A Complete Guide to Lime Mortar
What is Lime Mortar?
Lime mortar is a combination of lime (hydraulic or non-hydraulic), aggregate (sand, grit etc) and water. Due to the introduction of Portland Cement in the 19th Century, the use of lime declined. For the past 150 years modern, artificial cements have slowly been replacing traditional lime based mortars and plasters, to such a degree that now virtually all construction is carried out using only modern materials. Whilst many of these materials are perfectly suitable for modern buildings they have been found to be incompatible with the construction of old buildings.
The need to understand the different technology involved in historic and modern structures is essential if successful repair and maintenance programmes are to be carried out.
Lime has been the primary binder used in mortars and plasters for thousands of years and the vast majority of all buildings constructed before 1900 used lime. Despite this, in many cases today lime is ignored, so why Lime?
Modern cements are harder and less permeable than lime mortars, the general aim when selecting a traditional mortar or plaster is that it should breathe more freely than the material which it is applied to and that it should have less composite strength than the substrate with which it is used. This is essential if you are to prolong the life of the historic fabric.
Different Types of Lime
Non-Hydraulic Lime Putty - Although traditionally this type of mortar was used internally and externally, today it is generally used for internal plasterwork & cornice etc, pointing, bedding and renders in sheltered areas.
Non-Hydraulic With pozzolan - All of the above plus the ability to withstand more exposure to the elements.
Limestone is burnt in a kiln using temperatures of around 900C to produce lump lime, Calcium Oxide (CaO).
The production of lime putty is a hazardous procedure and should only be carried out by suitably trained personnel. This is not a procedure to be carried out on site where the work cannot be monitored and controlled.
The lump lime/burnt limestone (calcium oxide) is immersed in water. The mix is raked vigorously to aid the breakdown of the lump lime. When the resulting mix is the consistency of milk, the lime is then sieved to remove any un-slaked particles. The operatives should take great care as the mix reaches a very high temperature due to the chemical reaction taking place. The resulting material is called lime putty (calcium hydroxide).
This material is left to de-water/mature for at least three months in a bulk container known as an ark. The material will take on the appearance and consistency of cream cheese. Lime putty can be stored indefinitely in this state provided it is not exposed to air. When Carbon Dioxide, which was burnt off in the kiln, is reabsorbed back into the mortar/lime, this causes it to return to its original state, calcium carbonate.
Storing the lime putty in airtight containers allows the putty to mature and any un-slaked particles will have sufficient time to slake. The longer the putty is allowed to mature the better the finished product. Traditionally in Italy the best quality work was carried out using lime putty allowed to mature for generations.
Having manufactured and left the lime putty to mature for at least three months then this material can be used for mixing with suitable aggregate to make mortar. The best method of manufacturing a lime putty based mortar is to mix the ingredients in a mortar mill. The resulting mortar is of a high quality and very workable. No extra water should be necessary during the mixing process.
The amount of lime putty needed to bind the aggregate is determined by the amount of void space between the grains of sand, which needs to be filled. Most mixes use a ratio of two and a half parts aggregate to one part lime putty. This can vary slightly depending on the aggregate selected.
Health and Safety – Lump lime (calcium oxide) is a caustic alkaline. When it comes in contact with eyes or skin, it will cause a chemical burn. Wash the affected area with a solution of sugar water immediately and seek medical assistance. When working with lime the operative needs to wear personal protective equipment (eye protection, gloves, overalls and dust mask).
Non-hydraulic lime mortars were generally speaking the most common type of mortar used in the construction of historic buildings, these mortars were often a mixture of lime, earth, shells, crushed stone, pit or beach sand. The aggregate used basically depended on what was available locally and was often the direct result of years of local knowledge being passed down from generation to generation, as opposed to formal teaching techniques. These mortars performed and do perform in a very different way from modern cement or gypsum based mortars.
Non-hydraulic limes, sometimes called “fat limes” are limes which are very pure with regard to their calcium carbonate (CaCo3) content. The purest forms of this material can have upwards of 95% CaCo3, these “fat limes” have no or little chemical set in the way that modern materials harden, they harden by a process called carbonation i.e. reaction with the air.
The carbonation process is a slow ongoing reaction which can take weeks, months and to a degree years to complete. The mortars and renders therefore need good protection from the elements whilst carbonation is taking place.
This is the most traditional method of preparing mortar in Ireland. This process combines slaking and the mixing of the aggregate in one operation. This method was most commonly used in the preparation of mortar for the construction of rubble stone walling. The mixing took place on or very close to the construction site.
The aggregate was normally spread in a circle and the lump lime was added to the centre of the heap, water was added and the mixture was turned a number of times. The end product is a very workable and “plastic” mortar that is ideal for rubble wall construction. You can use this fresh or it can be stored and reworked at a later date. Traditionally the mixing of mortars was carried out in the winter and they were left to sour out in pits until the building season started in the spring.
When producing hot lime mixes judging the correct ratio of lime-aggregate is rather difficult. Lump lime increases in volume when it slakes. In general, a useful rule of thumb is to use four parts aggregate to one part lump lime.
This type of mortar is unsuitable for plastering as it may contain un-slaked particles. These un-slaked particles tend to blow and give the plastered surface a pock marked appearance.
There are many health and safety issues to be addressed when working with hot lime mixes and great care should be taken. Skilled operatives who understand the process and the dangers are essential when working with hot lime mixes.
In the past lime mortar was heaped up and allowed to stand with little more protection than a sheet thrown over it, and in many cases no protection at all, the outer skin hardened and the mortar underneath remained moist with just occasional wetting down. The reason for allowing the mortar to stand or mature is to allow a better bond to take place between lime and sand. When using large amounts of mortar the best way of storage is to construct large timber mortar bins, three sided, which will allow the mortar to drain off any excess water and with little more than a polythene sheet keep the mortar air tight, wetting the top in warm weather. When using small amounts, sealed plastic tubs offer the best way of storage.
Natural Hydraulic Limes (NHL)
Feebly Hydraulic Lime NHL 2 - Some internal/external work, pointing, bedding, rendering on soft brick or stone backgrounds in sheltered locations, grouts.
Moderately Hydraulic NHL 3.5 - External rendering, pointing, bedding, some moulded work in normal exposure conditions.
Eminently Hydraulic NHL 5 - Areas of extreme exposure on hard dense stonework, sea defences, canal work, coping, pointing (very strong mortar).
Production of Natural Hydraulic Limes
Natural Hydraulic limes differ from non-hydraulic limes in that they have a chemical set as well as the process of carbonation. The limestones from which natural hydraulic limes are formed naturally contain a varied range of minerals of which silica and alumina are the main ones for creating NHL. When these limestones are heated in a kiln at temperatures of around 1200℃, the resulting lime has different properties. From the pure limestone,silica and alumina combine with the lime to form active compounds. These compounds combine with water to create a chemical set. The percentage of silica and alumina contained in the limestone will determine the main characteristics of the lime and of course the resulting mortar or plaster.
The main characteristics:
- Setting time
- Frost resistance
Sand and larger sized aggregates make up the larger proportion of most mortars. Colour, texture and overall strength are all strongly affected by the choice of aggregate.
The aggregates most commonly used with hydraulic lime are sand and grit, although for the purpose of matching historic mortars various impurities may have to be added. A good sand should be a washed, sharp sand with angular grains to ensure good bonding qualities. Soft building sands should be avoided as their rounded grain shape can result in excessive shrinkage.Sands used should be well graded with a range of grain sizes, which for most plaster, render and mortar work will range from 5mm down to 75 micron. Larger sized aggregates may be used in some mortar or pointing work. As a rule of thumb for pointing, the maximum size of aggregate should be no bigger than one third of the joint width. Sands, which contain a clay or silt content of more than 4% should be avoided, as these will inhibit the contact between lime binder and aggregate.
Sands which have a high fines content should also be avoided as the larger surface area of these will require more water in the mixing. This higher water content will induce shrinkage and can affect flexural and compressive strengths. Monogranular sands should be avoided as they will possess poor workability qualities and will inhibit good vapour exchange i.e. the ability to breathe.
Use clean water. The addition of water should be considered carefully, as it will directly affect the ultimate strength and durability of a mortar. The more water introduced into the mortar mix, the weaker will be the final result. However too little water will prevent the chemical processes taking place and weaken the material. Generally, water should be added sparingly, until a usable consistency is achieved. Adjust quantities to give a workability suitable for the application. It is important to use the minimum amount of water necessary as to reduce shrinkage.
Modern buildings generally rely on an outer layer to prevent moisture penetrating the walls, whereas buildings constructed before 1900 generally rely on allowing the moisture which has been absorbed by the fabric to evaporate from the surface. In essence old buildings exposed to the elements are continually absorbing moisture and the ability for the moisture to evaporate again is crucial to the well being of the structure. Using cement based mortars and plasters in traditional buildings runs the risk of locking-in moisture which could result in dampness internally and general building fabric decay. Problems generally arise when the building has been “repaired” with inappropriate materials through lack of knowledge.
It is interesting to note that many structures built using lime technology 500+ years ago and maintained correctly are still in excellent condition today. It remains to be seen how modern structures will fare in 500 years!