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Lime refers to types of binder used in plaster, limewash, render and mortar made by burning limestone or chalk to make quicklime and then slaking this with water.

Mortar is usually composed of washed sand and other aggregates, with a binder to protect it from erosion by the wind and rain. In some areas of the country, coatings of the same material as the mortar are commonly applied over the stone or brick to form a coarse, exterior plaster known as render. This is often finished with limewash (lime mixed with tallow or linseed oil), coloured with natural earth pigments which produce delightfully soft, uneven colours.

Prior to the introduction of cement in the early 19th century, the binder used in mortar and render was almost invariably lime, and this material continued to be used widely until the end of the century.

Lime is made by first burning chalk or limestone to form quick lime (calcium oxide) and then slaking the quicklime with water (forming calcium hydroxide). If no clay is present in the original limestone or chalk, the resulting lime is said to be 'non-hydraulic'. This form stiffens and eventually hardens by reacting with carbon dioxide which is present in rainwater (in the form of a weak solution of carbonic acid) to form calcium carbonate once again; a process known as carbonation.

For conservation work, non-hydraulic lime is usually used in the saturated form known as 'lime putty'. This is supplied to site covered by a thin film of water in air tight tubs, to minimize the risk of carbonation. It is made by slaking the lime with a slight excess of water. When matured (lime putty continues to mature for months), the result is the purest form of non-hydraulic lime, ideal for making fine plasterwork and limewash, but also widely used for pointing masonry and making render, daub and other lime-based mortars.

To construct towns and cities at the rate required in the late 18th century, Gerard Lynch, the historic brickwork consultant, has convincingly argued that most lime must have been made on site and used immediately, without waiting for it to mature. Dry-slaking is ideal for this: lumps of fresh quicklime are slaked with a limited amount of water and then immediately covered over with damp sand; then, after screening to remove any remaining particles of unslaked quicklime, the mixture of sand and lime is knocked up with water ready for immediate use, although it was probably 'banked' to allow the lime to mature for a few days first.

Most builders merchants supply a dry form of non-hydraulic lime which can be used like lime putty if allowed to soak in water for a while. Known as 'dry-hydrated' lime or 'bag lime', it is generally considered to be inferior to lime putty, not least because an unknown proportion will have reacted with carbon dioxide by the time it reaches the site.

If the limestone contains particles of clay, after burning at 950-1200°C and slaking, the lime produced sets by reaction with water. Limestone containing the lowest proportion of clay (less than 12 per cent) results in a feebly hydraulic lime with properties close to non-hydraulic lime, which is relatively weak, permeable and porous. Higher proportions result in successively stronger and less permeable lime mortars. Because they react with water, hydraulic limes are usually supplied to site as dry powder. However, they can also be made by dry-slaking on site and may be knocked up with water and banked on site for a few days. Banking is not thought to harm the mortar despite the commencement of the set, as the bonds formed during banking are reformed later, after the mortar has been knocked up again. Indeed, the process may actually result in a better set ultimately, as the lime is more mature.

The Romans found that by adding volcanic dust from Pozzouli (near Naples) to non-hydraulic lime, it became artificially hydraulic. They also found that crushed brick or tile had a similar effect, and this type of additive is known as a pozzolan. Bricks and tiles are made from clay. When fired, crushed and mixed with lime, they give it similar properties to a lime with a natural clay impurity. Research has shown that the type of clay, the firing temperature and the particle size of pozzolans are all important factors and that clay as an additive or impurity without being fired, does not have a pozzolanic effect on a mortar. The hydraulic set takes place due to complex chemical changes involving the hydration of calcium silicates and aluminates in particular. A similar effect can be achieved by adding pozzolanic additives to non-hydraulic lime as these additives contain highly reactive silica and alumina. Pozzolanic additives include some types of brick dust, fired china clay, high temperature insulation, pulverised fuel ash, volcanic ash and pumice.

Mixtures of hydraulic and non-hydraulic lime were used in the past to create what English Heritage has termed 'hybrid' lime mortars. However, the performance of a hybrid mortar was called into question by English Heritage following a number of spectacular failures. The addition of a small amount of non-hydraulic lime (5-10 per cent) improves workability but anything above this level significantly impairs durability. Mixes containing 1:3:12 and 1:2:9 hydraulic lime:non-hydraulic lime:sand actually performed less well than a standard 1:3 non-hydraulic lime:sand mix in tests.

Generally, mortars for conservation and repair work should include the same range and types of aggregate particles as the original mortar, as well as the same binder and any pozzolanic additives, unless any of these are actually harmful. This is to ensure that the new mortar performs in the same manner as the old and is similar in appearance. The original mix is best determined by analysis. Common aggregates include local river sand and particles of brick (which may not have any pozzolanic effect), stone and old mortar, as well as extraneous material from the firing process in particular, such as specs of coal dust. The choice of aggregate has a significant effect on the performance and the appearance of lime mortar. In particular, any aggregate used should be well washed and graded, free from sulphates (this tends to rule out the addition of coal dust even if found in the original mortar), clinker and alkalis such as sodium and potassium hydroxide. Other factors which have a significant effect on performance include particle size and shape. The correct specification of the mortar for pointing or rendering old buildings is vital. Bear in mind that some proprietary mixes may contain cement, and that a mortar which is too hard or too impervious may cause extensive damage to historic masonry and other structures.

Lime is soft, porous and flexible. It allows building fabric to move and 'breathe'. History has shown us that fat limes can be used successfully in the majority of straight forward applications. However, there are areas where fat limes are inappropriate or not durable enough (e.g. they will not set in damp or wet conditions). In these areas naturally or artificially hydraulic mixes are essential.

There are a few simple rules to follow when choosing mortars or plasters:

    1. A mortar should never be harder or less porous than the building elements - bear in mind that the overall compressive strength of a wall does not diminish much as the strength of the mortar is reduced.
    2. Good masonry relies on skill and bonding - the mortar evens out the discrepancies and fills the voids.
    3. A mortar that is too weak may require repointing in due course, whereas a mortar that is too hard can destroy the bricks or stones of the wall.
    4. Plasters and renders should be strong enough to resist damage, but flexible enough to cope with any movement.
    5. Plasters and renders should be porous enough to allow the masonry behind to breathe, i.e. no water should become trapped by the plaster or render.

Lime obviously has a major role to play in the repair of old buildings. It is now widely understood that inappropriate cement based materials may have a tendency to crack and trap water which can lead to decay and damage by salts and frost, whereas lime based materials are compatible and help to protect historic structures and fabric.