Salt Crystallization in Substitution Renders for Historical Constructions.

Abstract

Salt crystallization in substitution renders for historical constructions Ana Fragata1, Maria do Rosário Veiga2, Ana Luísa Velosa3 1 Buildings Department, Laboratório Nacional de Engenharia Civil (LNEC), Portugal, afragata@lnec.pt 2 Buildings Department, Laboratório Nacional de Engenharia Civil (LNEC), Portugal, rveiga@lnec.pt 3 Department of civil Engineering, Universidade de Aveiro, GeoBic, Portugal, avelosa@ua.pt Abstract In Portugal, rehabilitation and conservation interventions are assumed of increasing relevance. Renders are used as decorative and protective coats, acting as sacrificial layers, particularly exposed to climatic actions and mechanical and environmental impact, becoming the most vulnerable constituents of historical buildings. Incompatible repair render solutions are still being specified, nowadays, for old buildings, not taking into account the specific situation, the background and the pre-existing materials. Salt damage is one of the major causes of renders decay, not only near the sea but also in continental areas. One of the most abundant and common salts present in walls is sodium chloride. The crystal growth is usually induced by changes in ambient temperature or relative humidity. These salts can appear in the walls from different sources: from the ground due to rising damp, carried by the wind as salt spray, flooding or originally present in materials, like unwashed beach sand or due to the salt transport behaviour of the substrate/render and the surrounding ambience severity (temperature and relative humidity) producing salt crystallization. In this paper an experimental laboratory campaign was developed with perforated red bricks rendered on both sides with four different render compositions and different permeability, submitted to capillary absorption in a sodium chloride solution. Particular attention is given to the influence of different renders solutions when contaminated with NaCl and along the several dissolution-crystalization cycles: (a) in the location of sodium chloride crystallization in the specimens; (b) crystallization quantification of different specimens. The final goal is to understand the relation between the more or less permeable renders and NaCl dissolution-crystallization cycles damage mechanisms, in order propose possible solutions to stop or slow down the salt decay process in walls contaminated with this salt and subjected to changes in ambient temperature and relative humidity.