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

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

#71

Post by iamflimflam1 » Fri, Jul 17, 2026, 8:50 AM UTC

ErroneousBosh wrote: post #56

Animats wrote: post #54

Earlier quoted context omitted.

Stainless steel rebar is a thing. It's expensive, but structures built with it last much longer. Washington State now has a policy that bridges over salt water will be built with stainless steel rebar. Stainless steel costs much more, but total project cost goes up less than 10%. Epoxy coated rebar looked promising for a while, but it's on the way out. Water gets in at cuts and joints. So all field joints have to hav…

> Stainless steel costs much more, but total project cost goes up less than 10%. I think people are often surprised how little materials cost affects the total cost of a job. I'm having this argument at work, where I want some more expensive cabling installed in a bunch of offices across Scotland so it doesn't need to be done again in another ten year's time, but "that stuff is so expensive, do we really need it?" is…

I hope you’re prepared for a huge amount of bike shedding… but if we pick this color, we can save a couple of dollars…

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

#72

Post by rkangel » Fri, Jul 17, 2026, 8:51 AM UTC

ErroneousBosh wrote: post #56

Animats wrote: post #54

Earlier quoted context omitted.

Stainless steel rebar is a thing. It's expensive, but structures built with it last much longer. Washington State now has a policy that bridges over salt water will be built with stainless steel rebar. Stainless steel costs much more, but total project cost goes up less than 10%. Epoxy coated rebar looked promising for a while, but it's on the way out. Water gets in at cuts and joints. So all field joints have to hav…

> Stainless steel costs much more, but total project cost goes up less than 10%. I think people are often surprised how little materials cost affects the total cost of a job. I'm having this argument at work, where I want some more expensive cabling installed in a bunch of offices across Scotland so it doesn't need to be done again in another ten year's time, but "that stuff is so expensive, do we really need it?" is…

In general, in the UK, labour costs dominate. How do people not understand that?!

This is what's interesting with AI pricing at the moment - it has gone from "a fraction of the equivalent labour cost" and so people have tried to cut staff, and is moving to "on par with labour cost" and all the calculations change.

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

#73

Post by rkangel » Fri, Jul 17, 2026, 8:52 AM UTC

Izmaki wrote: post #68

bombela wrote: post #15

> 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 us…

> but that is more costly and and less efficient. Maybe this is the clue as to why our concrete crumbles after 100 years: it's not economically efficient to make it last longer?

A broader view of this point would be "the economic structures in some countries do not allow for investments with a long payback".

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

#74

Post by iknownothow » Fri, Jul 17, 2026, 8:53 AM UTC

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

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

#75

Post by iknownothow » Fri, Jul 17, 2026, 8:58 AM UTC

iknownothow wrote: post #74

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…

I forgot another important reason why Portland cement is preferred over lime these days. Lime takes a while to set (a few days?), cement sets faster.

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

#76

Post by beeforpork » Fri, Jul 17, 2026, 9:06 AM UTC

Barbing wrote: post #23

eru wrote: post #8

Earlier quoted context omitted.

Yes, but you have to replace the sacrificial anode every so often. https://en.wikipedia.org/wiki/Galvanic_anode

Like the plate of food on the grass for the bees at the barbecue

Wasps, not bees.

(Sorry, I'm a bit of a nerd. But I am sure you can cope with that.)

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

#77

Post by PunchyHamster » Fri, Jul 17, 2026, 9:10 AM UTC

ErroneousBosh wrote: post #56

Animats wrote: post #54

Earlier quoted context omitted.

Stainless steel rebar is a thing. It's expensive, but structures built with it last much longer. Washington State now has a policy that bridges over salt water will be built with stainless steel rebar. Stainless steel costs much more, but total project cost goes up less than 10%. Epoxy coated rebar looked promising for a while, but it's on the way out. Water gets in at cuts and joints. So all field joints have to hav…

> Stainless steel costs much more, but total project cost goes up less than 10%. I think people are often surprised how little materials cost affects the total cost of a job. I'm having this argument at work, where I want some more expensive cabling installed in a bunch of offices across Scotland so it doesn't need to be done again in another ten year's time, but "that stuff is so expensive, do we really need it?" is…

> I think people are often surprised how little materials cost affects the total cost of a job.

It's because they frankly never did it. Nobody that had to contract something around the house is surprised by cost like that, it often costs more than price of renting equipment + you doing it yourself 3 times slower than an expert would did it, even if you're paid well.

There is reason there is so many DIY channels, labour costs are high and install costs of some stuff can be ridiculous (like 2 hour job to install AC costing more than cheap AC unit).

That's also partly the reason the more expensive materials are used - if they offer savings in labour (say way faster to put in), they might be worth it vs paying someone for more hours

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

#78

Post by cm2187 » Fri, Jul 17, 2026, 9:11 AM UTC

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.

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

#79

Post by PunchyHamster » Fri, Jul 17, 2026, 9:14 AM UTC

rkangel wrote: post #73

Izmaki wrote: post #68

Earlier quoted context omitted.

> but that is more costly and and less efficient. Maybe this is the clue as to why our concrete crumbles after 100 years: it's not economically efficient to make it last longer?

A broader view of this point would be "the economic structures in some countries do not allow for investments with a long payback".

Or simpler: people don't exactly want a house layout that was popular 100 years ago. And definitely not the level of insulation it provided. Nor the fact putting any new wiring and piping thru concrete is PITA

Tastes change and so do other requirements.

but, if we actually built for longevity in use, we could build a very durable shell of a house and then use more perishable but easier to modify methods for inside. Build a nice durable brick and concrete shell then use wood and plasterboard for room walls and floors

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

#80

Post by metalman » Fri, Jul 17, 2026, 9:18 AM UTC

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.