[Gibbon1]

And after the fall of Rome the economy was smaller and more fragmented.

Last time I looked into this I came away with the observation that preindustrial concrete was very labor intensive. We use it for it's physical properties and because it's 'cheap'. Roman concrete wasn't cheap at all.

The central Roman government was able to marshal the effort necessary to produce it on a ongoing basis for particular uses. But I understand Romans mostly used brick and mortar for ordinary construction.

After the Roman Empire fell apart concrete probably didn't make sense.