Substance-preserving restoration of hydrophobic, weathered natural stones with the use of a non-destructive, depth-graded characterization of the pore structure

在之前的DFG项目中，有机硅化合物的性能在长达30年的自然风化疏水砂岩上进行了全面的研究。除了毛细管吸水外，重点是长期风化引起的地表岩石破坏的程度和增加，这是通过200微米步长的深度相关核磁共振测量记录的。调查包括5种不同矿物组成的砂岩类型，用11种疏水剂处理，在7个地点暴露，并在风化2年、17年和24/30年后进行测量。从7600多个实验室测量结果可以得出结论，石块内试剂的疏水效果在30年内不会减弱。对于所有被分析的石头类型，疏水处理不能阻止表面附近的风化过程。相应地，2年后确定的破坏深度是30年风化过程中的数倍。这种近地表的损害意味着水在地表附近的渗透，会导致剥落、表面粗糙、打磨、石膏风化、结壳和结垢、污垢和生物生长。一些处理剂，特别是那些应用于鲍伯格尔砂岩、施莱里特砂岩和艾琴布勒砂岩的处理剂，即使在未风化的条件下，也表现出不充分的拒水效果。在这种情况下，损坏的深度更加明显。在后续项目中，将进一步调查有价值的原始材料(450个自然风化棱镜)。除了先前分析过的棱镜前表面外，还将检查通常风化较严重的屋顶表面，以补充和扩展现有知识。后续项目的重点是对风化的天然石料进行清理，以及随后对样品进行疏水处理。在清洗过程中，采用了4种不同的清洗方法，考察了不同清洗方法对石材性能的影响。清洗引起的孔结构变化将通过单面核磁共振和待开发的评估程序来表征。疏水后处理使用类似的试剂以及水基系统和包括杀菌剂在内的系统进行。所使用的试剂有清楚的描述，并可通过其CAS编号重现。最后，将样品再次暴露在自然室外风化中。该项目能够在不破坏实际建筑结构的情况下，在确定的边界条件下，为修复长期风化的天然石材立面/物体开发适当的物质保护程序。此外，在项目结束时将提供一种非破坏性测量方法，这将使砂岩孔隙结构随深度变化的高分辨率量化成为可能。

In a previous DFG project, the performance of organosilicon compounds was comprehensively investigated on up to 30 years natural weathered hydrophobic sandstones. Besides capillary water uptake, the focus was on the extent and increase of surface stone damage due to long-term weathering, which was recorded with depth-dependent NMR measurements in 200 µm steps.The investigation consisted of 5 sandstone types with different mineralogical composition, treated with 11 hydrophobic agents, exposed at 7 locations and measured after 2, 17 and 24/30 years of weathering. From more than 7600 laboratory measurements it can be concluded that the hydrophobic effect of the agents within the stone does not decrease over 30 years. For all analysed stone types, the hydrophobing treatment cannot stop weathering processes near the surface. Accordingly, the damaged depth determined after 2 years increases many times over in the course of a weathering of 30 years. This near-surface damage, which means the penetration of water near the surface, causes spalling, surface roughening, sanding, gypsum efflorescence, crusts and scaling, soiling and biogenic growth. Some agents, especially those applied on Baumberger, Schleeriether and Eichenbühler sandstones, show, even in unweathered condition, an insufficient water-repellent effect. In this case the damaged depths are more pronounced. In the follow-up project, the valuable source material (450 naturally weathered prisms) will be further investigated. In addition to the previously analysed front surfaces of the prisms, the roof surfaces, which are generally more severely weathered, will be examined in order to supplement and expand on existing knowledge. The follow-up project focuses on the cleaning of the weathered natural stones and a subsequent post-hydrophobing of the samples. During the cleaning process 4 different methods are used and their effect on the stone properties is investigated. The change of the pore structure due to cleaning will be characterized by means of single-sided NMR and evaluation routines to be developed. The post-hydrophobic treatment is carried out with comparable agents as well as with a water-based system and a system, including a biocide. The agents used are clearly described and reproducible by their CAS numbers. Finally, the samples are exposed to natural outdoor weathering again. This project enables the experimental development of suitable, substance-preserving procedures for the restoration of long-term weathered natural stone facades/objects under defined boundary conditions without destroying actual building fabric. In addition, a non-destructive measuring method will be available at the end of the project, which will enable the quantification with high resolution of depth-dependent changes in the sandstone pore structure.