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Lime Putty Mortars 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 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 plasters 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 sand. 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’ mortar 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 repointing 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, brick 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 calcium carbonate so they too produce lime Burning calcium carbonate (CaCO3) at a movement. Lime mortars even have the extracted from the when burned in a lime kiln. 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 plasterwork 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 cement or gypsum. 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 Portland cement, 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 carbon dioxide (CO2) to produce calcium because modern buildings are designed to lime plaster 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 LH111-Lime.indd 3 02/03/2017 14:56 LIME 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 sands, in ensuring compaction and in Peter Bell than cement resulting in 20% less CO2 output.
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