An assessment of lime mortars for masonry repair

Most historic and traditional mortars were made with lime. Due to their nature and function lime mortars weather and need to be replaced. Ordinary portland cement mortars can often be incompatible with historic masonry causing structural and aesthetic damage. Petrographic analysis of original mortars from Clonmacnoise Monastery and Ardamullivan Castle evidenced that they were fabricated with feebly hydraulic and magnesian lime respectively. Within a National Monuments’ conservation programme, the original mortars were replicated and tested and the best performers selected to undertake repairs. This paper investigates mortars made with four types of lime binder: fat, feebly-hydraulic, moderately-hydraulic and magnesian lime. The results indicate that lime mortars conduct moisture to a greater extent than OPC mortars. The feebly-hydraulic lime mortar has the lightest microstructure and highest porosity and absorption, being capable of holding the greatest amount of moisture thus being more susceptible to failure by water ingress. The results suggest that both fat and mg-lime binders would perform superiorly in areas subject to moisture ingress. As expected, the hydraulic set provides mortars with an additional strength and therefore both mg and fat limes are weaker in compression than hydraulic limes. However, the strength of the gauged mg-lime mortars is slightly higher than the values typically achieved by pure fat limes suggesting that mg-lime mortar may posses a certain amount of hydraulic set. The lime mortars tested are both physically and chemically compatible with the materials comprising the original masonry at Ardamullivan and Clonmacnoise. The chemical composition of the original mortars is similar to that of the new repair mixes. In addition, all new mixes are physically compatible with the limestones and sandstones comprising the monumental fabrics as they are substantially less dense, more porous and permeable and mechanically weaker than these rocks. Any of the lime mortars tested could therefore be selected to undertake repairs however, the magnesian and fat lime repair mixes are closer in composition to the original mortars and, if a conservative approach is to be taken, these are the best choices to undertake conservation work at the monuments.