Synergistic and Environmental Impacts of Industrial Solid Waste and Cement Clinker in Shield Muck Solidification: A Case Study in Shijiazhuang City

Abstract

Traditional landfill disposal of muck uses a significant amount of land and pollutes the environment, while current solidification methods heavily depend on energy-intensive cement. This study introduces a novel approach for synergistically solidifying muck using cement, fly ash, and steel slag, aiming to utilize waste resources and achieve low-carbon disposal. Experimental optimization identified the optimal ratio (cement:fly ash:steel slag = 2:2:1). The findings indicate that cement is crucial for early strength, while industrial waste materials enhance long-term performance through continued reactions. At a total solidifying agent content of 4–6%, the material exhibits optimal mechanical properties and durability, with only a 4% strength loss after 12 dry–wet cycles. Microscopic analysis indicates that several gels and polymers with cementing properties are produced, collectively enhancing the material’s structure. Additionally, this material effectively immobilizes heavy metals, including chromium, lead, arsenic, and cadmium, with leaching concentrations that are well below safety thresholds. This approach provides a dependable and eco-friendly method for large-scale disposal of construction waste muck and industrial solid waste, offering significant potential for engineering applications. Further studies could investigate additional solid waste types and formulations suitable for high-moisture materials like sludge.

1. Introduction

The rapid pace of urbanization has led to a steep rise in construction muck from subway projects. This waste, exceeding ten million cubic meters annually, is marked by its heterogeneous composition, containing 30–60% clay minerals and heavy metals. Inadequate disposal not only consumes substantial land resources through landfilling and delays construction but also triggers secondary environmental issues like soil compaction and groundwater pollution, endangering ecosystems and human safety [1]. These challenges underscore the critical importance of addressing construction waste muck management to support sustainable urban development. Urgent research is needed to ensure its safe disposal and utilization.

The solidification/stabilization (S/S) technology holds considerable potential for the treatment and disposal of construction muck. This approach consolidates muck particles into an integrated structure through chemical and physical processes, achieving muck solidification via mechanisms such as chemical precipitation, adsorption, and physical encapsulation using cementing materials. This technology can effectively enhance the macroscopic properties of the target muck, meeting construction objectives like increased soil strength, suppressed deformation, and reduced permeability within the required time frame [2,3]. Notably, this method facilitates on-site resource utilization of construction muck, substantially reducing transportation and disposal costs, while also mitigating the environmental pollution risks associated with the transportation process [4].

This technology has garnered significant academic interest. Gong Xing et al. [5] effectively solidified soft clay using Portland cement, lime, and gypsum, mitigating the ecological impact of Nansha soft soil. Lanh Si Ho et al. [6] explored cement’s role in muck solidification, examining the interactions between cement hydration, pozzolanic reactions, and carbonation, and their effects on unconfined compressive strength and pore structure. Their findings indicate that carbonation positively influences the strength development of cement-based solidified muck. Guo Shaohua et al. [7] demonstrated that combining cement with a curing agent significantly enhances compressive strength, achieving 6.91 MPa at 28 days. Zhu et al. [8] employed citric acid-modified materials to solidify soft clay. After a 28-day curing period, the compressive strength of the specimens reached 2.4 MPa, addressing challenges associated with the stabilization and limited durability of soft clay soils.

Traditional Portland cement-based materials continue to dominate muck stabilization, yet the drawbacks of relying solely on this system are increasingly evident. Most research has centered on optimizing cement-modified muck, revealing contradictions in engineering practice. The hydration kinetics of pure cement-stabilized muck lead to slow early strength development, inadequate for rapid traffic reopening. Additionally, the heavy reliance on cement dosage escalates material costs and limits the broader use of industrial waste like fly ash and slag in solidifiers.

To break through the dependence of muck solidification on cement and explore its low-carbon potential, existing studies have focused on two major technical approaches: (1) the use of alkali-activating materials, such as sodium hydroxide and water glass, to replace traditional cement-based activators. By supplementing the muck with OH⁻ and SiO₄, the mineral phases are depolymerized and recombined into a cementitious phase with a three-dimensional network structure, significantly improving the macroscopic properties of the solidified muck [9,10]; and (2) the introduction of industrial solid wastes with pozzolanic activity, such as fly ash [11,12], slag [13,14,15], carbide slag [16,17], and waste gypsum [18], as solidification components. Through the collaborative disposal of these industrial solid wastes, researchers have explored whether they can reduce cement consumption while ensuring the muck solidification effect, thereby optimizing resource efficiency and environmental benefits simultaneously.

Researchers like Chen Zhongqing [19] developed geopolymers from alkali residue and fly ash to enhance the early performance of solidified muck. Jin Shenghe [20] and his team utilized a slag-desulfurized gypsum-calcium carbide residue solidifier for clay stabilization. Scanning electron microscopy revealed abundant Hydrated calcium silicate (C-S-H) and ettringite (AFt) formation, resulting in a denser microstructure and superior performance compared to cement soil. Further studies [21] indicate that optimal calcium carbide residue content maximizes the unconfined compressive strength (UCS) of the steel slag-slag system, with ductility notably surpassing that of cement soil.

Researchers focusing on industrial solid waste often prioritize enhancing the performance of solidified muck through the addition of various waste materials and investigating the underlying mechanisms, such as hydration reactions and pozzolanic effects [6,22,23,24]. However, they frequently neglect the environmental pollution risks posed by heavy metals in these wastes when used to solidify engineering muck. The effectiveness of engineering muck in immobilizing heavy metals and reducing their mobility is also often overlooked. Industrial wastes like fly ash commonly contain heavy metals such as Cr, As, Cd, Ni, and Cu, which pose significant ecological and health risks [25,26,27]. For instance, Risto Pöykiö [25] reported that fly ash contains high proportions of extractable Cd and As, indicating substantial environmental migration risks. Similarly, Cao Yang et al. [28]. found that waste from carbide slag sites poses severe Cd and Hg pollution risks to nearby soil, with the parent soil material being a primary pollution source.

Researchers have explored using cement, alkali activation, and industrial solid waste to solidify contaminated muck, addressing environmental pollution. For instance, Chang et al. [29] utilized fly ash-derived active silicate to remediate Pb-contaminated muck, while Yin-Juan Sun [30] employed carbide slag and metakaolin for Cu-contaminated sites. Similarly, Suo et al. [31] treated Cu-contaminated muck using red mud and cement. These studies demonstrate the potential of solid waste-based cementitious materials to solidify and stabilize heavy metals.

Although solid waste-based cementitious materials have demonstrated the capacity to chemically stabilize heavy metal ions, current research overlooks certain aspects of industrial solid waste resource utilization. Predominantly, studies emphasize laboratory assessments of heavy metal solidification efficiency, neglecting the potential for secondary release from the cementitious materials themselves. In constructing the “contaminated muck—solidifying agent” reaction system, researchers often use idealized ratio designs without establishing a dynamic coupling model between the strength development of solidified muck and pollutant migration behavior. This oversight poses a risk of “failure upon reaching the standard” in practical applications, highlighting the need for technical advancements to reconcile “solid waste resource utilization” with “muck harmless treatment.”

In utilizing industrial solid waste for engineering muck solidification, it is crucial to systematically evaluate the environmental risks posed by heavy metals alongside performance optimization. Current research insufficiently addresses the forms, long-term stability, and release mechanisms of these metals, creating a significant barrier to technological advancement. Future studies must enhance the comprehensive evaluation of solidified bodies’ environmental safety, delve into the immobilization mechanisms and long-term effectiveness of the muck matrix on typical heavy metals, particularly As and Cd due to their high migration risks, and develop robust environmental risk assessment and control methods to ensure the technology’s sustainability and environmental compatibility.

This study initially employed Portland cement, fly ash, and steel slag as constituents of a solidifying agent to stabilize engineering muck. It examined the effects of the solidifying agent on the stabilization outcomes of the muck and confirmed the technical feasibility of utilizing multiple industrial wastes in its treatment. Additionally, the study assessed the impact of various solidifying agent additives on the macroscopic properties of the solidified muck. The changes in strength, reaction products, and microstructure of the solidified muck were characterized and analyzed using unconfined compressive strength tests (UCS), X-ray fluorescence analysis (XRF), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and infrared spectroscopy (IR) to elucidate the mechanisms influencing the macroscopic property alterations of the solidified muck.

This study assessed the environmental pollution risks of solidified sludge from various solid waste sources using leaching tests. The objective was to verify its environmental safety and evaluate the potential heavy metal pollution risks in industrial solid wastes. Specifically, it examines how industrial solid waste co-solidifies with construction waste muck, focusing on the mechanisms that immobilize and stabilize heavy metal ions. The findings aim to offer a new perspective on the interaction mechanisms of various industrial solid wastes and highlight the environmental risks associated with using solid waste in recycled products, encouraging further research in this area.

2. Materials and Methods

2.1. Raw Material Testing and Analysis

The muck used in this study primarily originated from the soil pressure balance shield’s construction process within the subway tunnel. According to the geological survey report of the shield construction, the collected muck consisted of a mixture of strongly weathered argillaceous siltstone and sandstone. The physical properties of the collected muck were tested and analyzed following the standard [32], and the results for their basic characteristics are presented in

Table 1. Basic properties of muck.

Property	Value
Natural water content (%)	20
pH	8.2
Liquid limit, wl (%)	46.97
The plastic limit, wP (%)	22.67
Plasticity index, IP	24.3
Optimum water content, wopt (%)	26.7
Maximum dry density rd (g/cm3)	1.3

. Fly ash (FA) and steel slag (SS) were utilized as the primary admixtures for solidifying the muck. These solid waste materials were sourced from Shijiazhuang Yuanjing Mineral Products Co., Ltd. located in Shijiazhuang, China. Additionally, ordinary Portland cement was employed as an admixture, obtained from Zhongheng Pipe Piles Company situated in Weihai, Shandong, China. The content analysis of major elements within both the muck and solid waste admixtures was conducted using a Niton XRF XL5 portable X-ray fluorescence spectrometer (Purchased from Langdu Scientific (Beijing) Co., Ltd., Beijing, China). The chemical composition is illustrated in

Table 2. Raw material chemical composition.

Oxide	Value (%)
	Muck	Portland Cement	FA	SS
SiO2	55.73	22.14	48.23	50.38
Al2O3	28.24	0.81	36.87	10.03
CaO	0.75	62.80	3.94	9.95
MgO	0.02	1.4	0.52	4.36
SO3	0.1	2.1	3.37	0.73
Fe2O3	12.4	2.93	5.46	23.05
Others	2.76	7.82	1.61	1.5

, while Figure 1 depicts the mineral composition of the muck and solid waste admixture, respectively.

Muck primarily comprises albite (Ca₃Si₂O₇) and montmorillonite clay (CaAl₂Si₂O₈·4H₂O) minerals. The mineral composition of solid waste varies significantly depending on its production process. For example, fly ash predominantly contains feldspar silicate minerals (CaSi₂O₇), CaAlSiO₄(OH), calcium carbonate, and some amorphous SiO₂ and Al₂O₃ components [33,34,35]. In Portland cement, used for testing, the main mineral phases are dicalcium silicate (C₂S) and tricalcium silicate (C₃S). Steel slag, a by-product of steel production with stable chemical properties, offers acid-base neutralization and adsorption capabilities. Its surface is rich in oxides and hydroxyl groups [36,37,38], and its primary mineral composition includes calcium silicate minerals like Ca₆(Si₂O₇)(OH)₆ and dihydrate calcium feldspar.

2.2. Preparation of Solidified Muck Samples

2.2.1. Preparation Procedure and Curing Conditions

The solidified muck samples were prepared in accordance with the specific requirements of standard JTG 3441-2024 [39] for molding and maintenance of solidified muck. In the experiment, the muck, Portland cement, and an admixture primarily composed of industrial solid waste were ground to a particle size less than 0.1 mm prior to mixing. The mixed raw materials for solidified muck were then homogenized and blended in a mortar mixer at a speed of 100 r/min for 3 min. Water was added to the mixture and stirred at a speed of 100 r/min for 3 min followed by 200 r/min for an additional 2 min until achieving uniformity in the solidified muck. The resulting mixture was filled into a cylindrical test mold measuring 50 mm (Height) × 50 mm (Diameter), compacted using a press, and subsequently demolded using a stripper. Each formed sample was wrapped in plastic wrap and placed inside a standard curing box where it underwent curing under controlled environmental conditions (temperature: 20 ± 2 °C; relative humidity: 95 ± 3%). The solidified muck specimen is shown in Figure 2.

2.2.2. The Design of the Curing Agent Mixture

In the orthogonal design method, the proportions of Portland cement, FA, and SS in the admixture are investigated and designed as four parameters. Each parameter corresponds to three levels: 0%, 3%, and 6%.

Table 3. Orthogonal experimental matrix for parameter optimization.

Number	Portland Cement	FA	SS	Blank Group
SJZ-1	0	0	0	—
SJZ-2	0	3	6	—
SJZ-3	0	6	3	—
SJZ-4	3	0	6	—
SJZ-5	3	3	3	—
SJZ-6	3	6	0	—
SJZ-7	6	0	3	—
SJZ-8	6	3	0	—
SJZ-9	6	6	6	—

presents the mixing ratios for a L9 (4,3) orthogonal design experiment with a total of 9 groups denoted as SJZ1–SJZ9.

Subsequently, based on the compressive strength results obtained from solidified muck samples after 7 days of curing, the optimum proportion for each mixture was determined.

In the orthogonal experiment, by regulating the independent dosage (0%, 3%, 6%) of the three components of Portland cement, fly ash and steel slag, the optimal curing agent formula was optimized to determine the relative optimal proportion of the three components. To further explore the influence law of the total dosage of the curing agent (non-single-component) on the properties of the solidified muck, the study designed comparative experiments of 2%, 4%, and 6%, focusing on analyzing the gradient influence of the total dosage of the curing agent on the macroscopic properties of the solidified muck.

2.2.3. Statistical Analysis

Using the different ratios obtained from the orthogonal experiment, three independent replicate experiments were conducted under each experimental condition. This study was completed using the SPSSAU (25.0) online statistical analysis platform.

Prior to data analysis, the Shapiro–Wilk test confirmed the normality of data within each experimental group, while the Levene test verified homogeneity of variance across groups. Results indicated that all datasets satisfied the assumptions for parametric tests, with both normality and homogeneity of variance tests yielding p > 0.05.

This study utilizes Multifactor Analysis of Variance (ANOVA) to assess the primary effects of cement, fly ash, and steel slag contents on the 7-day UCS of specimens, using the General Linear Model in SPSSAU. The orthogonal experimental design focuses on efficiently estimating main effects, thus excluding interaction terms between factors.

The statistical analyses were conducted with a significance level of α = 0.05. All analyses reported specific F-values and p-values, and standard deviation (SD) was included in the orthogonal test results to assess data dispersion.

2.3. Experimental Testing of Solidified Muck

2.3.1. Analysis of Macroscopic Properties of Solidified Muck

In order to assess the suitability of the prepared solidified muck as road material, the unconfined compressive strength (UCS) of the solidified muck was tested and analyzed according to standard JTG 3441-2024 [39]. The UCS test for 7-day and 28-day cured muck samples was conducted using a CMT6104 microcomputer (Purchased from Weihai Shengwei Testing Machine Co., Ltd. in Weihai City, China)-controlled universal press. The loading method employed constant displacement control at a speed of 5mm/min. Loading was halted when visible damage occurred on the surface of the solidified muck specimen or when strain exceeded 20%, with the maximum bearing capacity at that point considered as the maximum bearing pressure for that particular specimen. Each group of tests consisted of six parallel samples, and the average value from these tests represented the unconfined compressive strength of solidified muck in each corresponding group. Reference provided guidance on determining compressive strength grade and application scenarios based on combined use with a muck curing admixture (CJ/T 486-2015 [40]).

2.3.2. Hydro-Stability Assessment Test

Hydro-stability assessment was conducted in strict accordance with JTG 3441-2024 specifications to evaluate water resistance compliance for road construction applications.

Two distinct curing regimes were implemented: (1) Standard curing cohort (20 ± 2 °C, RH ≥ 95%, 7d duration); (2) Immersion conditioning cohort (6d standard curing + 24 h water immersion at 20 ± 2 °C). Pre-test surface moisture removal for immersion specimens was achieved through standardized blotting procedures (ASTM D559 [41]).

Both cohorts underwent UCS evaluation using a CMT6104 servo-hydraulic testing frame (MTS Systems Corporation) with displacement-controlled loading at 5 mm/min, terminating at 20% axial strain or visible macro-cracking. Hydro-stability coefficients (η) were calculated per Equation (1). Performance thresholds were established as η > 0.7 for conventional subgrades and η > 0.8 for premium highway applications (CJ/T 486-2015 [40]).

η = UCS_immersed/UCS_standard

(1)

2.3.3. Drying-Wetting Test

To assess the stability of solidified muck under fluctuating water conditions and examine its long-term degradation, dry and wet cycling tests were conducted following established standards [41,42]. The procedure involved: (1) Soaking: After curing samples with various formulations for 28 days, they were submerged in water at 20 ± 2 °C for 18 ± 0.5 h; (2) Drainage: Samples were removed and allowed to drain for 15 min; (3) Drying: The drained samples were placed in an oven at 60 ± 2 °C for 6 ± 0.5 h; (4) Cooling: Samples were cooled at room temperature for 1 h, completing one cycle. After completing each cycle, the unconfined compressive strength and mass loss of the specimens were tested.

2.3.4. Analysis of Mineral Phases in Solidified Muck

In order to characterize the mineral composition of solidified muck samples, we conducted an analysis on the influence of curing time and chemical reactions in solidified muck. The solidified muck samples were characterized using a Bruker D8 X-ray diffraction analyzer with a Co target for testing. The equipment was operated at an accelerated voltage of 40 kV, with a measurement angle set between 5°and 90°, and a scanning speed of 5°/min. The phase composition of the solidified muck was analyzed through X-ray diffraction.

2.3.5. Analysis of FTIR in Solidified Muck

In order to investigate the reaction mechanism of solid waste admixture during solidification, Fourier transform infrared spectroscopy was employed to analyze the chemical bonds in muck samples after solidification. Semi-quantitative analysis was conducted on the solidified muck to determine relative changes in characteristic groups based on spectral intensities. This study aimed to explore the degree of polymerization of Si-O tetrahedrons and Al-O tetrahedrons within gel groups such as C-S-H and C-A-S-H present in the solidified muck samples, as well as examine mineral phase alterations. These analyses were performed to identify the underlying cause for changes observed in macroscopic mechanical properties of the solidified muck.

2.3.6. Microstructure Characterization and Analysis of Solidified Muck

In the experiment, solidified muck specimens preserved for 7 and 28 days were subjected to pre-treatment in anhydrous ethanol. The microstructure of solidified muck at different curing ages was characterized and analyzed using a Hitachi S-4800 (Produced by the Japanese company Hitachi, Tokyo, Japan) field emission scanning electron microscope (SEM) to investigate the influence of gelling substances generated in different solidified muck on their internal microstructure. The changes in C-S-H, C-A-S-H, and other gelling groups under the influence of various admixtures were analyzed to understand their impact. Micro-morphological changes in different solidified muck samples were observed to verify the key components that affect the macroscopic mechanical properties of solidified muck and further support the reaction mechanism within it.

2.3.7. Environmental Impact of Solidified Muck

To investigate the impact of mixed solid waste on the environmental risk associated with solidified muck, this study employed a portable X-ray fluorescence analyzer to detect and analyze heavy metal elements present in muck, admixtures, and solidified muck samples. This analysis aimed to elucidate the environmental pollution risk posed by raw materials and samples used for solidification, as well as examine the effect of the solidification reaction on fixing and stabilizing heavy metal elements in solid waste. For X-ray fluorescence analysis, each ground solidified muck sample was placed in a test sample box measuring less than 1mm. Each sample underwent testing for 60 s at a time, with the average value representing the content of each heavy metal element within that specific tested sample.

In order to further investigate the environmental impact of solidified muck on groundwater and surface water, leaching toxicity tests were conducted on solidified muck samples in accordance with the national standard “Leaching Toxicity Method for Solid Wastes” (HJ/T 299-2007) [43]. The pH value of the leaching solution was adjusted to 3.2 ± 0.5. A mixture of 30 g of the tested sample and 0.3 L of extractant was placed in a 500 mL open conical flask, which was then positioned on a magnetic stirrer. The stirring speed was set to 200 r/min, and the flask was maintained at 25 ± 2 °C for 20 h. The filtrate was passed through a disposable needle-punched filter with a pore size of 0.65 μm. The concentrations of Cr, Pb, As, and Cd elements in the filtrate were determined using graphite furnace atomic absorption spectrometry (AAS). By analyzing the leaching behavior of different solidified muck, their environmental effects could be further validated.

3. Results

3.1. Orthogonal Experimental Result and Strength Characteristics

Solidified muck is often repurposed as landfill liner or subgrade material, with unconfined compressive strength (UCS) serving as the primary measure of solidification efficacy and engineering applicability. According to relevant standards [42], the 7-day soaked strength for pavement base materials must meet or exceed 2–3 MPa, establishing a definitive strength criterion.

The results of the orthogonal experiment analysis revealed the influence of cement, fly ash, and steel slag on the unconfined compressive strength (UCS) of the specimens (

Table 4. Orthogonal Experimental Results with Range Analysis.

Number	Portland Cement	FA	SS	Blank Group	UCS (MPa)	SD (%)	CI (95%)	Hs	SC
1	0	0	0	1	0.34	7.2	(0.27, 0.41)	0.19	0.58
2	0	3	6	2	0.37	8.8	(0.27, 0.47)	0.17	0.46
3	0	6	3	3	0.36	6.8	(0.29, 0.43)	0.21	0.59
4	3	0	6	3	3.01	4.9	(2.56, 3.46)	3.08	1.02
5	3	3	3	1	3.49	11.9	(2.22, 4.76)	3.36	0.96
6	3	6	0	2	3.33	9.1	(2.41, 4.25)	3.19	0.96
7	6	0	3	2	3.57	10.1	(2.48, 4.66)	3.48	0.97
8	6	3	0	3	3.43	10.9	(2.29, 4.57)	3.11	0.91
9	6	6	6	1	3.73	5.5	(3.11, 4.35)	3.64	0.97
K1	1.06	6.92	7.10	7.56
K2	9.83	7.29	7.42	7.27
K3	10.74	7.42	7.11	6.80
k1	0.35	2.31	2.37	2.52
k2	3.28	2.43	2.47	2.42
k3	3.58	2.47	2.37	2.27
R	3.23	0.17	0.11	0.25

(UCS—Unconfined compressive strength; SD—Standard Deviation; Hs—Hydro-stability; SC—Softening Coefficient).

and

Table 5. Analysis of Variance (ANOVA).

Curing Period	Source	DF	SS	MS	F-Value	p-Value	Significance
7d	Portland cement	19.035	2.000	9.518	193.352	0.005	p < 0.05
	FA	0.046	2.000	0.023	0.466	0.682	p > 0.05
	SS	0.022	2.000	0.011	0.220	0.820	p > 0.05
	Blank Group	0.098	2.000	0.049	1.000

(p < 0.05, Remarkable).

). Cement content was the dominant factor, with a range analysis influence weight (R = 3.23) 19.0 and 29.4 times that of fly ash and steel slag, respectively. Analysis of variance (ANOVA) confirmed the statistical significance of cement’s influence (F = 193.352, p = 0.005 < 0.05). Similarly, steel slag did not exhibit a significant impact (F = 0.220, p = 0.820 > 0.05). Notably, a 6% dosage of steel slag even resulted in a decline in strength (k₃ = 2.37 MPa < k₂ = 2.47 MPa), potentially due to volume expansion or inhibition of reaction activity at higher dosages [44].

The conclusions of this study align with similar research, such as Lv C [45], which also highlights cement’s primary role in strengthening industrial sludge during solidification. However, our findings diverge from studies utilizing steel slag or fly ash as partial replacements [46]. This discrepancy likely arises from the distinct chemical composition, initial particle size distribution of the treated muck, and the varying dosages employed.

In conclusion, this study demonstrates that incorporating approximately 3% cement to solidify muck satisfies the strength requirements for subgrade materials, indicating significant potential for large-scale application. While fly ash and steel slag did not markedly enhance strength under the study’s conditions, their potential as fillers or for enhancing other properties, such as heavy metal solidification, merits further investigation.

This study employs the optimal curing agent ratio derived from the orthogonal experiment (Portland cement: fly ash: steel slag = 2:2:1). The curing agent is added at 2%, 4%, 6%, and 8% of the total shield muck to be solidified. The 7-day and 28-day strengths of the solidified muck specimens with varying curing agent additions are tested and analyzed, as depicted in Figure 3.

Variations in curing agent concentrations significantly affected the compressive strength of solidified muck at 7 and 28 days. With increasing curing agent, the 7-day compressive strength initially rose, stabilized, and then declined. Maximum strength of 2.04 MPa was observed at 4% to 6% curing agent, although strength variability also increased. Over time, the muck’s mechanical properties improved, reaching a 28-day compressive strength of 4.19 MPa at 8% curing agent—175% higher than at 7 days. Enhanced mechanical stability was more pronounced with higher curing agent levels.

To assess the feasibility of employing industrial solid waste for co-solidifying engineering muck, this study evaluated the durability of solidified muck through dry–wet cycle tests, as depicted in Figure 3b. The results reveal that after three cycles, specimens without a solidifying agent completely disintegrated. This demonstrates that muck alone fails to meet the mechanical strength needed for engineering applications and exhibits inadequate durability to withstand extreme environmental conditions.

The durability of solidified muck exhibits a marked improvement with increasing curing agent content. During the initial dry–wet cycling (0–3 cycles), the compressive strength of solidified muck with varying curing agent contents (2–8%) demonstrates divergent growth rates (7–13%). Specifically, the compressive strength of the muck with 8% curing agent increases by 5.36 MPa, corresponding to a 13.3% rise. This phenomenon suggests that the curing agent not only facilitates the formation of cementitious materials within the muck, but also possesses the capacity for secondary hydration. This secondary hydration potential enables the solidified muck to withstand external environmental erosion to a certain degree, thereby delaying and mitigating the decline in the muck’s performance.

Under continuous dry–wet cycling, specimen strength gradually declines. For specimens with a 2% admixture, solidified muck strength decreased by 5.2% between the 6th and 9th cycles. By the 12th cycle, strength fell to 3.12 MPa, an overall reduction of about 12%. Notably, increasing the curing agent admixture extends the compressive strength growth cycle. Additionally, strength reduction in solidified muck specimens does not begin until 9–12 cycles, with a decline rate of only 4%.

3.2. Microscopic Characteristics

3.2.1. X-Ray Diffraction (XRD) Analysis

The study employed X-ray diffraction analysis to determine the mineral phase composition of solidified muck specimens and to assess how these phases change with varying curing times and admixture amounts. The results are presented in Figure 4a,b.

In Figure 4a, the 7-day cured solidified muck specimens primarily consist of muscovite (KAl₂(AlSi₃O₁₀)(OH)₂), anorthite (CaO·Al₂O₃·2SiO₂), C-A-S-H gel, lawsonite (CaAl₂Si₂O₇·2H₂O), C-S-H gel, clinoptilolite (Ca_0.5AlSi₂O₆·H₂O), chlorite ((Mg,Fe)₅Al(Si₃Al)O₁₀(OH)₈), and N-A-S-H gel mineral phases. In contrast to muck, these specimens include C-S-H and C-A-S-H cementitious materials and N-A-S-H geopolymers. However, due to the low reactivity of certain mineral phases, original components like anorthite, lawsonite, and muscovite persist in the reaction system.

As curing time increases, the hydration impact of industrial solid waste on solidified muck becomes more pronounced. For instance, the secondary hydration of fly ash continuously produces cementitious materials like C-S-H, sharpening the diffraction peaks of these materials. Concurrently, the secondary hydration releases substantial CH into the reaction system, enhancing the pozzolanic activity of steel slag and generating geopolymers, which further contribute to the strength development of the solidified muck.

As the curing agent content increases, the diffraction peaks of cementitious materials and geopolymer mineral phases do not exhibit significant sharpening, as shown in Figure 4b. The diffraction peaks of C-S-H and C-A-S-H decrease, while new mineral phases, such as N-A-S-H, chlorite, and heavy metal-rich phases, emerge. These peaks gradually transform into broad diffuse scattering peaks. This transformation likely results from increased industrial solid waste content, intensifying hydration. Alkali activation affects clay minerals and active silicon-aluminum components, leading to the formation of silicon-oxygen and aluminum-oxygen monomers. These monomers combine with cations like Na⁺ and Fe³⁺, producing a substantial amount of amorphous geopolymers [47].

Certain heavy metal ions, such as chromium (Cr) and lead (Pb), can form complexes through ion exchange and chemical precipitation, thereby disrupting the crystalline phase of cementitious materials within the reaction system [48,49]. This process suggests that solidified muck can effectively stabilize and immobilize heavy metal elements present in industrial solid wastes. Consequently, this encapsulation mechanism enables the stable existence of heavy metals within the solidified muck, contributing to the safe management of industrial solid wastes and offering a scientific foundation for their benign and resource-efficient utilization.

3.2.2. Fourier Transformation Infrared Spectroscopy (FTIR) Analysis

The infrared (IR) spectra of the solidified muck specimens cured for 7 and 28 days are presented in Figure 5a. Characteristic absorption peaks are observed at 3698, 3620, 3441, 1034, 1007, 913, 797, 694, 538, 470, and 429 cm⁻¹. The peak at 3441 cm⁻¹ corresponds to the O-H stretching vibration of crystal water in various mineral phases and cementitious materials, such as C-S-H and C-A-S-H [50,51]. The broad peak in the range of 3720–3500 cm⁻¹ is attributed to the O-H stretching vibration of Ca(OH)₂, indicating that the calcium hydroxide generated by Portland cement hydration effectively stimulates the pozzolanic reaction of fly ash and steel slag [52].

The absorption peak at 1034 cm⁻¹, attributed to the asymmetric stretching vibration of Si-O-Si in the silica tetrahedron [50,51], indicates the presence of both the geopolymer formed by active silicon-aluminum components in the muck and the hydration product C-S-H gel. Stretching vibration peaks between 115 and 800 cm⁻¹ are linked to the layered/chain stretching vibrations of the Si-O bond in the polymerized silica tetrahedron [53], suggesting the formation of [SiO₄]⁴⁻ monomers. The peaks at 913.2 cm⁻¹ and 429 cm⁻¹ arise from the out-of-plane and in-plane bending vibrations of the Al-O bond in the [AlO₄]⁵⁻ group, respectively [54,55]. These FTIR characteristics align with the gelling phase evolution observed in XRD analysis, confirming the formation of hydration products such as CH and C-S-H in specimens across different curing periods.

The 7-day and 28-day specimens exhibited peaks at 3441 cm⁻¹ (O-H vibration) and 1034 cm⁻¹ (Si-O-Si vibration), indicating that the gelation reaction in the solidified muck persists from early to middle curing stages. The changes in O-H vibration intensity indirectly reflect the evolution of bound water in hydration products, while the stable Si-O-Si vibration peak suggests that the polymerization degree of the silica-oxygen network stabilizes over time, aligning with typical hydration dynamics of cement-based materials.

The presence of the Ca(OH)₂ characteristic peak at 372–350 cm⁻¹ alongside the polymer peak at 1034 cm⁻¹ suggests a synergy and competition between cement hydration and the reaction of pozzolanic ash from solid waste. While calcium hydroxide promotes the dissolution of silicon and aluminum in fly ash/muck, its consumption is limited by the solid waste content. This observation offers a theoretical foundation for optimizing the composition of curing agents to balance the reaction pathways in solidified muck.

Figure 5b illustrates the impact of curing agent dosage on chemical bond structure. The absorption peak at 1421 cm⁻¹ was minimal at a 2% doping level but became more pronounced and sharper as the doping increased. Concurrently, absorption peaks at 1033 cm⁻¹ and 1007 cm⁻¹ (Si-O bond stretching in [SiO₄]⁴⁻) and at 430 cm⁻¹ (in-plane bending of the Al-O bond) also sharpened. This suggests that fly ash and steel slag, as industrial solid wastes, contribute essential siliceous and aluminum-rich minerals to the reaction system. These materials, through cement hydration, facilitate the abundant formation of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ monomers, accelerating the production of C-S-H cementitious materials and enhancing the mechanical strength of solidified muck samples. Moreover, the stable presence of the [AlO₄]⁵⁻ group (429 cm⁻¹) and the broad peak of amorphous silica-alumina gel imply that cement-based solidified muck inherently fixes heavy metals via ion exchange and physical encapsulation, ensuring the environmental safety of solid waste-derived curing agents.

3.2.3. Scanning Electron Microscope (SEM-EDS) Analysis

Figure 6A reveals the distinctly layered internal structure of the original muck, characterized by a stacked distribution of particles of varying sizes and noticeable pits and pores. This indicates an incomplete microscopic skeleton, low cementation, and ordinary macroscopic strength, preventing its direct use. However, the loose microstructure offers numerous attachment sites and reaction surfaces for industrial solid wastes and hydration products. This facilitates the formation of a dense microstructure between cementitious substances and muck, providing sites for immobilizing and stabilizing heavy metal elements in industrial solid wastes, highlighting its potential application [37,56,57].

When the solidification agent, made from industrial solid waste like fly ash and Portland cement, is mixed with muck, the resulting solidified muck specimen exhibits particle agglomeration. The previously loose voids and pits in the specimen become tightly filled, as illustrated in Figure 6B. This indicates that the microstructure of the solidified muck is enhanced by the microaggregate and pozzolanic effects of industrial solid waste through cement hydration.

The microstructural analysis presented in Figure 6B reveals the formation of flaky and flocculent structures at the aggregate-matrix interface, along with the presence of unreacted fly ash particles. This observation suggests that the microstructural optimization of the solidified muck is primarily driven by two key mechanisms. Firstly, the hydration of cement facilitates the interaction and agglomeration of various industrial solid waste constituents within the curing matrix, leading to the synergistic formation of C-S-H gels. These hydration products are extensively distributed across the surfaces and interfaces of the aggregates, effectively filling the pores of varying sizes among the muck particles. This microstructural development enhances the macroscopic properties of the solidified muck.

The cementitious material was analyzed using EDS, revealing that its chemical element composition aligns with the characteristics of C-S-H cementitious material. The EDS results also indicate the presence of Cr and Zn heavy metals, suggesting that these materials can potentially fix and stabilize heavy metal ions within the solidified muck.

The microstructural analysis of the solidified muck after 28 days of curing (Figure 7) reveals substantial densification characteristics. The internal porosity is reduced, and the pore size distribution is further optimized compared to the early curing stage. This microstructural evolution is attributed to the continuous hydration reaction of the solid waste particles in the solidified muck. The cementitious material forms a skeletal framework with the waste particles, and the unreacted solid waste particles fill the gaps of the skeleton as micro-aggregates. The acicular AFt (Figure 7a) and the amorphous C-S-H gel (Figure 7b) work in tandem through physical filling and chemical bonding [58,59,60] to effectively bridge adjacent particles and plug connected pores, thereby weakening the stress concentration effect. This microscopic mechanism explains the significant increase in 28-day compressive strength.

EDS analysis (Figure 8) reveals that AFt is rich in calcium, sulfur, and aluminum, while C-S-H primarily consists of calcium and silicon, aligning with the typical properties of sulphoaluminate and silicate products. The needle-like structure of AFt creates a three-dimensional network within the pores, enhancing the deformation resistance of the solidified muck. Meanwhile, C-S-H gel, known for its high specific surface area and bonding properties [61,62], encapsulates solid particles, mitigating defects in the interface transition zone. AFt occupies macropores, and C-S-H infiltrates micro- and nano-sized pores, establishing a multi-stage densified structure. This synergistic interaction markedly enhances the interfacial bonding strength of solidified muck.

SEM observations reveal the deposition of new C-S-H gel on fly ash particle surfaces (Figure 7B), signifying a pozzolanic reaction of active SiO₂ and Al₂O₃ in fly ash under alkaline conditions during later curing stages. This reaction consumes Ca(OH)₂ in the pore solution, which facilitates ongoing C-S-H gel formation and prevents AFt from transforming into monosulfoaluminate by reducing system basicity, thereby preserving the stability of the early-stage strengthened structure. The secondary reaction products coat the fly ash surface, enhancing interface adhesion between inert particles and the matrix, and elucidating the later strength increase in solidified muck at the material scale.

3.3. Environmental Effect

The contents of heavy metals in the muck samples (generated from construction waste, Portland cement, fly ash and steel slag) and the cured muck specimens after 7 days of curing are shown in

Table 6. Various raw materials and test specimens’ heavy metal content (unit: mg/kg).

Sample	Cr	Cu	Zn	As	Pb	Cd
Muck	19.96	34.83	8.66	—	12.29	—
Fly ash	18.54	45.27	14.43	0.06	27.30	—
Steel slag	17.87	32.58	9.80	—	10.58	—
7d-Solidifield muck	16.93	30.81	22.38	—	2.70	—
Standard-1 [63]	-	2000	100	20	400	20
Standard-2 [64]	150	50	200	30	70	0.3

(Ref. [63]—Screening values for the first category of land with muck pollution risks; Ref. [64]—muck environmental quality standards for agricultural land for muck pollution risk control, “—“ indicates not reaching the detection limit).

.

This study employed XRF to measure heavy metal concentrations in muck, industrial solid waste, and solidified muck samples (see Table 6). The analysis reveals significant differences in heavy metal concentrations between muck and industrial solid waste. According to muck environmental quality and solid waste resource utilization standards, the total concentrations of Cr, Cu, Zn, As, Pb, and Cd in industrial solid waste, muck, and 7-day cured solidified muck samples remain below the respective risk screening thresholds.

The utilization of industrial solid waste for muck solidification poses a relatively low risk of direct muck pollution from the heavy metals it contains. Specifically, in the context of engineering muck recycling for road engineering backfill, the total heavy metal concentrations are within acceptable levels according to relevant standards. However, it is important to note that this risk assessment is based solely on the total heavy metal concentrations and does not account for the potential leaching behavior and bioavailability of these contaminants.

This study conducted a leaching behavior analysis of solidified muck with varying curing periods to assess the environmental risk posed by heavy metals in solid waste-based solidified muck. The results, depicted in Figure 9, reveal a reduction in the leaching rates of Cr, As, Pb, and Cd in cured muck samples compared to uncured muck. Specifically, after 7 days of curing, the leaching rates of each heavy metal decreased by 11.5% (Cr), 26.1% (As), 41.3% (Pb), and 35.7% (Cd) relative to uncured raw materials. Furthermore, the leaching concentrations of Cr, Pb, and Cd in the solidified muck samples exhibited a continuous decline with prolonged curing, reaching levels of 189.5 μg/L (Cr), 8.1 μg/L (Pb), and 0.9 μg/L (Cd), respectively. In contrast, the leaching concentration of As demonstrated an increasing trend, rising to 36.1 μg/L. These findings underscore the variations in the solidification and stabilization of heavy metals within solidified muck.

This study aimed to investigate the dynamic release characteristics and short-term environmental risks associated with heavy metal ion leaching in solidified muck. Specifically, the research focused on varying leaching time intervals to examine the migration patterns of heavy metal ions such as chromium (Cr), lead (Pb), arsenic (As), and cadmium (Cd). The findings are presented in Figure 10.

Figure 10 illustrates the variation in leaching concentrations of heavy metal ions (Cr, Pb, Cd, As) in solidified muck samples’ leaching solutions, alongside the corresponding pH changes during 7-day (7d) and 28-day (28d) curing periods. Initially, the pH of the solution increased before stabilizing. Notably, a significant pH shift occurred within the first -10 min of leaching for the 7d sample, registering 0.67 units higher than that of the 28d leachate. This discrepancy can be attributed to the early-stage hydration reaction in the 7d specimen, where a substantial amount of soluble alkaline compounds (e.g., CaO, NaOH) and the hydration product Ca(OH)₂ were influenced by the porous microstructure (as observed in the SEM results of the 7d specimen in Figure 6), leading to a rapid pH surge. In contrast, the 28d samples exhibited a dense network structure formed by cementitious materials (C-S-H, C-A-S-H) resulting from the Portland cement and fly ash reaction, effectively impeding the diffusion of alkaline substances in the samples.

The pH of the leachate from the 28-day sample surpassed that of the 7-day sample during a leaching period of 80 to 120 min. Subsequently, the pH peaked at 7.24 between 240 and 360 min of leaching. This observation indicates that the 28-day sample exhibits enhanced sustained alkali release and greater alkaline buffering capacity, thereby facilitating the precipitation of heavy metal ions.

The leaching process of heavy metal ions Cr, Pb, As, and Cd in solidified muck exhibits distinct variations in ion concentrations. In the 7-day sample, the concentration of Cr ions peaked at 126.37 μg/L after 360 min, followed by a gradual and slow decline. Conversely, in the 28-day sample, the maximum Cr ion concentration of 95.89 μg/L was observed at 240 min. A comparison between the two samples reveals a reduction of approximately 24% in the maximum leaching concentration of Cr ions, which also occurred 120 min earlier in the 28-day sample.

The Pb ion concentration in the 7-day sample peaked at 7.19 μg/L after 600 min and subsequently declined by approximately 12% over time. In contrast, the Pb ion leaching concentration in the 28-day sample reached a maximum of 5.26 μg/L at 240 min, representing a 26.8% decrease compared to the 7-day sample. The Pb ion concentration in the leaching solution exhibited a continuous decrease during the leaching process, reaching 4.87 μg/L after 840 min. The behavior of Cd ion concentration in the leaching solution mirrored that of Cr and Pb; however, the distinction between the 7-day and 28-day samples was less pronounced. Specifically, the maximum Cd ion concentration in the leaching solution was 0.441 μg/L at 7 days and 0.406 μg/L at 28 days.

In the case of arsenic (As), the leaching solution’s As concentration rises with prolonged curing time. Specifically, in muck samples cured for 28 days, the As concentration peaked at 31.2 μg/L after 600 min, marking an 80% increase compared to samples cured for 7 days. These findings suggest that the formation and maturation of cementitious materials in solidified muck may impede the fixation and stabilization of As elements.

4. Discussion

4.1. Cement-Dominated Solidification Mechanism and Its Threshold Effect

This study demonstrates that the rapid enhancement of the macroscopic properties of solidified muck is primarily due to the swift hydration kinetics of Portland cement. Within seven days, the hydration degree of cement’s main mineral phases (C₃S, C₂S) reaches approximately 70%, significantly surpassing that of fly ash and steel slag [65]. This process produces substantial amounts of C-S-H gel and ettringite (AFt), which effectively decrease muck porosity and enhance muck compactness. Additionally, these gel materials bond muck particles of varying sizes, further strengthening the muck’s skeletal structure [66,67,68].

During muck solidification, Portland cement content exhibits a threshold effect. At higher cement levels (3–6% in this study), a C-S-H gel layer quickly forms on the surface of unhydrated cement particles, acting as a “diffusion barrier” that impedes water migration into the particles. This transition shifts the cement hydration reaction from being interface-controlled to diffusion-controlled, limiting the degree of cement hydration. As a result, the macroscopic strength increase in the solidified muck decelerates [69,70,71].

The threshold effect of cement content is closely linked to the properties of the muck being solidified, such as liquid and plastic limits and particle gradation. This effect varies significantly under the combined influence of different industrial solid wastes. Future research should concentrate on understanding how these synergistic interactions affect the cement hydration process and the structural evolution of hydration products. Such insights are crucial for optimizing the critical cement content to achieve low-carbon, high-efficiency solidification.

4.2. Reaction Limitations and Potential of Industrial Solid Wastes at Different Curing Periods

This study elucidates the limitations and mechanisms of industrial solid wastes in augmenting the solidification strength of construction waste muck. The primary active components of fly ash, namely SiO₂ and Al₂O₃, require a pozzolanic reaction with Ca(OH)₂ in an alkaline environment to generate C-S-H and C-A-S-H gels. This reaction, however, exhibits minimal efficiency in the initial stage (≤7 days) [72]. Although steel slag encompasses cementitious minerals such as C₂S and C₃S, its hydration rate is merely 1/3 to 1/5 that of cement [73,74], leading to an insignificant contribution from early hydration products. Initially, cement and a minor fraction of industrial solid wastes hydrate to produce C-S-H gels and Ca(OH)₂, as described in Equations (2) and (3). Predominantly, fly ash and steel slag serve as fillers, enhancing the microstructural compactness by diminishing the porosity of the construction waste muck. Concurrently, the generated OH⁻ ions facilitate the depolymerization and polycondensation of active silicon-aluminum substances within the construction waste muck, forming [SiO₄]⁴⁻/[AlO₄]⁵⁻ monomers, as depicted in Equations (3) and (4). These monomers interact with cations (Ca²⁺, Na⁺, K⁺) within the reaction system to form geopolymers, thereby improving the mechanical properties of the solidified muck.

As industrial solid waste increases, effective adhesion sites within construction waste muck diminish. Excess unreacted solid waste, hindered by a delayed pozzolanic effect [75], fails to promptly form cementitious materials, destabilizing the internal structure. Unreacted fly ash particles enhance aggregate fluidity via the “rolling ball effect” [76], while unreacted solid waste within the specimen creates weak points under pressure, compromising the solidified muck’s performance.

Extending the curing time to 28 days initiates the secondary hydration of fly ash and steel slag, producing substantial C-S-H gel and releasing Ca(OH)₂. This process greatly improves the compactness of the solidified muck structure and offers additional attachment sites for other free industrial solid wastes. The Ca(OH)₂ release further activates the silicon-aluminum activity in the muck, fostering the formation of numerous geopolymers. The combined effect of hydration products and geopolymers significantly boosts the unconfined compressive strength of the solidified muck at 28 days compared to 7 days, with this enhancement becoming more pronounced as the solidifying agent dosage increases.

This elucidates the mechanism behind the “abnormal” increase in compressive strength of solidified muck specimens during wet–dry cycles after the addition of a curing agent. In the wet phase, water infiltration repeatedly triggers the secondary hydration of industrial solid waste, generating gel materials and geopolymers that fill both newly formed and original pores. A higher curing agent content enriches the system with active components, leading to a nonlinear increase in muck strength. As hydration products fill cracks from drying shrinkage, the structural damage threshold of the solidified muck rises with more cycles, delaying the critical failure point as curing agent content increases. This demonstrates that industrial solid waste imparts a self-repair function to the solidified muck, distinguishing it from the deterioration patterns of traditional materials.

This study demonstrates that activating industrial solid waste markedly improves the mechanical properties and durability of solidified muck. Future research should prioritize analyzing the rapid activation processes of various industrial solid waste components and the synergistic interactions among them to establish a theoretical framework of “composition-structure-performance.” Additionally, exploring the resource utilization potential of industrial solid wastes is crucial to precisely design the macroscopic properties of solidified muck, thereby enhancing its long-term performance in challenging environments like dry–wet and freeze–thaw cycles.

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

(2)

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

(3)

$$\equiv \mathrm{S}\mathrm{i}-\mathrm{O}-\mathrm{S}\mathrm{i}\equiv +3{\mathrm{O}\mathrm{H}}^{-}\to {\left[\mathrm{S}\mathrm{i}\mathrm{O}{\left(\mathrm{O}\mathrm{H}\right)}_3\right]}^{-}$$

(4)

$$\equiv \mathrm{A}\mathrm{l}-\mathrm{O}-\mathrm{A}\mathrm{l}\equiv +4{\mathrm{O}\mathrm{H}}^{-}\to {\left[\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4\right]}^{-}$$

(5)

4.3. Solidification and Stabilization of Heavy Metals Under the Synergistic Effect of Industrial Solid Waste and Construction Waste Muck

The study results (Figure 11 and Figure 12) demonstrate that the leaching rates of Cr, Pb, and Cd significantly decrease as the curing period of the solidified muck increases. This reduction is due to various physicochemical synergistic effects, preliminarily confirming the effectiveness of the solidification technology for the short-term immobilization of these heavy metals:

Muck initially exhibits a loose and porous structure. Hydration products from industrial solid wastes like fly ash, including C-S-H gel, also possess high specific surface areas and porosity [77]. These pore structures offer numerous adsorption sites for heavy metal ions such as Cr, As, Pb, and Cd, which are immobilized on the pore surfaces via van der Waals forces and electrostatic attraction. This process effectively inhibits the migration and transformation of heavy metal ions.

Calcium hydroxide generated from cement hydration raises the internal pH of solidified muck to over 10, creating an alkaline environment. In this setting, heavy metal ions tend to combine with OH⁻ ions, forming insoluble hydroxide precipitates and thus becoming immobilized [78].

Hydration products, such as calcium-silicate-hydrate (C-S-H) gels, can undergo ion-exchange reactions with heavy metal ions present in the stabilized muck. This enables the heavy metal ions to either replace cations like Ca²⁺ within the gel structure or combine with anionic groups like [SiO₄]⁴⁻ and [AlO₄]⁵⁻, subsequently becoming incorporated into the crystal structures of minerals such as calcium silicate hydrate [79].

The extended curing period of the solidified muck facilitates the continuous hydration reaction, leading to the formation of additional hydration products and a densified microstructure (Figure 6B). This enhanced physical barrier impedes the migration of heavy metals across the media, effectively immobilizing them within the solidified matrix.

The evolving patterns of heavy metal leaching concentrations under laboratory conditions require reassessment within real-world environmental contexts. The long-term environmental safety of solidified bodies hinges not only on initial fixation but also on the enduring stability of the solidified muck and the solidification mechanism amid complex conditions. Studies by researchers like Dunja Rađenović [80] have evaluated both the short- and long-term stability of solidified, highly polluted sediments. Their findings indicate a risk of degradation in the heavy metal solidification mechanism due to dynamic environmental changes, resulting in the gradual release of pollutants.

Future research on the long-term leaching of heavy metals should focus on the degradation mechanisms and failure cycles of physical sealing structures, as well as the chemical adsorption effects of cementitious materials in complex environments. Although a dense structure formed after 28 days of curing (Figure 9) effectively reduces heavy metal leaching, environmental factors such as wet–dry cycles, freeze–thaw cycles, and carbonation can induce microcrack formation. This increases the porosity of the solidified body, potentially re-exposing heavy metals previously contained by physical sealing. For pH-sensitive elements like Pb and Cd [81], increased acidity—such as from acid rain—can dissolve their hydroxide precipitates, leading to the short-term release of “fixed” heavy metals over time.

Laboratory leaching tests in this study are typically conducted under controlled pH conditions, over short durations, and with a constant liquid-to-solid ratio, differing markedly from actual site conditions. In the field, precipitation is continuous, and the liquid-to-solid ratio can theoretically be infinite, suggesting that the cumulative leaching of heavy metals is substantially higher than laboratory predictions. Additionally, complex organic matter or ions such as Cl^- and CO₃²⁻ present on-site may form soluble complexes with metals like Cr, Pb, and Cd, enhancing their mobility. This complexity is challenging to replicate in simplified laboratory settings.

The environmental implications of solidified waste materials warrant rigorous investigation across diverse application contexts. Future research should establish a multi-factor, coupled-degradation experimental framework to simulate the leaching behavior of solidified bodies under realistic environmental conditions. This approach will elucidate the failure mechanisms underlying heavy metal solidification and stabilization techniques, providing critical insights to enhance the long-term environmental safety of these materials.

The study revealed that in muck specimens treated with curing agents, the leaching behavior of arsenic (As) markedly differed from that of heavy metals like chromium (Cr), lead (Pb), and cadmium (Cd), exhibiting distinct migration and transformation characteristics. Notably, after 28 days of standard curing, the leaching concentration of As increased compared to the 7-day curing period (refer to Figure 9 and Figure 10). This observation challenges the conventional paradigm for assessing the solidification efficacy of traditional industrial solid waste, which posits that “the solidification period is positively correlated with the stability of heavy metals” [82].

This phenomenon aligns with previous studies. Zhang et al. [83] demonstrated through pH regulation experiments that in arsenic-contaminated muck solidification systems, the As concentration in leachate follows a U-shaped curve as pH increases. Weakly alkaline conditions (pH 8.5–9.5) favor the stabilization of arsenate ions, offering crucial insight into the anomalous data observed in this study.

This study examines the solidification of muck, primarily driven by cement hydration and the pozzolanic effects of industrial solid waste. During this process, the substantial production of Ca(OH)₂ raises the system’s pH, leading to the dissolution of low-solubility calcium arsenate compounds formed in the early stages of curing. Additionally, the elevated pH renders the surface of the solidified muck’s primary hydration products negatively charged, resulting in electrostatic repulsion with AsO₄³⁻. Consequently, arsenic ions cannot stably adhere to the gel material’s surface, causing their concentration in the solution to increase continuously [84].

The research findings suggest that using cement or alkali activation to solidify arsenic-contaminated muck or arsenic-rich industrial waste may underestimate long-term environmental pollution risks. In actual disposal sites and muck landfills, these solidified materials remain in a closed alkaline environment, potentially increasing the risk of arsenic’s secondary release over time.

A comprehensive multi-faceted monitoring system should be implemented to ensure long-term environmental safety. The standard 28-day curing period should be extended to at least 90 days, and the frequency of leaching toxicity assessments should be increased to mitigate the risk of ecological contamination due to solidification failure. Additionally, research should explore the use of auxiliary cementitious materials to moderate the excessive pH rise in the reaction system, thereby limiting the leaching of arsenic. Furthermore, the incorporation of iron-rich solid waste (e.g., red mud) should be investigated to promote the formation of the stable FeAsO₄ phase, thereby enhancing the solidification of arsenic within the cementitious matrix [85,86,87].

This study offers a novel approach to the environmentally friendly disposal of arsenic-laden solid waste. Future research should integrate microscopic characterization with macroscopic performance testing to develop a quantitative model predicting arsenic leaching behavior in the presence of cementitious materials.

While cementitious materials in solidified muck can sequester heavy metals, stabilizing them temporarily, this process has drawbacks: (1) Metal cations occupy C-S-H gel sites, disrupting structural continuity. This weakens some cementitious materials, making parts of the dense structure fragile and reducing the microstructural stability of the solidified muck, which indirectly affects the mechanical properties of the specimens. (2) The precipitation reactions consume OH⁻ and reduce [SiO₄]⁴⁻/[AlO₄]⁵⁻ content, slowing the hydration process and hindering early strength development in the solidified muck.

The influence of heavy metals on the mechanical properties of solid waste-based materials and the primary solidification and stabilization mechanisms of various heavy metals warrant further investigation. Optimizing the proportions of industrial solid waste constituents could enhance resource utilization efficiency. This approach may enable the safe disposal and recycling of diverse industrial solid wastes through solidification processes.

4.4. Analysis of Engineering Application Potential

This study’s findings can be directly applied to large-scale urban engineering projects, offering substantial value in utilizing construction waste muck and mitigating environmental impacts. In applications such as road base construction, waste disposal sites, and land restoration, construction waste muck can be mixed on-site or at centralized plants with a low solidifying agent dosage (4–6%) and then compacted. This approach is cost-effective and straightforward, significantly improving muck mechanical properties while reducing pollution risks. The chemical bonding and physical encapsulation of heavy metal ions by hydration products and geopolymers ensure enhanced environmental safety.

The solidified muck in this study demonstrates clear self-repairing strength characteristics during wet–dry cycles. Micro-cracks formed during drying create channels for water penetration, which in turn stimulates ongoing hydration reactions of industrial solid waste and the formation of geopolymers, thereby repairing damage and restoring strength. This property makes the material particularly suitable for regions with variable climates, significantly extending the lifespan of engineering projects, reducing maintenance needs, and offering notable technical and economic benefits.

The cement–fly ash–steel slag solidification system proposed in this study offers diverse material sources, compatibility with existing construction technologies, and strong scalability and engineering applicability. Future pilot-scale verification and comprehensive life-cycle environmental and economic assessments could enhance its application in large-scale urban waste disposal and resource utilization projects, supporting the development of “zero-waste cities” and sustainable infrastructure.

5. Conclusions

This study introduces a novel method for the synergistic solidification of shield engineering muck using cement, fly ash, and steel slag to improve mechanical properties while mitigating environmental risks. Systematic experiments and microscopic analysis yielded the following key findings:

(1) Cement is essential for enhancing strength, yet more is not always advantageous. At a 3% addition, the 7-day strength of solidified muck suffices for subgrade material requirements. Beyond this point, further increases in cement content yield diminishing returns, as initial hydration products inhibit subsequent cement reactions. This highlights a cost-effective “optimal dosage.”

(2) Industrial solid wastes serve as “long-acting enhancers.” Individually, fly ash and steel slag show limited early-stage effectiveness. However, when combined with cement in a specific ratio (2:2:1), they demonstrate strong synergistic effects. Initially, they primarily fill spaces, but over time, ongoing reactions produce additional gel substances, significantly enhancing the long-term strength and durability of the materials.

(3) The treated solidified muck effectively immobilizes heavy metals like chromium, arsenic, lead, and cadmium, with leaching concentrations significantly below national sewage discharge limits. Microscopic analysis reveals that heavy metals are encapsulated within the solid matrix and converted into stable forms via chemical reactions, substantially minimizing environmental migration risks.

(4) This technology offers a scalable and environmentally friendly approach to transforming construction waste and industrial solid waste into valuable resources. Its straightforward process and manageable costs make it suitable for extensive urban projects, including subgrade filling and site leveling. By minimizing land use and environmental pollution, it also reduces reliance on natural sand and gravel, aligning with the strategic objectives of “Zero-Waste City” and sustainable development. Future research could investigate additional solid waste types and formulas for high-moisture materials like silt.