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Estimation of the strength of Konya pyroclastic rocks from P-wave velocity
Published in Charlie C. Li, Xing Li, Zong-Xian Zhang, Rock Dynamics – Experiments, Theories and Applications, 2018
S. Kahraman, T.K. Gun, B. Gunes, I. Ince
Different engineering structures are constructed on or in pyroclastic rocks in some volcanic areas. Pyroclastics are also commonly used as building materials. For this reason, developing some relations between P-wave velocity (Vp) and rock strength will be helpful for engineers and researchers.
Mechanical behavior of volcanic rocks
Published in Tatiana Rotonda, Manuela Cecconi, Francesco Silvestri, Paolo Tommasi, Volcanic Rocks and Soils, 2016
The two groups exhibit a very different mechanical behavior. The first group is formed by materials with much higher densities and strength, though highly dependent on their weathering degree. On the other hand, pyroclastic rocks are formed by fragmented materials with very different grain sizes and textures and with very high porosities and low densities. Commonly, these ones are easily weathered rocks, with very low strength and high deformability, with the exception of the group of ignimbrites (welded and non-welded), which corresponds to hard rocks and has been studied included in the first group.
Pyroclastic Stones as Building Materials in Medieval Romanesque Architecture of Sardinia (Italy): Chemical-Physical Features of Rocks and Associated Alterations
Published in International Journal of Architectural Heritage, 2022
Stefano Columbu, M. Palomba, F. Sitzia, G. Carcangiu, P. Meloni
The investigations evidence that the rhyodacitic ignimbrites used as construction geomaterials in the San Nicola Church show a chemical-physical decay mainly on the surface of stone ashlars, while overall the monument shows a good durability. On the macroscopic base, the decay is mainly represented by surface decohesion, exfoliation, and flaking processes, these latters generally responsible of the rock breakdown, and subordinately by the presence of efflorescence. The RDR and RDB lithofacies do not apparently show marked differences in the alteration degree. However, the results of research highlight that the in situ decay processes mainly affect the RDR lithofacies, due to its different petrophysical features, characterzied by a lower welding degree and then by a greater porosity than RDB lithofacies. Thus, water absorption capacity and water open porosity are the main features strongly affecting the durability of the ignimbrites. Open porosity to water, in particular, is the key-element in the channeling of saline solutions through fractures and micro-fractures of the rocks that are, as also the SEM studies evidence, detrimental for the affected materials. The greater open porosity in the RDR (ranging from 19–29 vol%) than RDB lithofacies (15–24 vol%) affects the physical decay process. For this reason, the RDR lithofacies were probably used less than the RDB in the construction of the Church. In any case, since both the lithofacies have a relatively high absolute porosity, the presence of circulating water-solutions may cause damages, due to the cyclic crystallization/solubilization of salts. That fact involved destructive effects inside the pore network of rock, with an exponential increase of the effective porosity between the crust/flake and “fresh” portions, calculated up to about 50%. This mechanism favors the stone decay in both RDR and RDB, in particular, advancing the decohesion and exfoliation processes in the ashlar stone surface. Obviously, physical-mechanical decay is more intense and faster in rocks characterized by intrinsic high porosity (> 25 vol%), due to an easier absorption of liquid phases.Integrated petrographic and analytical studies prove that the chemical-mineralogical alteration state of pyroclastic rocks is mainly due to devitrification degree of glassy matrix of these rocks, which started during syngenetic or immediately post-genetic processes, by geochemical transformations and formations of newly formed phases (i.e., phyllosilicates belonging to smectite, illite, zeolite, celadonite groups, etc.). Furthermore, the alteration of these ignimbritic stones can be subordinately attributed to epigenetic/weathering processes, which cause an increase of the chemical transformations on the stone surface, favoring the formation of clay minerals, frequent albitization of plagioclase, oxidation of mafic phases (e.g., piroxene, amphibole). The presence of gypsum, besides the recombination of calcium of the feldspars and volcanic glass with sulphur of acid rains, can be also attributed to sulphation process of ancient lime-mortars.