Lime mortar

Lime mortar is a type of mortar. It was used in the construction of the vast majority of brick and stone buildings worldwide from ancient times until the widespread adoption of Portland cement in the late nineteenth century. It is still used today, for the repair of such buildings and occasionally for new construction.

Most data and information about lime mortars has been via hearsay, folklore, trade knowledge and the vast number of old buildings still standing and being maintained. It has only been in the last decade that scientific and empirical testing has proven lime mortar's durability and made it more widely known.

There are several types of lime mortar, detailed below.

Non-Hydraulic & Hydraulic

In the context of cement, the term 'hydraulic' means to 'harden under water'. Thus, a non-hydraulic mortar or cement will not harden under water; whereas a hydraulic mortar or cement will. Traditional lime mortar is non-hydraulic: that is, it will not harden under water in normal conditions.

Non-Hydraulic Lime

Non-hydraulic lime is primarily composed of calcium hydroxide (generally greater than 95%). Non-hydraulic lime is produced by the heating of sufficiently pure limestone (calcium carbonate) to between 954° and 1066°C, driving off carbon dioxide, to produce quicklime (calcium oxide). As well as calcium based limestone, dolomitic limes can be produced which are based on calcium magnesium carbonate. This is done in a lime kiln. The quicklime is then slaked – thoroughly mixed with water to produce lime putty (calcium hydroxide), or with less water to produce dry hydrated lime. The slaking process involved in creating a lime putty is an exothermic vigorous reaction which initially creates a liquid of a cream consistency. This then has to be matured for between 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 displays a physical property known as "thixotropic" which means that when a lime putty is physically 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 and apply. If left to stand following agitation a lime putty will slowly revert from a thick liquid back to a putty state. It is always advised that a lime mortar should be "knocked up" prior to its use.

Non-hydraulic lime is produced in two forms: hydrated lime and lime putty. A frequent source of confusion regarding lime mortar stems from the similarity of the terms hydraulic and hydrated, however the two terms, in this context, have different meanings. Hydrated lime is any lime other than quicklime, so can refer to either hydraulic (hardens underwater) or non-hydraulic (doesn't harden underwater) lime. Stored lime putty is always non-hydraulic (since hydraulic putty sets quickly after mixing) and, 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 less water is used, then the result is a dry material (any excess water escaping as steam in during heating). This is ground to make hydrated lime.

Hydrated non-hydraulic lime can be mixed with water to form lime putty. Before use it is usually left in the absence of carbon dioxide (usually under water) to mature. Putty can be matured for anything from 24 hours to many years, an increased maturation time improving the quality of the putty. There is however an argument that a lime putty which has been matured for an extended period eg 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 carbonation periods as well as lower compressive strengths.

Mix

Traditional lime mortar is a combination of lime putty and aggregate (usually sand). A typical 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 1/3rd of the volume of the sand. The lime putty when mixed at a 3 to 1 ratio, fill these voids to create a compact mortar. 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 1) The sand being poorly graded or with too small a particle size 2) The mortar being applied too thickly (Thicker coats increase the possibility of shrinkage, cracking and slumping)3) Too much suction from the substrate 4) High air temperatures or direct sunlight which force dry the mortar5) High water content in the lime mortar mix6) Poor quality or unmatured lime putty

Pozzolans can be added to the mix of lime mortar. These are substances which when combined with lime produce a hydraulic (cementitious) set. They include powdered brick, heat treated clay, silica fume, flyash, and volcanic materials. The chemical set imparted ranges from very weak to almost as strong as Portland cement.

etting

Non-hydraulic lime mortar sets by reaction with atmospheric carbon dioxide (commonly called 'carbonation'). This is in contrast to the setting of Portland cement and hydraulic lime which sets by reaction with water in the mix. The reaction with carbon dioxide produces calcium carbonate – the raw material used at the start of the process to create lime. This process is much slower than that in Ordinary Portland Cement (OPC); depending on the thickness of the mortar and climate conditions, an initial skin hardening may take from a couple of hours to several days to gain an initial hardness. This tends to delay construction progress. Although the setting process can be slow, the drying time of a lime mortar must be regulated at a steady 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 with damp hessian sheeting or sprayed with water to control the drying rates. The full drying and hardening of the thickness process can continue, albeit at a slower rate, for many years with the mortar continuing to gain strength.

One of the advantages of lime mortar is that in the event of cracking, mortar deeper in the join is exposed and reacts with the air to crystallise and bond across the crack, reducing the loss of strength.

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. 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.

Properties

Lime mortar is not as strong in compression as OPC mortar, but both are sufficiently strong for construction of non-high-rise domestic properties.

Lime mortar nor does it adhere as strongly to the masonry as OPC. This is an advantage with softer types of masonry, where use of cement in many cases eventually results in cement pulling away some masonry material when it reaches the end of its life.

Under cracking conditions, OPC breaks, whereas lime often produces numerous microcracks if the amount of movement is small. These microcracks then recrystallise on exposure to air, effectively self-healing.

Historic buildings are frequently constructed with relatively soft masonry units (eg soft brick and many types of stone), and minor movement in such buildings is quite common due to the nature of the foundations. This movement breaks the weakest part of the wall, and with OPC mortar this is usually the masonry. When lime mortar is used, the lime is the weaker element, and the mortar cracks in preference to the masonry. This results in much less damage, and is relatively simple to repair.

Scrapped lime mortar is simply chalk and sand, which are normal constituents of soil. Cement mortar on the other hand presents a disposal issue.

Lime mortar is more porous than cement mortars, and it wicks any dampness in the wall to the surface where it evaporates. Thus any salt content in the water crystallises on the lime, damaging the lime and thus saving the masonry. Cement on the other hand evaporates water less than soft brick, so damp issues are liable to cause salt formation on brick surfaces and consequent disintegration of bricks. This damp evaportion ability is widely referred to as 'breathability.'

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 mositure 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.

Hydraulic Lime

:"Main article: Hydraulic lime"In the context of lime or cement, the term 'hydraulic' means to 'harden under water'. 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. 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.

Lime with cement

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 & lime mixes have been widely used.
* 1:1:6 is a satisfactory mortar mix for various purposes, but behaves as cement based mortar.
* 1:2:9 has been found to be prone to premature failure, and is no longer recommended
* 3:1 + 5% cement makes a lime mortar that firms up quickly, avoiding delays to work, but the addition of cement has been found to be associated with premature failures, and the practice is no longer recommended.

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.