# Delayed-Expansion Capsule Sealing Material for Coal Mine Overburden Isolated Grouting
## 4 Preparation and Properties of Sealing Material
### Properties  
To investigate the effect of particle size and overlay ratio on the delay effect, expansion capacity, and fluidity performance of the SAP sealing material, this study designed the plugging material capsule cores using commonly used granular filling materials for grout filling. The capsule cores consisted of pure SAP material with particle sizes of 20 mesh, 40 mesh, and 60 mesh. The wall material used was #52 sliced paraffin wax (density of 0.88-0.91 g/cm\({}^{3}\), melting point of 52 \({}^{\circ}\)C) with overlay ratios of 20%, 30%, and 50%. Additionally, a control group consisting of pure SAP material with the same particle size as the sealing material was included for comparison. The film cover ratio was 20%, 30%, and 50%, respectively. The control group was designed to be pure SAP material with the same particle size as that of the sealing material. The results of the two experiments were compared to validate the feasibility of the SAP sealing material in grouting and leakage sealing after film covering. In the overburden isolation grouting and filling project, the fracture aperture of the filling is small, and the practice shows that the appropriate particle size of the slurry should  
Figure 9: SAP granule film covering process. be less than 0.2 mm. If the particle size is too small, it is obvious that the leakage fracture cannot be blocked. If the particle size is too large, there is a risk of blockage in the grouting channel. That is why particle sizes of 20 mesh, 40 mesh, and 60 mesh were selected in this paper. Regarding the film covering ratio, it is mainly determined by laboratory experiments. If the film covering ratio is too small, the expansion will be very fast, and the purpose of delay expansion cannot be achieved. In this case, there is a risk of blocking for the grouting channel in the engineering. If the film ratio is too large, the expansion time will be too long; the sealing material will not expand after flowing into the leakage fracture, and the leakage cannot be sealed. According to the experiment, the appropriate film ratios were determined to be 20%, 30%, and 50%. The expansion rate of pure SAP material is approximately 100-150 times its original mass. Therefore, in the expansion rate experiments, the quantity of water should be controlled to be at least equal to or greater than 100 times the mass of SAP dry material to ensure sufficient conditions for complete expansion. To conduct the swelling test, 100 g of tap water was prepared and placed in each of nine 100 mL measuring cylinders. Each cylinder received a different sealing material, added at a 1:100 ratio (1 g of SAP material per 100 g of water). The materials varied by mesh size and overlay ratio as follows: 1 g of 20 mesh (20%), 20 mesh (30%), 20 mesh (50%), 40 mesh (20%), 40 mesh (30%), 40 mesh (50%), 60 mesh (20%), 60 mesh (30%), and 60 mesh (50%). The swelling of each material was observed and recorded continuously during the testing process, and the swelling status of the material was recorded at 1 min intervals until the material had fully expanded. Samples of sealing material with particle sizes of 20 mesh, 40 mesh, and 60 mesh at overlay ratios of 20%, 30%, and 50% were tested. Throughout the experiment, the expansion of each sample in water was observed continuously and recorded at 1 min intervals. This process continued until each sample had absorbed water to its maximum expansion limit, and the slurry had fully solidified, at which point the experiment was concluded. The pure SAP material started to expand rapidly upon contact with water, and expanded completely within only 1 min. Additionally, after contact with water, the SAP plugging material did not immediately undergo water-absorbing expansion, but was instead suspended in the water surface. The main reason for this behavior is that the outer surface of the SAP material was coated with low-density paraffin wax.