Click on an image
The use of building limes as the principal binder in mortars and renders has increased considerably over the past five to ten years. These simple, basic materials are fundamentally important to the long term survival of historic buildings, yet there are many who still regard their use with suspicion, generally unnecessary; a fad propagated by eccentrics and adopted by enthusiastic young fogies. It is particularly disturbing to find that such views are held by many of the professionals and the practitioners who are charged with the care and repair of much of our old building stock.
Lime has been used in building for thousands of years, and it is only in the past 50 years or so that its use has declined. During this period the UK’s building industry has witnessed the growth of huge cement producing companies and the decimation of the numerous sources of building limes which were active in the past. These small provincial industries had produced a whole range of limes, the properties of which varied according to the geology of the region, including many with some ‘hydraulic’ properties (see glossary). Now, although the number of sites producing building limes is generally increasing, despite the decline in the number of raw material sources, which will be seriously affected with the further decline of the British steel industry, a major user of quicklime.
The production of hydraulic lime in the UK remains limited to one source in Somerset, with a wider range being imported from continental Europe.
SO WHAT IS WRONG WITH CEMENT?
Cement or, to use its technical term, ordinary Portland cement was discovered in the early 19th Century. Its great advantage is that it sets rapidly on the addition of water to provide a very hard and rigid binder for mortars and concrete. It was, and remains a simply superb material for numerous construction needs including engineering works in particular.
However, mortars and renders made with cement are much more dense than those made with lime and are much less permeable. That is to say that the passage of air and moisture through the material is restricted, and a coating of cement mortar effectively blocks the pores of stone brick and old lime mortars and traps damp, holding it captive. This characteristic and the hardness of its set pose serious problems for traditional structures which need to ‘breathe’.
The need for permeability is paid more attention now than it had been in the past, however, consideration is rarely given to the need for a structure to accommodate movement and the actual suitability of the building mortars relative to their function.
In an industry driven by the need for speed, lime putty is too often ruled out ‘because lime takes so long to set’, despite the availability of pozzolanic additives and hydraulic limes which provide an excellent alternative, and in a much more acceptable way than cement.
THE NEED FOR WALLS TO BREATHE
Traditionally constructed masonry buildings used permeable materials such as stone, brick or cob (dried earth) in conjunction with permeable mortars and sometimes renders to keep out the weather. Moisture is both freely absorbed by these materials and readily released by evaporation from their surface. Damp is kept under control and prevented from reaching the interior by the depth of the wall and by the rate of evaporation.
Traditional materials are notoriously difficult to fully weatherproof, even by the application of modern materials such as silicones. Any impermeable coating such as paint, cement render or sealant inhibits the ability of the surface to breathe and prevents evaporation, causing damp in the interior, and in time, it is inevitable that it will crack, flake off or break down, allowing further moisture penetration to compound the problems. Any attempt to weatherproof a traditional wall is therefore pointless. A permeable render provides far more effective protection.
Lime mortars used as the bedding material for masonry and for pointing the joints provide a permeable route for the passage of moisture from the core of the wall to the surface where it evaporates. Where the stone is very hard and impervious, this may be the only route available. Where the stone is relatively soft, a more porous mortar will always be preferred to reduce the amount of evaporation which takes place through the stone.
Stone decay is perhaps most often caused by the use of impermeable mortars. Two basic mechanisms are involved.
Moisture absorption and evaporation is concentrated in the stone
immediately adjacent to the mortar joints.
Salt crystallisation can occur following the introduction of potentially damaging salts in the mortar and the dissolution of salts already present in the wall during repointing.
Salts cause stone decay where water
evaporates, as the salts are left behind in increasing concentrations until
crystallisation takes place. If crystallisation occurs within the pores of stone
(a process known as cryptofluorescence), crystals quickly fill fine pores and
further growth causes the pores of the walls to collapse.
Lime mortar, being much more porous than stone, absorbs water and subsequently encourages evaporation to occur through the joints rather than through the stone itself. Damage caused by crystallisation within the mortar joint (or in a render) is less serious as the mortar is replaceable.
A cement mortar often demonstrates the complete opposite. Being more dense and less porous than a lime mortar, a cement mortar traps moisture within the fabric. This in turn will reduce the thermal performance of the wall, and in cold weather the temperature of the wall may fall to the point where moisture in the interior starts to condense on its face, soaking in and encouraging the development of timber decay and other damp related problems.
Finishes are also important. The evaporation from the surface of masonry and renders is prevented by an impervious coating and encouraged by a textured, porous finish such as a traditional limewash. Limewashes also serve to consolidate and protect lime renders and limestone.
Timber framed buildings are particularly prone to damage by the use of cement render and impermeable finishes to the external infix panels. Gaps between the render and the timber are inevitable, and an impervious render or paint traps moisture, causing timber decay.
THE NEED TO ACCOMMODATE MOVEMENT
All materials expand and contract with changes in moisture content and temperature. Modern walls, built with Portland cement, are relatively rigid and rely on their strength to resist movement although, in long walls, expansion joints need to be introduced to accommodate the proportionally greater expansion.
Traditionally built masonry walls work in an entirely different way: they are much more flexible and will accommodate minor structural and seasonal movement by minute adjustments over many joints. Hairline cracks in the mortar may subsequently be resealed by the precipitation of lime.
The use of cement in modern construction works well if the brick or stone is stronger than the mortar, but problems arise when a hard cement mortar is used to repoint a traditional wall. This is because the shallow depth of hard cement mortar at the surface is rigid , whilst the body of the wall behind is flexible. Any movement which occurs introduces stress in the narrow band of rigid material at the face, which is only relieved by the failure of either the stone, the brick or the mortar. As the stone or brick is usually less hard than the cement, it is this which normally fails.
THE RIGHT SPECIFICATION
Techniques, skills and knowledge that were close to being lost entirely now appear to be more widely available and are generally being accepted (but all too often not adopted) by many professionals. This revival has been led by some serious academic research which presents us all with an opportunity to better understand how these materials perform and why they are right for the repair of historic buildings.
As a rule, mortars used in repairs should always match those used originally, unless it is clear that the original mortar had been incorrectly specified. Using simple scientific procedures, small samples of existing mortars may be analysed to identify the proportion of lime to aggregate. More detailed scientific analysis is available through dedicated organisations.
For interior work and some exterior work, non-hydraulic lime mortars (lime putty and aggregate) are generally most appropriate. They are however, much softer and due consideration should be given to their function within the environment they are to be used. At first the lime acts as a sticky binder holding the aggregate together, as there is no initial set. Hardening takes place slowly over weeks and months as the lime reacts with carbon dioxide in the air. For external work however, lime putty mortars and renders, are more vulnerable, and their use should be properly understood by those carrying out the works.
In less sheltered conditions certain materials may be added to provide an initial set. Brick dust was commonly used in the past, but there are a variety of other ‘pozzolanic’ additives (see glossary) which achieve the same effect.
Where there is a high degree of exposure to weathering, the use of a hydraulic lime should be seriously considered. Their use has been tried and tested over the last 200 years.
Whilst being stronger and denser than ‘fat’ limes (lime putty), hydraulic limes demonstrate a greater flexibility and permeability than cement mortars. Hydraulic limes are supplied as a dry powder added to sands that sets on the addition of water in a manner similar to cement, but, being softer they still offer the physical properties required of a traditional lime mortar or render.
Here at the Cornish Lime Company we only stock the pure natural material produced by St.Astier as NHL 2 - NHL 3.5 and NHL 5.
The classification of hydraulic limes is fundamental to their correct specification as their properties and their performance vary greatly according to their individual composition. Some varieties include other materials such as cement and gypsum which may be harmful in some applications, and the degree of hydraulicity varies from one to another. With the new European standards (ENV459/1 in particular) these ingredients and the characteristics of each source should be clearly identified. With only pure and natural materials allowed the suffix NHL. Materials that have been blended with external substances will be sold as NHL-Z.
The use of lime in repairs is fundamental to the long term survival of older buildings and their fabric. Although the use of lime is not particularly complicated, failures do arise due to either an operative’s lack of knowledge or bad site practice and, very occasionally, due to the infinite variables arising from the use of a material dependent on its geological origin. Only by using the correct materials and fully understanding their properties can we ensure that future generations will be able to enjoy the very rich and varied history that is to be found in the superb built heritage of this country.
Forms of lime which are supplied as dry powder and which set when water is added. (The term ‘hydraulic’ refers to the ability of this type of lime to set under water without air). Unlike lime putty, hydraulic lime is suitable for damp and wet conditions and, because it sets hard quickly, it is better able to withstand exposed conditions. It is made by burning naturally occurring deposits of limestone (calcium carbonate) which contains impurities: first carbon dioxide is driven off and the calcium carbonate converts to quicklime (calcium oxide - CaO); then at temperatures of between 950oC and 1,250oC some constituents of the impurities combine with the quicklime to form silicates and aluminates of calcium, the reactive compounds that give hydraulic lime its chemical setting properties. It is then slaked with just enough water to convert the quicklime to calcium hydroxide without causing a set, and it is then dried and ground. Setting and hardening occurs primarily by crystallisation of the silicates on the addition of water.
A mixture of lime (calcium hydroxide - Ca(OH)2) in water which is used for the production of lime plasters, renders, mortars, grouts and limewash. Lime putty is made by burning limestone or chalk (both forms of calcium carbonate - CaCO3) at a temperature of around 900oC to make quicklime (calcium oxide - CaO) which is then ‘slaked’ with water. It is best matured for several months by storing it in mass.
The material which fills the gaps between the stones or bricks in a masonry wall and binds them together. Its principal ingredients are usually aggregate (eg gritty sand) and a binder (eg lime) in a given ratio by volume of say; three to one, with small proportions of other additives (eg brick dust) as required.
A fine powdered material which is added to non-hydraulic lime mortars to accelerate the set. The material possesses little or no cementitious value, but in a finely divided form it will react with calcium hydroxide (lime putty) in the presence of moisture to provide a chemical set. The term derives from the Italian region of Pozzuoli, near Naples, where the effects of the local volcanic ash on lime mortars was first recognised by the Romans. Other pozzolanic materials in this category include finely crushed brick or clay tile dust, pulverised ash (PFA) with a low sulphate content, and ‘high temperature insulation’ (HTI). ECC Building Products Ltd currently produces an excellent material for this purpose known as Metastar.
A durable coating of lime and aggregate, sometimes reinforced with animal hair, which provides a protective covering to the walls of a building. Common substrates include rubble stonework, cob, or in the case of a timber framed building, wattle and daub.
PHIL BROWN is Director of The Cornish Lime Company.