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Lime, which is derived from the raw materials limestone, chalk or coral (all rich in calcium carbonate), has proved itself throughout history and into the present day to be one of the most versatile, durable and adaptable building materials. It is mixed with sands and aggregates to form both a bedding and pointing mortar for masonry (stone, brick, etc.), and as a breathable, flexible internal and external protective finish for many building substrates. It is, in most instances, the suitable material to be used on period buildings.
A little history on the use of lime
Until the coming of the railways at the time of the industrial revolution, the production of lime was principally a localised industry. Each town or village, wherever there were suitable limestone deposits, would have had its own lime burning kilns. Every region produced distinct lime mortars. These encapsulated
the unique properties of the geological compositions of both the local limestone and the sands and aggregates added.
Lime, throughout history, has always been used on buildings of both formal, high architecture, as well as on the more simple dwellings of the rural populations. It was the material of choice for internal and external finishes because of its widespread availability (cement was only developed in 1824) and because of its characteristic ability to breathe and move in harmony with the building.
Why are cement and other non-breathable finishes not suitable for period buildings?
The use of cement rather than lime on traditional buildings, had, and continues to have, a disastrous effect on the health and overall structural integrity of the building fabric. There are two basic reasons why cement does not work well with historic buildings: firstly, due to its unforgiving strength, it is unable to accommodate the natural movement of the structure, and hence can crack and allow the ingress of moisture. Secondly, cement has a low permeability (it is unable to allow the free passage of liquid moisture or moisture vapour). Because of this, it does not allow the building fabric to breathe and relinquish any moisture that gets trapped behind the render/plaster façade.
Different types of lime
As has already been established, lime is produced from calcareous materials such as chalk, coral, shells, or limestone deposits. Chalk, corals and shells are composed of relatively pure calcium carbonate. Limestone deposits, however, vary in their purity. Some consist of pure calcium carbonate, whereas others are made up of calcium carbonate plus different amounts of impurities. These impurities consist of different types of clay minerals. The chemical composition of the parent material, combined with the method of production, will produce varying types of building limes, with different properties, for use in a wide variety of situations. As a general rule, lime produced from raw materials composed of relatively pure calcium carbonate, produce non-hydraulic lime (known as lime putty). This comes in a wet paste form. Lime produced from raw materials containing impurities produce hydraulic limes. There are varying strengths of natural hydraulic limes, the main ones being NHL 2, NHL 3.5 and NHL 5. These come in a powder form.
Some 'general builders' merchants supply a type of lime called 'hydrated' or 'bagged' lime. This also comes in a powder form. It is a non- hydraulic lime. It is thought by many, (ourselves included), to be an inferior product, because it has inferior settting and workability properties. It is therefore not suitable for use as a plaster, render or limewash.
What type of lime should I use?
Non-hydraulic lime putty is the most breathable and flexible lime. It is called non-hydraulic because it will not set in the presence of water, but sets solely by reacting with atmospheric CO2 through a process known as carbonation.
It is most suitable for use on the more flexible and porous walling materials such as cob, wattle and daub, soft brick and stone. It can be used externally where walls are fairly sheltered, and is ideal as an internal plaster. It should not be applied in temperatures of 5℃ or below (and where such temperatures are likely for 2-3 months after application), or in excessively wet conditions, as it will not carbonate (cure) effectively.
Natural hydraulic limes (NHLs) are so-called because they set primarily through a process of chemically reacting with water. This process is known as hydration. NHL 2 is the most breathable and flexible of the natural hydraulic limes , and NHL5 the least. NHLs are suited to a wide variety of external situations where there is exposure to more extreme weather conditions, (although they can also be used internally). They are also used on less porous and flexible walling materials such as certain types of stone and brick. They are also useful where continued dampness is an issue. Even though NHLs will set more effectively in wetter/cooler climatic conditions, it is still essential to respect the rule of not applying them in temperatures of 5℃
or less.
How lime plasters and renders are made
Lime plasters and renders are made by combining lime with a well-graded aggregate (coarser aggregate for base coats and external finishing coats, and finer aggregates for internal finishing coats), water (when the lime is in powder form) and some form of fibre to provide tensile strength to the mix. Traditional practices use animal hair (horse and goat).
Lime plasters/renders can be made in a range of different mechanical mixing machines, by hand with a spade, or purchased ready mixed up.
Health and safety issues when working with lime
Lime is extremely alkaline and therefore highly caustic. It will dehydrate and even burn the skin if there is prolonged contact. It is therefore essential to wear waterproof gloves when handling as well as protective overalls or clothes that maximise skin coverage. Safety glasses should always be worn. This is particulary the case when mixing/slaking lime mortar. Beginners should generally seek direction and supervision from those experienced with the processes.
Conclusion
Period buildings should always be repaired with like-for-like materials. This will ensure that the building not only remains healthy and structurally sound, but also ensures that the original aesthetics and inherent beauty continue to shine through. In the May issue of PH we will explore the use of limewashes.
