SALTS IN THE DETERIORATION OF POROUS MATERIALS: AN OVERVIEW
A. ELENA CHAROLA
4 3. MOVEMENT OF SALTS IN POROUS BODIES
Salts can enter and move through porous bodies only when dissolved in water. Therefore, understanding the movement of this fluid in the material in question is of prime importance.
4.1 3.1 WATER MOVEMENT
Water can enter a porous material either as liquid or vapor. In the liquid state, two mechanisms can be operative: capillarity and/or infiltration. While the first is a result of the attraction of the water and the capillary material as well as the surface tension of the liquid, the latter requires a hydrostatic pressure and depends on the permeability of the material. Both mechanisms have been thoroughly studied and clearly described in standard texts (Feilden 1982; Amoroso and Fassina 1983; Weber 1984; Massari and Massari 1993; Henriques 1994).
In the vapor state, water can enter a porous material through two main mechanisms: condensation and hygroscopicity. Two types of condensation should be distinguished: surface condensation and microcondensation (or capillary condensation) in pores. Both are clearly understood and differentiated (Camuffo 1998). Hygroscopicity, on the other hand, is a broad term used to cover different processes for “absorbing or attracting moisture from the air” as defined by the American College Dictionary (1962). First of all, the material itself can sorb—i.e., ad- and/or ab-sorb—a certain degree of moisture depending on its nature, porosity, and internal surface. Second, salts can also absorb moisture, especially when the relative humidity increases above their equilibrium RH. Then, particularly the very soluble ones may deliquesce—i.e., absorb so much water vapor that they form a saturated solution. The term “deliquescent” is preferable to “hygroscopic” to describe this phenomenon because it identifies the condition of “becoming liquid.” Finally, concentrated salt solutions have a lower vapor pressure than pure water and have a higher tendency to condense water vapor from the environment to reach equilibrium—i.e., to equalize the activity of water within the solution to that in the vapor phase. This last process has also been called “classical osmosis,” particularly in German texts (Weber 1984, 29).
It is important to understand how water will move once inside a porous material. If it moves as a liquid, it will be able to transport salts; if as a vapor, it may be retained through hygroscopicity. In the first case, the mechanism relies on capillarity, and, in the second, on diffusion. The transition point between these two mechanisms defines the critical moisture content, Ψc, of a porous material. This parameter is a constant for each material and depends mainly on porosity and pore-size distribution (Snethlage and Wendler 1997).
The actual distribution of moisture within stone depends on porosity, pore-size distribution, and environmental conditions. As R. Snethlage and E. Wendler (1997) discuss, the maximum of moisture content resulting from wet-dry cycling is closer to the surface in denser stones, and deeper and broader in coarse, porous materials.
Lastly, it should be remembered that when porous materials absorb moisture or liquid water, they expand. This effect is known as hygric dilatation, if moisture is absorbed, or hydric dilatation, when it is produced by liquid water. When the material dries, it will shrink. Upon subjection to wet-dry, or moist-dry cycling, the resulting expansion and contraction can induce material fatigue (Snethlage and Wendler 1997). This effect can be compounded or inverted when salts are present, as discussed in section 7.
4.2 3.2 SALT MOVEMENT
Salts within a porous material will increase the amount of water in it, partly by enhancing capillary rise, as suggested by G. Massari and I. Massari (1993), but also because of hygroscopicity (Weber 1984). This is a particularly important point when decisions have to be made for moisture mitigation in a building. If moisture in a wall is caused mainly by high salt content, an expensive damp-proof barrier will be useless in solving the humidity problem.
Two main mechanisms are responsible for the introduction of soluble salts into the porous material of a building: capillary rise of groundwater and infiltration by rainwater. The former is responsible for introducing soil or deicing salts, while the latter will contribute salts resulting from air pollution or marine aerosols (Behlen et al. 1997). Both these processes rely on capillary transport of water into the pore system as described by B. Vos (1971) and, in more detail, by D. Camuffo (1998). A third mechanism that can introduce, or reintroduce, surface salts into the materials is surface condensation. Camuffo (1998) addresses this issue thoroughly.
Once a salt is in a porous material, its movement will be strongly dependent on ambient conditions—i.e., temperature and relative humidity—as well as the presence of other salts. Changes in relative humidity result in its partial crystallization and dissolution. These phenomena take place preferentially in medium to large pores and result in movement of solution from smaller pores to the larger pores (Camuffo 1998). From actual field observations, it has been established that salt crystals form mainly in pores 1–10 μm in diameter (Zehnder and Arnold 1989). The presence of a second salt results in the lowering of the relative humidity required for precipitation (Price and Brimblecombe 1994; Steiger and Dannecker 1995), thereby increasing the frequency of the crystallization-dissolution cycling.
Creep—i.e., salts crystallizing on glass walls above the surface of a solution—has also been postulated as a model for the mechanism of salt movement within a porous structure (Pühringer 1983a). This phenomenon can be readily observed for solution films drying out on glass surfaces. Salts crystallize at the edge of the film, forming a fine-grained microporous salt body. This enhances the capillary flow toward the edge of the film, advancing the crystallizing front. As J. Pühringer (1983b) graphically states, the salt carries the water.