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Lime mortar or torching is composed of
lime
Lime commonly refers to:
* Lime (fruit), a green citrus fruit
* Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide
* Lime (color), a color between yellow and green
Lime may also refer to:
Botany ...
and an
aggregate such as
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
, mixed with water. The ancient Egyptians were the first to use lime
mortars, which they used to plaster their
temple
A temple (from the Latin ) is a building reserved for spiritual rituals and activities such as prayer and sacrifice. Religions which erect temples include Christianity (whose temples are typically called churches), Hinduism (whose temples ...
s. In addition, the Egyptians also incorporated various limes into their religious temples as well as their homes. Indian traditional structures built with lime mortar, which are more than 4,000 years old like
Mohenjo-daro
Mohenjo-daro (; sd, موئن جو دڙو'', ''meaning 'Mound of the Dead Men';[Pakistan
Pakistan ( ur, ), officially the Islamic Republic of Pakistan ( ur, , label=none), is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a population of almost 24 ...]
. It is one of the oldest known types of mortar also used in
ancient Rome
In modern historiography, ancient Rome refers to Roman civilisation from the founding of the city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD. It encompasses the Roman Kingdom (753–509 B ...
and
Greece
Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders with ...
, when it largely replaced the
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
and
gypsum
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywall. ...
mortars common to ancient Egyptian construction.
With the introduction of
Portland cement
Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th c ...
during the 19th century, the use of lime mortar in new constructions gradually declined. This was largely due to the ease of use of Portland cement, its quick setting, and high compressive strength. However, the soft and porous properties of lime mortar provide certain advantages when working with softer
building material
Building material is material used for construction. Many naturally occurring substances, such as clay, rock (geology), rocks, sand, wood, and even twigs and leaves, have been used to construct buildings. Apart from naturally occurring materia ...
s such as
natural stone
This is a geographical list of natural stone used for decorative purposes in construction and monumental sculpture produced in various countries.
The dimension-stone industry classifies stone based on appearance and hardness as either "granit ...
and
terracotta
Terracotta, terra cotta, or terra-cotta (; ; ), in its material sense as an earthenware substrate, is a clay-based ceramic glaze, unglazed or glazed ceramic where the pottery firing, fired body is porous.
In applied art, craft, construction, a ...
. For this reason, while Portland cement continues to be commonly used in new
brick
A brick is a type of block used to build walls, pavements and other elements in masonry construction. Properly, the term ''brick'' denotes a block composed of dried clay, but is now also used informally to denote other chemically cured cons ...
and
concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...
construction, in the repair and
restoration
Restoration is the act of restoring something to its original state and may refer to:
* Conservation and restoration of cultural heritage
** Audio restoration
** Film restoration
** Image restoration
** Textile restoration
* Restoration ecology
...
of brick and stone-built structures originally built using lime mortar, the use of Portland cement is not recommended.
Despite its enduring utility over many centuries, lime mortar's effectiveness as a building material has not been well understood; time-honoured practices were based on tradition, folklore and trade knowledge, vindicated by the vast number of old buildings that remain standing. Only during the last few decades has empirical testing provided a scientific understanding of its remarkable durability. Both professionals and do-it-yourself home owners can purchase lime putty mortar (and have their historical mortar matched for both color and content) by companies that specialize in historical preservation and sell pre-mixed mortar in small batches.
Etymology
''Lime'' comes from Old English ''lim'' 'sticky substance, birdlime, mortar, cement, gluten', and is related to Latin ''limus'' 'slime, mud, mire', and ''linere'' 'to smear'.
["Lime". Online Etymology Dictionary. Accessed 5/10/2014]
/ref> ''Mortar'' is a mixture with cement and comes from Old French ''mortier'' 'builder's mortar, plaster; bowl for mixing' in the late 13th century and Latin ''mortarium'' 'mortar'. Lime is a cement
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mix ...
which is a binder or glue which holds things together but ''cement'' is usually reserved for Portland cement.
Uses
Lime mortar today is primarily used in the conservation of buildings originally built using lime mortar, but may be used as an alternative to ordinary portland cement. It is made principally of lime (hydraulic, or non hydraulic), water and an aggregate such as sand. Portland cement has proven to be incompatible with lime mortar because it is harder, less flexible, and impermeable. These qualities lead to premature deterioration of soft, historic bricks so traditionally, low temperature fired, lime mortars are recommended for use with existing mortar of a similar type or reconstruction of buildings using historically correct methods. In the past, lime mortar tended to be mixed on site with whatever sand was locally available. Since the sand influences the colour of the lime mortar, colours of pointing mortar can vary dramatically from district to district.
Hydraulic and non-hydraulic lime
Hydraulic lime
Hydraulic lime (HL) is a general term for calcium oxide, a variety of lime also called quicklime, that sets by hydration. This contrasts with calcium hydroxide, also called slaked lime or air lime that is used to make lime mortar, the other common ...
contains substances which set by hydration Hydration may refer to:
* Hydrate, a substance that contains water
* Hydration enthalpy, energy released through hydrating a substance
* Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound
* ...
so it can set under water. Non-hydraulic lime sets by carbonation
Carbonation is the chemical reaction of carbon dioxide to give carbonates, bicarbonates, and carbonic acid. In chemistry, the term is sometimes used in place of carboxylation, which refers to the formation of carboxylic acids.
In inorganic ch ...
and so needs exposure to carbon dioxide in the air and cannot set under water or inside a thick wall. For natural hydraulic lime (NHL) mortars, the lime is obtained from limestone naturally containing a sufficient percentage of silica and/or alumina. Artificial hydraulic lime is produced by introducing specific types and quantities of additives to the source of lime during the burning process, or adding a pozzolan
Pozzolans are a broad class of Silicon_dioxide, siliceous and aluminium oxide, aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically ...
to non-hydraulic lime. Non-hydraulic lime is produced from a high purity source of calcium carbonate such as chalk, limestone or oyster shells.
Non-hydraulic lime
Non-hydraulic lime is primarily composed of (generally greater than 95%) calcium hydroxide
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
, Ca(OH)2.
Non-hydraulic lime is produced by first heating sufficiently pure calcium carbonate to between 954° and 1066 °C, driving off carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
to produce quicklime (calcium oxide
Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, Caustic (substance), caustic, alkaline, crystalline solid at room temperature. The broadly used term "''lime (material), lime''" co ...
). This is done in a lime kiln
A lime kiln is a kiln used for the calcination of limestone (calcium carbonate) to produce the form of lime (material), lime called quicklime (calcium oxide). The chemical equation for this chemical reaction, reaction is
:Calcium carbonate, Ca ...
. The quicklime is then ''slaked'': hydrated by being thoroughly mixed with enough water to form a slurry (lime putty), or with less water to produce dry powder. This hydrated lime (calcium hydroxide) naturally turns back into calcium carbonate by reacting with carbon dioxide in the air, the entire process being called the ''lime cycle''.
The slaking process involved in creating a lime putty is an exothermic
In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
reaction which initially creates a liquid of a creamy consistency. This is then matured for 2 to 3 months—depending upon environmental conditions—to allow time for it to condense and mature into a lime putty.
A matured lime putty is ''thixotropic
Thixotropy is a time-dependent shear thinning property. Certain gels or fluids that are thick or viscous under static conditions will flow (become thinner, less viscous) over time when shaken, agitated, shear-stressed, or otherwise stressed (ti ...
'', meaning that when a lime putty is agitated it changes from a putty into a more liquid state. This aids its use for mortars as it makes a mortar easier to work with. If left to stand following agitation a lime putty will slowly revert from a thick liquid to a putty state.
As well as calcium-based limestone, dolomitic
Dolomite () is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite. An alternative name sometimes used for the dolomiti ...
limes can be produced which are based on calcium magnesium carbonate
Dolomite () is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite. An alternative name sometimes used for the dolomiti ...
.
A frequent source of confusion regarding lime mortar stems from the similarity of the terms hydraulic and hydrated.
* ''Hydrated'' lime is any lime other than quicklime, and can refer to either hydraulic (hardens under water) or non-hydraulic (does not harden under water) lime.
* Lime putty is always non-hydraulic and will keep indefinitely stored under water. As the name suggests, lime putty is in the form of a putty
Putty is a material with high plasticity, similar in texture to clay or dough, typically used in domestic construction and repair as a sealant or filler. Although some types of putty (typically those using linseed oil) slowly polymerise and be ...
made from just lime and water.
If the quicklime is slaked with an excess of water then putty or slurry is produced. If just the right quantity of water is used, the result is a dry material (any excess water escaping as steam during heating). This is ground to make hydrated lime powder.
Hydrated, non-hydraulic lime powder can be mixed with water to form lime putty. Before use putty is usually left in the absence of carbon dioxide (usually under water) to mature. Putty can be matured for as little as 24 hours or for many years; an increased maturation time improves the quality of the putty. There is an argument that a lime putty which has been matured for an extended period (over 12 months) becomes so stiff that it is difficult to work.
There is some dispute as to the comparative quality of putty formed from dry hydrated lime compared with that produced as putty at the time of slaking. It is generally agreed that the latter is preferable. A hydrated lime will produce a material which is not as "fatty”, being a common trade term for compounds have a smoother buttery texture when worked. Often, due to lengthy and poor storage, the resulting lime produced by hydrated lime will exhibit longer carbonatation periods as well as lower compressive strengths.
Non-hydraulic lime takes longer to set and is weaker than hydraulic lime, and should not be allowed to freeze before it is well set. Although the setting process can be slow, the drying time of a lime mortar must be regulated at a slow rate to ensure a good final set. A rapidly dried lime mortar will result in a low-strength, poor-quality final mortar often displaying shrinkage cracks. In practice, lime mortars are often protected from direct sunlight and wind with damp hessian
A Hessian is an inhabitant of the German state of Hesse.
Hessian may also refer to:
Named from the toponym
*Hessian (soldier), eighteenth-century German regiments in service with the British Empire
**Hessian (boot), a style of boot
**Hessian f ...
sheeting or sprayed with water to control the drying rates. But it also has the quality of autogeneous healing (self healing) where some free lime dissolves in water and is redeposited in any tiny cracks which form.
Oyster Shell Mortar
In the tidewater region of Maryland and Virginia, oyster shells were used to produce quicklime during the colonial period. Similar to other materials used to produce lime, the oyster shells are burned. This can be done in a lime rick instead of a kiln. Burning shells in a rick is something that Colonial Williamsburg
Colonial Williamsburg is a living-history museum and private foundation presenting a part of the historic district in the city of Williamsburg, Virginia, United States. The Colonial Williamsburg Foundation has 7300 employees at this location and ...
and the recreation of Ferry Farm
Ferry Farm, also known as the George Washington Boyhood Home Site or the Ferry Farm Site, is the farm and home where George Washington spent much of his childhood. The site is located in Stafford County, Virginia, along the northern bank of the Ra ...
have had to develop from conjecture and in-the-field learning. The rick that they constructed consists of logs set up in a circle that burn slowly, converting oysters that are contained in the wood pile to an ashy powder. An explanatory video of how the rick was built for the Ferry Farm can be foun
here
The burnt shell can then be slaked and turned into lime putty.
Mortars using oyster shells can sometimes be identified by the presence of small bits of shell in the exposed mortar joint. In restoration masonry, the bits of shell are sometimes exaggerated to give the viewer the impression of authenticity. Unfortunately, these modern attempts often contain higher than necessary ratios of Portland cement
Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th c ...
. This can cause failures in the brick if the mortar joint is stronger than the brick elements.
Hydraulic lime
Hydraulic lime sets by reaction with water called hydration.
When a stronger lime mortar is required, such as for external or structural purposes, a pozzolan
Pozzolans are a broad class of Silicon_dioxide, siliceous and aluminium oxide, aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically ...
can be added, which improves its compressive strength and helps to protect it from weathering damage. Pozzolans include powdered brick, heat treated clay, silica fume
Silica fume, also known as microsilica, (CAS number 69012-64-2, EINECS number 273-761-1) is an amorphous (non-crystalline) polymorph of silicon dioxide, silica. It is an ultrafine powder collected as a by-product of the silicon and ferrosilicon a ...
, fly ash
Fly ash, flue ash, coal ash, or pulverised fuel ash (in the UK) plurale tantum: coal combustion residuals (CCRs)is a coal combustion product that is composed of the particulates (fine particles of burned fuel) that are driven out of coal-fired ...
, and volcanic materials. The chemical set imparted ranges from very weak to almost as strong as Portland cement.
This can also assist in creating more regulated setting times of the mortar as the pozzolan will create a hydraulic set, which can be of benefit in restoration projects when time scales and ultimately costs need to be monitored and maintained.
Hydraulic lime can be considered, in terms both of properties and manufacture, as part-way between non-hydraulic lime and Portland cement. The limestone used contains sufficient quantities of clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
and/or silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
. The resultant product will contain dicalcium silicate
Calcium silicate is the chemical compound Ca2SiO4, also known as calcium orthosilicate and is sometimes formulated as 2CaO·SiO2. It is also referred to by the shortened trade name Cal-Sil or Calsil. It occurs naturally as the mineral larnite.
...
but unlike Portland cement not tricalcium silicate
Alite is an impure form of tricalcium silicate, , sometimes formulated as ( in cement chemist notation), typically with 3-4% of substituent oxides. It is the major, and characteristic, phase in Portland cement. The name was given by Törnebohm in ...
.
It is slaked enough to convert the calcium oxide to calcium hydroxide but not with sufficient water to react with the dicalcium silicate. It is this ''dicalcium silicate'' which in combination with water provides the setting properties of hydraulic lime.
Aluminium and magnesium also produce a hydraulic set, and some pozzolans contain these elements.
There are three strength grades for natural hydraulic lime, laid down in the European Norm EN459; NHL2, NHL3.5 and NHL5. The numbers stand for the minimum compressive strength at 28 days in newtons per square millimeter (N/mm2). For example, the NHL 3.5 strength ranges from 3.5 N/mm2 (510 psi) to 10 N/mm2 (1,450 psi). These are similar to the old classification of feebly hydraulic, moderately hydraulic and eminently hydraulic, and although different, some people continue to refer to them interchangeably. The terminology for hydraulic lime mortars was improved by the skilled French civil engineer Louis Vicat
Louis Vicat (31 March 1786, Nevers – 10 April 1861, Grenoble) was a French engineer.
He graduated from the École Polytechnique in 1804 and the École des Ponts et Chaussées in 1806.
Vicat studied the setting of mortars and developed his own ...
in the 1830s from the older system of water limes and feebly, moderately and eminently. Vicat published his work following research of the use of lime mortars whilst building bridges and roads in his work. The French company Vicat still currently produce natural cements and lime mortars. Names of lime mortars were so varied and conflicting across the European continent that the reclassification has greatly improved the understanding and use of lime mortars.
Mix
Traditional lime mortar is a combination of lime putty and aggregate (usually sand). A typical modern lime mortar mix would be 1 part lime putty to 3 parts washed, well graded, sharp sand. Other materials have been used as aggregate instead of sand. The theory is that the voids of empty space between the sand particles account for a 1/3 of the volume of the sand. The lime putty when mixed at a 1 to 3 ratio, fill these voids to create a compact mortar. Analysis of mortar samples from historic buildings typically indicates a higher ratio of around 1 part lime putty to 1.5 part aggregate/sand was commonly used. This equates to approximately 1 part dry quicklime to 3 parts sand. A traditional coarse plaster mix also had horse hair added for reinforcing and control of shrinkage, important when plastering to wooden laths and for base (or dubbing) coats onto uneven surfaces such as stone walls where the mortar is often applied in thicker coats to compensate for the irregular surface levels.
If shrinkage and cracking of the lime mortar does occur this can be as a result of either
* The sand being poorly graded or with a particle size that is too small
* The mortar being applied too thickly (Thicker coats increase the possibility of shrinkage, cracking and slumping)
* Too much suction from the substrate
* High air temperatures or direct sunlight which force dry the mortar
* High water content in the lime mortar mix
* Poor quality or unmatured lime putty
A common method for mixing lime mortar with powdered lime is as follows:
* Gather your ingredients, sand, lime, and water
* Measure out your ratio of sand to lime, for example 3 buckets of sand, and 1 bucket of lime for a 3:1 ratio.
* Mix the dry ingredients thoroughly so all the sand is coated with lime, and there are neither chunks of sand or lime visible.
* Reserve some portion of the dry ingredients by removing it from your mixing vessel. The amount reserved can vary, but a safe starting point is about 1/4 of the batch. This will be added in later to fine tune the dryness of the mix.
* Measure out water. How much depends on how wet you want your mix to be, and how damp/wet your sand is. A good starting point is 1 quart of water per gallon of sand.
* Add about 2/3 of the water to your dry ingredients and mix until even consistency.
* Add the reserved dry ingredients and/or the remaining water to get a mix you like. It takes time to know what works well, and the recipe can change depending on the temperature, humidity, moisture in the sand, type of brick, and task at hand (laying brick may warrant a wetter mix, while pointing may require a drier one.
* To test the mix as you are making it, you can use a trowel, or pat the mortar with your hand to see how much moisture and "cream" come to the surface.
* Remember to thoroughly wet your brick prior to using lime mortar. Old brick can be extremely porous, a 4lb brick can hold a pint of water. The bricks should be saturated, but dry on the surface prior to laying or pointing. Excess water can cause the lime to run and leave streaks.
Hair reinforcement
Hair reinforcement is common in lime plaster
Lime plaster is a type of plaster composed of sand, water, and lime, usually non-hydraulic hydrated lime (also known as slaked lime, high calcium lime or air lime). Ancient lime plaster often contained horse hair for reinforcement and pozzolan ...
and many types of hair and other organic fibres can be found in historic plasters.[Bennett, B., 2002, Lime Plaster and Render Reinforcement, Building Conservation Directory] However, organic material in lime will degrade in damp environments particularly on damp external renders. This problem has given rise to the use of polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins and ...
fibres in new lime render
Lime render is the first coat of lime "plaster or the like" applied to the external surfaces of traditionally-built stone or brick buildings.
It allows the building to 'breathe' – as lime is porous, it allows for the collection and evaporation ...
s
Properties
* Lime mortar is not as strong in compression as Portland cement based mortar, but both are sufficiently strong for construction of non-high-rise domestic properties.
* Lime mortar does not adhere as strongly to masonry as Portland cement. This is an advantage with softer types of masonry, where use of cement in many cases eventually results in cement pulling away some masonry material when it reaches the end of its life. The mortar is a sacrificial element which should be weaker than the bricks so it will crack before the bricks. It is less expensive to replace cracked mortar than cracked bricks.
* Under cracking conditions, Portland cement breaks, whereas lime often produces numerous microcracks if the amount of movement is small. These microcracks recrystallise through the action of 'free lime' effectively self-healing the affected area.
* Historic buildings are frequently constructed with relatively soft masonry units (e.g. soft brick and many types of stone), and minor movement in such buildings is quite common due to the nature of the foundations. This movement breaks the weakest part of the wall, and with Portland cement mortar this is usually the masonry. When lime mortar is used, the lime is the weaker element, and the mortar cracks in preference to the masonry. This results in much less damage, and is relatively simple to repair.
* Lime mortar is more porous than cement mortars, and it wicks any dampness in the wall to the surface where it evaporates. Thus any salt content in the water crystallises on the lime, damaging the lime and thus saving the masonry. Cement, on the other hand, evaporates water less than soft brick, so damp issues are liable to cause salt formation and spalling on brick surfaces and consequent disintegration of bricks. This damp evaporation ability is widely referred to as 'breathability'.
* Lime mortar should not be used below temperatures of and takes longer to set so it should be protected from freezing for three months. Because of its faster set, hydraulic lime may not need as much time before freezing temperatures begin.
Usually any dampness in the wall will cause the lime mortar to change colour, indicating the presence of moisture. The effect will create an often mottled appearance of a limewashed wall. As the moisture levels within a wall alter, so will the shade of a limewash. The darker the shade of limewash, the more pronounced this effect will become.
A load of mixed lime mortar may be allowed to sit as a lump for some time, without it drying out (it may get a thin crust). When ready to use, this lump may be remixed ('knocked up') again and then used. Traditionally on building sites, prior to the use of mechanical mixers, the lime putty (slaked on site in a pit) was mixed with sand by a labourer who would "beat and ram" the mix with a "larry" (a wide hoe with large holes). This was then covered with sand and allowed to sit for a while (from days to weeks) - a process known as 'banking'. This lump was then remixed and used as necessary. This process cannot be done with Portland cement.
Lime with Portland cement
The combination of Portland cement and lime is used for stabilization and solidification of the ground through establishing of lime cement columns or stabilization of the entire upper mass volume. The method provides an increase in strength when it comes to vibrations, stability and settling. When building e.g. roads and railways, the method is more common and widespread (Queen Eufemias street in Central Oslo, E18 at Tønsberg etc.).
For preservation purposes, Type N and Type O mortars are often used. A Type N mortar is 1 part Portland, 1 part Lime and 6 parts sand or other aggregate (1:1:6). A Type O mortar is 1 part Portland, 2 parts Lime and 9 parts sand or other aggregate (1:2:9). Straight lime mortar has no Portland, and 1 part Lime to 3 parts sand or other aggregate. The addition of cement or other pozzolan
Pozzolans are a broad class of Silicon_dioxide, siliceous and aluminium oxide, aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically ...
to decrease cure times is referred to as “gauging.” Other than Portland, ash and brick dust have been used to gauge mortars.
For historic restoration purposes, and restoration work involving repointing or brick replacement, masons must discover the original brick and mortar and repair it with a similar material. The National Park Service provides guidance for proper masonry repointing throug
Preservation Brief 2
In general, Brief 2 suggests that repointing
Repointing is the process of renewing the pointing, which is the external part of mortar joints, in masonry construction. Over time, weathering and decay cause voids in the joints between masonry units, usually in bricks, allowing the undesirable e ...
should be done with a similar or weaker mortar. Therefore, a straight lime mortar joint should be repointed in kind. Due to the popularity of Portland cement, this often is not the case. A wall system needs a balance between the mortar and brick that allows the mortar to be the weak part of the unit. When mortar is stronger than the brick, it prevents any natural movement in the wall and the faces of the brick will begin to deteriorate, a process known as spall
Spall are fragments of a material that are broken off a larger solid body. It can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ball ...
ing, the process by which the outer face of a brick degrades and can flake off or turn to powder. There is also a natural movement of water through a masonry wall. A strong Portland cement mix will prevent a free flow of water from a moist to dry area. This can cause rising damp
Structural dampness is the presence of unwanted moisture in the structure of a building, either the result of intrusion from outside or condensation from within the structure.
A high proportion of damp problems in buildings are caused by ambient ...
to be trapped within the wall and create system failures. If moisture can not escape into the air, it will cause damage to a wall structure. Water freezing in the wall is another cause of spalling.
In restoration work of pre-20th century structures, there should be a high ratio of lime and aggregate to Portland. This reduces the compressive strength of the mortar but allows the wall system to function better. The lime mortar acts as a wick that helps to pull water from the brick. This can help to prevent the older brick from spalling. Even when the brick is a modern, harder element, repointing with a higher ratio lime mortar may help to reduce rising damp.
It may not be advisable for all consumers to use a straight lime mortar. With no Portland in the mix, there is less control over the setting of the mortar. In some cases, a freeze thaw cycle will be enough to create failure in the mortar joint. Straight lime mortar can also take a long time to fully cure and therefore work needs to be performed at a time of year where the weather conditions are conducive to the mortar setting properly. Those conditions are not only above freezing temperatures but also drier seasons. To protect the slow curing mortar from damp, a siloxane
A siloxane is a functional group in organosilicon chemistry with the Si−O−Si linkage. The parent siloxanes include the oligomeric and polymeric hydrides with the formulae H(OSiH2)''n''OH and (OSiH2)n. Siloxanes also include branched compoun ...
can be added to the surface. With historic structures, this may be a controversial strategy as it could have a detrimental effect to the historic fabric.
The presence of Portland allows for a more stable mortar. The stability and predictability make the mixed mortar more user friendly, particularly in applications where entire wall sections are being laid. Contractors and designers may prefer mixes that contain Portland due to the increased compressive strength over a straight lime mortar. As many pre-Portland mix buildings are still standing and have original mortar, the arguments for greater compressive strength and ease of use may be more a result of current practice and a lack of understanding of older techniques.
See also
*Energetically modified cement
Energetically modified cements (EMCs) are a class of cements made from pozzolans (e.g. fly ash, volcanic ash, pozzolana), silica sand, blast furnace slag, or Portland cement (or blends of these ingredients). The term "energetically modified" ar ...
*Hempcrete
Hempcrete or hemplime is biocomposite material, a mixture of hemp hurds ( shives) and lime, sand, or pozzolans, which is used as a material for construction and insulation. It is marketed under names like Hempcrete, Canobiote, Canosmose, Isochan ...
*Plastering
Plasterwork is construction or ornamentation done with plaster, such as a layer of plaster on an interior or exterior wall structure, or plaster decorative moldings on ceilings or walls. This is also sometimes called pargeting. The process of ...
* Sticky rice mortar
*Whitewash
Whitewash, or calcimine, kalsomine, calsomine, or lime paint is a type of paint made from slaked lime ( calcium hydroxide, Ca(OH)2) or chalk calcium carbonate, (CaCO3), sometimes known as "whiting". Various other additives are sometimes used ...
References
;Further reading
* Burnell, George Rowdon; ''Rudimentary Treatise on Limes, Cements, Mortars, Concretes, Mastics, Plastering, Etc.''
* Dibdin, William Joseph; ''Lime Mortar & Cement: Their Characteristics and Analyses. With an Account of Artificial Stone and Asphalt''
* Gilmore, Quincy A.; ''Limes Hydraulic Cement and Mortars''
* Hodgson, Fred T.; ''Concrete, Cements, Mortars, Artificial Marbles, Plasters and Stucco: How to Use and How to Prepare Them''
* Lazell, Ellis Warren; ''Lime Mortar & Cement : Their Characteristics and Analyses. With an Account of Artificial Stone and Asphalt''
External links
* The following are mid-19th-century technical articles on the respective subjects
lime mortar
an
{{DEFAULTSORT:Lime Mortar
Building materials
Cement
Masonry