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134 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 HYDRAULIC LIME PRODUCTION COMING FULL CIRCLE? CRISTIANO FIGUEIREDO, ALISON HENRY and STAFFORD HOLMES N ATURAL HYDRAULIC limes (NHLs) are now used widely for conservation work. We know that limes with hydraulic properties (literally, the ability to set under water) were used in the UK in the past, but are modern NHL mortars suitable for replicating historic mortar or maintaining historic fabric? To answer these questions, we need to consider the nature of the building lime used in the past and compare it with the properties and performance of NHLs. HISTORIC BUILDING LIMES From before the medieval period to the beginning of the 20th century, limestone for lime burning was mostly sourced and burned in kilns close to the construction site. The type of lime produced varied from place to place depending on underlying geology, but most were low strength. Even limestone which, owing to its composition, could have yielded a faster, stronger-setting hydraulic material if burned at high temperature was usually burned in small-scale kilns at comparatively low temperatures. Therefore, the lime produced did not generally achieve a hard or rapid set. The second half of the 18th century saw increasing demand for civil and military engineering structures and industrial buildings. These specialist applications, including bridges, canal and harbour structures, warehouses and factories (some with high, relatively thin walls), demanded stronger and more durable mortars than had been used before. The civil engineer John Smeaton (1724- 1792) carried out extensive testing of building lime, such as the Blue Lias of Somerset and south Wales, and various pozzolans, including Dutch trass and Italian pozzolana. He discovered that mortar hydraulicity was due to reactive clay minerals, either contained naturally in the source limestone or added to the lime in the form of a pozzolan. Smeaton’s work was highly significant. Now engineers could predict with some certainty which limestones would yield hydraulic lime with the quick set and high strength they needed. They began to experiment and classify limes according to their properties, making it easier to specify the most appropriate material for particular applications. French civil engineer Louis Vicat devised the first formal classification system for lime, published in England in 1837, but it was not until 1927 that the terms ‘feebly’, ‘moderately’ and ‘eminently’ hydraulic emerged when AD Cowper completed the Building Research Station Special Report LIMESTONES CRETACEOUS (chalk) JURASSIC & LATE TRIASSIC PERMIAN (Magnesian limestone) CARBONIFEROUS DEVONIAN SILURIAN CAMBRIAN The location of historic limestone quarries and lime production can be related to regional geology (British Geological Survey © NERC 2017. Permit number CP17/067. All rights reserved. Contains Ordnance Survey data © Crown copyright and database right 2017) In the southern and eastern counties of England chalk beds predominate (shaded moss green), producing both non-hydraulic and feebly hydraulic limes. Extensive deposits of carboniferous limestone (shaded blue) producing mostly non-hydraulic lime or lean lime occur in deep pockets in the Midlands and further north, across the north Pennines and in many areas of Scotland and Ireland. There is a belt of magnesian limestone in the Pennines running from Nottingham in the south to Catterick in the north (shaded orange). The Jurassic and late Triassic strata (shaded teal) running from the Humber to the south coasts of Dorset and south Wales include deposits of Blue Lias, which produce mainly hydraulic lime. The Oolitic limestones which overlie them produce non-hydraulic and lean limes. Carboniferous deposits in Anglesey, Pembrokeshire and other areas to the north and south of Wales produce mostly non- hydraulic limes.
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