3.1. Strength Evaluation
The strength of CPB is one of the most important factors in the application of CPB [
34], which can mainly be affected by activity of the binder [
41], component proportions [
42] and slurry concentration.
3.1.1. Test Results of Unconfined Compressive Strength
3.1.2. Evaluation of Test Results
The cement mixture can have a beneficial effect on the UCS of CPB. It was revealed that the UCS increased as the mass fraction increased, while the UCS also declined when there was an increase in the tailings-cement ratio. The content of cementitious material has an important influence on the strength, but this effect is weakened after the tailings-cement ratio is equal to 10. The hardening ability of ordinary cement is comparatively poor, as the UCS increased slowly and there was almost no strength when the tailings-cement ratio was higher than 10.
The cement mixture has the advantage of early strength, with 7-day strength being about twice as high as ordinary cement. However, there is a phenomenon of strength retraction after 28 days. The strength increases rapidly during the first few days. On the 10th day, the strength reaches its peak, with subsequent decline in strength after this time. Finally, the strength remains basically stable after 28 days. For ordinary cement, the improvement of UCS is consistent and slow, with the strength remaining basically stable after 28 days. Additionally, the UCS of ordinary cement is mostly lower than that of the cement mixture, with only the 28-day UCS being a little higher when the tailings-cement ratio is equal to 4:1.
According to the conditions in mining sites, apparent concentration of slurry and test results, there are two types of specimens that could be handy. The specimens with a mass fraction of 70%–72% and tailings-cement ratio of 6:1 have significant strength and are thus recommended as high-strength CPB. The high-strength CPB is used to carry out efficient continuous mining in addition to producing the surface layer and pillar, due to it being able to satisfy the requirement of early strength. The specimens with a mass fraction of 70%–72% and tailings-cement ratio of 15:1 can satisfy the conditions for common uses of CPB and are thus recommended as low-strength CPB. The low-strength CPB is taken into account for economic purposes and is designed to fill the most of the goaf.
3.2. Analysis of Microscopic Results
A series of hydration of products and micro structures were observed, with a set comparison diagram of X-ray diffraction (XRD) patterns shown in
Figure 4.
Figure 5 shows a set of scanning electron microscope (SEM) images of the cement mixture specimen with a mass fraction of 68% after curing for 28 days. In general, the micro-structure characteristics and the hydration products of the cement mixture specimens are close to those of ordinary cement, but there are some obvious differences.
The main hydration products of the cement mixture specimens are quartz, chlorite, calcite, cordierite, fluorophlogopite, dolomite and iron oxide. Quartz accounts for the highest proportion, with the other products being found in similar proportions to those in ordinary cement. However, specimens of the cement mixture contain more dolomite.
The phenomenon of a large amount of ettringite being formed earlier in the cement mixture specimens was found in the State Key Laboratory of High-Efficient Mining and Safety of Metal Mine.
According to the analysis of material composition, it is known that there is relatively more MgO in the cement mixture, with the effects of MgO being mainly represented in two aspects. One is a certain degree of expansion, which is conducive to the improvement of early strength by compressing pores inside the backfill body during the early period of hydration, although this can also be detrimental for later strength. The expansion [
43] is limited as Mg(OH)
2 and granite were not found in the hydration products, which have obvious features of volume expansion. The other aspect is that as an active component, MgO can promote the activity and vitrification of slag by reducing the viscosity of the solution to some extent.
Additionally, a high content of CaO contributes to the early strength if activated fully, although it can adversely affect volume stability.
Furthermore, the stimulation of alkali ions and SO4− allow for significant activity of slag. In perfect cooperation with the physicochemical properties of unclassified tailings and the micro-expansion of MgO and CaO at the same time, the cement mixture causes a significant hardening of unclassified tailings.
Hence, the development process of strength is clear. In the beginning, OH
− and SO
4− emerge and excite the glass phase in slag after being mixed with neutral water. When combined with the active component (CaO, Al
2O
3, and MgO), a hydration phenomenon of high activity is realized. After this, C–S–H gels and ettringite form earlier due to being influenced by the environment. Furthermore, pores shrink and are affected by micro-expansion of MgO and CaO, resulting in the strengthening of the mixture. On the 10th day, the positive effect of expansion and the reaction reach their critical values, with the UCS reaching its climax. With continuance of the micro-expansion coupled with physicochemical interference, a little less damage appears, accompanied by slowing down of the physicochemical reaction. Finally, a new balance is reached. The strength changes with the hydration process (
Figure 6).
3.3. Fluidity Evaluation
From multiple perspectives, the recommended slurry parameters were measured and listed in
Table 7. These parameters [
37,
38,
39,
40] are considered superior with regards to pipeline transportation and ability to spread out over surfaces.
We defined
as the concentration of apparent ratio, which can directly reflect the apparent morphology of the slurry. The closer to 1
is, the thicker the apparent state of the slurry is. Concentration of the apparent ratio is calculated by the following equation:
where
is mass fraction of the slurry and
is the threshold concentration.
The tests of the main slurry parameters show that the recommended proportions have good performance with regards to fluidity, while stratification and segregation are hard to create when transported through the pipeline.
The value of being between 90% and 95% proves that the slurry has a proper structure, a small bleeding rate and a better fluidity than paste backfill.
Showing features of “Structural Flow” when transported, the recommended slurry is consistent with having a paste-like flow state [
44].
The fluidity parameters show that the recommended slurry is very conducive to pipeline transportation and spreading out in goafs.
3.4. Application of Backfill
3.4.1. Backfilling System
There are several common problems in the mining operations of CPB in underground mines, such as low speed of solidification of backfilling body, roof-contacted technique and filling costs. It is clear that the cement mixture is very suitable for backfill in the studied mine due to micro-expansion being helpful in contacting the roof of stopes, early strength being high, and raw material being cheap and widely available.
According to the research results, the optimal aggregates, with a mass fraction of 70%–72% and tailings-cement ratio of 6:1 and 15:1, are recommended for CPB in Sijiaying Mine. The backfilling application is realized depended on a backfilling system. First, unclassified tailings are poured into a deep-cone thickener by pipeline transportation from the dressing plant, with a high efficiency of dehydration being necessary. The cement mixture is transported and unloaded in the cement warehouse after intensive mixing. Another cement warehouse for storing ordinary cement is recommended. After this, the cement mixture, unclassified tailings and neutral water are accurately measured and then mixed using mixing equipment with high performance. After five minutes of mixing, the backfilling slurry is transported into stopes through pipelines with the help of pumps.
Most of the goaf is filled by low-strength CPB, with the surface layer being made up of high-strength CPB. Combined with the required pillars that are formed by high-strength CPB, it is confirmed that high-strength CPB accounts for 46% of the structure, in order to achieve safety and economic benefits. The process of crafting the backfilling system is shown in
Figure 7.
3.4.2. Application Evaluation
The price of the cement mixture is 74% [
45] of the ordinary cement price and has better strength and practicality. The scale of high-strength CPB accounts for about 46% and the scale of low-strength CPB accounts for about 54%. The integrated price of backfill is
$5.99/m
3, which is calculated based on the market prize.
The added scale of goaf is about 6 million cubes each year. If an effective coefficient of 80% is taken into account for rough calculation, then there will be a cost of 7.73 million tons of tailings and 0.86 million tons of blast furnace slag for backfill per year. Simultaneously, the utilization rate of tailings will reach 64.5%.
Under the recommended parameters, the slurry has good collapsed slump, diffusion degree, bleeding rate and concentration of apparent ratio, which are very conducive to pipeline transportation.
The cement mixture can cause effective hardening of the unclassified tailings. First, the CPB can be provided with 7-day strength of 1.5 MPa under high-strength CPB. Following this, a micro-expansion is conducive to the improvement of the roof contacting, which provides significant stability. Apart from this, the early strength can safeguard efficient continuous production, without a long period of conservation. Several application cases prove that this method is feasible, with economic and environmental benefits possibly being obtained by using the cement mixture.