How does salt affect the durability of dimension stone, and when is serious damage likely to occur? Explore the Australian/New Zealand Standard for salt durability testing and get expert insight to help you prevent salt attack damage in your next project.
Article: Jim Mann
Damage to dimension stone through salt crystal growth is serious – historically, many iconic buildings and monuments such as the Sistine Chapel and the Egyptian Pyramids have decayed as a result of salt damage. It is important to understand the mechanisms by which salt harms natural stone, so that we can plan ahead and prevent similar damage when constructing new buildings. Salt damage is largely thought to occur via two mechanisms: crystal growth and changing hydration states.
It is first important to understand the difference between the two different salts used in the Australian/New Zealand Standard for salt durability testing. The standard test method AS/NZS 4456.10 uses one of two different salts: Sodium Sulfate (Na2SO4) and Sodium Chloride (NaCl). Sodium Sulfate has two stable forms: Thenardite (Na2SO4), which is anhydrous and only precipitates from solution at temperatures above 32.4 degrees Celsius and Mirabilite (Na2SO4.10H2O), which forms below 32.4 degrees. Sodium Chloride has only one stable form (NaCl).
Build-up of crystallization pressure
Damage by crystal growth is caused by the build-up of crystallization pressure against pore walls when salt nucleation and growth take place in a confined space. Experiments have been carried out studying crystal growth in confined spaces, which show that if a thin, supersaturated solution film (nanometers thick) is maintained at the pore-salt interface, a crystal can begin to grow against the confining pressure of the pore. The force of crystallization is the result of deposition of matter on the growing crystal surface at the crystal–pore wall interface. Both NaCl and Na2SO4 can damage stone in this way, as the mechanism only relies on the crystals growing against the pore walls.
Changing hydration states
Another mechanism for salt damage is changing hydration states. This mechanism involves the transition between two different crystal structures, which leads to a volume change, resulting in increasing pressure against the pore walls of the stone, leading to damage of the stone. Sodium chloride cannot damage stone through this mechanism as it has only one hydration state.
In the case of sodium sulfate, the transition from thenardite to mirabilite involves the incorporation of 10 water molecules, leading to a volume increase of 320%. The transition happens at a low temperature (32–35 degrees) and is highly dependent on humidity. In the first cycle, the stone is immersed in sodium sulfate solution and forms thenardite (smaller cubes) when dried. In the second cycle the thenardite turns into mirabilite (larger prisms) through a dissolution–precipitation process, which happens after addition of solution to the stone. In the third cycle, enough thenardite is present to cause damage during the immersion cycle. It should be noted that damage will also occur during the heating due to crystal growth and the diagram is a guide only – not every different stone will begin to degrade after the second or third cycle.
Influence of dimension stone properties
In addition to the mechanisms of salt damage, properties of the dimension stone can influence the potential of salt attack damage to dimension stone. For example, the porosity of the stone plays a large part in controlling the extent of salt damage. Studies have shown that salt crystallization will initially take place in larger pores, forming large crystals which that are supplied additional salt through capillaries. When the large cavities are filled, the crystals will not grow into the smaller capillaries, as this would require a significant increase of surface area relative to the small volumetric increase, resulting in a large increase in the chemical potential of the crystal. Hence, the large crystals will continue growing, generating pressure against the pore walls, resulting in damage to the host material. The pressure caused by this crystal growth is calculated according to Equation 1.
∆P=2σ(1/R-1/r) Equation 1
Where R = radius of the large pore connected by a capillary of radius r (cm), σ = interfacial tension between solution and growing crystal and ∆P is the pressure due to crystal growth. Equation from Wellman, H. W. and Wilson, A. T., 1965, “Salt weathering, a neglected geological erosive agent in coastal and arid environments,” Nature, 205, 1097-1098.
If the radius of pores is large, this decreases the pressure (since ΔP is proportional to 1/R), which can be explained by the size of pores being too large for the salt crystals to grow to the size where they may grow against the pore. Also, if the radius of capillaries is small, this leads to a decrease in pressure as insufficient salt solution can reach the growing salt crystals. Hence, stones with large, non-interconnected pores should be more durable than stones with small, highly interconnected pores.
Supersaturation also has a large impact on the damage to dimension stone. Supersaturation occurs when the amount of salt in solution is greater than the volume of liquid that can naturally support it. This phenomenon occurs during evaporation. In the case of sodium sulfate, as it is dried the mirabilite is dehydrated to form thenardite. However, in the conditions set out in AS/NZS 4456.10 Method A, a limited drying time allows only a certain percentage to dehydrate, leaving some of the mirabilite embedded in the stone. When the stone is re-immersed in solution, the bulk of the thenardite will dissolve first (as it is more water soluble), leaving mirabilite and a small amount of thenardite in their crystalline form. This remaining mirabilite and thenardite then act as a nucleation sites to form mirabilite from the now supersaturated solution.
This process is highly dependent on temperature, as at lower temperatures there is a higher solubility difference between mirabilite and thenardite. This leads to a higher supersaturation due to increased partial dissolution during the immersion of the stone in salt solution, which causes greater overall damage to the stone.
In conclusion, no stone is impervious to salt damage – whether the salt present is sodium chloride or sodium sulfate. It should be noted that sodium sulfate is generally more damaging to stone, however, in a real-life circumstance, the salt present in the stone will never be one type, but rather, a combination of various salts. Also, the environment in which the stone is found will dictate the salt types present. The multiple hydration states of sodium sulfate are key to its higher damage potential. The porosity of the stone should be carefully considered. Many small factors such as humidity, temperature, imperfections in the stone and exposure to wetting/drying cycles can also have a significant effect on the long-term durability of the stone.
See more about salt attack here.