Growth of salt crystals in small pores generates stress only because there are repulsive (disjoining) forces between salt and the mineral crystals in the pore walls.The repulsion results from the difference in the crystal structures of the salt and the minerals in the pore wall, which means that each solid would rather be in contact with the solution than with teh other solid. If the crystal were to come directly into contact with the pore wall, it could not continue to grow, because it would not be possible for additional ions to be transported to the surface. However, the film of solution surrounding the growing crystals allows them to continue pushing on the pore wall, generating stress that can produce damage of the sort shown above. The theory of crystallization pressure is discussed in detail in several papers from our group.
We are studying methods for reducing or eliminating the disjoining forces to protect the stone. Recently, we have had very promising results with thin coatings (a few nm) of polyacrylic acid with very low molecular weight. The photo above compares the performance of blocks of Cordova Cream limestone exposed to capillary rise of a solution of sodium sulfate. The thickness of the treatment increase from left (zero) to right (a few nm).