# Delayed-Expansion Capsule Sealing Material for Coal Mine Overburden Isolated Grouting
## 1 Introduction  
Mining subsidence caused by the development of coal resources is a major cause of environmental problems in mining areas [1, 2, 3]. Fill mining is an important technical approach for addressing mining subsidence. After decades of development, filling and mining methods such as solid filling, paste filling, high-water material filling, and overburden isolation grouting filling have been gradually developed [4, 5, 6, 7]. Among them, overburden isolation grouting and filling technology have been extensively applied [8, 9, 10], and can effectively control surface subsidence and protect ground structures through the implementation of high-pressure grouting and the filling of mining fracture cavities under the selected key layer, forming a compaction-bearing structure to support the key layer. However, during the grouting process, abnormal fracture channels connect under the effect of faults, geological boreholes, and other unfavorable factors. This results in the failure ofthe filling cavity's tightness and the spread and leakage of slurry along the overburden rock fracture channels, leading to the loss of pressure in the filling cavity, which compromises the effectiveness of control measures. Therefore, a method for sealing abnormal fractures during the grouting process is an important prerequisite for ensuring the effect of grouting and filling. There are two main types of sealing methods for rock fracture channels, namely, the natural sedimentary slow-sealing method and the chemical agent rapid-sealing method [11]. The principle of natural deposition slow-sealing method is straightforward: the solid-liquid two-phase flow slurry is continuously separated and deposited during the flow process, and the solid particles are used to seal the fracture channel. The treatment process of this method is slow, and the control process is complex. Excessive grouting pressure can easily cause the reopening and expansion of sealed fracture channels, undermining the effectiveness of the sealing process. However, because the pressure and flow rate of overburden isolation grouting are high, this method is not suitable for fracture sealing in this grouting process. Rapid sealing with chemical reagents relies on chemical reactions either between the reagents themselves or between the reagents and substances within rock fractures. This process quickly solidifies the reagents used to fill the fractures, with some reagents specifically reacting with fracture minerals to enhance sealing effectiveness. Reagents and ions in the rock fractures react with each other to generate insoluble substances, which precipitate in the fractures and play a role in sealing. As for overburden isolation grouting engineering, the slurry is required to spread as far as possible along the dominant flow channel to achieve a better filling effect, with fewer boreholes. Therefore, directly using a quick-setting sealing material can easily lead to clogging in the boreholes or in the dominant flow channels before reaching the leakage fractures, resulting in the failure of the subsequent filling process. In summary, the natural sedimentary slow-sealing method is suitable for low grouting pressure and flow conditions, which is significantly different from the coal mine overburden isolated grouting. The rapid sealing method of chemical reagents may cause the problem of grouting filling channel blockage, and face problems such as high cost and potential adverse effects on the environment. Therefore, a suitable sealing material must be developed to accommodate the challenges posed by large fractures and high flow rates in coal mine overburden isolation grouting run-through fractures. There are many rock fracture sealing materials, and different materials have their own unique properties and application scenarios. Among them, some traditional sealing materials are widely used in sealing rock fractures. However, with the development of technology and the diversification of needs, various new materials have gradually emerged.