Kirsten Linnow

In situ Raman observation of the crystallization in NaNO3–Na2SO4–H2O solution droplets

Several double salts have been detected in building materials and most of these salts are incongruently soluble compounds. In contrast to single salts, however, no systematic investigations of the crystallization behavior and deleterious effects of incongruently soluble double salts exist. To assess the damage potential of these salts, a systematic investigation of their highly complex behavior is desirable. This paper deals with the crystallization behavior of various solids in the ternary mixed NaNO3–Na2SO4 system including the formation of the double salt darapskite, Na3NO3SO4·H2O. The crystallization sequence during droplet evaporation experiments at room conditions was determined using Raman and polarization microscopy. The basic idea of this research is to use deviations of the crystallization sequence of a salt or a mixed salt solution from the equilibrium pathway as an indicator to detect the degree of supersaturation. The observed crystallization pathway includes the formation of the metastable phases Na2SO4(III), Na2SO4(V) and darapskite. The experimental observations are discussed on the basis of the NaNO3–Na2SO4–H2O phase diagram and the results provide evidence for crystal growth from highly supersaturated solutions in both systems. If the crystals growing under these conditions are confined, these supersaturations result in substantial crystallization pressures.

The mode of formation of thecotrichite, a widespread calcium acetate chloride nitrate efflorescence

Abstract The widespread occurrence of thecotrichite, Ca3(CH3COO)3Cl(NO3)2·7H2O, in the museum environment is explained theoretically by construction and examination of its phase diagram. Thecotrichite formation was simulated in the laboratory to identify the key factors involved in its production. This efflorescence occurs on porous limestone or calcareous artefacts such as pottery, stored in wooden cabinets that generate acetic acid vapour. Salt production depends on the moisture content of the object and the concentration of acetic acid in its surroundings. Furthermore, for thecotrichite to form the artefact must contain soluble chloride and nitrate salts.

Monitoring of Deformations Induced by Crystal Growth of Salts in Porous Systems Using Microscopic Speckle Pattern Interferometry

Electronic speckle pattern interferometry (ESPI) has been used to monitor micro-deformations and surface micro-structure changes produced by crystallization or hydration pressure of magnesium sulfate in a porous material. Samples of fritted glass were chosen as a standard porous substrate because of its mean grain size, its porosity distribution, and its negligible humidity expansion. The glass samples, soaked with salt solution, were exposed to changes in the relative humidity of the surrounding air. The fullfield ESPI measurements were combined with cryogenic SEM visualizations. Results from these investigations were partly not expected theoretically and give new insight in the underlying salt transition processes.