3.1. AC Impedance of the Filling Body after 7 Days of Curing
Figure 4 shows the electrochemical impedance results of the filling body with different mass concentrations of SA (a), K
12 (b), SJ (c) and without AEA (d) for a curing period of 7 days. As can be seen from
Figure 4, with the increase in frequency, the AC impedance of the filling body with different mass concentrations of SA, K
12 and SJ showed a similar trend to that of without AEA, both of which decreased gradually with the increase in frequency. This indicates that whether AEA is added or not within the frequency range of 10
−1–10
5 Hz, the variation trend of AC impedance of backfill remains unchanged, and the filling body with the same proportion of AEA has a similar topological structure. In the low-frequency range of 10
−1–10
4 Hz, the impedance value of the filling body without AEA is the maximum for the same frequency. Furthermore, the higher the mass concentration of SA and K
12, the lower the impedance value. However, with the increase in mass concentration, the AC impedance value of SJ first increased, and then decreased. Within the range of 10
4–10
5 Hz at high frequency, the impedance value of the filling body without AEA was the highest. The variation trends of impedance of the filling body for the three AEAs were basically consistent with each other, indicating that the AC impedance of the filling body decreased with the increase in the mass concentration of AEA.
3.2. AC Impedance of the Filling Body for 28 Days of Curing
Figure 5 shows the electrochemical impedance results of the filling body with different mass concentrations of SA (a), K
12 (b), SJ (c) and without AEA (d) for the curing age of 28 days. As can be seen from
Figure 5, with the increase in frequency, the AC impedance of the filling body with different mass concentrations of SA, K
12 and SJ showed the same variation trend as that of without AEA, both of which decreased gradually with the increase in frequency. This indicates that whether AEA is added or not within the frequency range of 10
−1–10
5 Hz, the variation trend of AC impedance of backfill remains unchanged, and the filling body with the same proportion of AEA shows a similar topological structure. In the low-frequency range of 10
−1–10
4 Hz, the impedance value of the filling body without AEA is the maximum for the same frequency. Adding SA and K
12 did not show regularity in the whole low-frequency region; however, regularity was observed only in a certain frequency range. In the high frequency range of 10
4–10
5 Hz, the AC impedance variation patterns of the filling body for the curing periods of 28 and 7 days were consistent with each other.
3.4. Equivalent Circuit Analysis of the AC Impedance
As shown in
Figure 8, the conductive schematic of the microstructure of the filling body is mainly composed of three conductive modes, namely the continuous conductive channel (CCP), the discontinuous conductive channel (DCP), and the insulator channel (ICP), in which water in isolated pores forms a solid–liquid contact interface with the backfill.
According to the measured impedance value of the filling body based on the electrochemical AC impedance data, Zview software was used to simulate the equivalent circuit, and the corresponding results are shown in
Figure 9. The equivalent circuit diagram of the filling body shows samples with SA, K
12 and SJ with the mass concentration of 0.6% and for different curing ages. In the equivalent circuit model shown in
Figure 9,
Ws is the warbug impedance of the diffusion process occurring in the filling body, CPE1 is the original normal-phase angle reflecting the properties of the solid–liquid interface in the backfill, and
Rs is the pore water resistance. Furthermore, the resistance of charge transfer in the backfill is represented by
Rp.
By simulating the equivalent circuit model of the filling body, it can be seen that when the concentration of SA, K
12 and SJ was 0.6% for the same curing age, the equivalent circuit diagram remained unchanged. When the curing ages for the filling body with the same AEA were 7 days and 28 days, the equivalent circuit diagrams remained the same, and the chi-square coefficient had the maximum value of 0.46%. The results show that the main structure of backfill did not change due to different AEAs and curing ages. This means that the components in the equivalent circuit were consistent. However, the internal microstructure of the filling body changed due to changes in AEA and curing ages.
Table 5 presents the values of each property parameter of the circuit components. For example, for 0.6% mass concentration of SA for the curing age of 28 days,
Rs was 2365 Ω, whereas that for the curing age of 7 days was 560.2 Ω, which is almost 4.2 times less than that for 28 days. These results indicate that longer the curing age, the more adequate the hydration reaction of the backfill, the smaller the internal porosity and the weaker the connectivity.
3.5. Analysis of the Porosity of the Filling Body
In this paper, the porosities of water-filled backfill with different concentrations of SA, K
12 and SJ cured for 7 and 28 days were measured using the NMR instrument, and the corresponding results are shown in
Figure 10. For the same curing age, the porosity inside the backfill gradually increased with the increase in AEA. For the curing age of 7 days, the minimum porosities of SA and SJ were greater than that of 8.686% without AEA, whereas the porosity inside the backfill was greater than that without AEA when the concentration of K
12 was only 0.6%. This shows that SA and SJ can increase the porosity of the filling body more significantly than K
12. When the curing age was 28 days, the porosities inside the backfill with SA, SJ and K
12 were all greater than that without AEA. This shows that all three AEAs can increase the porosity of backfill, and their effect is found in the following descending order: SA > SJ > K
12. Obviously, the porosity of backfill with the curing age of 7 days is greater than that of the filling body with the curing age of 28 days. This indicates that the longer the curing age, the more sufficient the hydration reaction and the smaller the porosity.
SEM was used to analyze the pores in the filling body with the curing ages of 7 and 28 days. The changes of pore structure in the filling body with the addition of AEA and curing age were obtained. It can be seen from
Figure 11 that the filling body without AEA for the curing period of 7 days had a smaller porosity with a large number of pores and an uneven distribution. However, the filling body with AEA was more porous with a higher number of pores and even distribution. Due to a certain volume of backfill, the connectivity between the pores inside the filling body will be enhanced. From the microscopic point of view, it is reasonable that the AC impedance value of the filling body with AEA at the same curing age was less than that without AEA. It can be clearly seen from the comparison between the curing period of 7 days and 28 days for 0.6% SA that the longer the curing period, the more sufficient the hydration reaction inside the backfill, which will lead to a smaller porosity. Meanwhile, the larger pore size will gradually decrease, and the smaller pore size will gradually increase. Additionally, the connectivity between the pores will gradually weaken. This indicates that the longer the curing age under the same conditions, the smaller the porosity of backfill and the larger the AC impedance. Liu et al. (1998) [
10] showed that the hydration reaction can reduce the pore diameter of the filling body. The experiment in this paper proves this point from the microscopic view, and shows that the filling body with AEA has stronger fluidity, which is conducive to improving the current situation of the long-distance transportation of the filling body.
For the frequency of 10
5 Hz,
Figure 12a,b show that the relationship between the porosity of the filling body with different AEA concentrations and the AC impedance for the curing age of 7 days, and the relationship between the porosity of the filling body with different AEA concentrations and the AC impedance for the curing age of 28 days, respectively. The AC impedance and porosity of the filling body adding SA are given after the curing ages of 7 days, as shown in
Table 6. Because some of the pores in the filling body are small or closed, the porosity measured by NMR is smaller than the true value. Therefore, the change in filling porosity studied in this paper is relative, mainly based on the nuclear magnetic resonance technology to study the change law of filling porosity. According to
Figure 12a, when the curing age was 7 days, the experimental data were fitted linearly, and the values of correlation coefficient (R
2) were 0.7116, 0.9129 and 0.9487, respectively. This indicates that porosity is linearly correlated with AC impedance, and the AC impedance decreases with the increase in porosity. As can be seen from
Figure 12b, when the curing age was 28 days, the experimental data were fitted linearly. However, such a feat could not be achieved for SJ. The values of correlation coefficient for SA and K
12 were 0.9571 and 0.9655, respectively. The larger the porosity of the backfill with SA, the higher the impedance, while the larger porosity of the backfill with K
12, the smaller the impedance.
The experimental results show that the porosity and AC impedance at the curing age of 7 days show a negative linear correlation, while at the curing age of 28 days, the porosity and AC impedance do not show a good regularity, but are related to the type of AEA added. The porosity and AC impedance of backfill with the curing age of 7 days with different concentrations of AEA show a linear negative correlation. However, the relationship between the porosity and AC impedance of the filling body with different concentrations of AEA for a curing age of 28 days at high frequency is affected by the type of AEA added to the system.
3.6. Analysis of the Strength of the Filling Body
In this paper, the electro-hydraulic servo material testing machine of 200 kN was used to measure the uniaxial compressive strength of water saturated backfill with different concentrations of SA, K
12 and SJ and the curing ages of 7 and 28 days, respectively. The results are shown in
Figure 13. For the same curing age, the uniaxial compressive strength of the filling body decreases gradually with the increase in AEA concentration. When the curing age was 7 days, the uniaxial compressive strengths of the backfill with SA, SJ and K
12 with the concentration of 0.4% were close to that of the backfill without AEA at 1.23 MPa. This shows that the three AEAs in this paper have similar effects on the uniaxial compressive strength of the filling body. When the curing age was 28 days, the relationship between the uniaxial compressive strength of the filling body and the AEA was similar to that of the curing age of 7 days. When the concentration of AEA was 0.4%, the strength of the filling body was close to that of the sample without AEA. Obviously, with the same type of AEA, the uniaxial compressive strength of the backfill with the curing age of 28 days was greater than that for the curing age of 7 days. The results show that the longer the curing time, the more sufficient the hydration reaction inside the backfill, the smaller and more uniform the porosity inside the filling body, and the stronger the backfill strength. While Şahin et al., (2011) [
16] showed that the moderate concentration of AEA results in the optimal UCS of concrete, in accordance with the experimental results, this shows that the filling body with 0.4% concentration of AEA can improve the production of the mine due to the insufficient strength of the filling body compared with the filling body without AEA, thus relieving the pressure of mining on the ecological environment.
For the frequency of 10
5 Hz,
Figure 14a,b show the relationship between the UCS and the AC impedance of the filling body with different concentrations of AEA for the curing age of 7 days, and the relationship between the UCS and the AC impedance of the filling body with different concentrations of AEA for the curing age of 28 days, respectively. As can be seen from
Figure 14a, when the curing age was 7 days, the experimental data were fitted linearly, and the values of correlation coefficient (R
2) were 0.8542, 0.9984 and 0.981, respectively. These results show that the UCS was linearly correlated with the AC impedance, and the AC impedance increased with the increase in strength. As can be seen from
Figure 14b, the experimental data for the curing age of 28 days were linearly fitted (except for the data for SJ). The values of correlation coefficient of SA and K
12 were 0.9979 and 0.8668, respectively. The higher the strength of the backfill with SA, the lower the impedance value was, while the higher the strength of the filling body with K
12, the higher the impedance value.
The experimental results show that the UCS was positively linearly correlated with the AC impedance value for the curing age of 7 days, while the UCS and impedance value at the curing age of 28 days do not show a good regularity, but are related to the type of AEA added to the system. The porosity and impedance of backfill with the curing age of 7 days with different concentrations of AEA were positively linearly correlated. The relationship between the UCS and AC impedance of the filling body with different concentrations of AEA for a curing age of 28 days at high frequency was affected by the type of AEA added to the system.