Lime mortar

Lime mortar or torching is a masonry mortar composed of lime and an aggregate such as sand, mixed with water. It is one of the oldest known types of mortar, used in ancient Rome and Greece, when it largely replaced the clay and gypsum mortars common to ancient Egyptian construction.

With the introduction of Portland cement during the 19th century, the use of lime mortar in new constructions gradually declined. This was largely due to the ease of use of Portland cement, its quick setting, and high compressive strength. However, the soft and porous properties of lime mortar provide certain advantages when working with softer building materials such as natural stone and terracotta. For this reason, while Portland cement continues to be commonly used in new brick and concrete construction, its use is not recommended in the repair and restoration of brick and stone-built structures originally built using lime mortar.

Despite its enduring utility over many centuries (Roman concrete), lime mortar's effectiveness as a building material has not been well understood; time-honoured practices were based on tradition, folklore and trade knowledge, vindicated by the vast number of old buildings that remain standing. Empirical testing in the late 20th century provided a scientific understanding of its remarkable durability. Both professionals and do-it-yourself home owners can purchase lime putty mortar (and have their historical mortar matched for both color and content) by companies that specialize in historical preservation and sell pre-mixed mortar in small batches.

Lime comes from Old English lim ('sticky substance, birdlime, mortar, cement, gluten'), and is related to Latin limus ('slime, mud, mire'), and linere ('to smear'). Mortar is a mixture with cement and comes from Old French mortier ('builder's mortar, plaster; bowl for mixing') in the late 13th century and Latin mortarium ('mortar'). Lime is a cement which is a binder or glue that holds things together but cement is usually reserved for Portland cement.

Lime mortar appeared in Antiquity. The ancient Egyptians were the first to use lime mortar about 6,000 years ago; lime mortar was used, for example, to plaster the pyramids at Giza. In addition, the Egyptians also incorporated various limes into their religious temples as well as their homes. Traditional Indian structures were built with lime mortar, some of which are more than 4,000 years old (such as Mohenjo-daro, a heritage monument of Indus Valley civilization in Pakistan).

The Roman Empire used lime-based mortars extensively. Vitruvius, a Roman architect, provided basic guidelines for lime mortar mixes. The Romans created hydraulic mortars that contained lime and a pozzolan such as brick dust or volcanic ash. These mortars were intended to be used in applications where the presence of water would otherwise not allow the mortar to harden (carbonate) properly.

Lime mortar today is primarily used in the conservation of buildings originally built using it, but may be used as an alternative to ordinary portland cement. It is made principally of lime (hydraulic, or non hydraulic as explained below), water, and an aggregate such as sand. Portland cement has proven to be incompatible with lime mortar because it is harder, less flexible, and impermeable. These qualities lead to premature deterioration of soft, historic bricks so traditionally, low-temperature-fired lime mortars are recommended for use with existing mortar of a similar type or reconstruction of buildings using historically correct methods. In the past, lime mortar tended to be mixed on site with whatever sand was locally available. Since the sand influences the colour of the lime mortar, colours of pointing mortar can vary dramatically from district to district.

Hydraulic lime contains substances which set by hydration, so it can set underwater. Non-hydraulic lime sets by carbonation and so needs exposure to carbon dioxide in the air; the material cannot set underwater or inside a thick wall. For natural hydraulic lime (NHL) mortars, the lime is obtained from limestone naturally containing a sufficient percentage of silica and/or alumina. Artificial hydraulic lime is produced by introducing specific types and quantities of additives to the source of lime during the burning process, or adding a pozzolan to non-hydraulic lime. Non-hydraulic lime is produced from a high purity source of calcium carbonate such as chalk, limestone, or oyster shells.