The difference between hydraulic and non-hydraulic lime

The difference between hydraulic and non-hydraulic lime

Lime is a building material that’s been used for centuries. There are two main types of lime: hydraulic and non-hydraulic. The difference between them is the way they set after being mixed with water.


It may be confusing if you are unaware that there is a difference.  However, we have considerable knowledge when it comes to lime renders and plasters. Firstly, in construction, there are two broadly used categories regarding lime. Our first of the two limes is hydraulic lime.

Hydraulic lime sets through hydrolysis, a chemical reaction where water molecules break one or more chemical bonds. Hydraulic lime is made from impure lime, the process of hydrolysis causes a faster initial setting speed of the product.

Robust and strong, hydraulic lime can set in harsh, extreme conditions including being completely submerged under water. For this very reason, many builders will utilise hydraulic lime on exterior walls where weather conditions can affect non-hydraulic lime putty.

Hydraulic lime is measured using a European NHL (natural hydraulic lime) scale. There are three classifications, NHL 2, NHL 3.5 and NHL 5.

NHL 2 is softer and slower when setting. We can use this lime putty for internal applications where more delicate care is needed.

NHL 3.5 is a midway point. Brick facings, blockwork and general building work is perfect for this hydraulic lime.

NHL 5 is very dense. Strong and fast setting, this is suitable for paving, roofing, bridges, sea fronts and more. Anywhere there is severe exposure to water and weathering, use NHL 5.

Hydraulic lime is typically sold as a bag of powder whereas non-hydraulic is sold as a putty. Important to note, that hydraulic lime is easier for most builders to work with as it’s similar to working with cement.

If you’re on a tight timescale then use hydraulic lime as it’s faster setting.


Made from pure limestone, this product is sold as either hydrated lime or lime putty. Non-hydraulic lime is produced by burning pure limestone in a kiln and then slacked with water.

Unlike hydraulic limes, this lime will set and harden through the absorption of carbon dioxide from the air.

Softer in consistency, this material sets at a much slower speed than hydraulic lime and will only set when certain conditions are met. Ultimately this means the lime remains softer for far longer but with this comes many benefits.

The fatty nature of non-hydraulic lime lends itself to working particularly well in plasters and renders. Not only this but this lime is more breathable and is flexible enough to withstand subtle movements which tend to be common in older buildings with weaker foundations.

The difference between hydraulic and non-hydraulic lime


As you can see, these are two very different kinds of lime. While they both have their merits, it’s important for builders to know which one they need for each job so that they can get the right results.

Using the wrong lime can be detrimental to your building. Lime that is too strong can cause damage to older structures.

If you’re looking for more information about how to use lime in your building project then visit 


Ecoright have been working with architects, housebuilders and specialist brickwork contractors for many years, so their knowledge of lime and its uses is extensive. If you have any questions about how to use lime or its suitability to a particular project, please get in touch, they will be pleased to advise. You can read more about Lime Renders here:

Lime Renders are available through Ecoright’s sister companies:

Lime Render – the natural choice for exterior and interior work Lime Render – the natural choice for exterior and interior work

Lime mortar is highly aesthetic and can be exact colour-matched to bricks or other adjacent materials making it a good choice for conservation or new build.