A Study on the Performance of Self-Leveling Mortar Utilizing Tungsten Tailings as the Aggregate

Abstract

A significant quantity of tailings is produced during the development of different metal mines in China. In particular, fine-grained tailings pose challenges to the sustainable development of the mining industry. This study examines the utilization of finely ground tungsten tailings as a replacement for natural aggregates in self-leveling mortar (SLM). The study examined the impact of the aggregate-cement ratio, cement mix ratio, and varying substitution levels of different grain sizes of tungsten tailings on the flow properties, mechanical properties, and dimensional change rate of SLM. Additionally, the role of tungsten tailings in SLM was analyzed using XRD, FTIR, and SEM methods. The findings demonstrated that the utilization of sulphoaluminate cement (SAC) had a notable impact on improving the initial strength of the SLM. Additionally, a high aggregate-cement ratio negatively affected the fluidity of the SLM. The doping of tungsten tailings improved the grading relationship of the SLM. Substituting tungsten tailings of 38–75 μm grain size for natural aggregates in the preparation of SLM did not have a negative impact on its performance. In fact, substituting 60% tungsten tailings had a positive effect on the 28-day mechanical properties of the SLM. The compressive and flexural strengths of the SLM after 28 days were 26.53 MPa and 9.06 MPa, respectively, which were enhanced by 18.81% and 26% compared to the control group (C0). According to the environmental leaching test, SLM can effectively fix the heavy metal ions in tungsten tailings, and the leaching concentration of heavy metals is significantly reduced after long-term curing. The doping of finely fragmented tungsten tailings accelerated the process of hydration, resulting in the creation of hydrocalcium zeolite crystals in the latter phases of hydration. Furthermore, an increase in tailings substitution resulted in the production of a greater amount of hydration products, specifically C-S-H gels.

1. Introduction

Tungsten is an essential and valuable metal that is widely utilized in the aerospace, defense industry, and specialty chemicals sectors. However, the tungsten ore is generally of low grade, with the tailings making up more than 90% of the extracted ore [1]. This results in the acquisition of extensive territory and the deterioration of air, water, and soil quality, which contradicts the prerequisites of societal advancement and environmental preservation [2,3]. Responding to the sustainable development of the mining industry, scientists have extensively investigated the potential of using tailings in concrete to maximize the efficient use of resources [4,5,6]. A recent finding reveals that coarse-grained tailings can be converted into tailings sand, which can serve as a viable alternative to natural aggregates. However, the successful usage of fine-grained tailings has not been accomplished up to this point [7]. Zeng et al. [8] used tungsten tailings, which had a particle size ranging from 0 to 78.07 μm and made up 90% of the original material, to create polymers for the purpose of mine backfilling. Nevertheless, there is still a fraction of fine-grained tailings that cannot be efficiently remediated. Peng et al. [9] found by utilizing 85.21% of tungsten tailings with particle sizes ranging from 38–150 μm, it is feasible to produce cementitious materials that exhibit mechanical characteristics on par with conventional 42.5 silicate cement. Nevertheless, the process of activation, which involves mechanical grinding, results in the excessive consumption of energy. Wang et al. [10] produced porous ceramics using tungsten tailings, which had an average particle size of 39.12 μm. Yet, the extensive use of this technique was impeded by the significant fuel demand needed to provide thermal energy during the high-temperature sintering procedure. In addition, particular research efforts have focused on extracting valuable metals from tailings [11]. Nevertheless, the high concentration of fine mud and clay in these tailings with tiny particles poses significant difficulties in achieving effective recycling, leading to a utilization rate of less than 5% [12]. Therefore, it is crucial to rapidly develop a cost-effective and sustainable technique for utilizing fine-grained tailings.

Europe, the United States, and Japan were the first to introduce SLM. This is a specific kind of mortar that is made up of either inorganic or organic cementitious materials as a foundation, combined with aggregates and other chemical admixtures [13]. By maintaining a specific water–binder ratio and utilizing water agitation, SLM can effortlessly level and fill spaces due to its inherent gravitational force. Due to its exceptional fluidity, self-compacting properties, and low requirement for manual labor, this material is extremely appropriate for many sectors such as schools, major shopping centers, parking lots, and residential floors. Reports indicate that the utilization of fine-grained aggregates can lead to increased flow rates and higher mobility values. Furthermore, circular aggregates have larger flow widths in comparison to angular aggregates [14]. Chen et al. [15] used 90% of waste rock powder, with a particle size of less than 65 μm, as a replacement for river sand in the production of SLM. By including waste rock powder, the flow characteristics and mechanical properties of SLM were improved. The best results were obtained when waste rock powder was substituted at a rate of 20%. Lawrence et al. [16] suggested that the presence of small particles might facilitate the nucleation process by lowering the energy barriers, hence accelerating the reaction of mortar hydration. However, Scolaro et al. [17] emphasized that the larger surface area of small particles requires a larger quantity of cement paste to completely cover the surfaces of the particles, leading to a decrease in mortar compatibility. Li et al. [18] used limestone particles with a size lower than 75 μm to replace natural river sand in the manufacture of SLM. Research has shown that a substitution rate of 15% or below has a positive effect on improving the flow characteristics and mechanical properties of SLM. The inclusion of small particles, facilitated by a water-lowering agent, decreases the accessible volume for water [19], leading to a more compact SLM. Nevertheless, going over this substitution rate results in a decline in the manageability and structural integrity of SLM. Therefore, it is crucial to analyze the influence of the overall particle size, shape, and doping on the effectiveness of SLM.

This study examines the use of fine-grained tungsten tailings as fine-grained components in SLM. The performance of SLM containing tungsten-doped tailings is rigorously scrutinized and assessed. It specifically focuses on how the aggregate-cement ratio, cement mix ratio, and varying substitution levels of different grain sizes of tungsten tailings affect the performance of SLM. Environmental leaching tests were carried out on SLM containing tungsten tailings, which proved that the material did not have harmful effects on the environment. The objective is to develop a technique for utilizing and reclaiming fine tungsten tailings and producing high-performance mortar for use in the construction industry. This aims to address the issue of effectively utilizing fine tungsten tailings.

2. Materials and Methods

2.1. Materials

The study employed commercially available mortar mixtures, which consisted of PO42.5 ordinary Portland cement (OPC), R.SAC42.5 sulphoaluminate cement (SAC), anhydrite gypsum as cementitious components, and natural sand (80–270 μm) and limestone powder (38 μm) as aggregate components. The chemical admixtures used included dispersible latex powder, a polycarboxylic acid high-efficiency water-reduction agent, a defoamer, a retarder (tartaric acid), and hydroxypropyl methyl cellulose ether (HPMC-400 viscosity). The tungsten tailing material in this study comes from Ganzhou, Jiangxi, China. Tungsten tailings are acquired from tailings ponds and undergo a series of procedures, which include precipitation, filtration, and drying at a temperature of 105 °C. Subsequently, they were kept at the ambient temperature for a period of 24 h before being used.

Table 1. Chemical composition of fine-grained tungsten tailings (wt.%).

Materials	SiO2	Al2O3	K2O	Fe2O3	CaO	MgO	SO3	MnO	Na2O	TiO2	LoI
Tungsten tailings	65.80	20.42	4.72	3.37	2.37	1.57	0.44	0.29	0.24	0.20	-
OPC	21.60	5.15	-	3.11	62.37	1.29	3.22	-	-	-	3.21
SAC	7.23	18.60	-	4.30	45.30	1.35	12.50	-	-	-	10.05

presents the chemical compositions of the fine-grained tungsten tailings, OPC, and SAC.

The crystal structure of tungsten tailings and natural sand was investigated using a Japan Rigaku SmartLab series X-ray diffractometer (XRD), as shown in Figure 1. The XRD analysis indicated that the main components of the tungsten tailings are quartz (SiO₂), ammonium gypsum ((NH₄)₂Ca(SO₄)₂H₂O), and muscovite (KAl₂(AlSi₃O₁₀)(OH)₂). The primary crystalline component found in natural sand is quartz (SiO₂).

The LS-609 laser particle size analyzer (Omec, Zhuhai, China) was employed to assess the particle-size distribution of the tungsten tailings. Based on the data provided in Figure 2, the tungsten tailings had a D₁₀ value of 11.03 μm, a D₅₀ value of 56.90 μm, and a D₉₀ value of 146.52 μm. Out of these, particles measuring 98.08 μm in size had the highest percentage, roughly 11.09%.

2.2. SLM Mixing Ratio Design

Figure 3 depicts the process of preparing SLM by utilizing tungsten tailings. Blend the cementitious materials and aggregates with a precise amount of water and chemical admixtures. Afterward, the mixture was aggressively stirred and put into the mold. The air bubbles on the surface of the mortar were removed and the mixture was left to cure at room temperature for 24 h. Ultimately, the SLM, which is made from tungsten tailings, is subjected to a 28 day maintenance period in a controlled environment where temperature and humidity are carefully maintained.

Table 2. SLM mixing procedure/percent.

No.	Gelling Material			Aggregate
	Overall Amount	Cement	Gypsum	Overall Amount	Mineral Filler	Limestone Powder
		Ordinary Silicate Cement: Sulphoaluminate Cement			Quartz Sand: Tungsten Tailings
A1	60	35:15	5	40	5:5	10
A2	55	35:15	5	45	5:5	10
A3	50	35:15	5	50	5:5	10
A4	45	35:15	5	55	5:5	10
A5	40	35:15	5	60	5:5	10
B1	55	41:9	5	45	5:5	10
B2	55	38:12	5	45	5:5	10
B3	55	35:15	5	45	5:5	10
B4	55	32:18	5	45	5:5	10
B5	55	29:21	5	45	5:5	10
C0	55	38:12	5	45	10:0	10
C1-1	55	38:12	5	45	7:3 (Tailings size: 150–270 μm)	10
C1-2	55	38:12	5	45	7:3 (Tailings size: 75–150 μm)	10
C1-3	55	38:12	5	45	7:3 (Tailings size: 38–75 μm)	10
C1-4	55	38:12	5	45	7:3 (Tailings size: −38 μm)	10
C2-1	55	38:12	5	45	6:4 (Tailings size: 150–270 μm)	10
C2-2	55	38:12	5	45	6:4 (Tailings size: 75–150 μm)	10
C2-3	55	38:12	5	45	6:4 (Tailings size: 38–75 μm)	10
C2-4	55	38:12	5	45	6:4 (Tailings size: −38 μm)	10
C3-1	55	38:12	5	45	5:5 (Tailings size: 150–270 μm)	10
C3-2	55	38:12	5	45	5:5 (Tailings size: 75–150 μm)	10
C3-3	55	38:12	5	45	5:5 (Tailings size: 38–75 μm)	10
C3-4	55	38:12	5	45	5:5 (Tailings size: −38 μm)	10
C4-1	55	38:12	5	45	4:6 (Tailings size: 150–270 μm)	10
C4-2	55	38:12	5	45	4:6 (Tailings size: 75–150 μm)	10
C4-3	55	38:12	5	45	4:6 (Tailings size: 38–75 μm)	10
C4-4	55	38:12	5	45	4:6 (Tailings size: −38 μm)	10
C5-1	55	38:12	5	45	3:7 (Tailings size: 150–270 μm)	10
C5-2	55	38:12	5	45	3:7 (Tailings size: 75–150 μm)	10
C5-3	55	38:12	5	45	3:7 (Tailings size: 38–75 μm)	10
C5-4	55	38:12	5	45	3:7 (Tailings size: −38 μm)	10

displays thirty mixing ratios of SLM, with each ingredient indicated as a percentage of its mass. The tungsten tailings in C1–C5 are subjected to a screening process to divide them into four distinct categories based on particle size: 150–270 μm, 75–150 μm, 38–75 μm, and −38 μm. The combination was fortified with dispersible latex powder, a polycarboxylic acid high-efficiency water-reducing agent, a defoamer, a retarder, and hydroxypropyl methylcellulose ether in proportions of 1%, 0.2%, 0.15%, 0.05%, and 0.03% of the total solid mass, respectively. A water-to-material ratio of 0.3 was employed.

2.3. Test Methods

(1)

Fluidity

After pouring the fresh mortar into the test mold, which is placed horizontally in the center of the plate glass, lift the test mound vertically to allow the mortar to flow freely without any restrictions. Measure the vertical diameters in both directions using calipers before and 20 min after pouring the mortar without any impediment. Compute the mean of these measurements to ascertain the initial and 20-min flow angles, in accordance with the standards specified in the Cementitious Self-Levelling Mortar for Ground (JC/T 985-2017 [20]).

(2)

Flexural and compressive strengths

The newly formed mortar was placed into the test mounds, with one set of samples prepared for each mixing method to assess the strength after 24 h and 28 days. The flexural strength of a set of specimens was determined by computing the mean of their three flexural values using Equation (1).

$${\mathrm{R}}_{\mathrm{f}}=\frac{1.5{\mathrm{F}}_{\mathrm{f}}\mathrm{L}}{{\mathrm{b}}^3}$$

(1)

R_f represents the flexural strength, measured in megapascals (MPa). F_f denotes the maximum load at fracture, measured in newtons (N). L represents the distance between the supporting cylinders, which is 100 mm (mm). Lastly, b represents the side length of the prismatic square section, which measures 40 mm.

The specimens that were subjected to the flexural test were subsequently used in the compressive test, and the average of the six test results was considered as the compressive strength. The compressive strength is calculated using Equation (2).

$${\mathrm{R}}_{\mathrm{c}}=\frac{{\mathrm{F}}_{\mathrm{c}}}{\mathrm{A}}$$

(2)

R_c represents the compressive strength of a material, measured in megapascals (MPa). F_c refers to the greatest force applied to the material until it breaks, measured in newtons (N). A represents the area across which the material is squeezed, namely 1600 square millimeters (mm²). These definitions are based on the “Test Method for the Strength of Cementitious Sand (ISO Method)” (GB/T 17671-2021 [21]).

(3)

Dimensional change rate

The recently made mortar was put into test molds equipped with shrinkage heads and allowed to cure for a duration of 24 h. Subsequently, the object was removed from the mold and its original length was determined using a vertical mortar shrinkage meter 30 min after the removal. The length of the mortar was measured after it had dried for 28 days, following the same normal experimental circumstances. The rate of dimensional change for each specimen was calculated using Equation (3).

$$\mathsf{\epsilon }=\frac{{\mathrm{L}}_{\mathrm{t}}-{\mathrm{L}}_0}{\mathrm{L}-{\mathrm{L}}_{\mathrm{d}}}\times 100\%$$

(3)

ε represents the percentage rate of dimensional change. L₀ is the initial length of the specimen in millimeters. L_t indicates the length of the specimen after 28 days of drying, while L is a fixed length of 160 mm. Lastly, L_d refers to the total depth of the two copper shrinkage heads, which is 20 mm. The experimental outcome was determined by calculating the average value of the three samples. The document “Cement-based self-levelling mortar for flooring” (JC/T 985-2017) is specifically mentioned.

(4)

Porosity and water absorption

The water content and porosity of SLM can be determined by employing the water-saturation method [22,23]. The specimen, which had been subjected to standard maintenance conditions for 28 days, was completely saturated with water for 48 h. The weight of the specimen, W_w, was promptly measured when it was suspended in water. Next, the specimen was extracted from the water and any residual moisture on its surface was eliminated using gauze. The weight of the specimen, W_s, was measured soon after it reached its maximum water-absorption capacity. Subsequently, the specimen was subjected to an oven at a temperature of 105 °C until it achieved a consistent weight, hence yielding the weight of the dehydrated specimen, denoted as Wd. The water contents of the specimens were calculated using Equations (4) and (5), respectively, while the porosity was also estimated.

$$\mathrm{P}=\frac{{\mathrm{W}}_{\mathrm{s}}{-\mathrm{W}}_{\mathrm{d}}}{{\mathrm{W}}_{\mathrm{d}}}\times 100\%$$

(4)

$$\mathrm{W}=\frac{{\mathrm{W}}_{\mathrm{s}}-{\mathrm{W}}_{\mathrm{d}}}{{\mathrm{W}}_{\mathrm{s}}-{\mathrm{W}}_{\mathrm{w}}}\times 100\%$$

(5)

The variables used in the equation are as follows: P represents the water content (measured as a percentage), W represents the porosity (measured as a percentage), W_s represents the weight of the specimen when saturated and surface dry (measured in grams), W_d represents the adiabatic mass of the specimen (measured in grams), and W_w represents the mass of the specimen suspended in water in its saturated condition (measured in grams).

(5)

Toxic leaching test for heavy metals

The leaching test was conducted according to the China Solid Waste—Leaching Toxicity Leaching Method—Horizontal Oscillation Method (Ministry of Ecology and Environment, China, 2010) [24]. The samples were crushed and ground so that all of them passed through a 3 mm pore size sieve. Deionized water was mixed with the samples and added into a conical bottle with a liquid–solid ratio of 10:1. The bottle was sealed with a cap, fixed vertically on a horizontal shaking device, shaken for 8 h, and left to stand for 16 h. The ionic concentration of the leachate was determined by ICP-OES. To verify reproducibility, one sample was tested three times, and the average of the results was calculated.

(6)

Other test and analysis methods

After a specific period of controlled curing, the samples were crushed and then soaked in anhydrous ethanol for three days to stop the hydration process. Afterward, the specimens were subjected to a drying procedure in a vacuum drying oven at a temperature of 95 °C in order to prepare them for subsequent testing, specifically X-ray diffraction (XRD), infrared (FIRE), and scanning electron microscopy (SEM) analysis.

3. Results and Discussion

3.1. Effect of Aggregate-Cement Ratio on SLM Performance

Figure 4 presents the results of the investigations on how the aggregate-cement ratio affects the fluidity of SLM. A recent finding reveals that an increase in the aggregate-cement ratio negatively impacts the fluidity of SLM, leading to a more significant loss of fluidity over time. More precisely, the fluidity decreases by 6.64% within a time span of 20 min when the aggregate-cement ratio to cement is 60:40. Increasing the ratio of mortar aggregate to cement resulted in a reduction in the quantity of cement slurries per unit volume. This resulted in an insufficient amount of slurry to cover the aggregates, the inability to create a lubricating layer that caused particles to move, and an increase in internal friction between the aggregates. These factors hindered the flow of the SLM [25]. In addition, the cement paste lacks sufficient cohesion to withstand the gravitational settling of the particles, leading to segregation. This phenomenon is depicted in Figure 5, wherein the submergence of the aggregates resulted in a reduction in fluidity.

The results of the experiments on the mechanical properties of SLM with various aggregate-cement ratios are shown in Figure 6. The compressive and flexural strengths of SLM exhibited a trend of initially increasing and later dropping with the rise in the aggregate-cement ratio. With an aggregate-cement ratio of 45:55, the material exhibits a compressive strength of 12.10 MPa and a flexural strength of 4.36 MPa after 1 day. Following a period of 28 days, the compressive strength exhibits an increase to 31.32 MPa, while the flexural strength demonstrates an increase to 8.86 MPa. The material experiences a significant decrease in mechanical strength as the aggregate-cement ratio continues to rise. When the aggregate-cement ratio is 60:40, the compressive strength of 1 day decreases by 49.34% and the flexural strength decreases by 52.52% compared to a ratio of 45:55. Similarly, the compressive strength of 28 day decreases by 49.14% and the flexural strength decreases by 35.89% compared to a ratio of 45:55. An increase in the aggregate-cement ratio resulted in a higher quantity of sand being utilized for each unit volume of mortar. Consequently, there was a decrease in the quantity of cement utilized. There was not enough slurry to completely cover the surface of the aggregate and an increase in the empty space between the particles, resulting in an augmentation in the porosity of the SLM [26]. The presence of a thin layer of cement slurry results in a fragile connection between the aggregate and the cement. Additionally, the interface transition zone between the aggregate particles and the cement slurry is susceptible to compression-induced damage [27]. These parameters combined to influence the level of compactness of the SLM process and ultimately led to a reduction in mechanical strength.

The findings of the studies on the influence of the aggregate-cement ratio on the rate of dimensional change of SLM are illustrated in Figure 7. The rate of change in the dimensions of SLM increases progressively as the aggregate-cement ratio increases. The excessive presence of aggregate leads to water leakage, the formation of numerous water leakage channels, increased porosity of SLM, and reduced homogeneity of the internal mixture of SLM. Consequently, the rate of size change during the SLM hardening process increases.

3.2. Effect of Cement-Mix Ratio on SLM Performance

Figure 8 demonstrates the influence of the OPC:SAC ratio on the fluidity of the SLM. As the OPC:SAC ratio increases, the fluidity of the SLM decreases progressively; liquidity losses have increased compared to the initial state. More precisely, when the OPC:SAC ratios are 32:18 and 29:21, the fluidity after 20 min is below 130 mm. Excessive amounts of SAC increase the rate at which mortar undergoes early hydration and quickly deplete the available water in the mortar [28]. Within a span of 20 min, the fluidity of the SLM experiences a notable reduction, while the SLM solidifies prematurely.

Figure 9 illustrates the influence of the OPC:SAC ratio on the mechanical properties of SLM. Progressively increasing the ratio of OPC:SAC improves the initial compressive and flexural strengths of SLM but reduces the strength at later stages. SAC demonstrates a greater rate of hydration in comparison to OPC, leading to a quicker formation of initial strength. Ettringite (AFt) is the main chemical formed during the hydration process of SAC [29]. The reaction occurs rapidly and requires significant amounts of water, as described in Equation (6) [30]. This ensures that SLM solidifies quickly during a 24-h period. Ettringite crystals commonly display a shape that is either prism-like, needle-like, or flake-like [31]. The substantial pores are rapidly occupied, supplying the SLM with initial structural integrity.

$$3\left(\mathrm{C}\mathrm{a}\mathrm{O}·{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3\right)+{3\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{O}}_4+{38\mathrm{H}}_2\mathrm{O}={3\mathrm{C}\mathrm{a}\mathrm{O}·\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3·{3\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{O}}_4·{32\mathrm{H}}_2\mathrm{O}+4{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$$

(6)

The inclusion of OPC content had minimal impact on the progress of the final compressive strength in the OPC:SAC blend system. However, the bending strength at 28 days for the OPC:SAC ratio of 29:21 reduced by 35.89% in comparison to the ratio of 41:9. The reduction of strength after 28 days may be attributed to the breakdown of AFt due to carbonation during the hydration process [32,33]. Another potential factor may be the presence of microcracking caused by excessive expansion [34].

The results of the investigation on the rate of dimensional change in SLM caused by the OPC:SAC ratio are shown in Figure 10. The specimens made of a mixture of OPC and SAC showed dimensional variations ranging from 0.069% to 0.043%. Furthermore, as the SAC content in the system increases, the rate of size change steadily decreases. The contraction and expansion of the cementitious material are mainly determined by the pore structure. The presence of the hydration product, AFt, limits the deformation of the SLM, leading to a decrease in the porosity of the material.

3.3. Effect of Tungsten Tailings on SLM Performance

Figure 11 presents the results of fluidity research on SLM, where different particle sizes of tungsten tailings were used as a replacement. More precisely, after replacing 60% of the original sand with tungsten tailings particles measuring between 38–75 μm, the initial fluidity and fluidity after 20 min attained their peak values at 163 mm and 155 mm, respectively. By substituting natural sand with tungsten tailings in the −38 μm size range, at a proportion of 30 to 70 percent, the fluidity decreased to less than 130 mm within a 20-min period. The presence of these tiny particles increases the specific surface area, which in turn increases the demand for water in the environment [35]. To ensure adequate coverage of the particles’ surface, a greater amount of liquid phase is necessary, which consequently reduces the fluidity of the SLM. Indeed, it is feasible to utilize more refined tailings as aggregates. However, doing this necessitates a careful equilibrium between the flow and stability of the mortar, which can be enhanced through the application of chemical admixtures, albeit frequently at a substantial expense. To produce the best gradation, the fine aggregate consists of tungsten tailings with a particle size ranging from 38 to 75 μm and natural sand with a particle size ranging from 80 to 270 μm. This reduces the void ratio and improves the density of the SLM. Furthermore, there is an improved attachment of cement paste to the aggregate surface, resulting in a greater distance between the aggregates and a decrease in friction. Consequently, the self-leveling mortar achieves an ideal level of fluidity.

Figure 12 presents the results of the investigations on the mechanical properties of SLM while using tungsten tailings with different particle sizes as alternative materials. The SLM 1 day flexural strength demonstrates a trend of initially increasing and subsequently decreasing as the level of tungsten-tailings replacement increases, throughout particle sizes ranging from 150–270, 75–150, and 38–75 μm. Nevertheless, the flexural strength of SLM 1 day exhibits a constant decline when the grain size diminishes to less than −38 μm. Across all grain sizes, the 28-day flexural strength exhibits a pattern of initially increasing and subsequently decreasing as the proportion of tungsten tailings substitution rises. The flexural strength reaches its maximum value when the tungsten tailings replacement reaches 60%. The SLM 1 day material has its best flexural strength within the grain-size range of 75–150 μm. However, its ability to further grow in strength over time is limited, with a range of only 3.90 to 5.18 MPa seen during a 28-day period of hydration. Substituting 30%, 40%, 50%, 60%, and 70% of natural sand with tungsten tailings significantly improved the late-strength properties of the 38–75 μm grain size. The improvements, relative to the control C0, were 8.48%, 13.63%, 21.56%, 26.01%, and 17.52%, respectively. The enhancement was accomplished by infusing the inner pore volume of the SLM with finely textured tungsten tailings, leading to increased strength as a result of the enhanced pore structure. However, this behavior can only be observed within the limitations of this particular particle size. When the grain size is reduced below −38 μm, there is a significant drop in strength. Comparatively, tungsten tailings exhibit a reduction in particle size, an augmentation in specific surface area, and an improved capacity to absorb unbound water in contrast to natural sand [36,37]. Research has indicated that aggregates with high water-absorption capacity can disturb the interface between aggregates and cement. This disruption negatively affects the process of hydration and ultimately leads to a decrease in strength [38,39,40].

Figure 13b shows that using tungsten tailings with grain sizes between 75–150 μm and 38–75 μm significantly improves the compressive strength of the SLM during the anaphase. The compressive strength of 28d SLM increased by 11.64% and 18.81% when 60% of the natural sand was substituted with tungsten tailings, in comparison to the control C0. The delayed increase in strength of the SLM was influenced by the particle size, chemical composition, and shape of tungsten tailings. The surface of tungsten tailings has a rough and uneven texture, which differs from the smooth texture of natural sand. As hydration progressed, tungsten tailings were increasingly interconnected with the cementitious material, leading to the creation of more densely packed reticulation. The aggregates were fully encapsulated by the cementitious material, resulting in an increased compressive strength.

Figure 14 displays the research findings about the SLM 28 day dimensional rate of change, which is based on varying amounts of tungsten-tailings replacement and their related particle sizes. More precisely, when the amount of tungsten tailings with different particle sizes increases, the rate of change in the dimensional of the 28d SLM initially decreases and then later increases. By replacing natural sand with finer particles of tungsten tailings, the correlation between the various sizes of tungsten tailings is strengthened, leading to a higher density of the mixture. Research indicates that increasing the number of minuscule particles in the mixture improves its capacity to retain water [41]. This reduces the probability of SLM segregation and water infiltration. Moreover, it decreases the quantity of unbound water that is lost from the SLM. However, the addition of tungsten tailings leads to a higher density of powder in the SLM. As a result, this creates a higher amount of cement paste, which then results in a faster rate of dimensional change in the SLM. Therefore, creating a logical relationship between the larger and smaller particles in SLM can significantly improve its dimensional stability.

3.4. Mechanism and Microanalysis of Tungsten Tailing Action

3.4.1. Porosity and Water Absorption

The influence of tungsten tailings on the porosity and water absorption of SLM was evaluated for different amounts of substitution and particle dimensions, as seen in Figure 15. The experimental findings indicate that when the quantity of tungsten tailings utilized as a replacement increases, both the porosity and water absorption of the SLM decrease. The reason for this phenomenon can be ascribed to the morphology of grain. Natural sand possesses a smooth and rounded surface, whereas tungsten tailings exhibit a rough and irregular shape. As per the principle of sphere stacking, the maximum porosity is attained when spheres of equal diameter are stacked [42]. Porosity is an essential determinant that directly influences the mechanical robustness and pace of dimensional alteration of SLM. When tungsten tailings were introduced at different percentages (30%, 40%, 50%, 60%, and 70%) to the SLM, the porosity at the grain level of 38–75 μm dropped by 13.60%, 14.07%, 15.95%, 21.58%, and 32.46% accordingly, compared to the control group without any tungsten tailings. Tungsten-tailings particles exhibit a reduced particle size and are uniformly distributed inside the cement paste, in contrast to natural sand. This leads to a significant decrease in the porosity of SLM. Moreover, the decreased porosity and denser microstructure may account for the reduced water absorption seen in SLM utilizing tungsten tailings, which is consistent with the porosity evaluations.

3.4.2. XRD Mineral Phase Analysis

Figure 16 illustrates the XRD analyses conducted at 1 day and 28 days of cure. The findings suggest that the SLM is comprised of quartz (SiO₂), calcite (CaCO₃), ettringite (AFt), hydrated calcium silicate or C-S-H gel, and gypsum (CaSO₄) after 1 day of curing. According to Figure 16b, there was no gypsum found in the SLM after 28 days of curing. Nevertheless, a hydrated calcium zeolite phase (CaAl₂Si₂O₈-4H₂O) was generated in the final stages of hydration. This phase corresponds to the prominent apex of hydrated calcium silica-aluminate in the particle diffraction file PDF#20-0452. Furthermore, the intensity of the distinctive peak of the C-S-H gel reaction product grew gradually as the doping of tungsten tailings increased at an angle of 2θ = 50. Chen et al. [43] state that the enhanced strength observed in the later stage of the mixture is mostly due to the strong bonding of hydration products such as ettringite, hydrocalcite zeolite, and C-S-H gel. This bonding leads to a more compact internal microstructure. The existence of natural sand and tungsten tailings is attributed to the occurrence of the quartz phase. The process of silicate cement hydration entails the chemical reaction of tricalcium silicate (7) and dicalcium silicate (8) [44]. Tungsten tailings consist of a substantial quantity of aluminosilicate minerals, such as feldspar, quartz, and mica. The main constituents of these minerals are predominantly SiO₂ and Al₂O₃. When the tailings undergo a reaction with Ca(OH)₂ during the hydration process, it results in the formation of hydrotalcite zeolite, as depicted in Equation (9). According to Liu [45], the crystal’s presence has been found to increase the strength of the mixture.

$${3\mathrm{C}\mathrm{a}\mathrm{O}·\mathrm{S}\mathrm{i}\mathrm{O}}_2+{\mathrm{n}\mathrm{H}}_2\mathrm{O}={\mathrm{x}\mathrm{C}\mathrm{a}\mathrm{O}·\mathrm{S}\mathrm{i}\mathrm{O}}_2·{(\mathrm{n}-3+\mathrm{x})\mathrm{H}}_2\mathrm{O}+{(3-\mathrm{x})\mathrm{C}\mathrm{a}\left(\mathrm{O}\mathrm{H}\right)}_2$$

(7)

$${2\mathrm{C}\mathrm{a}\mathrm{O}·\mathrm{S}\mathrm{i}\mathrm{O}}_2+{\mathrm{n}\mathrm{H}}_2\mathrm{O}={\mathrm{x}\mathrm{C}\mathrm{a}\mathrm{O}·\mathrm{S}\mathrm{i}\mathrm{O}}_2·{(2-3+\mathrm{x})\mathrm{H}}_2\mathrm{O}+{(2-\mathrm{x})\mathrm{C}\mathrm{a}\left(\mathrm{O}\mathrm{H}\right)}_2$$

(8)

$${\mathrm{C}\mathrm{a}\left(\mathrm{O}\mathrm{H}\right)}_2+{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3+{\mathrm{S}\mathrm{i}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}={\mathrm{C}\mathrm{a}\mathrm{A}\mathrm{l}}_2{\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_8·{4\mathrm{H}}_2\mathrm{O}\left(\mathrm{G}\mathrm{i}\mathrm{s}\mathrm{m}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{e}\right)$$

(9)

3.4.3. FTIR Infrared Analysis

Figure 17 demonstrates how replacing tungsten tailings with different grain sizes affects the infrared vibrational bands of the SLM. The identification of silicate and quartz in the mortar mixture is confirmed by the peaks observed at wave numbers 475 cm⁻¹ (Al-O) and 535 cm⁻¹ (Si-O) [46]. The absorption peaks detected at 713 cm⁻¹, 875 cm⁻¹, and 1420 cm⁻¹ in Figure 17a,b are indicative of distinct vibrations of the C-O bond in carbonate. The peak at 713 cm⁻¹ corresponds to the in-plane bending vibration, the peak at 875 cm⁻¹ corresponds to the out-of-plane bending vibration, and the peak at 1420 cm⁻¹ corresponds to the out-of-plane asymmetric stretching vibration [47,48]. It is important to mention that the maximum levels of light transmission are detected at the highest points of group C4-3, as determined by the XRD study. This is where the absorption peak of calcite, which occurs at approximately 2θ = 29°, aligns with group C4. The presence of Si-O tetrahedra can be attributed to the absorption peak observed at a wavenumber of 996 cm⁻¹ [49], as referenced in [35]. The strong peak observed at 1200 cm⁻¹ corresponds to the asymmetric stretching vibration of Si-O-T, where T might be either Si or Al [50]. This indicates that silicate minerals and aluminate minerals are actively involved in the process of hydration. The band seen at 1640 cm⁻¹ is most likely attributed to the stretching vibration of the H-O-H bond in the water found in the mortar mixture [51]. However, the presence of the band seen at 2510 cm⁻¹ suggests the formation of either CaCO₃ or Na₂CO₃ [52]. The dominant source of vibrations responsible for the peak of about 3400 cm⁻¹ is hydrogen bonding in the H-OH or Si-OH groups [53]. The intensity of the peak at 3400 cm⁻¹ decreases at 28d compared to 1 day, suggesting that the free water in the system converts into bound water in the form of hydroxyls as hydration advances. The absorption peak detected at 3644 cm⁻¹ is caused by the stretching vibration of the hydroxyl (-OH) group in calcium hydroxide (Ca(OH)₂) [49].

3.4.4. SEM Microstructure Analysis

Figure 18 displays scanning electron microscope (SEM) images of C0 and C4-3 specimens taken at different stages of the curing process. Figure 18a,c clearly show that a large amount of prismatic calomel is produced, filling the pores of the SLM and contributing to the initial mechanical strength within the first day of hydration. Figure 18a illustrates the existence of Ca(OH)₂ with a hexagonal crystal system structure. Figure 18b,d display a substantial amount of interconnected, grouped C-S-H gels. Figure 18d clearly displays the prominent presence of the hydration product C-S-H (calcium silicate hydrate). The minute particles of tungsten tailings were nearly entirely enveloped by the hydration product known as C-S-H. The work of Bazzoni [54] indicated that during the acceleration phase, clusters of C-S-H (NEEDLE) progressively form around the cement particles and quickly increase in size until they reach a specific length and envelope a significant portion of the mineral’s exterior. This corresponds to the shift from Figure 18c,d, where the C-S-H gel transforms from the prismatic form seen in Figure 18c to the needle-shaped form depicted in Figure 18d. The main result of the hydration process in SLM is the formation of calcium-silicate-hydrate (C-S-H), which enhances the bonding between particles and thus enhances the mechanical properties of SLM.

3.4.5. Leaching Toxicity Test Results

Group C4-3 mortar samples were tested for toxic leaching procedures. The leaching concentrations of four heavy metals, lead, cadmium, chromium, and arsenic were explored for mortar samples at 3-, 28-, and 90-day curing ages. The average values of the samples at different curing ages were compared with the Hazardous Waste Landfill Pollution Control Standards [55] and Hazardous Waste Identification Criteria—Leaching Toxicity Identification [56], as shown in

Table 3. Heavy metal leaching results of self-leveling mortar sample (mg/L).

Inspection Items		Pb	Cd	Cr	As
Test results	3 day	0.141	0.073	0.450	0.053
	28 day	0.122	0.021	0.364	0.033
	90 day	0.023	0	0.22	0.009
Limits 1 [55]		1.2	0.6	15	1.2
Limits 2 [56]		5	1	5	5

.

At different ages, the concentration of heavy metals in the sample leaching solution was lower than the limit value. SLM can solidify the heavy metals in the tungsten tailings and reduce the leaching capacity of the tailings. After curing for a long time, the heavy metal content in the leaching solution was greatly reduced, and the curing rate of cadmium ion reached 100% when the curing period was 90 days.

4. Conclusions

(1)

The flow properties of SLM are adversely affected by a high aggregate-cement ratio. The mechanical qualities of SLM are optimal when the aggregate-cement ratio is 45:55. The formation of water leakage channels is the key factor in increasing the porosity of SLM.

(2)

The early strength of SLM increased rapidly with the higher SAC content. However, excessive SAC caused a quick depletion of free water, worsening the loss of SLM fluidity. At OPC:SAC ratios of 32:18 and 29:21, the 20-min fluidity fell below the minimum limit of 130 mm.

(3)

When 60% of natural sand is replaced by tungsten tailings, the grain grade of 38–75 μm demonstrates exceptional working performance. The initial and 20 min fluidity were 163 mm and 155 mm, respectively, while the 28d compressive and flexural strengths were 26.53 MPa and 9.06 MPa. Moreover, the dimensional shrinkage rate over a period of 28 days remained consistently below 0.1%.

(4)

The cement-based self-leveling mortar made from tungsten tailings has excellent immobilization effectiveness for heavy metals such as lead, cadmium, chromium, and arsenic. Following a 90-day curing period, no traces of cadmium were found in the leaching solution, indicating a 100% cure rate for cadmium.

(5)

The accumulation of fine particles in the tailings is a constant problem. A novel method for producing self-leveling mortar using tungsten tailings with a particle size of −75 μm was suggested as a means of treating aluminosilicate tailings. This approach not only tackles the problem of environmental contamination resulting from the accumulation of tailings but also reduces the production costs of the construction industry. Future endeavors should focus on assuring the long-lasting stability of tailings application in SLM.