The Rediscovery of Ancient Roman Concrete Many Roman harbors, bridges, and buildings have survived for two thousand years, although they were exposed to waves, rain, and earthquakes. Their durability has led researchers to examine Roman concrete, made from lime, volcanic ash, water, and rock. Modern concrete also contains cement and stone, but it often develops cracks within several decades, especially in marine environments. One explanation for Roman concrete’s strength concerns the way its ingredients react over time. When seawater enters tiny cracks, it interacts with volcanic minerals in the concrete. This process can produce new crystals that fill empty spaces and bind the material more firmly together. Instead of simply weakening the structure, limited exposure to water may therefore help repair it. Researchers have also studied white pieces of lime found throughout Roman concrete. These pieces were once considered evidence of careless mixing. More recent experiments suggest that they may serve an important function. When a crack reaches a lime fragment, water can dissolve part of it and carry the material into the damaged area, where it hardens again. Scientists do not expect Roman methods to replace construction techniques. However, understanding these ancient materials may help engineers design concrete that lasts longer and requires fewer repairs.

6. Which of the following best states the main idea of the passage?

7. The word “durability” in the passage is closest in meaning to