The durability of eleven different water repellents applied on one sandstone type was studied after a long-term weathering at seven different locations in Germany. By measuring colour changes, it could be shown that the formation of black crusts, the deposition of particles and biogenic growth caused a gradual darkening as well as significant changes in total colour over time. Additionally, the water absorption behaviour was investigated with two different methods: applying a low pressure using the pipe method and capillary water absorption measurements from a wet underlay. Afterwards, the test results were analysed with four different evaluation methods: calculation of the protection degree from pipe method and capillary water absorption, determination of the velocity of water uptake during capillary water absorption and calculation of the damaged depth of the stone surface using single-sided NMR technique. The growing damaged depth leads to an increase of the water uptake velocity and to a decrease of the protection degree of the applied hydrophobing agents. Three protective agents based on isobutyltrimethoxysilane showed already after two years of outdoor weathering a clear loss of performance, which significantly increased after 30 years of exposure.

This study deals with the effect of using different silicic acid ethyl esters (SAE) on diverse sandstones and to estimate their influence on the properties of the treated stones. Prismatic samples of Baumberger Sandstone (BST), Sander Sandstone (SST) and Nievelsteiner Sandstone (NST) with the dimensions 50 mm×50 mm×100 mm were treated with three different consolidating agents based on silicic acid ethyl ester (KSE 100, KSE 300, KSE 510) in three different procedures (vacuum, 1- and 5- time impregnation). The aim of this study is to demonstrate that differences in application, for instance varying stone consolidation agents, other treatment procedures as well as using sandstone samples with different binders (BST: calcareous, SST: clayey, NST: quartzitic), cause in each case different results concerning the strengthening effect and the success of a consolidation action. Laboratory measurements were performed on treated and untreated material in order to estimate the effectiveness of stone consolidation actions. To detect the influence of subsequent treatment procedures, water vapour diffusion resistance (WVDR) and capillary water absorption (CWA) measurements were carried out. Furthermore, the effectiveness of a stone consolidation was analysed by measuring ultrasound velocity, compressive strength, flexural strength, bond strength by pull-off and the porosity of the stone samples. Due to varying treatment procedures the investigated sandstones showed different petrophysical and mechanical properties (no strengthening effect up to “over-treating”). Different treatment procedures lead to increasing amounts of strengthening agent in the pore space of the investigated stones and as a result to higher values in WVDR (except BST and NST samples), ultrasound velocity and to an improvement in mechanical strength (except compressive strength of NST). This applies in particular to 1-time, 5-time and vacuum impregnated SST and 5-time treated NST and BST, regardless of the used stone strengthener. On the other hand, these different treatment procedures lead often to a decreased CWA and to a reduction in total porosity. The performed measurements indicate a development in strength in case of 5-time and vacuum treatment, but also an increase of the possibility of sealing the pores, especially for SST samples.

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.