“All Lime Mortars Are Not Created Equal: Choosing the Right One”

All lime mortars are not equal, but which one should you use? Lime putty and Natural Hydraulic Lime (NHL) are both materials used in construction and masonry work, but they have different properties and applications. Choosing which type of lime mortar to use when repairing traditional buildings needs to be considered carefully. Most NHL mortars will not be suitable for most conservation work due to their high compressive strength and reduced porosity.

  Lime Putty:

–   Source:   Lime putty is made by slaking (mixing with water) quicklime (calcium oxide) to create a putty-like consistency.

–   Type of Lime:   It is typically made from non-hydraulic lime, also known as “fat lime” or “air lime,” which is quicklime that does not have hydraulic properties.

–   Setting:   Lime putty sets through a slow and gradual process known as carbonation. It hardens as it absorbs carbon dioxide from the air.

–   Flexibility:   Lime putty is relatively flexible and can accommodate minor structural movements, making it suitable for restoration and conservation work on historic buildings.

  Natural Hydraulic Lime (NHL):

–   Source:   NHL is made by heating limestone containing clay or other impurities to produce a binder with hydraulic properties.

–   Type of Lime:   It is a hydraulic lime, which means it sets through a combination of hydraulic (water-related) and carbonation processes.

–   Setting:   NHL sets more rapidly compared to lime putty, and the setting process is a combination of hydraulic setting and carbonation.

–   Strength:   NHL is typically stronger and has a faster initial set than lime putty, making it suitable for various modern construction applications.

In summary, the main difference between lime putty and NHL lies in their source and setting properties. Lime putty is made from non-hydraulic lime and sets slowly through carbonation. It is often preferred for traditional and historic restoration work where flexibility and authenticity are essential. NHL, on the other hand, is a hydraulic lime that contains impurities and sets more rapidly. It is often used for modern construction projects where a faster initial set and greater strength are required. The choice between the two depends on the specific needs of the project and the type of construction being undertaken.

It was very common in the last 50 years for cement mortars to be used on traditional buildings. Most buildings constructed before the 1920s will use lime mortars, and even up to the late 1930s, lime mortar was being used, sometimes with a dash of cement. After the inter-war period, cement then became more popular than lime. Up until the late 1990s, most traditional buildings were being repointed or repaired using mortars that contained cement and bagged lime, which is not the same as lime putty or NHL. Cement traps moisture within buildings, and this has caused all kinds of moisture-related problems for traditional buildings, resulting in expensive remedial works. If you own a property built before 1940, it is strongly recommended that you use only compatible materials when undertaking any remedial works on the building.  The rule of the thumb is to match any replacement materials to the original material.

Most builders are now aware of the issues associated with using cement mortars in traditional buildings. However, we cannot overemphasize the importance of making the correct selection when it comes to the choice of lime mortars for your restoration project. Not all lime mortars are created equally, and this is where some builders may encounter challenges.

If we can be of any further assistance with your restoration project we would be pleased to advise further.  We can be reached on by email at bsullivan@hsa-surveyors.com or 01727 613088.