Riddle solved: Why was Roman concrete so durable?news.mit.edu/2023/roman-concrete-durability-lime-casts-0106
To prove that this was indeed the mechanism responsible for the durability of the Roman concrete, the team produced samples of hot-mixed concrete that incorporated both ancient and modern formulations, deliberately cracked them, and then ran water through the cracks. Sure enough: Within two weeks the cracks had completely healed and the water could no longer flow. An identical chunk of concrete made without quicklime never healed, and the water just kept flowing through the sample.
I knew Roman concrete was cool, but not that cool!