TAN 1 : PREPARATION AND USEOF LIME MORTARS
the mortar and the affected areas of stone obtained before proceeding with repointing. The feebly to moderately hydraulic lime mortars may be more appropriate than pure non-hydraulic lime mortars in these situations since they Inay be less susceptible to lime leaching. In practice some argillaceous (clay bound) sandstones are susceptible to continuing decay, independently of the mortar. It has been suggested that clay mortars would be beneficial for these stones.
Specijyirzg blended or conzposite rnixes A range of nort tar strengths can be achieved using the hydraulic limes now available, but none of these replicate the complexity or characteristics of traditional lime mortars, where durability was often achieved without the use of a strong hydraulic set. One of the characteristics of historic mortars is the inclusion of particles of lime with varying properties, derived from traditional lime burning and slaking processes. Whilst currently these mechanisms are not fully understood, it appears that this complexity of structure contributes to the performance characteristics of old lime mortars. Modern lime mortars combining two, or more, different types of lime, or limes from different sources, can go some way towards replicating the complexity of structure found in historic mortars. It is normally recommended that, in these composite mortars, the different limes should be used in roughly equal proportions, and certainly at not less than l to 2. The final performance of the mortar will be closer to that of the weaker lime component than that of the stronger, perhaps achieving no more than 30% to 40% of the potential strength of the hydraulic lime itself. Even though they have some degree of hydraulic set, these composite mortars rely on adequate carbonation to achieve durability. They require the same level of care in production, use and curing as do plain non-hydraulic mortars. Properly used, composite mortars containing hydraulic lime can be useful in damp or exposed locations, but they are not directly equivalent to historic hydraulic limes and should not be used indiscriminately.
Choice of aggregates Historically, most building sands were probably obtained from a source local to the construction work. Investigation of local commercial sand quarries may identify a good source of matching ~naterialfor repair, but carefill evaluation of the properties of any sand is required before including it in the specification. As outlined in Section 2.6, the specification of sands or aggregates 'to BS1199/1200' or 'to BS882' is not in itself adequate. Choice of sand or aggregate should generally be determined by suitability in terms of
physical and geological characteristics, by colour and, where appropriate, by matching to sands in existing mortar. Unless a specific material with known gradings has been chosen from a particular quarry source the specification should always include further information on the gradings required. The lnaximun~size of grains should normally be no Inore than 35% of the joint width, but for the majority of situations it will be sensible to specify sands which fall within a readily available particle size range, such as the 2.36rnin to 150ym envelope specified in BS882 or the 5.00mm to 75ym envelope in BSI 199/1200. The use of carbonate aggregates (crushed limestone, crushed marble, crushed old well-carbonated lime mortar, calciferous sand and shell) or of air entraining aggregates such as crushed lightly fired brick, should be considered where these materials have been identified in the original mortar or where there is a need to modify the properties or improve the durability of lime mortars. These materials tend to improve durability without necessarily increasing hardness or reducing permeability. If clean old crushed lime mortar is available this makes an ideal replacement for part of the sand aggregate. The use of freshly crushed limestone, preferably from one of the softer nonhydraulic limestones, has been demonstrated to improve the carbonation rate and the overall frost resistance of lime mortars.
Pozzolarzic irzgredierzts Similarly the use of pozzolanic additives can impart a more positive set to non-hydraulic, or, in appropriate situations, hydraulic lime mortars. The specification of pozzolanic materials for use in lime mortars is based on certain general principles:- the material should be as finely ground as possible, it should be fisshly ground, and where quantities in excess of around 25% of the lime content are being used, the sand content of the mortar should be reduced accordingly. Pozzolanic additives should therefore be fully specified by type of material, source, fineness and storage and handling requirements as well as by proportion required. (See Section 2.7.) Freshly ground soft-fired brick dust is an effective pozzolan and should be included in repair mortars where the original material contained brickdust. It can also be considered where work is required to soft sandstones in an exposed location. Cornrnercially available pulverised fuel ash (PFA) is someti~nesused as a pozzolan in modern structural grouts but it should be used with caution in repair mortars for traditionally built structures, as the finer particle sizes can result in a hard brittle material. Advice on the potential strength of PFA-gauged grouts should be obtained from the suppliers