Anoverviewofbuilding limes Continuing the discussion of the types and uses of lime, Ian Pritchett explains the role of the Building Limes Forum and contributes to the debate. Following recent articles in Context on the use of lime, it has become clear that many people are still very confused about types oflime, terminology, properties and potential uses of lime. The Building Limes Forum (BLF) came into being in 1992 to promote the appropriate use of building limes, based on an exchange of understanding and expertise between its members. The BLF now has over 300 members ranging from research scientists through architects, surveyors, lime manufacturers, conservators to builders, practitioners etc. The BLF is not a profit-making or trade organisation; it is simply a forum for debate. As a member of the Forum, I am attempting to clarify the situation and dispel some myths. There is a tendency for one to accept papers in journals such as Context as being the gospel truth. I believe that they should be treated as discussion documents to promote ongoing debate. Certainly it was some of the opinions expressed in previous articles which prompted this piece. Firstof all what is LIME? Lime is produced by heating calcium carbonate (limestone, chalk, shells, coral etc.) in a kiln to a temperature of approx. 900°C. At this temperature carbon dioxide gas is given off and the calcium carbonate is chemically changed, or calcined, to form calcium oxide (known as quick lime or lump lime). The raw material - calcium carbonate - will vary according to its point of origin and will contain impurities of various types and in varying quantities. It is these impurities which give the range of different building limes discussed below. To avoid over-complicating the subject I will concentrate on calcium-based limes and avoid magnesian or dolomitic limes. Lime can arrive and be used in different forms, but they all originate from quicklime. In the past it would have been commonplace for quicklime to be delivered to sites, but today we 28 tend to process it beforehand. Water and quicklime are comoined in a process known as hydration to produce hydrated lime. If only an exact amount of water is added, the end product is a dry powder which is sold in bags and generally known as hydrated lime or lime hydrate. If an excess of water is used (always putting the quick lime into the water for safety's sake) the •process is normally referred to as slaking or slacking and the end product is a colloidal gel, often sold in plastics tubs and known as lime putty. The properties of the dry hydrate or putty will depend mainly on the source of calcium carbonate burned and in particular the impurities it contained. Ve1ypure sources of calcium carbonate, e.g. Buxton limestone, will produce pure quicklime and hence pure putties. Pure lime putty (calcium hydroxide) is known as fat lime and will not set under water. For this reason, it is stored with a layer of water over the top. Mortars and plasters made from fat lime will not set under water and thus can be kept indefinitely ifcovered with wet sacking. These pure limes set by a two stage process; the first part (the initial set) is simply the excess moisture leaving the mortar by either evaporation or migration into the surrounding masorny. The mortar is then firm to the touch but can still be marked with a fingernail. The second stage (carbonation), is a very slow chemical reaction, caused by carbon dioxide from the atmosphere entering the damp mortar in solution and reacting with the lime (calcium hydroxide) to convert it back to calcium carbonate. This will depend on the temperature, the pore structure and the moisture within the mortar and can take many years to complete. Limestones which contain clay produce building limes which are known as hydraulic, because they have the ability to set under water. The clay impurities contain silica and alumina which give the limes a more complicated chemistry. The calcium ions react with the silica and alumina to form complex calcium silicates and calcium aluminates. This reaction can take place under water and thus hydraulic limes cannot be stored for any length of time as putties or wet mortars because they will start to set. The formation of the above compounds will take place relatively quickly, but will then be followed by the slower carbonation process, whereby the remaining free lime (calcium hydroxide) within the mortar will react with atmospheric carbon dioxide (as with fat lime). The properties of hydraulic limes are a direct consequence of the amount of clay impurity in the burned limestone (raw material) and of the burning temperature. The more clay impurity the faster the set and the harder the mortar, if the limestone is burned at the appropriate temperature. Thus, historically, we have had a whole spectrum of building limes available ranging from natural cements such as Roman Cement (with a setting time of 20-30 minutes) to fat lime which will keep indefinitely under water. Vernacular buildings would have been constructed from the locally available sources of lime. Very often an individual quarry would yield different types of lime from different strata. Where locally available lime did not have the required properties for a given task, either the required lime would have been transported from a source further afield,-or the local lime would have been modified by the addition of other materials (I will discuss these later). Mortars and plasters are almost always lumped together because they are often similar materials. However, this can lead to confusion, thus I am going to treat them separately. Plasters/rendersand finishes Plasters/renders and finishes are, in general, thin materials which cover a large surface. They have a very large surface area compared with their volume. They are what we see when we look at a building or wall. Historically, they were where the money was spent, Context54 June1997
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