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TAN 1: PREPARATION AND USEOF LIMEMORTARS
Cold mixing Preparation of mortar from lime putty or from hydrated lime are the methods most commonly used on site. In both these techniques the lime content of the mortar is slaked before mixing and the heat generated by slaking is not utilised in the production of the mortar. These techniques are described as cold mixing.
Lime putty mixes Traditional techniques for making coarse stuff from lime putty generally involved beating, chopping and ramming on a wooden board or trough for at least an hour until the mix was sticky and workable. For best results the mix was then matured before use. For modem site use similar troughs may be constructed or small quantities of mortar may conveniently be made in a plasterer's bath. For larger quantities of material a roller pan mixer is recommended. To produce a modem putty lime mortar, fresh or mature lime putty is combined with sand (or other aggregate) by one of these methods. Slaked lime putty is widely available from traditional producers and is frequently used for production of mortars on site: however, the practice of making lime mortars by mixing lime putty and sand and using immediately, or even within a few days, or weeks, of production is not recommended. If, exceptionally, this method is used it is essential that the mortar be mixed
m 13. Traditional mortar trough and mortar making tools.
in a compression type mixer, such as a roller pan mill, to achieve a well beaten mortar with a low water content. Mortars made from putty and sand should normally be allowed to mature for around 12 weeks before use to improve the bond between sand grains and the lime binder, and to allow excess water to drain from the mix. I f appropriate maturing conditions are provided, specifically conditions which allow the mortar to drain off excess water, and storage in these conditions for at least 3 months is possible, then putty lime mixes can be made in a modem rotary drum (cement) mixer. These mixes will have an excessively high water content and should not be used, even for harling, until they have been adequately matured. After mixing, putty lime mortars for general use were, traditionally, matured in earth or timber lined pits or timber vats, or in covered heaps, all of which allowed excess moisture to drain from the mortar but prevented drying out or freezing. Modern methods of storage and maturing should replicate these conditions as far as possible. As an alternative to site mixing, ready made, matured mortars can be obtained from specialist suppliers.
Mixes based on dry hydrates Modern hydrated non-hydraulic lime (commonly known as builder's lime or hydrated lime) is not suitable for the production of traditional lime mortars,