###### Abstract  
In China, groundwater loss caused by underground coal mining is becoming increasingly serious. The key to groundwater restoration is to repair mining-induced water-conducting fractures (WCFs) in the overlying strata. In this study, the adsorption-consolidation sealing characteristics of chemical precipitates were used to conduct permeability reduction (PR) experiments, including adding mixed CaCO\({}_{3}\) and Fe(OH)\({}_{3}\) to a sandstone specimen with a single fracture at room temperature. An aqueous solution of Na\({}_{2}\)CO\({}_{3}\) was used as the simulated groundwater, and a solution of mixed CaCl\({}_{2}\) and FeCl\({}_{2}\) was used as the repair reagent to simulate the water seepage conditions of a fractured rock mass. The two aqueous solutions were simultaneously injected into a single-fractured rock specimen at a constant flow rate. The experimental results show that the Fe(OH)\({}_{3}\) colloid encapsulated CaCO\({}_{3}\) crystals in a mixed precipitate, reducing the overall structural stability of the mixed precipitate and restricting repair and PR efficiency. However, the Fe(OH)\({}_{3}\) precipitate had better PR efficiency in the initial stage of the experiment. Therefore, a better scheme was put forward to repair the WCF, utilizing a mixed Fe(OH)\({}_{3}\) and CaCO\({}_{3}\) precipitate with a molar ratio close to 1:4 in the early stage and a single CaCO\({}_{3}\) precipitate in the later stage. Keywords:water-conducting fracture (WCF); mixed precipitate; permeability reduction (PR); restoration of aquifer ecological function; green mining +
Footnote †: journal: Applied sciences  
grouting methods boast excellent adaptability in fracture plugging and water inrush prevention in floor-based examples [11; 12], but they usually face difficulties in plugging WCFs in roof-based examples. Due to large-area coal mining, WCFs are widely distributed in roof rock strata, possessing large macroscopic openings and strong hydraulic interconnectivity. The grouting environment of roof rock strata is worse than the relatively closed trap environment around the floor of fractured rock masses (except at the grouting end of the floor; the fractures do not communicate with other free spaces). Consequently, injected slurry often rushes into the goat because of gravity flow, resulting in the \"grout runout\" phenomenon and, thus, a poor water-plugging effect [13]. In recent years, many studies have been conducted on organic polymer chemical materials that can easily \"reside\" in fracture channels [14; 15; 16; 17]. However, the high costs and toxic pollution problems caused by massive grouting are inevitable [18]. Therefore, it is particularly important to seek reliable technologies or materials suitable for efficiently plugging WCFs that develop in large overburdened areas. Many engineering studies [19; 20; 21] have found that chemical precipitates such as Fe(OH)\({}_{3}\) and CaCO\({}_{3}\) can form scales in fractures and other flow channels through \"adsorption-consolidation\". This phenomenon undoubtedly provides a way to repair and reduce the permeability of WCFs in overburdened areas. Therefore, an aquifer ecological function restoration method was proposed [22]. This method promotes the formation of precipitates and plugs WCFs by injecting repair reagents that can react with groundwater in the fractured rock mass. Specifically, preliminary experimental research was conducted to verify the feasibility of using chemical precipitates to plug WCF channels in mining rock mass. On this basis, the repair and PR effects of CaCO\({}_{3}\) and Fe(OH)\({}_{3}\) precipitates in the same fractured rock sample were compared and studied [23]. The results indicate that this CaCO\({}_{3}\) precipitate is typically crystalline, relying on seed adsorption and crystal growth to consolidate and nucleate. The formed consolidated body possesses a tight structure and good stability, and it can easily achieve good repair and PR effects in fractures.