What is the difference between hydrated lime and hydraulic lime?

Two products with confusingly similar names that do completely different jobs: one is an additive for cement mortar, the other is a binder in its own right.

Difficulty: intermediate
Time: 12 min
Read: 3 min
Safety: warning

Short answer

Hydrated lime is a bagged powder added to a cement mortar to make it more workable and forgiving — it is an ingredient, not a binder. Natural hydraulic lime, sold as NHL 2, NHL 3.5 or NHL 5, is a binder used instead of cement, with the number indicating strength. For repointing an old solid wall you want NHL, not hydrated lime.

Do this first

What is lime mortar, and how is it different from cement mortar?

The names cause real confusion at the merchant, and the consequence is a mortar that either has no lime binder in it at all or is far stronger than the wall needs. The distinction is simple once you see it: one is something you add to cement, the other is something you use instead of cement.

Safety

All limes are strongly alkaline and burn skin and eyes. Hydrated lime in powder form is easily blown into the face when a bag is opened or tipped — wear goggles and a mask, and open bags at arm's length and downwind. Splashes must be flushed with plenty of clean water, and anything in the eye is an immediate hospital matter, not something to watch and see.

Step by step

Identify hydrated lime by what it is for.Sold as a bagged white powder, also called builder's lime or bag lime. It is non-hydraulic, so it will not set under water and sets very slowly in air. It is added to a cement-and-sand mortar in a proportion such as 1 cement : 1 lime : 6 sand to improve workability, water retention and forgiveness of movement.
Identify natural hydraulic lime by its NHL number.NHL 2 is feebly hydraulic and softest, for sheltered internal work and very soft masonry. NHL 3.5 is moderately hydraulic and the general-purpose choice for repointing and rendering most exposed solid walls. NHL 5 is eminently hydraulic and strongest, for chimneys, copings, below ground and severe exposure.
Understand lime putty as the third option.Non-hydraulic lime slaked and matured under water into a putty, sold in tubs. It is the most permeable and most flexible, it sets purely by carbonation, and it is used on the softest historic fabric — usually on advice, and normally by someone with experience of it.
Match the NHL grade to the masonry, not to the weather you fear.The rule is unchanged: the mortar must be weaker than the units it beds. A soft handmade brick wants NHL 2 or 3.5. Specifying NHL 5 because the wall faces the prevailing wind is how people reproduce the cement problem in a lime product.
Never substitute one for the other.A sand-and-hydrated-lime mix with no cement and no hydraulic set will not harden usefully on an exposed wall. An NHL used as a plasticiser in a cement mortar over-strengthens it. They are not interchangeable in either direction.
Check what a bag labelled just "lime" actually is.Merchants stock hydrated lime under many names. If the bag does not say NHL and a number, treat it as hydrated lime and as an additive only.

Common mistakes

Buying hydrated lime to repoint an old wall with no cement in the mix — Non-hydraulic lime sets extremely slowly by carbonation and, mixed on site from bag lime rather than matured as putty, will not develop enough strength on an exposed wall. It washes out.
Defaulting to NHL 5 for everything outdoors — It is much stronger than most historic brick and stone, and it recreates the hard-mortar problem lime was chosen to avoid. Grade to the masonry.

Alternatives

Hydrated lime, in a cement mortar: The everyday improver. A 1:1:6 cement:lime:sand mortar is more workable, holds water better on an absorbent brick, and tolerates small movement better than a plain 1:4 cement mix. Good practice on modern work.
NHL 2: Feebly hydraulic. Very soft and permeable. Internal work, sheltered elevations, and the softest historic brick and stone.
NHL 3.5: Moderately hydraulic. The general-purpose choice for repointing and rendering solid-walled buildings in ordinary UK exposure.
NHL 5: Eminently hydraulic. Chimneys, parapets, copings, below ground and severe exposure. Approaching a weak cement mortar in strength, so not for soft brick.
Lime putty: Non-hydraulic, matured under water. The softest and most permeable option, for the most sensitive historic fabric. Sets only by carbonation and needs long protection.

Questions people ask

Which NHL should I use to repoint a Victorian brick house?

NHL 3.5 with a well-graded sharp sand is the usual starting point for ordinary exposure, dropping to NHL 2 on very soft handmade brick and rising to NHL 5 only for chimneys and copings. A supplier's technical line will advise on a sample.

Is hydraulic lime the same as hydrated lime?

No. Hydrated lime is a powder additive for cement mortars. Hydraulic lime is a binder used instead of cement. The similarity of the names is responsible for a great many wrong purchases.

Can I add hydrated lime to concrete?

There is no benefit. Lime improves the workability of a mortar. Concrete workability is handled by grading, water content and a plasticiser where needed.