There actually IS a contemporaneous source on how to do it, without exact proportions, unfortunately. They didn't know <i>why</i> it worked, of course.<p><a href="https://en.wikipedia.org/wiki/Vitruvius" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Vitruvius</a><p><a href="https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106" rel="nofollow noreferrer">https://news.mit.edu/2023/roman-concrete-durability-lime-cas...</a><p>"pozzolanic material such as volcanic ash from the area of Pozzuoli, on the Bay of Naples." They shipped this all over the Empire.<p>Now we have a better idea why it got better in seawater, instead of deteriorating.
It woyld be funny to me if it turned out the secret was urine instead of seawater. One of the bits of received wisdom i got from my grandfather was peeing on your hands toughened them up. He claimed it as a bit of advice he was given on his first day chopping hardwood for a steam engine at a tin mine. I figured it as a first day prank like being sent to find headlight fluid in modern times but he said it was true.<p>Before major industrialisation, people used what was at hand. I think the peeing on your hands thing has been long debunked but in roman concrete it may have worked.
We have a tendency to promote premature failure of seawater-exposed concrete structures by reinforcing them with rust-prone materials.<p>I don't think any amount of secret Roman concrete recipes is going to prevent crumbling apart due to the expanding forces of rotting rebar.
Old article from 2017. More advances were published earlier this year:<p><a href="https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106" rel="nofollow noreferrer">https://news.mit.edu/2023/roman-concrete-durability-lime-cas...</a>
It is fascinating how some ancient techniques had remarkable properties even though they didn’t have the tools or science to explain it back then. I recently learnt that ancient concrete has self healing properties because of the reaction of air/water with lime and volcanic ash.<p>Makes you wonder how far we can get with trial and error.
See also "Why was Roman concrete so durable?" from January 2023:<p>* <a href="https://news.ycombinator.com/item?id=34280239">https://news.ycombinator.com/item?id=34280239</a>
sadly no concrete evidence on the recipe - not surprised. Romans were excellent engineers but as so often, we're left with little documentation in this area (sciences). The humanities (law, politics, literature etc) has fared better, with way more surviving records.
The story mentions volcanic ash. If the Roman recipe became popular again for marine use, I wonder how much ash is currently available without terrible mining operations.
Relevant tidbit: hot mixing with quicklime was key for this self-healing property[1], as it allowed the creation of small lime clasts across the material.<p>[1] <a href="https://arstechnica.com/science/2023/01/ancient-roman-concrete-could-self-heal-thanks-to-hot-mixing-with-quicklime/" rel="nofollow noreferrer">https://arstechnica.com/science/2023/01/ancient-roman-concre...</a>
Obligatory mention of leading concrete-engineering YouTuber <a href="https://youtube.com/@TylerLey">https://youtube.com/@TylerLey</a> . He’s discussed Roman concrete in eg. <a href="https://youtu.be/U86tlUiFM1s" rel="nofollow noreferrer">https://youtu.be/U86tlUiFM1s</a> .