Mechanical properties of standard and commonly formulated NHL mortars used for retrofitting

Selecting materials for retrofitting of historic and heritage buildings can be challenging. These materials must be sufficiently compatible and durable without risk of damage to the existing fabric. Therefore, mechanical properties of the retrofitting mortars are of great importance.
Natural Hydraulic Lime (NHL) binders are classified according to their compressive strength at 28 days of ageing and lime content using standard EN 459-1. The standard test, however, although important for quality assurance and consistency of binder production, does not reflect the performance of mortars manufactured and used on-site, since these use different aggregates and water/binder ratios.
This study investigates binder classifications, NHL 2, 3.5 and 5, from a single supplier and compares the standard formulation as defined in EN 459-1, with a formulation commonly used as a conservation mortar with 1:2 binder:aggregate ratio.
The 28 day compressive strength of mortars manufactured using a formulation typical for conservation differed in strength from the standardised samples used to classify the binders. At later ages, some mortars were found to have a greater compressive strength than that implied from their classification.
This study concludes that the prediction of aged mortar properties using the standard classification is problematic. The basis for development of a model to predict the performance of aged mortars based on chemical and physical properties of the binders is identified. The model we propose to develop from this work will allow conservators to predict strengths more accurately and reduce the risk of building damage attributed to the use of mortars with inappropriate strength.