Three samples of Cordova Cream limestone after exposure to a solution containing 12 wt% sodium sulfate in an atmosphere at ~40% RH. The stones were pretreated with solutions containing 0, 0.75, and 1.75 wt% PAA. Both treated samples are undamaged, while the untreated stone is destroyed. (Meg McNall is seen in the background.)

Salt damage is one of the leading causes of deterioration of monuments and historical buildings. While this problem is widespread, proper prevention methods are not understood, leading to remedies that sometimes cause additional harm.

Princeton University is tackling the problem by using strategies to better understand the salt/water/pore interface. University researchers are using a PTT Grant to decrease or eliminate salt damage in cultural resources.

“The idea arose from our theoretical examination of the origin of salt damage,” George Scherer, W.L. Knapp Professor of Civil and Environmental Engineering, said. “Salt is the main cause of deterioration in the Mediterranean basin, as well as in many other parts of the world.”

The research will also clarify the causes of salt damage in porous materials, and provide a procedure to protect stone and brick structures from being damaged. This treatment involves using polymeric film to reduce the interfacial energy, or stress, that exists between salt and mineral surfaces. Scherer explained the stress is caused when a salt crystal grows inside a stone’s pores.

“We concluded that salt crystals exert stress on stone because of the difference in crystal structure between the salt and the minerals of the stone: they can’t bond together, so the surfaces repel each other,” Scherer said.

A liquid film forms between the pore wall and the crystal, creating a disjoining force between the two surfaces. The polymeric film would replace the liquid film, with a dry, low-energy interface, eliminating this stress. The salt crystals would touch the pore wall and stop growing, thus stopping deterioration.

The process has shown much promise in preliminary experiments, and plans to evaluate its durability and initiate field tests in other studies are underway. This will include testing different polymers and different molecular weights for their effectiveness with various salt mixtures, and observing their performance on silicate rocks and carbonates. Those that perform the best will be used in the field study.

Scherer hopes the research will decrease the effects of salt damage on historic sites.

“If successful, our method would make it possible to protect stone against damage from salt, which is one of the major causes of damage to monuments around the world,” he said. “Our approach attacks the cause of the problem, rather than the symptoms, so we are preventing, rather than repairing the damage,” he said.

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