Читать книгу Monument Future - Siegfried Siegesmund - Страница 167

The cation exchange capacity (CEC) value depends on both clay minerals and zeolites, however, the CEC method used is not suitable for CEC analysis of zeolites because the large colored Cu-trien cations cannot enter the cavities of zeolites (Meier and Kahr 1999). For the analysed samples a clear trend between CEC and hydric expansion could not be observed (Fig. 5a). On the other hand, a linear dependency of hydric expansion and hygroscopic sorption on the specific surface area is observable (Fig. 5b, Fig. 6b). The specific surface area increases when clay minerals and zeolites are present in a volcanic tuff, because they yield a high share of micropores (often in the nanometer-scale) within the rock fabric. The higher the specific surface area is, the more water can interact with the rock and the expansion is resultingly larger. Zeolites can adsorb and release substantial amounts of water and therefore passively lead to hydric expansion, whenever water is released during dehydration.

The results of this study show, that the presence of both zeolites and swellable clay minerals has a combined effect on the water-related weathering behavior of tuff building stones. The porespace properties (e. g. pore radii, specific surface area) and the water transport and storage properties 124of volcanic tuff rocks are significantly influenced, by either of them. Volcanic tuff rocks with a significant amount of swellable clay minerals as well as zeolites show extremely high hygroscopic water sorption and hydric swelling. Thermal effects like shrinkage and fracturing during drying are particularly high. This study showed that the identification of the clay mineral and zeolite content, as well as their location and shape within the rock fabric is an important measure for the prediction of the weathering behavior of tuff building stones. For the planning of effective conservational treatment of tuff building stones a clay mineral and zeolite analysis should be considered substantial.

Figure 5: Correlation between a: hydric expansion [mm/m] and CEC [meq/100 g], b: hydric expansion [mm/m] and BET [m²/g].

Figure 6: Correlation between a) hygroscopic water sorption at 95 % rh [wt.-%] and CEC [meq/100 g], b) hygroscopic water sorption at 95 % rh [wt.-%] and BET [m²/g].