A study of the mechanism behind crystal lifting: Crystallization pressure of confined KAl(SO4)2⋅12H2O crystals

Salt crystallisation 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(SO 4 ) 2 ⋅ 12H 2 O (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. • Reproducible experimental results on crystal lifting. • Only lifting in combination with side-facet growth. • Only lifting when interface energies allow liquid nanolayer. • Atomic scale model for crystal lifting.