22 C O N T E X T 9 7 : N O V E M B E R 2 0 0 6 Hydraulic limes always consist of a mixture of lime and belite acting in parallel. This dual curing process should be taken into account whenever they are used, ensuring that both the initial hydraulic set and then the slower carbonation of the free lime can take place, although for practical purposes the latter may now be less critical. When the proportion of belite is low, the mix is more workable. It cures slowly and softly to give a very permeable and flexible finished material. A higher proportion of belite gives a faster and harder set at the expense of workability, permeability and flexibility. Carbonation continues to follow the hydraulic set at its own natural pace until the material reaches its finished, cured state, so protection and tending are required on site with even a relatively strong hydraulic lime. Modern ordinary Portland cement (OPC) does still contain belite and lime, but the speed and strength of the alite set is deliberately designed to swamp the effects of either of the two weaker, slower processes. Such a material is extremely well suited to the production of modern reinforced concrete, and it can be used with little protection or curing on site, making it very tolerant of poor workmanship. Although convenient and economic to use, it is debatable whether OPC is at all appropriate to even modern masonry construction in the long term. Finished work is inevitably of inferior quality. In order to improve the economy and environmental sustainability of their production, modern OPCs now tend to be manufactured from a wide range of extracted and waste materials.This introduces complex and sometimes undesirable chemistry, about which the manufacturers are not always entirely open. Gypsum is often added to retard the speed of the natural set (which can otherwise take only seconds). This results in a material that is not only unnecessarily hard, brittle and impermeable, but which also introduces a significant load of soluble sulphates, causing at best efflorescence on facework when these are mobilised by water, and at worst, severely degrading adjacent brick and stone by recrystallising within their pores. The latter is a very common failure mechanism where modern concrete or cement has been introduced into historic masonry. Most mass-produced bricks are now manufactured with an exaggeratedly high resistance to salt damage, partly in order to cope with the cement, and this is a principal reason for the difficulty in matching their historic softer-fired predecessors. Pozzolans are separate activated silicates, usually derived from fired clays, which increase the hydraulicity of a mix by combining with the free lime during the curing process to form belite (ground brick dust and natural volcanic earths such as those from Pozzuoli near Naples, and trass, from Germany) and sometimes even alite (ground granulated blast-furnace slag, pulverised fuel ash, and similar high temperature materials). Any material containing a proportion of free lime can be moved to the right on the spectrum by the addition of suitable pozzolanic additives. Vicat’s classification of hydraulic limes into ‘feebly’, ‘moderately’ and ‘eminently hydraulic’ classes was widely adopted from 1820 until the demise of most of the traditional lime industry in the UK, as it was both practical and appropriate to the end uses of the materials. This classification has now been superseded by the natural hydraulic lime classifications of NHL2, 3.5 and 5 enshrined in the Euronorm EN 459-1: 2001. It should, however, be noted that these tripartite classifications are not equivalent. The weakest modern class of NHL2 is more closely equivalent to Vicat’s ‘moderately hydraulic’ class, while NHL3.5 is nearer to ‘eminently hydraulic’. NHL5 limes can easily reach strengths equivalent to natural cements. As can be seen from the diagram, there is now a critical gap in the modern classification between pure limes and NHL2, where Vicat’s ‘feebly hydraulic’ class used to sit comfortably. This gap is particularly important within the UK as many of the limes that were highly regarded historically, and greatly sought after, were classed byVicat as ‘feebly hydraulic’. Chief among these is perhaps the Dorking lime from the grey chalk of Surrey which was used to build so much of London, from the British Museum to the West India Docks, as well as being exported far further away. These ‘lean’ limes (as opposed to ‘fat’ or putty lime) are also a particularly interesting category for their working characteristics. Whereas modern NHLs are invariably produced by hydration like bag lime, and sometimes even ground as modern cements, the feebly hydraulic limes were traditionally slaked on site and used as young putties, giving a crucial mixture of the workability of putty with the convenience of a slow hydraulic set. It is particularly for this mix of qualities that they were so highly prized. Normal working practice within London was to slake lime on a Friday for use during the following week. It is galling that EN 459-1 omits this class not because there is no use for these limes, but because the test method used to assess compressive A simplified representation of the spectrum of limes and cements that have been historically available, based on the principal setting chemistries at work (as it is from this that their various qualities arise). Hydraulic limes:Vicat’sclassification (1820) “Feebly hydraulic” “Moderately hydraulic” “Eminently hydraulic” Hydraulic limes:classification toEN459-1:2001 NHL2 NHL3.5 NHL5 Cements incomparisonbydate range 1840-1870 1870-1910 1910-1940 1940-2000 (Naturalcements) Lime Belite Alite PureLime HydraulicLimes Cements Firing temperature THE LIME SPECTRUM (Diagrammatic) Compressive strength, impermeability, brittleness,setting speed.
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