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.
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.).
Hydrated (Aerial) Lime Mortars
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.
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.
Hydraulic (Pozzolanic) Lime Mortars
Mortars containing only lime and sand harden slowly in contact with the air (carbonation). Because their hardening is slow and they can't harden underwater at all, they were unsuitable in very damp environments or for specialist applications such as for marine works, ports, canals, water tanks etc.
The Phoenicians (2500 - 500 B.C.) were first to discover the recipe of a mortar capable of hardening in contact with water or even underwater - such mortars being called hydraulic mortars. This was achieved by mixing the lime with volcanic ashes or sands, known as natural pozzolans. If volcanic sands were not available, they used powdered tiles or pottery fragments (known as cocciopesto), which produced a similar effect. 1Blezard, R. G. (2003). The History of Calcareous Cements. In Lea’s Chemistry of Cement and Concrete (pp. 1–23). Elsevier Ltd.
The Greeks then the Romans have perfected the use of pozzolans. Pozzolanic lime mortars have been extensively used by the Romans in very demanding environments including sewers, ports, spas and aqueducts. For structural waterproofing – for harbour works, baths, basements and foundations – the sand has been partially or fully replaced with pozzolans or cocciopesto.
Cocciopesto plasters have also been extremely popular in Venice, well suiting the humid and aggressive environment of the Venetian lagoon. Many palaces of the old Venetian Republic have been built and plastered with cocciopesto (lime and cocciopesto mix) which, while developing good mechanical strength, have also retained the full breathability of 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.
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 favor 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
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 pozzolanic 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.
Natural Hydraulic Limes (NHL)
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. 2Trout, E. A. R. (2019). The history of calcareous cements. In Lea’s Chemistry of Cement and Concrete (pp. 1–29). Elsevier.
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.
Natural Cements (19c Roman Cement)
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) 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.
Water Resistance vs Breathability
Here is how the various plasters compare.
Modern plasters trade water resistance for breathabiity.
- On one end of the scale we have hydrated (aerial) lime plasters with maximum breathability and minimum water resistance.
- On the other end of the scale we have Portland cement with minimum breathability and good water resistance.
- In between the two we have the various strength NHL plasters trading water resistance for breathability, Increasing the amount of of impurities (higher clay content) results in stronger but less breathable NHL plasters.
Traditional Roman pozzolanic lime plasters (green) are no-compromise solution. They offer full breathability and water resistance at the same time, keeping old buildings breathable while lasting a long time in the presence of water and salts, even under the most demanding circumstances.
- The development of lime mortars throughout history occurred along two distinct lines, these being developed for very different applications: 1. lime putty mortars for general use, hardening only on air; 2. hydraulic lime mortars for wet environments, able to harden both on air and underwater.
- The oldest hydraulic waterproofing mortars in history (invented by the Phoenicians and perfected by the Romans) were the pozzolanic lime mortars, a mix of lime putty and volcanic ashes. These mortars, despite being waterproof, retained the full breathability of lime plasters.
- During medieval times the quality of lime plasters declined, followed by a revival during the Renaissance.
- Starting with the Industrial Revolution a gradual transition occurred from "traditional" to "modern"; from lime through NHL to cement. Increasing the firing temperatures resulted in harder, less porous, less breathable materials - many of them being non-compatible with older buildings.
- Not knowing nor understanding the fundamental differences between older and newer materials can result in incorrect renovations that within a few short years or decades can irreversibly damage old buildings.