Crystallization of sodium sulfate in two-dimensional interconnected pore system: Insights into localized, transient, and anisotropic stress generation and material deterioration

Crystallization-induced stress within confined spaces of porous materials presents significant challenges across various disciplines, including conservation science, geomorphology, geotechnical engineering, and concrete durability. Despite its significance, understanding how localized, transient, and anisotropic stress is generated in pores due to salt crystallization, along with its mechanisms of damage, has remained elusive due to the lack of direct observation and quantitative measurement in porous media. This study introduces an innovative approach using a lab-on-a-chip (LOC) with a two-dimensional interconnected pore system, providing a customized porous domain that mimics real-world porous structures. It enables real-time spatially resolved observations of drying pattern evolution and sodium sulfate crystallization under repeated drying-wetting cycles. This LOC system allows us to explore the effect of efflorescence on drying and salt crystallization, quantitatively demonstrating that efflorescence influences the drying rate, particularly in hydrophilic porous media, which in turn affects salt growth kinetics. During drying at an elevated temperature (65 \ifmmode^\circ\else\textdegree\fi{}C), thenardite (phase III) forms initially and then dissolves, and then thenardite (phase V) precipitates. During rewetting, the thenardite crystals formed during drying act as nucleation sites for accelerated mirabilite growth, leading to localized, transient, and anisotropic crystallization pressures up to 210 MPa. However, this high crystallization pressure alone does not guarantee macroscopic damage unless it is accompanied by substantial salt accumulation within pores, which is necessary for crack initiation and stress propagation. These findings underscore the significance of localized, transient, and anisotropic stress generation in porous media, providing critical insights into the long-term performance and deterioration mechanisms of porous materials in salt-laden environments. This work lays the groundwork for future research into the durability of porous structures and the development of targeted preservation strategies.