The Effect of Copper Tailings Sand on the Workability and Mechanical Properties of Concrete

Abstract

Concrete materials are widely used in engineering projects, with fine aggregates (sand) being a key component currently in short supply. Copper tailings sand, a waste by-product of copper mining, accumulates in large quantities in tailings dams. Recycling and reusing this waste sand is crucial for environmental sustainability. This paper investigated the incorporation of copper tailings sand into concrete as a partial replacement for fine aggregates and evaluated its effects on concrete’s workability and mechanical properties. The experimental results indicate that the addition of copper tailings sand reduced the workability and compressive strength of concrete. Specifically, at a 60% substitution rate, the slump of the concrete was reduced by 15%, and the axial strength was closest to that of ordinary concrete, with a reduction of 2.5%. As the proportion of copper tailings sand increased from 0% to 80%, the average axial strength decreased from 37.3 MPa to 34.9 MPa, and stiffness decreased by approximately 6.43%. A complete stress–strain curve equation was proposed based on fitting relevant parameters, closely aligning with experimental data. Moderately adding tailings sand can help address the issue of large amounts of waste sand accumulating in tailings dams while maintaining acceptable concrete properties.

1. Introduction

With the rapid economic development and accelerated urbanization in China, the number of industrial and civil buildings has surged, leading to a dramatic increase in the demand for concrete. Currently, the annual sand consumption reaches 2 billion tons [1,2,3]. Traditionally, natural river sand has been used as fine aggregate in concrete, but extensive mining has severely damaged the ecological environment, prompting many cities to ban its extraction [4,5,6]. Additionally, the vast amounts of tailings produced by mines across the country not only occupy arable land but also pollute the environment.

To address the shortage of natural river sand and reduce the environmental harm from tailings, using tailings sand as a concrete aggregate has become an effective solution [7,8,9]. Copper tailings sand, due to its similar material properties to natural sand, is considered a potential substitute for fine aggregate. Although copper tailings sand typically has higher mechanical strength, its cracks and defects can adversely affect the mechanical properties and durability of concrete if used excessively [10,11,12]. Therefore, it is crucial to adjust the ratio of natural sand to tailings sand to produce concrete with excellent performance. Zhao et al. [13] found that iron mine tailings could replace up to 40% of natural aggregate in ultra-high-performance concrete (UHPC) while maintaining similar mechanical properties to the control group. Research also indicates that replacement ratios above 40% significantly reduce the workability and compressive strength of concrete. However, with steam curing, samples substituting below 40% showed an increase in flexural strength, with a decrease in compressive strength not exceeding 11%. Similarly, Li et al. [14] studied the workability and compressive strength of copper tailings sand concrete at different replacement rates by using ground powdered iron tailings to replace cement and iron tailings sand to replace fine aggregate. Huang et al. [15] configured plastic concrete with different tailings sand ratios and cement content, which exhibited low elastic modulus, good impermeability, and plastic deformation characteristics. They conducted tests on the cubic compressive strength, axial strength, and elastic modulus of tailings sand plastic concrete specimens. Additionally, Zhao et al. [16], through their research on the characteristics of iron ore tailings (IOT) and their application in concrete, found that iron ore tailings concrete possesses good workability, mechanical properties, and durability. Studies show that an appropriate proportion of iron ore tailings substitution can enhance the overall performance of concrete without compromising its quality. These results indicate that using mine tailings sand to replace fine aggregates in concrete is a new method to achieve comprehensive utilization of solid waste from mines. To further explore the mechanical properties of tailings sand concrete, researchers have conducted in-depth studies on the uniaxial compressive stress-strain curves of concrete mixed with tailings sand.

Currently, focusing on the uniaxial compressive stress–strain full curves of concrete with tailings sand, Kang et al. prepared concrete with strength grades ranging from C20 to C55 using iron tailings sand as a substitute for ordinary sand. They analyzed the uniaxial compressive stress–strain full curves of copper tailings sand concrete at different strength levels. Similarly, Wang et al. [17] studied the full stress–strain curves of iron tailings sand concrete at different strength levels. They found that the ascending segment of the stress–strain curves was almost identical across strengths, with a faster descent in higher strength grades, similar to the characteristics of ordinary concrete stress–strain curves.

Furthermore, Zhang [18] conducted bending tests on iron tailings sand concrete beams and found that the stress–strain characteristics of the iron tailings sand concrete beams were fundamentally the same as those of ordinary concrete beams, except that the strain in the longitudinal reinforcement of the iron tailings sand concrete beams was slightly higher than that in ordinary concrete beams. Wang [19] analyzed the load-deflection curves of iron tailings sand reinforced concrete beams, finding that the cracking, yielding, and ultimate load of the iron-tailings-sand-reinforced concrete beams at the same strength level were roughly the same as those of ordinary reinforced concrete beams. Meanwhile, many foreign scholars and engineers have also conducted extensive research on the full curves of uniaxial compressive stress and strain in concrete. For instance, Thomas et al. [20], through the analysis of the uniaxial compressive stress–strain curves of copper tailings concrete, demonstrated its good mechanical properties and durability. However, current research on the uniaxial compressive stress–strain full curves of tailings sand concrete is mostly concentrated on testing concrete of different strengths, without testing concrete with different tailings sand contents.

To analyze the impact of different copper tailings sand contents on the uniaxial compressive stress–strain full curves of copper tailings sand concrete, this experiment prepared copper tailings sand concrete of the same strength grade with copper tailings sand contents of 0%, 20%, 40%, 60%, and 80%. By conducting full-range uniaxial compression tests on the copper tailings sand concrete blocks and combining existing theoretical models, the effects of different copper tailings sand contents on the uniaxial compressive stress–strain full curves of the concrete were analyzed.

2. Materials and Methods

2.1. Materials

(1) Cement: P 042.5 cement produced by Xi’an Cement Factory was used. The chemical composition and basic physical properties of the cement are shown in

Table 1. Chemical composition of cement (%).

Fe2O	Al2O3	SiO2	CaO	MgO	SO3	Other
3.37	5.31	19.72	62.33	2.33	3.33	3.61

and

Table 2. Basic physical properties of cement.

Density/(g/cm3)	Specific Surface Area/(m2/kg)	Setting Time/min	Flexural Strength/MPa	Compressive Strength/MPa
3.07	361	210	5.9	29.4

, respectively.

(2) Mine tailings sand: The copper tailings sand used in the experiments was sourced from a tailings storage facility of Ansteel Group. Tailings morphology is shown in Figure 1. Upon acquisition of the tailings sand, its density and particle size distribution were determined using a pycnometer method and a Mastersizer 3000E laser particle size analyzer, respectively. The density of the tailings sand is 3.1730 g/cm³, and the particle size distribution of the copper mine tailings sand is shown in Figure 2.

From the particle size distribution curve, the following values were determined: d₁₀ = 6.05 μm, d₃₀ = 21.7 μm, d₆₀ = 63.9 μm. The quality of the tailings sand’s granularity is also characterized by the coefficient of uniformity (C_U, which describes the range of particle sizes, generally considered well-graded when greater than 5) and the coefficient of curvature (C_C, which describes the continuity of the material grading, generally considered well-graded when between 1 and 3, indicating a high degree of compaction of the material) [21].

$$C_U=\frac{d_{60}}{d_{10}}$$

(1)

In the formula, C_U represents the coefficient of uniformity of the tailings sand;

$$C_C=\frac{d_{30}^2}{d_{10}\times d_{60}}$$

(2)

In the formula, C_C represents the curvature coefficient of the tailings sand. The calculations yield a C_U value of 10.5620 and a C_C value of 1.2180 for the copper mine tailings sand, indicating a well-graded particle size distribution.

To determine the performance of concrete prepared with copper tailings, the experiments mainly used XRD (X-ray diffraction) and chemical element calibration methods for analysis. The basic chemical composition determined quantitatively is shown in

Table 3. Chemical properties of tailings (%).

Fe2O3	Al2O3	SiO2	CaO	MgO	S	Other
23.41	10.18	42.77	4.56	3.28	0.92	14.88

.

(3) Fly ash: Selected for use based on its effectiveness in reducing the heat of hydration in large-volume concrete, minimizing slump loss over time, and increasing the workability of fresh concrete under economical conditions, Class II fly ash was chosen. The mix design began with tests for chemical and physical-mechanical properties, with the results shown in

Table 4. Chemical properties of fly ash (%).

Fe2O3	Al2O3	SiO2	CaO	MgO	SO3	Other
5.20	27.32	53.47	0.79	4.21	4.32	4.69

and

Table 5. Physical and mechanical properties.

Density/(kg/m3)	Specific Surface Area/(kg/m2)	Fineness/(%,50 μm)	Activity Index/(%)
2250	406	25.3	86

.

(4) Aggregates: The fine aggregate used was ISO standard sand provided by Xiamen Aisou Standard Sand Co., Ltd., Xiamen, China with a maximum particle size of 1.18 mm, apparent density of 2.66 g/cm³, and fineness modulus of 2.0. The coarse aggregate used is continuously graded crushed granite, ranging from 5 mm to 10 mm, with an apparent density of 2.68 g/cm³.

(5) Water-reducing agent: The third-generation polycarboxylic acid provided by Subote New Materials Co., Ltd., Xiamen, China.

(6) Water: Conventional tap water from Xi’an city.

2.2. Design Scheme

The designed strength for the copper tailings sand concrete was C30. The experiment was divided into 5 groups, each comprising 6 blocks. The specific mix proportions are shown in

Table 6. Concrete mix design.

No.	Water/(kg/m3)	Cement/(kg/m3)	Fly Ash/(kg/m3)	Coarse Aggregate (kg/m3)	Fine Aggregate/(kg/m3)	Tailings Sand/(kg/m3)	Water-Reducing Agent/(kg/m3)
1	165	300	100	1170	630	0	10
2	165	300	100	1170	504	126	10
3	165	300	100	1170	378	252	10
4	165	300	100	1170	252	378	10
5	165	300	100	1170	126	504	10

.

2.3. Experimental Procedure

2.3.1. Slump Test Method

The slump cone used in the test had a height of 300 mm, a top diameter of 100 mm, and a bottom diameter of 200 mm. The detailed procedure for the slump test is described as follows [22,23,24]: Initially, the necessary tools, including the concrete sample, a small slump cone, a metal rod, a trowel, scales, a measuring tape, and other equipment, were prepared. The concrete sample was prepared on-site and tested within 30 min of mixing. The concrete was poured into a clean, oil-free slump cone; filled to the top; and gently vibrated with a metal rod to remove air pockets and level the surface. To measure the slump, the bottom of the slump cone was opened, allowing the concrete to fall freely onto the measuring tape below. The height of the settled concrete was then measured to determine the slump. The slump for each sample was recorded, and the average was calculated. After the test, the slump cone, metal rod, trowel, and other tools were thoroughly cleaned with water and dried. Photos of the slump and spread from this experiment are shown in Figure 3.

2.3.2. Uniaxial Strength Test

The concrete specimens used in the test had dimensions of 150 mm × 150 mm × 150 mm, as shown in Figure 4. Six specimens were prepared for each mix ratio, totaling 30 specimens, which were cured under standard conditions for 28 days. One day before testing, the specimens were taken out to air dry. The tests were conducted using a 300 ton MTS electro-hydraulic servo testing machine, ensuring the total stiffness of the loading device was greater than the maximum negative stiffness of the descending segment. To capture the full compressive curve of the specimens, the test initially applied a load at a rate of 0.5 kN/s. When the applied load reached approximately 50% of the ultimate load, the control was switched to displacement at a rate of 0.03 mm/min. All specimens were loaded to the residual strength stage. Data were collected and processed using the press’s microcomputer control system and IMC data acquisition system, and the stress–strain curves of the specimens were generated using Origin software version 2022.

The axial compression test is conducted in accordance with the “Standard for Test Method of Mechanical Properties of Ordinary Concrete” (GB/T50081-2002) [25]. The test data calculation formula is as follows:

$$f_{\mathrm{cp}}=\frac{F}{A}$$

(3)

where $f_{\mathrm{cp}}$ is the axial strength of the concrete, F is the failure load of the specimen, and A is the bearing area of the specimen.

3. Experimental Results and Analysis

3.1. Flow Characteristics of Copper Tailings Sand Concrete

The flow properties of copper tailings sand concrete were tested based on the experimental design plan, and the resulting flow characteristics curves are depicted below.

As shown in Figure 5, with the increase in the substitution rate of copper tailings sand, the slump and spread of the copper tailings sand concrete initially increased and then decreased. The flowability of the copper tailings sand concrete was optimal when the content of copper tailings sand reached 60%. Beyond this point, as the content of copper tailings sand increased, the flowability of the concrete decreased. This behavior can be explained as follows: The increase in slump and spread as the copper tailings sand content increased from 0% to 60% can be attributed to the particle filling effect, which outweighs the increase in friction caused by the particles’ specific surface area [26,27]. The good gradation of the tailings sand particles enhanced the flowability of the concrete. However, when the copper tailings sand content increased to 80%, the total specific surface area of the tailings sand particles also increased, enlarging the contact area between particles and water. This resulted in an increased water demand for the mixture. The excessive amount of aggregate surpassed the positive effects of particle filling, and the high content of tailings sand made the mixture overly dense, preventing the water from sufficiently wetting all particles [28,29].

3.2. Mechanical Properties Experimental Results

3.2.1. Failure Process and Morphology

Figure 6 illustrates the failure morphology of copper tailings sand concrete. The first visible crack on the specimen’s surface appeared after the load reached its ultimate point and the stress–strain curve began to enter its descending phase. This crack was vertical to the ground and was located at the center of the concrete casting surface. As the loading continued, the crack’s development compromised the bond between the coarse aggregate and the cement mortar, weakening the concrete’s shear resistance. This led to the formation of two new diagonal cracks, typically positioned in the upper left and lower right (or upper right and lower left) of the initial vertical crack. With increasing strain, the diagonal cracks rapidly developed and eventually merged to form a main diagonal crack that spanned the entire interface.

The failure mode of both copper tailings sand concrete and ordinary concrete was primarily due to the bond failure between the coarse aggregate and the cement mortar, with no fractures observed in the coarse aggregate. This indicates that the failure morphology of copper tailings sand concrete was similar to that of ordinary concrete. Due to the uniqueness of each specimen, the failure process cannot be exactly the same for all specimens. Local defects in the specimens may influence crack formation, but the overall shape of the stress–strain curve remained largely unchanged.

3.2.2. Axial strength of Copper Tailings Sand Concrete

Figure 7 presents the axial strength of concrete with varying percentages of copper tailings sand. Generally, concrete containing copper tailings sand exhibits a lower axial strength compared to ordinary concrete. Analyzing the average uniaxial strength of concrete samples with different mix proportions, the following observations can be made:

When the percentage of copper tailings sand ranged from 0% to 40%, the axial strength decreased progressively with the increase in copper tailings sand content. At 40% incorporation, the strength of the copper tailings sand concrete was reduced by 11.9% compared to ordinary concrete. However, between 40% and 80% incorporation, the axial strength first increased and then decreased, peaking at a 60% incorporation level. At this level, the axial strength of copper tailings sand concrete was 10.7% higher than at 40% incorporation and only 2.5% lower compared to ordinary concrete, marking the smallest deviation from ordinary concrete strength under all studied proportions. At 80% incorporation, the strength decrease was 7.2% compared to ordinary concrete.

This pattern can be attributed to the rough and angular nature of copper tailings sand particles, which have higher internal friction. When a small amount of copper tailings sand was added, the primary fine aggregate was ordinary sand, and the friction between copper tailings sand and ordinary sand was relatively low, leading to a reduction in axial strength. As the percentage of copper tailings sand surpassed 40%, the primary contributing factor became the friction among copper tailings sand particles, which slightly improved as their proportion increased. However, when the incorporation exceeded 80%, almost all the fine aggregate was copper tailings sand. Since the fineness modulus of copper tailings sand was less than that of ordinary sand, the cohesiveness of the copper tailings sand concrete was lower than that of ordinary concrete, resulting in lower axial strength. Additionally, the reduced consistency of the copper tailings sand concrete also significantly impacted its axial strength. As the percentage of copper tailings sand increased, the flowability and workability of the concrete decreased, affecting its compaction and pore structure. At 40% incorporation, the reduction in consistency led to an increased air content within the mixture, decreasing the concrete’s density and thus impacting its structural integrity and load-bearing capacity. The lowered consistency facilitated the formation of microcracks and damage under stress, further reducing the axial strength. Therefore, adjustments in the proportion of tailings and control of consistency are crucial for optimizing the mechanical properties of copper tailings sand concrete.

3.2.3. Effect of Different Copper Tailings Sand Contents on the Uniaxial Stress–Strain Full Curves

For ease of analysis, the stress–strain full curves in this experiment are represented using dimensionless coordinates. The uniaxial compressive stress–strain full curves of copper tailings sand concrete are shown in Figure 8. This visualization allows for a direct comparison of the mechanical behavior across different mix ratios, illustrating the varying degrees of concrete ductility and strength resilience as the copper tailings sand content changed.

Figure 8 presents the full stress–strain curves for concrete under uniaxial compression. In this study, Concrete Mix 1, made without copper tailings sand, served as the ordinary concrete reference. Concrete Mix 2 contained 20% copper tailings sand, Mix 3 contained 40%, Mix 4 contained 60%, and Mix 5 contained 80% copper tailings sand. Comparative curves are shown in Figure 8 (note that although six samples were prepared for each mix proportion, only one sample from each mix was selected for mechanical property analysis).

As depicted in Figure 9, the stress–strain curves of concrete with a small amount of copper tailings sand did not significantly differ from those of ordinary concrete. In these mixes, the strength reduction in the initial stage of the descending segment was slow, and in the latter stage, it nearly aligned with that of ordinary concrete. The area under the curve was also similar to that of ordinary concrete. As the content of copper tailings sand increased, the initial failure characteristics of the concrete resembled those of ordinary concrete. However, in the later stages of the descending segment, the concrete failed rapidly and stabilized quickly. Additionally, the area enclosed by the curve and the coordinate axis progressively decreased with increasing copper tailings sand content. This indicates that as the amount of copper tailings sand increased, the concrete’s stiffness decreased, its ability to resist elastic deformation weakened, and its brittleness increased.

3.2.4. Theoretical Model Analysis

Here, a stress–strain equation with fewer parameters, clear significance, and good agreement with experimental results was adopted from the “Code for Design of Concrete Structures” (GB50010-2010) [30].

$$y=ax+(3-2a)x^2+(a-2)x^3\hspace{1em}(0⩽x⩽1)$$

(4)

$$y=\frac{x}{b{(x-1)}^2+x}$$

(5)

In the formula, $x=\epsilon /{\epsilon }_0$, $\epsilon$ denotes the strain at any given point, while ${\epsilon }_0$ represents the peak strain. $y=\sigma /{\sigma }_{\max }$, $\sigma$ denotes the stress at any given point, while ${\sigma }_{\max }$ represents the peak stress. a and b are empirical coefficients related to the strength of concrete. The coefficient a indicates the ratio of the initial elastic modulus to the peak secant modulus, while b is a parameter that reflects the brittleness of the concrete, with higher values indicating increased brittleness. By modifying the values of a and b, different complete stress–strain curves for concrete can be generated. These coefficients a and b can be calculated using Formulas (4) to (5) based on the data from the stress–strain curves obtained in experiments. These calculations are typically performed using the least squares method. The computed values are presented in

Table 7. Values of a and b for different copper tailings sand contents.

Empirical Coefficients	Copper Tailings Sand/%
	0	20	40	60	80
a	0.6	0.5	0.4	0.3	−0.7
b	0.25	0.2	0.3	0.4	0.5

as follows:

The comparison between the theoretical curve and the actual full stress–strain curve under uniaxial compression is shown in Figure 10.

Figure 9 illustrates the comparison between the theoretical curves and actual curves for uniaxial compressive stress–strain relationships. Through fitting analysis, values of the concrete parameters a and b were derived under different copper tailings sand contents. It was observed that the a value decreased with an increase in the copper tailings sand content, indicating a reduction in the elastic modulus and elasticity of the concrete. Conversely, the b value increased as the copper tailings sand content increased, suggesting an increase in the brittleness of the concrete.

From Figure 9, it can be seen that the stress–strain full curves of the copper tailings sand concrete matched well with the theoretical curves in the ascending phase and the early descending phase (before the main diagonal cracks appear). However, there was greater variability in the later descending phase (after the main diagonal cracks appeared). This variability in the descending phase of the stress–strain curves of the copper tailings sand concrete was closely related to the development of internal microcracks, such as initial microcracks, voids, the arrangement of coarse aggregates, and the adhesion between aggregates and cement mortar, all of which affect the progression of microcrack development.

Furthermore, the main diagonal cracks and macroscopic main diagonal failure planes in the concrete specimens, which appear after the specimens reach the peak load and then undergo further loading, only affected the residual strength and later deformation of the specimens. These are considered late failure modes that impact only the latter part of the descending phase of the full stress–strain curves, having virtually no effect on the ascending phase [31,32,33]. Due to the weaker bonding capacity between aggregates and cement mortar in copper tailings sand concrete compared to ordinary concrete, the load-bearing capacity in the descending phase was reduced more rapidly. Additionally, in the later stages of concrete structures, when cracks permeated and the internal structure was essentially unable to continue bearing load, this stage did not affect the load-bearing capacity of the structure. This indicates that the model from the “Code for Design of Concrete Structures” is still applicable to the copper tailings sand concrete used in this experiment, although the values of the constitutive parameters differ.

4. Conclusions and Discussion

This study explores the feasibility of using copper tailings sand as a fine aggregate in concrete through experimental methods and analyzes its effects on the workability and mechanical properties of concrete. The main conclusions are as follows:

(1) The addition of copper tailings sand adversely affects the workability and compressive strength of concrete, yet at a 60% substitution level, the compressive strength was closest to that of ordinary concrete. The inclusion of copper tailings sand led to a reduction in concrete stiffness, a decrease in its resistance to elastic deformation, and an increase in brittleness. Particularly at a 40% incorporation level, the reduction in consistency resulted in decreased concrete density and compromised structural integrity, which in turn impacts its axial strength. Therefore, adjusting the proportion of tailings and controlling the consistency of the concrete are crucial for optimizing the mechanical properties of copper tailings sand concrete.

(2) The failure mode of copper tailings sand concrete is fundamentally similar to that of ordinary concrete, demonstrating that copper tailings sand concrete shares similar structural characteristics with ordinary concrete, especially within the 40% to 60% substitution range. When the substitution level exceeded 60%, the strength and workability of the concrete significantly decreased, necessitating adjustments in the mix proportions to optimize performance.

(3) Stress–strain curve analysis revealed that copper tailings sand concrete exhibits initial hardening behavior similar to ordinary concrete under compressive stress, but it showed more pronounced brittle failure characteristics as it approached peak stress. This phenomenon is related to the rough and angular characteristics of copper tailings sand, which increase internal friction within the concrete but also reduce its overall cohesion.

(4) The use of copper tailings sand not only helps alleviate the shortage of natural sand but also promotes the resource utilization of mining waste, aligning with the requirements of sustainable development. Particularly under environmental and economic pressures, finding effective pathways for the resource utilization of waste materials becomes increasingly important.

Future work can further explore the potential applications of different types of tailings sand in concrete and how to optimize the use of these materials through improved mix designs, thereby enhancing the comprehensive utilization of industrial waste. Additionally, given that mine tailings are waste products screened by mineral processing plants containing certain chemical reagents, future studies should investigate the impact of residual chemical reagents in tailings sand on the durability of concrete.