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shop lime productsWhat is NHL lime?
NHL Lime – An Introduction
Let’s start by clarifying the correct terminology when it comes to non-hydraulic and natural hydraulic lime. Both could theoretically (but incorrectly) be acronymised to ‘NHL’, sometimes leading to confusion, but they are 2 different things and it’s crucial to remember the following:
- Natural Hydraulic Lime is NHL Lime
- Non-hydraulic lime is *NOT* NHL lime!
Hydraulic vs Non Hydraulic Lime
This article is an introduction to Natural hydraulic lime (NHL) so we’ll focus specifically on that. But first, here is a very brief overview of the broad differences between hydraulic and non hydraulic lime.
Natural hydraulic lime (NHL) sets by hydrolysis, which is essentially the reaction with water. These limes will generally come in ‘powdered’ form and set much faster and harder than non-hydraulic limes. Because of this, they are often more frequently used with exterior works, particularly in exposed areas. Due to their comparative ease of use, they have grown in popularity over recent years.
Non Hydraulic limes have a number of different names which include Fat Lime, hot lime mixes and Lime Putty. These set by reacting with Carbon in the air and won’t set under water. This type of lime will generally be supplied to the end user pre-mixed (by specialists using a mixture of lime, sand and water) and, as already indicated, set much slower and softer. It is often regarded as the most suitable lime to use where maximum permeability is required, for example in the conservation of old buildings.
What are the different grades of NHL lime?
Natural hydraulic lime is categorised into three distinct grades:
NHL 2 (feebly hydraulic)
NHL 3.5 (moderately hydraulic)
NHL 5 (eminently hydraulic)
The number preceding ‘NHL’ indicates the compressive strength when measured in MPa (Megapascals) or N/mm² (Newtons per square millimetre) after 28 days, in accordance with the current British Standard. These grades denote their varying degrees of hydraulicity—AKA their capacity to set in water without air exposure.
What are the advantages of NHL lime?
Other than the afore mentioned faster drying time and harder set, the main advantage of NHL lies in the fact that each grade is tailored for specific types of masonry and varying weather conditions. This allows for the selection of a mortar or render that is compatible with the masonry type, while ensuring maximum durability.
NHL 2 is characterised as being soft and permeable, making it ideal for internal applications, low exposure projects and conservation efforts involving stone walls and softer bricks. Typical applications for NHL 2 include hydraulic lime plaster, chalky stones, and soft, crumbling, or scaling sandstone, limestone, or bricks.
NHL 3.5 is a great general purpose option when neither NHL 2 or 5 are required. It is suitable for medium density masonry with moderate exposure levels. Common uses for NHL lime 3.5 encompass bricks, facings, commons, blockwork, sandstone, limestone, terracotta, and various building tasks, including cavity and solid wall construction, bedding, pointing, and re-pointing.
NHL 5 is the strongest and fastest setting option which is most suited for dense, durable materials, or areas that face large amounts of exposure to weather. Typical applications for NHL 5 include granite, engineering brick, basalt, flint, paving, roofing, chimneys, parapets, cornices, balustrades, copings, plinths, bridges, harbours, marinas, canals, dams, and sea-fronts.
How is Natural Hydraulic Lime Made?
NHL is made by heating impure limestone which contains elements such as clay, silica & alumina. Then slaking (a process which involves adding water) occurs to create a powder. This combination of heating and slaking in the production process creates active compounds within the lime that will in turn allow the mixture to set hard when water is added at the point of use.
Why use Lime Mortar Rather than Cement?
When re-pointing old stone walls, it is critical to make certain the correct type of mortar is used. For example using cement in conjunction with stone can be much more than just an eye-sore, it will lead to the stone being physically damaged over time.
This is because water can’t penetrate into the cement, whereas it can penetrate lime. The correct lime mortar will be able to absorb and manage the moisture levels throughout the year, so it essentially takes one for the team by removing the need for the stone to absorb more moisture than is necessary. If cement is used then all of the moisture needs to be absorbed by the stone because the cement is unable to do so. This will lead to excessive intake of water, which expands when it freezes. This can ultimately lead to the face of the stone ‘blowing’ under the internal pressure from the expanding water.
We have a blog post on pointing a stone wall with lime.
