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.
There are several types of lime mortar, detailed below
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 to produce quicklime (calcium oxide). The quicklime is then slaked – thoroughly mixed with water to produce lime (calcium hydroxide).
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 are, in this context, definitely not interchangeable. Hydrated lime is a dry powder and can refer to either hydraulic or non-hydraulic lime. Lime putty, however, is always non-hydraulic and, as the name suggests, is in the form of a putty. 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 must be 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 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, however if hydrated lime is correctly stored and prepared it should produce a comparable result.
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.
Pozzolans can be added to the mix of lime mortar. These are substances which when combined with lime produce a cementatious i.e. hydraulic set. They include powdered brick, heat treated clay and volcanic materials. The chemical set imparted ranges from very weak to almost as strong as Portland cement.
Setting
Non-hydraulic lime mortar sets by reaction with atmospheric carbon dioxide. 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 slower than that in OPC; taking several days to gain an initial hardness. The 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 reduce the effect of the crack.
Properties
Lime mortar is not as strong in compression as OPC mortar, nor does it adhere as strongly to the masonry as OPC. The mortar is softer so more able to accommodate movement and thus less prone to cracking than OPC. The 'softness' and reduced adhesion of lime mortar can be beneficial when used with old bricks or masonry which tend to be themselves softer than modern bricks and prone to damage by harder mortars. To say nothing of it being capable of recycling. This softness is a virtue in that it creates a less hard environment to live, less echo but more important it can "breath" This is vital for all buildings. Condensation within the architectural environment is a serious problem.
Lime mortar or mortars containing lime are more water absorbent than OPC mortar, acting to draw damp out of the brickwork or stone.
Hydraulic Lime
In the context of 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 and is heated to a higher temperature than in the production of non-hydraulic lime. 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.
There are three strength grades for Natural Hydraulic Lime (NHL): 2, 3.5 and 5. These are termed feebly hydraulic, moderately hydraulic and eminently hydraulic. The compressive strength of NHL3.5 mixed 1:3 after 90 days will be in the region of 8.3N/mm².