Salt Weathering in Anisotropic Calcarenite: Bedding-plane Controls on Sodium Chloride Precipitation Patterns

This study investigates the impact of bedding plane orientation on sodium chloride (NaCl) precipitation in a calcarenite stone, subjected to salt weathering cycles. It involves conducting wetting-drying cycles using sodium chloride on two series of specimens sampled parallel and perpendicular to the bedding plane. Capillary imbibition was carried out using saline solutions of two concentrations (15 g/L and 45 g/L). SEM observations show that, across all contaminated samples, halite precipitates mainly on the surface, in the form of efflorescence, while subflorescence remains negligible. The analysis identifies two distinct halite morphologies: (i) cubic crystals of 2 to 10 µm at grain boundaries and (ii) xenomorphic aggregates on pore walls, reflecting that the size and morphology of halite crystals vary according to local nucleation conditions, influenced by the mineralogical composition of the substrates and the degree of supersaturation reached during the cycles. X-ray diffraction analysis revealed significantly higher halite precipitation in samples oriented perpendicular to the sediment bedding (4.53–5.22%) than in those oriented parallel (2.71–4.17%), indicating that bedding plane orientation is a determining factor in weathering processes and the evolution of petrophysical properties. These results demonstrate that capillary transport is intrinsically anisotropic in calcarenite, with bedding orientation controlling both the amount of precipitated salt and the location of crystallizations. This study thus establishes a solid mechanistic framework for predicting salt weathering patterns in stratified heritage stones, and offers concrete perspectives for optimizing conservation strategies in coastal environments.