Clay swelling mechanism in tuff stones: an example of the Hilbersdorf Tuff from Chemnitz, Germany

Abstract

Hydric expansion is considered an important factor for the weathering behavior and deterioration of tuffs, which are utilized as building stones and typically associated with the presence of swellable clay minerals. Two types of swelling mechanisms are discussed when swellable clay minerals are present: stepwise intracrystalline swelling and continuous osmotic swelling. A mechanism that can cause expansion in the absence of swellable clay minerals, which is characterized by interaction of surface forces, is the disjoining pressure. The identification of the primary mode of swelling is important for understanding and finally preventing the swelling damage in tuff stones. The hydric expansion of the Hilbersdorf Tuff can exceed the values of typical volcanoclastic materials by multiples, but partly shows the absence of swellable clay minerals. In this study, extensive mineralogical and petrophysical investigations on five varieties of the Hilbersdorf Tuff were performed. Swelling experiments demonstrated that intracrystalline swelling plays a major role when swellable clay minerals are present. Furthermore, the importance of the structure and location of the clay minerals in the rock fabric is highlighted. The investigation suggests that the disjoining pressure plays an important role for the hydric expansion of two varieties, which are free of swellable clay minerals but show high microporosity and pores smaller than 2 nm.