# Delayed-Expansion Capsule Sealing Material for Coal Mine Overburden Isolated Grouting
## 4 Preparation and Properties of Sealing Material
### Properties
When the sealing material was submerged in water, the paraffin wax layer encountered water first, preventing the underlying SAP material from immediate water exposure and subsequent absorption and expansion. This delay in water interaction distinguishes the coated SAP from uncoated, pure SAP, which absorbs water and expands upon initial contact. However, the sealing material did not maintain the state of non-expansion indefinitely. Soon, the dense SAP small particles continued to expand and sink. After 5 min, some of the expanded material settled at the bottom of the beaker, while a portion of the sealing material remained unexpanded at the water's surface. After 15 min, the sealing material had essentially expanded completely. The reason for this behavior is that, unlike the pure SAP material, the sealing material was covered by paraffin wax and the tightness of the package was insufficient, which led to the rapid expansion of the SAP material in the capsule upon exposure to water and the cracking of the wax wall. Eventually, the wax material was suspended in the water, and the expanded material precipitated to the bottom of the liquid system. As shown in Figure 10, by comparing the expansion of pure SAP material with the 20-mesh (20%, 30%, 50%), 40-mesh (20%, 30%, 50%), and 60-mesh (20%, 30%, 50%) SAP sealing material, it can be found that, under the same material particle size, the film cover ratio is the key factor influencing the expansion rate. In other words, a higher film cover ratio of SAP materials with the same particle size leads to a better delayed-expansion effect, resulting in an extended time for the material to reach its maximum expansion limit. Additionally, with the same film cover ratio, larger particle sizes exhibit greater time-delayed expansion ability in the developed sealing material compared with the pure SAP material at the same particle size. The experimental results in Figure 11 indicate that, with overlay ratios of 20%, 30%, and 50% and water mixing ratios of 1.0% or 1.5%, the change in slurry fluidity follows a similar trend across different particle sizes. However, by comparing the 60-mesh SAP material with the 20-mesh and 40-mesh sealing materials with different film cover ratios and dosages, it can be found that the 60-mesh SAP material initially absorbed and swelled  
Figure 10: Differences in water absorption and expansion of sealing materials under different particle sizes and film cover ratios. (**a**) Particle size of 20 mesh, (**b**) particle size of 40 mesh, (**c**) particle size of 60 mesh. more rapidly in water, resulting in an immediate and sharp decrease in slurry fluidity. Subsequently, the slurry was fully swollen after 5 min and almost completely lost its fluidity. The flow of the 20-mesh and 40-mesh sealing materials with different film cover ratios and dosages tended to stabilize with time. In summary, the results for the flowability of sealing materials with different particle sizes and film cover ratios in water and fly ash slurry reveal that a higher film cover ratio results in better slurry flowability after the sealing materials are doped into the water or fly ash slurry. Additionally, the slurry's flowability is continuously weakened as the doping ratio of the sealing materials increases, while the film cover ratio of the sealing materials remains the same.