Natural Hydraulic Lime - An Explanation


Natural hydraulic lime is produced by burning impure limestone containing clay minerals such as silica, iron and aluminium, at relatively low temperatures, 800 – 900°C compared to 1450°C for Portland cement. The lime is then slaked, sprayed or immersed in water, to produce a mixture of Dicalcium Silicate and Calcium Hydroxide and other compounds, which when mixed with sand and water are used to produce a lime mortar. A Natural Hydraulic Lime mortar therefore contains no cementitious compounds due to the low burning temperature.


The majority of limestone in the UK and the primitive way that it was historically manufactured, produced limes of varying hydraulicity providing sets ranging from feeble to eminent. These allowed the craftsmen to produce mortars that were more suited to withstand inclement/harsh weather conditions with a reduced risk of failure. They also induced a faster hydraulic set when required by added reactive materials such as pozzolan, trass, brick dust, etc., a method dating back to Roman times.


Developments in cement technology for volume construction in the 19th and early 20th century, together with the perceived advantage of increased mechanical strength, served to eliminate the need for the traditional skills of craftsman to create a lime mortar. This has led to a distortion of the concept of mortar to such an extent that today a ‘traditional mortar’ is defined as a blend of cement/sand/lime whereas it is little more that a modern process adopted to satisfy the demands of mass building. From the beginning of this century until the mid 1970’s the benefits of lime in conservation works were widely ignored with the result that many ancient structures were damaged by the application of incompatible mortars of cement and epoxy.


The advantages of lime for conservation projects is becoming more widely acknowledged, but due to its 50 - 60 year absence from UK production for construction use, for many the knowledge regarding performance of Natural Hydraulic Lime is not fully appreciated. Many people considered Hydraulic Lime as being dense, strong and cementious, their source of information generally referring to the later 18th and 19th century Roman Cements used throughout the UK. These claims are generally true of Roman Cements and some artificial or man made hydraulic limes due to the production methods used, where the clay content and burning temperature were increased. Unilit products however, are Natural Hydraulic Lime a mortar wholly distinct in its performance characteristics from cement and Roman Cement.


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