WHY DO PURE ANHYDRITES DIFFER IN THEIR SWELLING CAPACITY

Abstract: Each rock has its own history. Also each anhydrite rock has undergone different sedimentation- and diagenesis-conditions and sometimes even varying degrees of metamorphism. The question arose if different anhydrite rocks would differ in their swelling properties and how these differences could be explained and verified. Therefore anhydrite rocks from 3 geological formations (Gypsum Keuper- Baden-Wuerttemberg, Reichenhall Formation – Northern Calcareous Alps (NCA) and Haselgebirge- NCA) have been investigated intensively. To determine the swelling capacity the purified anhydrite samples were prepared similarly and emplaced in the powder swell apparatus according to Thuro (1993). After a testing period of about 1.5 years all the samples show a distinct swell heave. The swell heave varies notably between the different anhydrite samples. Concurrent the differences of the samples in mineralogy (using thin section and x-ray diffraction analysis) and texture (using thin section analysis and scanning electron microscopy) were determined and compared. It showed that the size of the single crystals gives the decisive difference. It was also possible to gain correlations between the specific surface (air-permeability method after Blaine, DIN EN 196-6) and the swelling capacity. It seems that the swelling potential depends mostly on the crystallinity and thus on the former rock cover. The results of the study will be presented in this paper. Zusammenfassung: Wieso unterscheiden sich reine Anhydrite in ihrem Quellvermögen