Salt crystallization pressure is one of the main sources of weathering in porous building materials. In our work we study the origin of this phenomenon using single crystals of KAl(SO4)2⋅12H2O (potassium aluminium sulphate dodecahydrate) as model system. Crystals were grown immersed in a solution of a specific supersaturation (σ=10−80%). Evidence of crystallization pressure was the vertical lift of weights on top of the growing crystal, which was measured by in-situ displacement sensors. A clear supersaturation threshold of σ≃30% for a 0.5 N weight is required to initiate lifting in our model system. Above this value, the maximum displacement increases nonlinearly with σ and depends on crystal orientation, interfacial wettability, and applied load. Hydrophobic interfaces suppress crystal growth by limiting mass transport, whereas hydrophilic confinement supports continuous growth until the local supersaturation is exhausted. In saturated solutions, on the other hand, a load leads to dissolution. Based on the observations, we propose a mechanism for crystallization pressure for which the growth of the unloaded faces adjacent to the confined face is key. These results refine our understanding of crystallization pressure in confined spaces and can help explain and mitigate salt-induced damage in porous materials.

Salt crystallization represents 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 mechanistic understanding, 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 271‐ASC 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 mass transport. Implications are discussed in relation to the most used accelerated salt crystallization tests in an attempt to better define which field exposure conditions that these tests best represent and may be used for, or define effective test procedures representing specific field conditions. 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, typically losing strength and cohesiveness. The implications of these two phases are discussed in relation to the selection of appropriate salt weathering tests and conservation interventions.