Experimental evidence of crystallization inhibition by salt mixtures in mural substrates: mechanisms vs. single salts

This study investigates the inhibitory mechanisms of a mixed-salt system (Na 2 HPO 4 ·12H 2 O and Na 2 SO 4 ·10H 2 O) on crystallization damage within simulated mural plaster under thermal cycling. Laboratory analyses, including induction period measurements, SEM, and MIP, reveal that the salt mixture significantly prolongs crystallization onset. Two synergistic mechanisms explain this inhibition. First, while the common-ion effect (shared Na⁺) reduces solubility of each salt, ion activity reduction and ion pairing lower the effective concentration of crystallizing species, thereby weakening the thermodynamic driving force. Second, spatial competition during concurrent crystallization refines crystal habit and size, dispersing crystallization pressure across numerous micro-crystals and preventing localized stress concentration. Consequently, the mixed-salt specimen retained a finer pore structure (average diameter 203.06 nm vs. 421.88 nm) and much lower permeability (908.90 mDarcy vs. 2480.23 mDarcy) compared to its single-salt counterpart. These findings highlight how salt mixtures can mitigate crystallization damage through coupled thermodynamic and mechanical mechanisms.