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

What lime mortar actually does that cement cannot — and why it is the correct material on a pre-1919 solid wall rather than a heritage affectation.

Difficulty: intermediate
Time: 20 min
Read: 4 min
Safety: warning

Short answer

Lime mortar is soft, permeable and slightly flexible. It lets a solid wall absorb rain and give it back by evaporating through the joints, and it accommodates small movements by cracking harmlessly and healing itself rather than shearing the brick. Cement mortar is hard and largely impermeable, so on an old wall it forces both moisture and movement through the brick faces instead.

Lime mortar is not a nostalgic choice. Buildings before roughly 1919 were built as solid walls with no cavity and no damp proof course, and they worked by being permeable — rain soaked into the outer surface and evaporated back out. Lime mortar is the component that makes that work, because the joint is more permeable than the brick and therefore takes the wetting and drying. Replace it with cement and you have closed the only easy route out, which is what the diagram shows.

Safety

Lime is more caustic than cement, not less. Wet lime mortar and especially lime putty and quicklime cause serious alkali burns and can cause permanent eye damage very quickly. Wear goggles rather than glasses, waterproof gauntlets and long sleeves, and keep clean water to hand for splashes. Quicklime reacts violently with water, generating enough heat to boil it, and should not be slaked by anyone who has not been shown how.

Why hard cement mortar destroys soft brick

Step by step

Understand what makes lime different.It is softer than the masonry around it, it lets water vapour through freely, and it stays slightly flexible for a long time. Cement mortar is harder than most old brick, nearly vapour-closed, and rigid.
Understand why softness is the point.A mortar joint is designed to be sacrificial. It takes the frost, the salt and the movement, it erodes over decades, and it is repointed. That is far cheaper than the alternative, which is the brick faces taking it and blowing off permanently.
Recognise autogenous healing.Fine cracks in lime mortar re-seal themselves over time as free lime dissolves and recrystallises. A hairline crack in a lime joint is often self-repairing. The same crack in cement is permanent and channels water.
Know the two families you will meet.Non-hydraulic lime — lime putty — sets slowly by absorbing carbon dioxide from the air. Natural hydraulic lime, sold as NHL with a number, sets partly by reacting with water and is easier to use on a wall exposed to weather. Which one to use is its own decision.
Expect it to work differently on the trowel.Lime mortar is stickier and more plastic than cement mortar, it does not go off on a timescale you can rely on, and it will not gain useful strength for weeks rather than days. You cannot rush it and you cannot leave it unprotected.
Protect new lime work properly.It must be kept damp and shaded and shielded from wind, rain and any frost for weeks, normally under damp hessian with polythene over the top held clear of the face. Lime that dries too fast never carbonates properly and simply crumbles.
Work only in the season for it.Lime work is a spring-to-early-autumn job in the UK. It has no meaningful frost resistance until it has carbonated, and a frost within the first weeks destroys it. Do not start lime pointing in October to get it done before winter.
Get advice before touching anything listed.On a listed building or one in a conservation area, the mortar specification can be a consent matter. A conservation officer or a lime supplier's technical line will specify the mix and the sand for the specific building.

Tips

Lime is a material and a technique together. If you have only ever used cement mortar, do a test panel somewhere inconspicuous before starting on a front elevation.
Ready-mixed lime mortars are sold in tubs and bags matched to common historic sands and colours, and for a small repair they save a great deal of trial and error.
The sand matters as much as the lime. Historic mortars used sharp, well-graded, locally dug sand, and a modern soft building sand gives a weaker, shrinkier result than the original.
Lime mortar can be gauged with a small proportion of cement to speed the set, but a gauged mix loses most of the permeability that made lime the right answer. Use NHL instead if you need a faster set.

Common mistakes

Repointing a Victorian solid wall in cement because it is stronger — It closes the joints, forces evaporation through the brick face, and blows the faces off over the following winters. It is the most damaging thing a well-meaning owner routinely does to an old house, and the damage is permanent.
Letting new lime work dry out in the sun — Lime needs to stay damp for weeks to carbonate and gain strength. Dried out early it stays powdery and washes out of the joint within a season.
Starting lime work late in the season — Fresh lime has no frost resistance. A hard frost within the first few weeks turns the whole job to dust.

Questions people ask

How do I know if my house should have lime mortar?

Broadly, solid-walled construction built before about 1919, with soft handmade or early machine-made brick, thick walls with no cavity, and no visible damp proof course. Check the wall thickness at a window reveal and look at what the surviving original mortar is.

Can I use lime mortar on a modern house?

You can, and it does no harm, but there is no need. A modern cavity wall with hard, frost-resistant brick works differently and a conventional cement-and-lime mortar is appropriate.

Is lime mortar weaker?

In compression, yes, considerably. That is deliberate. A wall does not need a strong mortar — it needs a mortar weaker than the units it beds, and lime is comfortably strong enough for a solid masonry wall carrying its own weight.

How long does lime mortar take to set?

It stiffens in hours but gains strength over weeks to months, and non-hydraulic lime keeps carbonating for years. Do not judge it, load it or expose it after a few days as you would cement.