Mortar
Mortar, in various forms, has been used for thousands of years and has been fundamental to the development of masonry construction. The ancient Egyptians used gypsum-based mortars, while the Romans, Phoenicians, and Greeks refined sand–lime mortars of varying strength and permeability by adding pozzolanic materials. In contrast, the Inca civilisation largely dispensed with mortar, achieving structural stability through the remarkable precision of their ashlar masonry joints. Recent research in Scotland has revealed that clay based mortars were extremely common prior to 1800.
Clay had the advantage of being sourced locally. It would soaked overnight before being pummeled, probably by hand, into a smooth paste. It would be mixed with sand in a 1 to 3 ratio, sometimes with a lime additive. It is thought to have been robust, permeable, flexible and a good insulator. Research suggests that many dry stone structures may not have been so, at all, and only appear dry because the clay has long gone from the joints.
Certainly, Lime mortars were also used in Scottish historical construction, although a little less is known about their precise composition or production. In the late eighteenth century, the English engineer John Smeaton pioneered the reintroduction of hydraulic limes for waterproof construction. Portland cement was not developed until 1824.
The lime process begins with quarrying chalk limestone (calcium carbonate). This is fired in a kiln at high temperature (typically around 900°C), driving off carbon dioxide and producing calcium oxide, or quicklime, a highly reactive material. From this point, several pathways are possible. Adding a small amount of water slakes the quicklime to form hydrated lime (also known as fat lime, slaked lime or air lime) in powder form, while adding excess water produces lime putty. An alternative and more hazardous approach is 'hot-liming', carried out on-site, in which quicklime is mixed directly with sand and water. This produces an intense exothermic reaction during slaking. In all of these cases, sand is then added, and the end product is a soft, workable paste in which calcium hydroxide is the key ingredient. Once applied, the mortar cures gradually through the absorption of atmospheric carbon dioxide, a process that converts calcium hydroxide back into calcium carbonate (limestone, again). This binds the sand and stone together. The curing process typically requires a minimum of 90 days. The resulting material is relatively weak, flexible, and porous.
Hydraulic Lime is a little different. Silicate impurities, naturally present in the original limestone, subjected to the same process, produce a harder, stiffer, more durable material, which may also become water-resistant when the level of impurity is high. This product is known as Natural Hydraulic Lime (NHL). There is an alternative approach; once hydrated lime (air-lime) has been produced, impurities such as pozzolans or brick dust can be added to the end product to achieve a controlled increase in strength and water resistance. These 'guaged' additives enable a rapid set through a chemical reaction with water rather than with air. This second hydraulic-lime approach was pioneered by the Romans, who even developed mortars capable of setting underwater. The key hardening component is Calcium Silicate Hydrate Gel (also present in Portland Cement)
For centuries, the preparation and application of lime mortar was a highly specialised craft, transmitted through generations of masons. A wide variety of grades were adjustable, producing mortars with differing degrees of hardness and resistance to moisture. They did not have the science to explain their experience, but they proved that certain mixes and different sources worked best for particular situations. From approximately 1850 onwards, increasing construction demands led to the gradual standardisation of mortar mixes, diminishing the role of skilled craftsmanship.
Analysis of historic mortars seems to suggest that in Scotland, as much as 75% of pre-1800 mortar was clay based - particularly in traditional local vernacular structures. However, both air limes and hydraulic limes were certainly employed during the medieval period. Pozzolan type additives were were rare. Remnants of horsehair, ox blood, pig blood and animal fats have been found in English mortars, but less often in Scotland. Hydraulic lime mixes would have been essential for bridges , at least at the waterline and below.
Contemporary conservation practice emphasises the principle that mortar should be softer than the masonry units it binds, thereby functioning as a sacrificial material. Mortars that are excessively strong can induce spalling and delamination of the stone, accelerating material deterioration. It is also now recognised that flexibility is a critical characteristic of historic masonry, allowing for minor movements at the joints without structural damage. Moreover, the porous nature of softer lime and clay mortars facilitate the diffusion of water-vapour, thereby reducing the risk of freeze–thaw damage. Conversely, hard, dense, and less vapour-permeable materials—such as highly hydraulic limes and Portland cement ( which is very hard, impermeable and inflexible) —can be particularly harmful to historic stonework and should generally be avoided. Mortars that are excessively soft, however, may render them structurally ineffective or susceptible to being washed away. Achieving an appropriate balance between strength, flexibility, and permeability is therefore essential, especially in situations requiring enhanced water resistance as on bridge masonry.
Page last updated Dec. '25