Some of the most iconic structures from history are made using lime mortars or Roman cement. The Great Pyramid of Giza, the Parthenon, and the Colosseum are just a few examples of the beautiful and enduring structures built with this versatile material. But how did prehistoric people first discover limestone, and how did it change how they built their homes?

Limestone is a sedimentary rock formed over time from the remains of marine organisms. Early humans discovered limestone in its natural state as a soft stone that was easy to shape and carve. They quickly realized that limestone could be used for more than just surrounding fires; they used it to build durable homes and megalithic structures. The discovery of lime mortar changed the way humans built their homes, and it also had a profound impact on the development of civilization.

The use of hot lime mortar in construction dates back thousands of years, with the first mentions appearing in 6500 BC. Lime mortars were used extensively in the construction of the Indus Valley Civilization and in ancient Egypt and Greece. The Roman Empire also extensively used lime mortars, which helped give their buildings lasting strength and stability. During the early Middle Ages, few records of lime were being used in construction, mainly because most people could not read or write. However, the alchemists of the time were aware of the acidic properties of lime and used it for soap-making. Lime was later rediscovered when masonry walls began to be constructed. No evidence suggests any lime kilns in England before the Roman period. However, by the end of the Middle Ages, lime was widely used in construction once again, thanks to its versatility and strength.

Lime Production

Lime mortar is made by heating limestone (a rock rich in calcium carbonate) to a high temperature. This process produces quicklime (calcium oxide), which is then mixed with water to form lime mortar. Lime kilns were usually built near sources of limestone, such as quarries, and were powered by wood or coal. The use of lime mortar declined after the introduction of Portland cement in the 19th century. However, there has been a recent revival in lime mortar, as it is more environmentally friendly than Portland cement. There are now over 200 lime kilns in the UK.

Lime kilns have been a part of the British landscape for centuries, serving an essential role in the agricultural process. Before the Industrial Revolution, lime was primarily used for agricultural purposes, spread on fields to neutralize the soil and improve crop yields. With the advent of the industrial age, lime began to be used in various industries, including iron and steel production, paper manufacture, and construction. Today, there are over 200 lime kilns in the United Kingdom, many of which are still in use. While they may no longer play a prominent role in the industry, they remain an essential part of Britain’s history and heritage.

Past Research into Lime Mortars

In 1678, Joseph Moxon described a hidden fire in lime that appears with the addition of water. Since the 17th century, lime has been the subject of various studies by Englishman Black and Frenchman Lavoisier. They studied and wrote about the chemical reactions involved in the processing of lime in the 1800s. In the 19th century, Debray and Lechatelier discovered other qualities and added them to their work. Besides being used as a binding item in structures, ancient civilizations used lime for plasters and decorations, including paintings, marble imitations, ornaments, and frescoes linking it into a world of arts and crafts. It’s considered a traditional building material and was predominantly used until the 19th century. However, with the industrial revolution came new materials, such as cement, which resulted in a decline in the use of lime. There has been a renewed interest in lime and its potential benefits in recent years due to its environmentally friendly properties.

Engineer John Smeaton, born at Austhorpe lodge near Leeds, conducted a series of experiments in 1756 while preparing to build the Eddystone lighthouse; he did this to help them overcome the problems of building out in the sea, he carried out research into creating an effective mortar that would withstand the severe conditions and have a much faster setting time, John Smeaton patented the first hydraulic lime.
In 1796 James parker patented a natural hydraulic cement known as Roman Cement. This cement was created by burning Septaria made from calcined nodules of argillaceous limestone from the Thames estuary; he then later sold the patent to Samuel Wyatt, who, with his cousin, Charles Wyatt, produced cement in the name of Parker & Wyatt; Parker emigrated to America in 1797 and passed away shortly after.

From 1807 several people experimented with making artificial versions of this cement; in 1813, Vicat created an artificial hydraulic cement in France, James Frost introduced the same procedure in England, where he had about 20 patents from 1811 to 1822, including one for British Cement.

Hydraulic Lime Mortar

In the past, lime mortar for building masonry was produced by calcining limestone in a kiln to drive off carbon dioxide and leave calcium oxide. This ‘quicklime’ was then slaked with water to form calcium hydroxide (‘putty’), which was used to make mortars. The terms’ fat’ and ‘lean’ describe the proportions of sand and lime in a mortar. A ‘fat’ mortar has more lime and is softer, while a ‘lean’ mortar has more sand and is more arduous. Adding water to a lime putty makes it fluid and workable. Still, it also starts the process of carbonation, whereby the calcium hydroxide reacts with carbon dioxide in the air to form calcium carbonate. This process continues even after the mortar has set, making it more complex over time. Hydraulic limes (NHLs) are made by adding clay or other minerals to quicklime before it is slaked. This enables the NHL to set under water or in contact with damp masonry, making it ideal for wet or exposed conditions. Depending on their intended use, NHL mortars can be either fat or lean. While NHLs are suitable for many conservation applications, they should not be used where a highly breathable mortar is required, such as in Lime washing or similarly breathable renders or plasters.

New research into the effects of NHLs on historic masonry has raised several concerns among conservationists. One of the main problems is that the data collected so far is unreliable, as it is difficult to assess the long-term effects of NHLs on stone accurately. In addition, it is now clear that NHLs can significantly increase the strength of masonry, which could lead to structural problems if misused. Another issue is that NHLs can cause staining of historic masonry, which can be challenging to remove and may damage the stone. As a result, it is essential to use NHLs with care and consider all potential impacts before using them on historic buildings.

Hydraulic Lime mortar leaching out through the wall in Richmond, North Yorkshire

The History of Portland Cement

Joseph Aspdin was a Leeds bricklayer credited with the invention of Portland cement. In the early 19th century, Aspdin set up a home and business in the Slip Inn yard, now known as the Packhorse Inn yard. This ancient passage linked Briggate with Lands Lane in the centre of Leeds. Here, Aspdin began experimenting with improvements to Roman cement in everyday use. He would burn limestone and clay at a high temperature, let it cool, and then grind and mix it with water. This created a robust and fast-setting mortar that he named Portland cement.

When Aspdin was satisfied with the results, he patented it in 1824 and decided to go into manufacturing; he then moved to Wakefield with his family and opened a factory in Kirkgate.
The first time Portland cement was used on a large scale was for building the Thames tunnel in the mid-1820s; this was opened to the public in 1843.
Lime mortar and natural cement were still widely used at this point, sometimes even mixed; it was only during and after the war that cement and gypsum-based products started appearing in all construction; this was due to the lack of skilled masons and the need for rapid building at the time. Though Aspdin’s invention was met with some initial scepticism, it eventually caught on and became the standard for making concrete. Today, Portland cement is one of the world’s most commonly used building materials.

Cavity Walls

The cavity wall was first introduced in the late 19th century to improve buildings’ insulation. The cavity wall consists of two divisions with a space between them, and insulation is placed in the area to help keep the building warm. The walls are held together with ties, which are placed in the mortar between the bricks. Cavity walls are now used in most modern facilities as they provide good insulation and help to reduce energy costs.

A cavity wall is composed of two layers, with airspace between them. The cavity is typically filled with insulation, which helps to keep heat from transferring from the interior to the home’s exterior. In addition, the cavity wall is designed to collect any moisture that may enter the house, stopping it from transferring to the internal walls. The ties that hold the two layers of the fence are zagged so that water droplets fall into the cavity, and the insulation is pinned to the internal wall, leaving a void for water droplets to fall and air to circulate. Air vents are also installed to allow air movement through the cavity, which helps to prevent moisture buildup.

Damp Proof Courses

Damp-proof courses were first introduced in 1875 to prevent moisture damage to buildings. Initially, engineering brick and slate mixed with lime mortar were used, but these materials were eventually replaced with bitumen and plastic. While damp-proof courses are now an essential part of the construction, they have also been controversial. Some experts believe they can encourage dampness by trapping moisture in the walls, leading to problems like mould and mildew. Others argue that modern damp-proof course materials are not as effective as traditional ones and that they can even cause structural damage to buildings over time. Whatever the case, it is clear that damp-proof courses have played a significant role in construction history and will continue to do so for many years.

Solid wall construction

The solid wall construction method is a traditional way of building in which a wall has no cavity and usually no damp-proof course. This means that the walls are not relying on modern materials, and moisture needs to move freely between the walls. This construction method was commonly used in older homes and is still used in some parts of the world today. The solid wall construction method is known for its durability and its resistance to fire. However, this construction method is also known for its poor insulation properties. This can be a problem in colder climates where heat loss through the walls can be a significant issue. In recent years, some advances in solid wall construction methods have addressed this issue by using insulation materials that are better at trapping heat. However, the solid wall construction method is still not as popular as it once was due to its poor insulation properties.

Moisture and air movement in a traditional building

The Problem With Cement-Based Mortars in Traditional Homes

The construction of older buildings often uses materials that are not compatible with modern cement-based renders and mortars. As a result, using these materials in older buildings can cause several problems, including the erosion of masonry joints and units. In addition, the use of modern cement-based renders and mortars can also lead to an increased risk of dampness and mould growth. As a result, it is essential to be aware of the potential risks before using these materials on older buildings.

Traditional buildings rely on porous materials and capillary action. Permeable materials like stone and porous brickwork with lime plaster and lime washes or clay paints allow moisture to be absorbed through them, the water is then transferred into the masonry walls, and capillary action draws it out through the mortar joints; it is essential to use lime plaster on your walls with permeable paints or lime washes and not silk or plastic paints otherwise you will create a barrier where condensation can quickly form, good air circulation inside and outside your building aids capillary movement.

There are micro cracks in cement mortars. These form when they shrink and harden. They then turn into Hairline cracks in your render or mortar due to freezing, thaw and thermal expansion; they will then draw in water and become trapped behind the material and can cause damp patches to appear in your building. This is because there is no capillary movement to draw it back out, which then causes further erosion to the masonry.
Timber and earthen mortar constructed buildings will also suffer from structural damage if the moisture builds up behind the cement; in no circumstances should you put impermeable materials on these buildings.

Historic England prepared a long and in-depth presentation about mortars. Below you can find links to these presentations so you can dive even deeper into the history and technical aspects of mortars.

Mortars for Conservation Part 1

Mortars for Conservation Part 2