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.US Park Service Preservation Brief 2
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.Peter Ellis, The Analysis of Mortar: The Past 20 Years, 2002 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.
Etymology 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')."Lime". Online Etymology Dictionary. Accessed 5/10/2014 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"Cement, n." def. 1. Oxford English Dictionary Second Edition on CD-ROM (v. 4.0) © Oxford University Press 2009 which is a binder or glue that holds things together but cement is usually reserved for Portland cement.
History
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. Traditional Indian construction used brick dust, called surkhi, as a pozzolan in their mortars.
Uses 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 bricksSickels-Taves, L. B. and Sheehan, M. S., "Specifying Historic Materials: The Use of Lime". Masonry: Opportunities for the 21st Century, ASTM STP 1432. D. Throop and R.E. Klingner, Eds., ASTM International, West Conshohocken, PA, 2002. p. 3, 9. Print. 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 and non-hydraulic lime 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.
Non-hydraulic lime Non-hydraulic lime is primarily composed of (generally greater than 95%) calcium hydroxide, Ca(OH)2. Non-hydraulic lime is produced by first heating sufficiently pure calcium carbonate to between 954° and 1066 °C, driving off carbon dioxide to produce quicklime (calcium oxide). This is done in a lime kiln. The quicklime is then slaked: hydrated by being thoroughly mixed with enough water to form a slurry (lime putty), or with less water to produce dry powder. This hydrated lime (calcium hydroxide) naturally turns back into calcium carbonate by reacting with carbon dioxide in the air, the entire process being called the lime cycle.
The slaking process involved in creating a lime putty is an exothermic reaction which initially creates a liquid of a creamy consistency. This is then matured for 2 to 3 months—depending upon environmental conditions—to allow time for it to condense and mature into a lime putty.
A matured lime putty is thixotropic, meaning that when a lime putty is agitated it changes from a putty into a more liquid state. This aids its use for mortars as it makes a mortar easier to work with. If left to stand following agitation a lime putty will slowly revert from a thick liquid to a putty state.
As well as calcium-based limestone, dolomitic limes can be produced which are based on calcium magnesium carbonate.
A frequent source of confusion regarding lime mortar stems from the similarity of the terms hydraulic and hydrated. Hydrated lime is any lime other than quicklime, and can refer to either hydraulic (hardens under water) or non-hydraulic (does not harden under water) lime. Lime putty is always non-hydraulic and will keep indefinitely stored under water. As the name suggests, lime putty is in the form of a putty made from just lime and water.
If the quicklime is slaked with an excess of water then putty or slurry is produced. If just the right quantity of water is used, the result is a dry material (any excess water escaping as steam during heating). This is ground to make hydrated lime powder.
Hydrated, non-hydraulic lime powder can be mixed with water to form lime putty. Before use putty is usually left in the absence of carbon dioxide (usually under water) to mature. Putty can be matured for as little as 24 hours or for many years; an increased maturation time improves the quality of the putty. There is an argument that a lime putty which has been matured for an extended period (over 12 months) becomes so stiff that it is difficult to work.
There is some dispute (Roman concrete) as to the comparative quality of putty formed from dry hydrated lime compared with that produced as putty at the time of slaking. It is generally agreed that the latter is preferable. A hydrated lime will produce a material which is not as "fatty”, being a common trade term for compounds have a smoother buttery texture when worked. Often, due to lengthy and poor storage, the resulting lime produced by hydrated lime will exhibit longer carbonatation periods as well as lower compressive strengths.
Non-hydraulic lime takes longer to set and is weaker than hydraulic lime, and should not be allowed to freeze before it is well set. Although the setting process can be slow, the drying time of a lime mortar must be regulated at a slow rate to ensure a good final set. A rapidly dried lime mortar will result in a low-strength, poor-quality final mortar often displaying shrinkage cracks. In practice, lime mortars are often protected from direct sunlight and wind with damp hessian sheeting or sprayed with water to control the drying rates. But it also has the quality of autogenous healing (self healing) where some free lime dissolves in water and is redeposited in any tiny cracks which form.
Oyster shell mortar
In the tidewater region of Maryland and Virginia, oyster shells were used to produce quicklime during the colonial period. Similar to other materials used to produce lime, the oyster shells are burned. This can be done in a lime rick instead of a kiln. Burning shells in a rick is something that Colonial Williamsburg and the recreation of Ferry Farm have had to develop from conjecture and in-the-field learning. The rick that they constructed consists of logs set up in a circle that burn slowly, converting oysters that are contained in the wood pile to an ashy powder. The burnt shell can then be slaked and turned into lime putty.
Mortars using oyster shells can sometimes be identified by the presence of small bits of shell in the exposed mortar joint. In restoration masonry, the bits of shell are sometimes exaggerated to give the viewer the impression of authenticity. Unfortunately, these modern attempts often contain higher than necessary ratios of Portland cement. This can cause failures in the brick if the mortar joint is stronger than the brick elements.
Hydraulic lime
Hydraulic lime sets by reaction with water called hydration.
When a stronger lime mortar is required, such as for external or structural purposes, a pozzolan can be added, which improves its compressive strength and helps to protect it from weathering damage. Pozzolans include powdered brick, heat treated clay, silica fume, fly ash, and volcanic materials. The chemical set imparted ranges from very weak to almost as strong as Portland cement.
This can also assist in creating more regulated setting times of the mortar as the pozzolan will create a hydraulic set, which can be of benefit in restoration projects when time scales and ultimately costs need to be monitored and maintained.
Hydraulic lime can be considered, in terms both of properties and manufacture, as part-way between non-hydraulic lime and Portland cement. The limestone used contains sufficient quantities of clay and/or silica. The resultant product will contain dicalcium silicate but unlike Portland cement not tricalcium silicate.
It is slaked enough to convert the calcium oxide to calcium hydroxide but not with sufficient water to react with the dicalcium silicate. It is this dicalcium silicate which in combination with water provides the setting properties of hydraulic lime.
Aluminium and magnesium also produce a hydraulic set, and some pozzolans contain these elements.