The Building Conservation Directory 2020

PROTECTION & REMEDIAL TREATMENT 4.1 135 C AT H E D R A L C O MM U N I C AT I O N S 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 is around 90 per cent of the final material strength. Alitic mortars should be kept damp in the early stages to promote full hydration of the hydraulic components. CLASSIFICATIONOF LIMES UNDER EN459 EN459 is a manufacturing standard designed to ensure that production is consistent and reliable. Complying with the standard permits the CE marking of products and allows manufacturers to sell these binders within the EU economic area. There is a common misunderstanding amongst many who use limes that NHL designations are the given strength for a binder and are indicative of real-life mortar performance. The number classification assigned to NHLs is only indicative of their strength when mixed at approximately 1:1.3, with an engineered, unrealistic sand and with a water/binder ratio which is unworkable. The mortar is then mechanically dropped to remove air from the samples and then tested at 28 days. This classification permits manufacturers to ensure their product is consistent with the standard but nothing else; diligent manufacturers recognise this and supply data to show longer-term strengths when used with realistic sands, mix ratios and workabilities. COMMON CONCERNS A growing number of people are concerned about how limes are classified as the test data has no relevance to real practical application. Some of these concerns are valid and some not, below we will examine some of the commonly highlighted issues. There is a permitted overlap between NHL binders in EN459; this means that one binder could be sold as all three strengths. While this is technically true, it is very unlikely to actually happen. NHLs are made from a naturally occurring material, so they can vary in strength when taken from different parts of the quarry and tested in the method specified in EN459. Quality assurance data is used to form a bell curve which is overlaid onto the EN459 banding structure; this bell curve will broadly fall into one of the categories. To ensure the standard is followed and to ensure test compliance, all NHL binder manufacturers are externally audited yearly and are subject to unannounced spot checks throughout the year. Lime binders and mortars are tested at 28 days; this is nowhere near completely cured/carbonated and give unrealistic results. This is correct; the current standards for both lime binders (EN459) and mortars/renders (EN998) require testing at 28 days even though they are not fully reacted. What must be realised is that both EN459 and EN998 are manufacturing standards and are only relevant for compliance of manufacturing; they are not designed for specification purposes. A 28 day period is reasonable for manufacturing compliance, but requiring test data to be produced for six or 24 months would be impractical and, from a quality-assurance perspective, meaningless: the aim of the standards are to prove the product is consistent and not that it performs in a certain way. NHLs keep reacting over time; they increase in density and decrease in porosity This is also true; however, they still maintain an open pore structure. To elaborate on this we must refer back to the previous section about reactions, setting and curing; belitic binders release some lime while they hydrate and cure, this lines the pore structure of a mortar similar to the build-up of cholesterol in an artery. Where this differs from an alitic binder is that alite produces much more lime, meaning that rather than just lining the pore it blocks it, often overflowing out of the hardened binder matrix and exhibiting as lime bloom. This also partially explains why air lime mortars are the most porous; they don’t have any additional lime being deposited in the initial hydration phase. If NHL mortars were impervious they wouldn’t be able to carbonate; at 20mm depth you can expect an NHL mortar to be completely carbonated within 1.5–2 years, a 20mm depth of modern Portland cement mortar would need around 400 years to fully carbonate. A severe marine environment would pose particular problems for an air lime mortar. For the repair and consolidation of these chimney stacks on a scheduled monument on Lundy Island, a hydraulic set was essential. (Photo: Old Light Building Conservation) NHL strengths as classified by EN459