worth preserving.

by joshellington

2 subcomments

There must be a better format and distribution method than this. Ideologically a strong brand name and domain, yet ads every two sentences. Even with ad blocking, there is constant aggressive attempts at attention.
I propose the communal brain rot is less to do with short form video, and more to do with the everyday experience of trying to read something enlightening, tickle your curiosity - and be just absolutely fucking hammered with autoplay interstitials and 720x90 and 300x250 bullshit.

by metalman

1 subcomments

corrosion of rebar is so significant that there are specs for the acidity of rock used for agregate in the concrete. Older rock that has been "washed" by billions of years of water permiating it , is then used, fetching a premium. Look into it and there are periability numbers for everything, which is how water has gained it's reputation as the universal solven, and granite, hard as it is, is also a bit of a sponge. What is harder to figure is the use of a lot of carbon in rebar, which makes it harder and with a higher tensile stength, but with greater rates of corrosion, somehow the calculation is to not use an ultra low carbon steel with some nickle and larger cross sections along with low acid chemistry in the concrete.

by flippyhead

2 subcomments

I heard once that if the Roman civilization had continued, the state of current technology would be something like 500 years more advanced. I don't know if I ever believed it, but sometimes I do.

by Mistletoe

4 subcomments

I’ve often wondered why every good sidewalk I see has a WPA stamp on it from the 1930s and the modern ones are all crumbled and uneven.
https://share.gemini.google/5g0gxGyOmAPD

by daft_pink

1 subcomments

Is anyone else wondering why their house isn’t made of Roman Concrete everytime they read a story like this.

by eleveriven

1 subcomments

Yet carbonation is not universally helpful

by aizk

0 subcomment

Thinking of the ww2 plane with holes meme right now.

by 0xbadcafebee

0 subcomment

A lot of comments here on Roman concrete are missing the killer application: marine use. Any other concrete we know of will degrade in seawater due to different failure modes acting at once (leaching minerals, pH imbalance, freeze-thaw cycles), and none have the same extent of self-healing.
Roman concrete (pozollanic material, quicklime, seawater) is the only one that resists all failure modes and will sit in the ocean happily for millennia. The main downsides are it's not very strong, it takes a long time to cure, and volcanic ash is hard to come by. The specific of ash you use changes the result, ash is not always that easy to get, and Neapolitan Volcanic ash just happens to be extremely effective at this application.
There are alternatives you can make today. You can make Roman concrete today, but it's still kind of tricky and has the aforementioned downsides. Fly-ash concrete is like volcanic ash concrete but still not as good, and we're gonna run out of fly-ash as coal mines close. High-slag concrete works well but will degrade over time. Alkali-activated concrete is really promising as a Roman concrete alternative but doesn't have long-term test data. Ultra-high-performance concrete is brittle and won't self-heal.
So in truth we still don't know everything about Roman concrete and we still can't make its equivalent without traveling to southern Italy.

by SoftTalker

0 subcomment

Survivorship bias? The only Roman Concrete we see today is the stuff that has lasted.

by dukeofdoom

1 subcomments

How do we know Egyptian pyramids were not build from a form of concrete?

by penguin_booze

0 subcomment

Imagine how it'd be if BigConcrete were to make concrete. Every year at ConcreteCon, yet another release that lasts for... one year!

by gaoqian2580

0 subcomment

This is truly a great miracle

by rewtraw

1 subcomments

we can't figure out roman concrete
we don't know how ancient megaliths were built
centuries old cathedrals and holy places have more intricate masonry and carvings than anything built today (with allegedly more primitive tech)
are historians retarded? what's going on here? time is flowing backwards and civilization is declining, but humanity is under a collective trance and can't even see it.