BCD 2018

54 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 • pozzolans (crushed brick, etc) • Prompt (natural/Roman cement) • CL90 crushed limestone (chalk dust) • inert sand aggregate. As explained in the article on page 134 (‘Hydraulic Lime Production’), hydraulic lime is no longer manufactured from the UK’s traditional sources, and imported NHLs can have unexpected consequences for the unwary. There is widely available data on the minerology of the various indigenous limestones used in mortar-making for bridge- building throughout the UK. Although there is limited available data on the as-calcined properties of the quicklimes produced in the past, chemical engineering allows us to predict their likely characteristics under given burn controls (temperature, time). Further research into the performance characteristics of original bridge-building mortars is required. However, a working approximation for target as-calcined characteristics of hydraulic quicklimes can be made using the historical classifications for hydraulic quicklime and review of original mortar recipes (see Further Information: Eckel, Dibdin). The palette of modern NHL classifications cannot match the free lime content of the historical quicklime classifications, and natural cements contain practically none. As free lime is a vital function-imparting component, giving breathability, drying out capability, sacrificial behaviour, deformability and autogenous healing, it is important to carefully consider the context when specifying them. The lack of lime richness in NHL and natural cement binders should be compensated for where used above the water line, for example by using an aggregate of crushed limestone (CaCO 3 ). Quicklime CL90 grade (>95% CaO) typically doubles in lime yield upon slaking according to the Cowper ‘Class A’ (fat) classification, and therefore should be the first choice ingredient for raising the free lime content of the mortar, to replicate the functionality of historic examples. Gauged quicklime-pozzolan mortars are very effective for achieving lime-rich mortar mixes and a controlled amount of hydraulicity. Modern pozzolans such as metakaolin tend to be more reactive than the traditional brick dust/fuel ash counterparts and the lime-demand of each should be recognised. From an aesthetic perspective, lime inclusions are visually authentic which is a welcome by-product of getting the composition right. Stone for replacement masonry should be geologically compatible or matched to the original, and should comply with BS 5390:1976, selected on the basis of proven track record. Mortar joints should be fully filled and flush pointed (the most durable). To conclude, significant practical challenges are presented in the repair of masonry engineering works and a pragmatic approach to repair mortar design is needed. Historic lime mortars are binder-rich and the binders themselves are typically lime-rich. It is the lime-richness above the water-line which actively dries the masonry out, and preserves the masonry units by weathering sacrificially. The original lime-richness should be replicated in the design of compatible repair mortars to ensure this continued functionality. While the original hydraulic lime binders are not currently available commercially, the existing palette of CL90 quicklime, NHLs, pozzolans, Prompt natural cement and microporous calcitic aggregates can be adapted to obtain a compatible mortar which is a fair replication of the original, in terms of mineral composition and hence functional behaviour in service. Further Information AD Cowper, Lime and Lime Mortars , Donhead, Shaftesbury, 1927 W Dibdin, The Composition and Strength of Mortars , RIBA, London, 1911 E Eckel, Cements, Limes and Plasters , John Wiley & Sons, New York, 1922 A Henry and J Stewart (eds), Mortars, Renders & Plasters , English Heritage, Ashgate, Farnham, 2012 S Pavia, ‘Repair Mortars for Masonry Bridges’, in N Nuallain et al (eds), Bridge and Infrastructure Research in Ireland: Symposium 2006 , Trinity College, Dublin, 2006 J Smeaton, A Narrative of the Building and Description of the Construction of the Edystone Lighthouse with Stone , H Hughs, London, 1791 AM Sowden, The Maintenance of Brick and Stone Masonry Structures , E & FN Spon, Cambridge, 1990 DAVIDWIGGINS BSc(Hons) PhD IEng MICE undertook doctoral research into the functional behaviour and technical conservation of heritage masonry at Glasgow Caledonian University. He is a structural engineer, part of the Conservation and Heritage team at Curtins Consulting Ltd (see page 43). His work centres around the structural repair and conservation of architectural and civil engineering heritage. GLOSSARY Fat lime a relatively pure lime (calcium hydroxide, Ca(OH) 2 ) containing no more than six per cent calcium silicates and calcium aluminates: also known as non-hydraulic lime , it sets solely by carbonation, a chemical reaction with carbon dioxide which, in the presence of moisture, slowly converts calcium hydroxide to calcium carbonate. Hydraulic the ability of a lime mortar to set and harden primarily by chemical reaction with water: the property may be a consequence of using a lime (calcium hydroxide) containing calcium silicates and calcium aluminates, or it may be achieved by gauging a fat lime (relatively pure calcium hydroxide) with a pozzolan. NHL a natural hydraulic lime made from a limestone rich in aluminosilicates which sets primarily by chemical reaction with water, but also by carbonation (see Fat lime , above). NB the acronym is unfortunate, easily confused with non-hydraulic lime. Pozzolan a material containing finely powdered silicate and aluminate minerals which can react with lime (calcium hydroxide) in the presence of water to produce a hydraulic set. Quicklime principally calcium oxide, an unstable material which reacts vigorously with water to form calcium hydroxide: it is made by ‘calcining’ limestone (calcium carbonate) by heating it at around 800–1,000°C in a kiln. The bridge over the South Tynedale Railway Bridge from the first illustration as repaired, with rehabilitation work for the narrow-gauge steam railway underway. The bridge repair contract included a waterproofed saddle and better deck drainage. Note the brightened colour of the bridge fabric after the masonry has dried out.