An Innovative High-Content Steel Slag Alkali-Sulfate Composite Activated Binder: Hydration Behavior and Application

TL;DR

This study develops a new binder using steel slag and alkali-sulfate activation, achieving good strength and heavy metal immobilization for sustainable construction.

Contribution

A novel alkali-sulfate activation method enables high steel slag content (>50%) in a binder with good performance and environmental benefits.

Findings

A binder with 60% steel slag achieved 12.85 MPa compressive strength after 28 days.

The binder effectively immobilized heavy metals in lead–zinc tailings with >80% efficiency.

Hydration products like C-A-S-H and ettringite formed a dense structure, ensuring volume stability.

Abstract

The low activity and expansion risk of steel slag limit its large-scale utilization in cementitious systems. This study developed an alkali-sulfate synergistic activation method to prepare binder with steel slag content exceeding 50 wt%. The effects of alkali activator dosage, modulus, steel slag and flue gas desulfurization gypsum content on the mechanical properties and workability were systematically investigated. With a mix of 60% steel slag, 30% fly ash, 10% desulfurization gypsum and activated by additional 20% alkali activator with modulus 1.0, the 28-day compressive strength reached 12.85 MPa, along with excellent volume stability. Microstructural characterization revealed that the main hydration products are C-A-S-H and ettringite, which jointly form a dense microstructure. When used to solidify lead–zinc tailings for backfill, the binder yielded satisfactory strength and effectively immobilized heavy metals (Pb, As, Cd, Zn), with leaching concentrations meeting environmental standards and immobilization efficiencies > 80%. Heavy metals were primarily immobilized through physical encapsulation, ion exchange, and co-precipitation. This study elucidates the hydration and mechanisms of high-content steel slag systems under alkali-sulfate synergistic activation, providing a sustainable technical framework for large-scale utilization of steel slag and tailings management.

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Full text

1. Introduction

The accumulation of solid waste, as well as the risks of underground voids from ore extraction, increasingly threaten both industrial progress and ecological safety. Such subsurface voids can induce mine collapses, ground sinkholes, and surface subsidence, posing notable risks to the current mineral extraction work at the mine site and threatening the surrounding surface infrastructure for an extended period following the closure of mine [1]. Therefore, backfilling mines is important for the safety and sustainability of the mining industry. However, traditional cement-based backfilling materials face urgent challenges regarding high energy consumption, high emissions, and high costs [2]. In light of the continuously growing global demand for cementitious materials, the exploration of potential alternatives to conventional cement has attracted considerable attention [3]. Extensive studies have demonstrated that solid-waste-derived cementitious materials are capable of meeting these strength criteria and relevant engineering specifications [4,5,6,7]. Incorporating solid waste into the fabrication of cementitious materials effectively eliminates the need for the energy-intensive cement manufacturing process, thereby contributing to carbon recycling and cost reduction while concurrently satisfying the performance demands of large-scale engineering applications.

Steel slag (SS), due to its rich mineral composition and potential self-cementing properties, has become an ideal substitute for cement. Annual SS generation in China reached nearly 100 million tons, constituting over half of the total output of the world [8]. In fact, in developing countries such as China, over 70% of SS is not effectively utilized and often indiscriminately stockpiled, significantly lagging behind that of other developed nations [8]. According to a recent study, there is an urgent need to effectively utilize more than 190 million tons of SS annually [9]. By making full use of the synergistic effect between SS and various solid waste components, and through reasonable mix design, the prepared cement-based materials meet the performance requirements of backfill [10]. Currently, extensive research has been conducted on the development of low-carbon cementitious materials using solid wastes such as steel slag [11]. DUAN et al. [12] developed an all-solid-waste cementitious material utilizing ultra-fine ground SS, desulfurization gypsum (DG), and fly ash (FA), whose compressive strength was four times that of ordinary Portland cement (OPC). ZHANG et al. [13] adopted a binder formulation consisting of 35% SS, 50% ground granulated blast furnace slag (GBFS), and 15% DG, with the 3-day, 7-day, and 28-day UCS reaching 6.69, 12.05, and 16.36 MPa, respectively. However, a review of the literature indicated that existing technologies fall short of meeting the substantial demand for large-scale SS utilization. In conventional binder formulations, the incorporation ratio of SS is typically limited to only 10–30 wt% [14,15,16,17]. The two main challenges hindering the application of high content SS in binder are its expansive components and inherently low reactivity [18,19]. Unlike cementitious materials dominated by tricalcium silicate (e.g., Portland cement), SS, which primarily exhibits the β-dicalcium silicate (β-C_2_S) crystalline phase, demonstrates inferior hydration activity. Furthermore, the critical expansion caused by free lime (f-CaO) in SS poses a durability risk to binder in the presence of moisture [9]. Therefore, achieving high SS incorporation (>50 wt%) while maintaining sound performance is a key challenge.

Alkali-activated materials (AAMs) are currently the most widely used alternative binder, and they are cementless binders with high activity and strength, derived from alkaline solid waste, industrial by-products, and other aluminosilicate precursors [20]. AAMs formation occurs through alkali-mediated dissolution of aluminosilicate, followed by polycondensation into a stable inorganic polymeric structure [21]. Steel slag contains a substantial amount of CaO along with a certain proportion of SiO_2_ and Al_2_O_3,_ and has been used as a precursor material and incorporated into AAMs [19]. Meanwhile, low-calcium FA is a silicoaluminous material that is commonly used as a cement additive due to its ability to react with alkaline calcium oxide, forming C-S-H and C-A-H [22]. Furthermore, under strongly alkaline conditions, FA can polymerize into geopolymers, which exhibit superior durability, compressive strength, and impermeability compared to OPC [23]. However, FA is plagued by inherent drawbacks of slow pozzolanic reactivity and prolonged setting time [24]. The use of sulfate activators such as flue gas desulfurized gypsum (FGDG) could enhance the reactivity of aluminosilicate components and promote the formation of ettringite [25,26]. Although alkali activation is a common method for synthesizing AAMs, excessive alkali could inhibit silicate polymerization in high-calcium systems, potentially leading to incomplete reactions or increased water demand in high-SS formulations, thereby further promoting cracking [27]. At the same time, the effect of FGDG on the alkali-salt combined excitation of FA-SS is not clear [28]. The amount of SS that can be carried in the alkali-salt synergistic excitation system is unknown [29]. Moreover, the existing literature lacks research on the volume stability of high-content SS cementitious materials under high alkali-sulfate conditions, and the synergistic effect of various solid wastes and their hydration mechanism are also unknown, which limits its practical engineering application [30]. The key point is that, despite the formulation difficulty of binder with high SS content (>50 wt%), the potential of an alkali–sulfate hybrid activation strategy has not been systematically investigated, representing a significant knowledge gap [31]. The synergistic activation method used in this study is expected to overcome the low activity and poor stability in the high SS system, break through the bottleneck of SS resource utilization, and provide a more effective way for the resource utilization of solid waste.

Therefore, this study employed SS as the primary precursor to prepare a high-content SS alkali-sulfate composite activated binder by regulating the dosage of SS and FGDG. XRD, FTIR, SEM, and other characterization techniques were systematically utilized to investigate the effects of alkali activator dosage, modulus, SS content and FGDG content on the mechanical properties, workability, hydration process, and microstructure of the binder, and evaluate the practical application of the optimized binder in solidifying lead–zinc tailings for mine backfill. This research aimed to reveal the underlying hydration mechanisms and provide scientific guidance for the large-scale and efficient utilization of SS and other industrial solid wastes.

2. Materials and Methods

2.1. Materials

The main chemical composition of raw materials was shown in Table 1. The SS came from Maanshan Iron & Steel Co., Ltd. (Maanshan, China), with a CaO content of 45.87% and a SiO_2_ content of 11.12%. The FA was obtained from Gongyi Bairun Refractories Co., Ltd. (Gongyi, China), classified as Class 1 low-calcium FA, with a SiO_2_ content of 51.54% and a Al_2_O_3_ content of 30%. FGDG was sourced from Guanbang Environmental Technology Co., Ltd. (Shangqiu, China), mainly composed of CaO (48.33%) and SO_3_ (45.39%). XRD analysis (Figure 1a) revealed that SS consisted mainly of C_2_S, C_3_S, FA was composed of mullite (Al_6_Si_2_O_13_) and quartz (SiO_2_), and FGDG was predominantly CaSO_4_·2H_2_O. The alkaline activator used was composed of analytical grade sodium hydroxide (flake) produced by the China National Pharmaceutical Group Chemical Reagents Company and industrial-grade water glass (modulus 3.26). Furthermore, the specific surface areas of raw materials were provided in Figure 1b. Microscopic morphology analysis showed that SS (Figure 1c) and FGDG (Figure 1d) had irregularly sized particles and a loose structure, while FA was primarily spherical (Figure 1e). In addition, lead–zinc tailings (LZTs) were used as inert aggregates to prepare backfill materials from the Chizhou tailings reservoir in Anhui Province, China. The mineral composition and toxicity leaching results of the LZTs are shown in Table 1 and Table 2 respectively. The mineral composition and particle size analysis are shown in Figure S1.

2.2. Preparation of Specimens

2.2.1. Preparation Binder

Firstly, SS, FA, and FGDG were homogenized in a JJ-5 cement mortar by dry mixing for 3 min. The mixture cannot be cemented without the addition of alkali. Then, the alkali activator was prepared by blending water glass with sodium hydroxide to achieve the target modulus, followed by cooling to room temperature. According to the preliminary test, the compressive strength of the specimen was better when the water–binder ratio was 0.3. The solid raw materials, deionized water, and alkali activator were mixed in cement mortar mixer for 5 min. The resulting paste was cast into 40 mm × 40 mm × 40 mm molds and consolidated by vibration. After casting, the samples (in molds) were placed in a standard laboratory environment (20 °C, 97% RH) for 24 h. The molds were covered with plastic film to prevent moisture loss. Demolding occurred after 24 h, and the specimens were then transferred to a YH-40B constant temperature and humidity chamber (20 °C, 97% RH) for continued curing. Compressive strength was tested at 3, 7, and 28 days of curing. The experimental design is presented in Table 3.

2.2.2. Tailings Solidification

The optimized formulation was employed as a binder to stabilize LZT. Solidified specimens were prepared at varying binder-to-tailings ratios. Samples were systematically labeled based on their composition; for instance, “SFGL4” denoted that the binder and tailings are mixed at a ratio of 1:4. The preparation protocol is illustrated in Figure 2.

2.3. Test Method

To systematically evaluate the developed binder and its application in tailings solidification, the experimental work was structured into two main streams. First, the alkali-sulfate activated binder itself was tested. This included investigations of its workability in the fresh state (fluidity and setting time), mechanical properties in the hardened state (UCS, at 3, 7, and 28 days), volume stability, softening coefficient and microstructure (via XRD, FTIR, SEM-EDS). Second, the optimized binder formulation was used to solidify/stabilize lead–zinc tailings (LZTs) at varying binder-to-tailings ratios. The performance of these composite backfill materials was assessed through UCS, fluidity, toxicity leaching tests, and microstructural analysis to elucidate the immobilization mechanisms. The following subsections detail the specific methods and procedures for each test:

Fluidity: To assess the transport performance of the sample, the fluidity of the fresh slurry was measured following the GB/T 8077-2012 [32].

Setting time: The setting time was determined using a Vicat instrument (ZKS100, Shanghai, China) in accordance with GB/T 1346-2011 [33].

UCS: After curing to the specified age, the samples were demolded, and their compressive strength was evaluated based on GB/T 50266-2013 [34]. A DYE-2000 electro-hydraulic servo pressure testing machine (Hebei Xianxian Di’ao Test Instrument Factory, Cangzhou, China) was employed for the test, applying a constant loading rate of 0.1 mm/min. The test was automatically terminated upon specimen failure, and the peak load (in kN) was recorded. The uniaxial compressive strength was calculated by dividing the peak load by the bearing area (1600 mm^2^). The reported compressive strength for each series represented the average value obtained from three identical specimens, and the error bars in figures indicate the standard deviation.

Volume stability: To monitor potential volumetric expansion during the reaction process, the dimensional changes in the material were examined to assess its volume stability. The volume change rate (M, %) was calculated using Equation (1) [11]:

$$[eqn]$$

where V_0_ represented the initial volume and V represented the volume of the samples after 28 days of curing (Note: The volume of the sample is calculated by measuring the side length with a vernier caliper, and the average value is taken).

Softening coefficient: In order to evaluate the water resistance of the material, the test was carried out according to GB/T 4111-2013 [35], and the softening coefficient of the material was calculated according to Equation (2) [12]:

$$[eqn]$$

where K represented softening coefficient, U_4_ represented the compressive strength of the sample after soaking in deionized water at 20 °C for 4 days, and U_0_ represented the compressive strength of the sample after being placed under natural conditions for 4 days

Leaching results: To investigate the efficacy of the samples in treating LZT, the same suite of macroscopic performance tests was conducted on the LZT-solidified bodies. The immobilization capability for heavy metals was evaluated by assessing their leaching characteristics according to the HJ/T 557-2020 [36]. The concentrations of lead, cadmium, arsenic, and zinc in the leachate were quantitatively analyzed using inductively coupled plasma mass spectrometry (ICP-MS, NexION 1000G, PerkinElmer, Waltham, MA, USA). The immobilization efficiency (W, %) was calculated using the following Equation (3) [37]:

$$[eqn]$$

where C_0_, and C_L_ represented the heavy metal concentrations (μg/L) in the leachates before and after solidification, respectively.

2.4. Characterization

The particle size distribution of raw materials was measured using a laser particle size analyzer (Malvern Mastersizer 2000, Malvern Instruments, Malvern, UK). The specific surface area was derived from the particle size data assuming spherical particles. Following the compressive strength test, hydration was arrested by immersing the samples in ethanol. The samples were then dried at 60 °C for 24 h and ground into powder. The fraction passing through a 0.075 mm sieve was collected for subsequent characterization. The chemical composition of raw materials was analyzed using X-ray fluorescence spectrometry (XRF, Panalytical Axios, Panalytical B.V., Almelo, The Netherlands). X-ray diffraction (XRD, Rigaku SmartLab SE, Rigaku Corporation, Akishima, Tokyo, Japan) was performed with Cu-Kα radiation, over a 2θ range of 5 to 80°. Fourier transform infrared spectroscopy (FTIR, Thermo Fisher Scientific Nicolet iS20, Thermo Fisher Scientific Inc., Waltham, MA, USA) was employed in the wavenumber range of 400–4000 cm^−1^ to identify the hydration products based on chemical bonds and functional group peaks. The FTIR analysis was performed using the KBr powder and pressed into a transparent pellet for measurement. Thermogravimetric analysis (TG, PerkinElmer STA 6000, PerkinElmer Inc., Waltham, MA, USA) was utilized to analyze hydration products. Microstructural analysis was conducted using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS, ZEISS GeminiSEM 360, Carl Zeiss Microscopy GmbH, Oberkochen, Germany).

3. Results

3.1. Fluidity and Setting Time

Figure 3 illustrates the effects of mix proportions on fluidity (Figure 3a) and setting time (Figure 3b) of high content SS alkali-sulfate composite activated binders. Each group of data was tested three times to take its average and show the standard deviation. A significant negative correlation exists between paste fluidity and SS content. With SS content increasing from 50% to 80% (groups S1–S4), fluidity decreases from 219 mm to 171 mm, accompanied by shortened setting time (initial setting from 211 min to 99 min; final setting from 305 min to 146 min). This is attributed to the higher specific surface area and water absorption of SS compared to FA, which increases paste water demand and accelerates hydration under alkaline activation [38]. High CaO content from SS leads to the formation of Ca(OH)2 in the alkaline environment, which can form strong hydrogen bonds with water molecules, initially contributing to fluidity [39]. Conversely, the polymerization of silicate species (from FA and the activator) forms Si-O-Si networks that bind water less strongly and increase viscosity. The interplay between these phases—modulated by the alkali-sulfate synergy—explains the observed trends in fluidity and setting time [39].

3.2. Mechanical Properties

In order to evaluate the mechanical properties of the cementitious system with high content of SS, the UCS of the material is shown in Figure 4. Figure 4a demonstrates the SS content on the UCS. At 60–80% SS content, 28-day strength surpassed 10 MPa, with early strength primarily coming from SS and FA derived C-A-S-H gel. Increased SS content elevated CaO levels, accelerating polymerization and enhancing the UCS, while unreacted FA particles provided pore-filling benefits [23]. However, beyond 60% SS, the excess calcium, despite the synergistic activation, began to dominate the system chemistry, potentially leading to rapid setting and incomplete reaction due to water consumption and space constraints, ultimately limiting further strength gain [12]. The optimal SS content was therefore 60 wt%. As shown in Figure 4b, UCS first rose and then declined with increasing alkali activator content. When the content increased from 10% to 20%, the 3-, 7-, and 28-day strength improved from 1.2 to 5.87 MPa, 2.64 to 9.09 MPa, and 8.53 to 12.85 MPa, respectively, it is better than the AAMs based on blast furnace slag shown in [11]. This enhancement occurred because higher alkali activator content promoted Si-O-Si bond formation, which contributes more to strength than Si-O-Al and Al-O-Al bonds [40]. However, further increasing the alkali activator to 25% reduced the UCS to 3.36, 4.27, and 10.23 MPa at the respective ages, suggesting excessive alkali inhibited Si-O bonding [12]. This highlights the need for optimal alkali dosage control in the high-SS system to balance activation and potential inhibition. Thus, the optimal alkali activator content was maintained at 20 wt%. The modulus of the water glass (SiO_2_/Na_2_O molar ratio) critically influenced UCS development. As shown in Figure 4c, low moduli promoted geopolymerization of SS and FA, forming zeolite-like structures and N-A-S-H gel while inhibiting CaO in SS [40]. Conversely, high moduli reduced both polymerization and hydration reactivity [22]. A modulus of 1.0 proved optimal for facilitating geopolymerization within the constraints of the high-calcium, high-SS environment. FGDG enhanced geopolymerization through sulfate activation and crystallization effects, accelerating hydration [41]. As shown in Figure 4d, the addition of FGDG significantly improved UCS from 1.49 MPa (SS–FA binary system) to higher values in the ternary system, this is consistent with [12]. However, exceeding 10 wt% FGDG led to a strength decrease, likely because excess sulfate could promote excessive ettringite formation or Al dissolution, potentially causing microstructural instability or efflorescence risks, demonstrating the need for precise sulfate dosage control within the high-SS co-activation framework [41].

3.3. Volume Expansion Rate and Softening Coefficient

Figure 5 illustrates the effects of mix proportions on volume expansion rate (Figure 5a) and softening coefficient (Figure 5b) of high content SS alkali-sulfate composite activated binders. When the SS content increased from 50% to 80% (S1–S4), the volume expansion rate increased from 2.26% to 5.59%, still lower than 40% [42]. For water glass content (groups S2, S5–S7) with optimal softening coefficient are achieved at 20% water glass, along with reasonable volume expansion rate. Regarding water glass modulus (groups S2, S8–S10) with other components fixed, the paste with modulus 1.0 exhibits the best comprehensive performance, with higher compressive strength and softening coefficient than those with S8, S9 and S10. An appropriate modulus balances solution alkalinity and silicate polymerization degree, promoting active mineral dissolution and stable hydration product formation [22]. For FGDG ratio (groups S2, S11–S13), the group S2 shows superior softening coefficient compared to groups without FGDG (S11) or unbalanced proportions (S12). Significantly higher volume expansion rates of Sample S13 and S8 may relate to excessive ettringite formation induced by unoptimized sulfate content or alkali concentration, leading to potential volume instability [42]. The alkaline environment provided by water glass promotes the dissolution of inactive SiO_2_ and Al_2_O_3_ in SS and FA to form reactive silicate and aluminate [12]. Sulfate from FGDG react with Ca^2+^ dissolved from SS to generate ettringite, which fills paste internal pores and improves compactness. In summary, mix proportion significantly affects the performance of SS-based alkali-sulfate composite activated binders. This confirms that SS achieves the best activation effect under alkali-sulfate composite excitation. The synergistic effect of alkali and sulfate activation enhances active mineral dissolution in SS and regulates hydration product formation and growth, thereby significantly improving binder compressive strength and durability. Insufficient or excessive activation, such as low water glass content, inappropriate modulus or unbalanced FA-to-FGDG ratio, weakens this synergistic effect and impairs performance. Optimal comprehensive performance is achieved with 60% SS, combined with 30% FA, 10% FGDG and 20% water glass (modulus 1.0).

3.4. Correlation Analysis

To evaluate the relative influence of various factors on UCS, a Pearson correlation analysis is performed (Figure 6). According to Figure 6a, the strongest correlations with 3-day strength are exhibited by SS content (r = 0.42) and alkali activator content (r = 0.4), identifying them as the dominant factors for early strength development, with results aligning with [43]. As curing progresses to 7 days (Figure 6b), the correlation coefficients evolved, showing a heightened effect for the alkali activator (r = 0.32) against a weakened effect for SS (r = 0.28). A further decline is observed in the contribution of the modulus. Following 28 days of curing (Figure 6c), FGDG content emerge as the dominant factor (r = 0.55). This is likely attributed to the consumption of the alkali activator, coupled with the strength contribution from ettringite formation [41]. The results indicate that the early age strength was primarily controlled by the alkali activator and modulus. In contrast, the influence of FGDG increased significantly over time. Although SS exhibits a stronger effect initially, its impact stabilized in later stages. The results not only indicate a negative correlation between the alkali and sulfate activators, suggesting antagonistic interactions, but also reveals a time-dependent behavior where dominance shifts from alkali to FGDG. This interplay underscores the complementary roles within the synergistic activation strategy for high-SS binder. Therefore, this observed antagonism highlights that the careful proportioning of alkali to sulfate activators was pivotal for achieving superior performance in high-content SS binder.

3.5. Characterization Analysis

3.5.1. Mineral Phase and Functional Groups Analysis

To investigate the formation mechanism of geopolymers, the mineral composition and functional group of the S2 samples at different curing ages were analyzed, as illustrated in Figure 7. From the mineral phase perspective (Figure 7a), the primary constituents of the geopolymers included CaCO_3_, SiO_2_, mullite, C-A-S-H, N-A-S-H, and ettringite. The presence of SiO_2_ and mullite indicated unreacted FA, the content of which gradually decreased with increasing curing age. The intensity of the CaCO_3_ peak at around 30° increased over time, which can be attributed to the progressive carbonation reaction occurring during sample preparation and curing. This process also consumes f-CaO present in the solid precursor [9]. The XRD pattern of the S2 sample in the low-angle region (5–10°) was provided in Figure S2, where no distinct ettringite peaks were detected, likely due to the low content of FGDG. In contrast, noticeable ettringite peaks were observed at approximately 16° and 22°. With prolonged curing, both the intensity and breadth of these ettringite peaks increased, corroborating its contribution to the later-stage strength development, a finding consistent with the mechanical behavior shown in Figure 5. Therefore, the key hydration products, C-A-S-H, persisted throughout the curing process, thereby providing the geopolymer with sustained mechanical strength. The functional groups of S2 samples at different curing ages were characterized by FTIR spectra, as presented in Figure 7b. At 28 days of curing, a broad absorption band centered at 3444.05 cm^−1^ was attributed to the O-H stretching vibration from the Si-OH-H_2_O and Al-O groups in ettringite [41]. In samples cured for 3 and 7 days, the band observed near 3384.25 cm^−1^ was associated with adsorbed water. Peaks located at approximately 1639.87 cm^−1^, 1638.81 cm^−1^, and 1644.14 cm^−1^ indicated the formation of CaCO_3_ [12]. The absorption at 969 cm^−1^ and 451 cm^−1^ reflected the hydration of C_2_S and C_3_S leading to the formation of C-S-H and Ca(OH)2 as intermediate products [9]. Furthermore, the Si-O-Si and Si-O-Al stretching vibration at 969 cm^−1^ confirmed the presence of C-A-S-H. These FTIR results were consistent with the XRD, collectively identifying C-A-S-H as one of the primary hydration products. In summary, the persistent presence of C-A-S-H and ettringite in a system with up to 60% SS provides direct proof that the synergistic activation mechanism functions effectively.

3.5.2. Microstructure Analysis

The microstructures of specimens at different curing ages are shown in Figure 8. At 7 days, the formation of C-A-S-H and the pore-filling effect of carbonate were observed. By 28 days, further microstructural densification occurred due to the combined presence of ettringite and C-A-S-H. These observations demonstrate how the synergistic action of alkali and sulfate activation products developed a robust microstructure in the SS-based binder. Compared with the raw SS (Figure 1), the hardened binder exhibited a more compact microstructure. As curing time increased, the porosity decreased, hydration products accumulated, and the microstructure became progressively denser. At 3 days, the specimen contained large pores and numerous unreacted SS particles, resulting in a loose structure and low compressive strength. By 7 days, abundant C-A-S-H had formed, and calcium carbonate had filled part of the pores, thereby enhancing the density. At 28 days, ettringite and C-A-S-H further contributed to microstructural densification.

3.6. Hydration Process

Based on above results. In the early stage of hydration, C_2_S and C_3_S in SS hydrate to form C-S-H and Ca(OH)2, while SO_2_ and Ca(OH)2 in FA react to form C-S-H [12], which is consistent with the conclusion of Figure 7b. This process can be expressed by Equations (4)–(6). At the same time, the generated Ca(OH)2 reacts with CO_2_ in the air to generate a large amount of CaCO_3_ (Equation (7)). This phenomenon can be clearly seen in Figure 7a, which also reflects the carbon storage characteristics of the material to certain extent [41]. Subsequently, under the action of strong alkali, the system produced a large amount of soluble Al^3+^. The [SiO_4_] skeleton of C-S-H is replaced by the isomorphous substitution of Al^3+^. In order to maintain the charge balance, Ca^2+^ will be attracted to insert into the interlayer of the gel to form C-A-S-H (Equation (8)). Therefore, C-A-S-H is the main hydration product, which is consistent with Figure 7. In addition, trace amounts of N-A-S-H may be produced due to Na^+^ from the alkali activator (Equation (9)). In the late reaction, CaSO_4_·2H_2_O in FGDG not only activated SS and FA but also reacted with Ca(OH)2 and calcium aluminate (from FA hydration) to form ettringite (Equation (10)). This reaction scheme highlights how the synergistic activation leverages the high calcium content of SS and the sulfate from FGDG to drive the formation of C-A-S-H and ettringite, effectively utilizing the components within the high-volume SS system. Therefore, the whole process can be described as:

$$[eqn]$$ $$[eqn]$$ $$[eqn]$$ $$[eqn]$$ $$[eqn]$$ $$[eqn]$$ $$[eqn]$$

3.7. Solidification of Lead–Zinc Tailings

In view of the conversion of C-S-H to C-A-S-H under high calcium and strong alkali environment, accompanied by the formation of ettringite, the corrosion resistance and heavy metal solidification ability of the material are significantly improved, which is suitable for mine filling [25,37]. In order to evaluate the LZT practical applicability of the binder, the performance of backfilling materials with different binder and tailings ratios was studied. A series of backfill specimens were fabricated by mixing the optimized binder formulation (60% SS, 30% FA, 10% FGDG, with 20% externally added alkali activator at a modulus of 1.0, and a water-to-solid ratio of 0.3) as the binder with LZTs at various mass ratios (1:1, 1:2, 1:3, 1:4). Subsequently, testing and analysis were performed on the samples in terms of macro performance and leaching toxicity.

3.7.1. Macro Performance

UCS and fluidity are two most critical performance parameters for backfill materials. As shown in Figure 9, the 3 days UCS exceeds 1 MPa, while the 28 days UCS far surpasses the typical mine backfill requirement of 3 MPa [38]. With the increasing incorporation of lead–zinc tailings, the 28d strength of the backfill materials reached 6.95, 7.26, 8.55, and 10.08 MPa, respectively, with minimal strength loss compared to the high-content SS-based binder. This is attributed to the activity characteristics of the tailings and the synergistic optimization effect of the binder formula. Under the strong alkaline environment, the high SiO_2_ and CaO contents in the tailings promote the dissolution of their active components, which participate in hydration reactions to fill pore defects induced by tailings incorporation. Additionally, the filling effect of fine tailings particles optimizes the slurry bulk density and reduces internal cracks, partially offsetting the structural porosity potentially caused by decreased fluidity. This strength variation characteristic offers significant advantages over traditional alkali-activated materials. The binder formula developed in this study converts tailings from “inert fillers” to “active admixtures,” thus minimizing strength loss [25]. Furthermore, the strength was enhanced by 3.21% compared to conventional Ca/Si-based binders [41]. The fluidity of the samples ranges from 139 to 192 mm. The linear decrease in the fluidity of backfill materials with increasing LZTs content results from the synergistic effect of three mechanisms: physical water demand, chemical reaction competition, and rheological property transformation. The high specific surface area of fine tailings particles significantly increases water adsorption, while their angular morphology enhances interparticle frictional resistance and disrupts the optimal packing state of the original particle gradation, leading to an implicit reduction in the effective water-binder ratio. Additionally, the synergistic effect of the SS–FA–FGDG ternary system renders the binder more sensitive to tailings content, resulting in a stable linear downward trend in fluidity. While the filling materials meets macro performance, toxicity leaching tests remained essential to ensure environmental compliance.

3.7.2. Leaching Analysis

After 28 days of curing, the leaching concentrations of lead and arsenic are presented in Figure 10a, while those of cadmium and zinc are listed in Table S3. The leaching concentration of Pb increases proportionally with LZT content but remains below the limit requirements. Notably, the leaching concentrations of Zn and Cd are both below the detection limit, whereas that of As is significantly reduced, reflecting distinct leaching behaviors between anionic and cationic heavy metals [44]. It is worth emphasizing that SFGL4 fully meets the Class III requirements of GB/T 14848-2017 Groundwater Quality Standard [45], demonstrating its suitability as a backfill material. Although the As leaching concentrations of SFGL1 and SFGL2 exceed the Class III standard limit (<0.5 mg/L), they are still lower than the hazardous waste threshold specified in GB 5085.7-2019 Identification Standards for Hazardous Wastes-Identification of Leaching Toxicity [46]. The leaching results of raw materials are provided in Table 2, and the solidification efficiencies of Pb and As are illustrated in Figure 10b. As shown, SFGL4 achieves a solidification efficiency of over 80% for both Pb and As, and nearly 100% for Cd and Zn, indicating excellent heavy metal stabilization performance. Compared with traditional alkali-activated materials and Ca/Si-based materials, the proposed binder exhibits comparable solidification capabilities for Pb, Cd, and Zn, while demonstrating superior As stabilization within a certain range [25,37,41]. The mechanisms underlying heavy metal solidification/stabilization will be discussed in Section 3.7.3.

3.7.3. Hydration and Heavy Metal Solidification Mechanism

The high-content steel slag-based alkali-sulfate composite activator binder exhibits distinct properties depending on the tailing content. Therefore, microscopic analyses of SFGL1 and SFGL4 were performed to investigate the strength development mechanism of the solidified matrix and the immobilization mechanism of heavy metals. The results are shown in Figure S3. XRD curves (Figure S3a) reveals that the formation of calcium carbonate (at approximately 30°) is associated with the generation of C-S-H [37], which is attributed to the sufficient supply of Ca and Si from the tailings. When the tailings incorporation is insufficient (SFGL1), the primary hydration product is C-A-S-H, with a greater proportion of C-S-H converted into C-A-S-H. In contrast, when the tailings incorporation is excessive (SFGL4), the supply of Al from fly ash becomes inadequate, leading to the coexistence of C-S-H and C-A-S-H. Given that C-A-S-H exhibits higher strength, this observation is consistent with the results presented in Section 3.1. A comparison of the phase compositions of SFGL1-28d, SFGL4-28d, and the LZT indicates the formation of new phases in the solidified matrix, including CaZn(CO_3_)2, FeAs_3_O_3_·4H_2_O, and Kegelite. This demonstrates that the tailings undergo phase reconstruction following activation by the binder, and heavy metals are likely immobilized by binding to hydration products, thereby achieving efficient solidification [40]. In addition, due to the action of sulfate, a weak peak of ettringite can be seen. The FTIR spectra are presented in Figure S3b. The O-H stretching vibration of water molecules in raw LZT (3551 cm^−1^) exhibits a distinct shift in the binder system, which is attributed to the O-H interactions between water molecules and C-S-H hydration products. Notably, the enhanced intensity of the peak around 3430 cm^−1^ in the solidified tailings matrix indicates an increased formation of C-S-H. The characteristic peak at 999 cm^−1^ in SFGL4 corresponds to the generation of the hydration products C-A-S-H and C-S-H. In raw LZT, the peaks at 1430 cm^−1^, 874 cm^−1^, and 713 cm^−1^ are assigned to CaCO_3_ [37]; however, a broader and more intense signal peak is observed after tailings solidification, suggesting that additional CaCO_3_ is generated during the hydration process. The DTG curves of SFGL samples (25–1000 °C) reveals six distinct decomposition intervals (Figure S3c): I (25–220 °C), II (220–390 °C), III (390–580 °C), IV (580–720 °C), V (720–840 °C), and VI (840–1000 °C) [23]. Interval I represents loss of physically adsorbed water and decomposition of ettringite and C-A-S-H [47]. Interval II shows continued C-A-S-H decomposition [48]. Interval IV corresponds to CaCO_3_ decomposition [49].

The SEM-EDS results of SFGL1 and SFGL4 cured for 28 days are presented in Figure S4. The dominant hydration product of SFGL1 is C-A-S-H, which exhibits predominantly flocculent and agglomerated morphologies. In contrast, the main hydration products of SFGL4 are C-A-S-H and C-S-H, accompanied by needle-like gels interspersed within the flocculent and agglomerated structures, consistent with the XRD results. C-S-H and C-A-S-H synergistically refine the pore structure of the solidified matrix and hinder the migration of Pb and Zn. Hydration product gels can effectively immobilize cationic heavy metals [50,51]. Specifically, Pb and Zn participate in the hydration reaction and form M-O-Si bonds (where M denotes Pb or Zn) with C-S-H and C-A-S-H [41], and the coexistence of C-S-H and C-A-S-H may enhance this bonding reaction. Elemental mapping results (Figure 11) confirm the immobilization of heavy metals, as their spatial distributions overlap with those of calcium and silicon. Additionally, CO_3_^2−^ derived from calcite facilitates Zn-Ca co-precipitation. Furthermore, Fe-As coupling effects indicate that As can be immobilized through the formation of Fe-As compounds. However, the relatively high alkalinity of SFGL1 may impede the efficient immobilization of As, which could be attributed to the suppressed formation of stable Fe-As compounds under strong alkaline conditions. In addition, ettringite may solidify Pb and AsO^2−^ through ion exchange [52]. Therefore, the immobilization of heavy metals in tailings by high-volume steel slag alkali-sulfate composite activator binder is the result of coupling of various products. The hydration process and heavy metal solidification mechanism are shown in Figure 12.

4. Conclusions

This study successfully developed binder incorporating high-volume steel slag through synergistic alkali-sulfate activation strategy, demonstrating its application for backfill materials. Key findings include:

• (1)The synergistic activation effectively addressed the reactivity challenges associated with high-content SS, enabling the formulation to achieve a 28-day UCS of 12.85 MPa. Excellent volume stability and water resistance were obtained under the optimal formula.
• (2)Early strength development was primarily governed by the alkali activator, while the sulfate activator (via ettringite formation) became dominant for later-age strength enhancement.
• (3)The hydration of steel slag provided a calcium source. The active components in fly ash promoted the dissolution of aluminum and enter the silicate skeleton in a strong alkali environment and form a gel structure with C-A-S-H as the main body through the charge balance mechanism. Sulfate was excited to form ettringite, which further filled pores and optimizes microstructure.
• (4)The preferred high-content SS binder with a binder-tailing ratio of 1:4 cooperated with LZT to prepare backfill materials with excellent performance, which meets the standards. Leaching experiments confirmed the excellent immobilization effect of heavy metals. Heavy metals were fixed by physical encapsulation, ion exchange, and co-precipitation mechanisms.