The mystery of 2,000-year-old Roman concrete was “solved by scientists”. It turns out seawater makes ancient concrete stronger, so it lasts while modern structures crumble into the sea. Roman concrete used for ancient sea walls was made by mixing lime, seawater, volcanic ash and rock. Elements in the ash react with sea water, which actually strengthens the material. In contrast, waves erode modern concrete. You can read more about this here. This post below about how they “figured it out” goes well with the Ancient Oral Histories post about why researchers and scientists should listen to ancient oral histories from native people:
There’s more than just a sea water and volcanic ash to Roman cement. It’s self-healing due to lime clast deposits considered as impurities for a long time. https://www.scientificamerican.com/article/ancient-roman-concrete-has-self-healing-capabilities/