Natural hydraulic lime, abbreviated NHL, is a lime that sets not only in air but also under water. It is burnt from limestone that naturally contains clay minerals. Those impurities form compounds during firing that react with water — the hydraulic part — while the remaining free lime hydrate carbonates conventionally with carbon dioxide from the air. NHL therefore sits between air lime and cement.
The strength classes
The European standard for building lime, EN 459-1, defines three classes. The number is the minimum compressive strength of the standardised mortar in megapascals (N/mm²) at 28 days:
- NHL 2 — the softest, from 2 MPa. For internal plaster, soft historic masonry and pointing in vulnerable stone
- NHL 3.5 — the widely used middle class, from 3.5 MPa (in practice 3.5 to 10 MPa). For general building and pointing in conservation work and in new work with soft brick
- NHL 5 — from 5 MPa (in practice 5 to 15 MPa). For more heavily loaded work, damp conditions, foundations and chimneys
The higher the class, the stronger and faster setting — but also the less flexible and less vapour-open.
Why NHL rather than cement?
In historic masonry the rule is that the mortar must be weaker than the unit. If the building moves, the crack should run in the joint, because a joint can be replaced; a stone cannot. Cement mortar is harder and denser than much old brick and stone. The consequence is well known: moisture that can no longer evaporate through the joint passes through the unit instead, and the face spalls in frost. NHL mortar is softer, vapour-open and self-healing across hairline cracks.
Working with it
- Choose the class to suit the unit, not the other way round. When in doubt, go one class down
- Typical mix for building and pointing mortar is around 1 part NHL to 2.5 to 3 parts sharp sand by volume
- It sets slowly. Allow weeks rather than days for full strength
- Aftercare is essential. Protect fresh work from sun, wind and rain for several days and keep it lightly damp; drying too fast gives friable pointing
- Do not work below about 5 °C or when frost is forecast within a few days
- Wear eye protection and gloves; lime is strongly alkaline and caustic
Related terms
- Lime
- Lime mortar
- Air lime
- Slaked lime
- Lime putty
- Cement
- Natural stone replacement
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