interestingasfuck

We know the recipe for Roman concrete, and we have better concrete recipes. This absolutely can last longer than Roman concrete.

It's self healing because of the poorer mixing, undissolved quicklime (calcium oxide) would remain in the concrete, and water getting in cracks would dissolve it and produce calcium hydroxide, which then combines with carbon dioxide to form more limestone aggregate (calcium carbonate) to heal the cracks.

There's a survivor bias when it comes to really old buildings of course so it's likely we're only seeing a small portion of the surviving buildings from that era.

We could make concrete that lasts multiple times the amount of time as Roman concrete, but we usually don't because it's more expensive (and modern construction seems to be about minimizing costs as much as possible and not worrying about anything more than a few decades out)