tl;dr: Researchers studying an undisturbed 1,900-year-old concrete sample from a latrine at Hadrian's Villa found that carbonation—atmospheric CO2 reacting with calcium compounds to form crack-sealing calcite—plays a larger role in Roman concrete's durability than previously thought, complementing the well-known pozzolanic reaction. The findings, published in Science Advances, could inform development of longer-lasting, lower-emission modern concrete, which currently accounts for roughly 8% of global CO2 emissions.