How to Know if You Should Use Lime Mortar (& When to Use it)

Wells Cathedral, UK. Recently restored using Lime mortar.

From the 1930s onward, Portland cement mortars gradually overtook lime mortars to become the most commonly used building mortar. This is mainly because Portland cement has a speedy set compared to some lime mortars, which allows for quicker (and often sloppier) building. So how do you know if you should use lime mortar or cement mortar?

Generally speaking, if your home is built before 1900 it should only ever be repointed, plastered or painted with lime mortars, plasters and paints. You can’t beat lime mortars when it comes to older buildings. Lime mortars are flexible and allow for slight movement, whereas cement mortars would crack and begin to break up. Older buildings should almost never be repaired using cement.

Even if your home is newer and has been built using cement, with issues such as cracking and moisture, it may be worth considering using a lime-based material. Let’s go a bit more in-depth as to whether you should use lime mortars.

How to Know When Lime Mortar Should be Used

Firstly, if your house was constructed before 1880, chances are it’s been built using lime mortar. Using cement mortar can be extremely damaging to buildings originally constructed with lime mortar. This is due to a number of factors. Cement mortars are extremely hard, rigid and will likely trap moisture within the wall. This lack of flexibility can cause cracks and water ingress, where lime mortar can flex and move with the building. Any building constructed with lime mortar should be repaired with a similar lime mortar, and never repaired using cement mortars.

Houses built between 1880 and 1940 may have been built using cement or lime. If you are in doubt as to whether the mortar is cement or lime, your best bet is to repoint using lime. This is because buildings constructed with cement will not be damaged by replacing the cement with lime. However, a building originally constructed using lime must be repaired using lime. Cement mortars could seriously damage the building and will likely lead to many more issues in the future.

If your home was constructed after 1940, chances are it was built using Portland cement mortars. You will be safe to repair any damage with cement, although, lime mortars can be far more beneficial for your building and the environment in the long term.

If you are wondering how long you should expect lime mortar pointing to last, click here!

How to Tell if a Mortar is Lime or Cement

It can be very challenging to tell whether a mortar is lime or cement based. Generally, as lime ages, it will weaken and may become more ‘crumbly’. Very old lime mortar will often begin to fall apart if you rub it with your finger. If you can rake some of the mortar out of the wall with a screwdriver or other similar object, the mortar is likely lime based and needs repointing. If you just leave a mark on the mortar and it is still very hard it’s more than likely cement.

Another way you can test a mortar at home is to spray a small amount of vinegar on it. Because lime is very alkaline, it will react with the acid in vinegar. If the mortar is lime-based, you should notice a small amount of fizzing. If nothing happens or there is a tiny amount of fizzing, the mortar is likely cement. This method of testing isn’t the best way to check and can give unreliable results.

While old lime mortar is fairly easy to spot, due to its crumbly nature, newer lime can look very similar to cement. In the cases where you just can’t tell if it’s lime or cement mortar, your best bet is to get it properly tested by an expert.

Related article: Why is Cement Used Instead of Lime in Modern Buildings?

What Are The Benefits of Using Lime Mortar

Lime mortar has many great characteristics. It has been used to construct buildings for thousands of years and was extensively used by the Romans. Many of the world’s greatest buildings have been constructed using lime and have stood for thousands of years, for example, the Pantheon in Rome. So what benefits does lime mortar provide for a building?

Roman Pantheon, built with Lime in 126-128 A.D.
  • Breathability. We have already touched on this characteristic. Lime mortar has high porosity and high permeability. This allows lime mortars to deal with moisture within the building. The breathability helps to dry out buildings, reducing levels of condensation and dampness. Any leaks in the building pose little risk as the water can freely be expelled by the lime mortar. This is a huge advantage over cement, which creates a seal, trapping moisture and leading to damage.
  • Durability. When the correct lime mortar is used for the job at hand, it is exceptionally durable. Many lime buildings are hundreds of years old and are still standing today. The Pantheon in Rome was built with lime and is still standing 900 years later!
  • Self Healing. This is a little-known fact about lime in general. When buildings constructed using lime move and flex many fine cracks can develop. Over time, ‘free lime’ within the mortar can fill up these cracks and self-heal them! This is why so many old buildings made using lime are still standing, they will often distort rather than collapse. If this happened to a building made with cement mortars, large cracks would appear and the building would eventually collapse.
  • Protection. Lime mortars can help protect buildings from harsh elements such as rain. The lime itself gets damaged rather than the building, acting sacrificially. If needed, the lime can then be replaced. This also helps to protect softer masonry and bricks. The lime will be destroyed instead of the actual stonework of the building.
  • Workability. Lime mortars are well known to have excellent workability. This means that they are smooth and mouldable, even when being applied to a surface which is porous and creates a lot of suction. Non-lime mortars often go stiff as the water is sucked away and become awkward to use. Good workability helps lime mortars to properly bond and fill the joints, creating a strong key. Lime mortars are often described as ‘sticky’ and ‘workable’ which is a characteristic that’s normally easier to feel when using the material than describe it.

Related article: What Are The 3 Main Types of Lime Used in Construction?