Mortar is a building material that has always been assumed to exist. The first mortar dates back to the birth of man in the form of rudimentary mixtures of clay and mud. The real birth of mortars, however, is linked to the use of lime, the "standard" ingredient in the creation of mortars, and the discovery of firing, slaking and hardening of lime, which in the course of history, probably happened randomly.
The history of lime as a building material goes back for thousands of years According to recent archaeological excavations, the use of lime plasters goes back to at least 12,000 years. The use of fire as a tool made possible the transformation of raw materials into new materials (e.g. cooking food, clay to pottery, lime stone to plaster etc.).
The production of lime mortars involved the burning of limestone rocks at high temperatures (>700°C), first producing quicklime (burnt lime stone) in the process. Adding water to it (slaking) releases heat, decomposing the quicklime into a soft white paste: the lime putty. Mixing the lime putty with sand and water results in the well-known lime plaster, which in contact with the carbon dioxide from the air slowly hardens (carbonates) becoming durable. This process is known as the lime cycle, describing the transformation of hard limestone into a soft putty and lime plaster, which in contact with air hardens, returning back to limestone.
The history of lime continues with the great antique civilizations. Over the millennia, various civilization have refined the production techniques of common air lime mortars, until it become the fundamental building material, used for the construction of many great buildings during the Babylonians (3500 - 700 BC), the Egyptians, the Mayans, the Chinese and the Phoenicians.
Traditionally, lime mortars have often been prepared and applied hot. Hot-mixed mortars have not only a good workability but being very sticky they are less liable of being washed out by rain.
Air lime mortars containing only lime and sand harden slowly in contact with air (carbonation). Because their hardening is slow they were unsuitable for very damp environments or for specialist applications such as for marine works. The Phoenicians (2500 - 500 B.C.) were first to discover the recipe of mortars capable of hardening in contact with water - such mortars being called water limes, today mostly known as hydraulic limes.
The Romans have built ton he Phoenician discoveries, developing the technology of water limes to a whole new level. Being interested in creating long-lasting buildings, Roman builders have observed that mortars were the "weak link" in construction—in the presence of moisture mortars decaying much faster than bricks or stones. To formulate more durable mortars, they started experimenting with different sands and materials. Particularly, by mixing the lime with volcanic sands and ashes—known as natural pozzolans—or crushed tiles or pottery fragments—known as cocciopesto—resulted in some exceptional water limes, some of these being able to harden underwater.1Blezard, R. G. (2003). The History of Calcareous Cements. In Lea’s Chemistry of Cement and Concrete (pp. 1–23). Elsevier Ltd. http://dx.doi.org/10.1016/B978-075066256-7/50013-8
Over the course of several centuries, the Romans refined the use of volcanic sands and ashes (natural pozzolans) to an art form. From around 200 BC to 200 AD, a period of approximately 400 years, they made significant advancements in mortar technology through careful observation and experimentation.
Between the 2nd and 1st centuries BC, the Romans primarily used air-lime mortars, similar to those used today. Being soft and friable mortars, very few structures of that period have survived, mostly crumbling into ruin. As time progressed, the mortars became less friable and more resistant, with red pozzolans introduced during Julius Caesar's reign, a period that marks the beginning of Pax Romana (The Roman Peace), a period of golden age and relative peace. During this period the quality of mortars have evolved further, becoming superior and non-friable, typically in red or red-brown tones, with notable structures like the Colosseum built during this phase. By the turn of the 2nd century AD, mortars reached their peak durability, becoming free of earthy materials. These good quality mortars remained in use for another 300 years till the end of the Roman Empire.
Roman lime mortars have been extensively used by the Romans in challenging environments, including sewers, ports, spas, and aqueducts. These mortars were adapted to suit various applications and structural needs - different types of mortars being used in different areas of the buildings. For e.g. Roman limes used for waterproofing purposes in harbours, baths, basements and foundations were very rich in volcanic pozzolans or cocciopesto—the plain sand being replaced, either partially or entirely, with pozzolans or cocciopesto.
While many Roman mortars were harder than plain air lime mortars, they still remained significantly softer than the clay bricks or stones they held together. Unlike modern mortars, which often contain additives or industrial waste products (such as artificial pozzolans, often misleadingly named "eco-pozzolans"), Roman mortars were made entirely of natural, breathable materials. This allowed them to work harmoniously with other natural building materials, as evidenced by the remarkable longevity of many Roman structures that have stood for nearly 2,000 years.
Cocciopesto plasters have been widely used in Venice, perfectly suited to the humid and challenging conditions of the Venetian lagoon. Many palaces of the former Venetian Republic were built and plastered with a lime and cocciopesto mix, which not only provided excellent mechanical strength but also retained the breathability of traditional lime putty plasters.
The Romans carried their knowledge of the preparation of mortar with them to the most remote parts of their empire.
The Roman cement, as described by Roman architect Vitruvius in his writings “The Ten Books on Architecture”, is also an entirely lime-based mixture, made of fat lime, pozzolans, cocciopesto, sand and water.
After the fall of the Roman Empire in 476 AD, during the Middle "Dark Ages" the history of lime takes an interesting turn. The Middle Ages (5th - 15th centuries) saw a widespread and gradual decline in the quality of lime mortars. After a 400-year-old Romano–British civilization, during the following Germanic Anglo-Saxon culture, the use of Roman mortars were gradually abandoned and forgotten. The decline was due to the increased use of impure sands, poor mixing, low kiln temperatures (resulting in incomplete burning) and an absence of natural pozzolanic materials (e.g. volcanic sands). As a result, during the 9th, 10th and 11th centuries the art of burning lime was almost completely lost.
During this time, construction practices being overwhelmingly timber based, the use of timber-frame and beaten earth (cob) buildings has increased. Brick was also abandoned in the favour of stone.
From the 12th century onward, with the introduction of stone castles by the Normans, the demand for mortars has increased, improving their overall quality. Medieval mortars made of non-hydraulic lime weathered easily, but in large buildings such as castles, cathedrals and churches - held together by stones in compression - this was only a minor issue.
The Renaissance (15th - 16th century) saw a revival of culture and a renewed interest in ancient architecture,
In 1414 a copy of “The Ten Books on Architecture” from Vitruvius was rediscovered and its first printed edition was published in 1486 in Latin, being successively translated into Italian, German, French, Spanish and eventually into English. Many architects of that time, including Andrea Palladio (1508–1580), who had a marked influence on both the Italian and English architecture, all cited Vitruvius in their writings.
The Industrial Revolution opens a new chapter in the history of lime and its evolution as a building material. During the second half of the 1700s there was a renewed interest in Roman mortars because after so many centuries most buildings built with Roman lime mortars were still remarkably well preserved throughout Europe, despite harsh climatic conditions.
Natural pozzolans, including trass - a volcanic tuff from Germany resembling the Roman pozzolans - became an increasingly accepted addition to lime in England during this time for hydraulic works.
John Smeaton was the first in England to scientifically investigate why certain limes would harden underwater. Commissioned to replace an old lighthouse near Plymouth, in 1756 he started a series of experiments to find the right lime mortar that would withstand the battling of storms off the coast of Plymouth.
After testing 300 different lime stones, including "impure" lime stones naturally containing 6% - 20% clay, he discovered that the best raw material for a lime plaster that can set underwater (water lime) is, in fact, impure limestone that contains clay; clay being the ingredient that makes lime resistant to water.
He also looked into the ancient Roman practice of combining lime with volcanic pozzolans. He found that for his purposes the best results were given by a mix of clay and volcanic pozzolans (50% lime and 50% pozzolans) added to the lime.
The construction of the lighthouse was completed in 1759. Smeaton’s natural hydraulic lime (NHL) mixture was so successful that it became standard specification for government contracts for almost a century, eventually being replaced by Portland cement in 1867.2 Trout, E. A. R. (2019). The history of calcareous cements. In Lea’s Chemistry of Cement and Concrete (pp. 1–29). Elsevier. https://doi.org/10.1016/B978-0-08-100773-0.00001-0
Despite its success, the adoption of natural hydraulic limes was slow, and traditional mixtures of lime and pozzolans retained their supremacy for a long time.
The next milestone in the development of hydraulic materials was the discovery of natural cement by James Parker in 1796. His invention was important because it showed that by firing an impure, high-clay-content limestone at relatively low temperatures (800 – 1,100°C)3 1.Callebaut, K., Elsen, J., Balen, K. V. & Viaene, W. Nineteenth century hydraulic restoration mortars in the Saint Michael’s Church (Leuven, Belgium) Natural hydraulic lime or cement? Cem. Concr. Res. 31, 397–403 (2001). https://doi.org/10.1016/S0008-8846(00)00499-3 resulted in a burnt lime stone with hydraulic properties without the need for slaking. Once ground and mixed with water, the new cement mix hardened within 10-20 minutes, both above or underwater.
Parker's new product was brought to market in 1798; in its promotional pamphlet being called Roman cement, presumably for its hydraulic properties, hinting that it could replace the pozzolans of its time. The name Roman cement nevertheless was inappropriate and misleading, as the product in no way resembled the original fat lime - pozzolanic mix of Roman cement from antiquity.
Moreover, this cement becoming known under many other names - such as quick-setting cement, rapid cement or stucco-cement - just created further confusion.
When Parker’s patent expired in 1810, his manufacturing process spread throughout continental Europe, natural- or Roman cements becoming the leading cement during the first half of the 19th century.
Many buildings built during that time have benefited from this excellent cement – which half a century later, for economic reasons, was gradually "phased-out" and replaced with the much harder (and non-breathable) Portland cement.
In 1824 Joseph Aspdin, a bricklayer or builder from Leeds, patented the material “Portland Cement”, named after its similarity to Portland stone, a limestone quarried in Dorset (Isle of Portland) with high reputation for quality and durability. Aspdin’s early Portland cement was very different from today’s Portland cement, essentially being an artificial hydraulic lime created from limestone used for repairing the roads and clay.
Aspdin's youngest son, William, following his father’s footsteps, in the 1840s has accidentally discovered that clinkered or “overburnt” material significantly increased the strength of his cement, resulting in increased firing temperatures during the manufacturing process. An 1848 examination of William Aspdin’s cement shows that his Portland cement was 2.4 times stronger than the best quick-setting Roman cement.
Further changes in cement manufacturing took place slowly during the 20th century which include better quality control, the replacement of traditional chamber kilns with modern rotary kilns, transforming the cement industry to what we know today.
After a century, we have learned that cement is not the right material for old buildings, because it’s:
As a result, during the past two decades, Historic England and conservation authorities have been worked relentlessly to reinstate the use of limes in old buildings, leading to a Lime Revival, going back to the pre-cement era of the Industrial Revolution. The use of cement is increasingly being banned from the heritage sector, reinstating lime and other traditional breathable materials in old buildings.
The ancient Roman water limes with natural volcanic ingredients are also increasingly used for water related renovation projects, complementing the air limes, reinstating the full spectrum of lime for historic building projects.
Here is how the various plasters compare.
Here are some of our projects where we have dealt with some of the issues discussed on this page:
