Mortar, an integral component in architectural stonemasonry, is a workable paste used to bind building blocks such as stones, bricks, and concrete masonry units together, fill and seal the irregular gaps between them, and sometimes add decorative colours or patterns in masonry walls. In its broadest sense, mortar includes pitch, asphalt, and soft mud or clay, as used between mud bricks. The word “mortar” comes from Latin mortarium, meaning crushed.
Mortar has been used in construction for thousands of years and continues to be a fundamental part of modern building techniques. Despite its seemingly simple composition, the science and art of mortar in architectural stonemasonry is a complex field, with a vast range of types, compositions, and uses. This article will delve into the depths of this fascinating material, exploring its history, types, composition, and role in architectural stonemasonry.
Historical Overview of Mortar
The use of mortar dates back to ancient times, with evidence of its use found in the pyramids of Egypt, the Great Wall of China, and the Roman Colosseum. These early mortars were made from mud or clay, or a mixture of lime and sand. The Romans were the first to discover the binding properties of volcanic ash with lime to create a hydraulic mortar, a precursor to modern cement.
Throughout the Middle Ages, the use of mortar in construction became more sophisticated, with builders discovering the benefits of adding substances like horse hair to prevent cracking. The Industrial Revolution brought about the invention of Portland cement, a stronger and more reliable form of mortar that is still widely used today.
The Role of Mortar in Ancient Architecture
In ancient architecture, mortar played a crucial role in the construction of monumental structures. The Egyptians used a simple form of gypsum mortar in the building of the pyramids, while the Greeks utilized a lime-based mortar in their architecture. The Romans, however, were the true pioneers in the use of mortar, creating a hydraulic mortar that could set under water, which was used in the construction of their extensive aqueduct systems.
The use of mortar in these ancient civilisations not only provided a means of bonding stones together, but also added structural integrity to their buildings. The flexibility of mortar allowed for the absorption of stress and movement within the structure, preventing the collapse of the building during events such as earthquakes.
The Evolution of Mortar in the Middle Ages and Beyond
During the Middle Ages, the use of mortar evolved with the introduction of new materials and techniques. Builders began to add organic materials, such as horse hair, to the mortar mix to prevent shrinkage and cracking as the mortar dried. This period also saw the introduction of pozzolanic mortars, which contained volcanic ash that reacted with the lime to form a strong, durable bond.
The Industrial Revolution marked a significant turning point in the history of mortar. The invention of Portland cement in the 19th century provided a stronger, more reliable form of mortar that could be produced on a large scale. This cement-based mortar remains the most common type used in construction today.
Types of Mortar
There are several different types of mortar used in architectural stonemasonry, each with its own properties and uses. These include lime mortar, cement mortar, gauged mortar, and aerated cement mortar. The type of mortar chosen for a particular project depends on the nature of the building materials, the environmental conditions, and the desired aesthetic effect.
Lime mortar, made from slaked lime and sand, is one of the oldest types of mortar and is known for its flexibility and breathability. Cement mortar, made from Portland cement, sand, and water, is stronger and more durable, but less flexible and breathable than lime mortar. Gauged mortar is a combination of lime and cement, offering a balance between strength, flexibility, and breathability. Aerated cement mortar, which contains a foaming agent, is lightweight and has good thermal insulation properties.
Lime Mortar
Lime mortar is a type of mortar composed of lime and an aggregate such as sand, mixed with water. It is one of the oldest known types of mortar, dating back to the ancient Egyptian and Roman eras. Lime mortar is known for its flexibility and breathability, which makes it ideal for use in older buildings where movement and moisture are a concern.
The process of making lime mortar involves the burning of limestone or chalk, a process which converts the calcium carbonate into calcium oxide. This is then slaked, or combined with water, to create calcium hydroxide, also known as slaked lime. The slaked lime is then mixed with sand to create the final product.
Cement Mortar
Cement mortar is a type of mortar that consists of Portland cement, sand, and water. The cement acts as a binder, holding the sand particles together and allowing the mortar to harden into a strong, durable material. Cement mortar is known for its strength and durability, making it a popular choice for modern construction projects.
The process of making cement mortar involves the mixing of Portland cement and sand in a ratio that typically ranges from 1:3 to 1:5, depending on the desired strength and workability. Water is then added to the mix to create a workable paste. The mortar is then ready to be applied to the building materials.
Gauged Mortar
Gauged mortar is a type of mortar that combines lime and cement in a single mix. This type of mortar offers the benefits of both lime and cement, providing a balance between strength, flexibility, and breathability. Gauged mortar is often used in restoration work, where the properties of lime mortar are desired, but the additional strength of cement is needed.
The process of making gauged mortar involves the mixing of slaked lime, Portland cement, and sand. The ratio of lime to cement can vary depending on the specific requirements of the project, but a common ratio is 1:1:6 (lime:cement:sand). Water is then added to create a workable paste.
Aerated Cement Mortar
Aerated cement mortar is a type of mortar that contains a foaming agent, which creates small air pockets in the mortar. This results in a lightweight mortar with good thermal insulation properties. Aerated cement mortar is often used in the construction of lightweight concrete blocks and panels.
The process of making aerated cement mortar involves the mixing of Portland cement, sand, water, and a foaming agent. The foaming agent, when mixed with water, creates a foam that is then mixed with the cement and sand. The resulting mortar is lightweight and has a high volume, making it ideal for use in lightweight construction.
Composition of Mortar
The composition of mortar can vary greatly depending on the type of mortar and the specific requirements of the project. However, all mortars consist of a binder (such as lime or cement), an aggregate (such as sand), and water. The binder acts as a glue, holding the aggregate particles together and allowing the mortar to harden. The aggregate provides bulk and strength to the mortar, while the water allows the mortar to be worked and shaped before it hardens.
The ratio of binder to aggregate can have a significant impact on the properties of the mortar. A higher proportion of binder results in a stronger, more durable mortar, but can also make the mortar less workable and more prone to cracking. A higher proportion of aggregate makes the mortar more workable and less prone to cracking, but can also reduce its strength and durability.
Binders
The binder in mortar is the material that holds the aggregate together and allows the mortar to harden. The most common binders used in mortar are lime and cement. Lime, made from burnt limestone or chalk, is a traditional binder that has been used for thousands of years. It is known for its flexibility and breathability, which makes it ideal for use in older buildings. Cement, made from limestone and clay, is a modern binder that is known for its strength and durability.
Other binders that can be used in mortar include gypsum and clay. Gypsum, made from the mineral gypsum, is a soft binder that sets quickly and is often used in plaster and drywall. Clay, made from decomposed rock, is a weak binder that is often used in earthen construction.
Aggregates
The aggregate in mortar is the material that provides bulk and strength to the mortar. The most common aggregate used in mortar is sand. Sand, made from crushed rock or mineral, is a strong, durable aggregate that is widely available and inexpensive. The size and shape of the sand particles can have a significant impact on the properties of the mortar, with larger, rounder particles providing more strength and smaller, sharper particles providing more workability.
Other aggregates that can be used in mortar include crushed stone, gravel, and recycled materials. Crushed stone and gravel are strong, durable aggregates that are often used in concrete and masonry. Recycled materials, such as glass and plastic, are lightweight aggregates that can be used in lightweight and sustainable construction.
Water
Water is a crucial component of mortar, as it allows the mortar to be worked and shaped before it hardens. The amount of water used in the mortar can have a significant impact on the properties of the mortar. Too much water can weaken the mortar and make it prone to cracking, while too little water can make the mortar difficult to work and can prevent it from bonding properly with the building materials.
The quality of the water used in the mortar can also affect the properties of the mortar. Water that contains impurities, such as salts or acids, can weaken the mortar and cause it to deteriorate over time. Therefore, it is important to use clean, potable water when making mortar.
Role of Mortar in Architectural Stonemasonry
Mortar plays a crucial role in architectural stonemasonry, serving both functional and aesthetic purposes. Functionally, mortar is used to bond stones together, fill gaps between stones, and distribute loads evenly across the structure. Aesthetically, mortar can be used to add colour and texture to a masonry wall, and to create decorative patterns.
The choice of mortar can have a significant impact on the performance and appearance of a masonry structure. A well-chosen mortar can enhance the durability and longevity of the structure, improve its resistance to weather and seismic activity, and contribute to its overall aesthetic appeal.
Bonding Stones
One of the primary functions of mortar in architectural stonemasonry is to bond stones together. The mortar acts as a glue, holding the stones in place and preventing them from moving or shifting. This is particularly important in structures that are subject to loads or stresses, such as walls and arches.
The strength of the bond between the stones is determined by the type and composition of the mortar, the size and shape of the stones, and the technique used to apply the mortar. A strong, durable mortar, combined with properly sized and shaped stones and a good application technique, can result in a strong, stable structure.
Filling Gaps
Mortar is also used to fill gaps between stones in a masonry structure. These gaps, known as joints, can vary in size and shape depending on the size and shape of the stones. Filling the joints with mortar helps to seal the structure against water and air infiltration, and helps to distribute loads evenly across the structure.
The size and shape of the joints can have a significant impact on the performance of the mortar. Larger, wider joints require a more workable mortar that can be easily packed into the joint, while smaller, narrower joints require a less workable mortar that can be easily applied with a trowel or pointing tool.
Distributing Loads
Mortar plays a crucial role in distributing loads evenly across a masonry structure. When a load is applied to a masonry wall, the load is transferred from stone to stone through the mortar. The mortar acts as a cushion, absorbing and distributing the load and preventing the stones from cracking or breaking.
The ability of the mortar to distribute loads depends on its strength and elasticity. A strong, rigid mortar can distribute loads more effectively, but may be prone to cracking under heavy loads. A more elastic mortar can absorb and distribute loads more evenly, but may not be as strong or durable.
Adding Colour and Texture
In addition to its functional roles, mortar can also serve an aesthetic role in architectural stonemasonry. By varying the type and composition of the mortar, and the technique used to apply it, it is possible to create a wide range of colours and textures in a masonry wall.
The color of the mortar can be influenced by the type of binder and aggregate used, and by the addition of pigments. The texture of the mortar can be influenced by the size and shape of the aggregate, and by the technique used to finish the mortar. By carefully selecting and combining these elements, it is possible to create a unique and distinctive look for a masonry structure.
Conclusion
Mortar is a fundamental component of architectural stonemasonry, with a history that spans thousands of years and a range of types and compositions that reflect the diverse needs and challenges of construction. From the ancient Egyptians to the modern builders, the use of mortar has been a constant, evolving with the times and adapting to new materials and techniques.
Today, the science and art of mortar in architectural stonemasonry is a complex field, with a vast range of types, compositions, and uses. Despite its seemingly simple composition, the role of mortar in architectural stonemasonry is anything but simple, serving both functional and aesthetic purposes and contributing to the strength, durability, and beauty of our built environment.
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