Spatial relationships of salt distribution and related physical changes of underlying rocks on naturally weathered sandstone exposures (Bohemian Switzerland National Park,Czech Republic)

Efflorescence, case hardening, and granular disintegration represent common weathering features of Upper Cretaceous quartz sandstones exposed in the Bohemian Switzerland National Park (NW Bohemia, Czech Republic). Salt species (sulphates: gypsum (CaSO₄·2H₂O), potassium alum (KAl(SO₄)₂·12H₂O), tschermigite (NH₄Al(SO₄)₂·12H₂O), alunite (K(Al₃(SO₄)₂(OH)₆), and alunogen (Al₂(SO₄)₃·17H₂O), minor nitrates: nitrammite (NH₄NO₃)) determined by X-ray diffraction exhibit vertical and geographic zoning. More soluble salts (chlorides, nitrates, tschermigite) crystallize preferentially on the cliffs exposed to the south, whereas the north face is characterized by the presence of less soluble phases: gypsum and K(Al₃(SO₄)₂(OH)₆. Vertical zoning of salt distribution on natural outcrops differs from the salt distribution in masonry. Salt distribution near the base of the cliff (profile to about 2–2.5 m above the ground) is affected by capillary rise from the ground level (first maximum of water-soluble salts at the level of 1–1.5 m above the ground) and by percolation of precipitation through the overhanging rock sequence (second maximum of 2–2.5 m above the ground). Percolation of salt solution from higher parts is affected by the asperity of the rock surface. The concentration of salts (determined by ion exchange chromatography) correlates to the changes of physical properties: bulk porosity, microporosity and water absorption. The porosity, microporosity, moisture content and absorption generally increase with the increasing volume of sulphates and nitrates.