Crystallization behavior and supersaturation in mixed electrolyte solutions in view of salt damage in porous materials

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Salt damage in building materials is the result of confined crystal growth in the pore space. Growing crystal can generate sufficient stress to the pore walls, the crystallization pressure, if they are in contact with a supersaturated solution. The degree of supersaturation directly controls the crystallization pressure and, consequently, the resulting damage. Thus, studying the supersaturation of salts in pore solutions provides information on the damage potential of a particular salt. For several single salts such investigations are available confirming that very high supersaturations are achieved. However, contamination of real objects with single salts are hardly ever observed. Typically, the salt systems in building materials comprise of complex salt mixtures. This is the main focus of the present research proposal. The major objective of the project is an investigation of the crystallization behavior and critical supersaturation of mixed salt solutions and the resulting damage potentials in comparison to single salts. It is assumed that a high initial supersaturation will also favor high crystallization pressure. The salt mixtures to be mainly studied are the very hygroscopic mixtures of various chlorides and nitrates typically found in historic buildings affected by rising damp. The crystallization behavior of such salt mixtures is simulated using a thermodynamic equilibrium model and the model predicted crystallization pathway will be validated using temperature and humidity controlled Raman-microscopy. Critical supersaturation of the various crystalline solids in the mixed solutions are determined under realistic conditions, i.e. within porous substrates such as natural stone. If supersaturation is induced by cooling, nucleation can be detected calorimetrically. Supersaturation by evaporation is investigated using glass capillaries as porous substrates and optical microscopy. In addition, evaporation from other porous substrates will be studied using calorimetry and gravimetry, both under controlled conditions of temperature and relative humidity. Experimental results are used to calculate the thermodynamic supersaturation in order to assess the damage potential of salts and salt mixtures, respectively. Finally, cyclic damage experiments will be carried out using stone specimen contaminated with just the salt mixtures that have been characterized before in order to validate the damage potentials.