The Influence of Traditional Substrates on the Behaviour of Lime Mortars

Abstract

ABSTRACTOne of the main functions of mortars is the protection of walls in-service conditions. Mortars, however, are associated with common pathological manifestations, which not only affect substrate strength and other properties, but also the comfort and healthiness of buildings users, being responsible for a significant fraction of rehabilitation expenses. In the last few decades, there has been a growing trend towards the rehabilitation of existing buildings instead of their demolition, not only for economic and urbanistic issues, but also for ecological and sustainability reasons due to the awareness of the limitation of available natural resources. In traditional buildings, lime-based renders (coating mortars) were the most common type of wall coating. Indeed, these renders have properties, such as low mechanical strength and stiffness and high-water vapour permeability, which are compatible with traditional substrates. However, inappropriate coating solutions, such as cement-based mortars, have been used in the rehabilitation of old buildings, leading to the development of anomalies in building walls. Nevertheless, there is an increasing awareness of the need to use coatings that are mechanical-chemical, physical and geometrical compatible with traditional substrates. Mortar characteristics, whether declared by the manufacturer, designer, or established by the user, are currently determined in the laboratory, according to applicable standards. When applied on substrates, mortars have their properties altered due to interaction with them, and various substrates influence differently the same mortar, as showed by extensive work developed in the scope of the IF MORTAR project. Thus, knowledge about in-service behaviour of mortars becomes crucial. The main aim of the research presented in this paper is to assess the behaviour of lime mortars applied on traditional substrates and to discuss how substrate properties affect mortar behaviour. In this way, there is a contribution to improve the understanding required to select materials for interventions in traditional buildings, assuring compatibility between traditional and new elements, with suitable durability. To achieve the above-mentioned aim, hydraulic lime mortars and hydrated air lime mortars, produced with two sands with different particle sizes, were applied on solid ceramic bricks and limestone slabs. These substrates were chosen because they are commonly found in traditional buildings. Physical and mechanical characterisation tests were carried out on the mortars hardened in laboratory moulds and on the mortars hardened on the substrates (after being detached). The substrates used were also characterised, as well as the aggregates and binders. The test results revealed that substrates have an impact on mortars characteristics, and this influence is greater for the more porous supports, and varies depending on the type of binder and aggregate used, being directly related to mortar porosity and pore distribution. For air lime mortars, there was an increase in the range of pores from 0.05 to 10 μm and a decrease for pores above 10 μm. The opposite occurred for hydraulic lime mortars. In addition, for air lime mortars, open porosity and water absorption by capillarity increased, in contrast to what happened to hydraulic lime mortars, the open porosity and water absorption by capillarity decreased, after applied to substrates and when subjected to standard curing. Regarding the type of aggregate, for the same ratio, the use of finer aggregates resulted in more porous mortars.