# Delayed-Expansion Capsule Sealing Material for Coal Mine Overburden Isolated Grouting
## 6 Conclusions  
Grouting technology is an important method for underground reinforcement. However, unexpected fractures may lead to substantial grout leakage, resulting in unsatisfactory grouting. This paper proposes a new sealing method for coal mine overburden isolated grouting engineering. By exploiting the high expansion and water absorption properties of SAP, a time-delayed sealing material was prepared using paraffin-coated SAP particles to achieve leakage sealing. The swelling capacity and fluidity performance of the material were tested. To assess the material's fracture sealing capability, a fracture sealing model was designed and investigated experimentally. The swelling colloid mass ratio, colloid volume ratio, and flowability of the SAP material mixed with water and fly ash slurry were tested, respectively. The results revealed that the SAP materials with a smaller particle size absorbed water faster, resulting in the larger mass-to-volume ratio of the colloid. While the mixture of SAP and water lost flowability quickly, the flowability of the SAP and fly ash slurry mixture remained unaffected. This study found that the expansion rate is controlled by the film cover ratio of SAP. A higher film cover ratio of SAP with the same particle size results in a longer expansion time and better flowability performance after being mixed with fly ash slurry. Overall, the flowability of the slurry decreased as the mixing amount of the sealing material increased. The film cover ratio is a critical factor influencing the sealing effect. A high film cover ratio can overly restrict the expansion of the sealing material, compromising its sealing  
Figure 17: Variation curves of leakage amount of SAP sealing material under different grain sizes and fracture aperture. (**a**) Particle size of 20 mesh with a film cover ratio of 20%. (**b**) Particle size of 40 mesh with a film cover ratio of 20%. (**c**) Particle size of 60 mesh with a film cover ratio of 20%. effectiveness, because an isolation layer is formed between the sealing material and the leakage fracture. Conversely, a low film cover ratio leads to fast expansion, which affects the sealing effect. The results obtained by the sealing simulation experiments revealed that fractures with apertures of less than 4 mm could be effectively sealed using a low mixing ratio (\(\sim\)1%) of the sealing material with different particle sizes. However, a larger mixing ratio of the sealing material was required for fractures with apertures of 4-6 mm. The findings of this study provide a theoretical basis for unexpected slurry leakage in coal mine overburden isolated grouting engineering. Compared with the traditional natural sedimentation slow-sealing method and the chemical agent rapid-sealing method, the proposed sealing method in this paper is suitable for the plugging of leakage fractures under the conditions of large grouting flow and high pressure. It does not block the grouting channel, and can fix the leakage issue at a relatively short time, which ensures better grouting continuity and reduces the negative impact on the grouting effect. The paraffin and SAP materials used have no adverse impact on the environment because of low content, and the cost is low. It should be noted that this method may not be suitable for a long lag time of \(>\)15 min, since the delayed expansion time of the material has a limitation. It is likely that the expansion occurs before the material flow into the leakage fracture, resulting in the blockage of the grouting channel. Conceptualization, D.X. and J.L. ; methodology, D.X. ; formal analysis, X.N. and K.L. ; investigation, X.N. and J.L. ; resources, X.N. ; data curation, X.N. and J.L. ; writing--original draft preparation, D.X. ; writing--review and editing, D.X. and K.L. ; visualization, X.N. and K.L. ; supervision, J.X. ; funding acquisition, D.X. All authors have read and agreed to the published version of the manuscript. This research is funded by the Natural Science Foundation of China (Grant No. 52374143). Not applicable.