Investigations on the swelling behavior of pure anhydrites

The swelling ability of anhydrite can be a major problem during tunnel construction. It's a fact that under humid atmospheric conditions every natural anhydrite (CaSO₄) sooner or later dissolves, or alters to gypsum (CaSO₄*2H₂O), which coincides with a calculated volume increase of 61%. The swelling capability depends greatly on the type of anhydrite itself. In order to explain and verify the differences in the swelling behaviour of pure anhydrite rocks, different anhydrite samples were investigated. The results show that the "maturity" of the anhydrite rock provides the decisive difference. The maturity is herein reflected in a larger former rock cover which led to high temperature and stress conditions. That again produces a massive rock with large crystal grains and a relatively low specific surface area. The result is a material with a low reactive surface area and thus more or less inert ("sluggish") to swelling.