Sodium chloride damage to porous building materials

Abstract

This research studied sodium chloride (NaCl) damage to porous building materials with the aim of: i) gaining a better understanding of the damage process and ii) developing an effective crystallization test. It has been definitely proven that NaCl modifies the hygric dilation of a material completely. Irreversible dilation occurs during drying and crystallization of the salt and leads to damage. In the presence of a crystallization inhibitor, neither dilation nor damage occurs. The absence of dilation corresponds to a different crystallization pattern: NaCl, instead of crystallizing as a layer on the pore walls, forms, in the presence of the inhibitor, agglomerations of crystals not adhering to the substrate. These results lead to a new theory on the salt damage mechanism based on the mechanical interaction between salt and pore wall. Salt layers appear able to transfer stresses to the pore walls and thereby to cause dilation; this effect does not occur in case salt crystallizes without adhering to the material. The research has also addressed the problem of salt damage in practice. Buildings showing NaCl damage were investigated. Criteria for the evaluation of the risk of salt damage were formulated and guidelines for suitable conservation interventions derived. Moreover, an effective NaCl crystallization test was developed. By means of this test it is possible to simulate, in laboratory and in few months time, the damage process occurring in practice in several years. The test provides a considerable help in selecting the most suitable materials for both new buildings and restoration interventions.