Chemical and physical characterisation of some NHL binders and the correlation with the mechanical properties of conservation mortars

The chemical and physical properties of Natural Hydraulic Limes (NHL) from a given manufacturer can change over time. Some quarries are deactivated or the extracted raw material properties vary depending on their geographical location. These variations in property make it harder for practitioners to predict how changing new products can be best used to match the requirements of a mortar for heritage conservation in terms of mechanical performance and durability. NHL in the UK market have been show to meet the requirements of BS EN 459-1. Although important for a consistent production quality this requirement employs tests based on standard samples which are unrepresentative in their nature. As a consequence, these seldom reflect the performance of mortars produced and used ‘on-site’. Mortars formed from the same class of NHL often exhibit distinct variations in property. In this work, a selection of NHL binders were characterised using X-ray diffraction, X-ray fluorescence and thermo-gravimetric analysis. In addition physical properties including bulk density, surface area and particle size were obtained. Initial chemical and physical properties were correlated to the final mechanical properties of the mortars produced with a readily available sand in a 1:2 mix ratio (binder:aggregate by volume). Distinct mechanical properties at the ages of 7, 14, 28 and 90 days were measured for the same class of NHL. These initial studies will be used to develop a model to predict the final mortar performance from an understanding of the chemical and physical properties of lime binders