What’s the difference between hydraulic and non-hydraulic lime (incl. hydrated or putty)?

Lime has been a building material for thousands of years and throughout the history of lime, it has gone through many stages and improvements. Here, we look at the two main types of lime- non-hydraulic and hydraulic - exploring how they are made, their differences and when is the best time to use one over the other.

For more information on lime plasters, mortars, renders and ancillaries, shop our lime products online or call us directly on 01793 847444.

What is non-hydraulic lime?

Non-hydraulic lime is produced by burning pure limestone (calcium carbonate) in a kiln. This results in quicklime (calcium oxide) which is then slaked with water to produce calcium hydroxide in the form of lime putty.

Once created, this can be mixed with aggregate to create a mortar that will become hard when in contact with the air (carbonation), resulting in a material similar to the limestone it was made from. This process is known as the ‘lime cycle’.

lime cycle diagram

It requires exposure to air in order to carbonate and does not set under water.

This lime is regarded as the most appropriate lime for old buildings or ‘softer’ substrates where maximum permeability, capillarity and flexibility is required.

Premixed ‘wet’ products are made from lime putty with the addition of an aggregate (or alternative) and/or fibre, andime wash and lime paints are also typically made from non-hydraulic lime.

Non-hydraulic lime is also known as slaked/high calcium/putty/air/‘fat’

What is hydraulic lime?

Hydraulic lime is produced in a similar way but from limestones with naturally occurring impurities and it’s these minerals that allow the mortar to set and harden through chemical reactions with water (hydration).

Available in powder form, hydraulic lime sets faster than non-hydraulic lime and has a higher strength but a lower permeability. Natural hydraulic lime (NHL) is sold in various strengths (NHL 2, 3.5 and 5) to suit different building’s needs.

Natural hydraulic lime (NHL)

NHL5 is the most hydraulic, then NHL3.5, and NHL2 the least hydraulic lime. They do not perform in the same way as modern cements, nor contain the same damaging components.

It should be noted however that limes marked with NHL-Z or just HL on the bag can contain some additions that could be potentially damaging and at worst be not much better than cement. Only use limes marked NHL - these meet the highest British and European standards.

How are they classified?

Natural hydraulic limes (NHL’s) are classified by British Standard according to their compressive strength. These grades are associated with the terms:

  • feebly hydraulic (NHL2)
  • moderately hydraulic (NHL 3.5)
  • eminently hydraulic (NHL5)

However, lime-based mortars have an inherent flexibility, are self-healing and strength is one of the less important characteristics.

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 3.5 is an all-round general mortar. It can be used with bricks, facings, commons, blockwork, sandstone, limestone, terracotta, internal & external building work, cavity & solid wall construction, bedding, pointing and re-pointing.

NHL 5 is a very strong lime mortar to be used where there is severe exposure to weather and water, above rooflines, below DPC, including copings and cappings, and earth retaining walls.

How are hydraulic lime and non-hydraulic lime different?

Comparison table:

Hydraulic Lime Non-hydraulic Lime
 Sets by hydration (the addition of water)  Sets by carbonation (through exposure to the air)
 Faster setting lime  Slower setting time
 Stronger/harder  Weaker/softer
 Less permeable  More permeable
 General and conservation work  For old buildings or ‘softer’ substrate







Hydraulic limes (so called because they set under water) are made in the same way as non-hydraulic lime but using different limestone. They are sold as hydrated lime and have an initial set when water is added, followed by hardening while they absorb carbon dioxide. The more hydraulic a lime is, the faster it sets and the higher its final strength, but this means that it is less breathable and flexible.

Non-hydraulic lime (CL or DL 70-90) is sold as either hydrated lime or putty lime; they set and harden through drying out and absorbing carbon dioxide from the air. This means they have a very slow set: CO2 is only absorbed when certain conditions are met. They are the softest, most breathable limes available.

Hydrated lime simply means that a controlled amount of water is added to quicklime to make a powder that is more stable and safe to handle. This can be done to hydraulic lime or non-hydraulic lime.

Different types of lime

Lime putty

Lime putty can be made from either type of lime, and is made by adding an excess of water to quicklime. Hydraulic lime putty will set underwater within hours or days making them impractical, whereas non-hydraulic lime putty will remain plastic and improve with age.

Pozzolans

Pozzolans are additions that may be added to achieve harder, faster sets to any sort of lime or cement. Pozzolans, when added, produce similar chemical reactions to those found in hydraulic limes, so they reduce breathability and flexibility in exactly the same way.

The disadvantage is that you will never know how strong, breathable or flexible a Pozzolan lime is beforehand, unless you have considerable experience or knowledge. Adding some types of Pozzolans or even the smallest amounts of cement can be very damaging or produce poor performing lime mortars.

Benefits of lime materials

Breathability

Lime materials are highly breathable. Their vapour permeability means they allow water to pass through them, as either a gas or a liquid. This avoids the build-up of moisture, reducing the risk of damp or condensation.

Durability

Lime is exceptionally durable. Buildings built with lime products are able to stand the test of time as they act to keep their underlying structure dry. This allows for a more sustainable building structure.

Workability

With a sticky, almost ‘fatty’, feel for the builder applying it, lime has remarkable workability. It’s the workability that showcases the craftsman’s skill in the beautiful aesthetic it lends any project, old or modern.

Age resilient

Avoiding the synthetic appearance offered by many of its alternatives, lime mortars and renders age gracefully. Like a fine wine, they often improve with age. Lime mortars, renders and plasters can also be produced in a variety of colours.

Considerations when buying lime

Lime is slower setting

Some limes require a more methodical working practice as they can take longer to set than their synthetic alternatives. This longer setting time is negated by working methodically.

Misleading terminology

Sometimes, it can be misleading as to what some so-called ‘lime’ mixes contain, particularly when dealing with imported products. As a result, it’s important to take care to ensure you don’t pay a premium for something that is simply white cement with a pinch of lime.

Fear of use

Sometimes, builders can be wary of switching to using lime products. They can dismiss them as too difficult. However, modern lime materials can be as easy to use as cementitious products which offer none of the long-term benefits of lime.

General guidelines about lime

Usage

Any lime mortar must always be softer and more porous than the main building material. This is why issues such as spalling can occur when using OPC mortar to repair masonry of soft brick. The more exposed to the elements a lime mortar or lime render is, the greater the need for a faster set and greater durability, to cope with harsher freeze thaw cycles.

The less hydraulic a lime is, the more it will flex and move with a building. Timber structures therefore need a more flexible and breathable lime.

Lime putty or hemp lime mixes should be used with caution in houses where damp is a problem, if a wall is permanently very damp, a putty mix may never set. Low suction backgrounds (hard stone or blue/engineering bricks etc.) and damp cool weather also make the use of lime putty very slow, there are number of lime mortars, plasters and renders that can be used in these situations including Lime Green Duro and Ultra and Unilit 30.

Composition

The majority of lime mortars, renders and plasters are made of:

Sand or aggregate – a key component for the strength and durability of the mix. Sand also prevents the material from shrinking as it dries out.

A binder – acting to hold the mix together. This can be a range of materials from clay or cement to silicone or acrylic but for a long time, and increasingly more now as we’re favouring healthy building, the material of choice was lime.

The three key ingredients to any lime mix are: lime, water and sand. When mixed, these ingredients set to form a substance similar to man-made limestone.

Although there are many different kinds of lime which differ in both chemistry and strength, all limes are made when limestone is heated in a lime kiln. The heating process produces quicklime to which water is then added to form slaked lime.

Setting

Non-hydraulic limes set as they absorb CO2 from the atmosphere. This can mean they have a slower setting time than other limes as very specific environmental conditions must be met for this absorption to take place.

Hydraulic limes initially set when water is added, or even while underwater, and then harden over time as CO2 is absorbed, giving a harder mix which is more weather resilient. Although made in the same way as non-hydraulic limes, these are burnt from a different limestone.

For any further information, don’t hesitate to get in touch with our team of experts.

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