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A remarkable risk in the field of rock engineering is the natural exposure of low-grade metamorphic rock to the freeze-thaw process. Examples of such materials are slate and phyllite, which are employed approximately mountainous permafrost zones. In order to study the weathering processes and mechanisms in low-grade metamorphic rock following freeze-thaw weathering processes, the freeze-thaw cycles were applied. Accordingly to this process, low-grade metamorphic rock partially or completely deteriorates, and mechanical properties, microstructure, and mineral content changes are commonly associated with freeze-thaw processes. This study shows that, following numerous freeze-thaw cycles processes, the quality, wave velocity, and compressive strength of rock reduce; calcite, mica, feldspar, and pyrite contents decrease; and clay minerals and gypsum content increase. Furthermore, the study also demonstrated that the changes in quality, wave velocity, and compressive strength caused by weathering of the rock followed the sequence slate > phyllite. However, the mineral contents of slate and phyllite alter in the same way. Because of this increase in volume and subsequent F-T cycles, new cracks form on the surfaces of pores due to outward pressure, and a lot of water is absorbed by clay minerals. The amount of water increases once again due to freezing as the temperature falls below zero degrees Celsius. This course duplicates with cyclic temperature changes and causes deterioration of low-grade metamorphic rock materials. By analysis of an examination of the damage variables based on wave velocity, it is possible to conclude that mineral changes during the freeze-thaw effect may make for the formation of voids and the deterioration of metamorphic rock. This research contributes to a better understanding of weathering processes and mechanisms of low-grade metamorphic rock following freeze-thaw weathering processes. [ABSTRACT FROM AUTHOR]

Earthen sites with historical, scientific, social, cultural, and artistic values can be destroyed by weathering. Chemical consolidation is an effective method to protect earthen sites, and organic–inorganic composites are often considered ideal. The primary aim of this study was to investigate the effect of polyvinyl alcohol (PVA) with a high degree of alcoholysis on the expansion and cracking behaviour of quicklime-treated soil. The expansion and surface cracking behaviour of the composite soil were investigated with a Crack Image Analysis System. Changes inside the soil samples were observed and analysed by X-ray computed tomography and ImageJ. The results indicated that the expansion of quicklime-treated soil decreased with increasing PVA content, and the cracking behaviour on the surface was different from the inside of the samples. Surface cracks were significantly weakened when PVA was added to soil with a quicklime content ≤ 5%. As the quicklime content increased to 10% or more, the PVA transformed long and wide surface cracks into short and narrow cracks. The development of internal cracks was not as evident as those developing at the surface. Soil samples with 1.0% PVA had no obvious internal cracks at 0–25% lime content. Samples with 0.5% PVA content had internal cracks only when the quicklime content was ≥ 20%. A mechanism for the cracking phenomenon was proposed considering the results obtained from X-ray diffractometry. Soil particles were cemented together through physicochemical reactions with PVA, and these soil particles shared the expansion resulting from the hydration reaction of quicklime instead of through local expansion. [ABSTRACT FROM AUTHOR]

Grottoes are widely distributed along the ancient Silk Road in Northwest China, and wall paintings are among the most precious cultural relics in these grottoes. On-site investigation of some grottoes has revealed that these wall paintings are painted on earthen plaster and that most of them have a double-layer structure of plaster, i.e., a base coarse plaster layer (with wheat straw added) and a surface fine plaster layer (with hemp fibers added). Meanwhile, studies regarding the improvement in shrinkage and cracking behaviors via ancient plaster craftsmanship have been scarce. To investigate the specific effects of different material compositions on the shrinkage and cracking properties of plaster during desiccation, a series of laboratory shrinkage and cracking tests is conducted on plaster specimens composed of Dengban clay and quartz sand, vegetal fibers, and a potential restoration material (i.e., calcined ginger nut), respectively. The results show that adding sand and vegetal fibers can significantly increase the shrinkage limit of Dengban clay and inhibit its volume shrinkage and that vegetal fibers play an important role in the inhibition of surface cracking on plaster. Sand inhibits shrinkage by forming a skeleton in soil and reducing the formation of primary cracks. Fibers inhibit soil shrinkage by overcoming the suction between soil particles and preventing the formation of primary cracks. However, wheat straw cannot deform with the shrinkage of soil (e.g., hemp fibers); consequently, many subcracks appear around the wheat straw on the surface of coarse plaster. As a typical lime-based restoration material, calcined ginger nut paste presents slight shrinkage and a low surface crack ratio during desiccation, which demonstrates its good application prospect in plaster restoration. • Different types of plaster specimens are prepared based on the original composition. • Sand and vegetal fibers can significantly increase the shrinkage limit of Dengban clay. • Vegetal fibers are more effective in inhibiting surface cracking. • Calcined ginger nut paste shows slight shrinkage and a low surface crack ratio during drying. [ABSTRACT FROM AUTHOR]