Chemical and physical characterisation of three NHL 2 binders and the relationship with the mortar properties

In conservation works, the physical properties of mortars, such as compressive strength and porosity, are by far the most important for compatibility with, and protection and durability of historic fabric. The classification of Natural Hydraulic Lime (NHL) binders by the EN 459-1, gives little information about these properties for mortars, due to the unrepresentative nature of the standard samples used to categorise these binders, especially after 28 days of aging. As a consequence, although important for quality assurance and consistency of binder production, the standard test tends not to reflect the performance of mortars as made and used on-site, since these use different aggregates and water/binder ratios.

In this work, three types of NHL 2 were analysed. These binders were characterised by means of X-ray diffraction and X-ray fluorescence. In addition to chemical tests, the surface area and bulk density data were also obtained. Mortar samples were manufactured using a sand aggregate which is appropriate for a conservation mortar with 1:2 ratio (binder:aggregate by volume). Sufficient water was added to produce a spread by flow table of 16510 [mm]. The chemical and physical properties of the binders were related to the physical characteristics of the mortars.

The chemical and physical properties of different binders with the same NHL classification were found to vary greatly as did the properties of mortars at ages of 7, 14, 28, 90 and 180 days made with those binders. The need to develop a model to predict the performance of aged mortars based on the chemical and physical properties of the binders was identified.