Strata migration and fracture development under continuous extraction and continuous backfill with CO2 mineralized backfill materials

Abstract

CO₂ carbonation is currently restricted in laboratory instead of industrial application at ambient conditions. Meanwhile, few evaluations for safe storage have been made after CO₂ carbonation backfill. Herein, the continuous extraction and continuous backfill (CECB) with CO₂ mineralization backfill materials (CMBM) to storage CO₂ was proposed. The CMBM samples were prepared and the uniaxial compressive strength (UCS) and CO₂ uptake rates at various curing times and fly ash (FA)/gangue ratios were tested. The early and later strength at all ratios is more than 1 and 3.6 MPa, respectively, satisfying the requirements in underground backfill. A higher FA proportion means a higher UCS and a more significant effect of curing time on UCS as the hydration products of cement and FA contribute primarily to the early and later strength, respectively. As FA content rises, the CO₂ uptake rate increases from 3.55 to 4.25 mg-CO₂/g-CMBM since the alkaline oxides such as CaO in FA are higher than those in gangue. An analogue model was then constructed to simulate the overburden deformation. The ratio of the similar materials of CMBM at 7 d and F6G4 was determined to Water: Sand: CaCO₃: CaSO₄ of 3.56: 13.56: 0.94: 0.51. The maximum horizontal deformation of aquifuge is lower than the threshold value of 0.2–0.3 mm/m for preserving aquifer. The strain-softening parameters including cohesion, friction, dilation, and tensile strength were determined to be 0.54, 30°, 0, and 0 for UDEC simulation. The envelopes of water-conductive fractured zone (WCFZ) are saddle shaped, and the height of WCFZ is 4, 9.5, 17, and 26 m, respectively. After backfilling, there are still entire strata with thickness of 5 m between WCFZ and aquifer II. The research offers a novel way to dispose CO₂ gas, solid wastes and mitigate overburden deformation, which is conducive to geological disposal of energy wastes.

Article Highlights

The UCS and CO₂ uptake rates of CMBM were analyzed.

The key ratio of physical similar materials of CMBM w as determined

The key ratio of physical similar materials of CMBM w as determinedThe key ratio of physical similar materials of CMBM w as determined

Strata migration and fracture development was illustrated under CECB with CMBM.

Introduction

The exploitation and consumption of coal resources have effectively promoted economic and social development while it has also brought a series of problems, such as excessive emission of CO₂ gas and coal-based solid wastes, overburden destruction, and water inrush (Li et al. 2013; Liu et al. 2010; Przyłucka et al. 2022; Duo et al. 2022; Cai et al. 2023; Ma et al. 2022). The CO₂ emissions throughout the country in 2019 is around 10.8 million tons, 75% out of which is from coal combustion. Whereas, the annual CO₂ storage capacity is only 300 thousand tons, which is far less than the established dual carbon target (Yang et al. 2022; Lyu et al. 2023). Meanwhile, the coal-based solid wastes are primarily coal gangue and FA, and there are over 7 billion tons and 1 billion tons are stockpiled in China, respectively (Bian et al. 2007; Qiu et al. 2021; Ram et al. 2022). The spontaneous combustion and leaching process is liable to pollute the air, soil, and water bodies, which is detrimental to the local ecological environment (Karfakis et al. 1996).

The CO₂ mineral carbonation allows the CO₂ to react with calcium ions and magnesium ions in solid wastes to form carbonates for permanent storage, and is therefore the safest one to sequestrate CO₂ (Lim et al. 2010; Uliasz et al., 2017). This method combine CO₂ sequestration and solid wastes disposal together and thus has more advantages over other storage methods, such as geological storage and ocean storage (Hassan et al. 2009; Liu et al. 2021a, b; Liu et al. 2018, 2022; Xie et al. 2015). Domestic and foreign scholars have conducted in-depth researches regarding the CO₂ carbonation and have gained fruitful achievements. Whereas, these researches generally concentrate on high temperature and high pressure (Mo et al. 2016), high concentration of CO₂ (Chen et al. 2022), high-calcium FA (Wang et al. 2010), and tends to laboratory-scale (Ukwattage et al. 2015). This is inconsistent with the field application scenery.

To date, developing cementitious filling materials by using solid wastes, and pumping them into the mined-out area to support roof is of great prevalence among collieries (Liu et al. 2021a, b, 2022). Whereas, less literature has reported that the underground backfill involves CO₂ carbonation storage. Hence, it is necessary to prepare CMBM at ambient conditions for on-site applications. Moreover, the CMBM may suffer from weakening when encountering roof water, which requires that the storage space is dimensionally restricted. This reaches a good agreement with the features of CECB as its mining roadways (MRs) are limited and flexible (Xu et al. 2023, 2024; Xie et al. 2022). Herein, a new idea of using CECB and CMBM to realize trinity green mining of CO₂ carbonation storage, wastes treatment, and strata migration amelioration were proposed (Xu et al. 2022a; Ngo et al. 2023; Wang et al. 2022). To provide a safe site for CO₂ carbonization storage, it is essential to maintain the overburden integrity and prevent water-conductive cracks from connecting the commonly occurring overlying aquifer and storage site (Xu et al. 2022b).

The characteristics of strata movement and fracture development have been investigated in many scenarios, such as longwall caving mining of shallow, deep, and extremely thick coal seam (Liu et al. 2015; Shi et al. 2012; Zhang et al. 2013), the coal mining beneath gullies and reservoirs (Wang et al. 2016; Xu et al. 2010), longwall solid backfill mining (Li et al. 2020), short-wall block mining (Zhang et al. 2018), and upward slicing longwall-roadway cemented backfill mining (Deng et al. 2017). Whereas, less literature regarding the overburden migration and fracture development under CECB with CMBM are available since it is a novel method. The mechanical properties of CMBM and their impacts on the overburden also need further research. Herein, the CMBM samples will be developed at ambient conditions, and their mechanical properties and CO₂ uptake rates will be tested. Then, the ratio of the similar materials of CMBM will be determined to accurately simulate the strata migration by using physical analogue simulation. Additionally, the strain-softening parameters for CMBM is about to be determined for the simulation of fracture development using UDEC software. The core objectives of this paper are to sequestrate CO₂ gas and dispose FA and coal gangue in the premise of mitigating the overburden migration and fracture development to ensure the safety of storage space. The research findings offer references for CO₂ carbonation backfill beneath aquifer and make significant contributions for the geological disposal of energy wastes such as CO₂ sequestration.

CECB for CO₂ sequestration and strata migration control

The three-dimensional map of using CECB and CMBM to dispose CO₂ gas and solid wastes, and ameliorate overburden migration is shown in Fig. 1. Prior to extraction, the entire mining panel is divided into many MRs along the strike direction, and then the MRs are classified into four mining phases. With full consideration of the width of the continuous shearer, the moving of working face, the roof subsidence before filling, the span of MR usually ranges from 4 to 7 m while the length is less than 150 m. The procedures of coal extraction and CMBM backfill are illustrated in Fig. 2.

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Fig. 1

Three-dimensional conceptual diagram of CECB

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Fig. 2

Sketch map of the mining and filling procedure of CECB

To overcome the mutual restrictions between filling and mining, the skip mining was employed to extract the MRs in one mining phase to realize simultaneous extracting and filling. The MR was sealed and backfilled with fresh CMBM immediately it was extracted for the sake of controlling strata movement. After all MRs in the first phase were mined and backfilled, the extraction and backfill of MRs in the second phase starts until all MRs in four mining phases were fully extracted and backfilled. Eventually, the coal mass was completely replaced by CMBM.

Sample preparation and basics properties of CMBM

CMBM development and experimental schemes

The CMBM samples were developed using coal gangue as aggregate, and CO₂ gas, FA, sodium silicate, and cement as accessories. The FA acted as a substitute for part of cement as a cementitious material. The ordinary Portland cement (P.O42.5) was acquired from Yangchun Co., Ltd. The Class F FA was collected from a thermal power plant in Zhengzhou City, Hennan Province, China. The coal gangue was acquired from Xutuan Coal Mine in Bozhou City, Anhui Province, China. The control variable method was employed and the ratio of cement to solid mass was set to 20 wt%, and the F/G ratio was set to F2G8, F4G6, F6G4, and F8G2, respectively. It was observed that the concentration of Na₂SiO₃ at 10 wt% was insufficient to prevent gangue particles whose diameter are greater than 1 mm from sinking. Therefore, the concentration was determined to 15 wt%. To ensure that CO₂ gas can fully contact with the slurry through bubbling, good fluidity of fresh slurry is necessary. Hence, the solid–liquid ratio was designed to 2:0.8. The experimental schemes were listed in Table 1.

Table 1. Experimental schemes

No	Ratio	Solid proportion (Cement: FA: Gangue)	Proportion of Na2SiO3 solution (wt%)	Solid–liquid ratio	CO2 time (min)	Curing time (day)
1	F2G8	2:1.6:6.4	15	2: 0.8	20	3/7/14/28/56
2	F4G6	2:3.2:4.8	15	2: 0.8	20	3/7/14/28/56
3	F6G4	2:4.8:3.2	15	2: 0.8	20	3/7/14/28/56
4	F8G2	2:6.4:1.6	15	2: 0.8	20	3/7/14/28/56

F2G8 denotes that the proportion of FA is 20 wt% of the total of both FA and gangue

The process of CMBM samples preparation and relative tests are shown in Fig. 3. First, weigh solid raw materials, including FA, cement, and coal gangue, and place them in a container to mix them evenly. Meanwhile, a 15 wt% Na₂SiO₃ solution was prepared by heating in a water bath. Then, pour the dissolved solution into the raw materials and stir continuously until a uniform paste was formed. The fresh slurry was then poured into a reactor and was stirred at 500 rpm while CO₂ gas was introduced for 20 min at a constant rate of 1 L/min. Subsequently, the mortar was decanted into standard molds with a dimension of φ × h of 50 × 110 mm and the samples were placed on a vibration table to expel and discharge the internal CO₂ bubbles. 24 h later, the demolding was implemented and the samples were put in the curing box (SHBY-40B) with temperature of 20 °C and humidity of 98%. The specimens were cured for 3, 7, 14, 28, and 56 d, respectively. The UCS experiment and thermogravimetric analysis were then carried out. The chemical reaction between CO₂ gas and fresh slurry is presented in Fig. 4.

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Fig. 3

Preparation and test processes of CMBM

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Fig. 4

Chemical reaction between CO₂ gas and fresh slurry

Unconfined compressive strength of CMBM

UCS of CMBM

The UCS is the core index of CMBM as it is the primary body to support the overlying strata. A weak strength may lead to the instability of CMBM itself and thus contributes to its incapability of supporting roof. Under the regulation of national standard GB/T1761-1999, the WAW-1000D electro-hydraulic servo motor test system was employed to carry out the UCS test of CMBM at different curing times and F/G ratios. All UCS tests were repeated three times, and the average UCS values were presented in Fig. 5.

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Fig. 5

UCS of CMBM at different curing times and F/G ratios

As curing time rises, the UCS also increases. For F2G8, the UCS at 56 d is 3.57 MPa, which is 179% higher than that of 1.28 MPa at 3 d. Moreover, the F4G6, F6G4, and F8G2 have increased by 217%, 273%, and 350%, respectively. A larger FA proportion indicates a more significant influence of curing time on UCS. In the early stage, the cementitious materials from hydration reaction are minor, contributing to a limited bonding effect. As age increases, it gradually grows and can bond the particles together and thus results in a higher strength. Meanwhile, more cementitious materials are produced as FA dosage ascends, thereby improving the UCS significantly. At same age, a positive correction exists between FA dosage and UCS, and the strength improvement is more notable in the later stage. At 56 d, the UCS of F8G2 is 211% higher than that of F2G8, which is higher than that of 130% at 3 d. The fine FA also acts as fine aggregate, which is conducive to improving the poor gradation of gangue and filling the pores between particles and hydration products. As FA content grows, it plays a stronger role in dense filling of fine powder, thus enhancing the strength.

The hydration reaction of cement, which is carried out quickly and in large quantities during the early stage, contributes primarily to the early strength. This results in small differences in UCS at various ratios as the cement proportion is constant. In contrast, the later strength is mainly provided by the hydration products of FA. As FA content rises, the amounts of C-S(A)-H and silica gel also grow. Meanwhile, as the gangue proportion decreases, the pores also diminish accordingly. The generated cementitious materials can fill almost all pores and bond the aggregates together, leading to a larger strength difference at different ratios.

The strength development of CMBM at various ratios are illustrated in Fig. 6.

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Fig. 6

Strength development of CMBM

As shown, the specimen at F2G8 and 3 d has already achieved 36% strength, while the F8G2 has only developed 22%. From F2G8 to F8G2, the strength developments at other ages show a decreasing trend, which is notably different from that of backfill without CO₂ gas (Qi et al. 2015). The addition of CO₂ can significantly enhance the strength, especially for the early strength. The main reason is that the added CO₂ reacts with Na₂SiO₃ to generate silica gel and thus boosts the strength. Moreover, the CO₃²⁺ obtained after CO₂ dissolution will react with Ca²⁺ to form CaCO₃. The concentration of Ca²⁺ decreases, promoting the continuous migration of Ca²⁺ generated by hydration reaction of cement clinker into the slurry. This accelerates the hydration reaction of cement, and thereby improving the early strength. The obvious effects of early strength enhancement means that the CMBM can play a faster role in supporting roof after being injected into the mined-out area.

To reduce the risk of liquefaction and maintain the stability of CMBM itself in early age, the UCS at 3 d is suggested to be higher than 0.3 MPa. Meanwhile, the UCS at 56 d shall be greater than 4 MPa to support the roof effectively. It can be found that CMBM at all ratios satisfy the requirements of early strength while F2G8 is lower than threshold values of late strength (Yu et al. 2017).

Specimen failure form

The damaged CMBM samples at all ratios and cured for 3 and 56 d were selected for failure morphology analysis (Fig. 7). It is observed that the splitting failure dominates the failure mode, and the main crack of the specimen at 56 d is more notable than that of 3 d. In the process of uniaxial compression, a longitudinal main crack that runs through the samples generate and expands laterally due to the tensile stress (Liu et al. 2023). Meanwhile, as the main crack expands, secondary cracks are generated, causing the poorly cemented part of the sample to start peeling off. In this context, although the sample has a certain degree of integrity, its load-bearing capacity decreases significantly. Furthermore, the main fracture at F2G8 did not develop to the lower part. The gangue dosage is large and the limited cementitious materials in the early stage is insufficient to support the suspended gangue aggregates and give rise to sink and segregate may account for this.

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Fig. 7

Physical diagram of failure form of CMBM sample

CO₂ absorption rates of CMBM

The CO₂ uptake rate at ambient conditions is significant as it determines the efficiency of CO₂ mineralization carbonation in the field applications of underground backfill. The grinding test block powders at 56 days and different ratios were heated at a rate of 10 ℃/min from 20 to 1000 °C, with an airflow rate of 200 mL/min. The TG curves of CMBM are demonstrated in Fig. 8.

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Fig. 8

TG image of CMBM under different F/G ratio

From the ambient temperature to 100 °C, the residual moisture evaporating accounts for mass loss. At a higher temperature between 300 and 500 °C, the mass loss is due to the decomposition of Ca (OH)₂. The mass loss from 500 to 850 °C is attributed to the decomposition of CaCO₃. According to the TG test results of CMBM, the CO₂ uptake rates can be calculated:

1

$$W=10·(W_1-W_2)M_1\left./\phantom{10·(W_1-W_2)M_1{M}_2}\right)M_2$$where W is the CO₂ uptake rate of CMBM; W₁ and W₂ is the contents of CaCO₃ in CMBM and N-CMBM (No CO2 introducing), respectively; M₁ and M₂ is the molar mass of CO₂ and CaCO₃, respectively.

The CO₂ mineralization rates at different ratios is shown in Fig. 9. As FA proportion increases, the CO₂ absorption rate have grown from 3.55 to 4.25. The main reason is that the alkaline oxides such as CaO in FA are higher than those in gangue.

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Fig. 9

CO₂ uptake rates in CMBM with various F/G ratios

Strata migration characteristic of CECB with CMBM

Main parameters of analogue simulation

The scale effect, which is also known as satisfaction degree of between the physical similarity model and the prototype, is vital for the successful analogue simulation (Deng et al. 2023). Based on the principles of similarity theory, an analogue model with dimension of 2.5 × 0.2 × 1.07 m (length × width × height) was constructed to simulate the overburden migration (Fig. 10). The geometrical ratio was determined to 1:100. The model is divided into three mining phases, with 9 MRs in each phase. The extraction and backfill are carried out simultaneously. The dimension of each MR is 5.5 × 5.0 m (width × height). The material mixture ratio of coal seam and strata in physical similar model is shown in Table 2. Seven monitoring lines were set in the front of the model. The lowest two lines consists of 47 measuring points to measure the deformation of immediate roof and basic roof accurately. The top five lines are comprised of 24 points to monitor the migration of the overlying strata, aquifuge, and surface. The displacement of each measuring point was recorded by the 3D Photogrammetry System.

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Fig. 10

Physical analogue simulation mode and the measuring points arrangement

Table 2. Material ratio of strata and coal seam

Number	Strata	Thickness (cm)	Calcium carbonate (kg)	Gypsum (kg)	Water (kg)	Sand (kg)	Total (kg)
1	Clay	50	19.66	7.56	32.76	252.52	312.5
2	Sandy mudstone	9	5.26	2.32	6.65	52.74	66.96
3	Siltstone	10	6.25	2.72	8.97	53.49	71.43
4	Mudstone	12	7.78	3.3	8.57	66.07	85.71
5	Siltstone	8	5.0	2.18	7.17	42.79	57.14
6	Arkose	9	8.12	3.54	11.66	69.54	92.86
7	Siltstone	3	1.88	0.81	2.69	16.05	21.44
8	Coal seam	5	1.89	0.8	2.08	16.06	20.83
9	Arkose	5	3.38	1.47	4.86	28.97	38.69

Determining the ratio of similar materials of CMBM is crucial for the accurately simulation of strata movement as the backfill restricts the roof subsidence. As previous research results show, the strength of filling body is lower than that of rock, so the binder proportion in the similar materials is relatively low (Ye et al. 2018). The water, river sand, calcium carbonate, and gypsum were employed as raw materials to prepared the standard cylindrical samples with size of 50 × 100 mm (φ × H). The mixture ratios are listed in Table 3.

Table 3. Ratio of similarity materials for CMBM at 7 d and F6G4 ratio

Raw materials	1#	2#	3#	4#	5#
Water	3.14	3.42	3.56	3.64	3.71
Sand	13.05	13.24	13.56	13.65	13.77
CaCO3	1.14	1.06	0.94	0.87	0.84
CaSO4	0.72	0.65	0.51	0.44	0.41

Adjusting the ratios until the curves of similarity materials reach good agreement with that of CMBM. The stress–strain curves of specimens at five ratios were tested and compared to that of CMBM at 7 d and F6G4, as shown in Fig. 11. It was observed that the 3 # similar materials were the most ideal option. After each MR is extracted, the similar material was immediately injected into the goaf to simulate the actual backfill of CMBM.

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Fig. 11

Comparison of stress–strain relationships of similar filling materials and CMBM

Results of aquifuge and surface deformation

After all MRs are extracted and backfilled, there are only few vertical fractures occurring across the immediate roof. Upward, the overlying strata are still intact without any cracks. The maximum subsidence of 7 lines during three mining phases are demonstrated in Fig. 12. After the first mining phase are extracted and backfilled, the maximum subsidence of the lowest three surveying lines is 8, 7, and 4.1 mm, respectively. From line 4 to line 7, the vertical displacement is too minor to detect as the long distance between CMBM storage space and the measuring points. The maximum value of each line is 19, 14, 13, 10, 8, 6, and 5 mm, respectively as the completion of second phase, showing an exacerbating mining-disturbance. After all coal mass in the site of CO₂ carbonation backfill is substituted with CMBM, the maximum subsidence from bottom to top line is 93, 84, 73, 55, 44, 41, and 32 mm, respectively. The overburden migration aggravates further.

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Fig. 12

Maximum subsidence of each survey line in each mining phase

The integrity and stability of the aquifuge (clay) is crucial for the space safety of CO₂ carbonation storage as it can prevent overlying water resources from penetrating into the backfill site. The subsidence, tilt, and curvature of aquifuge are presented in Fig. 13. As illustrated, the maximum value of the three indicators is 32 mm, 0.71 mm/m, and 0.059 mm/m², respectively. Moreover, the horizontal indexes including the horizontal displacement and horizontal deformation are too low to measure. It is universally recognized that the threshold of horizontal deformation to maintain the integrity of aquifuge is less than 0.2–0.3 mm/m. The weak stratum is prone to being damaged and loses its water-seepage-resistance ability if the value is more than the suggested one. The horizontal indexes meet the requirements for water preservation in the process of CO₂ mineral carbonation backfill. Additionally, as the grade I building damage of China stipulates, the tilt and curvature for surface should be less than 3.0 mm/m and 0.2 mm/m². It is obviously that the maximum values of the two indexes of ground are far less than that of specified. Hence, the CECB with CMBM at 7 d and F6G4 ratio can effectively ameliorate overburden migration and surface subsidence simultaneously.

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Fig. 13

The deformation curves of three indicators of the aquifuge (clay)

Strata fractures development characteristics of CECB with CMBM

Construction of UDEC numerical calculation model

The universal discrete element code (UDEC) was used to simulate the fractures development under CECB with CMBM. The overlying layers with similar lithology were merged into 13 strata from the immediate floor to ground surface. With full consideration of full mining and boundary effect, the dimension of the model is designed as 300 × 107 m (X × Y), as shown in Fig. 14. The length of CECB face is 120 m and 90 m protective coal pillars on its two sides were left unmined. The displacement and velocity of the left, right, and bottom boundaries of the model are fixed while the top boundary is set as free surface. The Mohr–Coulomb model was selected for the strata and coal seam. The virtual vertical joints are set every 6 m in the coal seam, and the MRs in four phases were grouped. The “null” code is used to simulate the excavation of coal body. After the calculation of extraction is completed, the goaf is assigned to the strain-softening model for next calculation to truly reflect the support of CMBM on the overlying strata. Subsequently, the next MR is extracted at an interval of 24 m (the width of four MRs), until all MRs at four mining phases are mined and backfilled with CMBM.

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Fig. 14

Numerical calculation model of CECB

To accurately determine the mechanical parameters of coal seam and strata, on-site samplings were carried out for UCS test. Meanwhile, the numerical simulation of UCS test of rocks with different lithology were implemented. The mechanical parameters of overlying strata were preliminarily assigned based on the experience of reference. The stress–strain curve and the failure form of each stratum from indoor test are compared with that from numerical simulation. If the indoor test results reach good agreement with that of numerical simulation, the assigned parameters are final. Otherwise, the parameters of blocks and joints in the numerical simulation model are adjusted continuously until the results are consistent with each other. The mechanical parameters of the block and joint are listed in Tables 4 and 5, respectively (Qin et al. 2023).

Table 4. The mechanical parameters of strata and coal seam (block)

No	Stratum	Bulk modulus (GPa)	Shear modulus (GPa)	Cohesion (MPa)	Friction angle (°)	Tensile strength (MPa)	Density (kg/m3)
1	Sandstone	12.2	10.8	2.5	42	3.6	2520
2	Clay	0.28	0.09	2.0	25	0.9	1900
3	Mudstone	13.5	11.7	1.3	23	1.0	2200
4	Coarse sandstone	15.3	8.3	2.4	31	1.6	2600
5	Sandy mudstone	12.3	10.5	1.5	22	1.3	2260
6	Siltstone	10.8	8.1	2.8	38	1.8	2510
7	Fine sandstone	21.1	13.5	3.2	42	1.3	2540
8	Coal seam	2.5	1.7	1.7	28	1.5	1400

Table 5. The and mechanical parameters of strata and coal seam (Interface)

No	Stratum	Normal stiffness (GPa)	Shear stiffness (GPa)	Cohesion (MPa)	Friction angle (°)	Tensile strength (MPa)
1	Sandstone	7	5	8	14	5.7
2	Clay	3	2	2	18	1.0
3	Mudstone	9	7	2	10	1.8
4	Coarse sandstone	6	5	5	25	5.4
5	Sandy mudstone	6	4	4	10	3.2
6	Siltstone	10	8	7	20	4.3
7	Fine sandstone	8	6	6	13	6.3
8	Coal seam	4	2	3	15	1.2

Determination of strain-softening parameters for CMBM

The constitutive models of various filling materials also vary in the process of UDEC simulation. Generally, the double-yield model is used for crushed gangue backfill, while the strain-softening model is employed for paste backfill. For CMBM at various curing times, the stress drops gradually after the peak point. This is consistent with the characteristics of strain-softening model. Whereas, differing from the ideal model, the CMBM samples cured less than 7 days present a concave while those with a longer setting time are a convex. This model is based on the Mohr–Coulomb model and the difference between them lies in the possibility that the cohesion, friction, dilation, and tensile strength may soften as the onset of plastic yield while those properties are assumed to remain constant in the latter (Fig. 15).

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Fig. 15

Mohr–Coulomb model and three types of strain-softening model

1. Yield criterion.

The yield criterion for this model corresponds to a Mohr–Coulomb criterion with tension cutoff.

2

$$f^s={\sigma }_1-{\sigma }_3\left[1+sin\left(\phi \right)\right]\left./\phantom{\left[1+sin\left(\phi \right)\right]\left[1-sin\left(\phi \right)\right]}\right)\left[1-sin\left(\phi \right)\right]+2c{\left\{\left[1+sin\left(\phi \right)\right]\left./\phantom{\left[1+sin\left(\phi \right)\right]\left[1-sin\left(\phi \right)\right]}\right)\left[1-sin\left(\phi \right)\right]\right\}}^{\frac{1}{2}}$$

3

$$f^t={\sigma }^t-{\sigma }_3$$where φ is the friction; c is the cohesion; σ^t is the tensile strength; σ₁ and σ₃ are the maximum and minimum principal stresses.

From Eqs. (2), it is found that the strength of CMBM depends on its cohesion and friction. In the elastic stage, c and φ are constants. As the onset of plastic stage, the material structure is damaged. The two parameters drop as the damage degree increases, resulting in a decrease in strength. This process continues until it enters the residual deformation stage. The strain softening behavior is realized through the process of c and φ decreasing from peak strength to residual strength.

1. Strain-softening model.

In this model, plastic parameters are introduced to describe the degree of CMBM failure, and plastic shear strain is generally employed:

4

$${\gamma }^p={\epsilon }_1^p-{\epsilon }_3^p$$where ${\epsilon }_1^p$ and ${\epsilon }_3^p$ represent the maximum and minimum plastic principal strains, respectively. In UDEC, the incremental form of plastic shear strain was adopted and defined as:

5

$$\mathrm{\Delta }{\epsilon }^{\mathit{ps}}={\left[\frac{1}{2}{\left(\mathrm{\Delta }{\epsilon }_1^{\mathit{ps}}-\mathrm{\Delta }{\epsilon }_m^{\mathit{ps}}\right)}^2+\frac{1}{2}{\left(\mathrm{\Delta }{\epsilon }_m^{\mathit{ps}}\right)}^2+\frac{1}{2}{\left(\mathrm{\Delta }{\epsilon }_3^{\mathit{ps}}-\mathrm{\Delta }{\epsilon }_m^{\mathit{ps}}\right)}^2\right]}^{\frac{1}{2}}$$where $\mathrm{\Delta }{\epsilon }_m^{\mathit{ps}}=1\left./\phantom{13}\right)3\left(\mathrm{\Delta }{\epsilon }_1^{\mathit{ps}}+\mathrm{\Delta }{\epsilon }_3^{\mathit{ps}}\right)$; $\mathrm{\Delta }{\epsilon }_j^{\mathit{ps}}\left(j=1,3\right)$ are principal plastic shear strain increments, which is associated with plastic shear strain ${\gamma }^p$. There exists a relationship between them as dilation is constant:

6

$${\epsilon }^{\mathit{ps}}=\frac{1}{\sqrt{3}}\sqrt{\frac{\left(1+N_{\psi }+N_{\psi }^2\right)·{\left({\gamma }^p\right)}^2}{{\left(1+N_{\psi }\right)}^2}}$$where $N_{\psi }=\left(1+sin\psi \right)/\left(1-sin\psi \right)$ and $\psi$ is the dilation, °.

The previous findings have shown that the confining pressure exerts a great influence on the dilatancy of filling bodies. As confining pressure ascends, the dilatancy tends to minimize. When it is large enough, the dilatancy basically disappears and dilation equals to 0. The CMBM in each mining phase stays in three-dimensional stress environment and the confining pressure from its surrounding coal body or CMBM is large enough. Hence, the dilation can be set to 0 and Eq. (6) can be simplified as:

7

$${\epsilon }^{\mathit{ps}}={\gamma }^p/2$$

To describe the strain-softening process where the CMBM strength gradually decreases as the damage degree grows, it is necessary to establish a relationship between the strength parameters c and ϕ and the plastic parameter ${\epsilon }^{\mathit{ps}}$.

The field experience shows that it takes 2 days for coal extraction and 1 day for CMBM backfilling of each MR. Taking CECB with 4 mining phases as an example, the distance between the MR being extracted and the MR backfilled 7 days ago is usually greater than 30 m. Consequently, the CMBM in the MR that is backfilled 7 days ago is not affected by the current mining. It only bears the static load from the overlying strata, and its mechanical environment tends to be stable. In this context, the CMBM compression reaches the maximum value. The CMBM at F6G4 and 7 d is therefore employed for numerical simulation.

Here, the cohesion and friction are defined as functions of softening parameters for measuring plastic shear strain. With reference to previous literature, two exponential functions of cohesion and friction with plastic shear strain were modified and obtained, respectively (Yin et al. 2022):

8

$$c_f=c^{\prime }\times exp\left(\alpha ·{\epsilon }^{\mathit{ps}}\right)$$

9

$${\phi }_f={\phi }^{\prime }\times exp\left(\beta ·{\epsilon }^{\mathit{ps}}\right)$$where c_f and c’ is the post-peak and pre-peak cohesion, MPa; φ_f and φ’ is the post-peak and pre-peak internal friction angle, °; α and β are the fitting coefficients to reflect softening law.

Based on the triaxial experiments results and previous findings, it has been determined that the cohesion of CMBM is 0.54 MPa and friction is 30°. The function curves between them with cumulative plastic shear strain are shown in Fig. 16.

[See PDF for image]

Fig. 16

Variation of cohesion and friction with cumulative plastic strain

Evolution law of overburden fractures under CECB with CMBM

Based on the hydrogeological conditions of the study area, there are two aquifers above the CO₂ carbonation backfill site, i.e., the Quaternary Salawusu Formation phreatic aquifer (aquifer I) and bedrock fissure aquifer (aquifer II). The latter is only 31 m away from the storage space and belongs to the extremely close aquifer.

In the process of simulation, the “open” and “slip” code were used to plot the mining-induced fractures. The first one is intended to show two kinds of open cracks. The first type is the bed-separation fractures between two adjacent strata. It is attributed to the fact that the subsidence of the underlying stratum is larger than that of its upper stratum. The second kind is the open fracture developing and occurring inside the stratum. According to the strength theory, it is mainly caused by the tensile stress of the internal joints greater than its tensile strength.

Furthermore, the “slip” code is employed to draw slip fractures in the stratum. It is mainly due to the shear stress greater than its shear strength. It was observed that the open fractures are generally horizontal, while the slip cracks tend to develop in the vertical direction. The WCFZ is defined as the fractured overburden group where water can penetrate through and flow into the storage space. The water-flowing fractures are so-called connected fractures that act the hydraulic connecting channels between the overlying aquifer and the underground backfill site. Inside the WCFZ, the transverse fractures intersect the longitudinal fractures, or even if there is no connection, they are extremely close to allow water percolating freely under the effect of hydraulic pressure difference.

The fracture distribution characteristics after each mining phase are demonstrated in Fig. 17. As shown, after the first phase are extracted and backfilled, a small range of fractures are formed above the mined-out area of MR and the fracture envelopes are saddle shaped. The height of WCFZ is 4 m, which is as same as the mining height (H_c). As the second stage completes, the height develops to 9.5 m (2.4 H_c). The fracture envelopes are also saddle shaped but larger. As the coal in the third phase is replaced with CMBM, the value develops gradually to 17 m (4.3 H_c). The highest fracture is situated at the two boundaries of the CO₂ carbonation backfill space. In contrast, the fracture height in the central is only 9 m, 47% lower than that in the open-off cut and stopping line. When the project of CO₂ carbonation backfill is finished, the height of WCFZ is 26 m (6.4 H_c). In this context, there are still entire strata with thickness of 5 m between the top boundary of WCFZ and the aquifer II, indicating the water preservation can be realized.

[See PDF for image]

Fig. 17

Numerical simulation results of the fractures distribution in four mining phases

Conclusions and suggestion

Conclusions

A new concept of using CECB and CMBM to realize CO₂ carbonation storage, solid wastes disposal, and the mitigation of overburden migration and fracture development was proposed. The method has the practical significance and is a beneficial exploration for CO₂ mineral storage pathways. The conclusions drawn from the research are as follows:

1. The early strength and later strength of CMBM at all ratios are more than 1 and 3.6 MPa, respectively, which is higher than that of suggested values in underground backfill. A higher FA dosage results in a greater UCS and a more significant effect of curing time on UCS. The hydration reaction of cement contributes primarily to the early strength, which contributes to small differences in strength. In contrast, the later strength is mainly provided by the hydration products of FA. The amounts of cementitious materials, including C-S(A)-H and silica gel, grows with the rising FA dosage, contributing to a greater strength.

2. As FA proportion increases, the CO₂ absorption rates of CMBM with different ratios have grown 20%, from 3.55 to 4.25 mg-CO₂/g-CMBM, which is currently too low compared to that at high temperature and pressure. The main reason is that the alkaline oxides such as CaO in FA are higher than those in gangue, contributing to more production of CaCO₃ and thus improving the CO₂ uptake rates.

3. The ratio of the similar materials of CMBM at 7 d and F6G4 were determined to be Water: Sand: CaCO₃: CaSO₄ of 3.56:13.56:0.94:0.51. The results of physical analogue simulation show that: the maximum subsidence, tilt, and curvature of aquifuge is 32 mm, 0.71 mm/m, and 0.059 mm/m², respectively. Additionally, the horizontal deformation is lower than threshold of 0.2–0.3 mm/m for water-preserving in the process of CO₂ carbonation backfill. Additionally, the tilt and curvature for surface should be less than 3.0 mm/m and 0.2 mm/m², respectively, to protect the surface buildings and structures. It is obviously that the maximum values of the two indexes of ground are far less than that of specified. Hence, the CECB with CMBM at 7 d and F6G4 can ameliorate overburden migration and surface subsidence effectively.

4. The strain-softening parameters including cohesion, friction, dilatancy, and tensile strength for CMBM were determined to be 0.54, 30°, 0, and 0 for UDEC simulation. The results show that: the envelope of WCFZ during four mining phases are saddle shaped, and the height of WCFZ is 4 m, which is as same as the mining height (H_c). The heights of the last three mining phases are 9.5 m (2.4 H_c), 17 m (4.3 H_c), and 26 m (6.4 H_c). there are still entire strata with thickness of 5 m between the top boundary of WCFZ and the aquifer II, indicating the water conservation can be realized.

Suggestion

To date, the CO₂ uptake rate of CMBM at ambient conditions is too low compared to that at high temperature and pressure. Whereas, the on-site practice cannot maintain such strict conditions. Hence, the intended readers can further our study to try more alkali-activators to improve the CO₂ uptake rate at normal conditions in the future.

Acknowledgements

The authors would like to acknowledge support from the Scientific Research Foundation for High-level Talents of Anhui University of Science and Technology (2023yjrc89); the National Natural Science Foundation of China (51874280) and the Fundamental Research Funds for the Central Universities(2021ZDPY0211).

Author contributions

Y.X. conceived the research and wrote the original draft. L.M., Y.W. and J.Z. revised and reviewed the manuscript. Z.Z. was responsible for data curation. All authors have read and agreed to the published version of the manuscript.

Funding

Scientific Research Foundation for High-level Talents of Anhui University of Science and Technology, 2023yjrc89, National Natural Science Foundation of China, 51874280,Fundamental Research Funds for the Central Universities, 2021ZDPY0211

Data availability 

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Competing interests

The authors declare no competing interests.

Abbreviations

CECB

Continuous extraction and continuous backfill

CMBM

CO₂ mineralization backfill materials

WCFZ

Water-conductive fractured zone

MRs

Mining roadways

UCS

Uniaxial compressive strength

F/G

Fly ash/gangue

FA

Fly ash

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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