Context 154 - May 2018

C O N T E X T 1 5 4 : M A Y 2 0 1 8 31 A common way of making mortar in the past was to place a ring of damp sand on the ground, pour the quicklime into the centre, add water and draw the sand over the quicklime to cover it. During slaking steam would be driven off as the heat dried the sand. After slaking it was an easy matter to mix the dry sand and the slaked lime, before adding more water to make mortar of the desired consistency. relevant standard) were much stronger than their historic counterparts, and were perhaps too strong for many conservation applications. Comparison of the compressive strength of mortars made with NHLs with that of the limes most widely used historically (non-hydraulic and ‘feebly hydraulic’ lime) reveals clear differences, and suggests that concern about excessive strength of NHL mortars might be well- founded. Further concerns about long-term strength gain were raised by recent research commissioned by Historic England¹. Furthermore, historic records and analysis of mortars indicate that in the past the stronger grades of hydraulic lime (termed ‘moderately’ and ‘eminently hydraulic’), which compare closely in strength to the modern NHLs, were reserved for high-strength applications such as military and civil works, and for work underwater or in persistently wet environments. Yet by the early 21st century NHL mortars were being widely specified for many conservation applications, sometimes even for internal plastering. Learning from the past Recognition that NHLs were not the hoped-for panacea forced a rethink.Why do modern conservation mortars fail to replicate the balance between durability and sacrificial behaviour seen in so many traditional mortars? The answer lies in the evidence presented by historical mortars; analysis of thousands of samples has shown that a large majority were very lime-rich – often in the region of 1 part of lime to 1½ parts aggregate – compared to the 1:3 proportions of typical modern lime-putty mortars. Furthermore,with the exceptions mentioned above,most historical mortars were non-hydraulic or only slightly hydraulic – certainly nowhere near the strength of even the weakest NHL mortars. So, durability in traditional mortars seems to derive from a comparatively weak binder but used in larger quantities than in most modern lime-putty mortars. However, it is almost impossible to make very lime-rich mortars using lime putty because one volume of putty contains too much water for 1½ volumes of aggregate, so the resultant mix is very sloppy. So, how were such mortars made in the past? Historical sources show that in many cases sand was mixed with lime in the form of Reference ¹ Figueiredo, C, Henry, A, Holmes, S (2018) ‘Hydraulic lime production: coming full circle?’ in Building Conservation Directory 2018 , Cathedral Communications, Tisbury The hydraulic lime most widely used in the past for general building work was termed ‘feebly hydraulic’ in the 19th century, and later (in a British Standard of 1966) ‘semi-hydraulic lime’. The maximum strength of semi-hydraulic lime equals the minimum strength of NHL2, and most NHLs are very much stronger than this.This demonstrates the disparity between lime binders commonly used in the past and those in widespread use today for much conservation work.