BCD 2017

38 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 7 volume not weight, mix proportions should be varied accordingly. (Kibbled expands to approximately 2.7 times its original volume, while powdered expands approximately 2.1 times.) There are technical benefits and drawbacks to using either option as outlined in Table 1. From a historic perspective, however, kibbled generally provides a better match for mortars which contain lime inclusions. These are particles of quicklime which have combined insufficiently during mixing. Hot mixing mortar is not difficult, but it is more labour intensive than mixing putty or NHLs as the material is stiffer and stickier. While there are health and safety concerns due to the heat, with the adoption of appropriate measures on site the risks can easily be managed. The most significant risk is when mixing the powdered quicklime as it can plume into the air. If this is inhaled, able to dwell on skin, or comes into contact with the eye it can pose serious health risks. DOES IT HAVE TO BE USED HOT? The short answer in no; hot mix is a term that has been coined due to the heat generated during the mixing phase, there are benefits to using the result immediately, while still hot. Using it hot has the advantage that water content is easier to control, and the mortar is stickier and the work is quicker to finish. No storage facilities are required as the mortar is made in small batches. However, the mortar has to be made on site, close to the point of use, so work is slower. If used cold, large batches can be made either on site or by a commercial supplier, and pop-outs are less likely because the mortar has had more time to slake. IS HOT-MIXED SUPERIOR TO NHL MORTARS? One key argument for the use of hot-mixed mortars over NHL-based mortars is that of strength. The problem is that the strength specified for an NHL at 28 days may have little bearing on its full strength, which may not be achieved for several years. As an example, the two-year compressive strength of a St Astier NHL3.5 mortar at 1:3 is approximately 4 N . This is too high for soft masonry units as the compressive strength of the mortar could cause disruption to the host fabric. However, most common building stone will be much harder than this (typically a poor sandstone has a compressive strength of 25 N ), so strength is not usually an issue. Closely allied to this is the issue of permeability. For a mortar the general rule is that the higher the strength, the lower the vapour permeability, and it is claimed that NHL mortars can trap moisture within historic fabric. However, mortars made with NHL3.5 or weaker have been proven over many years to have sufficient vapour permeability not to trap moisture. There are claims that hot-mixed mortars do not need protecting in the same way as putty or NHLs, but this is not the case. All limes require babysitting in their infancy especially during winter months as a compressive strength of around 2 N is required before they can cope with the impacts of frost. While NHLs may achieve this rapidly through a hydraulic set, non-hydraulic limes rely solely on carbonation. For this to occur at any depth, carbon dioxide must dissolve within the pore structure of the mortar, so a hot-mixed mortar must be kept damp. In cold temperatures moisture can hold less carbon dioxide and reactions occur more slowly. Even when warm, this is a very slow process, so the mortar is vulnerable to the effects of rain and frost for several months and it can take several years before full strength is completely achieved. With very high binder content and relatively low strength, a hot mix will therefore be more prone to failure as a result of frost. During rain spells it will also be more likely to exhibit lime bleed due to the high binder content. Hot-mixed mortars also have a high rate of absorbency. Under laboratory testing, a hot-mixed mortar at 1:3 became fully saturated in under 15 minutes. As a comparison, a typical putty mortar at 1:3 will take around 30 to 35 minutes and an NHL3.5 mortar at 1:3 around 60 to 80 minutes. While vapour permeability is a significant factor in mortar selection (as is its ability to wick moisture out of a wall), in exposed locations absorbency cannot be ignored. HISTORIC PRECEDENT? A point often raised is that hot-mixed mortars were used originally. However, we are now using very pure limes (CL90) which have no traces of hydraulic components. This type of lime cannot accurately be compared to historic limes, as most would have contained impurities which resulted in some hydraulic element, and aggregates were often used which would have had some pozzolanic effect. We cannot claim to be using the material of our forefathers unless we revert back to the traditional methods of producing quicklime, and given the time and costs involved this would not be viable. CONCLUSION This article is not intended to deter anyone from using hot-mixed mortars. There is most certainly a place for their use in conservation, especially for mass wall building; they make fantastic bedding and pointing mortars. It really comes down to selection and location; the same factors we use to decide whether putty or NHL is best suited. A pure hot-mixed mortar used in a location that is not subject to wind-driven rains should work very well. And its high absorption and vapour permeability characteristics can be used to help wick moisture from a wall in many situations. For more exposed situations, given that the majority of historic hot-mixed mortars analysed contained some hydraulic element, should we not be adding this back into the mix through the addition of a pozzolan or NHL? This is a practice that is carried out today and has been extensively used for the last 20 years in Scotland. A form of gauged hot mix will often be closer to the original than a pure lime hot mix. There has been a consistent cycle within the lime movement of changing binders. It started with lime putty, which through improper use or inadequate specification resulted in some failures. We then moved towards NHL mixes to try to overcome these, which worked well for many applications. Now we are moving towards hot-mixed mortars. All of these binders were at some point hailed as ‘the solution’, and while our knowledge of these materials has significantly increased we are still constantly learning. Ultimately, anyone aware of the virtues of lime and using or specifying the material is looking for the most appropriate mortar to conserve, enhance and protect a building. Hot mixes most definitely have a place in conservation and they offer another choice, but selection is critical and they should only be used where appropriate. We cannot adopt a one-size-fits-all approach. ADAMBROWN BSc(Hons) is a qualified building surveyor who works for the Cornish Lime Company Ltd (see page 140), which supplies a full range of lime products including hot-mixed mortars, and offers technical consultancy. This article is based on a dissertation he prepared for his degree in building surveying. TABLE 1 – POWDERED VERSUS KIBBLED QUICKLIME POWDERED QUICKLIME KIBBLED QUICKLIME Reacts faster than kibbled – a quicker reaction can be harder to control and mix as it stiffens Reacts a little more slowly than powder No hotspots – as a powder it is all of similar granulometry Hotspots – larger pebbles can reach higher temperatures during slaking causing spitting Plumes – powdered quicklime can be thrown into the air during mixing, posing significant health and safety issue (eyes, inhalation, skin) No plumes – granules and pebbles are heavier and denser than the powder Limited pop outs – less prone to latent expansion after the mortar has been placed as almost all of it converts to lime Pop outs – some larger pebbles can be slow to slake, causing delayed expansion which can disrupt the mortar face Kibbled (left) and powdered (right) quicklime (Photo: Cornish Lime)