Mode-I fracture toughness and mechanisms of Salt-Rock gypsum interlayers under real-time high-temperature conditions

In this study, an attempt was made to investigate the influence of temperature on the stability and compactness of gypsum rock. The fracture toughness was tested using a real-time online high temperature system, and the sample was characterized by scanning electron microscopy and X-ray diffraction in order to further investigate the fracture characteristics and mechanisms. The results showed that (1) The change in gypsum fracture toughness (K-IC) with temperature can be divided into four stages: a slow decrease with increasing temperature between 20 and 100 degrees C, a sharp decrease from 100 to 300 degrees C, a slow decrease from 300 to 600 degrees C, and a rapid decrease from 600 to 700 degrees C; (2) When gypsum is at the low-temperature stage (20-300 degrees C), the fracture surfaces are smooth and flat. The main failure mode is trans-granular fracture. In the high-temperature stage (400-700 degrees C), the fracture surface is rough. The fracture morphology is complex and diverse, with fibrous stripes and secondary cracks being observed on the fracture surface. Intergranular fracture is the main fracture mode. (3) When the temperature of gypsum is between 100 and 300 degrees C, weakening occurs by thermal dehydration and calcium sulfate dihydrate is dehydrated to beta-calcium sulfate and gamma-calcium sulfate. At temperatures ranging from 300 to 700 degrees C, the weakening mechanisms are thermal fracturing and crystal distortion of calcium sulfate. The results of this study provide theoretical guidance for evaluating the stability and compactness of nuclear waste storage.