Hydraulic Lime is produced from limestone containing clay and other impurities which enables it to set without exposure to air. It is used for providing a faster initial set in more extreme conditions (including under water). There are three classifications of hydraulic limes, as follows: Feebly hydraulic lime (NHL2) - less than 12% clay content making it slower to set (about 20 days in water), Moderately hydraulic lime (NHL3.5) - 12%-18% clay content resulting in a slightly faster set (15-20 days in water) and Eminently hydraulic lime (NHL5) - up to 25% clay content providing a fast set (hours / days).