What is the difference between hydraulic and non-hydraulic lime?

Hydraulic lime (NHL) contains natural silicates, so it sets partially by reaction with water and partially by carbonation. It sets in days and is ready to work with in weeks. Non-hydraulic lime (lime putty) contains no silicates, so it sets only by carbonation (absorbing CO2 from air). It takes months to fully cure. Hydraulic lime is more practical for general building work. Lime putty is used for conservation work on very soft substrates.

Which NHL should I use: 2, 3.5, or 5?

NHL 3.5 is most common (moderate strength, suits most UK heritage buildings). Use NHL 2 for very soft substrates (cob, very soft brick, lime putty repairs). Use NHL 5 for exposed locations (coastal, hilltop, driving rain) or hard substrates (hard stone, engineering brick). If unsure, NHL 3.5 is the safe choice for repointing and rendering on pre-1919 solid walls.

Can I mix different NHL types together?

Not recommended. Mixing NHL types produces unpredictable strength and setting behaviour. If you need intermediate strength between NHL 3.5 and NHL 5, use NHL 3.5 and increase the lime:sand ratio slightly (e.g. 1:2.5 instead of 1:3). If a specific strength is required, consult a lime specialist or conservation officer.

Hydraulic lime mortar types: NHL 2, 3.5, 5 differences

What is hydraulic lime?

Hydraulic lime (NHL, Natural Hydraulic Lime) is made from limestone that contains natural clay impurities (alumino-silicates). When the limestone is heated in a kiln, these silicates remain in the quicklime. When mixed with water, the silicates react (hydraulic set), giving the lime early strength. The lime also continues to harden by carbonation (absorbing CO2 from air) over months and years.

This dual-setting mechanism makes hydraulic lime faster and more practical than non-hydraulic lime (lime putty), which sets only by carbonation and takes months to fully cure.

Hydraulic vs non-hydraulic lime

Hydraulic lime (NHL)

Hydraulic lime sets partially by water reaction (hours to days) and partially by carbonation (weeks to months). It's touch-dry in 24-48 hours and ready for the next coat in 7-14 days. Full carbonation takes 4-6 weeks. Hydraulic lime is stronger and faster-setting than non-hydraulic lime, making it suitable for most heritage building work.

Non-hydraulic lime (lime putty)

Non-hydraulic lime (also called fat lime or lime putty) contains no silicates. It sets only by carbonation, which takes 3-6 months for full cure. It's softer and more flexible than hydraulic lime. It's used for conservation work on very soft substrates (cob, very soft brick, wattle and daub) or for high-end finishes (Venetian plaster, tadelakt).

Non-hydraulic lime is supplied as putty (thick paste) and must be matured for at least 3 months before use (ideally 6-12 months). It's more expensive and slower to work with than hydraulic lime.

NHL classification: 2, 3.5, 5

Hydraulic limes are classified by compressive strength under BS EN 459. The number (2, 3.5, 5) refers to the minimum compressive strength in MPa (megapascals) at 28 days.

NHL 2 (softest)

Strength: 2-7 MPa at 28 days.

Setting time: touch-dry in 48-72 hours, ready for next coat in 14-21 days. Slower than NHL 3.5 or NHL 5.

When to use: very soft substrates (cob, rammed earth, very soft brick, earth mortar repairs). Internal plastering on soft substrates. Conservation work where a very weak mortar is needed to match original.

Availability: less common than NHL 3.5. May need to be ordered from specialist lime suppliers.

NHL 3.5 (moderate strength)

Strength: 3.5-10 MPa at 28 days.

Setting time: touch-dry in 24-48 hours, ready for next coat in 7-14 days. Full carbonation in 4-6 weeks.

When to use: most UK heritage buildings (pre-1919 solid walls, listed properties). Repointing brickwork or stonework. Rendering external walls (sheltered to moderately exposed). Internal plastering. This is the most common NHL for general heritage work. See the full guide: NHL 3.5 lime mortar.

Availability: widely available from builders' merchants and lime suppliers.

NHL 5 (hardest)

Strength: 5-15 MPa at 28 days.

Setting time: touch-dry in 12-24 hours, ready for next coat in 5-10 days. Sets faster than NHL 3.5 or NHL 2.

When to use: very exposed locations (coastal, hilltop, driving rain). Hard substrates (hard stone like granite, engineering brick). Chimney repointing (high exposure). External render top coats in exposed locations. Use NHL 5 where NHL 3.5 would erode too quickly.

Availability: widely available but less common than NHL 3.5.

Comparison table

Type	Strength (MPa)	Setting time	Use case
Lime putty (non-hydraulic)	0.5-2	3-6 months	Conservation work, very soft substrates, high-end finishes
NHL 2	2-7	14-21 days	Very soft substrates (cob, very soft brick)
NHL 3.5	3.5-10	7-14 days	Most UK heritage buildings, general repointing/rendering
NHL 5	5-15	5-10 days	Exposed locations, hard substrates, chimneys
Cement mortar (for comparison)	15-25	1-2 days	Modern cavity walls, not suitable for heritage buildings

How to choose

Match the mortar to the substrate

The mortar should always be weaker than the masonry it protects. This is the SPAB (Society for the Protection of Ancient Buildings) principle: the mortar acts as a sacrificial layer, eroding over time rather than damaging the brick or stone.

Very soft brick or earth: NHL 2 or lime putty.
Soft brick, soft stone, lime-built walls: NHL 3.5.
Hard stone, hard brick: NHL 5 or NHL 3.5 (NHL 5 if very exposed).

Match the NHL to the exposure

Exposed locations (coastal, hilltop, driving rain) need harder lime to resist erosion.

Sheltered (urban, south-facing, under eaves): NHL 3.5 or NHL 2.
Moderately exposed (rural inland, some weather exposure): NHL 3.5.
Very exposed (coastal, hilltop, driving rain, chimneys): NHL 5.

Match the NHL to the original mortar

For listed buildings, Historic England guidance requires like-for-like repairs. Analyse the existing mortar (visual inspection, crush test, or lab analysis) to determine its approximate strength and composition. Match the NHL type to the original mortar.

If the original mortar is very soft (crumbles easily), use NHL 2 or lime putty. If it's moderately firm, use NHL 3.5. If it's hard, use NHL 5 (but check it's not cement, which should not be matched on heritage buildings).

Can you mix NHL types?

Not recommended. Mixing NHL 2 and NHL 5, for example, produces unpredictable strength and setting behaviour. The different hydraulic components may not blend evenly, causing weak spots or cracking.

If you need intermediate strength between NHL 3.5 and NHL 5, adjust the lime:sand ratio instead. A richer mix (1:2.5 instead of 1:3) increases strength. A leaner mix (1:3.5 instead of 1:3) reduces strength.

For precise strength specification (e.g. conservation work on high-grade listed buildings), consult a lime specialist or conservation officer. They may recommend custom blends or lime putty with pozzolanic additives (brick dust, volcanic ash) to achieve specific properties.

Where to buy NHL

NHL 3.5 is widely available from builders' merchants (Jewson, Travis Perkins, etc.) and specialist lime suppliers (Ty-Mawr, Lime Green, Mike Wye). NHL 2 and NHL 5 are less common and may need to be ordered.

Prices: £10-15 per 25kg bag. Lime putty costs £15-25 per 25kg tub. Buy from a reputable supplier to ensure the lime meets BS EN 459 standards. Cheap imported limes may not meet UK strength classifications.

Summary

Hydraulic lime (NHL) sets by water reaction and carbonation. It's faster and stronger than non-hydraulic lime putty. NHL 2 is softest (for very soft substrates). NHL 3.5 is most common (moderate strength, suits most UK heritage buildings). NHL 5 is hardest (for exposed locations or hard substrates).

Choose NHL based on substrate strength and exposure. Match the mortar to the original mortar for listed buildings. Do not mix NHL types. For more detail on application, see lime mortar vs cement and lime rendering technique.