Lime has been used widely in the construction and finishing of Scotland’s traditional buildings. Cement mortars have become more widespread than lime mortars only since the 1900s.
Building limes usually take the form of a mortar, used for pointing or bedding masonry, finishing internal or external walls, or on roofwork with pantiles. It is also used as a limewash.
Mortar is a generic term given to workable material that you can trowel in place and which hardens in situ. The vital lime mortar components are: a binder, an aggregate and water. Additives may also be used to improve performance.
Choosing a suitable mortar for repair works is important for the conservation of traditional buildings. Using incompatible mortars can speed up masonry decay and damp problems.
Most lime mortars currently used in Scotland are based on natural hydraulic limes, sold in bags as a dry hydrate powdered lime. Other types of building lime are lime putty and hot-mixed lime mortars. These traditional methods of preparation can still be used in building and repair work today.
Using hot-mixed lime mortars may have benefits over other lime mortars. They are known for their workability, good fill, frost resistance and economy, among other qualities.
Lime has been used widely in the construction and finishing of Scotland’s traditional buildings.
Building limes usually take the form of a mortar. It is also used as a limewash.
Lime mortar may be used for:
pointing masonry walls
bedding stone or brick
finishing walls internally – as plaster
finishing walls externally – as render
parging (filling) the underside of pantiles, bedding slates and other roofwork
Methods for mixing and applying lime were perfected over generations to aid the performance of the traditional buildings. Cement mortars have become more widespread than lime mortars only since the 1900s.
Lime mortars are made by mixing lime with sand or another aggregate. They are used in a malleable state and once in place will harden to form a robust and durable material. Our medieval castles and cathedrals as well as everyday ‘stane and lime’ buildings vouch for the resilience of lime mortar.
This longevity is linked to reparability: renewal of lime mortar is possible and is indeed required from time to time. For example, a building can be repointed with lime mortar or coats of lime harling can be patched.
The principle use of lime mortars was to weatherproof (rather than waterproof) a traditional building. Lime mortars are able to hold and evaporate moisture, thus keeping the building fabric dry. This quality is often called ‘breathability’.
Mortar joints act sacrificially to protect the external masonry, so a correctly specified mortar will weather at a faster rate than the adjacent masonry. Mortars may therefore require periodic maintenance, i.e. repointing of joints.
Rubble masonry buildings in particular were often finished with external coats of lime harling or internal coats of plaster, which made walls more breathable. The lime harling gave a degree of protection, while the internal plaster would help to control condensation, by absorbing some moisture and redistributing it inside and out.
Masonry set in lime mortar can accommodate some movement – both structural and thermal – without significant damage. Any movement is compensated for by minute adjustments over many joints and beds. As the mortar can yield more than the masonry, cracking doesn’t spread through the walls.
Even large areas of traditional masonry don’t need the regular movement joints required by modern builds made with brittle binders such as Ordinary Portland Cement (OPC).
No modern material can rival the aesthetic properties of building limes.
Lime mortars can be fashioned to:
look like carved stone
create fanciful decoration
give vibrant coloured finishes with limewash
blend in naturally with the surroundings
As lime mortar is softer, its use is fully reversible. So masonry blocks can be recovered and reused when adapting a traditional building constructed using lime.
Mortar is a generic term given to workable material that you can trowel in place and which hardens in situ. Lime mortar will be required in most cases for the repair of traditional buildings.
Mortar can be used for:
The exact composition of each mortar varies, but all mortars have three main components:
binder – e.g. lime, cement, clay, earth
aggregate – e.g. sand, gravel, crushed stone
Additives, used to give particular properties, may also be part of the mix.
Understanding the effects of the different components will help in choosing an appropriate mortar for a specific purpose.
A binder holds the mortar together and supports the aggregate grains. The type of binder used significantly affects the properties of a mortar, both when it is wet and when it hardens. The permeability (degree of water absorption) and flexibility of a binder are often more significant aspects than its strength.
Air limes or natural hydraulic limes have a proven track record in conservation work and so are generally best to use for traditional buildings.
The range of lime binders available on the market is constantly changing. Modern materials, classed as hydraulic lime or formulated lime, can vary greatly in lime content and may contain additives that affect their properties. Such limes also typically have lower permeability and flexibility.
Historic cement mortars were generally weaker and more permeable than the Ordinary Portland Cement (OPC) mortars made today. The two materials aren’t always compatible, and using a modern cement mortar to repair masonry built with much earlier cement mortar may not be appropriate.
Selecting a suitable mortar for repair works is important for the conservation of traditional buildings.
Both lime and cement mortars can perform well when used appropriately. Lime mortar will be required in most cases for the repair of traditionally built masonry structures. Using incompatible mortars can lead to accelerated masonry decay and damp problems.
Modern cement mortars are less permeable. Rather than waterproof a building, this barrier stops the small amounts of moisture that get in from evaporating. The moisture builds up behind the mortar as a result and prevents the wall from drying out, and this speeds up stone decay.
Using cement mortar with traditional stonework can also produce poor aesthetic results. The often dull grey finish of cement mortars and renders, and the different styles used for cement pointing often don’t match the colours, textures and styles achievable using lime mortars.
Assessing the repair site
Repairs are usually most effective when they use the materials and methods employed in the original construction. Original cement mortars are often just as worthy of conservation as lime mortars. But care must be taken to establish what is actually original.
If replacing a failed mortar, consider:
why it failed
the condition of the masonry
the climate and exposure
the desired visual appearance – i.e. texture and colour
Analysis of the existing mortar can help with matching for patch repairs, and give an idea of the original mix proportions. But simply copying the mix may not always be the best option, especially if the function of the mortar has changed or the mix sample was degraded.
Engaging an experienced buildings consultant or stonemason at this stage can be a good investment. Getting the right specification for materials and application will ensure that the lime mortar performs better in the longer term, and will save on future maintenance and repair costs.
Most lime mortars currently used in Scotland are based on natural hydraulic limes (NHLs), which are sold in bags as a dry hydrate powdered lime. These products have become commercially available only fairly recently.
Before NHLs were introduced, lime mortars were prepared using either lime putty or quicklime mixes, the latter made as a ‘hot mix’. These traditional methods of preparation can still be used in building and repair work today and result in long-lasting mortars.
Lime is made by burning calcium carbonate (limestone, chalk, shells) at around 850ºC to produce quicklime.
Quicklime is mixed with water, causing an exothermic reaction (heat is generated). This so-called slaking process produces lime putty (if excess water is added) or a dry powder (dry hydrate powdered lime).
The lime putty is then mixed with sand, or the hydrated lime with sand and water, to produce a mortar. In the past, quicklime was often mixed directly with sand and water on site to produce a hot-mixed lime mortar. This is still a useful technique, and it most accurately replicates the performance and look of a historic mortar.
Purity of the raw material
Most quicklime commercially available in Scotland today is imported and it is very pure.
Limes made from pure sources of calcium carbonates (e.g. chalk) are called air limes or non-hydraulic limes. Used to make a mortar, the air lime will harden solely as a result of reacting with carbon dioxide in the air (carbonation). This process virtually returns the lime to its former state, forming a material similar to the original limestone.
Limes made from limestones that have clay impurities are called natural hydraulic limes (NHLs). These set partially as a result of a complex chemical reaction with water (hydration) and partially because of carbonation.
Hot-mixed lime mortars
Hot-mixed lime mortars are prepared on site by mixing together specific quantities of quicklime, aggregate and water. The quicklime generates heat as it slakes, and binds with the aggregate to produce a mortar.
The mortar can be used while still warm (hot lime) or stored for later use (hot slaked lime). After being allowed to cool, hot slaked lime is re-mixed and then applied in the same way as a cold-mixed mortar.
For some purposes, especially internal plasterwork, lime putty is usually stored and left to mature for several months, allowing the lime to ‘fatten up’ as it continues to absorb water.
Mixing the matured lime putty with aggregate to produce a mortar doesn’t generate heat. Lime putty mortars, being air limes, rely solely on carbonation to harden, so can be stored indefinitely if not exposed to the air. You can store lime putty in sealed containers or beneath a layer of water. This is often done before a big plastering project.
Ready mixed natural hydraulic lime mortars
A ready mixed natural hydraulic lime mortar (NHL) is supplied as a fine powder and mixed with aggregate and water in specific proportions to produce a mortar. There is no slaking reaction as the lime is already hydrated.
Mortar made from NHL or other limes with hydraulic properties can’t be stored for long periods as the setting process begins when the water is added.
Most hydrated lime available from builders' merchants is not sold for use as a mortar but as an additive for concrete. Such hydrated lime is not a substitute for NHLs or quicklime. Ask a specialist contractor if in doubt about the suitability of a lime product.
Hot-mixed lime mortars were used in the construction of buildings until the 1930s and 1940s. Today, they are still thought to have some benefits over other lime mortars.
A range of lime products is available, all of which have a place in the repair and maintenance of traditional buildings. But you may wish to consider a hot-mixed lime mortar especially when repairing traditional mortars and harls.
Hot mixes differ from mortars prepared from lime putty or dry hydrate powdered lime. This is due to the effect on the aggregates and other components of the heat generated by slaking, and the lime’s high alkalinity.
Most hot-mixed lime mortars used in Scotland are prepared as gauged mixes, or less often with pozzolans.
Quicklime can be combined with a natural hydraulic lime (NHL) to produce a gauged mortar. Traditional mortars derived from Scottish limestones were often slightly hydraulic. Hydraulic quicklime is not readily available in the UK, so gauging an NHL with quicklime is the closest you will usually get to a traditional hydraulic hot lime mortar.
Pozzolans are natural materials such as brick dust and pulverised fuel ash, or manmade additives that are added in small quantities to lime mortars just before use. Their purpose is to impart hydraulic properties.
Benefits of using a hot-mixed lime mortar
Hot-mixed lime mortars are thought to have some benefits not generally found in mortars prepared from dry hydrate lime.
Hot mixes are known for their:
workability – the sticky, workable mortars produced adhere well to masonry
good fill – quicklime expands as it slakes, and the volume of a hot mix can continue to increase even after it’s been laid (when used hot)
early stiffening – hot mixes take up water rapidly as they slake and so the mortar stiffens up quickly, allowing an efficient build rate
frost resistance – pure and gauged hot mixes seem to have good frost resistance, although the reasons for this aren’t well understood
incorporation of additives – the heat generated by slaking aids this process, e.g. tallow or casein will melt, improving their dispersal in the mortar
economy – quicklime is usually cheaper than lime putty, NHLs or other lime products, and less is required as it’s volume can double during slaking
authenticity – analysis of historic lime mortars and harls often points to the use of a hot-mixed lime mortar
There are two main methods of preparing a hot-mixed lime mortar. Both produce a mortar that can be used for various types of lime work including bedding, pointing and harling.
Method 1: Hot lime mortar
This type of hot mix is typically used for building or pointing rubble masonry, and for harling.
Quicklime is added to sand and water in the specified proportions and thoroughly mixed.
Care must be taken to use the right amount of water: too little and the mix can overheat, too much and the heat of the slaking reaction may be ‘drowned’.
When the quicklime has largely slaked, but is still warm and well mixed, the hot lime mortar can be used on site.
Method 2: Hot slaked mortar
Hot slaked mortar retains most, but not all, of the characteristics of hot lime mortar. It is usually used for bricklaying or for finer work.
Quicklime is added to damp sand measured to specific proportions (typically in the ratio 1:3).
Left to slake on a board or in a tub on site, a dry mixture of sand and hydrated lime is produced.
Water is added to the lime/sand combination, and this is mixed to the desired consistency.
The mortar is usually covered with waterproof sheeting and left to mature for a period of time (days, weeks or months).
Before use, the hot slaked mortar is ‘knocked up’ (re-mixed) on site.
Skills required to make hot mixes
The preparation and use of hot-mixed lime mortars on site today is a craft skill that’s currently being revived in Scotland by some specialist contractors.
The site practice and manual skills required when preparing hot mixes are essentially similar to those needed to prepare other lime or cement mortars. With appropriate training, a skilled contractor will be able to offer the preparation of hot-mixed lime mortars as one of their services.
Quicklime is classified as a hazardous – but not dangerous – material. It can be used on site as long as appropriate health and safety procedures are followed.
Quicklime is a highly caustic material and can cause irritation or burns if it comes into contact with the eyes or skin. Personal protective equipment – gloves, breathing apparatus/masks and goggles – must be worn for all work involving quicklime. Sugar-water eyewash should always be available on site (saline eyewash is ineffective for use with lime).
Slaking of quicklime (and mixing of dry powdered lime) should be carried out in a well-ventilated area. Hot mixing is a further building site risk as temperatures can reach 300ºC.
Other materials used as binders are similarly hazardous. The same safety precautions apply when mixing cement or other types of lime.
Lime mortars have been used in buildings for thousands of years and continue to be used for the repair and maintenance of traditional buildings.
Lime mortar has been used with stone since Roman times for building in Scotland. Many of Scotland’s traditional buildings were constructed and finished using lime mortars, for bedding, pointing, harling and renders.
Rise and fall of lime mortar
Lime mortar was used as bedding for masonry when building Scotland's imposing medieval churches, cathedrals and civic buildings. Lime harling was often applied to the exterior of these structures to add an extra layer of protection.
Mass production of lime was made possible in the 1700s and 1800s, causing the cost of making the material to fall. This led to its widespread use, and the majority of surviving buildings from the 18th and 19th centuries feature lime mortar. Many were also coated internally with lime plaster, or externally in lime harling or limewash for extra protection.
Most building sites – even well into the 20th century – had their own pit where lime was prepared.
Then Ordinary Portland Cement (OPC) became available from the mid-1800s, though it was initially used for civil engineering and industrial purposes. By the early 1900s, its use was more common for foundations, floors and structural reinforcement.
Following the First World War, and the introduction of modern construction methods, OPC has been used increasingly in all sectors.
A resurgence in lime mortars
OPC was the norm for new construction by the mid-20th century, and it began to be used to repair traditional buildings originally built with lime mortar. The properties of OPC and lime mortar were little understood at this time.
Decades later, the incompatibility of OPC mortar with many stone types has become evident from the many cases of masonry decay linked to its use.
Recently, the growing understanding of the importance of stone and mortar compatibility has led to a resurgence in the use of lime mortars – particularly on traditional stone buildings.
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