Assessing salt mixture-induced degradation in masonry facades using hygrothermal simulations

Salt-induced deterioration is of significant concern when assessing the degradation risk of masonry facades. Despite extensive knowledge about salt behaviour mechanics, risk assessment of the occurrence of potential damaging cycling remains challenging. Previous studies mainly evaluate single salts, though masonry walls typically contain a mixture of different salt types. Salt damage arises as a consequence of repeated crystallisation-dissolution cycles, which exert pressure on the pore surface of porous building materials. Relative humidity is a major driving factor for the frequency and intensity of these cycles. An extensive study analysed over 11,000 drill samples collected across Belgium identified four typical salt mixtures in heritage structures, categorised as five- and six-ion systems: two with an excess of sulphate (T1v and T1vi) and two with an excess of calcium (T2v and T2vi), with respect to gypsum crystallisation. This paper combines the research on the thermodynamic behaviour and kinetic variables of these salt mixtures with hygrothermal simulations. For the first time, the dynamic risk assessment is conducted using range-based indicators and salt mixtures, rather than single salts with a single threshold criterion offering more realistic insights into the interaction between climate change, interior insulation, salt damage and the hygrothermal properties of masonry constructions.