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Article

Development and Performance Evaluation of Translucent Concrete Incorporating Activated Copper Tailings as Cementitious Material

by
Guangdong An
,
Siyang Li
,
Zhaorui Li
,
Zhaohui He
,
Kai Li
,
Ping Ning
,
Xiangyu Wang
* and
Xin Sun
*
Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(18), 10228; https://doi.org/10.3390/app151810228
Submission received: 25 August 2025 / Revised: 11 September 2025 / Accepted: 15 September 2025 / Published: 19 September 2025
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Abstract

This study reports a method for producing translucent concrete using alkali-activated copper tailings, aiming to advance the valorization of solid waste and the development of sustainable construction materials. Under optimal conditions—600 °C calcination, 10 wt% CaO, and a 1:2 water-to-solid ratio—the material achieved a maximum 28-day compressive strength of 52.7 MPa, accompanied by a significantly reduced setting time. Leaching tests indicated that Cu, Zn, Pb, and As concentrations were well below the standard limits, ensuring environmental safety. Further optimization revealed that incorporating 40 wt% cement and 2 wt% polypropylene fibers (1 mm in diameter) provided the best balance between light transmission and mechanical performance. Microstructural analyses (XRD and SEM) confirmed the formation of C–S–H and C–A–S–H gels with minor Ca(OH)2, which densified the matrix and enhanced strength. Despite these promising results, potential variations in the tailing composition and challenges associated with industrial-scale implementation must be considered. Overall, this work elucidates the hydration and solidification mechanisms of copper-tailing-based translucent concrete and highlights its potential for environmentally sustainable and functional construction materials.

1. Introduction

Cement is the most widely consumed anthropogenic material, yet its production accounts for approximately 8% of global CO2 emissions due to limestone calcination and fossil fuel combustion [1,2,3]. Concurrently, mining industries generate large volumes of copper tailings, which are often stockpiled, occupying land and posing environmental risks from heavy metals [1,4]. Conventional utilization of copper tailings is largely limited to low-value applications, underscoring the urgent need for high-performance valorization pathways that advance circular economy principles [5,6].
Aluminosilicate-rich wastes, such as copper tailings, can serve as precursors for alkali-activated binders, producing amorphous or semi-crystalline networks with a significantly reduced carbon footprint and excellent heavy metal immobilization compared to OPC [7,8]. At the same time, light-transmitting concrete (LTC), produced by embedding optical fibers within a matrix, offers aesthetic and energy-saving benefits, but conventional OPC-based LTC suffers from high fiber requirements and mechanical limitations [9,10,11]. Previous LTC studies have predominantly focused on fiber arrangement and geometry [12], while the use of low-reactivity copper tailings as a primary binder for sustainable LTC remains unexplored.
This study addresses this gap by developing a novel alkali-activated copper-tailing-based light-transmitting concrete (AACT-TC). The specific objectives are (i) to optimize alkali-activation conditions through the water-to-binder ratio and activator composition, (ii) to fabricate AACT-TC specimens incorporating optical fibers, and (iii) to evaluate the effects of key design parameters on compressive strength, heavy metal immobilization, and light-transmitting performance. We hypothesize that synergistic hydration between copper tailings and cementitious components will densify the matrix, thereby enhancing both strength and optical functionality and ultimately providing a sustainable route for functional building materials. It should be noted, however, that other critical engineering properties—such as flexural strength, durability under wet–dry cycling, and resistance to thermal variation—are beyond the scope of the present work and will be systematically investigated in future studies.

2. Materials and Methods

2.1. Raw Materials

The copper tailings (CT) utilized in this study were sourced from Yunnan Tin Group (Holding) Company Limited (Mengzi, China). The alkaline activators, including sodium hydroxide (NaOH, purity ≥ 96.0%) and sodium silicate (Na2SiO3), were purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). The light-transmitting material used was a polypropylene-fiber-based plastic optical fiber, supplied by Toray Industries ((China) Co., Ltd., Shanghai, China.) For comparison, ordinary Portland cement (P.O 42.5 grade), produced by Huaxing Environmental Protection Engineering Technology Co., Ltd. (Pingxiang, China), was employed as the reference cementitious material.
The chemical composition of CT was determined through X-ray fluorescence (XRF) analysis (Table 1), revealing that the major oxides were Fe2O3 (43.14%), SiO2 (36.36%), and CaO (3.41%). Given that environmental impact assessment is crucial for tailing utilization, we conducted a quantitative analysis of potentially toxic heavy metals using ICP–MS. The results showed significant concentrations of Fe (1520 mg/kg), Zn (149 mg/kg), Mn (167 mg/kg), and Mg (198 mg/kg), while As (10 mg/kg), Cr (58 mg/kg), and Cd (12 mg/kg) were also detected. These heavy metals may be released during material preparation processes, thereby posing potential risks to ecosystems and human health. Therefore, the quantitative analysis of these elements provides a relevant basis for the subsequent assessment of material toxicity. Brunauer–Emmett–Teller (BET) analysis (Figure 1a,b) indicated that the CT possessed a specific surface area of 1.25 m2/g, with pore size distribution predominantly centered around 10 nm. X-ray diffraction (XRD) patterns (Figure 1c) revealed that the primary crystalline phases in the copper tailings were fayalite (Fe2SiO4), magnetite (Fe3O4), and a Ca–Fe–Mg silicate mineral [Ca(Fe,Mg)Si2O6]. Notably, the detected heavy metals are likely associated with these phases—for instance, Cu and Pb may be encapsulated within iron oxides and silicates or exist as trace sulfide forms (such as chalcopyrite or galena), which directly influences their leaching behavior. Furthermore, scanning electron microscopy (SEM) images equipped with energy-dispersive spectroscopy (EDS) (Figure 1d) demonstrated that the CT particles exhibited irregular morphology, with smaller particles adhering to the surfaces of larger ones.

2.2. Sample Preparation

2.2.1. Preparation of Cementitious Specimens and Experimental Methods

This study primarily employed single-factor experiments to investigate the effects of calcination temperature, activator dosage, and water-to-solid ratio on the properties of the material through a series of systematically designed paste preparation tests:
  • Raw material measurement
Copper tailings and alkali activators were weighed according to the designed proportions (Table 2). Distilled water was added in volumes of 30, 35, 40, 45, and 50 mL, depending on the specific experimental group.
2.
Mixing process
The copper tailings and activator were first placed into a cement paste mixer and dry-mixed for 5 min to ensure uniform dispersion. Pre-measured distilled water was then gradually added, followed by an additional 15 min of mixing to obtain a homogeneous slurry.
3.
Molding and compaction
The slurry was poured into cubic molds (20 × 20 × 20 mm) and vibrated for 90 s to eliminate entrapped air bubbles and achieve full compaction. The initial and final setting times were recorded.
4.
Curing
Specimens were first placed in a standard curing chamber (20 ± 2 °C, relative humidity ≥ 95%) for 24 h. After demolding, they were further cured under the same conditions until the designated testing ages (7 d and 28 d).
5.
Testing
After curing, the specimens were subjected to compressive strength testing, heavy metal leaching analysis, and mineralogical characterization.

2.2.2. Preparation and Experimental Methods for Light-Transmitting Concrete Specimens

  • Fiber embedding
Polypropylene fibers (diameter: 1 mm), chosen for their transparency, flexibility, and corrosion resistance, were used as the light-transmitting medium. Plastic molds (20 × 20 × 20 mm) were drilled with holes on four sides (front, back, left, and right), through which the fibers were inserted in a predefined arrangement.
2.
Mixing and casting
A specified mass of alkali-activated copper tailings and cement (Table 2) was weighed. Distilled water (50 mL) was added after the dry mixing stage (5 min), followed by 15 min of wet mixing to obtain a uniform paste. The prepared paste was poured into the fiber-pre-embedded molds and vibrated for 90 s. The initial setting time was recorded immediately.
3.
Curing and testing
Specimens were cured in a standard curing chamber (20 ± 2 °C, relative humidity ≥ 95%) for 24 h. After demolding, they were further cured until 7 and 28 d. Finally, compressive strength tests and heavy metal leaching analyses were carried out to evaluate both mechanical performance and environmental safety.
In summary, Table 2 presents the precise dosages of the various raw materials together with the corresponding liquid-to-solid ratios, thereby offering a comprehensive reference for the subsequent experimental design and analysis.
Table 2. Mixing proportions of cementitious materials and light-transmitting concrete specimens.
Table 2. Mixing proportions of cementitious materials and light-transmitting concrete specimens.
SchemeTotal Mass/gActivatorActivator Content in Total Binder/wt%Water-to-Solid RatioCement Content in Light-Transmitting
Concrete/wt%		Polypropylene Fiber Content/wt%
Cementitious paste A100NaOH103:10––
100NaOH207:20––
100NaOH302:5––
100NaOH409:20––
100NaOH501:2––
Cementitious paste B100CaO103:10––
100CaO207:20––
100CaO302:5––
100CaO409:20––
100CaO501:2––
Cementitious paste C100Na2SiO3103:10––
100Na2SiO3207:20––
100Na2SiO3302:5––
100Na2SiO3409:20––
100Na2SiO3501:2––
Cementitious paste D100Na2CO3103:10––
100Na2CO3207:20––
100Na2CO3302:5––
100Na2CO3409:20––
100Na2CO3501:2––
Concrete specimen100CaO101:29:1–
100CaO101:28:2–
100CaO101:27:3–
100CaO101:26:4–
100CaO101:25:5–
Light-transmitting concrete specimen100CaO101:26:41
100CaO101:26:42
100CaO101:26:43
100CaO101:26:44
100CaO101:26:45

2.3. Characterization of Material Methods

The pore size distribution and specific surface area of the materials were determined using the Brunauer–Emmett–Teller (BET) method with a surface area and porosity analyzer (ASAP 2460, Micromeritics, Norcross, GA, USA). Phase composition analyses of the copper tailings, alkali-activated cementitious materials, and translucent concrete specimens were conducted using an X-ray diffractometer (XRD, X’Pert PRO, PANalytical, Almelo, The Netherlands) equipped with Cu–Kα radiation, operated at 40 kV and 40 mA. The XRD patterns were collected over a 2θ range of 10–80°, with a step size of 0.02° and a scanning rate of 2°/min. Elemental compositions of the materials were examined by X-ray fluorescence spectroscopy (XRF, Axios, PANalytical, Almelo, The Netherlands). The microstructural characteristics of the raw copper tailings, cementitious materials, and translucent concrete were observed using a scanning electron microscope (SEM, Regulus8100, Hitachi High-Technologies, Tokyo, Japan). Energy-dispersive spectroscopy (EDS) was employed in conjunction with SEM to analyze the surface elemental distribution and morphological features of hydration products.

3. Results and Discussion

3.1. Optimization of the Performance of Copper-Tailing-Based Cementitious Materials

3.1.1. Study on the Effect of Water-to-Solid Ratio on the Performance of Cementitious Materials

To investigate the influence of the water-to-solid ratio on the performance of copper-tailing-based cementitious materials, a systematic study was conducted under varying water-to-solid ratio conditions (3:10, 7:20, 2:5, 9:20, and 1:2). The setting time, mechanical strength, and heavy metal leaching behavior of the materials were comprehensively evaluated. The results revealed that both initial and final setting times increased with increasing water content. Specifically, the initial setting time was prolonged from 1012 min to 1176 min, while the final setting time extended from 1056 min to 1268 min (Figure 2a). This trend is primarily attributed to the dilution effect induced by the increased water content, which decreases the concentration of reactive ions in the system. In this context, the term reactive ions refers mainly to species that actively participate in the alkali-activation and hydration processes, including SiO44−, AlO45−, OH−, and Ca2+. These ions are considered reactive because they directly contribute to the dissolution of precursors, the formation of intermediate species, and the subsequent polymerization or precipitation reactions that govern the setting and hardening behavior [13,14]. As their concentration decreases, the nucleation rate and ionic diffusion are hindered, thereby retarding the hydration kinetics and prolonging the setting process [15,16].
In terms of mechanical performance, both the 7-day and 28-day compressive strengths exhibited a progressive increase with a higher water content (Figure 2b). When the water-to-solid ratio reached 1:2, the material achieved its maximum compressive strengths of 19.6 MPa at 7 days and 38.7 MPa at 28 days. This enhancement can be attributed to the synergistic contribution of two factors: (i) the additional water facilitated the dissolution and transport of reactive species (Si, Al, and Ca), thereby promoting more complete alkali-activation reactions and the formation of greater amounts of binding gels [17,18,19]; and (ii) the improved particle packing reduced the volume of interconnected pores. Together, these processes resulted in a denser microstructure with superior mechanical strength [20,21,22]. The 7-day compressive strength provides insights into the early-stage hydration and structural development. Although the 28-day strength is commonly used as a conventional benchmark for cementitious materials, the strength development and microstructural evolution of alkali-activated composites may continue beyond 28 days [23,24]. Nevertheless, for the purposes of comparison in this study, the 28-day strength was selected as an important indicator of mid-to-late-stage performance. The continuous increase in strength from 7 to 28 days suggests that sufficient water not only promotes early hydration but also sustains long-term hydration and microstructural densification, ultimately improving overall mechanical performance [25,26,27].
Furthermore, the analysis of heavy metal leaching behavior at 28 days showed a significant reduction in the leaching concentrations of Cu, Zn, Pb, and As with increasing water content (Figure 2c). Under the condition of 50 mL water addition, the lowest leaching concentrations were recorded as follows: Cu = 0.037 mg·L−1 (Cu), 0.073 mg·L−1 (Zn), 1.348 mg·L−1 (Pb), and 0.168 mg·L−1 (As), all of which are well below the limits specified by GB/T 5086.3-2007 [28]. This reduction is ascribed to the higher hydration degree, which leads to a denser matrix and the formation of stable hydration products, thereby enhancing both the physical encapsulation and chemical immobilization of heavy metals [29,30].

3.1.2. Investigation of the Effects of Different Activators and Their Dosages on the Performance of Cementitious Materials

To assess the influence of activator dosage, all experiments were performed at a fixed water-to-solid ratio of 1:2. The initial and final setting times were determined using the Vicat needle method following GB/T 5086.3-2007 [28] to ensure reproducibility. With the increase in activator dosage, all types of activators showed a decreasing trend in both initial and final setting times. Among them, the CaO activator resulted in the longest setting time, indicating a slower dissolution rate and delayed initiation of the hydration process (Table 3). This phenomenon can be attributed to the relatively low concentration of Ca2+ released in the early stage, which leads to a slower increase in the system’s pH. As a result, the nucleation rate of hydration products is reduced, thereby prolonging the setting process [31,32].
Regarding compressive strength, a notable decreasing trend was observed in most activator systems as the dosage increased (Figure 3a), a phenomenon that contrasts with some conventional findings. We hypothesize that this inverse relationship may be attributed to excessive alkalinity disrupting the formation of a well-structured gel phase and potentially leading to premature crystallization or increased micro-porosity [33,34]. In contrast, the CaO-activated system exhibited superior mechanical performance, achieving compressive strengths of 19.6 MPa at 7 days and 38.7 MPa at 28 days when 10 wt% CaO was incorporated. The initially lower 7-day strength is likely due to slower, incomplete early-stage hydration typical of CaO-activated systems [35,36]. The significant strength gain at 28 days indicates progressive pozzolanic reactions and ongoing microstructural densification, culminating in a more stable and robust matrix [37,38]. Consequently, CaO was identified as the optimal activator for subsequent investigations into heavy metal immobilization efficiency across different curing stages.
At 28 days, heavy metal leaching was found to be inversely related to compressive strength (Figure 3b). As strength increased, the leaching concentrations of Cu, Zn, Pb, and As decreased significantly. In the CaO-activated system, the lowest leaching levels were observed: 0.037 mg·L−1 (Cu), 0.073 mg·L−1 (Zn), 1.348 mg·L−1 (Pb), and 0.168 mg·L−1 (As). These results indicate that a denser matrix enhances both the physical encapsulation and chemical immobilization of heavy metals [39,40].
X-ray diffraction (XRD) analysis supported these findings (Figure 3c). In the CaO-activated samples, the original olivine phase (Fe2SiO4) decomposed, while new phases such as magnetite (Fe3O4) and calcium silicate appeared. These changes suggest effective activation of amorphous components in the tailings. The sharp and intense diffraction peaks indicate improved crystallinity and significant structural transformation. The formation of calcium silicate hydrate (C–S–H) significantly contributed to both strength development and porosity reduction, enhancing immobilization through a dual mechanism of physical entrapment and chemical bonding. The presence of the C–S–H phase was confirmed by X-ray diffraction (XRD) analysis, which showed broad and characteristic hump-like peaks centered around 29° 2θ, consistent with poorly crystalline C–S–H, as reported in previous studies [41,42]. Although C–S–H is typically poorly crystalline and may not present sharp diffraction peaks, its presence was supported by the attenuation of quartz peaks and the emergence of diffuse scattering in the diffractograms, indicative of gel formation [43].
Mechanistically, Ca2+ ions released from CaO promote the dissolution of Si- and Al-rich phases in the tailings, leading to the formation of C–S–H and crystalline calcium silicates. These hydration products form compact, impermeable networks that physically entrap heavy metal ions [44]. Simultaneously, heavy metals may substitute into the interlayers of C–S–H or precipitate as stable, insoluble phases (e.g., M–OH and M–Si species), reducing their mobility. This immobilization mechanism aligns with findings reported in related studies [30,45,46].

3.1.3. Effect of Calcination Temperature on the Performance of Cementitious Materials

As shown in Figure 4a, both the initial and final setting times first increased and then decreased with increasing calcination temperature. The maximum values were recorded at 600 °C, reaching 1286 min and 1319 min, respectively. This indicates a nonlinear evolution of hydration kinetics and microstructural densification with temperature. A similar trend was observed in compressive strength (Figure 4b). At 600 °C, the compressive strength peaked at 19.6 MPa (7 days) and 38.7 MPa (28 days), reflecting excellent early-age and long-term mechanical performance.
Meanwhile, as shown in Figure 4c, the leaching concentrations of Cu, Zn, Pb, and As decreased significantly with rising temperature. At 600 °C, the concentrations dropped to 0.037 mg/L (Cu), 0.073 mg/L (Zn), 1.348 mg/L (Pb), and 0.168 mg/L (As). This inverse relationship between strength and leaching indicates that improved mechanical properties are accompanied by the enhanced encapsulation and stabilization of heavy metals [47,48].
XRD patterns (Figure 4d) further confirmed that with increasing calcination temperature—particularly at 600 °C—the olivine phase (Fe2SiO4) in the copper tailings decomposed, while new crystalline phases such as magnetite (Fe3O4) and calcium silicates were formed. These phase transitions enhanced the structural integrity and demonstrated the pozzolanic activity of the system. The results indicate that the addition of CaO not only promoted the thermal decomposition and phase transformation of the original minerals in CT but also facilitated the in situ formation of C–S–H gel [49,50]. While CT itself is not traditionally classified as a pozzolanic material, the thermal activation and CaO treatment induced reactive silica and alumina phases in the system, thus imparting pozzolanic characteristics necessary for the formation of cementitious gels [51,52]. These findings suggest that the addition of CaO acted as a fluxing agent, lowering the thermal decomposition temperature of the original minerals in CT and thereby accelerating their breakdown during heating. This process led to phase reconstruction, referring to the transformation and reorganization of mineral phases into new, reactive compounds, such as calcium silicates and aluminates [53,54]. These newly formed phases subsequently facilitated the in situ formation of C–S–H gel, enhancing the material’s mechanical properties and stability. This dual effect contributed to improved mechanical strength and environmental stability [55,56]. Therefore, the optimal activation condition was achieved at a calcination temperature of 600 °C with a CaO content of 10 wt%. Under these conditions, the material exhibited superior strength, effective heavy metal stabilization, and a dense microstructure, offering a promising pathway for the resource utilization of copper tailings in engineering applications.

3.2. Investigation of the Performance of Copper-Tailing-Based Light-Transmitting Concrete

3.2.1. Effect of Different Cement Contents on the Setting Time of Light-Transmitting Concrete

The setting time and compressive strength of translucent concrete specimens prepared with different copper tailing–cement ratios are presented in Figure 5a. As the cement content increased, the initial setting time decreased from 1205 min to 1086 min, while the final setting time dropped from 1284 min to 1157 min. This reduction in setting time is attributed to the accelerated hydration kinetics induced by a higher cement content, which promotes faster structural development.
Figure 5b indicates that the compressive strength at both 7 and 28 days increased initially and then slightly declined. The highest strength was observed at a 6:4 ratio, reaching 32.8 MPa at 7 days and 52.7 MPa at 28 days. This improvement is closely related to the enhanced formation of hydration products, such as C–S–H gel, which improves pore filling and crystal bonding, thus strengthening the matrix. This trend is consistent with the findings of Liu et al., who reported that cement addition promotes strength development in slag-based binder systems [57].
In addition, Figure 5c shows the leaching concentrations of heavy metals (Cu, Zn, Pb, and As) across different mix ratios. The variations were minor, and all concentrations remained well below the national regulatory limits. This indicates that the translucent concrete matrix, regardless of cement dosage, effectively immobilizes heavy metal ions by forming a dense microstructure and providing a favorable encapsulation environment. The stabilization mechanism is primarily attributed to the synergistic effects of the alkaline environment and the encapsulating action of cement hydration products. The high alkalinity promotes the precipitation of metal ions as hydroxides or other insoluble compounds, thereby reducing their solubility [29,58,59]. Simultaneously, the formation of C–S–H and related hydration gels provides a physical encapsulation effect, entrapping metal ions within the dense gel network [60,61]. Moreover, chemical interactions such as ion exchange and surface complexation further contribute to immobilization, resulting in both reduced mobility and the long-term passivation of heavy metals within the matrix [62].

3.2.2. Effect of Light-Transmitting Specimen Mix Proportions on the Setting Time of Light-Transmitting Concrete

As the cement content in the binder increased from 10 wt% to 50 wt%, the initial setting time decreased from 1205 min to 1086 min, and the final setting time was shortened from 1284 min to 1157 min (Figure 6a). This continuous decline is primarily attributed to the higher cement content, which introduces a greater amount of reactive calcium silicates into the system. During the early hydration stage, these calcium silicates readily react with water to form calcium silicate hydrate (C–S–H) gel, accompanied by the release of Ca(OH)2 [63]. The rapid formation of C–S–H gel accelerates the nucleation and growth of the solid matrix, while the released Ca(OH)2 further enhances the alkalinity of the system, thereby synergistically promoting faster setting and consolidation of the binder matrix [64].
Figure 6b shows the development of compressive strength at 7 and 28 days under different cement-to-copper tailings ratios. With increasing cement content, the compressive strength initially increased and then slightly decreased. Specifically, as the cement content increased from 10 wt% to 50 wt%, the 7-day compressive strength initially rose from 22.4 MPa to 32.8 MPa before slightly decreasing to 31.4 MPa, while the 28-day strength increased from 41.6 MPa to a peak of 52.7 MPa and subsequently declined to 48.9 MPa. The highest strength values were observed at a ratio of 6:4, indicating a synergistic interaction between cement and copper tailings. An appropriate cement content accelerated hydration and improved the pore structure, thereby enhancing mechanical performance. However, excessive cement may lead to increased hydration heat, early shrinkage cracking, or heterogeneous microstructures, slightly reducing long-term strength [65,66].
Figure 6c presents the leaching concentrations of heavy metals (Cu, Zn, Pb, and As) in translucent concrete under varying cement contents. The leaching behaviors of heavy metals Cu and Zn follow a similar trend. The results indicate that all leaching concentrations remained at low levels regardless of cement dosage. This environmental stability is mainly attributed to enhanced hydration and microstructural densification at higher cement contents, which promote physical encapsulation and the chemical immobilization of heavy metals within C–S–H gels and other hydration products [4]. Moreover, secondary minerals such as ettringite and Fe–Si gels may further stabilize multivalent metal ions through ion exchange or surface complexation.
Notably, the leaching behavior was largely unaffected by cement dosage, demonstrating a robust stabilization capacity. This effectiveness is specifically attributed to the unique chemical environment provided by Portlandite (Ca(OH)2) in ordinary Portland cement (OPC), which may not be fully replicated by other alkali activators (e.g., sodium silicate or sodium hydroxide). The key mechanisms include the following: (1) a persistently high-pH environment buffered by Portlandite, which not only promotes the precipitation of metal hydroxides but also prevents a pH drop that could lead to re-dissolution, a critical advantage over non-buffered activators [67]; (2) the formation of a dense, highly cross-linked C–S–H gel, which is characteristic of OPC hydration and provides superior physical encapsulation; (3) the isomorphic substitution of Ca2+ by various metal cations within the C–S–H structure, facilitated by the specific Ca/Si ratio and layer structure of OPC-derived C–S–H. However, some alternative activators may produce lower-dimensional or less stable reaction products (e.g., N–A–S–H gel in geopolymers) with differing metal incorporation capacities. Even at a low cement content of 10%, this synergistic multi-mechanistic system within the OPC matrix proved highly efficient, underscoring its distinct advantage for reliable heavy metal immobilization and promising application in sustainable construction.

3.2.3. Effect of Light-Transmitting Material Diameter on the Performance of Light-Transmitting Concrete

The geometric dimensions of polypropylene (PP) fibers, serving as light-guiding media in translucent concrete, have a decisive impact on the hydration kinetics, mechanical performance, and environmental stability of the composite. Experimental investigations on fiber diameters in the range of 1–5 mm revealed a significant negative correlation between fiber thickness and the mechanical performance of the material (Figure 7b). As the fiber diameter increases from 1 mm to 5 mm, the initial setting time extends from 1109 min to 1217 min, and the final setting time increases from 1157 min to 1277 min, showing a consistent delay in hydration (Figure 7a). Meanwhile, the compressive strength at 7 and 28 days decreases monotonically, with the 28-day strength dropping from 52.7 MPa (1 mm fiber) to 42.3 MPa (5 mm fiber) (Figure 7b).
This hydration retardation and strength loss are attributed to the quality variation of the interfacial transition zone (ITZ) formed between the fiber and cement matrix. Coarser fibers raise the local water–cement ratio and hinder the efficient packing of cement particles, resulting in a porous and weak region enriched with oriented Ca(OH)2 crystals [68]. This wall effect not only delays the formation of a continuous C–S–H gel network, thus prolonging setting time, but also acts as a stress concentration site and microcrack initiation zone under loading, reducing the overall load-bearing capacity. In contrast, finer fibers (1 mm diameter) with a higher surface area promote the heterogeneous nucleation of hydration products and generate a denser, better-bonded ITZ, thereby maximizing mechanical strength.
In contrast to their pronounced effect on mechanical properties, the fiber diameter shows no clear correlation with the leaching behavior of heavy metals (Cu, Zn, Pb, and As) (Figure 7c). The leaching behaviors of heavy metals Cu and Zn follow a similar trend. All tested samples exhibit metal concentrations well below national regulatory limits, indicating excellent environmental safety. This can be attributed to the physical encapsulation and chemical adsorption of heavy metals by cement hydration products, along with microstructural densification induced by PP fibers. Particularly at a fiber diameter of 1 mm, the denser matrix and reduced porosity significantly inhibit the migration and release of heavy metals. Therefore, using finer PP fibers not only enhances compressive strength and accelerates setting but also maintains effective heavy metal immobilization while preserving mechanical performance [69,70,71].
To further reveal the microstructural characteristics and performance mechanisms of translucent concrete made from activated copper slag–cement blends with PP fibers, XRD and SEM analyses were performed on light-transmitting concrete specimens incorporating fibers with a diameter of 1 mm and containing 40 wt% cement after 28 days of curing. The XRD patterns indicate the presence of magnetite (Fe3O4), calcium silicate, and minor Ca(OH)2 phases. The sharp diffraction peaks suggest high crystallinity, implying sufficient hydration (Figure 7d). The decomposition of fayalite (Fe2SiO4) promotes the formation of Fe3O4, enhancing the compactness and mechanical stability of the matrix. Meanwhile, the hydration of calcium silicate produces C–S–H gel, which fills voids and improves strength. The precipitation of Ca(OH)2 raises alkalinity, activating the secondary hydration of latent components and forming additional C–A–S–H phases, which synergistically reinforce the material.
These observations are supported by SEM images, which reveal a uniform distribution of both granular and blocky hydration products, with minimal porosity and microcracks (Figure 7e). At the microscale, a continuous C–S–H gel network is formed. Polypropylene (PP) fibers act as three-dimensional reinforcing elements, limiting crack propagation and improving stress distribution, thereby enhancing both the mechanical strength and structural stability of the material. Previous studies have also demonstrated that the incorporation of polymer fibers can refine the microstructure and promote early-stage hydration in cement-based systems [72].

4. Conclusions

In this study, a novel translucent concrete was successfully developed using alkali-activated copper tailings as a sustainable cementitious material. Under optimized conditions, the binder exhibited a maximum compressive strength of 38.7 MPa at 28 days. Characterization techniques such as XRD and SEM revealed the activation mechanism. The development of mechanical strength was primarily attributed to the formation of a dense matrix of hydration products, including calcium silicate hydrate (C–S–H) and calcium aluminum silicate hydrate (C–A–S–H) gels, accompanied by depolymerization of the silicate network.
Based on the optimized binder, translucent concrete specimens were prepared by incorporating 40 wt% cement and embedding polypropylene optical fibers (1 mm in diameter, 2 vol%) via a post-insertion method. The resulting composites exhibited a compressive strength of 52.7 MPa at 28 days. Performance analysis indicated that the synergistic hydration between copper tailings and cement enhanced the overall degree of hydration, where the pozzolanic reactivity of copper tailings facilitated the dissolution of reactive silica and alumina. These species subsequently reacted with calcium hydroxide from cement hydration to form additional C–S–H and C–A–S–H gels, thereby accelerating calcium hydroxide consumption, densifying the microstructure, and improving the mechanical properties.
Furthermore, the leaching concentrations of heavy metals (Cu, Zn, Pb, and As) in all optimized formulations remained well below national regulatory limits, confirming good environmental safety. The light-transmitting performance of the composites was strongly correlated with both the fiber content and diameter, underscoring their potential for application in green and energy-efficient building design. Nevertheless, it should be noted that direct external validation through experimental benchmarking against existing translucent concrete systems was not performed in this study. Future research will address this limitation by conducting systematic cross-validation with representative reference materials to more comprehensively assess the magnitude of the advances achieved.

Author Contributions

Conceptualization, X.W. and X.S.; data curation, G.A. and S.L.; formal analysis, G.A., S.L., and X.S.; funding acquisition, K.L., X.S., and P.N.; resources, S.L. and G.A.; investigation, Z.L. and S.L.; methodology, G.A., S.L., and Z.L.; software, Z.L. and Z.H.; validation, Z.H., S.L., and Z.L.; writing—original draft, G.A.; writing—review and editing, K.L., X.W., and X.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant Nos., 52270106, 22266021, and 52204362), Natural Science Foundation of Yunnan Province (Grant No. 202201AU070015, 202301AU070060), Yunnan Major Scientific and Technological Projects (Grant No. 202505AT350002, 202401AT070400), and the Yunnan Key Laboratory of Phosphogypsum Recycling and Ecological Utilization (Grant No. 202449CE340028).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Due to the ongoing use of the data in further research, the data supporting this study are not publicly available at this time to avoid compromising the research progress. Researchers who require access to the raw data may contact the corresponding author to request it. We will provide the data on a case-by-case basis, ensuring compliance with all relevant regulations.

Acknowledgments

This work was supported by the Faculty of Environmental Science and Engineering, Kunming University of Science and Technology. We confirm that all individuals mentioned here have provided consent for acknowledgement.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 15 10228 g001
Figure 1. Physicochemical characterization of copper tailings: (a) N2 adsorption–desorption isotherms (black and white squares represent the adsorption and desorption curves, respectively), (b) pore size distribution, (c) XRD pattern, and (d) SEM micrograph.
Figure 1. Physicochemical characterization of copper tailings: (a) N2 adsorption–desorption isotherms (black and white squares represent the adsorption and desorption curves, respectively), (b) pore size distribution, (c) XRD pattern, and (d) SEM micrograph.
Applsci 15 10228 g001
Applsci 15 10228 g002
Figure 2. Effect of water-to-solid ratio on cementitious materials: (a) setting time, (b) compressive strength development, and (c) heavy metal leaching concentrations.
Figure 2. Effect of water-to-solid ratio on cementitious materials: (a) setting time, (b) compressive strength development, and (c) heavy metal leaching concentrations.
Applsci 15 10228 g002
Applsci 15 10228 g003
Figure 3. (a) Effect of activator content on the compressive strength of cementitious materials; (b) effect of varying CaO content on heavy metal leaching concentrations in cementitious materials; (c) XRD patterns of cementitious materials with different CaO contents.
Figure 3. (a) Effect of activator content on the compressive strength of cementitious materials; (b) effect of varying CaO content on heavy metal leaching concentrations in cementitious materials; (c) XRD patterns of cementitious materials with different CaO contents.
Applsci 15 10228 g003
Applsci 15 10228 g004
Figure 4. Effects of calcination temperature on the (a) setting time, (b) compressive strength development, (c) heavy metal leaching concentrations, and (d) XRD patterns of cementitious materials.
Figure 4. Effects of calcination temperature on the (a) setting time, (b) compressive strength development, (c) heavy metal leaching concentrations, and (d) XRD patterns of cementitious materials.
Applsci 15 10228 g004
Applsci 15 10228 g005
Figure 5. Effects of different cement proportions on the (a) setting time, (b) compressive strength evolution, and (c) heavy metal leaching of translucent concrete specimens.
Figure 5. Effects of different cement proportions on the (a) setting time, (b) compressive strength evolution, and (c) heavy metal leaching of translucent concrete specimens.
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Figure 6. Effects of varying polypropylene fiber contents on the (a) setting time, (b) compressive strength evolution, and (c) heavy metal leaching of translucent concrete specimens and red circles (representing Zn) are consistent.
Figure 6. Effects of varying polypropylene fiber contents on the (a) setting time, (b) compressive strength evolution, and (c) heavy metal leaching of translucent concrete specimens and red circles (representing Zn) are consistent.
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Figure 7. (a) Effect of polypropylene fiber diameter on the setting time of the specimens; (b) evolution of compressive strength over time; (c) variation in heavy metal leaching concentrations of light-transmitting concrete incorporating polypropylene fibers of different diameters (The trends of the black squares (representing Cu) and red circles (representing Zn) are consistent.); (d) XRD pattern and (e) SEM image analysis of light-transmitting concrete specimens containing 1 mm diameter polypropylene fibers with 40 wt% cement after 28 days of curing.
Figure 7. (a) Effect of polypropylene fiber diameter on the setting time of the specimens; (b) evolution of compressive strength over time; (c) variation in heavy metal leaching concentrations of light-transmitting concrete incorporating polypropylene fibers of different diameters (The trends of the black squares (representing Cu) and red circles (representing Zn) are consistent.); (d) XRD pattern and (e) SEM image analysis of light-transmitting concrete specimens containing 1 mm diameter polypropylene fibers with 40 wt% cement after 28 days of curing.
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Table 1. Composition and content (%) of copper tailings.
Table 1. Composition and content (%) of copper tailings.
Fe2O3SiO2CaONa2OMgOMnOZnOK2OPbO
Content43.1436.363.412.261.791.701.361.090.57
Table 3. Effect of different activator types and their contents on the setting time of cementitious materials.
Table 3. Effect of different activator types and their contents on the setting time of cementitious materials.
Activator Content in Cementitious Materials/wt%NaOHCaONa2SiO3Na2CO3
Initial Setting Time/minFinal Setting Time/minInitial Setting Time/minFinal Setting Time/minInitial Setting Time/minFinal Setting Time/minInitial Setting Time/minFinal Setting Time/min
1012871308141214781324136512231274
2012561289138914231294131511971213
3012071254130713861245128611671204
4011761203127613241189124311341178
5011211176123512851145118911131165
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An, G.; Li, S.; Li, Z.; He, Z.; Li, K.; Ning, P.; Wang, X.; Sun, X. Development and Performance Evaluation of Translucent Concrete Incorporating Activated Copper Tailings as Cementitious Material. Appl. Sci. 2025, 15, 10228. https://doi.org/10.3390/app151810228

AMA Style

An G, Li S, Li Z, He Z, Li K, Ning P, Wang X, Sun X. Development and Performance Evaluation of Translucent Concrete Incorporating Activated Copper Tailings as Cementitious Material. Applied Sciences. 2025; 15(18):10228. https://doi.org/10.3390/app151810228

Chicago/Turabian Style

An, Guangdong, Siyang Li, Zhaorui Li, Zhaohui He, Kai Li, Ping Ning, Xiangyu Wang, and Xin Sun. 2025. "Development and Performance Evaluation of Translucent Concrete Incorporating Activated Copper Tailings as Cementitious Material" Applied Sciences 15, no. 18: 10228. https://doi.org/10.3390/app151810228

APA Style

An, G., Li, S., Li, Z., He, Z., Li, K., Ning, P., Wang, X., & Sun, X. (2025). Development and Performance Evaluation of Translucent Concrete Incorporating Activated Copper Tailings as Cementitious Material. Applied Sciences, 15(18), 10228. https://doi.org/10.3390/app151810228

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