Why lime mortar is different from cement?
The key difference is how it behaves over time.
Lime mortar:
- Softer and more flexible
- Breathable (vapour-permeable)
- Self-healing (small cracks can re-seal)
- Weaker than masonry units (by design)
Cement mortar:
- Hard and rigid
- Traps moisture
- Cracks rather than flexes
What is lime mortar?
Lime mortar is a traditional masonry mortar made from:
- Lime (the binder)
- Sand (the aggregate)
-Water
It was the standard mortar for most buildings before Portland cement became common in the late 19th–early 20th century.
What are the Types of lime used in mortar?
- Non-hydraulic lime (Lime Putty / Fat Lime)
- Hydraulic lime (NHL – Natural Hydraulic Lime)
What are the commonly used strength grades for NHL – Natural Hydraulic Lime?
NHL 2 – Very feebly hydraulic (softest). Used for soft stone, internal or sheltered.
NHL 3.5 – Moderately hydraulic (most common). Used for external walls, most repointing.
NHL 5 – Eminently hydraulic (strongest). Used for severe exposure (rarely appropriate for historic brick).
Preparation rules?
- Remove hard cement mortar fully
- Dampen masonry (not soaking)
- Never apply to dry or dusty surfaces
Application rules?
- Press mortar firmly into joints
- Build up in layers if deep
- Finish slightly recessed or flush (not smeared)
Curing rules?
Protect from: Sun, Wind, Frost and Heavy rain
Keep damp for several days (mist spray / hessian)
When you should use lime mortar?
- Building is pre-1919
- Walls are solid (no cavity)
- Masonry is soft brick or stone
- Damp issues exist
-You want longevity and reversibility
When lime mortar is NOT appropriate?
- Modern cavity walls designed for cement
- Structural elements requiring high early strength
- Engineering brickwork
- Situations where vapour control is required (not breathability)
Lime mortar lifespan?
Properly used lime mortar can last:
50–100+ years
Why use in solid walls?
Traditional solid walls (brick or stone, no cavity) were never intended to be waterproof.
They work on this principle:
- Rain and ground moisture enter the wall
- Moisture moves through the wall
- Moisture evaporates back out
This only works if all materials are vapour-permeable. Lime mortar allows water vapour and moisture to pass through it.
Why does lime mortar need protection when curing?
Lime mortar does not “dry” like cement.
It hardens mainly by carbonation (absorbing CO₂ from the air), which only happens if the mortar:
- Stays damp
- Cures slowly
- Is protected from rapid moisture loss and temperature extremes
If lime dries too quickly, it:
- Fails to carbonate properly
- Becomes weak and friable
- Powdering and early failure occur
Why would you use hessian sheeting?
Hessian sheeting slows down moisture loss. It reduces wind and sun exposure, prevents rapid evaporation and helps keep mortar joints evenly damp.
This allows carbonation to proceed correctly.
Why Hessian and not another material?
Hessian is ideal because it is:
- Breathable
- Absorbent
- Non-reactive
What is hessian sheeting?
Hessian sheeting is a coarse, woven natural fabric made from jute plant fibres, commonly used in construction, agriculture, and conservation work.
