This has reappeared in TIL. The typical anecdote of concrete being completely lost when Rome fell.

I have a very hard time believing that this is true. First of all, there was a whole other half to the Empire that was possibly more well off at that point. They also clearly used mortar in all their buildings, and as far as I can tell, still used concrete in a structural manner, just not as widely (preferring brick over concrete). They even achieved Hagia Sophia without the use of a ton of concrete, so it'd make sense that they may have had the technology but saw no use in it if they could effectively use brick and have easier access to it. They also retained great engineering feats and good infrastructure like the vast sewers and cisterns in Constantinople.

The Roman Empire was just a lot bigger than just Rome and the Italian peninsula. Plus it was all soon reconquered (albeit briefly) by the East. It just really seems improbable that the West had a special technology that the East lacked and that was completely lost when one half's government fell.

So is it actually true that the technology of Roman concrete (and other technologies) was lost with the fall of the Western Roman Empire? Or was it retained by the Eastern half, and that just gets overlooked?