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

建筑材料中的盐损伤是由于孔隙空间中晶体生长受限造成的。生长中的晶体如果与过饱和溶液接触，会对孔壁产生足够的应力，即结晶压力。过饱和程度直接控制结晶压力，从而控制产生的损伤。因此，研究盐在孔隙溶液中的过饱和提供了特定盐的破坏潜力的信息。对于几种单一的盐，这样的研究证实达到了非常高的过饱和度。然而，实际物体被单一盐污染的情况几乎从未被观察到过。通常，建筑材料中的盐体系由复杂的盐混合物组成。这是本研究计划的主要焦点。该项目的主要目的是研究混合盐溶液的结晶行为和临界过饱和度，并与单一盐进行比较。假定较高的初始过饱和度也有利于较高的结晶压力。主要研究的盐混合物是在受潮气影响的历史建筑中常见的各种氯化物和硝酸盐的吸湿性混合物。使用热力学平衡模型模拟了这种盐混合物的结晶行为，并将使用温度和湿度控制的拉曼显微镜验证模型预测的结晶途径。各种晶体固体在混合溶液中的临界过饱和是在现实条件下测定的，即在多孔基质（如天然石材）内。如果过饱和是由冷却引起的，可以用量热法检测成核。利用玻璃毛细管作为多孔基质和光学显微镜研究了蒸发引起的过饱和现象。此外，将使用量热法和重量法，在控制温度和相对湿度的条件下，研究其他多孔基质的蒸发。利用实验结果计算了盐和盐混合物的热力学过饱和，分别评估了盐和盐混合物的破坏潜力。最后，我们将使用仅受盐混合物污染的岩石试样进行循环损伤实验，以验证损伤潜力。

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.