BCD 2019

142 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 9 C AT H E D R A L C O MM U N I C AT I O N S Lime for repairing a medieval wall at Culross Palace, Fife. We burnt the local Charlestown lime and slaked it over a few days to a hydrate, then made it into a mortar with local sand and water. The resulting mortar was very plastic and workable suitable for both re-building and repointing masonry. It is worth mentioning that all UK lime production today is of high calcium or air lime (CL80/90), because its high purity is prized for steel fluxing, water treatments and ground consolidation works. These are hard burnt limes (over 1100°C) and can often contain particles that are slow to slake, resulting in disrupted mortars if made as hot mixed mortars and used too soon after mixing. The Romans made a law that lime to be used for plastering should be matured for at least three years. We should take heed of these past practices. In terms of modern lime products available today, we have an enviable range to choose from, some designed for particular purposes, for example grouting, surface repair of brick and stone, one-coat render systems or just plain binders for making simple site mixed mortars. Many of our traditional mortar suppliers have ventured into producing an ever-increasing range of ready mixed, dry bagged mortars pertaining to their local area and utilising local sands and aggregates. These offer more reliable and consistent products that removes the chance of error in accurately proportioning out binders and sand on site. One contractor recently compared them to ‘Pot Noodle’: “you just add water – it’s a no brainer!” Incorrectly proportioned lime and sand mortars are in our experience a primary culprit in the failure of lime mortars, as well as the inadequate aftercare and protection of some which is often left to chance. They can also be a convenient solution where city centre sites have limited space for preparing mortars on site. In the 2010 revision of the EN 459:1 standard for building limes, two new categories were introduced, namely HLs (hydraulic limes) and FLs (formulated limes) which gave producers the opportunity to create binders with more exacting results than ever before, by the addition of other ingredients such as low sulphate cement or pozzolans (for earlier stiffening), air-entrainers, carbonate dust (ground chalk or limestone), water retainers and water repellents. In the case of HLs, the manufacturer is not required to reveal the individual constituents, and so is able to protect their secret recipe. In the case of FLs, these are a blend of constituents with a declared composition including cement. In reality, there are only a handful of HLs and FLs available in the UK (St Astier’s HL5 Hourdex, Tradiblanc and HL3.5 Tradeco, for example), although many more are available in continental Europe. While these newer products are primarily designed for new build applications, conservators have found them to be of use when trying to emulate the properties of early patented cements (now no longer made), where appropriate. The addition of water retainers is particularly useful in lime mortars as they prevent over-rapid drying, reducing the risk of shrinkage cracks. Water retention will also help a hydraulic lime mortar to acquire strength slowly by avoiding desiccation. It is of particular note that hydraulic lime mortars should be kept damp for at least 72 hours after placing to ensure the hydraulic set forms. If a mortar is allowed to dry out within this 72 hour period, re-wetting will not re-initiate the hydraulic set. Water retainers are particularly useful on high suction backgrounds or where protection of the new mortar work is particularly difficult to achieve. Illustrated here are three examples of modern lime products used in the context of building conservation, restoration, repair and renovation. In the case of the church spire (top right illustration) using a like-for-like mortar would have been a catastrophic mistake. For budget reasons, it was not possible to fully scaffold this spire and so we were forced to design a mortar that fulfilled the brief of appropriate repair, both technically and aesthetically and in a particularly challenging environment. Furthermore, the once fine joints were now much wider due to erosion, so if pointed with a traditional white ashlar mortar which matched the original, the joints would have been visible from the moon, detracting from the original design. Instead, we formulated a mortar based on NHL 5, with an air entrainer (reducing the strength down to the equivalent of NHL 3.5), colour matched to the building stone with a red iron oxide pigment and with a water retainer (as it was impossible to secure close covering for protection of the mortar as it was curing). In conclusion, there is a place for all mortar types be they hot or cold mixed, formulated or dry mixed ready bagged mortars, the specifier just needs to be fully informed and that’s up to each individual to investigate. And with 25 per cent of professional indemnity insurance claims being specification related, it’s well worth the effort. ROZ ARTIS MRICS RICS certified historic building professional IHBC is director of the Scottish Lime Centre Trust, one of its primary course tutors and the head of its building advisory service. Roz has written widely on the practical use of lime and other mortars. Due to erosion, a like-for-like traditional white ashlar mortar in the now widened joints of this church spire would have changed its character dramatically, and site conditions dictated the use of a carefully formulated hydraulic lime mortar to suit the harsh environment A colour matched lime based one-coat render system was selected for the north-facing roadside elevation on a very busy street with a narrow pavement, the other three elevations are traditional lime harl and limewash. To the untrained eye, there is no detectable difference. A mortar formulated for the surface repair of stone (or brick), colour matched to the original stone