Hydraulic limes are manufactured from chalk or limestone containing various proportions of clay impurities. The materials produced have some of the properties of Portland cement, and partially harden through hydration processes, rather than solely through carbonation as happens with non-hydraulic pure calcium oxide lime. Hydraulic limes rich in clay impurities are more hydraulic and set more rapidly than those with only a low silica and alumina content. Natural hydraulic limes (NHL) are traditionally categorised as feebly, moderately or eminently hydraulic depending on their clay content, which is in the ranges 0–8%, 8–18% and 18–25%, respectively. These traditional grades equate approximately to the 28-day compressive strengths of 2, 3.5 and 5 MPa, respectively, for NHL2, NHL3.5 and NHL5. Eminently hydraulic lime mortar is used for masonry in exposed situations, moderately hydraulic lime mortar for most normal masonry applications, and feebly hydraulic lime mortar is appropriate for conservation work and solid wall construction. Grey semi-hydraulic lime is still produced within the UK in small quantities from chalk containing a proportion of clay. It is used with very soft bricks and for conservation work.
Hydraulic lime (HL) is produced by blending lime with other constituents such as cement, blast furnace slag or fly ash in appropriate proportions. It is usually imported from France and is mainly used for the restoration of historic buildings, where the use of modern materials would be inappropriate. It is gauged with sand only, giving a mix which develops an initial set within a few hours, but which hardens over an extended period of time. The workable render or mortar mixes adhere well and, because the material is flexible, the risks of cracking and poor adhesion are reduced. The dried mortar is off-white in colour and contains very little alkali, which in Portland cement mortars can cause staining, particularly on limestone. Hydraulic lime may be used for interior lime washes, and also for fixing glass bricks where a flexible binding agent with minimum shrinkage is required. Unlike hydrated lime, hydraulic lime is little affected by exposure to dry air during storage.