C O N T E X T 9 1 : S E P T E M B E R 2 0 0 5 17 IHBC Annual School 2005 IHBC Annual School 2005 IHBC Annual School 2005 Richard Smith A lime for all seasons Lime is good for people, for tradesmen, for looks, for buildings and for the environment – and cement has been used instead of lime only for 100 out of the past 10,000 years. Lime is the single most important conservation material. Not because of the value of the lime mortars and plasters themselves, but because of the beneficial effect of lime in preserving the whole fabric of a building. Lime therefore has a key role in preserving our built heritage. The Building Limes Forum is not opposed to cement. Ordinary Portland Cement (OPC) is an excellent material, particularly for making concrete, and probably has more similarities with lime than differences, not least in the basic raw material – limestone – used to make it. Cement is at one end of a spectrum rather than being a totally different material. But it is totally unsuitable for the repair or alteration of old buildings. The strength of Portland cement has increased massively since it was invented by Joseph Aspdin in 1824. Great care needs to be taken when repairing buildings which were built with cement in the latter part of the 19th century and the first half of the 20th century. These earlier cements probably have more in common with the hydraulic limes of today than they do with current-day cement. Either hydraulic lime, weak cement mortars or composite mortars of lime and cement need to be used. Lime has been used as the principal binder for mortars and plasters for 10,000 years. It was only the introduction of cement in the middle of the 19th century that led to the decline in the use of lime, culminating in its virtual disappearance by the mid‑20th century. Emerging evidence in the 1970s of the damage caused to historic buildings by the use of cement mortars and modem plasters has led to what is now referred to as the lime revival over the past 20 years. There has also been a return to the use of lime for new build – so cement has been used instead of lime only for 100 out of the past 10,000 years. There are two principal types of lime: pure lime and hydraulic lime. Pure lime, Ca(OH) 2 , is also known as air lime, as it hardens on exposure to air by absorbing CO 2 ; fat lime, from its consistency in putty form; high calcium lime, from its chemical content; and non-hydraulic lime, as it does not set under water. Hydraulic lime is synonymous with water lime, as it sets under water. The production process for lime starts with the burning of limestone to produce quicklime. Lime is then created by adding quicklime to water, referred to as slaking. With hydraulic quicklime, sufficient water is used to produce a dry powder. Pure lime is produced either as putty or a powder. Pure lime powder is also known as builders’ lime, because it is generally available from builders’ merchants and is used by builders to gauge cement mortars; bagged lime; hydrated lime or dry hydrate (which is confusing because all lime is hydrated by virtue of having had water added). Pure lime mortars and plasters made from putty or A craftsman at the CharlestownWorkshops learns how to repair a deep cavity in a masonry wall using clay tiles and lime mortar. (Photo: Scottish Lime Centre Trust)
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