What is lime and why do we use this material?
At times the term ‘lime’ is used rather confusingly to refer to a variety of products made from limestone and chalk (both forms of calcium carbonate). In the context of building conservation, the term is most commonly applied to types of binder used in plaster, limewash, render and mortar that are made by burning limestone or chalk to make quicklime and then slaking this with water
Mortar is the stuff between the bricks or blocks of stone in masonry walls which closes the gaps and makes the structure wind-proof. It 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 or, in Scotland, harling. 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.
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 (such as metakaolin and HTI/’high temperature insulation’), PFA/’pulverised fuel ash’, volcanic ash and pumice.
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.
Mixtures of hydraulic and non-hydraulic lime were used in the past to create what English Heritage has termed ‘hybrid’ lime mortars (Historic Scotland describes them as ‘complex’ mortars). However, the performance of a hybrid mortar was called into question by English Heritage following a number of spectacular failures, after which it banned the use of these mixtures on grant-aided work. The results of a study by the Building Research Establishment and English Heritage, which are now being prepared for publication, show that 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 their 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. Several companies offer mortar analysis services – see The Building Conservation Directory or the Directory pages of this website for details. 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.