Why Lime Mortar Crumbles: Understanding the Causes


You’re not alone if you’re wondering why your lime mortar is crumbling. Failing or crumbling lime mortar is a common problem caused by several factors. Understanding the reasons behind the crumbling can help you take the necessary steps to prevent it from happening in the future.

One of the main reasons lime mortar crumbles is due to it drying out too quickly after application. Lime mortars gain strength via carbonation. Carbonation is slow and can only take place if the mortar remains damp. Allow fresh mortars to cure slowly over 2-4 weeks to allow for proper carbonation. Failure to do so will most likely result in a weak, crumbly mortar.

It’s important to note that very old lime mortars will naturally decay over time. Old lime mortars can be soft, and when disturbed, they may begin to crumble. The beauty of working with lime is that these joints can be easily raked out and repointed with fresh lime mortar without damaging the surrounding masonry!

Understanding Lime Mortar

What is Lime Mortar?

Lime mortar is a traditional building material that has been used for centuries. It comprises lime, water, and an aggregate such as sand. Lime mortar is typically used in conserving existing old buildings or recreating new ones using traditional methods.

Its use dates back at least 6,000 years to the Ancient Egyptians, who used lime to plaster the insides of pyramids. In addition, many other ancient civilisations like the Indus Valley civilisation, Romans and Greeks used lime in all manner of construction projects. So it would be safe to say lime is a time-tested building material!

Types of Lime Mortar

There are two main types of lime mortar: hydraulic lime and non-hydraulic lime. Each type has its own unique properties and uses.

Non-hydraulic lime is the most traditional lime. It’s made by burning limestone or chalk in a kiln and then slaking (mixing) the resulting quicklime with water. This creates lime putty, a thick white liquid which can be mixed with sands and aggregates to make mortars and plasters.

Hydraulic lime is made by burning limestone or chalk containing small amounts of clay impurities. When burnt in a kiln, these clay impurities become activated. This allows hydraulic limes to set via a process called hydrolysis – a reaction with water that is quicker than the carbonation of non-hydraulic limes. Hydraulic limes are sold in powder form.

For a more extensive overview of the types of lime used in construction, try this article: What Are The Main Types of Lime Used in Construction?

How Lime Mortar Works

Non-hydraulic lime mortars set by a process known as carbonation. When lime is mixed with water, it begins to re-combine with carbon dioxide introduced from the air around it. This process causes the lime to harden and set, creating a strong and durable mortar.

The strength of lime mortar depends on the amount of carbonation that occurs. If the lime does not carbonate properly, the mortar will have little, if any, strength and will crumble or fail. Non-hydraulic mortars can take 2-4 weeks to set properly, depending on the weather conditions.

Hydraulic limes are more like modern cement. They react with water in a process called hydrolysis but will also undergo a small amount of carbonation, unlike cement. This is quicker than non-hydraulic limes but still takes a good deal longer than cement. Both types of lime must be kept slightly damp as they cure.

Carbonation can only occur in the presence of water. If the mortars dry out, then sufficient carbonation cannot take place. This is a leading cause of crumbling mortars. Always make sure your lime mortars cure slowly (especially non-hydraulic ones)

In summary, lime mortar is a traditional building material used for centuries. It comprises lime, water, and an aggregate such as sand. There are two main types of lime mortar: hydraulic lime and non-hydraulic lime. Lime mortars set by a process known as carbonation or hydrolysis, and the strength of the mortar depends on the amount of carbonation that occurs.

Why Does Lime Mortar Crumble?

If you have noticed that your lime mortar is crumbling, it can be a cause for concern. Lime mortar is a popular choice for building and construction projects, but like all building materials, it will eventually begin to break down. In this section, we will explore the factors that cause lime mortar to crumble and the effects of weathering, ageing, and dampness.

Ageing and Dampness

Age is the main factor that causes old lime mortars and plasters to crumble. Many mortars are decades old, and many can even be centuries old! Lime is extremely durable. But it will begin to break down and crumble over time. This is normal, and there are solutions to fix crumbling mortar, such as reporting with fresh lime mortars.

While lime mortars are among the best building materials on the planet for dealing with moisture, they can still be damaged by persistent dampness. Usually, lime mortars and plasters help to remedy damp issues, as they can act like a wick to remove water from within the structure of the building and bring it to the surface, where it can evaporate away without causing damage.

However, in extreme conditions, even lime building materials cannot cope. If they remain constantly waterlogged, they are susceptible to frost damage and other water-related issues, like most other building materials. In these cases, the water source must be located and eliminated. Then, fresh lime mortars can be applied. Over time, they will help the building to dry back out.

Related article: How Long Will Lime Mortar Pointing Last?


Effects of Weathering

Weathering can also significantly impact the strength and durability of lime mortar. Exposure to rain, wind, and other elements can cause the mortar to break down over time. This can lead to cracks and gaps forming, allowing water to seep into the structure. This can cause further damage, as the water can freeze and expand, causing the mortar to crumble even more.

This is part of the normal ageing process. If weathering is causing extensive damage to your mortar, it may be time to remove and repoint it. Consider repairing with a stronger hydraulic lime mortar in extremely exposed conditions (think chimneys etc..). NHL 3.5 or 5 would be a good choice for these scenarios.

In conclusion, several factors can cause lime mortar to crumble, including a lack of carbonation, incorrect proportions of lime and aggregate, weathering, ageing, and excessive dampness. By understanding these factors and taking steps to protect the mortar, you can help ensure that it remains strong and durable for years to come.

Why Does Freshly Repointed Lime Mortar Crumble?

If you’ve recently repointed your masonry with lime mortar, you may find that it crumbles away soon after application. This can be frustrating and costly, there are several reasons why this might happen.

Drying Out Too Quickly

One of the most common reasons for freshly repointed lime mortar to crumble is that it has dried out too quickly. Lime mortar needs time to cure and harden; if it dries out too quickly, it won’t have a chance to carbonate properly. This can cause the mortar to become powdery and weak, leading to crumbling and eventual failure.

To prevent this, it’s important to take care when the mortar is curing. This might involve dampening the wall down with a mist sprayer to slow drying. Hanging damp hessian sheeting over the newly repointed area to keep it moist is also a good way to control the curing process. It’s also advisable to avoid repointing in very hot weather, as this can cause the mortar to dry out too quickly.

For non-hydraulic lime mortars, you should allow 2-4 weeks of curing. During this time, you may need to be hanging protective sheeting and dampening down the wall with mist sprayers. Hydraulic lime mortars won’t require attention for quite as long; the initial week is the most important time.

Poor Quality Aggregates or Lime

Another possible cause of crumbling lime mortar is the use of poor-quality aggregates or lime. If the aggregates are not sharp and washed (as opposed to round and containing organic matter like leaves etc..) or the lime is of low quality, the resulting mortar will be weak and prone to crumbling.

To avoid this, using high-quality materials when repointing with lime mortar is important. This might involve sourcing sharp-washed aggregates or using a high-quality mature lime (for non-hydraulic mortars).

Related article: What Type of Sand is Best For Making Lime Mortar?

As a quick note, if you are using hydraulic lime, ensure it is still in date. Additionally, if it has been stored in damp or wet conditions, it could have spoiled. If you’re in doubt, make a test mix to see how well the material sets. If you notice it crumbling or not binding together well, fresh lime should be bought.

Poor Weather Conditions as The Mortar Cures

Another possible cause of crumbling lime mortar is poor weather conditions, as the mortar cures. If the weather is too hot, cold, or wet, the mortar may not cure properly and become weak and crumbly.

To avoid this, choosing the right time of year to repoint with lime mortar is important. Ideally, this should be during mild weather conditions that are neither too hot nor too cold. It’s also important to protect the newly repointed area from rain that could wash it out until the mortar has cured properly.

If you need to use lime mortar on hot or cold days, you must take extra care to protect the mortar. This can be done by hanging damp hessian sheets, dampening the wall with a mist sprayer and insulating the mortar in cold conditions.

Incorrect Application Techniques

Finally, the crumbling lime mortar may be the result of incorrect application techniques. If the mortar is not mixed properly or applied too thinly or too thickly, it may not cure properly and become weak and crumbly.

To avoid this, following the correct application techniques when repointing with lime mortar is important. This involves using the right tools and equipment, mixing the mortar to the right consistency, and applying it evenly and consistently to the masonry.

If you mix your own lime mortar, ensure it has the correct ratio of lime to aggregates; a good ballpark figure to go by is 1 part lime, 3 parts aggregates and water to suit. If you are in doubt, speak to your local lime supplier or specialist.

Related article: Adding Water to Lime Mortars: Is it Necessary?

In summary, there are several reasons why freshly repointed lime mortar might crumble, including drying out too quickly, poor quality materials, poor weather conditions, and incorrect application techniques. By taking care to avoid these issues, you can ensure that your repointed masonry remains strong and durable for years to come.

Strength and Durability of Lime Mortar

Compressive Strength

Lime mortar has a lower compressive strength than cement mortar, but it has a higher flexural strength, making it more resistant to cracking. The compressive strength of lime mortar varies depending on the type of lime used, the aggregate size and type, and the curing conditions. The compressive strength of lime mortar can range from 0.5 to around 5 N/mm2.

Related article: Understanding the Strength of Lime Mortar vs Cement Mortar


Lime mortar is known for its durability. Lime mortar can last for hundreds and sometimes even thousands of years and has been used in many historic buildings and monuments. The durability of lime mortar comes from its ability to self-heal and softness.

Lime mortar can absorb carbon dioxide from the air, which reacts with the calcium hydroxide in the mortar to form calcium carbonate. This process, known as carbonation, can fill in small cracks and gaps in the mortar, making it stronger and more durable.

In addition, lime mortar is more flexible than cement mortar due to its softness, which allows it to accommodate movement in the building without cracking. This flexibility is particularly important in historic buildings, which are often subject to movement due to changes in temperature and humidity.

Overall, the strength and durability of lime mortar make it an excellent choice for historic preservation and restoration projects. Its ability to self-heal and accommodate movement in the building makes it a reliable and long-lasting option for masonry work.

Related article: The Reason Lime Mortars Are Expensive (The Hidden Costs)


In conclusion, lime mortar is a popular choice for historic buildings due to its compatibility with traditional building materials. However, it is important to understand why lime mortar crumbles in order to prevent any damage to the structure.

It is clear that failing or crumbling mortar can be caused by a lack of carbonation of the lime within the mortar, meaning that the mortar will have little to no strength. Efflorescence, the appearance of white salts on the surface of the mortar, can also contribute to the crumbling of lime mortar.

Overall, by understanding the causes of crumbling lime mortar and taking the necessary precautions, you can ensure the longevity and stability of your historic building.