Stabilization of water-based drilling cuttings using an alkali-activated GGBS–FA-based composite binder for road construction

Abstract

Water-based drilling cuttings (WBDC) are generated in large quantities during oil and gas development, but their reuse in road construction is restricted by insufficient bearing capacity, moisture sensitivity, and potential contaminant release. This study developed an alkali-activated GGBS-FA-based composite binder containing cement and sodium silicate to stabilize WBDC for road construction. The optimized binder was evaluated in terms of mechanical properties, water stability, wet-dry and freeze-thaw durability, environmental performance, and microstructural characteristics. At a total binder dosage of 15%, the stabilized WBDC achieved a 28d unconfined compressive strength of 10.17 MPa, a splitting tensile strength of 1.46 MPa, and a water stability coefficient of 94.44%. After nine wet-dry and freeze-thaw cycles, the corresponding strength retention ratios were 97.06% and 87.25%, respectively. Stabilization reduced the leachate chemical oxygen demand from 260 to 4 mg/L. SEM-EDS observations showed that products with gel-like and needle-like morphologies progressively coated and connected WBDC particles and improved matrix continuity. The results provide a low-energy route for the large-volume utilization of WBDC and indicate that a 15% binder dosage provides a practical balance between engineering performance and binder consumption for road construction.

1 Introduction

Water-based drilling cuttings (WBDC) are solid residues generated during oil and gas well drilling after fragmented formation materials carried by circulating drilling fluids are separated from the liquid phase. They mainly consist of fine rock particles, clay minerals, weighting materials, soluble salts and residual drilling-fluid additives (Wang et al., 2022; Zhou et al., 2024; Pang et al., 2025). Although these constituents give WBDC certain soil-like engineering characteristics, their water content, particle-size distribution, plasticity, mineral composition and environmental properties vary considerably with formation conditions and drilling-fluid systems (Liu et al., 2021; Liu W. et al., 2022). WBDC may also contain residual organic matter, soluble salts and trace metals, and its environmental risk depends not only on contaminant concentrations but also on their mobility under water exposure. Conventional stockpiling and landfilling occupy land resources and transfer potential risks to long-term containment. Therefore, the beneficial utilization of WBDC requires both engineering-property improvement and contaminant-migration control. The stabilization of WBDC is challenging because its fine particles, variable moisture content, soluble salts, residual drilling-fluid additives, and heterogeneous mineral composition can interfere with particle bonding and cementitious reactions. In addition, a suitable stabilization system must simultaneously provide sufficient load-bearing capacity and water resistance while restricting the migration of potentially soluble constituents. The mineral composition of WBDC provides a basis for its use in construction materials. Previous studies have incorporated WBDC into non-fired bricks, sintered bricks and supplementary cementitious materials (Liu Y. et al., 2022; Yang et al., 2023; Liu et al., 2026; Wang et al., 2026). These studies showed that treated WBDC can participate in cementitious reactions and improve product performance. However, sintering, calcination and intensive grinding usually require high energy input and complex processing. Product-oriented routes, such as brick production and cement replacement, are also limited by manufacturing capacity and the allowable WBDC content in final products. A low-energy route suitable for large-scale WBDC consumption is therefore needed.

Stabilization/solidification (S/S) offers a more direct route for converting loose drilling cuttings into integrated materials with load-bearing capacity. It can also reduce contaminant mobility through physical encapsulation, adsorption, precipitation and transport restriction. Leonard and Stegemann (2010); Huang et al. (2025) showed that the drill-cuttings-to-binder ratio, binder composition and curing age affected the strength, permeability and pore characteristics of cement-stabilized drill cuttings. Subsequent leaching tests indicated that high-carbon fly ash improved hydrocarbon immobilization, although highly mobile constituents such as chloride remained difficult to retain completely. Aboutabikh et al. (2016) further reported that treated oil-sands drill cuttings could be incorporated into cementitious grout at moderate contents, balancing strength development and heavy-metal leaching control. These findings indicate that mechanical performance and environmental stability should be considered together during binder design.

Road base and subbase construction requires large quantities of filling and cemented materials, providing a potential outlet for stabilized drilling wastes. Shon et al. (2016) found that untreated modified drilling waste did not meet the requirements for flexible base materials, whereas 3% cement markedly improved its mechanical properties and water stability. Chen et al. (2018) used stabilized bored-pile mud in a test road embankment and obtained compactness and deflection indices that satisfied engineering requirements. For shale-gas WBDC, Li et al. (2020) combined WBDC with phosphogypsum to prepare road cushion materials and evaluated their physical, microstructural and environmental properties. These studies suggest that drilling wastes can be used in road engineering after appropriate treatment. However, the resulting performance remains sensitive to waste composition, stabilizer type and moisture exposure.

Alkali-activated binders can consume industrial solid wastes such as ground granulated blast-furnace slag (GGBS) and fly ash (FA), while reducing ordinary Portland cement consumption (Provis, 2018; Li et al., 2025). Under alkaline conditions, reactive Ca-, Si- and Al-bearing species dissolve from GGBS and FA and form binding products such as C-(A)-S-H and N-A-S-H gels. GGBS is rich in calcium and contributes to early strength, whereas FA supplies Si- and Al-bearing phases and provides particle filling, with a more evident contribution at later curing ages. Ismail et al. (2014) reported that the reaction products and microstructure of alkali-activated GGBS-FA systems depend strongly on precursor proportions. Sodium silicate modulus and dosage further regulate alkalinity, soluble silicate availability, solution viscosity and reaction progress. A moderate cement addition can also provide early hydration products and an additional calcium source, improving early bonding while retaining industrial solid wastes as the main binder constituents.

Alkali-activated stabilization has recently been applied to waste drilling mud for road engineering. Zhang P. et al. (2024) used a FA-slag geopolymer to stabilize waste drilling mud for road subbase applications and found that increasing the slag proportion improved strength and resilient modulus. Prolonged curing also promoted progressive densification of the stabilized matrix. Zhang et al. (2025) further showed that increasing slag content enhanced strength and fracture resistance but reduced water stability and wet-dry durability. These findings indicate that a formulation optimized only for mechanical strength may not provide satisfactory overall durability. Most existing studies have focused on bored-pile mud, construction slurry and other engineering-derived waste muds. These materials differ from oil- and gas-derived WBDC in water content, mineral composition, residual additives and contaminant characteristics. Studies on alkali-activated stabilization of oil- and gas-derived WBDC using binders mainly composed of GGBS, FA and other industrial solid wastes remain limited. In particular, the effects of sodium silicate modulus, sodium silicate dosage, cement dosage and GGBS-to-FA ratio on strength development have not been systematically clarified. Previous work has also focused mainly on compressive strength and heavy-metal leaching, with less attention to splitting tensile strength, water stability, wet-dry durability, freeze-thaw durability and organic pollution indicators. In addition, the links among reaction-product distribution, interparticle bonding and macroscopic performance require further investigation (Zheng and Wu, 2021; Tan et al., 2026).

This study developed an alkali-activated GGBS-FA-based composite binder containing cement and sodium silicate to stabilize oil- and gas-derived WBDC for road construction. Single-factor tests were conducted to evaluate the effects of sodium silicate modulus, sodium silicate dosage, cement dosage, and GGBS-to-FA mass ratio on strength development. An orthogonal test, followed by range analysis and an additional verification test, was used to select the binder formulation. Based on the verified formulation, the unconfined compressive strength, splitting tensile strength, water stability, wet-dry durability, and freeze-thaw durability of stabilized WBDC were systematically evaluated at different binder dosages. Leachate COD and solid-phase contaminant contents were measured to assess environmental performance. Finally, SEM-EDS was used to characterize reaction-product morphology, elemental distribution, and interparticle bonding, thereby relating microstructural development to macroscopic performance.

2 Materials and experimental procedures

2.1 Raw materials

The WBDC used in this study were collected from a shale-gas drilling site in Dazu, Chongqing, China. After mechanical dewatering, the material was sealed and transported to the laboratory. Its appearance and particle-size distribution are shown in Figure 1.

FIGURE 1

Its basic physical properties, chemical composition, and initial contaminant characteristics are summarized in Tables 1–3, respectively. The liquid limit, plastic limit, and plasticity index of WBDC were 34.72%, 16.20%, and 18.51, respectively. According to the Test Methods of Soils for Highway Engineering (JTG 3430-2020), WBDC was classified as a low-liquid-limit clay. Because untreated WBDC has a loose structure, insufficient bearing capacity, and poor water stability, it does not meet the strength and water-stability requirements for subgrade materials specified in the Specifications for Design of Highway Subgrades (JTG D30-2015). Stabilization is therefore required before its road-engineering suitability can be assessed. The contaminant characteristics of WBDC were evaluated using DB65/T 3997-2017 as the reference criterion. As shown in Table 3, the measured solid-phase contaminant contents were below the corresponding reference limits. However, the COD concentration in the WBDC leachate was 260 mg/L, exceeding the reference value of 150 mg/L and representing the principal environmental constraint on direct utilization. Accordingly, the stabilized WBDC investigated in this study is primarily intended for potential use as a treated filling material in road subgrade engineering.

S95-grade GGBS, FA and P.O. 42.5 ordinary Portland cement were used as the main binder constituents. Their chemical compositions are provided in Table 2. Liquid sodium silicate, with an initial modulus of 3.29, served as the alkaline activator. The modulus was adjusted by adding NaOH with a purity of at least 98%. Sodium silicate and NaOH were supplied by Shanghai Aladdin Biochemical Technology Co., Ltd., China, and their main technical specifications are listed in Table 4.

2.2 Experimental program

Single-factor tests were first conducted to evaluate the effects of sodium silicate modulus, sodium silicate dosage, GGBS-to-FA mass ratio, and cement dosage on the unconfined compressive strength (UCS) of stabilized WBDC. The investigated factors and levels are presented in Table 5, and the 7 d UCS was used as the primary response. Unless otherwise stated, the sodium silicate and cement dosages are expressed as mass percentages of the total binder. The sodium silicate dosage refers to the mass of the sodium silicate solution used as the alkaline activator rather than its equivalent solid content. The GGBS-to-FA ratio refers to the mass ratio of GGBS to FA within the precursor fraction remaining after accounting for sodium silicate and cement.

Based on the trends obtained from the single-factor tests, three representative levels were selected for each factor to establish an L9(3⁴) orthogonal design for further optimization of the binder composition. The four factors included sodium silicate modulus, sodium silicate dosage, cement dosage, and GGBS-to-FA mass ratio, and the corresponding orthogonal test scheme is presented in Table 6. The UCS of stabilized WBDC specimens prepared with each formulation was measured after 7, 14, and 28 d of curing. Range analysis was conducted using the mean UCS values to determine the relative influence and preferred level of each factor.

Finally, the selected binder formulation was incorporated into WBDC at dosages of 5%, 10%, 15%, 20%, and 25%, designated G5, G10, G15, G20, and G25, respectively. WBDC stabilized with 10% ordinary Portland cement was used as the reference group and designated C10.

2.3 Test methods

2.3.1 Specimen preparation and curing

The WBDC was dried and pulverized, and the sieving procedure was conducted in accordance with the Test Methods of Soils for Highway Engineering (JTG 3430-2020) before specimen preparation. The required masses of WBDC and binder were calculated according to the maximum dry density, a target degree of compaction of 95%, and the designed binder dosage. Water was then added according to the optimum moisture content, and the materials were mixed thoroughly until a homogeneous mixture was obtained.

The mixture was compacted by static pressure into cylindrical specimens with a diameter and height of 100 mm. The applied pressure was maintained for 2 min, and the specimens were demolded after 24 h. The demolded specimens were sealed in plastic bags and cured in a standard curing chamber at 20 °C ± 2 °C and a relative humidity of at least 95% until testing. Three replicate specimens were prepared for each test condition, and the mean value was reported.

2.3.2 Compaction test

Heavy compaction tests were conducted in accordance with JTG 3430-2020 to determine the optimum moisture content and maximum dry density of the mixtures. These parameters were subsequently used to determine the water content and specimen mass required for static compaction.

2.3.3 Unconfined compressive and splitting tensile strength tests

The unconfined compressive strength (UCS) test was conducted in accordance with JTG 3441-2024 using an electronic universal testing machine. The specimens were loaded axially at a constant rate of 1 mm/min until failure, and the peak load was recorded.

The splitting tensile strength test was also performed according to JTG 3441-2024. Curved loading strips were placed symmetrically along the upper and lower generatrices of each cylindrical specimen, and the load was continuously applied until splitting failure occurred.

2.3.4 Water stability and durability tests

Water stability was evaluated by comparing the UCS of specimens subjected to standard curing with that of companion specimens immersed in water during the final 24 h of the corresponding curing period. The water stability coefficient, Kw, was calculated as Equation 1:where R₀ is the mean UCS of the specimens subjected to standard curing, MPa; R_w is the mean UCS of the specimens after water immersion, MPa; and K_w is the water stability coefficient, %.

Wet-dry cycle tests were conducted with reference to ASTM D4843-88. After standard curing for 28 d, the specimens were immersed in water for 12 h and subsequently dried at 60 °C ± 3 °C for 12 h. One immersion-drying process was defined as one wet-dry cycle. The UCS values of the specimens were measured after 3, 6 and 9 cycles. The wet-dry strength retention rate after n cycles, K_Dn, was calculated as Equation 2:where R_Dn is the mean UCS of the specimens after n wet-dry cycles, MPa; R₀ is the mean UCS before cyclic exposure, MPa; and K_Dn is the wet-dry strength retention rate after n cycles, %.

Freeze-thaw cycle tests were conducted with reference to ASTM D560/D560M-24. After standard curing for 28 d, the specimens were sealed to minimize moisture exchange during cyclic exposure. Each freeze-thaw cycle consisted of freezing at −20 °C ± 2 °C for 12 h, followed by thawing at 20 °C ± 2 °C for 12 h. The UCS values of the specimens were measured after 3, 6 and 9 cycles. The freeze-thaw strength retention rate after n cycles, K_Fn, was calculated as Equation 3:where R_Fn is the mean UCS of the specimens after n freeze-thaw cycles, MPa; R₀ is the mean UCS before cyclic exposure, MPa; and K_Fn is the freeze-thaw strength retention rate after n cycles, %.

2.3.5 Leaching test

Untreated WBDC and stabilized specimens cured for 7, 14 and 28d were subjected to environmental performance tests. The solid-phase contents of Cr(VI), Cu, Zn, Ni, Pb, Cd, As and benzo[a]pyrene were determined and compared with the limits specified in DB65/T3997-2017. Leachates were prepared using the horizontal vibration method specified in HJ 557-2010, and their COD concentrations were measured to evaluate the release of soluble organic constituents before and after stabilization.

2.3.6 SEM-EDS analysis

Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) was used to examine the microstructure and elemental composition of selected specimens. Small blocks of approximately 5 mm × 5 mm × 5 mm were cut from the specimens and attached to conductive carbon tape. Before observation, the samples were sputter-coated with an approximately 10 nm-thick gold layer. EDS was used to identify the distribution and relative contents of the main elements.

3 Results and discussion

3.1 Effects of binder parameters on the UCS of stabilized WBDC

The effects of sodium silicate modulus, sodium silicate content, GGBS-to-FA mass ratio and cement dosage on the 7d UCS of stabilized WBDC are shown in Figure 2.

FIGURE 2

Sodium silicate modulus had a pronounced non-linear effect on UCS (Figure 2a). Increasing the modulus from 1.0 to 1.3 raised UCS from 1.59 to 3.38 MPa, whereas further increases reduced UCS to 1.92, 1.36 and 1.46 MPa at moduli of 1.6, 1.9 and 2.2, respectively. The peak at a modulus of 1.3 indicates that strength development required a suitable balance between alkalinity and soluble silicate supply. At lower modulus, alkalinity was stronger but soluble silicate was comparatively limited. At higher modulus, lower alkalinity and higher solution viscosity may have restricted reactive-phase dissolution and hindered uniform activator dispersion (Arabani and Haghsheno, 2023).

Sodium silicate content showed a similar optimum (Figure 2b). UCS increased from 2.46 MPa at 2% sodium silicate to 3.78 MPa at 6%, then decreased to 3.13 and 2.51 MPa at 10% and 14%, respectively. At low contents, the activator was insufficient to dissolve the reactive Si-, Al- and Ca-bearing phases in GGBS and FA. A content of 6% enhanced precursor activation and supplied soluble silicate for cementitious-product formation. Excess sodium silicate, however, increased liquid-phase viscosity and may have promoted rapid precipitation around precursor particles, limiting further dissolution and reducing matrix homogeneity (Vaiciukyniene et al., 2026).

The 7d UCS increased continuously with the GGBS-to-FA mass ratio (Figure 2c). Increasing the ratio from 1:1 to 5:1 raised the UCS from 1.64 to 3.94 MPa, with values of 3.78 and 3.94 MPa obtained at ratios of 4:1 and 5:1, respectively. This trend reflects the higher early-age reactivity of GGBS relative to FA. The Ca-rich glassy phase in GGBS dissolves readily under alkaline conditions and supplies Ca, Si and Al species for the formation of calcium-bearing binding phases. FA contributes reactive aluminosilicate species and particle-packing effects, but its comparatively low calcium content and slower dissolution limit its contribution to early strength when present at high proportions (Li et al., 2026).

Cement dosage also produced a distinct optimum at 10% (Figure 2d). The UCS increased from 1.87 MPa without cement to 2.61 MPa at 5% cement and reached 3.78 MPa at 10% cement. Increasing the cement dosage to 15% and 20% reduced the UCS to 2.73 and 2.93 MPa, respectively. At moderate contents, cement hydration supplied early binding phases and Ca(OH)₂, which increased the availability of calcium and maintained an alkaline environment favorable for the reaction of GGBS and FA. At higher cement dosages, rapid hydration and local accumulation of calcium-rich products may have reduced the uniformity of the reaction products and restricted the continued participation of the solid-waste precursors (Samantasinghar and Singh, 2021).

3.2 Optimization of binder composition

3.2.1 Orthogonal test

Based on the single-factor results, the orthogonal test was conducted using the factor levels listed in Table 6. The corresponding UCS results are shown in Figure 3.

FIGURE 3

All nine orthogonal formulations exhibited increasing UCS with curing age, although the extent of strength development varied with binder composition. At 7 d, the UCS values ranged from 4.67 to 6.50 MPa. SSD7 exhibited the highest early strength of 6.50 MPa, followed by SSD8 and SSD1, with UCS values of 6.10 and 5.90 MPa, respectively, whereas SSD2 showed the lowest value of 4.67 MPa. At 14 d, SSD4 achieved the highest UCS of 8.10 MPa, followed by SSD8 and SSD7, with values of 7.30 and 7.13 MPa, respectively. At 28 d, SSD4 remained the best-performing formulation among the nine orthogonal mixtures, reaching 9.90 MPa, followed by SSD8 and SSD7, with values of 9.07 and 8.80 MPa, respectively.

The variation in strength ranking with curing age may reflect changes in the relative contributions of cement hydration and alkali activation. The relatively high cement dosage in SSD7 favored early strength development, whereas the factor combination used in SSD4 produced higher intermediate- and later-age strength. However, the best-performing formulation included in an orthogonal array does not necessarily represent the preferred combination of individual factor levels. Range analysis was therefore conducted to determine the relative influence and preferred level of each factor.

3.2.2 Range analysis

Range analysis was conducted to evaluate the relative influence of sodium silicate modulus (A), sodium silicate dosage (B), cement dosage (C), and GGBS-to-FA mass ratio (D) on UCS development.

The values of k₁, k₂, and k₃ represent the mean UCS values corresponding to Levels 1, 2, and 3 of each factor, respectively, and R is the difference between the maximum and minimum level means. A larger R value indicates a greater influence of the corresponding factor within the investigated levels. The results are summarized in Table 7. At 7 d, the factors followed the order C > B > A = D, indicating that cement dosage had the greatest influence on early-age UCS within the investigated levels, followed by sodium silicate dosage. At 14 and 28 d, the order changed to B > D > A > C. Sodium silicate dosage became the most influential factor, followed by the GGBS-to-FA mass ratio, whereas cement dosage showed the smallest influence. The R value of cement dosage decreased from 0.878 at 7 d to 0.178 at 28 d, indicating that its relative contribution to strength development decreased with curing age. These results suggest that the dominant factor governing strength development changed with curing age: cement hydration played a more important role in early-age strength by providing initial binding products and a Ca-rich alkaline environment, whereas intermediate- and later-age strength increasingly depended on sodium silicate activation and the continued reactions of GGBS and FA. Therefore, no single component dominated strength development throughout the entire curing period; the controlling effect gradually shifted from cement at early ages to the alkali-activated GGBS-FA system at later ages.

The preferred level combinations for the 7, 14, and 28 d UCS were A₃B₁C₁D₃, A₂B₁C₃D₃, and A₃B₁C₂D₃, respectively. Levels B₁ and D₃ were preferred at all three curing ages, indicating that a sodium silicate dosage of 4% and a GGBS-to-FA mass ratio of 6:1 consistently favored strength development within the investigated ranges. Because the 28 d UCS was selected as the primary response for binder formulation optimization, A₃B₁C₂D₃ was identified as the predicted preferred combination. Compared with the single-factor results, the preferred levels obtained from the orthogonal test showed some differences. The single-factor tests favored a sodium silicate modulus of 1.3, a sodium silicate dosage of 6%, a cement dosage of 10%, and a GGBS-to-FA mass ratio of 5:1 for 7 d UCS, whereas the orthogonal analysis based on 28 d UCS identified A₃B₁C₂D₃, corresponding to 1.4, 4%, 10%, and 6:1, respectively. These differences may result from the different evaluation ages and the combined influence of multiple binder components under simultaneous variation. However, because the L9(3⁴) design was mainly used to evaluate the main effects, specific interaction effects among individual factors could not be quantitatively separated in this study.

3.2.3 Verification of the selected binder formulation

The A₃B₁C₂D₃ combination corresponded to a sodium silicate modulus of 1.4, a sodium silicate dosage of 4%, a cement dosage of 10%, and a GGBS-to-FA mass ratio of 6:1. Because this combination was not included among the original nine orthogonal formulations, an additional verification group, denoted SSD10, was prepared and tested. The verification results are presented in Table 8.

SSD10 achieved 7, 14, and 28 d UCS values of 6.87, 9.74, and 10.17 MPa, respectively. Its 28 d UCS exceeded the maximum value of 9.90 MPa measured among the original nine orthogonal formulations, supporting the suitability of the formulation selected by range analysis. Therefore, the A₃B₁C₂D₃ binder formulation verified using SSD10 was adopted in the subsequent road-performance, environmental, and microstructural evaluations.

3.3 Road performance of stabilized WBDC

3.3.1 Unconfined compressive and splitting tensile strengths

The mechanical properties and representative failure patterns of stabilized WBDC are shown in Figure 4.

FIGURE 4

As shown in Figure 4a, UCS increased with curing age and binder dosage. At 7 d, UCS increased from 4.00 MPa for G5 to 6.87 MPa for G15, followed by only modest increases to 7.00 and 7.20 MPa for G20 and G25, respectively. At 28 d, G5, G10, G15, G20 and G25 reached 6.20, 8.00, 10.17, 10.50 and 10.80 MPa, respectively. Increasing the binder dosage from 5% to 15% raised the 28 d UCS by 64.0%, whereas a further increase from 15% to 25% produced only a 6.2% gain. G5 showed slightly lower UCS than C10 at 7 and 14 d, with values of 4.00 and 5.30 MPa compared with 4.30 and 5.50 MPa for C10. After 28 d of curing, however, the UCS of G5 increased to 6.20 MPa, slightly exceeding the 6.00 MPa obtained for C10. G10-G25 consistently outperformed C10 at all curing ages, and the 28 d UCS of G15 was 69.5% higher than that of C10. These results indicate that the GGBS–FA-based binder provided greater long-term strength development than cement alone. The increase in strength with binder dosage can be attributed to the greater supply of reactive Ca-, Si- and Al-bearing phases. Cement hydration provided an initially alkaline and Ca-rich environment. Meanwhile, the continued activation of GGBS and FA promoted the formation of additional binding products and strengthened contacts between WBDC particles. The diminishing strength gain beyond G15 indicates that the principal particle contacts had already been effectively bonded, reducing the contribution of additional binder to macroscopic load-bearing capacity.

The post-test appearances in Figure 4c further show the dosage-dependent differences in compressive failure. The specimens mainly developed longitudinal cracks parallel to the loading direction, accompanied by local edge spalling and surface detachment. G5 displayed more numerous and wider cracks, some extending over much of the specimen height. This failure pattern indicates that damage propagated more readily through its less effectively bonded particle framework. With increasing binder dosage, the specimens retained greater integrity after failure, while crack opening and local spalling became less pronounced. This change in failure morphology is consistent with the increase in UCS and reflects the progressive strengthening of interparticle contacts. The splitting tensile strength showed a similar dependence on curing age and binder dosage (Figure 4b). At 7 d, the splitting tensile strengths of G5, G10, G15, G20 and G25 were 0.74, 1.01, 1.04, 1.36 and 1.37 MPa, respectively, all exceeding the 0.50 MPa obtained for C10. At 28 d, the corresponding values increased to 1.12, 1.27, 1.46, 1.47 and 1.49 MPa, whereas C10 reached only 0.58 MPa. The improvement was pronounced up to G15. However, the differences among G15, G20 and G25 at 28 d were limited, indicating that tensile resistance approached a plateau once the binder dosage reached approximately 15%.

As shown in Figure 4d, most specimens developed a dominant crack along the loaded diameter, which is characteristic of indirect tensile failure. Lower-dosage specimens exhibited wider cracks, local edge collapse and, in some cases, partial separation of the fractured sections. With increasing binder dosage, the principal crack became more concentrated, and the fractured portions retained greater overall integrity. This change indicates that stronger interparticle bonding increased resistance to crack initiation and limited the lateral expansion of the fracture zone, consistent with the measured splitting tensile strengths (Xiao et al., 2022).

3.3.2 Water stability

The water stability results are presented in Figure 5.

FIGURE 5

As shown in Figure 5a, immersion reduced the UCS of all groups, but the extent of strength loss depended on binder dosage and curing age. Here, R₀ and R_w denote the UCS values before and after immersion, respectively, and K_w represents the water stability coefficient. At 7d, the R_w values of G5 and G10 were 2.96 and 4.14MPa, corresponding to K_w values of 74.07% and 75.34%, respectively. In contrast, G15 and G20 retained R_w values of 6.24 and 6.20MPa, with K_w values of 90.79% and 88.61%, both higher than the 79.63% obtained for C10. Although G25 exhibited the highest R₀ at 7d, its R_w decreased to approximately 5.52MPa, resulting in a comparatively low K_w of 76.60% (Figure 5b). This result indicates that a high early-age R₀ did not necessarily correspond to strong resistance to immersion-induced weakening. The lower K_w of G25 may reflect incomplete early-age reactions and local structural heterogeneity at the higher binder dosage, leaving some water-sensitive interfaces within the matrix.

After 28d of curing, both R_w and K_w increased markedly. The R_w values of G15, G20 and G25 reached approximately 9.60, 9.84 and 10.23MPa, respectively, with corresponding K_w values of 94.44%, 93.75% and 94.74%. All three values exceeded that of C10, which exhibited an R_w of 5.37 MPa and a K_w of 89.55%. The pronounced increase in the K_w of G25 from 76.60% at 7d to 94.74% at 28d indicates that continued curing improved matrix continuity and reduced the susceptibility of internal interfaces to water ingress. At 28d, the K_w values of G5 and G10 increased to 85.48% and 87.50%, respectively, but remained lower than those of G15-G25. These results indicate that the lower binder dosages were insufficient to form a comparably continuous and water-resistant matrix. Once the binder dosage reached 15%, the stabilized WBDC retained more than 90% of its R₀ after immersion, demonstrating substantially improved resistance to water-induced softening.

3.3.3 Wet-dry and freeze-thaw durability

The wet-dry durability results are presented in Figure 6. Figure 6a shows the UCS values before cycling and after 3, 6 and 9 wet-dry cycles, while Figure 6b presents the corresponding UCS retention ratios. After three cycles, G5 and C10 retained 86.79% and 56.67% of their initial UCS, respectively. The UCS of G10 remained unchanged, giving a retention ratio of 100.00%. In contrast, the retention ratios of G15, G20 and G25 increased to 106.86%, 126.67% and 127.78%, respectively. In contrast, the retention ratios of G15, G20 and G25 increased to 106.86%, 126.67% and 127.78%, respectively. This early strength increase indicates that continued hydration and activation of residual precursor phases initially outweighed deterioration caused by moisture migration and shrinkage-swelling deformation. Water introduced during immersion may have promoted further dissolution of unreacted mineral components, producing additional binding products and a temporary strength gain.

FIGURE 6

As the number of wet-dry cycles increased, the UCS retention ratios of the solid-waste-stabilized specimens generally decreased from their values after three cycles. After nine cycles, G5, G10, G15, G20 and G25 retained 73.58%, 78.57%, 97.06%, 102.86% and 106.48% of their initial UCS, respectively. G15 retained nearly all of its initial strength, whereas G20 and G25 still exceeded their zero-cycle UCS values. Although the retention ratio of C10 increased from 56.67% after three cycles to 70% after nine cycles, its UCS remained substantially below the initial value throughout the test. This partial recovery may be related to continued cement hydration during cycling, but it did not fully compensate for damage caused by repeated wetting and drying. Overall, G15-G25 showed better wet-dry durability, indicating that higher binder contents produced a more stable binding structure and reduced the adverse effects of repeated moisture migration and volumetric deformation.

The freeze-thaw durability results are shown in Figure 7. Figure 7a presents the UCS values before and after freeze-thaw cycling, and Figure 7b shows the corresponding retention ratios. After three cycles, G5 and G10 retained 86.79% and 97.14% of their initial UCS, respectively. The retention ratios of G15, G20 and G25 were 102.94%, 105.71% and 120.37%, respectively. The temporary strength increase observed for G15-G25 suggests that continued hydration and precursor activation remained dominant during the initial freeze-thaw cycles. By comparison, C10 retained only 76.67% of its initial UCS, indicating that the cement-stabilized specimen was more susceptible to early freeze-thaw damage than specimens containing sufficient composite binder. With increasing cycle number, the UCS retention ratios of all specimens progressively decreased. After nine freeze-thaw cycles, G5, G10, G15, G20 and G25 retained 54.72%, 75.71%, 87.25%, 93.14% and 94.44% of their initial UCS, respectively, compared with 61.67% for C10. G5 exhibited the greatest deterioration among the solid-waste-stabilized specimens. Its relatively low binder content may have produced insufficient binding products, a larger proportion of connected pores and more weak particle interfaces. These features would make the specimen more susceptible to damage caused by repeated pore-water freezing and thawing. In contrast, G15-G25 maintained higher residual strengths, indicating that their denser and more continuous binding structures restricted pore-water migration and reduced local stress concentration caused by ice formation.

FIGURE 7

A comparison between Figures 6, 7 shows that freeze-thaw cycling caused greater strength deterioration than wet-dry cycling. After nine cycles, the freeze-thaw retention ratios of all solid-waste-stabilized specimens were lower than their corresponding wet-dry retention ratios. Repeated freezing of pore water generates expansion pressure within the pore network, while subsequent thawing weakens particle contacts and promotes internal damage accumulation. Therefore, although continued hydration and secondary reactions partly compensated for deterioration during the initial cycles, they could not fully offset the cumulative damage caused by repeated ice formation and melting. Considering mechanical properties, water stability, durability and binder consumption, G15 provided the most appropriate balance between engineering performance and material dosage. It achieved a 28 d UCS of 10.17 MPa, a splitting tensile strength of 1.46 MPa and a water stability coefficient of 94.44%. After nine wet-dry and freeze-thaw cycles, G15 retained 97.06% and 87.25% of its initial UCS, respectively. Increasing the binder content to 20% or 25% further improved cyclic durability, but the additional engineering benefit was limited relative to the increase in binder consumption (Lu et al., 2023).

3.4 Environmental performance

The contaminant characteristics of stabilized WBDC at different curing ages are summarized in Table 9.

As shown in Table 9, all measured solid-phase contaminant contents and leachate COD values were below the reference limits adopted from DB65/T 3997-2017. The measured solid-phase contents varied with curing age but remained below the corresponding reference values. Because these measurements represent total solid-phase contents rather than leachate concentrations, the variations among curing ages should not be interpreted as quantitative immobilization efficiencies. Benzo[a]pyrene remained below the detection limit of 0.005 mg/kg at all curing ages. The COD concentration in the stabilized-WBDC leachate remained at 4 mg/L throughout curing, compared with 260 mg/L for untreated WBDC, corresponding to a reduction of 98.5%. These results indicate that stabilization rapidly restricted the release of soluble organic constituents and maintained a stable leaching performance during subsequent curing.

As shown in Table 9, all measured solid-phase contaminant contents and leachate COD values were below the reference limits adopted from DB65/T 3997-2017. The measured solid-phase contents varied with curing age but remained below the corresponding reference values. Because these measurements represent total solid-phase contents rather than leachate concentrations, the variations among curing ages should not be interpreted as quantitative immobilization efficiencies. Benzo[a]pyrene remained below the detection limit of 0.005 mg/kg at all curing ages. The COD concentration in the stabilized-WBDC leachate remained at 4 mg/L throughout curing, compared with 260 mg/L for untreated WBDC, corresponding to a reduction of 98.5%. These results indicate that stabilization rapidly restricted the release of soluble organic constituents and maintained a stable leaching performance during subsequent curing.

The marked reduction in leachate COD may be related to the formation of a hardened binding matrix that coated WBDC particles and restricted the migration of soluble constituents. Continued hydration and alkaline activation improved matrix continuity and may have reduced open transport pathways.

3.5 SEM-EDS analysis

SEM-EDS analysis was performed to examine the microstructural evolution and elemental characteristics of stabilized WBDC. SSD10 specimens, prepared using the binder formulation selected through range analysis and the additional verification test, were analyzed after 7, 14, and 28 d of curing. The SEM images of SSD10 at different curing ages are shown in Figure 8.

FIGURE 8

After 7 d of curing, SSD10 showed a relatively loose structure with clear particle boundaries, connected pores and unfilled regions (Figures 8a,b). Fine particles adhered to the surfaces of larger particles, while needle-like and rod-like products were locally distributed on particle surfaces and along pore edges. These features indicate that early hydration and alkali-activated reactions had begun, but the amount of reaction products was still insufficient to form a continuous binding matrix (Zhang L. et al., 2024).

After 14 d, the particle surfaces were increasingly covered by flocculent, clustered and gel-like products, and the particle boundaries became less distinct (Figures 8c,d). Particle contact gradually changed from physical packing to bonding through cementitious products. These products extended into interparticle pores, subdivided larger pores and improved the continuity of the solidified structure. After 28 d, SSD10 developed a denser microstructure (Figures 8e,f). Most particles were coated and connected by reaction products, while fewer large and visibly interconnected voids were observed. Continuous cementitious products formed bonding bridges between particles and filled or refined the original pores. This gradual improvement in particle bonding and pore filling is consistent with the increase in SSD10 strength with curing age.

EDS analysis was further performed on SSD10-28d to characterize the elemental composition of the observed reaction products. The corresponding elemental maps are shown in Figure 9, and the elemental composition obtained from the EDS mapping spectrum is summarized in Table 10. As shown in Figure 9 and Table 10, O, Ca, Si, Al and S were the main elements associated with the observed reaction products in SSD10-28d, with weight fractions of 49.34%, 11.96%, 10.32%, 4.16% and 3.97%, respectively. The spatial association of Ca, Si, Al and O, together with the flocculent and gel-like morphologies observed in the SEM images, is consistent with the possible presence of C-S-H/C-A-S-H-type gels. Meanwhile, the coexistence of Ca, Al, S and O, combined with the locally observed needle-like morphology, may be associated with AFt-type products (Yang et al., 2024). The features exhibiting gel-like morphologies covered the surfaces of the WBDC particles and extended into the interparticle voids, thereby weakening the original particle boundaries and improving the continuity of the binding matrix. The locally observed needle-like features were distributed within the pores and between adjacent particles, contributing to particle bridging and structural interlocking. The combined pore-filling and bonding effects reduced the number of visible voids and improved the apparent continuity of the cemented matrix.

FIGURE 9

4 Conclusion

Although the laboratory results demonstrate the feasibility of using the proposed binder for WBDC stabilization, further studies are required before large-scale field application. Future work should evaluate the influence of source-dependent variations in WBDC composition, verify the selected formulation under field mixing, compaction, and curing conditions, and investigate long-term performance under coupled moisture, temperature, and traffic loading. Long-term contaminant release, construction adaptability, economic feasibility, and life-cycle environmental benefits should also be assessed to establish appropriate design parameters and quality-control criteria for road construction applications.

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