[This corrects the article DOI: 10.1016/j.heliyon.2023.e22277.].

This study aims to investigate the evolution patterns of fluidity and rheological properties of AASCM under varying dosages of foaming agent and particle sizes of filling aggregate. The flow characteristics of AASCM are significantly affected by the filling aggregate's size and the foaming agent's dosage. Specifically, an increase in filling aggregate size (D(4,3) ϵ [26 μm, 69 μm]) enhances the fluidity of foamed AASCM, while an increase in foaming agent dosage reduces fluidity. These observed variations can be attributed to the presence of particle voids, the specific surface area of the aggregate, as well as the quantity and spatial distribution of bubbles within the slurry. A bubble-particle packing model is established, and by calibrating the simulation error coefficient to 1.1, the study investigates the evolution of water film thickness (WFT) in foamed AASCM with slurry expansion degree. It is observed that bubbles in the slurry affect the fluidity by altering the overall compactness and specific surface area of the foamed slurry, subsequently modifying the WFT.

This study aims to investigate the weakening of CO2-fly ash-based backfill (CFBF) material in cation water. Weakening is determined in an order of Na+ > Ca2+ > Mg2+ by slake durability and mechanical properties analysis. Na+ damages the microstructure of CFBF through the ion exchange phenomenon, causing the ejection of tetrahedral calcium. In Ca2+ and Mg2+ water, N-M-A-S-H gel is precipitated and filled in openings via a two electrical-layer mechanism, which mitigates the materials from further deterioration. On this basis, the suitable cation type and concentration range are proposed as criteria to implement CFBF in mine-water affected backfill coal mining.

The investigation of rock damage behavior and fracture precursor recognition are important parameters for ensuring stability and safety prediction in rock engineering. In this paper, the Damage Infrared Response Matrix (DIRM) of increasing stress in rock is defined to separate the Infrared Radiation (IR) temperature increment caused by crack development from the temperature field of the rock surface. On this basis, a new index, Infrared Energy Response of Damage (IERD), is proposed to describe the crack evolution state and recognize the fracture precursor within the rock. It is found that the IERD remains constant in the compaction and elastic stages, rises gradually in the stable crack propagation stage, and increases sharply in the unstable crack propagation stage, which is consistent with the changing trend of AE signals. The change characteristics of IERD in stable and unstable crack propagation stages can be regarded as initial and critical precursors for rock fracture, which are 67.1% and 84.1% of the peak stress, respectively. Then, the IERD is considered as a damage factor to establish a new theoretical characterization of the damage variable for rock materials. The damage variable based on the IERD (D-IR) can accurately describe the damage evolution process of rock under uniaxial compression. Finally, a statistical damage constitutive model of rock modified by D-IR is established, which gives the coefficient of performance of more than 0.94 and verifies the rationality of D-IR. The research results can provide a theoretical and experimental basis for monitoring potential rock engineering instability using IR.

The environmental concerns resulting from coal-fired power generation that produces large amounts of CO2 and fly ash are of great interest. To mitigate, this study aims to develop a novel carbonated CO2-fly ash-based backfill (CFBF) material under ambient conditions. The performance of CFBF was investigated for different fly ash–cement ratios and compared with non-CO2 reacted samples. The fresh CFBF slurry conformed to the Herschel-Bulkley model with shear thinning characteristics. After carbonation, the yield stress of the fresh slurry increased significantly by lowering fly ash ratio due to gel formation. The setting times were accelerated, resulting in approximately 40.6% of increased early strength. The final strength decreased when incorporating a lower fly ash ratio (50% and 60%), which was related to the existing heterogeneous pores caused by rapid fluid loss. The strength increased with fly ash content above 70% because additional C–S(A)–H and silica gels were characterized to precipitate on the grain surface, so the binding between particles increased. The C–S(A)–H gel was developed through the pozzolanic reaction, where CaCO3 was the prerequisite calcium source obtained in the CO2-fly ash reaction. Furthermore, the maximum CO2 uptake efficiency was 1.39 mg-CO2/g-CFBF. The CFBF material is feasible to co-dispose CO2 and fly ash in the mine goaf as negative carbon backfill materials, and simultaneously mitigates the strata movement and water lost in post-subsurface mining.

Static segregation of coal gangue-fly ash backfill (CGFB) material presents a significant impact on its mechanical performance for underground support. To resolve, a novel formulation was addressed using sodium silicate (SS) and CO2 as co-activator. Its setting behaviors and mechanical properties were investigated with respect to the coal gangue content, CO2 influx and the concentration of sodium silicate solution. The microstructure was characterized by SEM, EDX, XRD, and FTIR. The present method lowered the initial and final setting times to approximately 90% and 74% comparing to which of conventional activator. The compressive strength increased from 2.06 to 10.23 MPa with coal gangue ratio of 3.2 after 56 days curing. This mainly results from the mitigation of the effect of segregation through the generation of silica gel, which precipitated on the grain surface. The silica gel promoted the interparticle binding and rapid consistency, thus preventing gangue from settlement. Incorporating the microscale crystalline phase characterization, the carbonate products work as the filling particle and the coal gangue presents as the reinforcement after hardening, leading to the significant increase in material strength. This method not only ensures safe disposal of coal gangue and fly ash from segregation, but also mitigates overburden deformation and promotes CO2 utilization. Therefore, the coordinated development of coal resource development, environmental protection, and carbon footprint reduction is realized.

The consumption of coal resources has caused an increase in CO2 emissions. A scientific concept that can realize CO2 sequestration, the harmless treatment of solid wastes, and coal extraction under buildings, railways, and water bodies (BRW) is proposed. First, a novel CO2 mineralized filling body (CMFB) is developed by employing CO2 gas, fly ash, silicate additives, and cement. It is then injected into the mined-out mining roadways (MRs) of the continuous extracting and continuous backfill (CECB) mining method to ameliorate the overburden migration and thus extract the coal body under the BRW. The AHP-fuzzy comprehensive evaluation method was employed to construct a prediction model for the suitability of this concept. Subsequently, the evaluation model is generalized and applied to the Yu-Shen mining area. Each indicator affecting adaptability is plotted on a thematic map, and the corresponding membership degree is determined. The aptness for 400 boreholes distributed in the entire area was determined and a zoning map which divides the whole area into good, moderate, slightly poor, and extremely poor suitability was drawn. This paper puts forward a mathematical model for predicting the suitability of using CECB and CMFB to sequestrate CO2. Research results can provide references for determining the site of CO2 sequestration under the premise of maximizing the economic and ecological benefits, which is conducive to constructing ecological, green, and sustainable coal mines.

Coal mining under the aquifer in Northwest China has brought a series of ecological problems, such as the decline of groundwater levels and the death of surface vegetation. The study of the impermeability of rock strata between coal seams and the overlying aquifers is of great significance to solve these problems and realize water-preserving coal mining (WPCM). Based on mining-induced overburden damage and permeability deterioration, the concept of the “three seepage zones” of overburden is proposed, namely the pipe flow zone, water seepage zone and nominal water-seepage-resistance zone (NWSRZ). Meanwhile, the concept of water-seepage-resistance strata (WSRS) is put forward from the aspects of initial permeability, structural strength, swelling and the stratigraphic structure of the overlying strata. AHP-fuzzy comprehensive evaluation (AHPF) is employed to construct a model to evaluate the water-blocking capacity (WBC) of the WSRS. The model includes three secondary factors and nine tertiary indicators, and the weights and membership functions of the indicators are determined. Subsequently, the model is generalized and applied to the Yu-Shen mining area. The membership degrees are spatially visualized by means of thematic maps. The comprehensive evaluation values Φ of the WBCWSRS of 400 boreholes in the mining area under backfill mining, narrow strip mining, slice mining and longwall cave mining are calculated. Then, the Kriging method is employed to plot the zoning maps of Φ under four different mining methods. In view of different grades of WBCWSRS, three corresponding countermeasures, i.e., mining methods optimization, curtain grouting and underground reservoir construction, are put forward. The fluid–solid coupling embedded in FALC3D software is employed to establish a numerical calculation model to simulate the water table fluctuation of the underground aquifer under the four mining methods, and the reliability of the model is verified indirectly. In this paper, mathematical theory is combined with WPCM to develop an evaluation model of WBCWSRS, which provides a reference for the coordinated development of coal extraction and water resource preservation in arid and semi-arid mining areas.

This study aimed to prevent water flow in microcracks and simultaneously achieve CO2 capture during grouting (CCG). Using sodium silicate (SS) as the primary material, the microcracks were grouted by a two-step approach. The low-initial-viscosity (5 mPa s) SS was first saturated within the microcracks followed by CO2 injection at 2 MPa. Through CO2 dissolution, silica gel was developed and tolerated a hydraulic pressure of up to 5.5 MPa. The effects of aquifer ions (Na+, Ca2+, Mg2+, HCO3-, and SO42-) were equally evaluated at harsh conditions, and it was found that the strength of the silica gel was reduced, which was caused by salting out, low CO2 solubility, and precipitation. As a result, the hydraulic pressure was reduced to as low as 3 MPa. After 210 days, 16% of the silica gels (without ion inclusion) were reversible to the liquid phase, where a similar effect was found in the cases of Na+ and Mg2+ ions. The degradation increased with more Ca2+ ions (up to 55%) and decreased with more HCO3- and SO42- ions. Microcracks grouted with CCG extended the CO2 utilization in grouting application. Combined with the effect of dissolved ions, the proposed approach is feasible in the field implementation for underground engineering under water bodies.