Lime mortar has been popular in construction and masonry work for centuries. Its sticky consistency makes it stand out among other types of mortar. This is due to the combination of lime with other ingredients, such as sand, stone dust and water, which together create a workable, sticky, adhesive mixture.
The stickiness of lime mortar can be attributed to its etymology, originating from Old English ‘lim’, which translates to ‘sticky substance, birdlime, mortar, cement, gluten’. This attribute makes it an ideal substance for binding bricks and stones, together with increased durability and flexibility. Understanding how lime mortar works and utilising it properly can vastly improve the quality and longevity of your construction projects.
Understanding Lime Mortar
Components of Lime Mortar
A lime mortar is a mixture of lime, sand, and water. The primary component of lime mortar is lime, which can either be hydrated lime (also known as lime putty, fat lime, slaked lime, or calcium hydroxide) or quicklime (unslaked lime or calcium oxide). When you mix water with quicklime, it becomes hydrated lime.
Hydraulic lime mortars use hydraulic lime instead. This type of lime comes in 3 strengths and can set in extremely wet or even underwater conditions. Because quicklime (and hydrated lime) based mortars set by reacting to carbon dioxide in the air, they cannot set in very wet conditions. Non-hydraulic lime putty-based mortars are more sticky than hydraulic limes. This is due to their ‘fatty’ nature.
The proportion of lime and sand depends on the specific requirements of your project. In general, hydraulic limes produce a faster and harder set than non-hydraulic limes (hydrated and quicklime-based mortars). However, non-hydraulic limes are far more breathable and flexible, making them more suited to older buildings and new eco-home construction projects.
Main Types of Lime for Construction
- Non-hydraulic lime: Non-hydraulic lime mortars rely on the natural carbonation process to set, and they require time to absorb carbon dioxide from the atmosphere. These mortars are flexible and breathable, making them suitable for older buildings.
- Hydraulic lime: These limes contain impurities such as clay, which react with water, forming a stronger bond and faster set compared to non-hydraulic limes. There are two types of hydraulic lime:
- Natural Hydraulic Lime (NHL): It is derived from limestone featuring natural impurities. NHL is chemically identical to calcium lime but sets faster than non-hydraulic lime mortars. They are graded into NHL 2, NHL 3.5, and NHL 5, depending on their compressive strength.
- Artificial Hydraulic Lime: This is produced by introducing additional raw materials, such as pozzolans (e.g. volcanic ash), to create a faster-setting mortar.
When working with lime mortar, it’s essential to understand the properties of different types of lime and how they impact the performance of the mortar. By choosing the right lime for your project, you can ensure the longevity and integrity of your construction.
Related article: What Are The 3 Main Types of Lime Used in Construction?
The Role of Lime in Mortar
Improving Workability
Lime makes mortar more workable by increasing its plasticity and stickiness, which allows for easier application and better moulding. The addition of lime helps control the water content in the mortar, ensuring it retains sufficient moisture for optimal bonding. This workability is particularly useful when you’re working with irregular or uneven surfaces, as it helps the mortar adjust to any inconsistencies in the substrate.
Increased Flexibility
Lime mortar exhibits greater flexibility compared to cement mortar. This flexibility allows the mortar to better accommodate movements within the structure, such as thermal expansion and contraction and prevents cracks that could lead to water ingress or structural damage. Additionally, the increased flexibility means that lime mortar has better tensile bond strength, enabling it to withstand stresses and maintain the integrity of your masonry.
Breathability
One of the primary advantages of lime mortar is its breathability, allowing moisture to evaporate more quickly from the wall, ensuring that your masonry remains dry and in good condition. Breathability minimises the build-up of moisture within the structure, reducing the risk of water-related damage such as efflorescence, frost damage, and mould growth. The use of lime mortar ensures that your building can maintain a healthy internal environment.
Autogenous Healing
Lime mortar can self-heal, a process known as autogenous healing. When small cracks form, free lime in the mortar reacts with water and carbon dioxide to form additional calcium carbonate, sealing the cracks and preventing further damage. This self-healing property is beneficial for your masonry’s durability and long-term stability.
Mortar Materials
Cement Types
In the world of mortar materials, various types of cement are available. One of the most commonly used is Portland cement, which is widely applied due to its versatility and affordability. Ordinary Portland cement (OPC) is the most common type of Portland cement, and it plays an essential role in making mortar effective for use in masonry construction.
Apart from OPC, other specialised cement types are available that may be suitable for specific applications. However, when it comes to lime mortar, which is the subject of this article, the primary component that provides stickiness and workability is not cement but hydrated lime. It must also be noted that adding lime to cement is a common practice to help improve the workability and stickiness of cement motars.
Aggregate Choices
The selection of suitable aggregates significantly influences the mortar’s stickiness and overall strength. Typical aggregates used in mortars are sand and crushed stones. Your choice of aggregate should be based on factors such as particle size, cleanliness, and overall quality.
Sand is typically the preferred aggregate for mortar due to its fine particle size and easy availability. When selecting sand for your lime mortar, it’s essential to use clean, well-graded sand with particles ranging from fine to coarse. This variation in particle size ensures an even distribution and enhances the mortar’s bonding capabilities.
Moreover, it’s crucial to maintain the correct moisture content in your mortar mix. A moist mix is necessary for providing the best workability and adhesion between the components. This will result in better bond strength and improved durability of the final construction.
By considering these factors when choosing your cement type and aggregate, you can create a lime mortar mix that is sticky and effective in your masonry projects.
Related article: What Type of Sand is Best For Making Lime Mortar?
Lime Mortar Uses
Historic Masonry Conservation
Lime mortar has been widely used for centuries, particularly in conserving and restoring historic masonry buildings. You might find it in brick, stone, or masonry units and in the joints and pointing of these structures. When repairing or restoring heritage properties, using lime mortar helps maintain their authenticity and respect for the original techniques. It also helps prevent dampness issues, as lime mortar allows the building to breathe and manage moisture more efficiently than modern materials like cement.
Modern Construction
In contemporary construction, lime mortar provides unique benefits that make it an attractive choice for certain applications. Lime plasters are often used for plaster and stucco work, providing a smooth and durable finish that can withstand the test of time. Lime mortar is also popular in the construction of eco-builds due to its compatibility with a range of natural materials and ability to absorb carbon dioxide from the atmosphere, making it a more environmentally friendly option compared to cement.
When utilising lime mortar in your projects, be aware that it has a sticky texture, making it easier to work with when constructing or restoring masonry buildings. Additionally, its properties give it high levels of flexural bond strength, which can provide greater resilience and longevity in your modern construction projects.
Conclusion
In conclusion, lime mortar has been a popular choice in construction and masonry work for centuries due to its sticky consistency, which is a result of the combination of lime with other ingredients such as sand, stone dust and water.
This article highlights the importance of understanding the different types of lime and how they impact the performance of the mortar. Lime mortar provides increased flexibility, breathability, and autogenous healing, making it an ideal substance for binding bricks and stones with increased durability and flexibility.
While it is commonly used in the conservation and restoration of historic masonry buildings, it is also popular in modern construction due to its compatibility with a range of natural materials and its ability to absorb carbon dioxide from the atmosphere, making it a more environmentally friendly option compared to cement.
Overall, understanding how lime mortar works and utilising it properly can vastly improve the quality and longevity of construction projects.
