> the romans supposedly used a superior cement which we can no longer replicate.
I read a few years ago the 'secret' had been discovered and it was to do with volcanic ash which made it self healing.
Doing a quick search there seems to be a lot of articles from 2014 about this so possibly that what when the research was published.
https://archserve.id.ucsb.edu/courses/arthistory/152k/concre...
https://www.ibtimes.co.uk/why-pantheon-has-not-crumbled-roma...