Alveolar weathering of Cretaceous building sandstones on monuments in Saxony, Germany
The pattern of alveolar weathering on Cretaceous building sandstones from Saxony has been studied and related to the sandstone rock fabric and the spatial distribution of soluble salts at two monuments. The results demonstrate that uneven weathering of the stone surface is initiated by differences in porosity and mineral composition at macroscopic or microscopic scale within the heterogeneous material. Magnesium sulphate, the dominant salt, accumulates in areas with a higher content of clay minerals or ferric oxides/hydroxides, characterized by a higher amount of smaller pores. At an incipient stage, small pits in the surface are formed by salt weathering at these points. The stone dries more slowly from the base of these pits, thus accumulating more salt and leading to further material loss. Once deepened, the hole can shelter efflorescing salts from rain. The preferential weathering at the surface of the holes by salt crystallization and hydration pressure becomes a self-perpetuating process at this second stage, dependent only on changing moisture and climate. Building materials (mortars) and sulphur derived from air pollution are the main sources of the salts involved in the process of alveolar weathering on the investigated buildings. In contrast, salts that cause alveolar weathering of the Cretaceous sandstone bedrock in natural outcrops in Saxony mainly originate from long-term chemical weathering of the rock itself.