Types of Hydraulic Lime Mortar

Hydraulic lime mortar is available in three different grades; NHL 2, NHL 3.5 and NHL 5. The NHL stands for Natural Hydraulic Lime. The number relates to the compressive strength of the hydraulic lime in N/mm² (Newtons per square millimetre).

These grades are often associated with the terms feebly hydraulic (NHL 2), moderately hydraulic (NHL 3.5) and eminently hyrdraulic (NHL 5). These terms refer to each grade's degree of hydraulicity, that is their ability to set under water without exposure to air.

The most common question, “Which type of hydraulic lime mortar do I need?”

This all depends on the masonry you're working with, its hardness, weathering and its degree of exposure to the elements. Each grade of hydraulic lime is more suitable for certain building conditions.

You can follow this guide to help choose a suitable grade of hydraulic lime mortar. The best thing to do is to check the NHL 2 and NHL 5 first. If they don't sound suitable you probably need NHL 3.5, that is the most common mix and the one most people buy.

NHL 2

NHL 2 lime mortar is sutiable for any kind of stone or brick that is tending to scale or powder. It can be used for soft, chalky or flakey masonry in any exposure or location. Suitable for less durable sandstone, limestone and brick. Also used for internal renders, makes an excellent lime plaster.
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NHL 3.5

NHL 3.5 lime mortar is suitable for moderately permeable sandstone, limestone and brick masonry. It is considered a general purpose mix, suitable for most internal and external building work, walling, repointing, rendering and repairs. If you don't need to use NHL 2 or NHL 5 then use this.
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NHL 5

NHL 5 lime mortar is best for dense, durable or impermeable materials such as granite, engineering brick, basalt and flint. It is also used where the masonry has more severe exposure to water like a bridge, harbour wall, retaining wall, paving, plinth, roof, chimney or coping. Its high degree of hydraulicity makes this the best mortar for these types of exposed environments and situations.
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Things to consider

Remember when choosing mortars, strongest is not always best, mortars should be expendible.

It is important that the mortar is softer than the masonry. A softer hydraulic lime mortar will absorb and evaporate the moisture from the surrounding masonry, thus allowing it to breathe. It will also accomodate movement and thermal expansion and will not damage the masonry by wearing it away over time.

This doesn't mean that softest is always best either though.

It is also important that the mortar is strong enough to last. If the mortar is too soft then it won't hold up to the elements or the surrounding masonry. You need to consider the weather conditions and degree of exposure too. The mortar should be as soft as it needs to be, but no softer.

So in a nutshell, don't just choose the hardest or the softest grade of hydraulic lime mortar, choose the most suitable one. Our specialist team can help you make this choice, just make an enquiry We have tons of practical experience and we're here to help.