*2.5. Preparation and Study of the Backfill Material*

Previously, the optimal amount of nanomodifying additive was determined, which was 0.01% of the solid mass in the material. Therefore, the amount of nanomodifying additive remained unchanged during the experimental studies. Nanomodifying additive and magnesia binder were mixed for 5 min, after which the wastes from enrichment of water-soluble ores were added, and the mixing was continued for up to 10 min. Then, the salt brine was added, and the mixing was continued for an additional 10 min until a homogeneous mass was achieved.

Such a sequence of mixing is due to a sufficiently small amount of one of the components (nanomodifying additive astralene) and will lead to its better distribution in the entire volume of the material being prepared.

Mixing was carried out in a laboratory planetary mixer MICS-D-C (МИКC-Д-Ц (EN 196-1, EN 196-3, EN 413-2, EN 459-2, EN 480-1, EN-ISO 679, NF P15-314, DIN 1164-5, UNE 80801, UNE 83258, ASTM C305, AASHTO T162). Optimal and efficient mixing was achieved due to the characteristic planetary motion of the mixer, namely, a combination of circular motion and motion around its axis. The planetary rotation speed was 62 rpm with an increase to 125 rpm at an initial circular rotation speed of 140 rpm with an increase to 250 rpm. Then, the material was placed in cubes with faces of 10 cm.

The storage and hardening of the samples occurred in conditions close to those of the mine, provided with the methodology (T = 20 ± 2 ◦C; W = 95 ± 5%). The subsequent compression test was carried out at specified periods in accordance with the methodology: 7, 28, and 60 days [41]. The magnitude of the ultimate compression strength of the hardened mixture was tested by crushing samples of standard sizes (edge 10 cm) on the test press PI-2000-A.

Reliability was confirmed by the repeatability of the results with a sufficient number of experiments. The condition for obtaining high reliability of the results is a large number of experiments. In order to obtain the most accurate values close to the actual ones, 18 samples were made for each composition. Then, the average values were calculated and presented in tables.

#### *2.6. Microstructural Analysis of the Backfill Material*

Microscopic analysis—the study of the internal structure of the created material—was carried out using optical or electronic microscopes at magnifications from 100 to 1000 or higher. The method of microscopic analysis was used to study the structure and materialmineralogical composition of the material (coarseness, various inclusions or new formations, etc., invisible to the naked eye), which made it possible to give a more detailed and accurate characterization of the material properties and quality [42].

The study of the created material microstructure required the use of analytical methods and appropriate equipment allowing adequate determination of the shape, composition and structure of particles of both the original components and new formations in the size range from tens of microns to nanometers [43,44].

To study the microstructure of the nanomodified composite prepared on the basis of wastes from enrichment of water-soluble ores, structural-mineralogical (petrographic analysis) and X-ray analyses were used.

All microstructural studies were carried out on a fracture of samples of the investigated nanomodified material. The fracture was obtained by a mechanical method. The fine delaminated fractions and dust particles, formed on a fracture as a result of mechanical influence, were removed by a jet of air.

The application of scanning electron microscopy to diagnose textured material has become the most powerful method for studying the structure and physical and chemical features of solid materials, including nanostructures, in the last few years [35,39].

Operating peculiarities and research methods using electron microscopy are analyzed in [45–48]. Scanning electron microscopes present patterns in secondary electrons, which makes it possible to highlight light and dark contours.

#### 2.6.1. Structural-Mineralogical Analysis (Petrographic Analysis) of the Backfill

Structural-mineralogical analysis (petrographic analysis) is a method of visual or microscopic investigation of the mineralogy and composition of a created material on the basis of morphological features.

Petrographic analysis was carried out on a Polam R-211 polarizing microscope using the immersion method. The phases were identified by refractive indices, birefringence, basicity, sign, elongation, and extinction angles. Immersion liquids were used as standards. The quantitative ratio of the phases (crystallographic composition) was determined by the Stroyber method. In the study using a polarizing microscope Polam R-211, the maximum magnification was 720 times.

These researches were supplemented by studying the samples using a Philips SEM 515 scanning electron microscope. In this case, the maximum magnification was 2000 at an accelerating voltage of primary electrons of 20.00 kV. The pressure in the chamber at the time of the study was 2 × <sup>10</sup>−<sup>5</sup> Torr.

#### 2.6.2. X-ray Analysis

X-ray analysis is a method of studying the structure of matter by the distribution in space and intensity of X-ray radiation scattered on the analyzed object.

A DRON-3 diffractometer was used for X-ray phase analysis. Recording signals in digital form allowed data processing automatically. Further, the obtained data were processed manually using a graphical editor or decrypted using a specially program for X-ray phase analysis of new crystalline formations. The operation of the graphical editor and the program used are described in detail in the study [49].

#### **3. Results**

A set of experiments were carried out to determine the optimal quantitative composition of the nanomodifying additive astralene in the backfill and its effect on the strength characteristics.

For comparison, the data obtained in reference [50] were taken when studying the effect of the activating additive astralene on the backfill based on the waste from enrichment of water-soluble ores. Experimental data on the use of the nanomodifying additive astralene are juxtaposed in Table 3 and presented in Figure 4.


**Table 3.** Compositions with different contents of astralene.

**Figure 4.** The change in the samples' compressive strength depending on the astralene content at the age of: 7 days, 28 days60 days.

#### *3.1. Optimal Astralene Content*

The hardened samples were tested for uniaxial compression. The test results are shown in Table 3. From the analysis of the strength characteristics of the samples, it follows that the activation with nanomodifying additive astralene significantly increases them.

The dependence of the ultimate compressive strength of the samples at the age of 7, 28 and 60 days on the astralene content *C* are very well approximated by third-order polynomial Functions (1)–(3):

$$
\sigma\_{\text{comp},7} = 610,216 \cdot \text{C}^3 - 27,131 \cdot \text{C}^2 + 380.23 \cdot \text{C} + 0.0450 \text{ (R}^2 = 1.0000\text{)}\tag{1}
$$

$$
\sigma\_{\text{comp},28} = 468,815 \cdot \text{C}^3 - 21,934 \cdot \text{C}^2 + 323.27 \cdot \text{C} - 0.0029 \text{ (R}^2 = 1.0000\text{)}\tag{2}
$$

$$
\sigma\_{\text{comp},60} = \text{57}.272 \cdot \text{C}^3 - 3710.9 \cdot \text{C}^2 + 69.226 \cdot \text{C} + 0.0261 \text{ (R}^2 = 0.9667) \tag{3}
$$

where σcomp,7, σcomp,28, σcomp,60—ultimate compressive strength of the samples at the age of 7, 28 and 60 days respectively, MPa; *C*—astralene content. mass. % of waste. Values in brackets show the accuracy of approximation R2, respectively.

#### *3.2. Strength of Backfill with Different Component Contents*

The method for selecting the composition of the hardening backfill is standard and includes studies of the main characteristics and properties. One of the main ones is obtaining necessary or specified physical and mechanical characteristics.

The choice of the rational composition of the backfill implies methods for comparing experimental compositions, analogies with previously performed works, and the exclusion of compositions that do not meet the requirements or specified characteristics.

Laboratory studies of the physical and mechanical properties of raw materials assess the possibility of their use in backfill. Then, studies of materials and hardened samples based on the selected raw materials are carried out. The samples were studied after material solidification with different component contents: binder/additive/aggregate. The components were mixed in a certain sequence in various combinations and ratios in order to determine the optimal composition of the backfill. Then, the material was fabricated into cubes with faces of 7 cm and stored in conditions close to those of the mine.

Previously, the optimal amount of nanomodifying additive was determined, which was 0.01% of the solid mass in the material. Therefore, the amount of nanomodifying additive remained unchanged during the experimental studies. The nanomodifying additive and magnesia binder were mixed for 5 min, after which the waste from enrichment of water-soluble ores was added, with mixing continued for 10 min. Then, the mixture was gaged with brine, and mixing was continued for an additional 10 min until a homogeneous mass was produced.

The samples were tested for uniaxial compression after material hardening to determine the rational-optimal composition. The test results are presented in Table 4 and Figure 5.


**Table 4.** Research on compositions with different content of components.

**Figure 5.** Kinetics of the backfill strength set depending on the component content: 1—waste/magnesia cement/astralene: 98.99/1.00/0.01. 2—waste/magnesia cement/astralene: 99.49/0.50/0.01. 3—waste/magnesia cement/astralene 99.99/0.00/0.01. 4—waste/magnesia cement/astralene 99.00/1.00/0.00.

The optimal water–solid ratio was selected based on the requirements that ensure the necessary mobility of the composite—20 cm according to the Suttarda viscometer.

The dependence of the ultimate compressive strength of the samples (Composition 2, Table 4) on the hardening time is well approximated by a logarithmic function:

$$
\sigma\_{\rm comp} = 1.2444 \cdot \ln(\text{t}) - 2.1581 \text{ (R}^2 = 0.9916\text{)}\tag{4}
$$

where: σcomp—ultimate compressive strength of the samples, MPa. tduration of hardening, days. R2—accuracy of approximation.

A comparative analysis of the experimental results with the data obtained from early studies (Compositions 1a, 2a, 4a) allowed us to conclude that the use of a nanomodified additive makes it possible to reduce the magnesia binder consumption by at least 2 times while increasing the strength properties of the hardened mass. It may be also concluded that, despite some similarities to concrete, the time-dependent increase in compressive strength lasted longer than 28 days. Longer setting times resemble the case of other soil–cement composites with or without additives [51,52].

### *3.3. Microstructural Study of the Backfill Material*

Structural-mineralogical and X-ray phase analyses facilitated study of the influence of a separate component of the backfill material on the creation of structural bonds. We performed X-ray phase analysis of compositions No.2, No.3 and petrographic analysis of compositions No.2, No.3 and No.4 (Table 4).

To determine the crystallographic parameters, we used the constants of the optical properties of minerals combined in the Winchell A.N [53,54] reference book for inorganic compounds:


(**a**)

**Figure 6.** Microstructure of the studied samples. (**a**) Peripheral needle frame of magnesium hydroxy chloride. (**b**) Formation of a cryptocrystalline structural frame representing secondary NaCl crystals. (**c**) Brucite.

Analysis of the X-ray patterns of samples No.2 and No.3 allowed us to note that the bulk of the reflections with the highest amplitudes were crystals of sodium (NaCl) and potassium (KCl) salts, which was explained by the large amount of waste from enrichment of water-soluble ores (Figure 7a,b). Crystals of brucite and magnesium hydroxy chloride, which are the products of the magnesian binder hydration, were reflected with a lower amplitude. This proved that there is a compaction of pore voids between crystals of sodium (NaCl) and potassium (KCl) salts by filling them with brucite and magnesium hydroxy chlorides. As a result, the strength characteristics of the homogeneous mass increase during solidification.

**Figure 7.** Evaluation of samples by X-ray phase method: (**a**) waste/binder, (**b**) waste/binder/astralene.

In the studied samples, which did not contain a nanomodifying additive (Figure 7a), there were noticeably smaller numbers of magnesium hydroxy chloride reflections dα = 8.1315; 5.6427; 3.4670 Å, and a slight dominance of reflections corresponding to brucite was revealed dα = 2.3540; 1.3019; 1.4857; 1.2928 Å. In samples obtained after solidification of the material containing a nanomodifying additive (Figure 7b), the numbers of reflections that corresponded to the fundamental crystal structures of magnesium hydroxy chloride were preserved da = 8.1227; 2.9094; 2.0964 Å. At the same time, there was a significant reduction in reflections corresponding to brucite da = 1.4856 Å. Additionally, in these samples, reflections appeared, indicating the formation of new structures da = 5.6577; 5.5597; 3.4772; 1.6960; 1.2900 Å, not typical for samples without a nanomodifying additive.

The use of a nanomodifying additive, astralene, influenced the formation of a finecrystalline nanomodified structure of the fill mass. Structural-mineralogical and X-ray phase analyses made it possible to establish that astralene acts as an activating additive in

the backfill. In the hardening (hydration) process, brucite was formed along the peripheral zones. This created additional stable crystal structures of magnesium hydroxy chlorides (Figure 6a) and provided an increase in the strength of the created fill mass.

In addition, the cryptocrystalline frame was formed when astralene was injected on the surface of sodium salt (NaCl) grains. The frame represented secondary crystals of these salts (Figure 6c). The formation of this structure was favored by the mutual penetration of halite aggregates and hydration products of magnesium hydroxy chlorides into the pore space and their additional adhesion.

#### **4. Discussion**

Figure 6a,c show the microstructures of the fill mass, visually representing the marked crystalline new formations.

In the analysis of the X-ray study (Figure 7), it can be seen that upon introduction of the nanomodifying additive astralene into the composite, reflections from the new phase appeared (Figure 7b), testifying to the new formation in the composite being created. This new formation corresponded to development of a cryptocrystalline structural frame.

Upon activation of the backfill material with astralene, after its solidification, a denser and more homogeneous structure was formed (Figure 8a), in contrast to the composite that did not contain the nanomodifying additive (Figure 8b).

**Figure 8.** Microstructure of the studied samples: (**a**) with a modifying additive, (**b**) without a modifying additive.

When analyzing Figure 8a,b, it can be seen that image 8a is more even, while image 8b shows a sharp contrast. Dark contrasts (Figure 8b) indicate the presence of pores, and light contrasts turning into white indicate a high graininess of the material. The more even contrast in Figure 8a indicated that the composition of the material containing astralene hade less porosity and granularity. The combination of astralene with magnesian cement contributed to the formation of a dense, therefore, more durable structure. The setting time of the mixture was not changed significantly and required a long period of time. This was due to the fact that when mixing a mixture with a saturated solution of salts consisting mainly of halite, the process of hydration of magnesia binder takes a longer period of time in comparison with the setting time of magnesia-based mixtures with bischofite (saturated solution of MgCl salts).

Experiments proved that the use of the nanomodifying additive astralene in the backfill makes it possible to increase the strength properties of the created artificial mass with a decrease in binder consumption. Activation of the backfill with the additive astralene formed a fine-crystalline nanomodified structure and allowed creation of a completely new nanomodified material with stronger bonds.

Activation occurs by adding a nanomodifying additive to the backfill. The formation of a nanomodified artificial mass based on the wastes from enrichment of water-soluble ores occurs due to the formation of fine-structured bonds by filling its pore voids. As a result of the introduction of a nanomodifying additive (astralene) into the backfill, needle crystalline and cryptocrystalline frames were formed, which filled the pore space. These structures guaranteed the formation of stable structural bonds between the crystalline matrix components, which increased the strength of the mass by at least 1.76–2.36 times.

Testing of composite samples after 60 and 90 days proved that even after a standard 28 day period, an important increase in compressive strength may still be observed. The range of this increase is higher than that for standard cementitious materials such as concrete, and it is comparable to the results from the creation of soil–cement composites in the course of geotechnical works.

#### **5. Conclusions**

To study the possibility of creating and using nanomodified backfill material based on the waste from enrichment of water-soluble ores, the composition was selected, physical properties were studied, and micro-structural research was conducted. From the conducted research, the following conclusions can be drawn:


#### **6. Patents**

The presented results are the subject of Russian Patent RU 2754908 C1: "Backfill mixture with nanomodified additive". Authors of the patent: Elena A. Ermolovich, Albert M. Khayrutdinov, Yulia S. Tyulyaeva, Cheynesh B. Kongar-Syuryun.

Field of application: mining industry.

Substance: Invention relates to the mining industry, namely to backfill mixtures, and can be used to backfill a goaf in the development of mineral deposits. The filling mixture contains a saturated solution of halite waste salts and a solid mixture consisting of: halite waste from potash ore processing, a binder-magnesia cement, an additive, and the filling mixture contains a nanomodified additive, astralene, as an additive. The filling mixture contains, wt.%: 11.11—a saturated solution of salts of halite waste and 88.89—a solid mixture, which contains, wt.%: halite waste from potash ore processing—98.99–99.49; nanomodified additive astralene—0.01; magnesia cement—the remainder.

Effect: increasing strength of the filling mixture, reducing the consumption of the binder in the filling mixture, increasing completeness of utilization of potash ore processing waste.

**Author Contributions:** Conceptualization, E.A.E.; methodology, M.M.K.; software, C.B.K.-S.; validation, E.A.E. and Y.S.T.; formal analysis, M.M.K. and Y.S.T.; investigation, C.B.K.-S.; resources, E.A.E. and A.L.I.; data curation, C.B.K.-S. and M.M.K.; writing—original draft preparation, E.A.E. and M.M.K.; writing—review and editing, C.B.K.-S.; visualization, Y.S.T.; supervision, A.L.I.; project administration, Y.S.T. and A.L.I.; funding acquisition, A.L.I. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

