Moisture expansion of tuff stones and sandstones

Abstract

AbstractVolcanic tuffs and sandstones have been used as natural building stones in the construction of monuments and buildings over the millennia of human history. Progressive weathering over time (e.g., temperature fluctuations, moisture uptake, variations in humidity, precipitation, the input of modern-day pollutants or salt crystallization) lead to damages in the internal structure of the stone. Scaling and layer-parallel flaking of rock fragments result from these types of damage-causing processes, especially when swellable clay minerals are present that lead to the generation of stresses in the rock fabric. Tuff and sandstone samples that exemplify these types of damages are investigated in this study, which were used in the construction of cultural heritage sites in Armenia, Mexico, Germany and Switzerland. Comparisons are made between these rock types considering that they show strong variations in fabric heterogeneity, grain sizes, mineralogical composition (e.g., swellable clay- and zeolite-bearing minerals), and visible macroscopic features such as depositional layering. Comprehensive investigations have been carried out that include detailed petrographic analyses, XRD analyses for the determination of the swellable and non-swellable clay fraction and minerals of the zeolite group, the cation exchange capacity (CEC), detailed SEM surveys for determining the type and localization of clays and zeolites in the rock fabric, as well as defining the petrophysical properties (e.g., porosity, capillary water uptake, water absorption, hydric expansion and mechanical properties). Moreover, this study explores the possible interaction between swellable clay minerals and zeolites (mordenite, heulandite/clinoptilolite) by conducting swelling experiments with salts and solvents and fluid-exchange experiments with and without the presence of zeolites. To explain the damages caused by the expansion in the tuffs and sandstones, two principle modes of moisture expansion are discussed when swellable clay minerals are present in the rock. These are dominantly inner or intracrystalline swelling, and secondarily, intercrystalline or osmotic swelling. When no swellable clay minerals are present, the study explains the damages in tuffs and sandstones as the result of disjoining pressure, where a high percentage of microporosity plays a pivotal role.