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Lime plastering and Rendering...

 

The company has a wealth of experience in all aspects of traditional lime plasterwork including:

The Technology of Lime

Lime has been used for building for over 7000 years. Its uses include Roman civil engineering structures, medieval cathedrals and castles. Major 19th century industrial and engineering structures were built completely with lime mortars and plasters - including lighthouses, harbours and canals, and domestic buildings. Most of London is built with lime.

Traditional techniques of masonry construction made the maximum use of all available stone, including offcuts and chippings, with a minimum of mortar. Best quality stone was cut to ashlar blocks, lesser quality was utilised as dressed squared rubble or more informal rubble, and field boulders were used for steadings and dykes. Masonry was laid to horizontal courses, with course lines being more or less formalised depending on the nature of the masonry. All masonry included small pinnings to level up individual stones and courses and to minimise the amount of mortar. Stones were always laid to give a true straight face to walls and mortar joints were filled flush to the face of the stones. It is probable that before the mid 19th century a large proportion of domestic buildings were finished in harling (essentially pebbledash, but using lime render as a base - very predominant in Scotland and Wales, but a little less common in England), although some were simply limewashed onto the stone and some were left as exposed masonry.

Many masonry structures retain their original lime mortars within the structure of the wall, but a majority have been repointed with modern (often harder) materials. Very few buildings retain their original lime-based harl or render. The use of cement for building large structures became widespread after the end of the 19th century, and for domestic building generally by the mid 20th century. For a period of around 50 years between the 1930s and the 1980s the use of lime mortars for repair of buildings was unusual. Cement pointing and renders are frequently found on old buildings, and other materials such as 'plastic-based' paints, silicon 'waterproofers' and resin based coatings are common. All these materials are detrimental to old buildings.

Cement mortars are valued by the modern building industry for their fast set and relatively early frost resistance. However cement mortars have a hardness, brittleness and reduced permeability which are serious drawbacks when used in conjunction with natural stone or other permeable materials. Lime, although it may require more care in use and take longer to reach full maturity, produces more flexible and permeable mortars and renders. Despite their advantages, lime mortars will not, of themselves, solve all problems associated with old buildings. The detailing and maintenance of a building should be the first consideration, as many common problems are the result of inadequate maintenance of the elements (such as flashings and rainwater goods) which are intended to prevent water from affecting the building fabric.

Some buildings also have inherent problems, such as irreversible stone decay, serious salt contamination, or fractured impervious stone, which will not be permanently resolved by the application of either lime based, or cement based, mortars. The performance and effectiveness of lime mortars will be dependent on selection of an appropriate type of mortar for the specific location within a building. For example:-

The life expectancy of lime based mortars can also be directly influenced by any of the following:-

Traditional masonry structures were built as mass construction, generally with permeable finishes, and without the incorporation of membranes and damp-proof courses which modern construction employs. Most masonry materials and their binding mortars are porous and permeable to some degree, and therefore have an ability to take in and release water, and water vapour. All lime mortars, regardless of their strength, can handle and manage the movement of moisture in building fabric, although the softer (non-hydraulic) mortars are generally more effective in this way than the stronger (more hydraulic) mortars. This ability to absorb and re-evaporate moisture is important in old buildings because:-

Cement based mortars, when used for repointing or coating traditional masonry structures, nearly always lead to accelerated stone decay and to the penetration of water.

All old buildings move - due to thermal changes between night and day or between summer and winter- and due to changes in ground conditions. Lime mortars allow the building a small degree of flexibility to accommodate movement without cracking. Cement based mortars used in traditional masonry structures invariably result in the formation of cracks and, in consequence, in the penetration of water.

Lime production

All types of building lime are produced by heating ('burning') limestone, or other chemically similar material. Limestone gives up carbon dioxide when heated and converts to quicklime. This is a caustic, unstable material which reacts vigorously with water to form lime. (This is known as slaking.) The lime may be in the form of a dry powder if a minimum of water is used, or a putty if excess water is used for slaking.

Hydraulic and non-hydraulic limes

Lime produced from pure calcium carbonate materials is non-hydraulic or pure lime - that is it will harden only in air (by absorbing carbon dioxide). If the limestone contains certain other minerals, such as silica and alumina (reactive silicates), other reactions take place producing a chemical set in the lime mortar. This type of lime is known as hydraulic lime.

When wet lime is exposed to air carbon dioxide is reabsorbed and the mortar hardens. This process is known as carbonation. In hydraulic lime this hardening is supplemented by the chemical set. The lime may be more or less hydraulic depending on the amount of reactive chemicals present. The composition and characteristics of the original limestone will determine the type of lime produced, but the way in which the raw materials are processed, particularly the burning temperature, can also have a significant effect on the final properties of the lime and the same hydraulic limestone may be utilised to produce a variety of different types of lime. Cement is produced in a similar way but has a much harder set than hydraulic lime and all the lime content is used up in the chemical setting, leaving no free lime to carbonate.

Classification of limes

The accepted method of classification is based on theoretical strength of the lime mortar. In practice durability and other criteria, such as permeability, are generally more critical for work involving lime mortars. Four categories of lime mortars are recognised in the current European and British Standards, based on theoretical strengths achieved at 28 days. BS890 covers non hydraulic limes, but does not specify strength requirements. Well made and properly cured non hydraulic mortars may readily achieve strengths of up to 3 N/mm2 over a period of time. ENV459 covers:

Sources of lime

Limes continue to be manufactured in England and on the continent. They come in many types and strengths, although the properties of these modern limes can be very different from those of earlier materials. Modern English limes are mainly non-hydraulic, although there is one producer of hydraulic lime in Somerset. Hydraulic limes are imported from the continent.

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