Lime mortar has a rich history that dates back thousands of years. The ancient Egyptians, Greeks, and Romans all used lime mortar in their monumental structures, many of which still stand today. Lime mortar is made from lime, a caustic alkaline substance produced by burning limestone or seashells. When mixed with water and sand, it forms a workable paste that hardens over time through a process called carbonation.
Lime mortar was the dominant building material for centuries, used in everything from humble dwellings to grand cathedrals. Its popularity was due to its many benefits. Lime mortar is flexible and breathable, allowing buildings to move and moisture to escape. It is also self-healing, meaning that small cracks can repair themselves over time.
The Transition to Modern Mortars
The advent of the Industrial Revolution brought about significant changes in the construction industry, including the introduction of Portland cement in the early 19th century. Portland cement is a hydraulic binder that hardens quickly and is stronger than lime mortar. It quickly became the preferred choice for many builders, and by the mid-20th century, it had largely replaced lime mortar in new construction.
The transition to Portland cement was driven by several factors, including its lower cost, faster setting time, and perceived superior strength. However, this shift had unforeseen consequences for historic buildings.
Shortcomings of Portland Cement Mortars for Historic Buildings
While Portland cement is a suitable material for modern construction, it is often incompatible with historic masonry. Historic buildings were designed and built with lime mortar, which is softer and more flexible than Portland cement. When Portland cement is used to repair historic masonry, it can create several problems:
- Incompatibility: Portland cement is harder and less porous than lime mortar. This can lead to stress and cracking in the surrounding masonry, as the two materials expand and contract at different rates.
- Moisture Trapping: Portland cement is less breathable than lime mortar, which can trap moisture inside the wall. This can lead to dampness, mold growth, and accelerated deterioration of the masonry.
- Salt Damage: Portland cement can also contribute to salt damage. When moisture evaporates from the mortar, it leaves behind salts that can crystallize and damage the masonry.
- Loss of Historic Fabric: The use of Portland cement can lead to the loss of historic fabric. When repairs are made with Portland cement, the original lime mortar is often removed, which can irreparably damage the historic integrity of the building.
The Advantages of Lime Mortar for Masonry Repairs
In recent years, there has been a growing recognition of the shortcomings of Portland cement for historic masonry repairs. As a result, many preservationists are advocating for a return to the traditional lime mortar. Lime mortar offers several advantages over Portland cement for historic buildings:
- Compatibility: Lime mortar is softer and more flexible than Portland cement, making it more compatible with historic masonry. It allows the building to move and breathe, reducing the risk of stress and cracking.
- Breathability: Lime mortar is more breathable than Portland cement, allowing moisture to escape from the wall. This helps to prevent dampness, mold growth, and accelerated deterioration of the masonry.
- Self-Healing: Lime mortar has self-healing properties. Small cracks can repair themselves over time, as the lime reacts with carbon dioxide in the air to form calcium carbonate.
- Preservation of Historic Fabric: The use of lime mortar helps to preserve the historic fabric of the building. It allows the original lime mortar to remain in place, which is essential for maintaining the historic integrity of the structure.
- Aesthetics: Lime mortar can be matched to the color and texture of the original mortar, ensuring a seamless and aesthetically pleasing repair.
Types of Lime Mortar
There are several types of lime mortar available, each with its own unique properties. The most common types include:
- NHL (Naturally Hydraulic Lime) Mortar:
Source: Limestone with naturally occurring clay impurities.
Setting: Hardens through a combination of carbonation (reaction with air) and hydration (reaction with water).
Strength: Available in varying strengths (NHL 2, NHL 3.5, NHL 5), suitable for different applications and exposures. This is an imported product in North America.
Use: Ideal for general masonry repairs, repointing, and rendering, especially in damp or exposed conditions.
- PHL (Pozzolanic Hydraulic Lime):
Source: Non-hydraulic lime (lime putty) blended with pozzolans (volcanic ash, brick dust, etc.).
Setting: Hardens through both hydration and carbonation, similar properties to NHL and available Made in USA in comparable strengths.
Strength: Varies depending on the pozzolanic material and mix proportions.
Use: Ideal for general masonry repairs, repointing, and rendering, especially in damp or exposed conditions.
- Non-Hydraulic Lime Mortar (Lime Putty):
Setting: Hardens solely through carbonation, a slow process that requires air exposure.
Strength: The softest and most flexible of the three types.
Use: Primarily used for internal plastering, restoration of soft, historic brickwork, and creating breathable mortars.
The choice of lime mortar will depend on several factors, including the type of masonry, the exposure conditions, and the desired strength. It is important to consult with a qualified preservation professional to determine the most appropriate type of lime mortar for your project.
The Future of Lime Mortar
Lime mortar is increasingly being recognized as the superior choice for historic masonry repairs. Its compatibility, breathability, self-healing properties, and ability to preserve historic fabric make it the ideal material for preserving our architectural heritage. As more and more preservationists embrace the use of lime mortar, we can expect to see a resurgence in this traditional building material, ensuring that our historic buildings are preserved for future generations to enjoy.