Abstract
Salt crystallization is accepted to represent one of the major causes for the degradation of building and ornamental stone. As such, it has attracted the attention of researchers, who over the years have progressively unraveled most mechanisms involved in salt damage. Despite this, many questions subsist about how to quantitatively predict damage or its progression, and in particular how to relate performance on site to that in laboratory tests.
 In this context, a new RILEM TC has been started with the objective of defining laboratory tests that deliver more reliable predictions of field behavior. One deliverable of this TC, is to provide a theoretical insight into this question based on recent progress on the understanding of salt damage. This paper presents a summary of this work, highlighting key aspects relating to crystallization pressure, chemo-mechanics and transport. Implications are more specifically discussed in relation to existing accelerated tests in an attempt to better define the type of field exposure that they may best represent.
 A simple conceptual model for the development of salt damage is introduced. During an initial “induction” phase, transport of ions and accumulation of salt in the porous materials occurs without causing detectable damage until a critical point, termed “damage onset” is reached. Beyond this point, during the “propagation phase”, the material degrades increasingly. The implications of these two phases are discussed in relation to the selection of appropriate salt weathering tests and conservation interventions.
Highlights
Stones and masonry materials exposed to the environment are subject to physical, chemical and biological weathering that can damage these materials over time
During an initial “induction” phase, transport of ions and accumulation of salt in the porous materials occur without causing detectable damage until a critical point, termed “damage onset” is reached. Beyond this point, during the “propagation phase”, the material degrades increasingly. The implications of these two phases are discussed in relation to the selection of appropriate salt weathering tests and conservation interventions
The goal of this review is to contribute to better defining the questions and identifying possible approaches to designing the most meaningful possible tests for salt damage
Summary
Stones and masonry materials exposed to the environment are subject to physical, chemical and biological weathering that can damage these materials over time. Different deterioration patterns are known and, in the case of stone, well described in the ICOMOS‐ISCS glossary [1] In many situations, this degradation is due to the action of various types of soluble salts entrapped in masonry from internal or external sources or a combination of both. The accumulation rates and localization of supersaturation, which depend on exposure conditions, play important roles as drivers for crystallization pressure An overview of these factors is given, followed by a discussion of factors that control rates of salt transport in porous materials (section 4). Analyzing salt damage involves transport kinetics, environmental exposure, materials chemistry, and poromechanics In view of this complexity, numerical modelling of these coupled processes is essential to quantify “exposure conditions” and design relevant tests. The implications of these two phases are discussed in relation to the selection of appropriate salt weathering tests and conservation interventions
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