# Delayed-Expansion Capsule Sealing Material for Coal Mine Overburden Isolated Grouting
## 1 Introduction
Among them, the high-water-absorption resin (SAP) material plays a unique role in many fields owing to its properties [12; 13]. Known for their high swelling capacity upon water absorption, SAP materials hold significant potential for fracture sealing applications and have garnered substantial interest from researchers. [PERSON] et al. [14] investigated the effect of four different SAP concentrations (0.2%, 0.4%, 0.6%, and 0.8% by weight of cement) on the performance of hybrid fiber concrete (HFC) using highly absorbent polymers (SAPs) to evaluate the efficiency of crack healing based on the rate of crack closure and strength restoration. They reported that, for all investigated SAP concentrations, cracks measuring up to 465 \(\upmu\)m were completely sealed. [PERSON] et al. [15] analyzed the crack self-sealing mechanism of hydrogel, providing a comprehensive review of hydrogel design methods for enhancing crack self-sealing. They outlined the influence of hydrogel on concrete's impermeability, mechanics, and durability recovery. Additionally, they discussed the testing and evaluation techniques for crack healing and sealing, offering insights into practical applications and performance assessment. [PERSON] et al. [16] were the first to investigate the self-healing efficiency of SAP-merged concrete and reported that SAP can seal cracks immediately. [PERSON] et al. [17] used microfibers and 1.0% SAP in concrete with a relative humidity greater than 60%. The experimental concrete exhibited a faster healing rate compared with conventional concrete while maintaining all of its original properties, showing no signs of degradation. [PERSON] et al. [18] investigated the synergistic effect of highly absorbent polymers (SAPs) and crystalline admixtures (CAs) on the closure of macroscopic cracks (>0.5 mm in width) in cementitious materials (CBMs). The combination of CA and SAP resulted in good self-healing ability and satisfactory crack sealing. [PERSON] et al. [19] extensively evaluated the performance of highly absorbent polymers (SAPs) as self-healing agents for addressing macro-cracks in ultra-high-performance concrete (UHPC). Their assessment consisted of measurements of the crack closure rate, recovery of compressive strength, and detailed stereo-microscopic analysis. Their results revealed that the SAP significantly enhanced the self-healing ability of UHPC. [PERSON] et al. [20] conducted a comparative study between a mixture of SAP and nano-silica added to concrete and samples containing only nano-silica and reported that the former was able to achieve 100% crack closure. [PERSON] et al. [21] investigated and designed a novel capsule. The core material of the capsule is cement or a combination of cement and highly absorbent polymers (SAPs). The core material in the capsule reacts with water, exhibiting high sealing rates for cracks up to 400 \(\upmu\)m, and even bridging internal cracks with widths over 200 \(\upmu\)m. [PERSON] et al. [22] assessed the feasibility of using highly absorbent polymers (SAPs) to enhance the self-healing properties of microbial-induced calcium carbonate precipitation (MICP) in cementitious materials and reported that the incorporation of SAP into the MICP system reduced the MICP bacterial content without affecting the self-healing properties. [PERSON] et al. [23] used a fluidized bed-spraying process to coat highly absorbent polymers and characterized the encapsulated SAP to determine its solubility and quantify the delayed effect of water and Ca(OH)\({}_{2}\) solution absorption. Their results revealed that the swelling of mortar samples was substantially delayed, though this effect was brief. Importantly, the SAP film coating did not hinder the mortar's self-sealing efficiency. [PERSON] et al. [24] reported that fibers containing SAP admixtures enhanced the self-healing ability of concrete, confirming the potential value of this material in crack sealing.