Inside the project to bring 'self-healing' Roman concrete to American shorelines; Lessons from 2,000-year-old Roman material could help us build structures better suited for a waterlogged future.

ANCIENT ROMANS were masters of concrete, fashioning concoctions of sand, water, and rock into long-lasting marvels. Bridges, stadiums, and other structures they built with the stuff still stand tall—even harbors and breakwaters that have been soaked by tides and storms for nearly 2,000 years. This substance, robust to the microscopic level, far outlives the modern material, which generally requires steel supports in salt water and is still likely to corrode within decades...