Site Mixing: Lime Putty - Gauged Mixes - Hydraulic Limes
Lime Putty for site mixed mortars.1
Combining or mixing lime putty and sand on site usually requires considerable effort, kneading, chopping and beating the lime putty and sand together vigorously until a uniform colour is achieved. It is difficult to mix these materials to the correct consistency using normal modern site practice.
Mechanical mixing using a Roller Pan Mixer produces the best results. This type of mixer combines the materials with little or no additional water. Other mixing equipment will generally not provide a suitable mix without the addition of significant amounts of water, this can lead to high early shrinkage and poor bond characteristics and significantly increases the risk of freeze thaw failure. After initial mixing all lime putty mortars benefit from further maturing and further reworking immediately before use.
Once mixed, lime putty mortars can be stored indefinitely in airtight plastic buckets in frost-free conditions. They must be reworked before use.
Lime Putty Mortars for Gauging.1
Similar mixing practice is required for lime putty mortars that require a proportion of hydraulic lime to make them complete mortars. One significant difference is the method of combining the gauging material, in this case hydraulic lime. The proportions of hydraulic lime will be determined by the specifier, however the following procedure should be followed. The hydraulic lime should always be mixed with clean water to make a very stiff paste before mixing as previously described with the lime putty mortar.
Typically. MM104 Gowrie C/S supplied, as a 1 : 6 coarse stuff would normally be gauged with 1* part NHL 5 to 6 parts of the pre-mixed mortar, thus making a 1 : 1 : 6. (1 : 3)
Whereas. MM104 Gowrie C/S supplied as a 1 : 5 coarse stuff would normally be gauged with 1 part NHL 5 to 5 parts of the pre-mixed mortar, thus making a 1 : 1 : 5 (2 : 5)
Once mixed, gauged mortars can be reworked for many hours.(up to 48 hours depending on the gauging material used)
* In all instances the Gauging should be specified by weight. Very fine powders, such as all hydraulic limes are difficult to measure accurately by volume. If a hydraulic lime has an RBD (relative bulk density) of :710** it means that a litre of powder is 0.71 KG irrespective of its apparent volume, fine powders can bulk up with air to more than double their volume, so volume mixing of powders is inherently inaccurate and should never be relied upon.
Further lime putty mortars are stiff and very cohesive materials and do not accept powder combination readily, once the gauging material has been made into a paste similar in consistency to the lime putty mortar, combining these materials accurately becomes much easier. This is normally clearly visible when all the materials are the same colour.
** See Hydraulic Limes (Page 8) for RBD information.
Mixing Hydraulic Lime and Sand to Make Mortars on Site.1
Mixing mortars by volumes is a risky and inaccurate process unless it is carried out using full bags of lime and known quantities of sand accurately measured in full buckets. Sand can vary in bulk as a function of moisture content and it is possible to be as much as 30-40% out. At the very least, when full bag mixers are not available, properly reserved gauging boxes or strong buckets of equal volume should be used to determine the quantities of material being used.
Hydraulic lime mortars can be mixed in most types of mixers, in most countries paddle mixers and pan mixers are the norm for making mortars with rotating tilting drum mixers (most common in the UK) being generally classed as concrete mixers. In all instances, as the powders are fine and easily dispersed in the air, the mixing procedure should be carried out with care and in phases of sand, some water, then lime, then more sand, then more lime with the ingredients being mixed at each stage to a uniform colour and consistency.
1 We produce guides to site mixing of lime mortars, they are available upon request, please contact us for additional information, or visit our website at www.masonsmortar.co.uk
Dry Bagged Lime Mortars: See bag for site mixing instructions
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