Hydraulic lime is a type of lime that sets and hardens when mixed with water, thanks to the presence of clay and other impurities. Unlike non-hydraulic lime, which requires air to cure, hydraulic lime can set underwater, making it suitable for various construction applications, including masonry and plastering. This material has been used for centuries in building structures, particularly in historic European architecture. It offers good flexibility and breathability, allowing moisture to escape while maintaining strength. Hydraulic lime is often favored for restoration projects due to its compatibility with traditional materials.