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 (CO₂) 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.