Hydraulic Lime, Which NHL?
Hydraulic lime 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 compressive strength in MPa (Megapascals) or N/mm² (Newtons per square millimetre) at 28 days under the current British Standard.
These grades were historically associated with the terms feebly hydraulic, moderately hydraulic and eminently hydraulic respectively. These terms refer to each grade's degree of hydraulicity, that is their ability to set under water without exposure to air.
The beauty of NHL is that each grade is suitable for different kinds of masonry and weather exposure. This means you can select a mortar or render that is sympathetic to the masonry but with a maximum durability, the perfect mortar for the job.
Soft, Permeable Masonry
NHL 2 is softer and slow setting, suitable for internal applications or where conservation is a primary concern with soft or deteriorating stones and bricks
NHL 2, Typical Uses:
Hydraulic lime plaster, chalky stones, soft, crumbling, badly worn, flakey or scaling sandstone, limestone or bricks.
NHL 2 Products:
Medium Density Masonry
NHL 3.5 is for general building, suitable for moderately permeable masonry materials. Basically, if you don't need to use NHL 2 or NHL 5 then use this.
NHL 3.5, Typical Uses:
Bricks, facings, commons, blockwork, sandstone, limestone, terracotta, general building work, cavity & solid wall construction, bedding, pointing, re-pointing
NHL 3.5 Products:
Dense, Impermeable Masonry
NHL 5 is stronger and faster setting, more suitable for dense, durable or impermeable materials with severe exposure to weather or water.
NHL 5, Typical Uses:
Granite, engineering brick, basalt, flint, paving, roofing, chimneys, parapets, cornice, balustrades, copings, plinths, bridges, harbours, marinas, canals, dams, sea-fronts
NHL 5 Products:
Things to consider
Remember when choosing lime, strongest is not always best, mortars should be sacrificial.
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 accommodate 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, select 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.