C O N T E X T 9 7 : N O V E M B E R 2 0 0 6 23 strength (the crushing of a standard cube) is derived from cement-testing methods, and is simply unreliable below a crushing strength of two N/mm2. In recent years the practice of specifying NHL3.5 hydraulic limes as a middling solution for general use in almost any circumstances has become widespread, and is often even advocated by official agencies, usually without adequately considering the circumstances of any particular building. This does have the practical advantage of providing a material which is relatively easy for the inexperienced to use, and which has a conveniently quick initial set. Prior to around 1870, however, limes of this strength would have been quite rare throughout much of the UK, and would normally have been reserved for civil engineering works, including bridges, tunnels, docks and canals.They can be much stronger than necessary for masonry or render on historic buildings. This frequent error is compounded by another misleading characteristic of EN 459-1: the establishment of compressive strength of mortars only at 28 days old. This is also derived from cement chemistry, as an alite set will be substantially complete at this age. A belite set, however, will often continue gaining strength for 90 days or more, so all of the NHL classes of hydraulic lime tend to produce finished mortars which are much stronger, harder, and less permeable or flexible than are generally recognised. In spite of the widespread acceptance of the lime revival, therefore, we are still often using materials which are significantly harder than those originally used on historic buildings, and the consequences of this rather sloppy approach remain uncertain. In the unfortunate absence of any feebly hydraulic limes on the market, it is possible to obtain comparable results by adding NHL to a pure lime putty. This approach was called into question by English Heritage’s moratorium on the use of ‘hybrid’ mixes some years ago. It is crucial to recognise that under these circumstances the first proportion of hydraulic lime added will be inert; thus a putty-plus-NHL3.5 mix will give actual results close to those expected of a putty-plus-NHL2 mix. In all cases, however, other variables can cause problems, and site-specific trials are strongly recommended. One can only hope that a manufacturer of a high-quality feebly hydraulic lime will reappear before long. As with the limes, there has historically been a wide range of different types of cements, characterised by varying proportions of alite, belite and free lime. Another conspicuous gap in the spectrum of available limes and cements is that between NHL5 and modern OPC. This is particularly significant for the conservation of buildings in the later 19th century and the first half of the 20th, including those from the early modern movement. Although these often used cement as a critical component in their construction and appearance, the actual material available was very different from that found in builders’ merchants now. Few present equivalents exist. A NHL5, perhaps with added pozzolan, can be used to match closely the compressive strength of 19th century cements, although it is not always possible to match other qualities, including appearance and long-term behaviour. Masonry cements, which tend to be hybrid mixes of modern Portland cement and hydraulic or hydrated lime (usually with plasticisers and other additives) can be useful in this area. Their continued existence is a tacit admission of the relative unsuitability of OPC for laying brickwork or stonework. But the appropriateness of a specific proprietary mix to an individual historic building can not be guaranteed, and little variety is available. A recent international EU-funded research programme, represented in the UK by the University of Bradford, has been studying the ‘Roman’ cements widely produced during the 19th century, particularly in the UK from septaria occurring in London clay. These natural cements have a highly distinctive appearance, with a strong reddish-brown colour which is notoriously difficult to match. The project’s intention to recommence manufacture of the material is an exciting development. Roman cements were made effectively obsolete by the rise of early Portland cements after 1870, so this only partially fills the gap. Some other natural cements have survived on the UK market, principally to suit the potable water industry’s requirements for low soluble chemical content and rapid set. But the majority of these are supplied as premixed mortars tailored specifically for the repair of water-retaining structures, and are of limited use in conservation. That does not exclude the possibility that suitably formulated products may be developed in the future. As above, the question of ‘lime or cement?’ is no longer an adequate model for the specification of mortars, renders or plasters in building conservation. Instead we should ask: ‘which lime or cement?’. A suitable equivalent for many of the materials used historically, throughout all periods, is now available, but there are significant gaps in the overall spectrum which still remain to be filled. This relatively wide availability puts the building conservation community in a position of strength which has never previously existed. It must be recognised, though, that each of these varied materials does need to be understood in its own right in order to achieve appropriate results.There are grounds to hope that a full spectrum of appropriate limes and cements for both conservation and new build may actually be available in the foreseeable future, although much work still remains to be done. Postscript Since this article was written, and in response to a proposal put forward by British representatives, the Euronorm committee has agreed in principle to consider the revision of EN 459 to introduce a weaker class of natural hydraulic lime, provisionally designated NHL1. At least one manufacturer is understood to be actively considering the introduction of a suitable product. Building Limes Forum A list of references and further reading can be found on the website of the Building Limes Forum. All the issues raised here have previously been discussed within the pages of the forum’s journal. The forum is an international charity, based in the UK, which aims to encourage expertise and understanding in the use of building limes. It welcomes as members all those who would like to know more. See www. buildinglimesforum. org.uk. Ian Brocklebank is chairman of the IHBC technical committee and also chairs the Building Limes Forum.
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