Riddle solved: Why was Roman concrete so durable? (news.mit.edu)

MIT and collaborators report that the durability of Roman concrete likely came from “hot mixing” that used reactive quicklime and produced lime-rich mineral inclusions (“lime clasts”). These clasts create a brittle, reactive calcium source that helps the material spontaneously self-heal as cracks form by reacting with water to recrystallize calcium carbonate and further strengthen the surrounding composite. Experiments with hot-mixed samples showed cracks sealed within weeks, unlike concrete made without quicklime.