What is shell lime mortar?
Shell lime mortar (Dutch: schelpkalkmortel) is a masonry mortar made from shell lime (lime from burnt seashells), sand and water. For centuries it was the standard mortar in the coastal provinces of the Netherlands. An old Dutch slogan put it this way: “Built with shells? Trusted for centuries!”
Because shell lime is hardly produced today, the name is usually misleading for new mortar. What is meant is almost always an air lime (non-hydraulic lime) mortar: a lime mortar that hardens by absorbing CO2 from the air.
Properties
- Flexible: lime mortar is less rigid than cement mortar. Small movements and settlement are absorbed without cracking the bricks.
- Slow setting: the slow set means fewer hairline cracks from early settlement.
- Breathable: moisture can evaporate through the joints instead of being trapped in the bricks.
- Little efflorescence: lime mortars produce few salt deposits.
- Lower strength: the compressive strength is lower than that of cement mortar. For historic masonry with soft bricks, that is exactly what you want: the joint is the weakest, sacrificial and replaceable part.
Application
You find shell lime mortar in almost every historic building along the Dutch coast. For repair and repointing, follow these rules of thumb:
- Have the existing mortar analysed and choose a matching restoration mortar, such as an air lime mortar or one based on natural hydraulic lime (NHL).
- Mortar softer than the brick: the new joint must never be harder than the brick or the original mortar.
- Rake out with care: remove old joints gently without damaging the brick arrises.
- Aftercare: keep the work lightly damp for several days and protect it from frost, rain and strong sun.
Related Terms
- Lime mortar
- Air lime
- Lime
- Mortar
- Repointing
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