Mineralogische Aspekte zur Verwitterungsanfälligkeit des Schilfsandsteins aus dem Raum Bamberg

Abstract

SchilfSandstein of the Bamberg area, F.R.Germany, a commonly used
freestone for historic buildings and sculptures, has a fine polymineral
grain-structure with spezial Chemical and mechanical
properties. Ultrasonic velocity measurements and X-ray tomography,
which are both non-destructive, have been used to show that inhomogeneities
already exist in the unweathered rock.
Initial strength properties of SchilfSandstein appear to be dependent
upon the quartz and/or feldspar particle contacts. Open pores, microfissures
and the amount of clay matrix, which offen encloses mineral
grains, have a profound influence on the weathering behavior of this
rock and hence control the desintegration.
Depending upon the local microclimatic conditions two types of near
surface fissures were observed on weathered building rocks. High
moisture absorption is provoced by the mineralogical composition of
the matrix. A swelling clay mineral (vermiculit) has been found; its
high cation exchange capacities seem to influence the special
Chemical and mechanical reactivity of SchilfSandstein. Clay particles
may catalize the crystallisation of salts like gypsum. This is
interesting because orginaly very little calcium was detected within
the rock.
A drying-wetting stress seems to be a very important parameter during
the weathering of SchilfSandstein. In order not to enhance this
"natural" weakness by Conservation one must also consider possible
reactions between organic Solutions and the clay matrix in the porous
structure.