Fundamental Research on Salt Damage
 

Salt damage results largely from the growth of salt crystals within a porous structure. A broad variety of damage features—from granular disintegration to flaking and scaling—have been documented (Doehne 2002).

As part of the project's research on salt damage, the team used time-lapse imaging to create movies that showed that crystallization can take place within the stone's pores and can cause severe damage to historic masonry especially under conditions of rapid drying. Under more humid conditions salt crystallization is limited to surface efflorescence and is much less damaging. Pore size distribution and weather conditions (calm versus windy) significantly affected the severity of sodium sulfate damage (Doehne et al 2002; Rodriguez-Navarro and Doehne 2000, and Rodriguez-Navarro and Doehne 1999).

Project experiments indicated that some surfactants substantially increase or decrease salt damage to masonry (Rodriguez-Navarro et al 2000). Experimental evaluation has shown that potential crystallization inhibitors strongly affect the degree of salt damage in limestone (Selwitz and Doehne 2002). Tests also confirmed that crystallization inhibitors can change the location of sodium chloride deposition from inside the stone to the stone's surface (Rodriguez-Navarro et al 2002).

To gauge how changes in relative humidity affect sodium chloride-laden sandstone, the mechanical dilatation of the sandstone was measured. (Lombardo et al 2004). Since movement is correlated with damage, high amplitudes in dilatation indicate higher rates of flaking and damage in the sandstone. While most unweathered stones expand with increasing humidity and shrink during decreasing humidity, this behavior is reversed by the presence of sodium chloride in the stone's pore space. In addition, the amplitudes are increased dramatically, with shrinkage during wetting occurring faster than expansion during drying. This provides laboratory evidence that damage caused by sodium chloride is related to rapid drying. This research suggests that the mitigation of rapid drying in certain situations may extend the lifespan of works of art, such as wall paintings.

Quantification of relative humidity and temperature conditions in the environmental scanning electron microscope (ESEM) was conducted. Stability and environmental conditions in the ESEM chamber were characterized in order to allow reliable quantitative measurement of water vapor pressure, temperature, and relative humidity.

Salt experiments at atmospheric pressure and at lower pressure in ESEM show similar results. For example, sodium chloride deliquescence occurs at the expected relative humidity (RH) (approximately 75 percent) over a wide temperature, pressure, and grain-size range; the expected depression of the equilibrium RH for a sodium chloride and sodium nitrate mixture is also observed.

Work Completed

  • Time-lapse experiments with two stone types, two salts (sodium chloride and sodium sulfate), and several environmental conditions.
  • Search for potential crystallization inhibitors for sodium chloride and sodium sulfate.
  • Quantification of relative humidity and temperature conditions in the ESEM.
  • Dynamic electron and optical microscope experiments on the kinetic behavior of sodium chloride.
  • Depression of deliquescence points with salt mixtures (sodium chloride and sodium nitrate) determined.
  • Contrasting expansion/shrinking behavior of sandstone samples from Petra in Jordan—with and without sodium chloride characterized.
  • Formation of powdery sodium sulfate efflorescence by humidity cycling.
  • Nuclear Magnetic Resonance (NMR) experiments demonstrating transport of sodium chloride due to humidity fluctuations.

Work in Progress

  • Dynamic electron and optical microscope experiments on the kinetic behavior of magnesium sulfate.
  • Nuclear Magnetic Resonance experiments on transport of salts due to humidity changes.
  • Further work on mechanisms of sodium nitrate damage.
  • Analysis of damage caused by salt mixtures found at example field sites.

References:

Lombardo, Tiziana, Eric Doehne and Stefan Simon. 2004. The response of NaCl and Umm Ishrin sandstone to humidity cycling: Mechanisms of salt weathering. In STONE 2004, Proceedings of the 10th International Congress on Deterioration and Conservation of Stone, ed. D. Kwiatkowski and R. Löfvendahl, 20310. Stockholm: ICOMOS Sweden, Elanders Gotab.

Doehne, E. 2002. Salt weathering: A selective review. In Natural Stone, Weathering Phenomena, Conservation Strategies and Case Studies, ed. S. Siegesmund, A. Vollbrecht and T. Weiss, 5164. London: The Geological Society of London.

Doehne, E., C. Selwitz, and D. Carson. 2002. The damage mechanism of sodium sulfate in porous stone. Paper presented at the SALTeXPERT meeting, Prague.

Rodriguez-Navarro, C., L. Linares-Fernandez, E. Doehne, and E. Sebastian. 2002. Effects of ferrocyanide ions on NaCl crystallization in porous stone. Journal of Crystal Growth 243:50316.

Selwitz, C., and E. Doehne. 2002. The evaluation of crystallization modifiers for controlling salt damage to limestone. Journal of Cultural Heritage 3 (3): 20516.

Rodríguez Navarro, C., and E. Doehne. 2000. How does sodium sulfate crystallize? Implications for the decay and testing of building materials. Cement and Concrete Research 30:152734.

Rodríguez Navarro, C., E. Doehne, and E. Sebastian. 2000. Influencing crystallization damage in porous materials through the use of surfactants: Experimental results using sodium dodecyl sulfate and cetyldimethylbenzylammonium chloride. Langmuir 16:94754.

Rodríguez Navarro, C., and E. Doehne. 1999. Salt weathering: Influence of evaporation rate, supersaturation and crystallization pattern. Earth Surface Processes and Landforms 24:191209.

Last updated: January 2011