Hydraulic lime is a type of lime that can set and harden in the presence of water. It is made by heating limestone that contains clay impurities, which gives it the ability to bond with water. This property makes hydraulic lime suitable for use in construction, particularly in environments where moisture is present. Unlike non-hydraulic lime, which requires air to cure, hydraulic lime can be used in damp conditions and is often found in historical buildings. It is commonly used in restoration projects and is valued for its flexibility and breathability, making it compatible with traditional materials like mortar and plaster.