BCD 2018

136 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 standard specifies different curing conditions for NHL 2 compared to NHL 3.5 and NHL 5 mortars, which suppress the strength gain of the NHL 2s. Furthermore, all testing is carried out at 28 days, but research has shown that, unlike Portland cement (which gains 80% of its strength in the first 28 days), hydraulic lime mortars continue gaining strength for well over two years and can ultimately reach compressive strengths at least twice their 28 day strengths, sometimes considerably more. In addition to conservation professionals’ and practitioners’ concerns that NHL mortars are too strong for most conservation applications, there have been reports of other problems such as leaching of lime, migration of salts into adjacent masonry and retention of moisture in masonry walls where NHLs have been used. But there is little objective evidence to confirm whether, why or how this is happening, and little data about the properties of NHL mortars in use. To address this lack of information, Historic England, working through the Building Limes Forum, funded a PhD at the University of Bath to assess the properties and performance of natural hydraulic lime mortars for conservation. The research, completed in October 2017, involved chemical and physical characterisation of 16 NHLs that were on the market in 2014. Mortar samples were made using a representative selection of 11 of these. The aim was to learn more about the properties of ‘real’ mortars, rather than the ‘standard’ mortars used to test compliance with BS EN 459, so the samples were made using one part dry hydrated lime to two parts (by volume) well-graded silica sand mixed with sufficient water to obtain a workable consistency. They were then cured in controlled conditions and subjected to a range of tests to determine their properties. The results were surprising (below). At the end of 28 days, the two mortars with the highest compressive strength were those made with two brands of NHL 3.5. All the NHL 3.5 mortars and even one of the NHL 2 mortars were stronger than the weakest NHL 5 mortar. The situation proved to be even more surprising when the strengths of mortar samples at two years’ age were measured. The strongest sample was still an NHL 3.5 mortar, but the third strongest was an NHL 2 mortar, which had overtaken three NHL 3.5 and two NHL 5 mortars. In fact, whereas most of the NHL 3.5 and NHL 5 mortars gained strength fairly slowly after 90 days, all but one of the NHL 2 mortars continued gaining strength at a steady rate, overtaking many of the mortars made with binders in a higher strength class, including the mortars made with NHL 3.5 lime from the same manufacturer as each NHL 2 being tested. This demonstrates that not only is there significant variation in strength between limes from different manufacturers, but that even within an individual manufacturer’s product range, the NHL 2 binder may end up stronger than the NHL 3.5. To validate these results, ‘standard’ samples of mortars were made and tested at the university and by an independent commercial laboratory, strictly in accordance with the standard methodology required by BS EN 459. The binders used were all from the same manufacturer, but from different batches purchased from different suppliers. Both the NHL 2 samples tested exceeded the 2 MPa minimum compressive strength at 28 days required by the standard, although the values achieved by the university and the commercial laboratory were significantly different. While the values obtained for the NHL 3.5 were closer in both labs, the NHL 3.5 tested at the commercial facility failed to meet the minimum 3.5 MPa at 28 days specified by BS EN 459, and the NHL 5 tested in both labs fell far short of the required minimum strength. All three binders tested at the university achieved virtually the same compressive strength at 28 days when tested according to BS EN 459, regardless of the classification on the bag (see Comparison table, above). As well as compressive strength, other relevant properties assessed included permeability, porosity and capillarity. The research team plans to meet with the NHL manufacturers to discuss the research findings before publishing the detailed data. CAN NHLS BE SPECIFIED USING BS EN 459? These results throw up serious questions for people specifying and using NHLs. When selecting mortars for conservation (or indeed any other purpose), to what extent can we rely on the classification printed on the bag to understand the likely long-term properties of an NHL mortar? Our research suggests not at all. If you buy a bag of NHL 2, believing that this is the weakest NHL, there is a good chance that you could end up with mortar stronger than one made with an NHL 5. Can reliable information be gained from the product data published by the manufacturers? One manufacturer publishes two-year data for a range of mortars, but this appears to be many years out of date. Another supplier’s data changed significantly a couple of years ago, with no explanation or warning on the company website. So, even if someone had taken the trouble to check the properties of those particular binders, they could find themselves unwittingly using material with very different properties a few months later. Some manufacturers don’t publish any detailed long-term data about ‘real’ mortars. Specifiers are urged to request or demand better test data from manufacturers. This may help to exert pressure on the manufacturers to produce it and to learn more about the properties of their materials. WHAT ARE THE ALTERNATIVES TO NHL MORTARS? All limes can be given additional hydraulic qualities by incorporating a pozzolan in the mix. For centuries it was common practice to modify lime mortar mixes with a pozzolan if a greater hydraulic set was needed than could be expected from local lime. In broad terms, pozzolans contain minerals – mainly composed of alumina, silica and sometimes iron oxide – similar to those in the ‘active clays’ in limestones from which hydraulic limes are produced. Pozzolans were used by the Romans and records confirm that from at least the 17th century a wide range of ash from different industrial processes was Consolidation of decaying Blue Lias stone was carried out using NHL 2 repair mortars 14 years ago. The mortar is still intact but it is not acting sacrificially, and the stone continues to decay. (Photo: Alison Henry) Independent analysis commissioned by Historic England revealed that the actual strengths of sampled NHLs at 28 days bore no relation to the expected strengths.