Analyzing Near-Surface Regions of Hydrophobic and Long-Term Weathered Natural Stones at Microscopic Scale

Abstract

The visual appearance of building structures is an important attribute which reflects the character and identity of a region. Due to the influence of weathering, the surfaces of building stones alter, leading to aesthetic changes of the material surface such as discoloration or darkening. In this study, near-surface regions of weathered Baumberger (BST), Schleeriether (SST), and Obernkirchener Sandstones (OKS) have been analyzed at the microscopic scale in order to investigate the intensity and the extent of visual as well as structural changes and how both can be affected due to the presence of surface treatments with hydrophobing agents. It could be detected that aesthetic changes appeared already after 2 years of outdoor exposure, with the slightest variations on BST surfaces, followed by SST and OKS. The use of hydrophobing agents leads to a reduction in surface darkening in the short term. After long-term weathering, no significant changes are visible, as similar values in total color difference (ΔE*) were measured. Biogenic growth and the formation of black weathering crusts are the main reasons for color alterations in the case of the examined stones. The surface damages occur especially on calcareous (BST) followed by clayey (SST) and quartzitic (OKS) stone surfaces.

About the effectiveness of a hydrophobic surface treatment of Baumberger Sandstones

“…In addition it may also contain of up to 15% glauconite. Concerning the petrophysical properties, the Baumberger Sandstone has a total porosity of about 20%, with a bulk density of 2.17 g/cm 3 and an apparent density of 2.73 g/cm 3 , whereby the majority of the fine pore spaces are present in the micritic matrix (Braun et al 2020;Braun 2021). The capillary water absorption coefficient is about 2 kg/m 2 √h and after 1 h water absorption by pipe method values of 0.4 ml/cm 2 can be determined.…”

“…Due to its weakly consolidated microporous structure, the Baumberger Sandstone shows moderate to poor weathering resistance (e.g., black gypsum crusts, scaling, sanding) (Braun et al 2020), which illustrates the necessity of conservation measures. In contrast to siliceous stone types, stone protecting agents on an organosilicon basis are merely insufficiently chemically bound via the carbonate structure of the Baumberger Sandstone, which is why the success of water repellent surfaces treatments have to be regarded as problematic.…”

“…In addition, scanning electron microscopy (SEM) measurements conducted a further analysis with regard to near-surface weathering phenomena. The results of these investigations are explained in detail in Braun et al (2020).…”

“…The macroscopic and microscopic analysis of the samples, regardless of whether an hydrophobic surface treatment has been carried out and which hydrophobing agent was chosen, revealed the occurrence of weathering phenomena such as spalling, surface roughening, sanding, gypsum efflorescence, formation of crusts and scales, soiling and biogenic growth after 24 years of outdoor weathering (Fig. 8 and Braun et al 2020). The used hydrophobing agents did not cause a clearly detectable deceleration of the weathering processes on the Baumberger Sandstone, although the resulting capillary water absorption values partially showed water-repellent effects.…”

Analysing the Porosity Distribution in Stone Surfaces by Means of Unilateral NMR after Long-Term Outdoor Weathering

“…Analysis of the pore structure of mineral building materials is conventionally carried out using mercury porosimetry and additional microscopic transillumination of thin sections [1,2]. Furthermore, investigations using the NMR technique produced successful porosity analyses of concrete samples [3,4].…”