1. Introduction

2. Materials and Methods

2.3. The properties and characteristics of materials

The following details the characteristics of the materials used in this study, such as aeolian sand, coal gangue, cement, fly ash and chemical admixture.

2.3.1. Aeolian sand

The samples were collected from near the Shiyaodian coal mine in Shenmu, Yulin, Shaanxi, China. The mineralogical analysis of the aeolian sand was carried out using an X-ray diffraction (XRD) spectrometer (Japan Rigaku SmartLab), as shown in Figure 2. The particle size analysis was performed using a particle size analyzer (NKT5200-HF), which showed that the particle size of the aeolian sand was mainly distributed between 100 μm and 300 μm in Figure 6. The chemical composition of the aeolian sand was analyzed by X-ray fluorescence (XRF) spectrometer, as shown in Table 1.

2.3.2. Coal gangue

The coal gangue samples were collected from Shenmu Shiyaodian coal mine, Yulin, Shaanxi, China, and the coal gangue was crushed below 3 mm by mechanical processing, and the particle size analysis was carried out using a particle size analyzer (NKT5200-HF), and the results showed that 50% of the coal gangue was less than 500 μm in size in Figure 7. The mineralogical analysis of the coal gangue was carried out using an X-ray diffraction (XRD) spectrometer (Japan Rigaku SmartLab), as shown in Figure 3, and the chemical composition of the coal gangue was analyzed using X-ray fluorescence (XRF) spectrometer, as shown in Table 2.

2.3.3. Fly ash

Fly ash samples were collected from the Shenmu Guohua Power Plant in Yulin, Shaanxi, China, and particle size analysis was performed using a particle size analyzer (NKT5200-HF), which showed that the fly ash particle size mainly ranged from 0 μm to 300 μm in Figure 8. The mineralogical analysis of the fly ash was carried out using an X-ray diffraction (XRD) spectrometer (Japan Rigaku SmartLab), as shown in Figure 4 and the chemical composition of fly ash was analyzed using X-ray fluorescence (XRF) spectroscopy, as shown in Table 3.

2.3.4. Cement

The cement was ordinary Portland cement 425# (P.O. 42.5), the bulk density is 3100 kg/m3,and the initial and final setting time are 165 min and 231 min respectively, and the compressive strengths at 3d and 28d are 18.4 MPa and 46.4 MPa respectively, and the particle size analysis was performed using a particle size analyzer (NKT5200-HF), and the results showed that the cement particle size mainly below 100 μm in Figure 9. The mineralogical analysis of the cement was carried out using an X-ray diffraction (XRD) spectrometer (Japan Rigaku SmartLab), as shown in Figure 5 and the chemical composition of the cement was analyzed using X-ray fluorescence (XRF) spectrometer, as shown in Table 4.

2.3.5. Chemical admixture

Master Glenium 7500 (Master G), a product of the chemical company, Badische Andilin und Soda Fabrik (BASF), was used as the plasticizer in the CAFB mix. It is a full range water reducing admixture generally used for workability improvements and early strength achievement. It meets the ASTM requirements for high range water reducing admixtures[26,35].

Figure 2. X-ray diffractogram of aeolian sand.

Figure 2. X-ray diffractogram of aeolian sand.

Figure 3. X-ray diffractogram of coal gangue.

Figure 3. X-ray diffractogram of coal gangue.

Figure 4. X-ray diffractogram of fly ash.

Figure 4. X-ray diffractogram of fly ash.

Figure 5. X-ray diffractogram of cement.

Figure 5. X-ray diffractogram of cement.

Figure 6. particle size distribution of aeolian sand.

Figure 6. particle size distribution of aeolian sand.

Figure 7. particle size distribution of coal gangue.

Figure 7. particle size distribution of coal gangue.

Figure 8. particle size distribution of fly ash.

Figure 8. particle size distribution of fly ash.

Figure 9. particle size distribution of cement.

Figure 9. particle size distribution of cement.

2.5. Specimen preparation and experimental methods

The CAFB’s ingredients i.e. solid concentration C_w%, aeolian sand dosage B_w%, coal gangue dosage D_w%, fly ash dosage E_w%, cement dosage F_w%, plasticizer dosage SP(%) and water content G_w% which are used insubsequent sections of this paper are calculated from the following equations(Eq.(2) to Eq.(8)):

C_(W%)=(100×M_(dry-solid))/(M_(dry-solid)+M_water )

(2)

B_(W%)=(100×M_(aeolion-sand))/(M_(dry-solid)+M_water )

(3)

D_(W%)=(100×M_(coal-gangue))/(M_(dry-solid)+M_water )

(4)

E_(W%)=(100×M_(fly-ash))/(M_(dry-solid)+M_water )

(5)

F_(W%)=(100×M_cement)/(M_(dry-solid)+M_water )

(6)

SP(%)=(100×M_sp)/M_cement

(7)

G_(W%)=(100×M_water)/(M_(dry-solid)+M_water )

(8)

Where M_water is mass of water in the paste fill; M_dry-solid is mass of aeolian sand, coal sand, Portland cement, and fly ash. M_aeolian-sand is mass of aeolian sand, M_cement is mass of Portland cement, M_coal-gangue is mass of coal gangue, M_fly-ash is mass of fly ash and M_sp is mass of plasticizer. The amount of aeolian sand and coal gangue are varied to prepare different mix composition as shown in Table 5.

Table 5. CAFB mixture proportions.

Table 5. CAFB mixture proportions.

CAFB code	1#	2#	3#	4#	5#	6#	7#
$C_{W\%}$	77.5	77.5	77.5	77.5	77.5	77.5	77.5
$B_{W\%}$	46	43	40	37	34	40	40
$D_{W\%}$	5	8	11	14	17	11	11
$SP\%$	0	0	0	0	0	0.05	0.1
$E_{W\%}$	17.5	17.5	17.5	17.5	17.5	17.5	17.5
$F_{W\%}$	9	9	9	9	9	9	9
$G_{W\%}$	22.5	22.5	22.5	22.5	22.5	22.5	22.5

Table 6. Characteristic index of particle size gradation of CAFB.

Table 6. Characteristic index of particle size gradation of CAFB.

CAFB code	d10/μm	d30/μm	d60/μm	d90/μm	d50/μm
1#	53.771	80.586	179.465	361.146	138.018
2#	51.263	78.586	207.453	449.321	151.538
3#	48.755	76.587	235.442	533.496	165.059
4#	46.247	74.587	263.43	617.671	178.579
5#	43.74	72.594	291.507	702.089	192.149

Note: d₁₀, d₃₀, d₆₀, and d₉₀ are the corresponding particle sizes when the cumulative particle size content reaches 10%, 30%, 60%, and 90%, respectively.

2.5.1. Vane test to determine yield stress and viscosity

In the following, the procedural steps for developing a slurry material for rheological evaluation are outlined.

Step 1. Initial mixing: The air-dried coal gangue are thoroughly mixed with certain proportion of aeolian sand, cement, and fly ash using spatula in a mixing container. Then specified amount of tap water followed by a certain proportion of SP is added and homogenized thoroughly using the electric and blender at high shear (+1000rpm) for 5min.

Step 2. Final mixing: The slurry was then loaded into a 500 mL beaker, and finally the container loaded with the slurry was assembled with the rheometer for subsequent rheological tests, with a four-blade rotor gradually entering the middle of the beaker and inserted with the slurry sample undisturbed for 30 seconds to allow the mixed slurry to reach its structural balance and at least partially recover its initial structure and strain state.

Step 3. Determination of yield stress and viscosity: For each sample, rheological curves describing the correlation between shear stress and shear rate were obtained experimentally. The rheological parameters describing the rheological properties of the slurry included yield stress and viscosity. The yield stress is the stress value at which the material just starts to yield to a state transition, and the shear rate of the rheological curve is from 0.01s-1 to 150s-1 with an interval of 0.05s-1. The Herschel-bulker rheological model was used to fit the measured rheological curves. In the fitted curves, the intersection of the shear stress curve with the longitudinal coordinate (longitudinal intercept) is the yield stress, the viscosity is the slope of the fitted curve, and the viscosity is the ratio of shear stress to shear rate.

Step 4. The above process was repeated for samples from the same batch of slurry to obtain different parameters i.e., 3 min, 30 min, 60 min after sample preparation. the hydration age was chosen to cover a wide range of transport times encountered during the slurry backfill tests. The shear test was performed using a controlled shear rate by placing the rotor in a 500 mL beaker for rheological testing, rotating it at a variable shear rate, dispensing the slurry several times and taking the average value for multiple measurements to eliminate experimental errors, and recording the corresponding shear stress and viscosity in real time. The test is repeated three times for each slurry sample to ensure reproducible results.

2.5.2. Microstructural analysis

To gain a more in-depth understanding of the effects of different factors on the cement hydration process in CAFB, microstructure analyses, including thermal analyses thermogravimetry (TG) and differential thermogravimetry (DTG), were conducted on the prepared cement fly ash paste (CFP) samples of the CAFB. The CFPs were prepared at a water / binder ratio of 1, cured at room temperature for 1 h and 2 h, respectively. Before testing was carried out, the specimens were dried in an oven-dried at 45°C until a constant mass was attained[37].

The thermal analyses were carried out by using a simultaneous thermogravimetric analyzer and differential scanning calorimeter (SDT) from TA Discovery TGA 550. The dried samples were placed on a plate and heated under an inert nitrogen atmosphere at a rate of 10℃/min from room temperature to 800℃.

2.5.3. Zeta potential test

Zeta potentials of CAFB samples treated with a different dosage of the plasticizer were measured using Malvern Zetasizer Nano series. The device measures the electrophoretic mobility of suspended particles based on phase analysis light scattering (PALS). The zeta potential of the particles is evaluated using the Henry equation[38,39].

2.5.4. Monitoring program

To gain additional insight into the reaction mechanisms responsible for the rheological behavior of the CAFB with varying composition, a 5TE sensor was used to monitor the electrical conductivity (EC), while an MPS-6 sensor was used for suction. Each sensor was placed at the middle of a cylindrical plastic container of 10 cm diameter by 20 cm height filled with the CAFB and readings recorded from the time of cast until 2 h.

2.5.5. Scanning electron microscopy tests

The parts of the dried CAFB specimens were cut into small rectangular specimens (10 mm × 10 mm × 10 mm) with a naturally formed surface. The specimens were placed in a holder with its natural surface facing upwards. The base of the specimens was attached to the holder using conductive adhesive or glue. Finally, micrographs of the prepared SEM specimens were captured using a ZEISS Gemini SEM 300 scanning electron micro-scope with magnifications between 100x and 5000x.

3. Results

3.2. The influence of the amount of coal gangue added on the rheological properties of the slurry

From Figure 11, the relationship between shear stress and shear rate is different from different grades of slurry at different shear rate. The rheological characteristic curves of 3#, 4# and 5 # of slurry are basically the same, and the rheological characteristic curves of 1# and 2# of slurry are basically the same with similar basic laws. With shear rate from 10 to 60, the rheological curves of slurry are pseudoplastic body; with shear rate from 60 to 100, the rheological curves of slurry are Bingham body; with shear rate from 100 to 150, the rheological curves of slurry are pseudoplastic body with a certain yield stress; with the increasing shear rate, the shear stress is also increasing, but the particle gradation of the slurry is not consistent, resulting in the increase of the shear stress of the slurry from 1# to 5# is not the same degree, and the initial shear stress is not the same with the increase of the mass fraction of the unit volume in the slurry, and the initial shear stress of the slurry is also increasing with the increase of the unit volume mass fraction in the slurry, which is basically consistent with the particle size gradation. From 1# to 5# slurry, the viscosity is in the unstable stage, transition stage, stable stage, and the value is decreasing, and the stable stage is basically below 10 Pa·s. From Table 6, the median particle size of 1# to 5# slurry are 138.018 μm, 151.538 μm, 165.059 μm, 178.579 μm and 192.149 μm respectively, and the distribution of the particle size of coal gangue can be seen that 50% of the coal gangue is below 500 μm, and the particle size of the aeolian sand is mainly between 100 and 300 μm, from the slurry Proportional distribution, it can be seen that from 1# to 5# slurry, the proportion of aeolian sand is in the decline, but the proportion of coal gangue is in the rise. The mass fraction of unit volume is in the increase, so the median particle size of 1# slurry to 5# slurry is also increasing, and D10 particle size value is in becoming smaller, and D90 particle size value is in becoming larger, which indicates that the particle size distribution of the material is more uniform, but, with the increase of coal gangue the initial shear stress of the slurry is getting bigger and bigger, the reasonable particle gradation should not only ensure that the slurry has good rheology, but also make the yield stress of slurry to be in a reasonable range, so that the pipeline’s conveying resistance runs in a reasonable range and reduce the energy loss[30,40].

Figure 11. Distribution diagram of shear stress, viscosity and shear rate after standing for 60 minutes.

Figure 11. Distribution diagram of shear stress, viscosity and shear rate after standing for 60 minutes.

Figure 12. Distribution diagram of shear stress, viscosity and shear rate after standing for 30 minutes.

Figure 12. Distribution diagram of shear stress, viscosity and shear rate after standing for 30 minutes.

Figure 13. Distribution diagram of shear stress, viscosity and shear rate after standing for 3 minutes.

Figure 13. Distribution diagram of shear stress, viscosity and shear rate after standing for 3 minutes.

3.5. Time-dependent evolution of the rheological properties of CAFB with plasticizer

Viscosity and yield stress were used to study the rheology of fresh CAFB made with different ratios of plasticizer. At the early stages of preparation, the viscosity of CAFB without addition was a lot higher than that with addition of 0.05 % and 0.1 % as shown in Figure 17. With moderate additions of 0.05 %, the viscosity decreased from 0.148 Pa·s to about 0.04 Pa·s at 0 h, and this tendency continued for the rest of the maintenance period. Similarly, it shows the development of yield stress in the first 1 hour of mixing in Figure 16. Likewise, the adding of the plasticizer led to a proportionate reduction in the yield stress of CAFB[8,20]. In the maintenance period of 1 hour, the yield stress decreased more obviously. The yield stress of 0.05 % admixture content decreased from 652 Pa to 234 Pa, and the yield stress of 0.1 % admixture content decreased to 215 Pa. The increased fluidity observed from the 0.05 % and 0.1 % admixture content may be related to the dispersion effect of plasticizer. It can be seen from that the Zeta potential of CAFB with and without the mixture supports this point in Figure 18. The zeta potential of CAFB containing 0.05 % mixture was about −10 mV, while the untreated sample was −3.5 mV. This indicates that the repulsion between particles is much higher in the presence of plasticizers[42]. Another explanation for the change in CAFB fluidity caused by the adding of plasticizers is its effect on cement hydration. Due to the increase of repulsive force among cement particles, fly ash particles and gangue particles, less hydration by-products are produced, and the viscosity of CAFB decreases with the decrease of solid content[45,46,47,48]. In addition, the EC monitoring results support the conclusion that the plasticizer postpones the hydration reaction in Figure 19. The EC of CAFB containing 0.1 % plasticizer was obviously lower than that containing 0 % CAFB. Therefore, the mobility of ions is higher in CAFB without dopants[38,39].

Figure 16. Time-dependent evolution of yield stress of CAFB containing different dosages of plasticizer.

Figure 16. Time-dependent evolution of yield stress of CAFB containing different dosages of plasticizer.

Figure 17. Time-dependent evolution of viscosity of CAFB containing different dosages of plasticizer.

Figure 17. Time-dependent evolution of viscosity of CAFB containing different dosages of plasticizer.

Figure 18. Effect of the plasticizer on the zeta potential of fresh CAFB made with different admixture content.

Figure 18. Effect of the plasticizer on the zeta potential of fresh CAFB made with different admixture content.

Figure 19. Development of electrical conductivity in CAFB containing different dosages of plasticizer.

Figure 19. Development of electrical conductivity in CAFB containing different dosages of plasticizer.

It can also be seen that the yield stress of CAFB samples increase smoothly with the maintenance period owing to the hydration process of cement and the surface absorption of poorly crystalline C-S-H gels in Figure 16. Once the cement is in contact with water, the chemical reaction usually begins[24]. In addition, it can be seen by electron microscope scan that CAFB with maintenance age of 3 d can clearly see C-S-H, CH, ettringite filling around the aeolian sand and coal gangue, which affects the fluidity of CAFB slurry, as shown in Figure 20[20,46,48,49,50,51,52].

5. Conclusions

• 3#, 4# and 5# slurry flow index change pattern is basically the same, the index n which characterizes the flow property is greater than 1 after 3 min resting, belongs to swelling body; n is close to 1 after 30min resting, belongs to Bingham body; n is less than 1 after 60 min resting, belongs to pseudoplastic body; 1# and 2# slurry in 3-30min flow index is greater than 1, belongs to swelling body, After 60min of resting, the flow pattern of slurry changes n is less than 1, which belongs to pseudoplastic body.
• With the increase of shear rate and shear time, the viscosity first gradually decreases and then stabilizes, i.e., the rheological properties of the slurry have the characteristic of “shear thinning”; the rheological properties of the slurry process is the comprehensive embodiment of a variety of model composite properties, with the increase of shear rate, the rheological curve of the slurry shows an upward convex shape, showing a pseudoplastic body-Bingham body-Pseudoplastic body(swelling body).
• According to 2# and 3# mixed material, it will be grading configuration aeolian sand, coal gangue, fly ash filling slurry, to ensure long-distance pipeline conveying process slurry flow stability, to ensure smooth pipeline transport.
• Curing time (0h-0.5h) results in a higher yield stress of the CAFB. It is because a longer curing time is associated with greater degree of cement hydration products. Curing time (0.5h-1h) results in a lower yield stress of the CAFB. Is is because more hydration products is associated with an decrease in the aeolian sand inter particle frictional resistance of the CAFB.
• Addition of plasticizer to the CAFB significantly reduces the yield stress and viscosity of CAFB. An addition of 0.05% of the admixture results in over 65% reduction in yield stress at the time of preparation as well as after 1 h. A similar reduction was observed in the improvement of viscosity. The marginal reduction upon increasing the admixture to 0.1% is much less, indicating that an optimum percentage is around 0.05.

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