5 4. SALT SOLUTIONS

As is known, the water vapor pressure over a salt solution is lower than the vapor pressure of pure water at a given temperature. As the concentration of the solution increases, the water vapor pressure decreases, reaching its lowest value when the solution is saturated. This water vapor pressure, which depends on the nature of the salt in the solution and temperature, corresponds to what is known as the “equilibrium relative humidity” since water vapor pressure can be expressed as %RH. For example, at 20�C, the values can range from 99% for calcium sulfate dihydrate (gypsum) to 11% for lithium chloride monohydrate, depending on the interaction of the particular ions with the water molecules.

Seldom can the deterioration of porous materials be attributed to the presence of only one salt. In general, two or more salts are found to be present. The simultaneous presence of two salts affects their behavior in solution and, in particular, their individual solubility. The rule of thumb is that if the salts do not have any ions in common, as in the case of a mixture of sodium chloride and calcium sulfate dihydrate (gypsum), solubility of both salts increases because of the higher ionic strength of the solution. However, the increase in solubility will be far greater for the less-soluble salt, in this case gypsum. If the salts have an ion in common, such as sodium chloride or sodium sulfate, solubility of both will decrease. Again, the solubility of the less-soluble salt, sodium sulfate, will be affected more than that of the more soluble one. However, not all salts behave in this way; for example, the solubility of potassium nitrate increases in the presence of other nitrates (Steiger and Dannecker 1995).

A solution of a mixture of salts will not have a single “equilibrium RH” at a given temperature but will show a range of such humidities (Price and Brimblecombe 1994). This range will not necessarily fall between the levels of the equilibrium RH of the individual salts and can even be broader, as is the case for the mixture of potassium chloride and sodium chloride: the equilibrium RH ranges from close to 85% to 72.4%, while equilibrium RH for the individual salts is 84.3% and 75.3%, respectively (Steiger and Dannecker 1995). Further details about the model used to calculate the behavior of multiple salt systems can be found in C. Price and P. Brimblecombe (1994), M. Steiger and A. Zeunert (1996), and Steiger et al. (1998). The effect of a wider RH range of a mixed solution within the pore system of a material will be compounded by the hygroscopicity and capillary condensation capability of the latter, increasing the deterioration potential of the salt(s).

Also to be taken into account is the order in which salts will precipitate out of a multiple salt solution. This order depends on the composition of the original mixture present and has been studied by geologists for the case of evaporating lakes. Hence, mixtures of these soluble salts are referred to as “evaporites.” The order of precipitation of such mixtures can be found in most geochemistry textbooks (Krauskopf 1979) or in A. Goudie and H. Viles (1997).