BCD 2018

PROTECTION & REMEDIAL TREATMENT 4.1 C AT H E D R A L COMMU N C I AT I O N S C E L E B R AT I N G T W E N T Y F I V E Y E A R S O F T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 1 9 9 3 – 2 0 1 8 137 added to mortars in the UK. This included ‘forge scales’ and ‘smith’s ashes’, and various imported volcanic ashes such as German trass and Italian pozzolana. Generally, coarser particles provided bulk and the finer ones reacted with the lime to form hydraulic compounds, so the pozzolan was often the only aggregate in the mortar. Pozzolanic lime mortars were superseded in the 19th century by stronger hydraulic limes and cements. Since the lime revival in the late 20th century, pozzolans have once again been used but generally as very finely ground powder added in small quantities to a mortar to modify the properties of the lime binder rather than to provide any appreciable bulk. Modern practice in the use of pozzolans is variable, and there is little evidence from research or long-term trials of current relevance to building conservation. In some cases where research was undertaken, the pozzolans tested are no longer available. Other pozzolans are variable; the reactivity of brick dust for example depends on the type of clay used to make the bricks, their firing temperature and how fine the powder is. When used as a pozzolan, the results are often inconsistent so the results of trials are of limited value. The most consistent pozzolan for building purposes is probably metakaolin, but this is a far cry from the pozzolanic mortars of the past or from the feebly hydraulic lime used in so many parts of the UK. It has been found that some unburned clays can also assist a hydraulic set. It is therefore possible that where ‘active clay’ minerals are present (either included as part of the aggregate, for example if unwashed sand is used, or incorporated deliberately with it) stable compounds may be formed by combination with the lime, imparting hydraulic qualities to the mortar. When quicklime is used in the mix, which was common practice in the past, it raises the temperature and increases the possibility of a mild pozzolanic reaction and hydraulic set. Pozzolans have huge potential to create appropriate mortars for conservation, but our understanding and experience of them is limited. Some very useful research has been undertaken over the past 16 years at Trondheim Cathedral in Norway where long-term exposure trials of a variety of pozzolanic hot-mixed mortars have informed the development of repair mortars. However, this work is site- specific and tailored to the demands of the Norwegian climate, so although the findings are very informative, they are unlikely to be directly applicable to many UK situations without further trials. It is clear that if we wish to rely on pozzolans rather than NHLs to achieve a mortar with a feeble hydraulic set, more research is needed. Historic England is funding further research in this area. A FUTURE FOR FEEBLY HYDRAULIC LIME? Historic England is exploring the feasibility of re-starting production of feebly hydraulic lime. Inevitably, many obstacles will have to be overcome to achieve viable large-scale commercial production. However, there seems to be something of a grass-roots renaissance in lime burning under way. At the 2017 gathering of The Building Limes Forum, several case studies of small-scale and replica lime kiln constructions were presented, the lime produced being used for local conservation projects. There were discussions with people who have constructed their own kilns and with others who were inspired to explore the possibility. Perhaps these small enterprises, burning local lime in traditional kilns, may offer a way to obtain more authentic lime for conservation. If so, then lime production will have come full circle, giving new meaning to the term ‘the lime cycle’. Further Information AD Cowper, Lime and Lime Mortar, 1927 edition, Donhead Publishing, Shaftesbury, 1997 English Heritage, Practical Building Conservation: Mortars, Renders & Plasters , Ashgate, Farnham, 2012 Historic England, https://historicengland.org . uk/advice/latest-guidance/ S Holmes and MWingate, Building with Lime: A Practical Introduction , ITDG, London, 2002 MWingate, Small-scale Lime Burning , Intermediate Technology Publications, London, 1985 CRISTIANO FIGUEIREDO completed his PhD at the University of Bath in 2017. ALISON HENRY works in the Building Conservation and Research Team of Historic England. STAFFORD HOLMES is an architect, author and former chair of the Building Limes Forum. Mortar samples made with different batches of NHL from one manufacturer were tested strictly in accordance with BS EN 459 at the University of Bath and at an independent commercial laboratory. Neither batch of NHL 5 met the minimum strength required by the standard, and results from the two labs varied significantly. This is further evidence of the inconsistency of NHL binders and the fact that classification according to the standard cannot be used as a reliable guide to mortar properties. Analysis showed that measurements made at 28 days give no indication of final performance because NHL mortars continue to gain strength for many months and at varying rates. All NHLs commonly available in the UK have now been tested, and full details of each will be released by Historic England after consultation with their manufacturers. 18 years after being repointed in NHL 3.5 mortar, this sandstone is decaying while the pointing remains intact. The mortar is not acting sacrificially to protect the stone. (Photo: Sam Baxter) A chalk cob wall had been rendered with an NHL 3.5 render, trapping moisture inside the cob which led to its collapse. (Photo: Kevin Stubbs)