Pore-scale investigation of salt weathering in building heritage materials: combining AFM nano-indentation measurements and multiscale modeling.

Salt weathering is a main cause of damage in building heritage materials. Despite the large amount of research on this topic, the mechanism of damaging processes remains not fully understood in particular at the pore scale where the salt crystallization and dissolution occur. For this reason, we propose an innovative approach combining damage proxy measurements at pore-scale using Atomic Force Microscopy (AFM), Raman spectrometry and multi-scale numerical modelling, performed during weathering cycles. Imbibition-evaporation cycles are performed on carbonate stones (Savonnières and Saint Maximin limestones) with a 0.1 mol/L sodium sulfate solution at controlled room temperature and relative humidity. The stone samples are especially designed for the measurements at the pore-scale. Cylinder of 1.6 cm diameter and 1.5 cm thickness have been coated with very viscous epoxy resin. Then the two sides of the cylinder have been polished to obtained two free surfaces that allow the fluid circulation in the sample and the measurements. After each weathering cycle, nanoindentation experiments are performed on representative areas of several hundreds of square micrometers in order to monitor the mechanical properties evolution. A force of the µN order is applied in order to stay in the elastic deformation regime. Young modulus values can be then deduced from the slope of the force curves that occurs during the cantilever deflection. With this method, the effect of salt weathering on the mechanical properties of stone minerals is investigated at the pore scale and with no impact of the measurement on the phases structure (reversible indentations). The AFM results are then coupled with chemical Raman mapping to identify the present phases and assign them their mechanical properties. The obtained experimental data are then used in numerical modeling, to generate a numerical Young modulus field with the same properties than the experimental field. Finally, a new medium with higher dimensions will be generated to compare the results with the macro-scale observations on building heritage stones. AFM characterization shows that changes occurred on the topography of the samples between the first and the third alteration cycles. They are of the order of several tens of nanometers and correspond either to salt crystals deposits or in some cases to the loss of material that took place between the second and the third cycle. A decrease of the Young modulus is observed after each cycle that is of the order of 2 GPa between the first and the second cycles. More significant changes are observed after the third cycle especially in some areas where a decrease up to 3 to 4 GPa is estimated.