The ancient Roman office and shopping complex known as Trajan’s Markets is a monument to resilient architecture. More than 1,900 years old, this structure has withstood moderate earthquakes and floods. One secret to its staying power, scientists say, is the mechanical resilience of the mortar that binds together cobble-sized pieces of brick and porous rock that make up the market buildings. Researchers led by Marie D. Jackson of the University of California, Berkeley, used computed tomography and synchrotron X-ray microdiffraction to study a reproduction of the Roman mortar as it cured (Proc. Natl. Acad. Sci. USA 2014, DOI: 10.1073/pnas.1417456111). Over the course of 180 days, the mortar made from a simple recipe of hydrated lime and volcanic ash developed platelike crystals of strätlingite, a durable calcium-aluminosilicate mineral. These mineral plates provide reinforcement and prevent cracks from propagating through the mortar. The researchers observed these same plates of strätlingite in samples of the mortar taken from Trajan’s Markets. The Roman mortar suggests routes to making sturdy concrete via a process that releases less carbon dioxide than the standard process for making modern Portland cement.