Effects of ferrocyanide ions on NaCl crystallization in porous stone

Abstract

The effects of [Fe(CN) 6] 4− ions on the crystallization of NaCl in aqueous solution has been studied, particularly in the situation where the saline fluid percolates through and evaporates from a saturated porous body (i.e., an ornamental porous limestone). In concentrations ranging from 2.48×10 −4 up to 2.85 10 −3 M the additive was able to increase the solution critical supersaturation (up to 8%) resulting in a significant crystallization inhibition effect, which promoted efflorescence growth on the porous stone surface as opposed to subflorescence growth. The former induced no damage to the stone support while the latter always resulted in crystallization pressure development inside the pores, causing granular disintegration—a process also known as “salt weathering”. Significant changes in NaCl crystal growth morphology (from {1 0 0} towards {1 1 0}, {1 1 1} and {2 1 0} forms) were observed in the presence of ferrocyanide ions, which also promoted the growth of dendrites with their main growth axis parallel to 〈1 1 1〉, and branched along 〈1 0 0〉, at the highest supersaturation. The possible inhibiting mechanism of ferrocyanide on NaCl crystallization and growth is discussed based on the evolution of NaCl growth morphologies, and some practical applications of ferrocyanides to prevent salt damage and enhance desalination in ornamental stones are outlined.