What is shell lime?
Shell lime (Dutch: schelpkalk) is lime produced by burning seashells. In the coastal provinces of the Netherlands, where limestone was scarce, shell lime was for centuries the main lime used for masonry mortar and plaster.
From shell to lime
- Burning: shells consist of calcium carbonate (CaCO3). In a lime kiln they are heated to around 900 °C. The calcium carbonate breaks down into quicklime (calcium oxide, CaO) and carbon dioxide (CO2).
- Slaking: the quicklime is slaked with water to form hydrated lime (calcium hydroxide, Ca(OH)2).
- Setting: in the mortar, the lime slowly reabsorbs CO2 from the air and turns back into calcium carbonate. That is why it is called an air lime (non-hydraulic lime).
Because shells often carried some silt (clay), shell lime also contains silica and alumina compounds, which give it a slightly hydraulic character. It tends to be greyer and coarser than lime made from limestone.
Application
Shell lime was used for masonry mortar, plaster and limewash. For the lowest part of a wall, trass was often added to make the mortar more resistant to damp.
Shell lime is hardly produced any more. When restoring historic masonry, an air lime mortar or a mortar based on natural hydraulic lime (NHL) is usually specified instead. Tips:
- Have the old mortar analysed so the new mortar matches in strength and composition.
- Never too hard: a hard cement mortar in soft historic masonry can cause bricks to spall, because moisture and stress are forced through the brick instead of the joint.
- Be patient: lime mortar sets slowly. Protect fresh work from frost, strong sun and rapid drying.
Related Terms
- Lime
- Air lime
- Lime mortar
- Quicklime
- Lime kiln
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