Hydraulic lime (HL) is a general term for varieties of
lime (calcium oxide), or
slaked lime (calcium hydroxide), used to make
lime mortar which set through
hydration: thus they are called
hydraulic. The other common types of lime mortar set through carbonation (re-absorbing carbon dioxide (CO
2) from the air) and are sometimes called
air lime. Hydraulic lime provides a faster initial set and higher compressive strength than air lime and
eminently hydraulic lime will set in more extreme conditions including under water. Calcium reacts in the lime kiln with the clay minerals to produce
silicates that enable some of the lime to set through hydration; any unreacted calcium is slaked to
calcium hydroxide which sets through carbonation: These are sometimes called
semi-hydraulic lime and include feebly and moderately hydraulic lime, NHL 2 and NHL 3.5. The terms
hydraulic lime and
hydrated lime are quite similar and may be confused but are not necessarily the same material: hydrated lime is
any lime which has been slaked whether it sets through hydration, carbonation, or both.