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

• Applicant: Professorin Dr.-Ing. Jeanette Orlowsky
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 496342862

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.

• DFG Programme: Research Grants