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Article

Comprehensive Performance Evaluation of Lead–Zinc-Tailing-Based Geopolymer-Stabilized Aggregates

by
Zhengdong Luo
1,
Yuheng Yue
1,
Benben Zhang
2,* and
Yinghao Chen
1
1
School of Civil Engineering, Xiangtan University, Xiangtan 411105, China
2
School of Transportation, Southeast University, Nanjing 211189, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(3), 884; https://doi.org/10.3390/pr13030884
Submission received: 28 February 2025 / Revised: 13 March 2025 / Accepted: 15 March 2025 / Published: 17 March 2025
(This article belongs to the Special Issue Municipal Solid Waste for Energy Production and Resource Recovery)
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Abstract

:
As an innovative inorganic cementitious material, geopolymer holds significant application potential in the field of road engineering. Based on the theoretical basis of industrial solid waste resource utilization and combined with geopolymerization technology, this study investigates the feasibility of applying lead–zinc-tailing-based geopolymer–stabilized aggregate (LZT-GSA) in road engineering through systematic mechanical property tests, durability assessment, and microstructural characterization. The study focuses on the influence of cementitious material admixture on the unconfined compressive strength, splitting tensile strength, compressive resilient modulus, drying shrinkage, and freeze–thaw cycle resistance of LZT-GSA. The experimental results demonstrated that LZT-GSA exhibited excellent properties in terms of mechanical performance and durability, which were remarkably better than those of conventional cement-stabilized aggregates (CSA). However, the incorporation of a small amount of lead–zinc tailing alone can weaken the mechanical properties of CSA. The drying shrinkage of LZT-GSA was slightly higher than that of CSA due to the difference in the intrinsic reaction mechanism between LZT-GSA and CSA. The effective cementing and wrapping effect of geopolymer gel on discrete aggregate dramatically improves the structural compactness of LZT-GSA. The leaching concentration of heavy metals in LZT-GSA is far below the requirements of environmental protection standards. These research results not only provide theoretical support for the resource utilization of lead–zinc tailings, but also lay a technical foundation for its practical application in road engineering.

1. Introduction

Global road infrastructure is currently experiencing steady and continuous expansion, especially in developing countries. Cement-stabilized aggregate as a base layer is widely implemented in road infrastructure projects due to its high bearing capacity, excellent load-spreading performance, and high stability [1,2,3]. However, there is a huge environmental cost hidden behind the over-reliance on cement-stabilized aggregates. The production and use of traditional road foundation materials have become one of the main sources of carbon emissions in the engineering field [4,5]. It has been reported that the cement industry contributes about 8% of global CO2 emissions, consuming 3.0–4.5 GJ of energy and releasing 0.6–1.0 tons of CO2 per ton of cement produced [6,7]. The excessive emission of greenhouse gases exacerbates the greenhouse effect and has serious negative impacts on the global climate. In addition, the overexploitation of natural sand and gravel resources has led to a series of ecological degradation problems, such as the rapid depletion of sand and gravel resources in river basins. These phenomena demonstrate the lack of sustainability of traditional cement-based road material systems.
The application of geopolymers as green building materials in road engineering has become a hot research topic. These materials, often referred to as “green cement”, are mainly prepared by mixing industrial solid waste with external alkaline activators [8,9]. Industrial wastes usually include silica–aluminate minerals with potential cementing activity, such as slag, fly ash, and waste incineration fly ash [10,11,12]. The application of this material not only realizes the resourceful utilization of industrial solid waste, but also effectively controls the emission of greenhouse gases, thus strongly contributing to the sustainable development of road engineering. Compared with conventional cement-based materials, geopolymers exhibit outstanding mechanical properties [13,14], resistance to acid and alkali attack [15,16], durability in extreme environments [17,18], and the ability to sequester heavy metal ions [19,20]. As a result, many studies have validated the potential of geopolymers for applications in road engineering, environmental engineering, construction engineering, and foundation engineering [21,22,23].
In recent years, scholars have carried out a series of experimental studies around the application of geopolymers in road engineering. Hu et al. [24] comparatively analyzed the mechanical properties (including compressive strength, destructive strain, and dry shrinkage) of fly ash–red mud-based geopolymers and conventional cement- and lime-stabilized aggregates by means of mechanical testing and microscopic characterization. The results confirmed the potential of geopolymer for flexible pavement applications at ambient temperatures. Xiao et al. [25] investigated the effectiveness of geopolymer-stabilized glass slag in roadbed applications by preparing geopolymer using glass powder and fly ash, and by substituting glass slag instead of natural aggregate. Tiyasangthong et al. [26] utilized high-calcium fly-ash-based geopolymer-stabilized recycled concrete aggregate (RCA-FAG) as a pavement base material, and systematically investigated the effects of the ratio of fly ash to recycled concrete aggregate, the ratio of alkali activator, and the curing time on the mechanical properties of RCA-FAG. Meanwhile, the study by Sofri et al. [27] showed that the addition of fly-ash-based geopolymer could significantly enhance the mechanical properties of crushed stone aggregate, including unconfined compressive strength, flexural strength, and indirect tensile strength, making it an effective stabilizer for flexible pavements. Furthermore, the experimental results of Poltue et al. [28] confirmed that rice husk–fly-ash-based geopolymer-stabilized recycled concrete aggregate possesses excellent mechanical properties and is suitable for use as a lightweight pavement base material. Similar to the above studies, Tabyang et al. [29] further examined the mechanical properties of municipal solid waste incineration fly-ash-based geopolymer-stabilized recycled concrete aggregates and their microscopic characteristics. Yaowarat et al. [30] stabilized crushed stone by synergistically using fly-ash-based geopolymer and asphalt emulsifier and found that the proper incorporation of asphalt emulsifier enhances the confinement of the crushed stone particles and the strength of the geopolymer-stabilized crushed stone, but that the excessive incorporation of asphalt emulsifier produces negative effects. Ramulu et al. [31] comprehensively researched the strength, stiffness, stability, heavy metal leaching, and carbon footprint of geopolymer-stabilized aggregates and revealed that geopolymer-stabilized aggregates have better properties than cement-stabilized aggregates. Yue et al. [32] found that the mechanical properties, dry shrinkage performance, and freeze–thaw cycle resistance of slag–fly-ash-based geopolymer-stabilized aggregates are superior to those of cement-stabilized aggregates, which meets the requirements for the application of subgrade on highways and primary roads. Ji et al. [33] investigated the mechanical properties (compressive strength, elastic modulus, and cracking strength) and heavy metal leaching characteristics of domestic waste incinerator slag-based polymer-stabilized aggregate and confirmed that it meets the criteria for asphalt pavement applications.
As non-ferrous metals, lead–zinc tailings are rich in heavy metal ions and residual beneficiation chemicals, which are vulnerable to contamination of the surrounding soil and water bodies through rainfall leaching and wind erosion when exposed to the natural environment for a long period of time [34,35]. Moreover, the dust generated by simply stockpiled lead–zinc tailings may pose a threat to the health of the surrounding residents. It has been established that the preparation of lead–zinc tailings into geopolymer is an effective treatment method. This method can not only realize the resource utilization of tailings, but also effectively sequester heavy metal ions and reduce environmental pollution. Li et al. [36] confirmed, through mechanical properties and microstructural analysis, that lead–zinc-tailing-based geopolymer can be produced by adjusting the precursor raw materials and alkali activators to fully comply with the standards of building materials. Zhao et al. [37] found that the addition of multi-walled carbon nanotubes can significantly enhance the mechanical properties, high temperature resistance, and resistance to acid rain erosion of lead–zinc-tailing-based composite geopolymer. Deng et al. [38] produced geopolymer mortar via the alkali fusion activation of lead–zinc tailings and systematically investigated the effects of calcination temperature, alkali consumption, stagnation time, and slag content on the properties of geopolymer mortar by response surface methodology. Li et al. [39] further found that lead–zinc tailing–electrolytic manganese slag-based geopolymer achieved effective immobilization of lead–zinc ions mainly through physical sequestration and chemical bonding. Bah et al. [40] discussed the effect of alkali activator, curing temperature, silica–aluminum ratio, and curing time on the unconfined compressive strength of fly ash–lead–zinc-tailing-based geopolymer.
In summary, researchers and scholars have extensively studied the mechanical properties and durability of conventional geopolymer-stabilized aggregates, including indoor modeling tests and field applications, and have achieved remarkable research results. Further, some scholars have preliminarily explored the feasibility of preparing geopolymer from lead–zinc tailings, and the research mainly focuses on the performance of geopolymer net paste and mortar. However, these studies have been applied to a lesser extent in the resource utilization of lead–zinc tailings, making it difficult to achieve wide-scale engineering applications. Meanwhile, little research has been reported on the preparation of geopolymer-stabilized aggregates from lead–zinc tailings as precursor raw material and its application in road engineering. Therefore, based on geopolymerization technology and the concept of resource utilization of nonferrous metal tailings, this study proposes the use of lead–zinc tailings as a precursor raw material to prepare geopolymer-stabilized aggregates. The mechanical properties, durability, and drying shrinkage performance of lead–zinc-tailing-based geopolymer-stabilized aggregates (LZT-GSA) were comprehensively tested by mechanical tests and durability tests. In addition, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were performed to characterize the microstructure and mineral composition of LZT-GSA. The heavy metal leaching behavior of LZT-GSA was measured according to the toxicity characteristic leaching procedure. The aim of this study was to confirm the feasibility and effectiveness of LZT-GSA for road engineering applications, with a view to providing theoretical and technological support for the material design and engineering applications of lead–zinc tailings.

2. Experimental Materials and Methods

2.1. Materials

Geopolymer-stabilized aggregate (GSA) consists of a mixture of geopolymer binding material and aggregate, in which the geopolymer binding material is mainly composed of silica–aluminate mineral precursor raw materials and alkali activator. In this study, S105-grade slag and F-grade fly ash, which are of stable quality, widely available, and easy to use, are chosen as the main precursor raw materials, and lead–zinc tailings are chosen as the auxiliary precursor raw materials. In order to verify the effectiveness of geopolymers in stabilizing aggregates, ordinary silicate cement was also selected as a control group in this study. Slag and fly ash were purchased from Henan Zhengzhou Water Treatment Materials Co., Ltd. with the former in the form of white powder and the latter in the form of a grayish black powder. The lead–zinc tailings were obtained from a lead–zinc mining area in Hunan, China, and were grayish-white in color. The microstructure of the precursor raw materials was observed by scanning electron microscopy (SEM), and the results are shown in Figure 1. Specifically, slag is mostly irregular polygonal with a rough surface; fly ash is a spherical glassy body with a smooth and regular surface; lead–zinc tailings are prismatic with a rough surface and scattered distribution; and cement particles are mostly layered and stacked with a rough surface.
The chemical composition of the precursor raw materials was analyzed by X-ray fluorescence spectrometry (XRF), and the results are shown in Table 1. The analysis suggests that the lead–zinc tailings mainly contain chemical compositions such as SiO2, Al2O3, and CaO, which have the potential to be utilized as potential precursor raw materials. The compounding of calcium-rich and silica-rich slag with silica-rich and aluminum-rich fly ash is conducive to giving full play to the geopolymerization reaction, which promotes the improvement of the mechanical properties of cementitious materials.
Alkali activator is mainly applied to promote the dissolution and polymerization of precursor raw materials, and its nature plays a decisive role in the performance of geopolymer binders. In accordance with existing research reports, solid sodium hydroxide fragments and liquid sodium silicate were selected as the composite alkali activators in this experiment. Specifically, the solid sodium hydroxide fragments were analytically pure with a flaky appearance and a purity greater than 98%; the sodium silicate solution was transparent and viscous with an initial modulus of 3.31 and a Baume degree of 38.5, and the main chemical components were Na2O (8.42%) and SiO2 (27.84%). In order to ensure the full excitation and activation of the precursor raw materials, solid sodium hydroxide fragments and liquid sodium silicate were mixed in a certain proportion to prepare a composite alkali activator.

2.2. Synthesis Procedures

The aim of this study was to compare the difference in performance between GSA and cement-stabilized aggregate (CSA), and to investigate the effect of geopolymer admixture on the mechanical properties of GSA. Based on the results of existing studies and preliminary tests [41,42] and considering the mechanical properties and workability of geopolymer slurry, the dosage ratio of lead–zinc tailings, slag, and fly ash was set at 10:70:20 in the test; the modulus and admixture of the alkali activator (with respect to the mass of the precursor) were set at 1.2 and 40%, respectively; and the water–cement ratio was set at 0.4. The dosage of geopolymer was 3%, 4%, 5%, and 6%, respectively. The specific test protocol is presented in Table 2. This test was based on the construction technology of crack-resistant embedded cement-stabilized aggregate pavement base layer, and the grading design of the lead–zinc tailing-based geopolymer-stabilized aggregates (LZT-GSA) was carried out. The calculated gradation composition of LZT-GSA according to this design is shown in Table 3.
The preparation process of LZT-GSA can be summarized in the following steps. Firstly, the appropriate amount of coarse and fine aggregates and tap water were weighed and added into the mixing bucket and mixed well, with a smothering time of 1–2 h. Next, appropriate amounts of precursor raw materials and alkali activator were weighed and placed together in a blender and stirred and mixed for 3 min to prepare a geopolymer slurry. The smothered aggregate and geopolymer slurry were then poured into a mixing drum and mixed thoroughly for 20 min to prepare the geopolymer-stabilized aggregate. Further, the geopolymer-stabilized aggregate was poured into pre-lubricated 150 mm × 150 mm cylindrical molds in three stages and loaded for compaction. Finally, the geopolymer-stabilized aggregate was demolded and placed in a standard constant temperature and humidity curing box for continuous curing to a predetermined age. Through the above steps, it can be ensured that the preparation process of LZT-GSA is standardized and effective, providing a reliable basis for subsequent performance testing and application.

2.3. Test Methods

2.3.1. Mechanical Test

Mechanical properties are the most direct technical means to assess the suitability of LZT-GSA. The unconfined compressive test, split tensile test, and compressive resilient modulus test of LZT-GSA were carried out according to the Chinese standard TG 3441-2024 [43]. In this study, a WAW-1000 microcomputer-controlled electro-hydraulic servo universal testing machine manufactured by Shanghai Sansi Zongheng Machinery Manufacturing Co., Ltd. in China was adopted to carry out the unconfined compressive test, split tensile test, and compressive resilient modulus test on the LZT-GSA, and the specific test setup is illustrated in Figure 2. The size of the samples used in the mechanical tests is 150 mm × 150 mm cylindrical specimens. The loading rate was set at 1 mm/min for both the unconfined compression test and split tensile test, while the compressive resilience modulus test was conducted using the top surface method. Three specimens were used for each set of tests and the average value was calculated as the test value. These tests aim to comprehensively evaluate the mechanical properties of LZT-GSA and provide a scientific basis for practical applications.

2.3.2. Drying Shrinkage Test

The drying shrinkage test of the LZT-GSA was carried out according to the Chinese standard TG 3441-2024 [43]. The samples used in the drying shrinkage test were 150 mm × 150 mm × 150 mm rectangular specimens. The total drying shrinkage coefficient of the LZT-GSA was calculated by the following formula to evaluate its drying shrinkage performance. Three specimens were used for each set of tests and the average value was calculated as the test value. Here, αd represents the total drying coefficient, εi stands for the total drying strain, and ωi denotes the total water loss.
α d = ∑ ε i / ∑ w i

2.3.3. Durability Test

A freeze–thaw cycle test was conducted to assess the durability of LZT-GSA under cyclic environmental service conditions. The freeze–thaw cycle test of the LZT-GSA was carried out according to the Chinese standard TG 3441-2024 [43]. The samples used in the freeze–thaw cycle test were 150 mm × 150 mm cylindrical specimens. Three specimens were used for each set of tests and the average value was calculated as the test value. The procedure was as follows: the specimens were first cured in a standard curing chamber for 28 d, weighed on the last day of the curing period, and immersed in water for 24 h to achieve saturation. After saturation, the surface of the specimen was wiped clean and then placed in a rapid freeze–thaw tester. The tester is regulated in such a way that it is first frozen at −18 °C for 16 h, and then the temperature is raised to 20 °C and held for 8 h to complete the thawing process, which is a complete freeze–thaw cycle. After the freeze–thaw cycle test, the unconfined compressive strength of the specimen is tested. The ratio of the unconfined compressive strength of the specimen before and after the freeze–thaw cycle is calculated as the frost resistance coefficient.

2.3.4. Microstructure Analysis

In an attempt to define more clearly the physical composition, structural characteristics, and micromorphology of the geopolymer, SEM and XRD tests were performed on the LZT-GSA. The microstructure analysis of LZT-GSA was performed with reference to established studies [41,42]. Specifically, SEM was performed on a Zeiss Gemini 300 scanning electron microscope manufactured by Carl Zeiss, Germany, and an XRD test was conducted on an X’Pert PRO MPD X-ray diffractometer manufactured by Panalytical, Netherlands. The sampling procedure was as follows: firstly, the fragments were selected from the inside of the specimens after the mechanical tests, mechanically broken into granular specimens with a size of about 5 mm and with a natural cross-section, and then dried under low-temperature conditions. On this basis, the granular specimens were milled into powder form. The scanning range of the XRD test was from 5° to 90°, the scanning speed was 5°/min, and the scanning step width was 0.02°.

2.3.5. Heavy Metal Leaching Test

In order to verify the stabilization/solidification effect of geopolymer on heavy metals in lead–zinc tailings, a toxicity characteristic leaching procedure (TCLP) was applied in this study to determine the leaching concentration of heavy metals in the LZT-GSA samples. The leaching solution was prepared according to the Chinese standard HJ557-2009 [44]. The results of the experiment were scientifically evaluated according to the Chinese standard GB5085.3-2007 [45]. An ICP-OES inductively coupled plasma spectrometer manufactured by Thermo Fisher Scientific, United States was used for the detection of leached of heavy metal ions. The operating parameters of the instrument were optimized and set, which included the following: a plasma gas flow rate of 15 L/min, a nebulizing gas flow rate of 0.6 L/min, a lifting pump speed of 100 r/min, and a sample liquid rising rate of 1.2 mL/min.

3. Results and Discussion

3.1. Mechanical Test

The unconfined compressive strength of CSA, LZT-CSA, and LZT-GSA at different curing ages is shown in Figure 3. The results indicated that the compressive strengths of CSA, LZT-CSA, and LZT-GSA all exhibited a class-linear increase with the increase in the admixture of cementitious materials. Specifically, the macroscopic mechanical properties are positively correlated with the admixture of cementitious materials. The main reason for this phenomenon is that the increase in cementitious materials promotes the generation of hydration products, which positively affects the mechanical properties of the materials. Firstly, the hydration product gels and wraps the dispersed aggregate to form aggregate agglomerates. These aggregates are connected with each other under the cementing effect of the hydration products, constituting a stable spatial network structure. Secondly, the excess hydration products fill in the pores between aggregate agglomerates, which significantly improves the overall densification of the material. Therefore, the macroscopic mechanical properties of the stabilized aggregate were significantly improved with the increase in the admixture of cementitious materials.
Further analysis revealed that the compressive strength of LZT-CSA was lower than that of CSA when the admixture of cementitious materials was lower. Specifically, the compressive strength of LZT-CSA was lower than that of CSA when the admixture of cementitious materials was lower than 4%, and the strength decreased by up to 14%. However, with the increase in the admixture of cementitious materials, the gap between the compressive strength of LZT-CSA and that of CSA gradually decreased until the compressive strength of LZT-CSA exceeded that of CSA, with a strength increase of up to 15%. This phenomenon suggests that a moderate admixture of lead–zinc tailings can help improve the mechanical properties.
In addition, under the condition of the same admixture, the compressive strength of LZT-GSA is significantly higher than that of LZT-CSA, which is attributed to the fact that the compounding of calcium-rich and silica-rich slag and silica-rich and alumina-rich fly ash is conducive to giving full play to the geopolymerization reaction and promoting the generation of calcium- and sodium-based geopolymer gel products [46]. For example, the compressive strength of LZT-GSA is 2.36 times higher than that of LZT-CSA when the cementitious material is 6% and the curing age is 28 d. Moreover, the incorporation of lead–zinc tailings rich in silica–alumina minerals further supplements the silica–alumina source in the geopolymer system and promotes the increase in hydration products. The lead–zinc tailings have a high specific surface area, which can exert a fine aggregate filling effect and improve the overall densification between aggregate agglomerates [47]. It is worth noting that the fly ash particles with a spherical glassy structure can enhance the workability of the geopolymer gel, which is favorable for the full adhesion between the geopolymer slurry and the aggregate [41,42]. In summary, under the same conditions, the compressive strengths of LZT-GSA are higher than those of LZT-CSA, indicating that LZT-GSA has remarkable advantages in mechanical properties.
The splitting tensile strength of CSA, LZT-CSA, and LZT-GSA at different curing ages is demonstrated in Figure 4. The splitting tensile strengths of CSA, LZT-CSA, and LZT-GSA showed an asymptotic growth with the increase in the cementitious material admixture. This trend is consistent with the change in compressive strength, and the increase in cementitious materials promotes the generation of hydration products and enhances the cementing force on aggregates, thus enhancing the splitting tensile strength. The difference in splitting tensile strength between CSA and LZT-CSA was not significant at the same admixture of cementitious materials. However, the splitting tensile strength of LZT-GSA was dramatically increased compared with that of CSA. The results suggested that the incorporation of lead–zinc tailings alone had a small effect on the splitting tensile strength of CSA, mainly due to the fact that the alkalinity of cement hydration was not sufficient to effectively activate the lead–zinc tailings, resulting in a reduction in the generation of hydration products [47]. In contrast, under the action of the alkali activator, the precursor raw materials were able to fully dissolve and depolymerize, and the geopolymerization reaction occurred, generating a series of geopolymer gel products that significantly enhanced the macroscopic mechanical properties of the materials [48].
The compressive resilience modulus of CSA, LZT-CSA, and LZT-GSA at different curing ages is presented in Figure 5. The results indicated that the trends of compressive resilience modulus for CSA, LZT-CSA, and LZT-GSA with the admixture of cementitious materials were basically the same as those for compressive strength and splitting tensile strength. Specifically, the compressive resilience modulus of these three materials increases with the increase in cementitious material admixture. The main reason for this is that the increase in the admixture of cementitious materials promotes the generation of hydration gel products and, at the same time, enhances the bond between aggregates. Further analysis revealed that the difference in compressive resilience modulus between CSA and LZT-CSA was not significant under the same cementitious material admixture conditions. However, the compressive resilient modulus of LZT-GSA was significantly higher under the same conditions compared to CSA. This phenomenon may be attributed to the positive effect of specific components in LZT-GSA, which may have formed a denser structure during the hydration process, thus improving the overall performance of the material.

3.2. Drying Shrinkage Test

The drying shrinkage volume change rates of CSA, LZT-CSA, and LZT-GSA with different cementitious material admixtures are exhibited in Figure 6. The observed results indicated that, with the increase in dry shrinkage age, the volume change rates of all three materials showed the trend of increasing first and then stabilizing. Under the same conditions, the volume change rates were LZT-GSA, LZT-CSA, and CSA in order from high to low. For example, the volume change rates of LZT-GSA, LZT-CSA, and CSA were 0.524%, 0.357%, and 0.307%, respectively, after 168 h of curing at 6% cementitious material admixture. Moreover, the volume change rates of CSA, LZT-CSA, and LZT-GSA were positively correlated with the admixture of cementitious materials, i.e., the increase in the admixture of cementitious materials significantly increased the volume change rate of the specimens. Specifically, the incorporation of lead–zinc tailings led to an increase in the volume change rate of CSA, while the volume change rate of GSA was higher than that of CSA. The reason for this phenomenon is that the alkalinity of the system after cement hydration is not enough to fully stimulate the activity of lead–zinc tailings, which makes part of the lead–zinc tailings exist in the form of mineral admixture in CSA, thus increasing the water loss rate and shrinkage stress of CSA, and resulting in a greater rate of change in dry shrinkage volume of LZT-CSA [47].
The difference in volume change rate between LZT-GSA and LZT-CSA is mainly attributed to the different mechanisms and hydration products of geopolymerization and cement hydration reactions. In the geopolymerization reaction, water only serves as a mass transfer medium rather than a reaction-consuming component. The higher the amount of silica–aluminum precursor raw material, the higher the water content in the saturated state, and the greater the total water loss during drying. The equilibrium between external water vapor and saturated water pressure in the pores is disrupted, and the loss of water triggers the formation of water–air meniscus surfaces in the capillary pores, generating hydrostatic tensile stresses, which, in turn, induce isotropic tensile stresses in the rigid solid skeleton, leading to volume contraction. According to the drying shrinkage mechanism, an increase in water loss leads to a decrease in the internal moisture of the mix and a decrease in the free water in the matrix, which causes a greater shrinkage in the volume of the specimen [49]. In comparison, during the cement hydration reaction, cement clinker generates hydration products by consuming free water, which converts the free water into volume-stable bound water. In addition, the main hydration products in LZT-GSA prepared from slag, fly ash, and lead–zinc tailings are nanoscale hydrated silica–aluminate. As the reaction proceeds, these hydration products undergo a time-dependent irreversible particle rearrangement or recombination process [50]. Meanwhile, the alkaline cations in the reaction system cause the hydrated silica–aluminate layer to lose its stacking regularity, thus triggering structural collapse. Calcium hydroxide is the main component in the hydration products of cement, and its crystal structure and physical properties are relatively stable, so drying shrinkage does not occur easily. Therefore, under the same conditions, the volume change rate of LZT-GSA is larger than that of LZT-CSA.

3.3. Durability Test

The unconfined compressive strengths of CSA, LZT-CSA, and LZT-GSA under freeze–thaw cycle conditions are demonstrated in Figure 7. The analysis illustrated that the freezing resistance of these three materials gradually increased with the increase in the cementitious material admixture. Under the same conditions, the frost resistance, in descending order, is LZT-GSA, LZT-CSA, and CSA. For example, the freezing resistance of LZT-GSA, LZT-CSA, and CSA at 6% cementitious material admixture is 88.30%, 81.19% and 78.94%, respectively. The reason for this is that when the admixture of cementitious materials is small, the hydration products cannot fully cement the wrapped aggregate, resulting in poor overall connectivity between aggregate conglomerates, which increases the structural porosity and enhances the freezing and expansion force on the aggregate conglomerates.
With the increase in cementitious material admixture, the discrete aggregate forms a spatial network body under the cementing effect of hydration products, and the overall compactness is significantly improved, which effectively enhances the frost resistance. Taking CSA as a reference, the addition of an appropriate amount of lead–zinc tailings can effectively exert the physical filling effect to improve the densification of CSA, thus enhancing the frost resistance of LZT-CSA. The silica–alumina–calcium components in lead–zinc tailings, slag, and fly ash act synergistically with each other, so that the types and incorporation of geopolymer gel products are efficiently supplemented, which further enhances the cementation force on aggregates, thus notably improving the frost resistance of LZT-GSA.

3.4. XRD Analysis

The XRD pattern characteristics of CSA, LZT-CSA, and LZT-GSA with different admixtures of cementitious materials are shown in Figure 8. It can be clearly observed that the hydration products of CSA contain typical hydration product crystalline phases, which are mainly hydrated calcium silicate (C-S-H), hydrated calcium aluminate (C-A-H), and ettringite (AFt). It is worth noting that the intensity of the diffraction peaks of the hydration products of LZT-CSA decreased at an admixture of 3% cementitious materials. This indicates that a small amount of incorporated lead–zinc tailings failed to effectively stimulate the hydration reaction, and only acted as a mineral dopant to fill the inside of the aggregate and exert the physical filling effect. This phenomenon explains the intrinsic mechanism of the decrease in the mechanical properties of CSA due to a small amount of LZT-CSA. Nevertheless, a few diffraction peaks of hydrated calcium–silica–aluminate (C-A-S-H) appeared in the XRD pattern of LZT-CSA at 6% cementitious material admixture, which indicated that some of the lead–zinc tailings underwent a geopolymerization reaction after activation.
In contrast, the XRD patterns of LZT-GSA exhibit the generation of quartz, calcite, mullite, zeolite, and geopolymer gel products. The presence of quartz phase can be attributed to the non-participation of inert quartz in the raw material in the geopolymerization reaction, while the formation of calcite is closely related to the mineralization phenomenon during the specimen maintenance process. During the reaction, there is an interconversion mechanism between the zeolite phase and the geopolymer, and its final conversion to the geopolymer gel product is significantly influenced by the reaction conditions. It can be observed that the generation of C-S-H, C-A-S-H, and hydrated sodium–silica–aluminate (N-A-S-H) signifies the effective fusion of the calcium silica–aluminate component of the precursor raw material with the alkali activator, triggering the geopolymerization reaction [51]. It is noteworthy that the diffraction peak intensities of the geopolymer gel products showed an obvious positive correlation with the admixture amount of gelling materials. This phenomenon suggests that the activity of the geopolymerization reaction is enhanced with the increase in the admixture amount of gelling material, which, in turn, promotes the rapid improvement of the macroscopic mechanical properties of the specimens.
The above phenomena illustrate the strength generation mechanism of LZT-GSA from the perspective of the mineral crystalline phase. Specifically, the macroscopic mechanical properties of LZT-GSA are mainly affected by the type and quantity of hydration products. In other words, the increase in hydration products promotes the rapid improvement of the mechanical properties of LZT-GSA. The generation of hydration products increased with the increase in the cementitious material admixture, which was reflected in the enhancement of the diffraction peaks of the mineral crystalline phase. In summary, the increase in cementitious materials significantly enhanced the mechanical properties of LZT-GSA mainly by promoting the generation of hydration products.

3.5. SEM Analysis

The microscopic morphology characteristics of CSA, LZT-CSA, and LZT-GSA with different cementitious material admixtures are shown in Figure 9. The microscopic observation results revealed that the main hydration products in CSA at 3% cementitious material admixture included gum-flocculated C-S-H gel, plate-like Ca(OH)2 gel, and needle-and-rod-like AFt crystals. In particular, the discrete aggregates in the CSA were encapsulated by the C-S-H gel to form aggregate agglomerates, and some of the AFt crystals were filled in the voids between the aggregate agglomerates. However, the CSA still has relatively more porous defects on the whole, which is mainly attributed to the limited amount of gelling material that is not enough to completely gelatinize and encapsulate the aggregate. The overall densification of the CSA was effectively improved with the increase in the gelling material admixture up to 6%, which can be mainly explained by the increase in the hydrated gel yield.
In the LZT-CSA system, the distribution range of the C-S-H gel attenuates considerably when the cementitious material is incorporated at a concentration of 3%, resulting in the structural integrity becoming relatively loose. This phenomenon mainly stems from the fact that the lead–zinc tailing particles were not effectively activated and existed only as mineral admixtures inside the structure. However, when the admixture of lead–zinc tailings was increased to 6%, the lead–zinc tailings were effectively stimulated to generate flocculated hydration products and covered the aggregate surface due to the enhancement of cement hydration alkalinity, which significantly enhanced the spatial integrity of LZT-CSA.
Unlike CSA, LZT-GSA exhibits notable differences at 3% gelling material admixture. It exhibits an internally visible network of fiber-stacking gel products, mainly due to the precursor raw materials in the calcium silica–alumina component of the geopolymerization reaction generated by the calcium and sodium-based geopolymer gel, including hydrated calcium–silica–aluminate (C-A-S-H) and hydrated sodium–silica–aluminate (N-A-S-H) [41,42]. These gel products produced a conspicuous improvement in the compactness and porosity of the discrete aggregates while promoting the interconnection of the aggregate agglomerates. When the admixture of gelling materials was increased to 6%, the structural integrity and densification of LZT-CSA were further strengthened, and its macroscopic mechanical properties were also significantly improved.
To summarize, the changes in the cementitious material admixture exhibit a consistent correspondence with the development process of microstructural integrity and macroscopic mechanical properties. Specifically, when the cementitious material admixture is low, the precursor raw materials are not sufficiently activated, resulting in the limited generation of hydration products. Due to the insufficient amount of hydration products, the dispersed aggregates could not be completely wrapped up, which resulted in the formation of some void defects in the internal system and reduced the integrity of the spatial structure. Under this condition, the development of the macroscopic mechanical properties of LZT-CSA is restricted.

3.6. Heavy Metal Leaching Analysis

The heavy metal leaching results of LZT-GSA with a curing age of 28 d at different cementitious material admixtures are shown in Table 4. The experimental results revealed that the leaching concentrations of Pb and Zn in the untreated lead–zinc tailings reached 26.6372 mg/L and 216.5517 mg/L, respectively, which far exceeded the limits set by GB 5085.3-2007 (Pb: 5 mg/L, Zn: 100 mg/L). The results indicate that the lead–zinc tailings have leaching toxicity and are classified as hazardous waste. It was found that when the lead–zinc tailings were activated into a geopolymer, the leaching concentrations of Pb and Zn decreased significantly, and both met the requirements of environmental standards. Further, the heavy metal leaching concentration of LZT-GSA exhibited a stepwise downward trend with the increase in the admixture of cementitious materials. Specifically, the leaching concentration of Pb and Zn heavy metals in LZT-GSA is less than 1 mg/L. This phenomenon is mainly attributed to the increasing amount of physicochemically and chemically stable geopolymer gels, which can effectively solidify heavy metal ions and sequester them inside the structure. As a result, the lead–zinc-tailing-based geopolymer showed excellent immobilization ability for Pb and Zn heavy metals in the tailings and remarkably reduced the leaching risk of heavy metal ions. The geopolymerization technology not only realizes the resourceful utilization of lead–zinc tailings, but also ensures its environmentally sound treatment, which has important environmental significance and application value.

3.7. Strength Generation Mechanism

The strength generation mechanism of LZT-GSA can be mainly attributed to the friction-locking effect of aggregates, the cementation-linkage effect of geopolymer, and the filling–squeezing effect of fine ingredients. The friction-locking effect of the aggregates is achieved with continuous gradation and angular aggregates in the pressure molding process, through mutual friction and embedded locking, and the formation of the structural strength of the mixed aggregates to build a spatial network structure of aggregates that can effectively transfer the external load stress. This structure not only enhances the overall stability of the material, but also improves its load-bearing capacity. For the cementation linkage effect of the geopolymer, lead–zinc tailings, slag, and fly ash rich in silica–aluminate minerals under the action of alkaline activators, the precursor raw materials undergo depolymerization and condensation reactions to form the geopolymer gel product (C(N)-A-S-H). Geopolymer gels bond discrete aggregates with each other to form stable aggregate agglomerates and effectively fill the voids within the structure, thus significantly enhancing the macroscopic mechanical properties. For the filling–squeezing effect of fine ingredients, some of the precursor raw materials is distributed within the structure in the form of mineral admixture, which effectively fills and densifies the interface transition zone between the geopolymer and aggregate. Under the joint action of this series of mechanisms, the mechanical properties of LZT-GSA were rapidly improved.

4. Conclusions

The aim of this study was to investigate the feasibility of LZT-GSA application in the field of road engineering based on the concept of industrial solid waste resource utilization and geopolymerization technology. The differences in the mechanical and durability properties between LZT-GSA and conventional CSA were systematically compared and analyzed by means of indoor mechanical test methods and microscopic characterization. The study focuses on examining the influence law of cementitious material admixture on the unconfined compressive strength, splitting tensile strength, compressive resilient modulus, drying shrinkage, and freeze–thaw cycle resistance properties of LZT-GSA. The main research conclusions of this paper are as follows:
(1)
The mechanical properties of LZT-GSA mainly depend on the admixture of cementitious materials, and its unconfined compressive strength, splitting tensile strength, and compressive resilience modulus all exhibit significant positive correlations with the admixture of cementitious materials. Under the same test conditions, LZT-GSA demonstrated superior mechanical properties compared with traditional CSA. However, it should be pointed out that the filling effect of lead–zinc tailings adversely affects the mechanical properties of CSA when it is incorporated as a mineral admixture in small amounts.
(2)
The drying shrinkage of LZT-GSA presents a continuous increase with the growth of the curing age until it reaches the stabilization period. Meanwhile, the increase in the admixture of cementitious materials enlarges the drying volumetric shrinkage of LZT-GSA. The drying shrinkage of LZT-GSA is higher than that of CSA under the same conditions due to the different reaction mechanisms.
(3)
The frost resistance of LZT-GSA is positively correlated with the admixture of cementitious materials. That is, the increase in the admixture of cementitious materials is favorable to the enhancement of frost resistance.
(4)
The SEM and XRD analyses revealed that the precursor raw materials were activated by the alkali activator to generate geopolymer gels such as C-S-H, C-A-S-H, and N-A-S-H through geopolymerization reactions. The discrete aggregates are encapsulated by the geopolymer gels to form aggregate agglomerates, and the overall compactness of the structure is enhanced.
(5)
The leaching concentration of Pb and Zn heavy metal ions of LZT-GSA is much lower than the limit value of environmental protection standard, and it has a significant fixation effect on heavy metal ions.
To summarize, LZT-GSA, as an all-solid-waste-prepared and low-carbon and environmentally friendly road base material, provides broad application prospects for low-carbon road construction and the high-value utilization of solid waste. In other words, LZT-GSA can effectively realize the resource utilization of industrial solid waste while meeting the performance requirements of road engineering. In the application of road engineering, the reasonable selection of material ratios should be carried out according to the actual load-bearing requirements of different road environments and engineering costs. It should be noted that this study used corrosive strong alkaline as an activator, which may limit the feasibility of LZT-GSA in large-scale engineering applications. In the future, efforts should be made to develop geopolymers with low alkalinity and high strength as cementitious materials, for example, by utilizing alkaline solid waste, in order to improve their environmental adaptability and range of application. Furthermore, the long serviceability of LZT-GSA under the effect of complex environmental factors, such as environmental erosion and vehicle fatigue load, will be further studied by combining indoor tests and field applications. On this basis, the long-term tracking observation of LZT-GSA is carried out by paving test sections to verify its application effect in actual road projects. This will provide an important theoretical basis and technical support for the design and application promotion of solid-waste-based geopolymer materials in road engineering.

Author Contributions

Conceptualization, Z.L., Y.Y. and B.Z.; investigation, Z.L., Y.Y. and Y.C.; writing—original draft preparation, Z.L., Y.Y. and Y.C.; writing—review and editing, Z.L., Y.Y. and B.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Natural Science Foundation of Hunan Province, Grant No. 2023JJ50008, and the Construction Science and Technology Program of Hubei Province, Grant Nos. JK2024113 and JK2024114.

Data Availability Statement

The data are available on request from the corresponding author.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Processes 13 00884 g001
Figure 1. Microscopic morphology of precursor raw materials.
Figure 1. Microscopic morphology of precursor raw materials.
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Figure 2. Representative mechanical property tests.
Figure 2. Representative mechanical property tests.
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Figure 3. Unconfined compressive strength of CSA, LZT-CSA, and LZT-GSA at different curing ages.
Figure 3. Unconfined compressive strength of CSA, LZT-CSA, and LZT-GSA at different curing ages.
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Figure 4. Splitting tensile strength of CSA, LZT-CSA, and LZT-GSA at different curing ages.
Figure 4. Splitting tensile strength of CSA, LZT-CSA, and LZT-GSA at different curing ages.
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Figure 5. Compressive resilience modulus of CSA, LZT-CSA, and LZT-GSA at different curing ages.
Figure 5. Compressive resilience modulus of CSA, LZT-CSA, and LZT-GSA at different curing ages.
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Figure 6. Drying shrinkage volume change rates of CSA, LZT-CSA, and LZT-GSA with different cementitious material admixtures.
Figure 6. Drying shrinkage volume change rates of CSA, LZT-CSA, and LZT-GSA with different cementitious material admixtures.
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Figure 7. Unconfined compressive strength and frost resistance coefficient of CSA, LZT-CSA, and LZT-GSA under freeze–thaw cycle conditions.
Figure 7. Unconfined compressive strength and frost resistance coefficient of CSA, LZT-CSA, and LZT-GSA under freeze–thaw cycle conditions.
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Figure 8. XRD patterns of CSA, LZT-CSA, and LZT-GSA.
Figure 8. XRD patterns of CSA, LZT-CSA, and LZT-GSA.
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Figure 9. Microscopic morphology of CSA, LZT-CSA, and LZT-GSA with different cementitious material admixtures.
Figure 9. Microscopic morphology of CSA, LZT-CSA, and LZT-GSA with different cementitious material admixtures.
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Table 1. Chemical composition of precursor raw materials (%).
Table 1. Chemical composition of precursor raw materials (%).
CaOAl2O3SiO2MgOFe2O3SO3K2OOthers
Lead–zinc tailings5.665.8078.850.773.090.802.052.98
Fly ash3.3633.3654.610.701.330.572.093.98
Slag43.5514.5628.138.360.452.450.452.05
Cement49.4011.3227.481.193.39--7.22
Table 2. Test protocol.
Table 2. Test protocol.
Mix IDsLead–Zinc
Tailings (wt%)		Slag
(wt%)		Fly Ash
(wt%)		Cement
(wt%)		Alkali ActivatorWater/
Binder		Curing Time (Days)
ModulusContent (wt%)
CSA---1001.2400.47/28
LZT-CSA10--901.2400.47/28
LZT-GSA107020-1.2400.47/28
Table 3. Synthetic grading of geopolymer-stabilized aggregates.
Table 3. Synthetic grading of geopolymer-stabilized aggregates.
Aggregate Size
(mm)		Powder Material (%)Passage Rate in Hole Sieves of Different Apertures (%)
31.519.09.54.752.360.60.075
0~38100.0100.0100.0100.085.543.78.9
3~517100.0100.099.289.833.521.32.4
5~1022100.0100.099.415.313.112.70.3
10~2037100.074.80.20.07.87.90.0
25~3015100.010.20.00.014.213.30.0
Table 4. Heavy metal ion leaching test results of LZT-GSA.
Table 4. Heavy metal ion leaching test results of LZT-GSA.
Heavy Metal Ion Leaching Concentration (mg/L)
Admixture of
Cementitious Materials		PbZn
Leaching
Concentration		Standard
Concentration		Leaching
Concentration		Standard
Concentration
Original LZT26.63725216.5517100
3%0.08550.1128
4%0.04820.0565
5%0.02210.0325
6%0.00750.0086
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Luo, Z.; Yue, Y.; Zhang, B.; Chen, Y. Comprehensive Performance Evaluation of Lead–Zinc-Tailing-Based Geopolymer-Stabilized Aggregates. Processes 2025, 13, 884. https://doi.org/10.3390/pr13030884

AMA Style

Luo Z, Yue Y, Zhang B, Chen Y. Comprehensive Performance Evaluation of Lead–Zinc-Tailing-Based Geopolymer-Stabilized Aggregates. Processes. 2025; 13(3):884. https://doi.org/10.3390/pr13030884

Chicago/Turabian Style

Luo, Zhengdong, Yuheng Yue, Benben Zhang, and Yinghao Chen. 2025. "Comprehensive Performance Evaluation of Lead–Zinc-Tailing-Based Geopolymer-Stabilized Aggregates" Processes 13, no. 3: 884. https://doi.org/10.3390/pr13030884

APA Style

Luo, Z., Yue, Y., Zhang, B., & Chen, Y. (2025). Comprehensive Performance Evaluation of Lead–Zinc-Tailing-Based Geopolymer-Stabilized Aggregates. Processes, 13(3), 884. https://doi.org/10.3390/pr13030884

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