Damaged, porous materials: A salty tale

Damaged, porous materials: A salty tale. Many otherwise beautiful edifices and works of art constructed from porous materials wear the scars of salt damage. The Sagrario Chapel in Granada, Spain, part of which is shown in the figure, is one example. Salt water settles inside pores and can become supersaturated as it cools or as the water evaporates. When salts from the supersaturated solution precipitate, pressures associated with frustrated crystal growth create large, damaging forces on the pore walls. The evaporation route to supersaturation has heretofore not been much studied, but a team from the University of Granada, led by Carlos Rodriguez-Navarro, has now investigated in detail the evaporation of a confined sodium sulfate solution. Calorimetry and x-ray diffraction revealed that the sodium salt—responsible for much real-world damage—initially exists in solution as a metastable heptahydrate (Na₂SO₄ · 7H₂O) but undergoes a phase transition to less-soluble, stable mirabilite (Na₂SO₄ · 10H₂O) as evaporation proceeds; the result is a highly supersaturated solution. Analytic techniques allowed the Granada team to determine the mirabilite concentration and calculate the damaging strength of its crystallization pressure. In a follow-up experiment, Rodriguez-Navarro and colleagues added a compound to the initial salt solution to promote the early crystallization of mirabilite and mitigate supersaturation. Thanks to the additive, the precipitation pressure was not enough to damage most building materials. (M. Schiro, E. Ruiz-Agudo, C. Rodriguez-Navarro, Phys. Rev. Lett. 109, 265503, 2012.)