136 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 0 C AT H E D R A L C O MM U N I C AT I O N S Air lime mortars also decrease in porosity as they react, as there is approximately a 10 per cent volume increase in crystal size from calcium hydroxide to calcium carbonate. Furthermore the higher unslaked oxides available at time of placement in these mortars increases the risk of density change blocking pores and increasing strength through unintended densification. Pozzolanmortars with air limes can bemore porous thanNHLs, sowhy not just use them? Pozzolans are hugely varying in their reactivity, their workable behaviour and their local appropriateness. Pozzolans can impact every hardened mortar property, sometimes in ways which are undesirable in conservation. If you can find a good consistent source of pozzolan with repeatable results and performance then there is no reason not to use them, but make sure you understand how it affects the mortar before use. Pozzolans can be more or less consistent than NHLs in performance, it entirely depends on the source and quality-assurance methodology of each; like every manufactured product – some manufacturers and sources are better than others. Some pozzolans are not appropriate for use in most conservation. Blast-furnace slag (GGBS/GBFS) is one example of this; it is a limestone which has been calcined at 1500°C along with molten metal, and then cooled and ground. The temperatures and processes are almost exactly like those used to make a Portland cement, and it contains unwanted heavy metals and a high proportion of soluble sulphates. GGBS/GBFS can be used at up to a 95 per cent replacement for Portland cement and still maintains a 32.5N minimum strength (under EN197), the same strength as most CEM IIs on the EU market (these cements are required to contain a minimum of 65% Portland cement). Of the various limes and natural cements in general use in conservation, there are no inherently bad materials; there is only an appropriateness of specification. A wide range of available materials means we have a vast palette to choose from when deciding how to perform repairs, renovations or restorations. The one downside of this wide spectrum of materials and the complexities of their chemistry is that understanding the properties and interactions of everything on the market becomes nearly impossible. Developing a good specification depends not just on material performance but a collaborative approach between all parties involved. Sources and recommended reading English Heritage Practical Building Conservation: Mortars, Renders and Plasters, Ashgate 2011 How Hydraulic Lime Binders Work, Alan M Forster, Scottish Lime Centre 2004 http://bc-url.com/forster-hydraulic Concrete , Sydney Mindess, Francis Young and David Darwin, Pearson , 2nd edition 2002 Cement and Concrete Research Vol 41, Issue 12, “Supplementary cementitious materials”, Barbara Lothenbach, Karen Scrivener and RD Hooton, Elsevier 2011 Use of Prompt Cement with Natural Hydraulic Limes, Denis Sommain, The Louis Vicat Technical Centre Materials and Microstructures Laboratory, Special Binders Section, 2006 Chemistry and Technology of Lime and Limestone , Robert S Boynton, Wiley , 2nd edition 1980 EDWARDWALKER is Technical Manager at The Cornish Lime Company (www. cornishlime.co.uk ). As a materials chemist specialising in the conservation of historic masonry, Edward is primarily responsible for materials analysis, mortar matching, on site quality assurance procedures and product development. Left, a wall pointed with a mortar that was too hard and impervious for the masonry, forcing moisture to evaporate though the stones between: Right, free lime leaching from the core of a terracotta balustrade in Newcastle, which was probably caused by a breeze-concrete fill containing granulated blast furnace slag. (All photos this page: Jonathan Taylor) The greater the proportion of calcium oxide contained by a pozzolan, the more it acts as a cementitious material. When added to a mortar, those at the top of the chart act purely as pozzolans, while those at the bottom are inherently hydraulic. Pozzolans with higher hydraulic contents release lime which can block pores and densify a mortar.