Why was Roman concrete so durable?

With zero sarcasm, when I read “every time I go to Rome” I feel that I took a wrong turn in life somewhere.

Fucking awesome mate.

The Pantheon is just an example, but many other instances can be found in Rome: the Church of Santa Maria degli Angeli, the Basilica of Saints Cosmas and Damian, etc. (These are just examples you find in Rome, there are plenty of other examples here in Italy.)

The OP suggested that the loss of architectural art is mainly due to a clash of different cultures and to religion. While there were cases where this was surely true (the many churches destroyed during French Revolution!), in most of the cases the reasons were much more practical and did not involve conflicting cultures or the «long tentacles of the Catholic Church».

Before the concept of «historical preservation» was invented, what should have been the point to keep a temple to Venus in the crowded center of a city, when nobody worship her any longer and that very space could have been used for something more useful?

These acts of destruction happen all the time even within the same cultural context. For instance, the old Vatican Basilica, built in the IV century, was destroyed and rebuilt in the XVI century by the Pope himself! And the same applies to countless of other important ancient churches and public buildings (My favourite example: the astonishing church of St. Clemente [1], well worth a visit if you're in Rome! The XI-century building was erected over the old VI-century church.) Not only churches, but frescoes in churches have been continuously covered by newer frescoes, once the taste of the church goers changed. A few days ago I visited the beautiful church of Sant'Abbondio in Como [2]: the church dates back to the XI century, but the old frescoes were covered by newer ones in the second half of the XIII century.

Even the Romans did the same for many of their buildings: once they realized that they were no longer useful or usable, they either repurposed them or destroyed and built something else on the rubble. The Colosseum was not built on pristine land! And Nero destroyed a lot of houses to build his Domus Aurea, which was later destroyed by Vespasian and Titus and covered by newer constructions, with the intent of make people forget Nero. (No, as far as I know neither Nero nor Vespasian or Titus were catholic.)

It's a pity for scholars, but before historical preservation was a thing, it was often the best thing to do, as the older buildings did no longer fit their purpose. These acts of destruction made the world lose important pieces of art, but they were meant as a way to repurpose the spaces of the city according to the evolved needs of its inhabitants. This is particularly true for cities like Rome, which have been populated continuously for ~2500 years.

[1] https://en.wikipedia.org/wiki/San_Clemente_al_Laterano

[2] https://it.wikipedia.org/wiki/Basilica_di_Sant%27Abbondio

Well, I lived in Italy for 10 years. Going to Rome from time to time wasn't that hard.

Now I live in Spain and I continue to fly to Rome now and then.

In Europe, distances are easy to overcome.

Well, it's also wonderful they speak Italian!

Relevant thread: https://twitter.com/cleptok/status/1611622181665923075

Ok. Let's do a short thread on Roman cement for the 3 people out there that might be interested. You often hear that the Romans had some sort of superior and arcane knowledge on superior cement mixes that has been now lost. And still today we cannot replicate it. This is complete nonsense. Modern day cement (and any other building material, really) is far, far superior than anything from the past. There is simply no comparison. Above everything we understand the chemical composition, interaction and quality control for consistent results.

Romans were making mortars based on volcanic ashes. There was no set recipe. Every builder had their own. Volcanic ash from Pozzuoli, crushed brick, slaked like, animal fat, hair etc. Some basic rules existed (as recorded by Vitruvius on the 10 books on Architecture).

Millions of structures were constructed these way. With the exception of a handful, most of these do not exist anymore. But why the ones that exist are still standing strong?

Usually via a simple Darwinian process. Most of these times those builders got it wrong. But in a few cases, out of luck, they got the chemistry just right. This would result in a ok-ish material, with decent properties. Not only that. Unreinforced (without steel) concrete will not suffer any serious deterioration, with few exceptions. (Pantheon in Rome) Also the very coarse pozzolan they were using, will chemically react very very slowly producing low initial strengths Vs any modern material.

In the long term (centuries) though, it will continue gaining strength. Totally useless for the original purpose of the builders, but quite good for the few surviving buildings.

So what the latest MIT research told us? That lime in very small quantities will be "self-healing'. And this somehow is something that the Romans knew and used to create a superior cement.

This is simply not true. It is well known that lime expands (the pompous "self-healing" tag), so in very small and precise quantities it will counteract the shrinkage and cracking behaviour of cement binders. But you have to be pretty lucky to get it just right.

So not intentional. Just survivor bias in the remaining Roman cement structures. Beyond this, lime is a definite no-no for any concrete with steel in it -it corrodes it. The reason we use concrete today is precisely the embedded steel. It allows to build all modern structures.

And steel is the reason modern concrete deteriorates. It corrodes, expands and cracks the surrounding concrete. An unreinforced concrete (like the ones Romans used) with modern materials and understanding would last forever. But you wouldn't build anything worthwhile.

Final. Remember. Cement = the glue, the binder that holds everything together, activated by water. Concrete= the actual building material with binder, water, sand and stones. Mortar= only binder, sand and water (maybe small stones)