SALTS IN THE DETERIORATION OF POROUS MATERIALS: AN OVERVIEW
A. ELENA CHAROLA
6 5. SALT DISTRIBUTION IN MASONRY
On a wall with rising damp, salts crystallize at different heights depending on their solubility. A. Arnold (1982) has synthesized this distribution from his careful analysis and observation of many damp structures. He differentiates four zones: the first (A), close to the ground, shows less damage than the second (B), which in general is the most deteriorated one. Above this zone, the third area (C) is characterized by being darker than the others; it marks the upper limit of moisture rise. The fourth area (D) is sound wall. In area A, the more insoluble salts such as gypsum and the calcium and magnesium carbonates crystallize. In zone B, potassium nitrate and magnesium and sodium sulfates precipitate out, forming efflorescences and crusts. In zone C, the most soluble salts, sodium nitrate and chloride, as well as the deliquescent salts, magnesium nitrate and chloride, are found. The latter will tend to keep the wall moist except under very dry conditions. As Arnold points out, this distribution will be affected if alkaline salts are introduced into the system, as is the case when Portland cement or other alkaline materials are injected to prevent rising damp.
The distribution of salts within a wall is also dependent on the actual mixture of salts present and their origin. As Steiger (1996) discusses, on the basis of the thorough analysis of the north facade of a former convent in northern Bavaria and other monuments (Steiger et al. 1993), the presence of nitrate, potassium, and magnesium, as well as chloride and sodium, is the result of rising damp. The zone of maximum enrichment, around 2–3 m from the ground, reflects the capillary rise height. Fractionation during this ascending transport is revealed by the shift in the potassium/magnesium (K/Mg) ratio, which decreases with increasing height. Sulfate distribution, mainly on the surface of the wall, reflects deposition from the atmosphere. Given normal outdoor environmental conditions, only a small amount of potassium nitrate (niter) can be expected to crystallize, while most other salts, except the rather insoluble gypsum, remain in solution. This analysis confirms the findings of K. Zehnder (1993, 1996), who describes the formation of thin veils of gypsum in the upper zone of the capillary rise. These veils cause a slow decay that is particularly evident on painted stones and wall paintings, where the thin surface layer is especially susceptible to damage.
Steiger et al. (1997) and Behlen et al. (1997) present a careful analysis of the various pathways for sodium chloride accumulation in marine environments. Its distribution within a wall or walls of matched age and material results from the accumulation pathway followed. For example, the significantly higher salt accumulation in one building could be attributed to overall poor design that led to rising damp and increased direct rainwater flow from the roof.
Further studies by Arnold and coworkers (Arnold and Kueng 1985; Arnold and Zehnder 1985) report on efflorescing salts, their distribution over wall surfaces, and their various growth habits. These authors give a good description of the various types of crystals, including longer (fluffy) or shorter (bristly) acicular efflorescences, salt crusts, and salt pustules—i.e., salt crusts limited to small areas. Acicular (whisker) growth occurs at low supersaturation and low evaporation rate. Salt crusts will form in the damper areas of the walls—i.e., in the lower part—and are generally formed by the lesser soluble salts: calcite, gypsum, hydromagnesite, and nesquehonite. But any salt can produce crusts under the right conditions. Arnold and his coworkers also point out that efflorescences age with time and changing conditions due to dissolution and recrystallization processes that may affect the damage induced.
Arnold and Zehnder (1988) also present the results of a five-year monitoring program correlating microclimate and efflorescing salts in several buildings in Switzerland. They relate in situ–measured relative humidity and temperature to the “fractionation” pattern of efflorescing salts as saline solutions rise in walls. Sodium nitrate (nitronatrite or soda niter), for example, was found to effloresce only when the relative humidity dropped below 60% (equilibrium RH for this salt is 75.4% at 20°C). These authors also monitored the aging of the efflorescence as conditions changed. The acicular growth of the nitronatrite, for example, turned into a more isometric shape with time.
If a conservation treatment, such as consolidation or hydrophobization, is applied to salt-containing masonry, the distribution of the salts can be significantly affected. L. Franke and F. Pinsler (1998) proposed an in situ radiography method for measuring the movement of salts resulting from an imperfect surface hydrophobization or when unhydrophobized joints are present. Using salts such as lead nitrate and the hygroscopic sodium iodide for the study they found that salts accumulated at nonhydrophobized surfaces as well as behind the hydrophobized front.