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.