Artificial microcracking of granites subjected to salt crystallization aging test

Salt crystallization-induced decay of Vardavard granodiorite and Shirkouh monzogranite, two Iranian building stones, were assessed with two non-destructive methods: saturation-buoyancy technique and P- and S-wave velocity measurement. Moreover, polarized and fluorescence microscopy studies were used to evaluate the behavior of the studied stones at microscopic scale against a salt crystallization aging test. The aging test extended pre-existing microcracks and generated new ones. Intracrystalline microcracking was the most predominant microcrack type for both samples. Fine-grained Vardavard granodiorite experienced higher intercrystalline microcracking than coarse-grained Shirkouh monzogranite. The microcracking mechanism of feldspars substantially depends on their alteration degree and microstructural precursors. When a growing microcrack reaches a biotite, it propagates within the crystal if the growing microcrack coincides with the cleavage plane; otherwise, it propagates as an intercrystalline one. The increase in maximum microcrack length of the samples was higher than the increase in their mean microcrack length. Low-strength Vardavard granodiorite showed higher microcrack width after the aging test. Dry weight loss in low-strength Vardavard granodiorite was more pronounced than in high-strength Shirkouh monzogranite. Dry unit weight decreased at a higher rate than saturated unit weight with the increase of effective porosity. The reduction in ultrasonic wave velocities and the increment in effective porosity and water absorption were more pronounced for Vardavard granodiorite, indicating a higher degree of decay, i.e., higher microcrack generation, enlargement, and widening. Shirkouh monzogranite, which has large-sized crystals and pores, wider initial microcracks, high tensile strength, and low effective porosity and microcrack density, was more durable than Vardavard granodiorite

The influence of petrographic properties on mechanical characteristics and the durability of the greenschist subjected to simulated weathering tests

Despite the wide use of greenschists as a building stone, few studies have been conducted on their resistance to both physical and chemical weathering processes. The present study investigates the petrographic and physico-mechanical characteristics of the Abrangi building greenschist from Iran. In addition, it evaluates the resistance of the stone to three simulated weathering tests, including salt crystallization, frost action, and acid solution. The loss in integrity and esthetic damage caused by simulated weathering tests are quantified using P-wave velocity and colorimetry tests. Moreover, fluorescence microscopy (crack studies) and SEM–EDX techniques are applied to investigate the evolution of stone fabric caused by salt crystallization and wetting–drying in an acid solution, respectively. The results showed the quality of Abrangi greenschist is controlled by its compositionally banded texture. Mechanical damage was localized in tremolite-rich bands. Meanwhile, chemical decay was observed in granoblastic bands, where calcite and epidote were embedded within the bands. Furthermore, it was observed that tremolite-rich bands were highly damaged when the compressive stress was applied in the direction parallel to the longitudinal axis of the fibers. However, fiber bundles were cleaved into thinner or single fibers by applying the tensile stress perpendicular to the longitudinal axis of the fibers. It is noteworthy that this tensile strength is induced by the Brazilian tensile strength tester and salt crystallization weathering test. Wetting–drying in sulfuric acid caused the formation and accumulation of gypsum crystals on etched calcite and epidote crystals or adjacent voids in granoblastic bands.Iran National Science FoundationTarbiat Modares University of IranIGEO (CSIC-UCM)Coimbra UniversityTrás-os-Monte e Alto Douro UniversityFundaçao para a Ciencia e a TecnologiaDepto. de Mineralogía y PetrologíaInstituto de Geociencias (IGEO)TRUEpu

Ghaleh-khargushi rhyodacite and Gorid andesite from Iran: characterization, uses, and durability

Durability of building stones is an important issue in sustainable development. Crystallization of soluble salts is recognized as one of the most destructive weathering agents of building stones. For this reason, durability of Ghaleh-khargushi rhyodacite and Gorid andesite from Iran was investigated against sodium sulfate crystallization aging test. Petrographic and physico-mechanical properties and pore size distribution of these stones were examined before and after the aging test. The characteristics of the microcracks were quantified with fluorescence-impregnated thin sections. Durability and physico-mechanical characteristics of Ghaleh-khargushi rhyodacite are mainly influenced by preferentially oriented preexisting microcracks. Stress induced by salt crystallization led to the widening of preexisting microcracks in Ghaleh-khargushi rhyodacite, as confirmed by the pore size distributions before and after the aging test. The preexisting microcracks of Gorid andesite were attributed to the mechanical stress induced by contraction of lava during cooling. The number of transcrystalline microcracks was significantly increased after the aging test. The degree of plagioclase microcracking was proportional to its size. Durability of the studied stones depends on initial physico-mechanical properties, pore size distribution, and orientation of microcracks. Initial effective porosity is found to be a good indicator of the stones’ durability. Salt crystallization resulted in an increase in the effective porosity with a parallel decrease in the wave velocities. Surface microroughness parameters increased with the development of salt crystallization-induced microcracking. Gorid andesite showed higher quality and durability than Ghaleh-khargushi rhyodacite