Lime Mortar

Part of Lime & Cement

Making, proportioning, and applying lime mortar for masonry construction.

Why This Matters

Lime mortar is the glue that holds civilization’s buildings together. From the earliest stone walls to medieval cathedrals, lime mortar allowed humans to build structures that lasted centuries. Unlike modern Portland cement, lime mortar is forgiving — it flexes with settling foundations, self-heals hairline cracks through carbonation, and can be made from materials found nearly everywhere on Earth.

In a rebuilding scenario, lime mortar is your primary binding material for stone and brick construction. It requires no industrial infrastructure beyond a kiln hot enough to calcine limestone. The skills to make and apply it correctly mean the difference between buildings that stand for generations and walls that crumble in the first hard rain.

Understanding lime mortar also opens the door to plasters, renders, limewash paints, and eventually hydraulic cements. Master this material first, and you have the foundation for all masonry construction.

Types of Lime Mortar

Lime mortar falls into two broad categories based on the type of lime used and how it sets.

Non-Hydraulic Lime Mortar

Made from pure or high-calcium lime (calcium hydroxide), this mortar sets exclusively by carbonation — absorbing carbon dioxide from the air to slowly convert back to calcium carbonate. This process can take months or even years to complete fully.

Property	Characteristic
Setting mechanism	Carbonation (CO₂ absorption)
Setting speed	Slow — weeks to months
Water resistance	Poor when fresh, moderate when fully cured
Flexibility	High — accommodates movement
Self-healing	Yes — dissolved lime recrystallizes in cracks
Best uses	Above-ground walls, interior work, pointing

Hydraulic Lime Mortar

Made from limestone containing clay impurities (silica and alumina), hydraulic lime undergoes a chemical set when mixed with water, similar to Portland cement but slower and more flexible. It does not require air contact to begin setting.

Property	Characteristic
Setting mechanism	Hydraulic set + carbonation
Setting speed	Days to weeks for initial set
Water resistance	Good — suitable for wet conditions
Flexibility	Moderate
Self-healing	Partial
Best uses	Foundations, below-grade work, wet environments

Identifying Hydraulic Limestone

Limestone with visible clay bands or a slightly grey-blue color often contains the silica and alumina needed for hydraulic properties. Pure white limestone produces non-hydraulic lime.

Proportioning and Mixing

Getting the ratio of lime to aggregate correct is critical. Too much lime and the mortar shrinks and cracks. Too little and it crumbles.

Standard Proportions

The traditional ratio is 1 part lime putty to 2.5–3 parts sand by volume. This has been the standard since Roman times and remains the benchmark.

1. Measure lime putty — Use well-matured lime putty that has been slaked for at least two weeks, ideally three months or more. It should have the consistency of thick cream cheese.

2. Select sand — Use sharp, angular sand with a range of particle sizes from fine to coarse (up to 3-4mm). River sand or crushed stone works well. Avoid perfectly round beach sand — angular particles lock together better.

3. Combine — Add sand to the lime putty gradually, mixing thoroughly with a hoe or shovel on a flat mixing surface. The traditional method is to chop and fold repeatedly.

4. Test consistency — The mortar should hold its shape when squeezed but not be so stiff it’s hard to work. Press a trowel across the surface — it should leave a smooth, slightly shiny trail.

The Squeeze Test

Roll a ball of mortar about the size of a golf ball. Drop it from waist height onto a hard surface:

• Splatters flat: Too wet — add more sand
• Breaks apart: Too dry — add small amounts of water
• Flattens but holds together: Correct consistency

Sand Selection Guide

Sand Type	Suitability	Notes
Sharp river sand	Excellent	Best general-purpose aggregate
Crushed limestone	Excellent	Ideal for matching limestone masonry
Pit sand (angular)	Good	May contain clay — wash if necessary
Beach sand (round)	Poor	Weak bond, salt contamination
Desert sand	Poor	Too fine and rounded

Salt Contamination

Never use sand from saltwater sources without thorough washing. Salt attracts moisture and causes efflorescence (white crystal deposits) and long-term deterioration.

Mixing Methods

Hand Mixing on a Banker Board

The traditional method uses a flat wooden platform or stone slab called a banker board.

1. Form a ring of sand on the board, like a volcano crater
2. Place lime putty in the center
3. Using a hoe, chop the sand into the lime from the edges
4. Fold and chop repeatedly — this action is called “beating” the mortar
5. Continue for at least 15-20 minutes until completely uniform
6. The more you work lime mortar, the better it performs

Hot Lime Mixing

An older technique that produces exceptionally strong mortar:

1. Measure damp sand onto the mixing floor
2. Add quicklime (calcium oxide) pieces directly to the sand — roughly 1 part quicklime to 3 parts sand
3. Add water gradually — the quicklime will heat violently and steam
4. Mix immediately and vigorously as the lime slakes in place
5. The heat drives lime into the sand particles, creating an intimate bond
6. Allow to cool before use, or use warm for cold-weather work

Safety

Hot lime mixing produces temperatures exceeding 150°C and spatters violently. Wear full eye protection, heavy gloves, long sleeves, and boots. Keep children and animals well away. Always add water to lime, never lime to water.

Application Techniques

Laying Mortar Beds

For stone or brick walls:

1. Dampen the surface — Dry masonry units suck water from mortar too quickly, preventing proper curing. Spray or brush water onto stones and bricks before laying.

2. Spread the bed — Apply mortar 10-15mm thick for bricks, up to 25mm for rough stone. Use a trowel to create a slightly concave surface.

3. Butter the unit — Apply mortar to the end of each brick or stone before placing. Press firmly into position.

4. Tap into place — Use a wooden mallet or trowel handle. Excess mortar should squeeze out slightly at the joints.

5. Point the joints — After the mortar has firmed slightly (1-2 hours depending on conditions), use a pointing tool to compress and shape the joint surface.

Joint Profiles

Profile	Method	Best For
Flush	Cut excess level with wall face	Weather-exposed walls
Slightly recessed	Press in 2-3mm with flat tool	Traditional appearance
Weatherstruck	Angle top inward	Maximum rain shedding
Bucket handle	Press with rounded tool	Durable, attractive finish

Curing and Protection

Lime mortar requires careful curing — the carbonation process needs both moisture and carbon dioxide.

Critical First Two Weeks

• Keep mortar damp — Mist with water 2-3 times daily in warm weather. Cover with damp burlap or hessian if conditions are hot and dry.
• Protect from rain — Heavy rain washes uncured mortar from joints. Cover fresh work with boards or tarps during storms.
• Protect from frost — Freezing before initial set destroys mortar. In cold weather, cover with insulating material (straw, old blankets) and avoid working if temperatures will drop below 5°C within 48 hours.
• Avoid direct sun — Strong sunlight dries mortar too quickly, causing surface cracking before carbonation can occur.

Long-Term Carbonation

Full carbonation of lime mortar takes months to years depending on wall thickness. A 200mm thick wall might take 6-12 months to carbonate through. During this time:

• The mortar gradually hardens from the outside in
• Strength increases steadily — lime mortar reaches about 60% of its ultimate strength in the first year
• The self-healing property means that fine cracks from settling close themselves as dissolved lime migrates and recrystallizes

Testing Carbonation

Apply a few drops of dilute acid (vinegar works) to a cut surface. If it fizzes vigorously, the lime has carbonated back to calcium carbonate. If reaction is weak, the mortar is still largely calcium hydroxide and hasn’t fully cured.

Troubleshooting Common Problems

Problem	Cause	Solution
Cracking while drying	Too much lime, too little sand	Remix with more aggregate; keep damp
Mortar crumbles	Sand too fine or rounded; poor mixing	Use sharp sand; mix more thoroughly
Doesn’t stick to stones	Dry masonry absorbing water	Dampen surfaces before applying
White powder on surface	Efflorescence from salts	Usually cosmetic; brush off when dry
Mortar stays soft	No air reaching mortar (buried joints)	Ensure adequate air circulation; use hydraulic lime for enclosed joints
Frost damage	Froze before setting	Remove damaged mortar, relay in warmer conditions

Storage and Shelf Life

One of lime mortar’s greatest advantages: lime putty mortar can be stored indefinitely if kept wet. Cover mixed mortar with water in a sealed container. It actually improves with age as lime crystals become finer and more workable. Some historic lime putty was aged for 20 years or more before use. Dry-mixed hot lime mortar, by contrast, should be used within hours of mixing.