retain the benefits of high-calcium putty. Of course certain basic 'on site' practices are important and these should be emphasised at the beginning as well as monitored during the work. The main rules are: 0 Generally specify washed sharp sand with 3-4 mm grit (where the joints allow) and not too high a proportion of fines. 0 Mix very well and apply within a couple of hours - do not 'knock up again'. 0 Once the mortar or render is in place finish off (within another couple of hours) and then leave to set. D With render, the second or third coats can be applied when the previous one has set hard enough to resist a pressed finger. In temperatures over 12°C this should be the next day. As the ambient temperature drops the speed of set reduces and stops around 5°C. However, often the masonry will be warmer than the air temperature and this should be borne in mind. 0 Walls must be dampened (before starting), especially with porous masonry, and subsequent render coats applied only onto a damp previous coat. D Shrinkage can occur if the base is too dry or the mortar is made up too wet. Hydraulic lime mortars need mixing dryer than cement, commensurate with workability. It is important to emphasise that hydraulic limes set initially by a proportion of the calcium hydroxide combining with the other minerals that are in the lime. Thereafter the normal carbonation set associated with high calcium limes takes over (i.e. the remaining free hydroxide carbonates over a prolonged period). The conclusion to be drawn from this is that, although the hydraulic lime will withstand the effects of weathering especially rain, within a short time, it can still be vulnerable to frost. Atpresent English Heritage's advice is that if hydraulic lime is to be employed it should not be mixed with putty (in order to gain the advantages of a quick set). Generally most historic records show that blue lias hydraulic limes were used on their own - often 1:2, certainly not less than 1:3 and sometimes 1:1:5. It is quite wrong to specify 1:6, for example, whether on its own or mixed with high calcium 'coarse stuff', and expect any hydraulic lime to have a worthwhile effect. Surprisingly, such mixes are being used by specifiers and contractors who feel they are putting a 'toe in the water' but fail to understand the material they are using. Nevertheless, some specifiers and contractors are successfully mixing high calcium and hydraulic limes in higher concentrations. If this procedure is used, then as soon as the hydraulic lime powder is added, the resulting mortar must be applied in accordance with good practice for a pure hydraulic lime. Under these circumstances tamping down must not be attempted the next day because this will damage the hydraulic set. Lastly try to forget the commonly used phrases: "eminently, moderately and feebly hydraulic". In general terms, the important factors are: D Setting time underwater- the shorter the better, around three days is ideal. D Final strength for most work should not be too high, on an average 1:3 mix. three newtons per square metre (N/ m2 ) is satisfactory, figures approaching 10 N/mm2 are too high for general masonry work. New and accurate scientific data are becoming available regularly, enabling the industry to apply hydraulic limes to their full potential. Michael Farey is the managing director of Hydraulic Lias Limes Ltd based in South Somerset. He often gives presentations around the country on the subject of hydraulic lime. THE TRADITIONAL LIME Co. TLP • matured lime putties • mortars • renders • plasters • lime washes • pigments TRADLYM© TODAY'S BOND WITH THF. PAST Church Farm, Leckhampton, Cheltenham, Glos GLS 1 SXX Telephone: (01242) 525444 Fax: (01242) 237727 CONTEXT 52 TELLING LIME PRODUCTS LIMITED PRIMROSE AVENUE FORDHOUSES WOLVERHAMPTON WEST MIDLANDS WVlO 8AW TEL: 01902 789777 FAX: 01902 398777 NATURAL HYDRAULIC LIME BINDERS MORTARS GROUTS DECORATIVE COATINGS PAINTS POLISHED PLASTERS 27
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