Why has Roman concrete endured two thousand years, in surf and weather, while modern concrete structures decay within decades?

Recent materials research has produced strong partial answers, including reactions between seawater and volcanic ash that grow reinforcing minerals in Roman marine concrete, and lime fragments that may reseal cracks as they open, but the full recipe and practice remain incompletely understood, translation of the findings into modern durable and low-carbon concretes is unfinished work, and the research groups involved continue to publish competing accounts of which mechanisms matter most.