It still is somewhat of a mystery, but the current belief is that due to the nature of the way they created their concrete was the root of its durability. They didn't mix it all together and homogenize it in a tumbler the way we do today, they laid it down manually, throwing the ingredients like lime randomly via shovelfuls.
These structural inconsistencies created pathways for a low temperature self-healing process following the irregular pathways created during the curing process that baked in the inconsistency of the mixture which were leveraged by natural stresses and processes to actually repair cracks on its own.
(I hope I didn't botch that explanation) I couldn't find the original one I looked at a few years back, but this link describes it:
Fascinating stuff…
Imagine how far along we would be architecturally and engineering wise, if the barbarians hadn’t destroyed all of the Roman knowledge during the dark ages.
And also, if the great library of Alexandria hadn’t burned.
True story.
When I was stationed in Italy, my wife and I took an ITT trip to Rome for Christmas.
We had free time in the evening, so we went to the massive Boun Natale (Christmas) market in the Plaza Navona, which used to be the Stadium of Domitian. Anyway, it started to sprinkle, so we headed back to the hotel. Just then a real gusher of rain exploded from the sky, we started to run and saw a lot people running into these massive dark doors, but well lit on the inside... We followed.
It was the Pantheon. So beautiful on the inside, and, despite the heavy rain, nothing was coming down the open roof! They say it was purposely designed to let in light, but kept rain out due to the natural lifting of the air from below!
At first I didn’t even know where we were; it was around 8:00 pm at nighttime, so not a lot of people in it, especially tourists. I had to wander around and then read the English language sign to see where we were!
Here in Northern Illinois the roads need to be constantly patched or torn up and rebuilt. I guess the Romans didn’t have to deal with constant jobs programs for labor unions.