Lime mortar: hydraulic, non-hydraulic, and when to use which

How lime mortar works

Ordinary Portland cement sets by hydration: water triggers a chemical reaction with the cement compounds that produces calcium silicate hydrate crystals, and the mortar hardens within hours. It continues to gain strength for weeks, eventually reaching a hardness that exceeds most historic masonry.

Lime mortar works differently. Non-hydraulic lime — the type made by burning calcium carbonate to produce calcium oxide (quicklime), then slaking it with water to produce calcium hydroxide (lime putty) — sets by carbonation. Carbon dioxide from the air reacts with the calcium hydroxide to reform calcium carbonate. The process is slow, depends on access to air, and produces a mortar that remains relatively soft and flexible throughout its life. This is not a weakness. It is the point.

The flexibility of lime mortar means it can accommodate the movement that all masonry buildings make: thermal expansion and contraction, slight foundation settlement, moisture cycling. Instead of cracking the masonry unit, the mortar accommodates the movement within itself. When it eventually does need replacement, it can be raked out without damaging the brick.

Non-hydraulic lime (lime putty)

Non-hydraulic lime putty is the softest and most flexible of the lime types. It sets entirely by carbonation, which means it needs air to set and will not set underwater or in very thick sections. It is the correct choice for very soft, friable, or historic masonry where maximum flexibility and minimum strength are required.

London stock brick was originally pointed with non-hydraulic lime putty mortars. If you are repointing a Victorian terrace in London, this is almost certainly the correct type to use. The mortar should be weaker than the brick — this is intentional, not a compromise. A mortar that is weaker than the masonry unit will crack and flex before the unit does. It can be replaced. The brick cannot.

The practical disadvantage of non-hydraulic lime is its slow setting time. In summer, freshly pointed joints need to be kept damp for the first few days to slow the carbonation and prevent cracking. In cold weather, lime mortar should not be applied below 5°C. Winter pointing work using non-hydraulic lime requires protection from frost.

Feebly hydraulic lime (NHL 2)

Natural hydraulic limes (NHL) contain clay impurities that cause partial hydraulic setting alongside the carbonation mechanism. The designation — NHL 2, NHL 3.5, NHL 5 — refers to the approximate compressive strength in N/mm² after 28 days. NHL 2 is feebly hydraulic, meaning the hydraulic element is minimal and the mortar remains relatively flexible. NHL 5 approaches the strength of weak Portland cement and should not be used on historic masonry.

Feebly hydraulic lime (NHL 2 or a low-hydraulicity hot lime) is the appropriate choice for most repointing work on London's Georgian and Victorian building stock. It is slightly stronger and faster-setting than non-hydraulic lime putty, which makes it more practical in many situations, while remaining sufficiently flexible and weak to avoid damaging the masonry. It can also be used in conditions where non-hydraulic lime is difficult — slightly damp substrates, thicker sections, or where a somewhat faster initial set is needed.

Moderately hydraulic lime (NHL 3.5)

NHL 3.5 has a higher hydraulic content and achieves greater strength. It is appropriate for exposed, wet, or below-ground applications where more robustness is required: copings, below-damp-proof-course work, external renders on exposed elevations. It is not appropriate for the pointing of Victorian domestic brickwork, where the strength would be excessive and the flexibility insufficient.

The distinction matters because NHL 3.5 is widely available, easy to use, and often specified on historic masonry by contractors who don't understand why it is wrong. The result — over a period of years — is micro-cracking in the masonry unit adjacent to the mortar joint, as the mortar fails to accommodate movement. The damage is slow and subtle, but cumulative.

Matching the original mortar

The specification is not just about hydraulicity. The appearance of the mortar — its colour, texture, and aggregate composition — should also be matched to the original as closely as possible. This matters aesthetically, but it also matters practically: a very different-coloured mortar draws the eye to every joint and makes the repointing visible in a way that detracts from the building's character.

The correct approach is to take a sample of the original mortar from a sheltered area — under a sill, behind a downpipe — and analyse or assess the aggregate. In practice, this often means disaggregating a sample to identify the sand type, grading, and colour, then matching those characteristics in the new mortar. On important listed buildings, a petrographic analysis may be required. On most domestic work, a careful visual assessment is sufficient.

Hot lime

Hot lime — also called run lime or lime run — is quicklime (calcium oxide) mixed with aggregate and water on site, without the intermediate slaking stage. The exothermic reaction of the quicklime with water produces a very hot mix that is spread immediately. The resulting mortar is softer and more flexible than pre-slaked lime putty, with a distinctive workability and a very fine texture.

Hot lime mortars were used extensively in London's eighteenth-century and earlier building stock. They are increasingly favoured by conservation specialists for the most sensitive historic masonry repairs, particularly on Grade I and Grade II* buildings where the closest possible match to original materials is required. They require specialist knowledge and are not suitable for general contractors.