Natural stones of the Saale–Unstrut Region (Germany): petrography and weathering phenomena

Dimension stones of the central German Saale–Unstrut Region were investigated with regard to petrographic and petrophysical properties as well as their weathering behaviour, onsite and in laboratory. To evaluate the suitability of building stones in this region, different buildings (the Rudelsburg Castle, Memleben Monastery, and vineyard walls) and one rock relief (Steinerne Bibel), made of different types of sandstone or limestone were studied concerning the damage situations. Laboratory measurements include characterisation of pore space properties, water uptake or water vapour diffusion analyses and strength tests. Weathering simulation tests, such as hygric swelling and salt weathering resistance were performed and compared with on-site building observations. In general, the limestones investigated here are classified with a higher construction suitability due to their lower porosity and ability for water uptake, as well as their higher tensile strength, which again is proven by the weathering tests in laboratory. Laboratory analyses reveal for sandstone samples higher porosities and unfavourable pore radii distributions (high content of capillary pores/micropores), resulting in high rates of water uptake, and low resistance against mechanical stress. This behaviour is attributed mainly to the petrographical/diagenetical properties of sandstones investigated, where, e.g. low compaction or cementation (e.g. due to the presence of clay coatings) prohibit a reduction in intergranular volume and create high amount of capillary pores and, in some cases, a high content of micropores. In deviation to this, one dolomitic sandstone sample shows low porosity and water absorption, as well as high strength properties, making them, at a first glance, highly suitable for building purposes. The limestones predominantly exhibit, due to their micritic fabric, low porosity and water absorption but high strength, and, in consequence, hold a relatively high resistance against salt weathering and therefore a high weathering resistance during laboratory tests and onsite. The generally low hygric dilatation is also a favourable characteristic. Contrary, also limestones with low tensile strength or high porosity occur, which is explained with internal crack structures and loss of components (ooids), respectively. However, together with favourable pore radii distributions (e.g. few capillary/micropores), the construction suitability is evaluated to be better than for the sandstones. These findings on construction suitability are also reflected/supported by studies onsite. Observations at the, e.g. rock relief “Steinerne Bibel” confirm that the unfavourable petrophysical properties of the fluvial, immature sandstones lead to low resistance against salt weathering and together with hygric dilatation, to a poor condition of the rock relief. Additionally, the investigation of the rock relief “Steinerne Bibel” has shown that the preservation of the reliefs within the Solling Sandstone and Chirotheria Sandstone is a great challenge, since the rock interacts with pore water which is enriched by salts of the Röt saline unit. A direct interaction between the cement of natural stones and mortar could also be observed (Bernburg Formation, Memleben Abbey), which emphasises that the application of mortars should only be performed by the existing knowledge of the material properties. Moreover, the intensity and type of deterioration depends mainly on different rock properties and on the exposition.