Purpose of this paper -
This study was performed in order to understand what work has been completed regarding how lime mortar repair is carried out on historic buildings and why lime is used.
Journal articles and books were studied in order to understand the level of work that has so far been completed.
It was found that
Research limitations - There is a large amount of published work on this topic, consequently not all of the relevant articles and studies could be read for inclusion into the literature review. However it is thought that the important details and findings are included.
Practical implications -
Keywords - mortars, lime, refurbishment, historic buildings, maintenance.
The use of building limes as the principal component in mortars and renders has increased considerably over the past five to ten years. This traditional
basic material is fundamentally important to the long term survival of historic buildings, yet there are many who regard its' use with doubt, appropriate perhaps for a National Trust property but generally unnecessary. When we consider that nearly all the churches, abbeys, cathedrals, castles, stately homes and half the building stock in the U.K. was built with lime, including the now famous Coronation Street, it is hardly surprising that the use of lime is fast coming back.
This literature review is intended to synthesise published information regarding research carried out into lime mortar and its use in historic buildings. The emphasis in this document is on the analysis of the demands of building conservation activities, their relationship with and influence on the choice and application of technical and scientific measures needed to fulfill these demands.
Lime is the traditional, ancient binding medium of masonry (Ashurst, 1997). Lime is produced by burning limestone (calcium carbonate). This produces calcium oxide (otherwise known as quick lime) and this is then hydrated with water to produce calcium hydroxide (lime). This process is known as slaking. According to Taylor (2000), ‘if no clay is present in the original limestone or chalk, the resulting lime is said to be non-hydraulic'. Non-hydraulic limes set by reacting with carbon dioxide to form calcium carbonate, they will stiffen and eventually harden.
Lime otherwise know as calcium hydroxide, has its own classification system which was developed by Louis Vicat - a French civil engineer who researched hydraulic limes and cements. He classified limes according to their hydraulicity, which is their ability to set under water. The classes he identified consist of: non-hydraulic to feebly hydraulic lime, feebly to moderately hydraulic lime and moderately to eminently hydraulic lime. The hydraulicity of the lime depends on the amount of impurities present in the limestone from which it is burnt such as clay and silica. The more clay and silica which is present, the greater the hydraulicity of the lime. In general the more hydraulic the lime the harder the resulting mortar (Pavia and Bolton, 2000) and the shorter the setting period. This will affect where the mortar can be used on the building.
There are various definitions on what a mortar is, the most detailed being from the RILEM Technical Committee-167 COM (2001). It states that: ‘Mortar is a mix of organic and inorganic binders, mainly fine aggregates, water and admixtures and organic and inorganic additives mixed in order to give to the fresh mortar a good workability and to the hardened mortar adequate physical (porosity, vapour permeability etc.) and mechanical (strength, deformability, adhesion etc.) behaviour and good appearance and durability. This definition is the most relevant for mortars in historic buildings as it is based on recent research into requirements for a compatible mortar used for conservation of masonry.
Functions of mortar
The UK Limes research team (2003) state the three principal functions of a mortar are to provide an even bed so that the load on the wall is distributed evenly over the whole bearing area of the masonry units, to bond the units together and help them resist lateral forces and to seal the joints against the penetration of rain. Mortars in historic structures function in many different ways as renders on internal and external walls, bedding mortar of masonry, supporting material for pavements and mosaics, and watertight lining materials in cisterns, wells, aqueducts etc…(Moropoulou et al. 2000). Ashurst (1983) states that mortars can be used in the following conservation techniques: pointing, repointing, rough racking, tamping, bedding, replacement of masonry units, internal and external coating - plasters, renders and their repair, plastic repair, injection and grouting.
Repair of historic buildings & compatability
The most effective methods of repairing and maintaining traditional masonry buildings almost invariably involve the use of materials and techniques employed in their original construction. This is based on conservation principles such as the BURRA charter (1999). The BURRA charter states that the conservation policy of the building should identify the most appropriate way of caring for the fabric. Stirling, S (2002) also argues that the repair must be compatible with the original construction, its life span, and follow the principles of good conservation practice. The BURRA charter prefers traditional techniques and materials for the conservation of historic buildings. However Pickard (1996) has a more relaxed view of conservation, he feels that new materials and methods of repair can merit consideration if they have proved themselves over time and also if the benefit of using them outweighs any harm that they could cause to the building. The repair techniques should respect the original material and structures otherwise they become incompatible (Binda et al. 2000).
Understanding of material interaction with other materials and environments is much more significant for materials used in the modern building industry. The materials and techniques used in conservation must meet special requirements such as compatibility, reversibility, etc.
Van Hees (2000) suggested a definition of compatibility related directly to mortars as follows: “The new mortar should be as durable as possible, without (directly or indirectly) causing damage to the original material.”
There are still mortars currently applied in remedial conservation works which can cause damage or accelerate deterioration to the historic substrate.
Mortars in historic buildings
The majority of mortars in historic buildings are made from lime. Lime mortar has been used in Ireland from when early Christian churches were built and more commonly from when round towers were built (Pavia and Bolton 2000). Lime mortar technology arrived relatively late in Ireland (Dotter et al, 2009) since its use has been known since Egyptian times when both lime and gypsum based mortars were used in the building of the pyramids. The path of introduction of mortar technology to Ireland is unclear, though according to Pavia and Bolton (2000) the general hypothesis is that it was introduced from post-Roman Britain or Western Europe. The use of lime mortar was mastered by the day to day practice of craftsmen in prehistoric and medieval times more or less following the rules set by Vitruvius in his classic work ‘Ten books on Architecture'. Their use was complemented much later on by experimental research results from the end of the 18th century by Smeaton (1793) or Vicat (1837). Historic Scotland (Gibbons 1995) published a Technical Advice Note which revived the subject of the Use of Lime mortars. It especially stressed the use of non-hydraulic lime mortars made of lime putty. The publication became an authoritative source of information for preparation of lime mortars used in conservation. However, the variety of composition, techniques of production and application of mortar should not be underestimated. Lynch (1998) pointed out that historic mortars were not only mortars made of lime putty or 1:3 mix. The source of limestone and the local source of sand had an influence on its composition as well as the differences in mixing and preparation. There is now increasing evidence that hot mixing and dry mixing (Leslie and Gibbons 2000, Callebaut et al. 2000, Hughes et al. 2001) techniques were also applied to prepare many historic mortars. Hydraulic lime mortars were also used, especially in towns and cities. In areas devoid of natural hydraulic limes, ' artifical' hydraulic limes were utilized, being non-hydraulic lime mixed with a pozzolana (Lynch, 1998).
Furlan (1991) pointed out that the research into lime mortars has multiplied during the last two decades. Many publications suggest that new mortars for any remedial work on historic masonry should be based on the approach that pure lime mortars prepared in a traditional way show the closest performance properties to the historic masonry (e.g. Torraca, 1988). Conservation literature in many cases (e.g. Gibbons, 1995) simplifies the problem of compatibility as the decision to use lime-based mortar instead of cement mortar. This results from bad experiences of using cement mortars in the past. When assessing past conservation attempts the two following points often appear:
- Firstly the lime technology and skills were partially forgotten (Gibbons, 1995)
- Secondly portland cement suspended lime because of its superiority in strength (Gibbons 1995) but other qualities of lime mortar were not properly considered.
Correct application of lime mortar requires special training and care to be successful (Maxwell 1998, Gibbons 1995). Although the use of cement-based mortars was widespread. Their compatibility was not considered and they were often found later to be incompatable with the orginal masonry material. There is general agreement between scientific and conservation literature regarding these two points. A serious study of the use of cement mortar and the decay of stone and brick is still lacking, so statements regarding the incompatibility of cement and its rejection by the conservation practitioners are not backed by detailed evidence.
Research into mortar and concrete based on Portland cement appears much more advanced than that on lime (e.g. Hewlett 1998). The research reflects the use of cement and concrete in the modern building industry. This point illustrates the fact that there are no international lime oriented research journals, some national journals exist, e.g. The Journal of the Building Limes Forum. Research articles about lime and lime mortars are often found in journals oriented towards mainly concrete and cement such as Cement and Concrete Research, Concrete International, Materials and Structures, Magazine of Concrete Research, Construction and Building Research, Brick News, Magazine of Masonry Constructions, Masonry International, etc. In future it may appear that a lack of exact scientific knowledge of lime and lime mortars will be the greatest drawback in its proper use as a compatible material. A more detailed understanding of fundamental properties of new lime mortars is therefore required to back up the demands from conservation practice to use original materials for conservation works (Furlan 1991, Valek 2000).
Addition of brick dust
Addition of brick dust into lime mo Lime mortars & conservation
Lime based mortars are good examples of a traditional material used in conservation, where good skills and practical experience have a significant influence on performance. Burman (1995) described the advantages of using lime in various conservation techniques and the successful revival of lime into wider conservation practice. For traditional lime mortars, often the conservation technique, good site practice, workmanship and skills are considered to be the key instruments for their successful application (Gibbons 1995, Johnston 1995). Many failures of lime mortars are caused by their inappropriate use or by a lack of practice and training in their correct application (Gibbons, 1995).
The most basic specifications for new mortars are from practical building and masonry conservation guides (e.g. Stone buildings by Pat McAfee 1998). The usual advice is to carry out basic analysis to obtain the composition of the mortar (usually by dissolving the mortar in acid) in the first instance. If the original mortar has performed well then the new mortar mix should resemble it as closely as possible. It is advised that the new mortar should be softer than the stone. Ashurst (1990) in the ‘English Heritage Technical Handbook' specifies that the original aggregate should be copied, however the mix may need to be modified to improve the weathering characteristics. Milner (1972) points out that natural materials should be used to match the colour and texture. He also suggested that soft mortars should not be used for repairs on buildings originally constructed with hard mortar.
The basic specifications for mortar required by conservation practice can be summarized as follows:
- The formula of the new mortar should match that of the original one
- The new mortar should be softer than the original mortar or masonry but, on the hand, the mortar should not be too soft if the original masonry was constructed with hard mortar.
- Under no circumstances should the new mortar cause deterioration to the existing material
- Side effects and long-term effects caused by repair should also be considered
Holmstrom (1992) presented a list of compatibility criteria for lime mortar renders used by contractors in Sweden:
- Materials and components used must be removable, and must not change the physical or chemical balance of the building and must not change the aesthetics.
- Each layer of mortar should be weaker than the substrate.
- Materials with the same properties as the original should be used. If the original materials are not used, this must be justified and all relevant properties of the original and the substitute must be declared.
These suggestions support minimum intervention and the preservation of existing historic fabric. In addition Holmstrom's (1992) criteria emphasise the need for reversibility of the material applied. The final point makes demands for analysis of historic mortars in order to formulate compatible replacements that resemble the orginal. Basic performance testing and in-situ trials are common and recommended. There is less demand however for testing and characterizing the new mortar properties, as it is a copy of the orginal one and therefore it is assumed to perform in the same way. This may not alwaysbe correct. Valek (2000) discussed a compatibility model for two mortars, one designed on a ‘like to like' basis and one on a ‘compatible properties' basis. The performance of both mortars relied strongly on the curing and ageing conditions. However, the curing conditions of the new mortar may not match the ones of the past.
Other literature to be included in final literature review
According to the Northern Ireland Environment Agency (2009) in their work practice guidelines for remedial conservation works, lime mortar is the most important material to be used in remedial conservation works on historic structures. This is because it is water permeable and also allows for movement within the joints of the building.
The use of lime mortar in stone structures dominated the construction industry until the invention of Portland cement by John Smeaton in the 18th century. However it was John Aspen's commercial production of Portland Cement in 1824 which had the most dramatic effect on the construction industry. Its properties were to revolutionise the architectural landscape up to the present day, (Pavia and Bolton, 2000).
There is continual movement in the cracks of an historic building which have a seasonal envelope. When movement is cumulative in one direction it is making a significant deformation. Accordingly, the more elastic materials are, the more likely they are to be compatible with the structure as a whole. This is one of the advantages that lime mortars and plasters have over modern cement mixes. (Feilden, 2003)
The great merits of traditional elastic, relatively absorbent and easily renewed lime mortars for pointing and rendering should be emphasized. Portland cement mortars are, on the other hand, stiffer, almost impermeable and excessively strong, so that repairs tend to break away in large sections or subsequent weathering destroys the original, weaker, brick or stone rather than the pointing. (Feilden, 2003)
Different types of lime for different uses
Non-hydraulic to feebly hydraulic lime
This type of lime is available in the UK in the form of lime putty. Lime putty is made by hydrating the lime with a small amount of water. As lime putty is non-hydraulic it is a soft material and is not suitable for areas of extreme exposure on a building. The Northern Ireland Environment Agency (2006) recommends that its use should be in very sheltered locations due to its vulnerability to frost and salt damage during the carbonation process. Lime putty cures slowly in reaction to air thus giving an extended workability.