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

建筑材料中的盐害是限制晶体在孔隙空间生长的结果。如果晶体与过饱和溶液接触，生长的晶体会对孔壁产生足够的应力，即结晶压力。过饱和度直接控制结晶压力，从而导致损害。因此，研究孔隙溶液中盐类的过饱和度可以提供有关特定盐类潜在损害的信息。对于几种单一的盐，可以进行这样的研究，证实达到了非常高的过饱和度。然而，很少观察到单一盐类对真实物体的污染。通常，建筑材料中的盐体系由复杂的盐混合物组成。这是本研究方案的主要着眼点。该项目的主要目标是调查混合盐溶液的结晶行为和临界过饱和度，以及与单一盐相比所产生的损害潜力。高的初始过饱和度也有利于高的结晶压力。将主要研究的盐混合物是各种氯化物和硝酸盐的极具吸湿性的混合物，通常存在于受湿度上升影响的历史建筑中。用热力学平衡模型模拟了这类盐混合物的结晶行为，并用温湿度控制的拉曼显微镜对预测的结晶路径进行了验证。各种结晶固体在混合溶液中的临界过饱和度是在现实条件下确定的，即在多孔基质中，如天然石材。如果过饱和是由冷却引起的，则可以用量热法检测到形核。利用玻璃毛细管作为多孔衬底和光学显微镜，研究了蒸发过饱和现象。此外，在控制温度和相对湿度的条件下，还将使用量热法和重量法来研究其他多孔基质的蒸发。实验结果被用来计算热力学过饱和度，以评估盐和盐混合物的破坏潜力。最后，为了验证损伤势，将使用仅被先前表征的盐混合物污染的石材试件进行循环损伤实验。

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.