That thing people call 'Roman concrete' is usually just Roman mortar

I keep seeing posts online where people point at any old Roman wall and call it 'concrete' (opus caementicium), but most of the time it's actually opus incertum or opus reticulatum with a rubble core. The real distinction matters because concrete implies a specific mix with volcanic ash (pozzolana) that makes it set underwater, like at Caesarea Maritima in Israel. I only figured this out after reading Vitruvius' descriptions and realizing most forum floors are mortar over brick, not the hydraulic stuff. Has anyone else noticed how often museum labels get this wrong too?

2 comments

2 Comments

averymartin23d ago

Wait, aren't most of those surviving Roman walls actually just the inner core that was hidden behind stone facing? I thought the pozzolana concrete was mostly used for huge structures like the Pantheon or aqueducts... not your average forum floor. Museum labels really do seem to mess this up constantly, calling everything concrete when it's really just rubble packed with mortar.

davis.adam23d ago

Wait, haven't you heard the talk about some of those walls actually being repurposed later? I was listening to a history podcast the other day, @averymartin, and they mentioned how medieval builders would strip the stone facing off Roman structures to reuse in castles and churches, leaving the concrete core exposed. So a lot of what we see now is definitely the inner rubble, like you said. But I think the pozzolana concrete thing is still a bit wild, because even though it's not everywhere, the Pantheon dome is insane proof of how good it was. Those museum labels just say "concrete" and move on, but the real story is way more complicated and specific.