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Lime mortar is a type of mortar composed of lime and an aggregate such as sand, mixed with water. It is one of the oldest known types of mortar, dating back to the 4th century BC and widely used in Ancient Rome and Greece, when it largely replaced the clay and gypsum mortars common to Ancient Egyptian construction.
With the introduction of ordinary portland cement (OPC) during the 19th century the use of lime mortar in new constructions gradually declined, largely due to portland's ease of use, quick setting, and high compressive strength. However the soft, porous properties of lime mortar provide certain advantages when working with softer building materials such as natural stone and terracotta. For this reason, while OPC continues to be commonly used in brick and concrete construction, in the repair and restoration of brick and stone-built structures originally built using lime mortar, the use of OPC has largely been discredited.
Despite its enduring utility over many centuries, 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. Only during the last few decades has empirical testing provided a scientific understanding of its remarkable durability.
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 which holds things together but cement is often reserved for Portland cement such as the common term lime-Portland cement.
Lime mortar today is primarily used in the conservation of buildings originally built using lime mortar, but may be used as an alternative to ordinary portland cement. It is made principally of lime (hydraulic, or non hydraulic), 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 the 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.
Lime is commonly available in the United States as Type S hydrated mason's lime (ASTM C 207). However, this lime product is intended to be added to Portland cement to improve workability and several other reasons. Type S lime is not reliable as a sole cement because some manufacturers fire the lime above 1,100° C which begins to cause vitrification of impurities in the lime. This over-burned lime is sometimes called dead-burned because lime fired at these temperatures loses reactivity leading to poor or no bond strength and will not hold up to freeze-thaw weather cycles.
Hydraulic lime sets by hydration so it can set under water. Non-hydraulic lime sets by carbonatation and so needs exposure to carbon dioxide in the air and cannot set under water or inside a thick wall. For natural hydraulic lime (NHL) mortars, the lime is obtained from lime stone 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 pozzolana to non-hydraulic lime. Non-hydraulic lime is produced from high purity source of calcium carbonate such as limestone or oyster shells.
Non-hydraulic lime is primarily composed of calcium hydroxide (generally greater than 95%). 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 — a hydrated lime (calcium hydroxide). Hydrated lime naturally turns back into calcium carbonate, the entire process called the lime cycle.
The slaking process involved in creating a lime putty is an exothermic reaction which initially creates a liquid of a cream 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. It is advised that a lime mortar should be "knocked up" prior to its use.
A frequent source of confusion regarding lime mortar stems from the similarity of the terms hydraulic and hydrated.
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 24 hours to many years, with an increased maturation time improving 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 less workable.
There is some dispute as to the comparative quality of putty formed from hydrated lime compared to 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" and 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 cannot 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 autogeneous healing (self healing) where some free lime dissolves in water and is redeposited in any tiny cracks which form.
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 OPC. The limestone used contains sufficient quantities of clay and/or silica. The resultant product will contain dicalcium silicate but unlike OPC 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.
There are three strength grades for natural hydraulic lime, laid down in the European Norm EN459; NHL2, NHL3.5 and NHL5. The numbers stand for the minimum compressive strength at 28 days in newtons per square millimeter (N/mm2). For example the NHL 3.5 strength ranges from 3.5 N/mm2 (510 psi) to 10 N/mm2 (1,450 psi). These are similar to the old classification of feebly hydraulic, moderately hydraulic and eminently hydraulic, and although different, some people continue to refer to them interchangeably. The terminology for hydraulic lime mortars was improved by the skilled French civil engineer Louis Vicat in the 1830s from the older system of water limes and feebly, moderately and eminently. Vicat published his work following research of the use of lime mortars whilst building bridges and roads in his work. The French company Vicat still currently produce natural cements and lime mortars.  Names of lime mortars were so varied and conflicting across the European continent that the reclassification has greatly improved the understanding and use of lime mortars.
Traditional lime mortar is a combination of lime putty and aggregate (usually sand). A typical modern lime mortar mix would be 1 part lime putty to 3 parts washed, well graded, sharp sand. Other materials have been used as aggregate instead of sand. The theory is that the voids of empty space between the sand particles account for a 1/3 of the volume of the sand. The lime putty when mixed at a 1 to 3 ratio, fill these voids to create a compact mortar. Analysis of mortar samples from historic buildings typically indicates a higher ratio of around 1 part lime to 2 part aggregate/sand was commonly used. A traditional coarse plaster mix also had horse hair added for reinforcing and control of shrinkage, important when plastering to wooden laths and for base (or dubbing) coats onto uneven surfaces such as stone walls where the mortar is often applied in thicker coats to compensate for the irregular surface levels.
If shrinkage and cracking of the lime mortar does occur this can be as a result of either
Hair reinforcement is not found in lime mortars, but is common in lime plaster and many types of hair and other organic fibres can be found in historic plasters. However, organic material in lime will degrade in damp environments particularly on damp external renders. This problem has given rise to the use of polypropylene fibres in new lime renders 
Usually any dampness in the wall will cause the lime mortar to change colour, indicating the presence of moisture. The effect will create an often mottled appearance of a limewashed wall. As the moisture levels within a wall alter, so will the shade of a limewash. The darker the shade of limewash, the more pronounced this effect will become.
A load of mixed lime mortar may be allowed to sit as a lump for some time, without it drying out (it may get a thin crust). When ready to use, this lump may be remixed ('knocked up') again and then used. Traditionally on building sites, prior to the use of mechanical mixers, the lime putty (slaked on site in a pit) was mixed with sand by a labourer who would "beat and ram" the mix with a "larry" (a wide hoe with large holes). This was then covered with sand and allowed to sit for a while (from days to weeks) - a process known as 'banking'. This lump was then remixed and used as necessary. This process cannot be done with OPC.
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Addition of lime to cement mortar makes the mixture thicker and stickier while wet. Addition of cement to lime mortar acts as a pozzolan giving some degree of quick set, but this comes at a price, and is not recommended.
Three cement-sand-lime mixes have been widely used.
1:2:9 and 1:1:6 are frequently mistaken for lime mortars, but are considered cement mortars, as although they contain substantial amounts of lime, their properties are primarily those of cement mortar. 1:1:4 ( lime + O.P.C + sand ) is the usual mix for the experienced builder. Note: this calculation is based on weight, not volume.