An assessment of the Natural Hydraulic Lime binder-aggregate interface : utilising indigenous Scottish aggregate materials

Abstract

Naturally Hydraullic Lime (NHL) binder is commonly specified for use in Scotland where it is regarded as a suitable replacement material in the conservation of historic buildings. Compared to calcium limes (CL), NHL mortars are considered to possess favourable characteristics; such as greater resistance to wind, rain and damage by frost, leading to their prevalence in the harsh Scottish climate. Lime mortars are perceived as playing a leading role in reducing carbon footprints in the construction and conservation sectors as a whole, whilst an increased awareness of local raw, region specific aggregate materials could further assist in this reduction. This research has investigated the performance of NHL 5 mortars, including Scottish aggregate materials, on account of several criteria which are indicative of overall performance and durability. An emphasis has been placed on trying to isolate the effects of the Interfacial Transition Zone (ITZ), considered detrimental in cements, this was achieved by producing mortars which contained variable quantalities of aggregate and binder. Mortar specimens have been assessed on account of their strength, sorptivity, carbonation depth, porosity and electrical impedance as a means to attempt to isolate the effects of the ITZ; while a second series of testing has attempted to examine the strength and failure patterns of a scaled up interface between NHL 5 binder and natural stone. The research has highlighted that grading and textural features such as intra-clastic porosity can have more of a profound effect on the interface and overall strength achievable in lime mortars. Furthermore, the research suggests that a greater uptake of local, mineralogically varied aggregates could be used as part of a low carbon strategy.