Crystallization of hydrated and anhydrous salts in porous limestone resolved by synchrotron X-ray microtomography

Highlights: • In-pore salt crystallization in Savonnières limestone studied by synchrotron X-ray μCT. • Simultaneous phase-and-amplitude retrieval to segment solution and hydrated phases. • Confined and unconfined crystallization in multiple pore systems of Savonnières. • Salt solution and salt crystals are located in mechanically weak zones. • Pore scale observations corroborate salt weathering damage patterns in Savonnières. - Abstract: The crystallization processes of two anhydrous salts (NaCl and Na2SO4) and one hydrated (sodium sulfate) salt in the pore space of a natural building stone, Savonnières limestone, are studied. We imaged the salt solution distribution before and after crystallization and the solid crystal distribution in between repeated crystallization cycles using synchrotron X-ray microtomography. This technique proves to be very useful to study salt crystallization processes at the pore scale. The use of simultaneous phase-and-amplitude retrieval during X-ray tomographic reconstruction allows a clear segmentation of sodium sulfate solution and hydrated sodium sulfate crystals without the need for a dopant. Salt crystals can precipitate under unconfined as well as confined conditions in the multiple pore systems of Savonnières limestone, depending on their interconnection. Salt solution and salt crystals are located in mechanically weak zones of the limestone, which can be linked to damage patterns observed in this stone after repeated salt weathering cycles. The distribution and the process of pore filling by salt crystals that are revealed here advance the understanding of salt damage in porous media and may open ways to perform remediation