3.1. Uniaxial Compressive Strength
The strength of SWCPB directly affected ore body safety and continuity of mining. Figure 3
shows the curve strengths of SWCPB in groups A and B based on UCS test results.
Results shown in Figure 3
indicated that the strength for three days in group A observed no obvious trend, while with seven days of curing, the strength decreased with the increase of limestone powder replaced. The strengths of A2, A3, and A4 were similar after 28 d. Similarly, for group B, the strength of B4 with 28 d was higher than B2 and B3. Comparing the strengths of group A with B, all strengths of group A were higher than those of group B. It was interesting that there was an apparent retardation in strength development as a result of cement substitution. According to [10
], the hydration processes of stone powder cement tailing backfill can be divided into the following four stages: dissolution period, condensation period, infiltration period, and hardening period. At the early curing stage, the groups with cement substitution had less cement to increase the strength, and limestone powders just played a role as a crystal nucleus. However, at the later curing stage, hydration products reacted with SiO2
, which improved hydration reactions and strength.
In addition, according to mining experience, the target strength of seven days is 0.5 MPa, while for 28 days the required strength is 1.3 MPa. As a consequence, the seven-day strength of all groups met the requirements, while the 28 d strengths of B2 and B3 did not meet mining strength requirements of backfill.
3.2. NMR Analysis of Pore Properties
NMR analyses were done using an MiniMR-60 magnetic resonance imaging (MRI) analysis system manufactured by Shanghai Newmai Co. Ltd., China. As the samples were very small, three samples were tested for each group, and their mean values were used for analysis to minimize the error. NMR experiments were carried out for 28 d of curing samples of group A and B, and the pore structure parameters of SWCPB were obtained.
NMR total relaxation (T2) time is related to surface relaxation, loose relaxation of fluid procession, and diffusion relaxation caused by gradient fields [22
]. For the water-saturated samples, T2 relaxation time was directly proportional to the pore size and the magnitude of the T2 curve, which directly reflected the porosity of SWCPB samples. Pore size distribution maps were plotted by considering the average amplitude of NMR, as shown in Figure 4
and Figure 5
, for groups A and B, respectively.
Three peaks of T2 relaxation time for group A after 28 d of curing can be observed in Figure 4
, in which the first peak emerged at about 0.28 ms, the second peak observed at about 11.50 ms, and the third peak appeared at 204.91 ms.
The pore ratio of A1 was shown in Figure 4
a, in which the total porosity was about 1.99%. The main pores in A1 were micropores (1st peak accounted for 93.3%), the mesopores accounted for a relatively small proportion (2nd peak accounted for only 1.91%), and there were also many macropores (3rd peak accounted for 4.79%). The pore ratio of A2, as depicted in Figure 4
b, showed an increase in total porosity as the limestone powder consisted of fewer active ingredients than the cement. The addition of limestone powder increased the micropores but decreased the macropores, and the change of mesopores was not significant. By further increase of limestone powder to about 15%, as shown in Figure 4
c, the total porosity, proportion of micropores, and mesopores were increased, while the amount of macropores was found to be reduced. When the addition of stone powder increased and reached about 20% (Figure 4
d), the mesopores remained the same while the micropores increased and the macropores decreased, but the total porosity decreased overall.
The NMR distribution of T2 relaxation time for 28 d of the group B sample showed two peaks in Figure 5
, the first peak appeared in the vicinity of 0.30 ms, while the second peak emerged at around 204.90 ms.
Due to poor grading of group B (Cu > 40), there was no obvious difference between the micropores and mesopores. Therefore, the curve had two peaks, and the peak areas of the first peak and the second peak in each group were assumed for 95% or higher of the total area. By increasing the limestone powder, the distribution of micropores decreased, but the macropores increased slightly while the total porosity remained constant. However, when the addition of limestone powder reached 20%, the proportion of micropores increased significantly and the macropores decreased.
3.3. SEM Analysis of the Microstructure
After the initial gelatinization process, the pore structure was formed by the cemented hydration reaction, and it was involved in transferring water, unhydrated material, and storing water. Among the three, the water storage function of the pores maintained the hydration reaction and further enhanced the strength of CPB [22
]. In this process, the proportion of harmless pores could be increased by reducing the proportion of large pores in SWCPB and simultaneously increasing the proportion of small pores while the total porosity remains intact.
The 28 d curing samples of groups A and B were selected for SEM analysis, at 5000× magnification and a scale size of 10 μm, to study pore distribution, pore morphology, and other characteristics. Figure 6
illustrates SEM images of group A samples for 28 d curing while Figure 7
presents the samples of group B for the same.
The structures of A1, as shown in Figure 6
a, were denser, in which the distribution of cementitious materials was uniform, and some of the large particle size tailings were exposed without any connection. Most of the pores were less than 3 μm in diameter. The pore structure of A2 was poorer than A1, shown in Figure 6
b. Here, the porosity and pore diameter increased because the limestone powder consisted of fewer active ingredients than cement. When the particles of inert limestone powder were crystallized by the hydration process, the volume was increased, and the body of the crystal was difficult to dissolve as the pores between the tailings were filled. However, the hydration product decreased because of the decrease in cement incorporation; therefore, the microporous structure of A2 was poorer than A1. The pore structures of A3 and A4 were similar to A1, as shown in Figure 6
c,d, respectively. Among them, it was found that the macropores were significantly reduced while mesopores and micropores increased, as compared to Figure 6
The SEM images of 28 d curing age samples of group B showed a small number of large-sized tailings. In the samples of group B, the exposed crystals (mainly Aft) accounted for a large proportion, and the cementitious material (mainly C–H–S gel structures) was not obvious. The structure of group B was looser than that of group A. Without limestone powder particles in backfill, a lot of coarse particles were observed, and there was no particle that filled the gaps between tailings (Figure 7
a). The structure of group B4 was denser than that of B2 and B3. Groups B2 and B3 had more macropores, and a lot of macropores combined together to form a complex pore structure, as shown in Figure 7
, which meant that the pore structure of B4 was simpler than those of B2 and B3. The calculated NMR porosities of B3 and B4 were almost the same (Table 3
); however the average pore radii of B3 were higher than that of B4, which meant that pores of B4 were small, but the distribution was relatively uniform. In addition, group B3 had a large number of pores above 3 μm in diameter, as the number of specific particles that could fill the pores was insufficient. In B4, a large number of fine particles entered into the large pores, and the macropores were divided because there was an increase in the number of particles below 10 µm. Hence, most of the pores of B4 were found to be less than 1 μm in diameter, and the largest pore size reached less than 2 μm in diameter.
3.4. SEM Image Quantitative Analysis
SEM images of each group were binarized for quantitative analysis of pore topography data. SEM images (500×) were binarized by using the FRACLAB toolbox in MATLAB. By setting the nearest contrast threshold, NMR porosity was used to generate a binary image together with parallel color inversion. Figure 8
shows two samples that contrasted original SEM images and its binarized images. By implementing count instruction, the pixel count and the unit count of the bright area of binary images were calculated, followed by the calculation of porosity, and then the comparison with NMR porosity, as shown in Table 3
. The size of the SEM image was 779,264 pixel units with a total size of 1024 × 761, in which the scale bar was 100 μm length and 185 pixel units. Finally, a single 500× SEM image area was calculated to be 227,710 μm2
The binarized images had a clearer pore structure than the original images and were easy to measure, as shown in Figure 8
. Among them, A4 (Figure 8
a) had a uniform pore distribution with most of the pore diameters less than 5 μm and maintaining a compact structure. In a similar way, it was analyzed that B4 had more pores than A4, while for B4 (Figure 8
b) a large number of pores had sizes over 10 or even 20 μm.
shows that porosity was calculated by using the image processing method of Figure 8
. With the help of stereology principles, the calculated porosities of groups A1, A2, B3, and B4 were slightly larger than NMR porosity. The other four cases were reversed. Although there were differences between calculated porosity and NMR porosity, the values were almost the same. The calculated porosity, number of holes (counting units), and the hole radius of group B were significantly greater than in group A. The data shown in Table 3
followed that the average pore radius changed as a result of adding limestone powder. The average pore radius in group A was basically unchanged, but in group B the average pore size first decreased, then increased, but then finally decreased significantly.