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 8 135 in the current BS EN standard. For many conservators and specifiers today, this alone is enough to make them steer clear of NHLs for the majority of conservation applications, although they might still be appropriate where high strength is needed for engineering structures or in persistently wet environments. THE DEMISE OF GREY CHALK LIME PRODUCTION Needless to say, the lack of an appropriate standard for specifying feebly hydraulic lime did nothing to maintain demand for its production. But there had already been other factors contributing to its demise. Firstly, from the late 19th century there was the rapid adoption of Portland cement as an alternative to lime for many applications. Throughout the 20th century Portland cements became progressively stronger, which benefitted large- scale engineered structures but was disastrous when applied to mortars, renders and plasters for most traditional buildings. But by the time the damage to historic fabric caused by cement was understood, the material was firmly established as the ubiquitous binder in British construction. Secondly, a combination of legal obstacles in the UK and strong marketing from European natural hydraulic suppliers compromised the financial viability of the remaining small- and medium-sized UK hydraulic lime producers. Production of grey chalk lime at the Shillingstone Lime and Stone Company ended in 1990 and at Totternhoe Lime and Stone Company Limited in 1993. More recently, production of a true feebly hydraulic lime was attempted by Singleton Birch in Lincolnshire using a local lime. Unfortunately, there was little demand for this product – perhaps because specifiers were reluctant to specify a material that did not conform to a standard – and production ceased a few years later. There is currently no production of traditional natural hydraulic lime from UK limestone and all NHLs are imported. Gone is the local link between geology, lime and buildings, and with it generations of knowledge and understanding of which lime was best suited to the task in hand. THE PROPERTIES AND PERFORMANCE OF NHL MORTARS The method of testing binders for compliance with BS EN 459 does not give realistic data about ‘real’ mortars made with them. The ‘standard mortars’ tested are made with a laboratory sand and a water/binder/aggregate ratio that produces a mortar with unworkable consistency. The objective of this methodology (which is borrowed from the Portland cement standard) is not to produce data relevant to the end-user but simply to ensure that test results from any laboratory in Europe are comparable. There are a number of other problems with the test methodology. For example, the Comparison of strengths in BS 890 and BS EN 459 The remains of Smeaton’s Eddystone lighthouse today. John Smeaton carried out extensive research into hydraulic lime and pozzolans to inform his design of the third Eddystone Lighthouse (1756–1759), 12 miles south of Plymouth in Devon. He settled on Blue Lias lime mixed with Italian pozzolana. The lighthouse survived for over 120 years, and only had to be dismantled because the bedrock on which it was built was undermined by the sea, causing the tower to sway in strong seas. The upper part of it was reconstructed on Plymouth Hoe, but the lower part remains in situ, as it was found to be too strong to dismantle. (Photo: John Stewart) No 9, entitled Lime and Lime Mortars . His report confirmed that there are extensive Cretaceous deposits comprising an upper layer of white chalk, producing a non- hydraulic or ‘fat’ lime (Class A), below which is frequently a layer of grey chalk, containing some argillaceous (clayey) matter, which he classified as feebly hydraulic (Class C1). (‘Lean’ limes, which contain a significant amount of inert impurities but were not hydraulic, were assigned to Class B.) It is clear from various archive records, published technical references and analysis of historic mortars that this grey chalk lime and other feebly hydraulic limes were used across a large part of the UK for a wide range of applications, and were an important part of the spectrum of historic building limes. Although moderately and eminently hydraulic limes were produced (especially from the second half of the 18th century), they were reserved for civil engineering and military construction, or for works in continually wet environments (such as locks, harbours and water mills). BUILDING LIME STANDARDS Although many practitioners and specifiers in the UK will be familiar with Cowper’s descriptive terms, most of them were never incorporated into any British standard. BS 890:1940, Specification for Building Limes was the first on this subject. When revised in 1966, it distinguished two classes of building lime on the basis of hydraulicity, ‘high calcium lime (white lime)’ and ‘semi- hydraulic lime (grey lime)’, as well as a class for magnesian lime. The metric edition of BS 890, published in 1972, specified that ‘semi- hydraulic lime’ should be between 0.7 MPa and 2.0 MPa. (Strengths were then given in Newtons per mm 2 rather than megapascals, 1 MPa being equal to 1 N/mm 2 .) These definitions changed dramatically when BS EN 459, Building Lime was published in 1995 (see chart above). All references to ‘grey chalk’ and ‘slightly hydraulic’ lime (Cowper’s ‘feebly hydraulic’ lime) were omitted from this new standard. Lime with naturally hydraulic properties was termed ‘natural hydraulic lime’ and three types were introduced: NHL 2, NHL 3.5 and NHL 5. These had broad, overlapping compressive-strength requirements for classification, the lowest being NHL 2 with a compressive-strength requirement between 2.0 and 7.0 MPa, compared to a strength of 0.7-2.0 MPa for the BS 890 equivalent (semi-hydraulic). The strongest was NHL 5 with a requirement of 5-15 MPa. So, almost overnight, the maximum compressive strength of the strongest mortar in BS 890 became the minimum compressive strength of the weakest hydraulic mortar in BS EN 459, and there was no longer a standard applicable to feebly hydraulic lime, despite its previous widespread use. Some suppliers market NHL 2 as feebly hydraulic but this is incorrect; even the weakest NHL2 is significantly stronger than traditional feebly hydraulic lime (shown in red, above). It is clear that the majority of limes in general use until the last half of the 20th century were well below the strengths of those