Lime mortar has been used as a building material for thousands of years. It has many great qualities that allow for amazing longevity, even when compared to modern Portland cement-based mortars.
Lime mortars can last for thousands of years or only 50. Generally, factors such as exposure and the type of lime mortar used will influence how long lime mortar pointing will last. There are examples of Roman lime pointing still surviving well. Lime mortar that is reasonably sheltered can last for hundreds of years. However, you may find exposed areas such as copings and weatherings needing to be repointed every 50 or so years.
Let’s look at some of the factors that can cause lime mortar to last for a longer or shorter period of time in more detail.
Factors That Cause Lime Mortar to Deteriorate Quicker
Over time, all kinds of mortars will deteriorate. From Portland cement to Lime mortars. Lime mortars have stood the test of time, there are centuries-old buildings all across Europe and India that were constructed using lime mortars. Lime is one of the most versatile building materials on the planet, there are many different types of lime mortar, suited to a number of different applications. There is a ‘correct’ lime mortar to use in almost every scenario.
So what are the reasons that lime mortars deteriorate over time? Firstly, areas very exposed to severe rain and weather will find that they will need to repoint their building more regularly. Lime mortars protect the masonry and stonework of the building by being the weakest link. Rather than the stonework being broken down by the harsh weather, the lime mortar is. This sacrificial relationship makes lime an amazing building material. Where Portland cement mortars are used, you will often find the face of bricks and stonework are blown off. This is because the stones and bricks are the weakest links, and therefore they will be broken down instead of the mortar.
Acid rain can also negatively affect lime constructions. Because lime is a highly alkaline material, it will react with acid within rainwater. If you live somewhere with particularly high amounts of acid rain you may find you need to repoint your mortar more often. Again, the decay of the lime mortar itself helps to protect the brick or stonework of the building. It is far cheaper to replace the lime mortar than to rebuild the wall.
Related article: What Are The 3 Main Types of Lime Used in Construction?
Types of Lime Mortar and Where to Use Them
When selecting which lime mortar to use, it’s imperative that you make sure it’s softer than the masonry, stone or brickwork it is being applied to. There are 2 main types of lime mortar. Hydraulic and non-hydraulic limes.
Non-hydraulic limes are the traditional lime mortars that have been used for thousands of years. They are produced by mixing quicklime and water together which creates a reaction. The result of this reaction is ‘lime putty’ also known as ‘fat lime’. The lime putty has an appearance and texture similar to clotted cream. Lime putty is then mixed with aggregates such as building sand to make a mortar. Putty-based mortars set by a process called carbonation.
Lime putty mortars react with and absorb carbon dioxide in the air, causing them to slowly harden. These mortars are the softest and most suitable for very old buildings. However, when pointing or building in more exposed and damp conditions the mortar may never set, and that’s where hydraulic limes come in.
Hydraulic limes are best for building in exposed and wet conditions, this is because a non-hydraulic mortar will not set underwater. There are 3 different strengths of hydraulic limes, each with a larger amount of active clay impurities. The more clay, the harder and faster the hydraulic lime will set. Hydraulic limes initially set via hydration, a reaction with water. Over time they will also carbonate in the same way a putty-based lime would, continuing to harden over time. As a general rule, the harder the lime, the less breathability and flexibility, it is important to consider this when picking the right lime.
- Feebly hydraulic limes (NHL 2) are slightly harder than putty limes and will set in wetter conditions. These hydraulic limes are good for internal applications where there may be issues with moisture that would not allow putty limes to set. NHL 2 will still make a soft mortar, ideal for where conservation is still a high priority and used around very soft stone and bricks.
- Moderately hydraulic limes (NHL 3.5) are used to make a general building mortar. They can be used extensively on almost all masonry types. These hydraulic limes are well suited to areas that get moderate exposure to rain, where lime putty mortars would not set quickly enough.
- Eminently hydraulic lime (NHL 5) are the strongest limes and most similar to Portland cement. NHL 5 should only be used in severely exposed areas such as weatherings, areas below DPC, copings and bridges.
For a more extensive guide on how to know if you should use lime mortars click here.
How Long Can Lime Mortars Keep For?
Because lime putty sets via reaction to carbon in the air, as long as it’s in an airtight container, it will keep indefinitely. Many lime putty-based mortars come pre-mixed, which means the putty and various aggregates have already been mixed together. They are often sold in airtight plastic bags of various sizes. Pre-mixed putty mortars are very convenient and easy to use. As long as you keep the leftovers airtight, they will be good to use for years. Many putty-based mortars are matured for multiple years at a time. This helps improve workability.
Hydraulic limes are sold as powders. Like Portland cement, the user then mixes these powders with various aggregates to get their desired mortar. As soon as hydraulic lime comes into contact with water, it begins to set. Bags of hydraulic lime should be kept in dry non-humid conditions, once purchased they should be used within a year. It’s best to buy hydraulic lime as you need it, it will degrade over time as it’s exposed to moisture in the air.