# Delayed-Expansion Capsule Sealing Material for Coal Mine Overburden Isolated Grouting
## 3 Materials and Properties
### Flow Characteristics  
The mobility experiment investigated the behavior of the material over time in the slurry to assess the flow characteristics after incorporating SAP materials with different doping ratios. This evaluation aimed to ensure that the slurry maintained a certain degree of mobility during the pumping process, and that it loses its mobility when reaching a fracture, allowing for complete expansion and effective fracture sealing. The experimental results in Figure 6a reveal that, when the material particle size was 20 mesh, and the mixing ratio in water was set to 0.8%, 1.0%, 1.2%, and 1.5%, respectively, the stability times of the slurry flow were 16 min, 14 min, 12 min, and 10 min. At this time, the SAP material in water had reached the state of saturated water absorption in the corresponding time mentioned above. Therefore, as the SAP mixing ratio in water increased, the water loss efficiency of the slurry improved, resulting in the reduction in residual free water within the slurry and subsequently impairing its flow characteristics. For the SAP materials with the particle sizes of 40 mesh and 60 mesh, the flow behavior when mixed with water followed the same pattern observed in the 20-mesh slurry. Figure 7 shows the effects of SAP materials with different particle sizes at the same mixing ratio. As can be seen, under identical conditions, the flow behavior of the SAP in water is significantly influenced by the particle size. Specifically, a smaller particle size leads to a more rapid decrease in flow. Therefore, as the particle size of the SAP material decreases, the slurry loses its flow characteristics more quickly after SAP is added to water. Figure 5: Test results for colloidal volume ratio of SAP materials. The changes in the fluidity of SAP material mixed with fly ash slurry are illustrated in Figure 8. With the particle size of 20 mesh, the mixing ratios in the fly ash slurry were 1.0% and 1.5%, resulting in slurry fluidity stabilization at 18 min and 14 min, respectively. With the particle size of 40 mesh, the mixing ratios in the fly ash slurry were 1.0% and 1.5%, and the slurry fluidity stabilized at 14 min and 12 min, respectively. With a particle size of 60 mesh, the mixing ratios in the fly ash slurry were 1.0% and 1.5%, respectively, and the slurry fluidity stabilized at 8 min and 6 min, respectively. Therefore, a larger mixing ratio of SAP material in the fly ash slurry results in a larger reduction in the slurry's flow rate. Figure 6: Variation curves of flowability of SAP materials in water with different particle sizes. (**a**) Particle size of 20 mesh. (**b**) Particle size of 40 mesh. (**c**) Particle size of 60 mesh. Figure 7: Slurry flowability of SAP material under different particle sizes. (**a**) Doping ratio of 0.8%. (**b**) Doping ratio of 1.0%. (**c**) Doping ratio of 1.2%. (**d**) Doping ratio of 1.5%. The flow degree of SAP material added in fly ash slurry was approximately 80 mm. After the test had been carried out for 30 min, the flow degree of slurry was still floating at approximately 45 mm, indicating that the SAP material in the fly ash slurry did not absorb water as effectively as it did in pure water. This resulted in a more restricted decrease in flowability, although a downward trend was still noted. The addition of SAP materials at a mixing ratio of 1.0%-1.5% to the fly ash slurry had a minimal effect on slurry flow. The reason for this may be the smaller size of fly ash particles, which remained suspended in the slurry and tightly adhered to the surface of the SAP material. This adherence restricted the contact area between the SAP and water, ultimately impairing the water-absorbing capacity of the SAP. Additionally, after the occurrence of water secretion in fly ash, the fly ash particles were deposited to the bottom of the slurry, and the secretion water was in the upper part of the deposited slurry. This secretion water can be classified as free water, while the water in the slurry can be regarded as bound water.