Multiple Choice, Single Answer

Many Roman harbour walls and buildings made of concrete have survived for two thousand years, while modern concrete structures can begin to crack within decades. For a long time, the durability of the Roman material was put down to the use of volcanic ash, which reacts with lime to form strong binding compounds. Roman builders certainly valued this ash; the engineer Vitruvius recommended a particular type from the area around the Bay of Naples. Recent analyses, however, point to a second factor that had previously been dismissed. Roman concrete often contains small white lumps of lime, which earlier researchers interpreted as evidence of poor mixing. A team examining samples from an ancient city wall concluded that the lumps were the result of mixing lime while it was still hot and highly reactive. According to this view, when cracks later formed and water seeped in, the water dissolved some of the lime from the lumps, which then hardened again inside the crack, sealing it. The researchers tested the idea by making concrete with the same method, cracking it deliberately and passing water through it; within two weeks, the cracks had closed. If confirmed, the finding could matter beyond archaeology. Producing cement accounts for a large share of global carbon emissions, and concrete that repairs itself would need replacing less often.