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Home , Lime mortar, Lime plaster

Hydrated is available in different forms. It is commonly bought in powder form, sometimes referred to as bag lime, from the builders’ merchants (left). A 25Kg bag costs between £10 and £15. Lime putty (right), on Building lime. the other hand, is purchased in tubs from specialist suppliers (right). A 14 litre tub of lime putty costs What’s it all about? between £10 and £14

WHAT IS LIME? not readily available was masonry sometimes This process of lime burning, slaking and then condensation on windows and wall surfaces f you are involved in any Lime is used as a binder in mortars, bedded on mud or clay, but by far the majority carbonation is known as the ‘lime cycle’. is much reduced. restoration or conservation and renders. It is produced by burning of traditional mortars consisted of two • They are relatively soft and fl exible which Limekilns were found WHY IS IT IMPORTANT TO USE across the country work, large or small, it calcium based stones, normally limestone or ingredients: lime and . is essential in old buildings which tend LIME IN TRADITIONAL BUILDINGS? and many burned well helps to understand the chalk, in a kiln. What comes out of the kiln is to move a little with the seasons or with Lime produces a very ‘plastic’ into the 20th century. importance of lime. Whether quicklime, which reacts violently with water THE CHEMISTRY changes in ground conditions. Because lime which is excellent for bedding masonry Limestone or chalk you are brickwork, to produce either lime putty or bag lime. Feel free to skip this bit, but the more mortars are soft, they cushion and protect and wonderfully adhesive for plasters. The and fuel was fed into I technically minded may be interested to stone, or timber, all of which can plastering walls, decorating or advantage of using lime mortars and plasters the top of the kiln. replacing a floor, the likelihood is Seashells and corals are also made of understand the chemical processes involved. be damaged by hard mortars that resist Quicklime and ash was in traditional buildings cannot be overstated: that your conservation professional so they too produce lime Burning calcium carbonate (CaCO3) at a movement. Lime mortars even have the extracted from the when burned in a . Christopher temperature of about 950°C forms calcium aperture at the bottom. or builders will refer to lime. Lime • They retain the character of old buildings in ability to self-heal. Wren specifi ed ‘cockleshell lime’ for use in oxide (CaO) known as quicklime. When This small lime kiln in a way that modern materials cannot. Lime • Lime mortars remain workable for hours crops up in the context of mortars, prestigious in the 17th century water is added to quicklime in a process the Yorkshire Dales was mortars and plasters have a soft, warm and sometimes days or weeks. They can renders, plasters and washes, so to ensure its quality and consistency. known as slaking, it produces hydrated lime located close to natural character that is virtually impossible to be stored in sealed containers for months it’s good to go back to basics and (Ca(OH)2). This is the lime which you buy outcrops of limestone replicate using or . without going off. explain what it’s all about and HOW IS IT USED? either as lime putty or in dry powder form • They encourage evaporation from walls, so why lime is, and always has been, Prior to the development of , from the builders’ merchants. Hydrated lime keep buildings dry which is good for the Modern building materials based on important in traditional building virtually every masonry structure in Britain is added to sand/aggregate to form a mortar building and its inhabitants. This process is cement do not have the same characteristics from Roman times was built with lime which, when exposed to air, reacts with construction. often referred to as ‘breathability’. as lime. They do not need to breathe or fl ex mortar. Most buildings were plastered with (CO2) to produce calcium because modern buildings are designed to lime and then fi nished with limewash. carbonate (CaCO3) in a process referred to • Because they breathe, they also have the behave in a different way from traditionally Lime was used as the binder to fi ll the gaps as carbonation. In layman’s terms, it reverts benefi cial effect of absorbing moisture at constructed buildings. between the particles of sand and to hold back to being limestone and effectively sticks times when humidity levels are high in a them together. Only in areas where lime was the particles of sand together. building. Consequently, the risk of internal Continued >>

46 Listed Heritage Magazine March/April 2017

LH111-Lime.indd 2 02/03/2017 14:56 LH111-Lime.indd 3 02/03/2017 14:56 Hydrated lime is available in different forms. It is commonly bought in powder form, sometimes referred to as bag lime, from the builders’ merchants (left). A 25Kg bag costs between £10 and £15. Lime putty (right), on Building lime. the other hand, is purchased in tubs from specialist suppliers (right). A 14 litre tub of lime putty costs What’s it all about? between £10 and £14

not readily available was masonry sometimes This process of lime burning, slaking and then condensation on windows and wall surfaces bedded on mud or clay, but by far the majority carbonation is known as the ‘lime cycle’. is much reduced. of traditional mortars consisted of two • They are relatively soft and fl exible which WHY IS IT IMPORTANT TO USE ingredients: lime and sand. is essential in old buildings which tend LIME IN TRADITIONAL BUILDINGS? to move a little with the seasons or with Lime produces a very ‘plastic’ mortar THE CHEMISTRY changes in ground conditions. Because lime which is excellent for bedding masonry Feel free to skip this bit, but the more mortars are soft, they cushion and protect and wonderfully adhesive for plasters. The technically minded may be interested to stone, brick or timber, all of which can advantage of using lime mortars and plasters understand the chemical processes involved. be damaged by hard mortars that resist in traditional buildings cannot be overstated: Burning calcium carbonate (CaCO3) at a movement. Lime mortars even have the temperature of about 950°C forms calcium • They retain the character of old buildings in ability to self-heal. oxide (CaO) known as quicklime. When a way that modern materials cannot. Lime • Lime mortars remain workable for hours water is added to quicklime in a process mortars and plasters have a soft, warm and sometimes days or weeks. They can known as slaking, it produces hydrated lime character that is virtually impossible to be stored in sealed containers for months (Ca(OH)2). This is the lime which you buy replicate using cement or gypsum. without going off. either as lime putty or in dry powder form • They encourage evaporation from walls, so from the builders’ merchants. Hydrated lime keep buildings dry which is good for the Modern building materials based on is added to sand/aggregate to form a mortar building and its inhabitants. This process is cement do not have the same characteristics which, when exposed to air, reacts with often referred to as ‘breathability’. as lime. They do not need to breathe or fl ex carbon dioxide (CO2) to produce calcium because modern buildings are designed to carbonate (CaCO3) in a process referred to • Because they breathe, they also have the behave in a different way from traditionally as carbonation. In layman’s terms, it reverts benefi cial effect of absorbing moisture at constructed buildings. back to being limestone and effectively sticks times when humidity levels are high in a the particles of sand together. building. Consequently, the risk of internal Continued >>

Listed Heritage Magazine March/April 2017 47

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ARE THERE ANY DISADVANTAGES projects and modern cavity walled building has left a gap in the country’s skill base and TO USING LIME? construction but were entirely unsuited it is surprising how many builders have never • The curing of lime mortars is necessarily to traditional buildings which were built of used it, or shy away from using it because it slow because of the carbonation process. In relatively soft, porous and flexible materials. is unfamiliar to them. Listed property owners some circumstances this means that work It was only in the 1970s and 80s that people do not need to be experts in lime technology will progress more slowly. became aware of the serious damage cement but it will help to understand the basics so • Lime mortars are not totally frost resistant was causing to historic buildings and that that you can select the correct tradesmen and until carbonation has taken place which marked the beginning of the ‘lime revival’. avoid causing damage to your special buildings normally takes months (or even years if by using inappropriate materials. Look out for the mortar is in the middle of a deep wall). WHAT ABOUT LIME’S the next articles in this lime series which will Consequently, external lime working has to SUSTAINABILITY CREDENTIALS? explain more about the different limes that are be carried out between spring and autumn. Both lime and cement production result in available and how to use them. CO2 emissions through burning fossil fuels, • Skill is required in selecting appropriate but lime is burned at a lower temperature , in ensuring compaction and in Peter Bell than cement resulting in 20% less CO2 output. LPOC Conservation Advisor controlling the curing. Lime also absorbs nearly its own weight in • In its wet state lime is caustic, so proper CO2 during carbonation (curing). The LPOC Suppliers Directory includes safety precautions need to be observed details of lime suppliers, lime plasterers to avoid harm to skin or injury to eyes. Limestone is available worldwide and so lime and tradesmen who are familiar with lime is a relatively renewable resource. There is also technology. The Suppliers Directory can be Although these factors may result in extra less waste when working with lime mortars found in this magazine on page 105 and online cost, using lime is essential because of its as mixes can remain useable the following day www.lpoc.co.uk special qualities. It is often the only material and some may be stored indefinitely. Because to use which will protect the building for is soft, materials laid in lime can be generations to come. reclaimed at the end of their life. GLOSSARY HOW CAN I TELL THE DIFFERENCE For all these reasons lime mortars are Lime putty: the basic ingredient in lime BETWEEN LIME AND CEMENT? considered more sustainable than cement mortars, renders and washes. Produced Lime mortars tend to be softer and lighter in mortars and are widely used in the ecological by adding quicklime to water in a process colour with a varied texture. Cement mortars building industry. known as slaking. are hard and often grey in colour. If you put a lime scale remover (such as Viacal) on a lime SOME COMMON MISCONCEPTIONS: Lime mortar: used in laying masonry mortar it will fizz but not on a cement mortar. • Because lime is porous, it encourages damp. In fact the opposite is true. Traditional and in repointing. It is made by mixing lime with sand. DO THE BENEFITS OF LIME ONLY buildings rely on porous materials in order APPLY TO LISTED BUILDINGS? to allow damp, which is inherent in all walls, : used to create smooth and No. Virtually all solid walled, traditionally to evaporate. Any attempt to keep moisture decorative finishes on walls and ceilings. constructed buildings were built using soft out using non porous materials actually has It is made by mixing lime with sand and breathable materials which will be damaged by the effect of keeping moisture in. can include hair for added strength. the use of hard cement mortars and renders. • Lime mortars are not durable. Lime’s softness The advantages of using lime apply to most and flexibility should not be confused with Limewash: used as a decorative finish to buildings built before the First World War. lack of durability. Lime mortar survives cover internal and external walls. Widely from the construction of the pyramids in used in traditional buildings, it is made by WHY DID LIME FALL OUT OF 4000 BC and from Roman Britain; it is not diluting lime putty in water. A pigment can FASHION IN THE 20TH CENTURY? a short-lived material. Not only do lime be added to create a colour wash. Portland cement was patented in 1824. It mortars last a long time, they protect the had some similar qualities to lime in that it materials they surround. Carbonation: the process by which lime too could be used as the binder in mortar mortars, renders and washes set by and renders. By the mid to late-19th century, • A mortar gauged with cement and lime is absorbing carbon dioxide from the air cement was considered easier to use, cheaper, a lime mortar. Mortars mixed with lime as to produce calcium carbonate. harder and faster setting than lime. Small scale well as cement were widely used in the lime producers could not compete against the past and still appear today (often in the industrialised cement factories and there was proportion 1:1:6 or 1:2:9 cement:lime:sand). a widespread belief that cement was more In these mortars lime is used as a plasticiser effective. but the mortar has the characteristics of a cement mortar (hard and impermeable) Lime production continued to decline during rather than of a true lime mortar (soft and the 20th century. Many lime kilns closed porous). They should generally be avoided for the last time during the Second World in traditional buildings. War when there was a risk of the Luftwaffe • ‘Limelite’ is a lime plaster: Limelite is a using the red glow of burning lime kilns to trade name for a cement-based plaster navigate the country. Most lime kilns burned manufactured by Tarmac. Despite its name, continuously and, once cooled, were difficult it should never be confused with to recommission. Traditional lime mortars lime plaster. disappeared almost entirely by the 1950s. Lime is not outmoded technology, nor is it In reality, cement mortars were much harder complicated or difficult to use. However, the and better suited to some engineering decline in its use during the 20th century

48 Listed Heritage Magazine March/April 2017

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Traditional lime mortar used for re-pointing masonry

crushed pottery shards, would achieve a quicker, harder set. The calcined clay reacts with lime putty giving a slight hydraulic set. The Greeks and Romans found that the pumice from volcanic regions containing silicates and aluminates would also achieve a similar reaction. We take from their discovery the term ‘’ derived from Pozzuoli in the volcanic region of Naples. The Romans referred to this hydraulic building lime as ‘cementum’ and it was used in major building and engineering projects such as the construction of the Pantheon and Roman aqueducts.

Manufactured are now available and widely used these days, most commonly Lime putty mortars – metakaolin – thermally treated china clay, and the waste products of several major industries such as blast furnace slag, fl y ash strength in weakness and . The addition of pozzolans essentially kick-starts the strength gain so that carbonation can continue with less risk of frost damage to a young lime putty mix. Unlike cement, which initially sets lime render could take a couple of years to hydraulically with the addition of water, fully carbonate! Of course, it should not be Irrespective of the time of year, care should traditional lime putty products cure by expected to protect new external renders, always be taken to protect lime mortars absorbing carbon dioxide from the air. This lime pointing or bedding mortars for this and renders externally. Rapid drying from process of carbonation is lengthy (estimated length of time, but as more carbonation takes direct sunlight or strong winds can also cause to be one millimetre per month) and place the stronger and less vulnerable the lime mortars and renders to craze or crack therefore lime putty mortars and renders material becomes. if they are left exposed to the elements. remain weak and vulnerable to damage Lime needs a little more care and respect for signifi cantly longer compared to mixes For this reason, most specialist suppliers and as a construction material but should not containing cement. contractors will recommend that external be feared because of its relative weakness work is only carried out from spring through compared to many modern construction For example, tested under laboratory to autumn – the earlier in the year work takes materials. conditions, lime putty mortar has a place, the further the lime can carbonate and compressive strength of between 0.5-3 MPa strengthen before winter sets in. Lower strength lime mortars are highly vapour (megapascals) at 28 days (a megapascal is permeable (breathable), they are fl exible, equal to the force that would give a mass However, having a reduced strength mortar and they should be considered the sacrifi cial of one kilogram an acceleration of one is a distinct advantage for many applications element in traditional buildings – after all, it’s millimetre per second per second). By which require sympathetic consideration. much easier to re-point a wall with lime after comparison, a Portland cement mortar would Lime putty mortars are more fl exible and 100 years than it is to replace the stonework/ typically test between 17-20 MPa at 28 days. vapour permeable than mixes containing brickwork that would have deteriorated in cement – properties which can be essential a fraction of the time where cement based The slower curing and carbonation for traditional, solid wall constructions in mixes have been inappropriately used! process of traditional lime putty mixes mean minimising cracking and reducing issues arising that they can be more prone to failure caused from damp. Selecting a strong mortar to try Ryan Stojic by frost damage during the winter months and minimise the risk of frost damage can General Manager compared to cement or natural hydraulic be damaging to masonry and contribute to Mike Wye & Associates Ltd. lime mixes. Water contained within young condensation, mould growth and reduced lime mortars can freeze and expand causing thermal effi ciency. delamination, cracking and failure (often For more information contact: typifi ed by crumbling). The addition of natural materials used to Mike Wye & Associates Ltd strengthen lime is often advocated for all Tel: 01409 281644 If we work on the assumption that full external works. Early evidence suggests that [email protected] carbonation takes one millimetre per the Minoans had discovered that adding burnt www.mikewye.co.uk month, then a typical 25 millimetre thick clay, sometimes in the form of brick dust or

50 Listed Heritage Magazine March/April 2017 © The Listed Property Owners’ Club

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