The use of lime as building material is central to the practice of building conservation. It has been used over many centuries as the binding agent in mortars, renders, plasters, limewashes etc and so it's presence is almost guaranteed in any historic building. For this reason it is important to understand the properties of lime when undertaking any work on an historic building.
Lime mortar offers many advantages over modern cementitious alternatives. Lime mortars are generally less hard and more porous than cement-based mortars, this allows moisture to evaporate through the mortar rather than through the bricks or stones of a building. This prevents a build-up of moisture levels in the wall, which in turn prevents the build-up of harmful soluble salts in the masonry. These salts are often the cause of deterioration to the fabric of a building.
The specification and application of lime-based materials in repair and conservation work is not difficult, but requires a certain amount of patience and care and a little appropriate knowledge. Below, are some general guidelines on the use of lime.
The Production and Properties of Building Limes
Lime is manufactured from naturally occuring Calcium Carbonate ie limestone, chalk, sea shells). When this is heated in a kiln Calcium Oxide is produced when the carbon dioxide is burnt off, this is quicklime which when combined with water, a process called slaking, will in turn produce Calcium Hydroxide, called slaked lime. Calcium Hydroxide in the form of putty can be stored for many years if kept safe from frost and not exposed to the atmosphere (this is normally achieved by covering it with water). When it is exposed to the air it absorbs carbon dioxide from the atmosphere to become Calcium Carbonate once again, thus completing the lime cycle.
Slaked lime putty is the state in which lime is commonly used in building. When slaked, quicklime combines vigourously with water swelling and generating a great deal of heat. The result of this reaction is called milk of lime and this should be seived into a lime pit or container to remove any large pieces of over- or under-burnt quicklime. These larger particles will react slowly to the slaking and can cause problems when the putty comes to be used since imperfectly slaked particles can result in an unsound mortar.
When the milk of lime is left to stand the solution of lime in water settles so that a lime putty forms beneath a clear layer of limewater. The longer a putty is left to stand the better; it will fatten, continuing to swell and mature as it absorbs more water. This will give any imperfectly burnt particles of quicklime time to slake, and the longer a putty has to absorb water, the longer it will retain that water when it comes to be used. This is important since it means that the plasticity of the lime will be improved, thus improving the bond between a mortar and masonry by resisting suction from a building.
When putty is kept for a long time (years even) it can become very stiff. However lime putty is a thixotropic substance which means that when stirred, known as knocking up, the plasticity returns.
Gauging and Using Lime Mortars
Lime is available commercially in the two forms mentioned above - as a putty or as an hydrated powder.
Any existing pointing must be raked out to a depth usually equal to the width of the joint, but not less then 15mm if possible. The back of the joint should be roughly square in profile. Using a plugging chisel will help to ensure that the stone of bricks are not forced apart.
The joints must be thoroughly dampened with a water spray so that the masonry doesn't absorb too much moisture from the mortar which if allowed to dry too quickly will be weak. Allow time for the stone or brick faces to dry a little as this will help to prevent the lime from smearing and staining the masonry surfaces. The mortar should be as dry as is practical to point with. This allows maximum compaction in the joint, reduces shrinkage cracking and reduces the tendency to smear.
Gauging It is often beneficial to guage the lime putty mortar with a pozzolanic additive, this helps to prevent damage by frost and heat. A 3.5:1 mix of coarse sharp sand and lime putty should be guaged with a 10-25% addition of a pozzolan. The actual quantity added will vary according to the hardness of the masonry and the degree of exposure of a site. Hence, a wall that faces into the prevailing wind and rain should be pointed with a mortar that has more pozzolan that a wall in a sheltered position. However if pointing during the autumn and winter when the weather is likely to be cold and wet it can be beneficial to point even a sheltered site with a pozzolanic mortar as this will give the pointing a better chance of setting and subsequent survival.
Start at the top of the wall to allow for cleaning up and spraying to continue. Use a pointing key or a metal trowel to force the mortar into the joint from a hawk. Joints deeper than 20mm may need an initial dubbing out otherwise shrinkage may occur. Try to fill all cavities so that no pockets of air remain in the joints, as this can expand causing the mortar to fail once it has gone off. Initially it is wise to overfill the joints, again to allow for shrinkage during setting. This will also give you the option to choose whether to leave a rebate, which highlights the stones more, or whether to have the final finish flush with the mortar.
It is wise to monitor the mortar as it is going off, spraying it with a water spray as often as is appropriate to keep it moist. This is especially important in hot weather as a controlled, slow evaporation of the moisture from the mortar is essential to a satisfactory final set. If the mortar dries too quickly the bond within the joint could be weak and unreliable, and the mortar may fail.
When the mortar is "green hard", ie it has not set hard but is still slightly wet, use a hard bristled brush to tamp the mortar in to the joint. This will compact the mortar back into the joint to strengthen the bond between masonry and mortar and will also provide a soft finish to the joints, removing any excess lumps and bobbles of mortar, and enhancing the aggregate within the mortar making the final look less uniform and more interesting.