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

#161

Post by psd1 » Fri, Jul 17, 2026, 2:44 PM UTC

carlmr wrote: post #94

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…

>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. Can we use this instead of grout in the bathroom somehow?

In my experience, grout doesn't go mouldy but silicone sealant certainly does. It's the caulk and the sealant that you want, i think, to replace

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

#162

Post by 0xbadcafebee » Fri, Jul 17, 2026, 2:45 PM UTC

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.

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

#163

Post by paulgerhardt » Fri, Jul 17, 2026, 2:45 PM UTC

eleveriven wrote: post #140

iknownothow wrote: post #118

Earlier quoted context omitted.

I'm not sure as I'm not a builder or plasterer and it does depend on the use case. However I do know that some grout and some paints that say they are anti-mold contain Lime or calcium hydroxide as their primary anti-mold ingredient. I think pure Lime putty can be used as pointing material between bricks in humid basements. Lime plaster (lime mixed with sand) can also be used in bathrooms (with some considerations I…

I like the idea of treating limewash as maintenance rather than a permanent cure. That is probably closer to how these materials were historically used anyway

I just restored an 1800’s house and we redid the paint in limewash as part of that process. (Effectively zero VOCs but pretty caustic when wet.)

Historically (for interior paint) you would use regular limewash in dry spaces and add casein/milk in high traffic/wet spaces. It works in a bathroom but wouldn’t hold up in a shower (for that you would want to use “tadelakt“ - especially if you’re going for that Aman spa look).

The contemporary solution with modern limewashing is to use “mineral shield” - it uses silicone instead of casein - it makes water droplets bead up but still lets water vapor breath through.

It also doesn’t flatten out the nice velvet texture one gets with limewash paint from the calcite crystals.

That said, it’s extremely obvious if one touches up a patch of wall with limewash. No two batches are the same color unless you’re going pure white and even then…

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

#164

Post by mikepurvis » Fri, Jul 17, 2026, 2:50 PM UTC

cma wrote: post #30

litoE wrote: post #20

Earlier quoted context omitted.

As I understand it, concrete has excellent resistance to compression but fails easily on traction, while steel bars are exactly the opposite. That is why you put rebar in concrete: the steel handles the traction loads and the concrete handles the compression. This works well because both materials have similar coefficients of thermal expansion, so as the temperature changes they both expand and contract at the same r…

There's also stainless steel rebar

Even without the rebar rusting, I think you can still have issues with the steel and concrete having difference thermal expansion properties, particularly for outdoor strucutures where a freeze/thaw cycle is in play.

Basically the rebar works itself loose over time and creates micro-fractures in the concrete that then get moisture in them and can cause expansion damage— all without the rebar itself ever rusting or "failing".

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

#165

Post by adolph » Fri, Jul 17, 2026, 2:52 PM UTC

smalltorch wrote: post #149

skybrian wrote: post #2

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.

It only corrodes if it's not completely encased in concrete. The concrete stops the reaction. And, if one portion of the rebar is not encased it will eventually corrode through overtime.

This is something I wasn't aware of that is fascinating. The ph balance of the rebar's environment determines the progression of oxidation. There was a guy in the 1920's who observed this and had the idea to put cement in paint; it was so successful he renamed the company to cement spelled backwards.

  Inside the concrete, Ca(OH)₂, which is generated by the hydration of cement, 
  creates a strongly alkaline environment, with a pH value of more than 12.7. 
  In this environment, a passivation film forms on the surface of the rebar to 
  prevent corrosion. However, the pH of the concrete decreases as micro-cracks 
  develops and chloride from the outside infiltrates the rebar. Eventually, as 
  the environment inside the concrete becomes neutralized, the corrosion of the 
  rebar accelerates, which reduces the overall durability and stability of the 
  reinforced concrete structure.

https://www.sciencedirect.com/science/article/pii/S095006182...

  A sighting of preserved steel reinforcing bars sticking out of old concrete 
  rubble piqued the curiosity of our company founder, Albert C. Bean, Sr. After 
  investigating this finding, his company, Armor Oil & Chemical Company, began 
  producing a patented cement-filled coating formulation that, in its updated 
  form, still protects structures from corrosion today.

https://tnemec.com/about/

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

#166

Post by nly » Fri, Jul 17, 2026, 2:54 PM UTC

freedomben wrote: post #141

rkangel wrote: post #82

Earlier quoted context omitted.

There are some downsides, but most people on the UK would prefer houses that were 100+ years old. In the village I live in the houses immediately around me are "only" 50 years old. Most of the ones on the main street are well over 100 years old. Housing tastes don't really change that much. Yes over the years we've had to fit things like double glazing and better insulation but that's a whole lot cheaper (and better…

> but most people on the UK would prefer houses that were 100+ years old. Why do most people prefer older houses?

Primairly because old houses are on bigger plots and allow for extensions and conversions.

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

#167

Post by freedomben » Fri, Jul 17, 2026, 2:56 PM UTC

nly wrote: post #166

freedomben wrote: post #141

Earlier quoted context omitted.

> but most people on the UK would prefer houses that were 100+ years old. Why do most people prefer older houses?

Primairly because old houses are on bigger plots and allow for extensions and conversions.

ah interesting! That's quite different from the areas I've lived in the US. If you're in or near city lines the lots on older houses are small. It's not uncommon to see .15 acre lots. Newer houses aren't much better, but you can see .2 to .25 acres relatively commonly, though those are nearly always on the outer city or outside city lines so it could just be proximity to the city that's the factor there rather than age.

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

#168

Post by demosthanos » Fri, Jul 17, 2026, 3:23 PM UTC

justin66 wrote: post #151

demosthanos wrote: post #6

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 c…

This is a good link and the Practical Engineering guy doesn’t appear to reference a “myth.” Which is a good thing, because there’s nothing mythological about the high performance of Roman concrete.

The myth is that Roman concrete was 'better' and that we don't know how to recreate it. The facts are that it was not better in any way than what we can do now and we have always known how to replicate it. Grady addresses both of those aspects of the myth in the video, and the first part is literally the title of the video: "Was Roman Concrete Better?"

The answer is no. What is true is that some Roman concrete structures (but far from all of them) are extremely durable because they were optimized for a different set of requirements than modern buildings usually have, notably "needs to last forever as a symbol of the emperor's power". From the 19th century on that has very rarely been a design constraint, so we optimize for other things instead.

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

#169

Post by flippyhead » Fri, Jul 17, 2026, 3:41 PM UTC

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.

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

#170

Post by fsckboy » Fri, Jul 17, 2026, 3:41 PM UTC

IAmBroom wrote: post #144

carlmr wrote: post #94

Earlier quoted context omitted.

>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. Can we use this instead of grout in the bathroom somehow?

Pure silicone caulk is mold-proof, even in the white variation. The various "improvements" in many caulks allow mold to get a foothold.

does "mold proof" mean mold won't grow on the surface, or that mold will not affect the silicone?