Abstract
Steel slag (SS), coal gangue (CG), and construction waste (CW) were investigated as partial replacements in cement-stabilized aggregate mixtures for highway semi-rigid bases, with incorporation rates ranging from 0% to 80%. Performance assessments included unconfined compressive strength, flexural strength, drying shrinkage, and freeze-thaw resistance, followed by optimal material selection using a comprehensive fuzzy algorithm. Results indicated that SS synergistically enhanced overall performance. Active components within SS induced micro-expansion, reducing drying shrinkage by 29.5%–40.0%. Furthermore, continued hydration increased 360-day compressive strength by 16.0%–36.2%, flexural strength by 42.1%–52.9%, and the freeze-thaw strength ratio by 17.3%. Conversely, CW content exceeding 60% increased drying shrinkage by 10.8% due to fine aggregate hydration, yielding only marginal compressive strength gains (4.4%–7.3%) at 360 days. High CG levels (>40%) caused significant degradation in mechanical and durability properties due to layer structure disintegration; compressive strength decreased by 30.9%–46.9%, flexural strength fell by 40%, and freeze-thaw resistance declined by over 10%. Drying shrinkage for all mixtures conformed to the EXPASSOC model (R2 > 0.99), exhibiting a three-stage pattern where 85% of shrinkage occurred within 60 days, followed by stabilization after 100 days. A derived cracking resistance index confirmed that SS dosages of >40% provided optimal anti-cracking performance. Fuzzy comprehensive algorithm analysis identified SS (40%–80% incorporation) as the optimal material choice. Consequently, CG requires strict limitation to ≤40%, whereas CW is permissible at ≤ 40% (excluding fine aggregates). These findings provide theoretical support for the classified resource utilization of multi-source solid wastes in highway base courses.
1 Introduction
Steel slag (SS), coal gangue (CG), and construction waste (CW) represent typical large-volume solid wastes in China, whose continuous accumulation has triggered severe environmental issues. Notably, these solid wastes possess considerable mechanical strength, rendering them suitable for processing into recycled aggregates for highway bases. This approach not only facilitates waste recycling and environmental protection but also promotes the development of green highways (; ). Currently, SS, CG, and CW are emerging as significant alternative materials for semi-rigid highway bases. However, due to the inherent variability in their material properties, ensuring durability remains a critical challenge when utilizing them in base courses. Furthermore, selecting the optimal waste material based on road classification and economic considerations is pivotal for engineering applications.
Existing literature predominantly investigates the road performance of individual solid waste streams. demonstrated that incorporating 18% fly ash effectively mitigated the expansion rate of steel slag. Pretreated steel slag could replace up to half of the aggregate in cement-stabilized materials (). reported a 40% decrease in steel slag expansion rate after coating it with waterborne polyurethane (). Prepared steel slag powder–cement stabilized macadam by partially replacing cement with mechanically ground steel slag powder, concluding that its addition improved the shrinkage properties of the mixture (). Used carbonation pretreatment to control the expansion rate of steel slag (). employed alternative modifications to inhibit its expansivity, addressing volume stability in engineering (). Coal gangue has been extensively studied for its strength, frost resistance, and expansibility in highway subgrades. explored the freeze–thaw damage mechanisms and dynamic mechanical properties of concrete incorporating coal gangue as recycled aggregate, attributing the reduced frost resistance of concrete to the porous layered structure and high water absorption of coal gangue (; ). utilized cement-coal gangue to stabilize expansive soil, effectively suppressing the expansion rate of the soil (). studied the blending methods and durability of coal gangue in pavement base mixtures, revealing an inverse correlation between coal gangue content and mechanical strength (). examined the mechanical properties of cement-stabilized coal-based solid waste base mixtures containing coal gangue, finding that coal-based solid wastes negatively impacted mixture strength, albeit with diminishing effects over longer curing ages (). reported that the recycled construction waste aggregate in concrete enhanced interfacial bonding via secondary hydration of residual cement components, although higher fine aggregate content increased drying shrinkage strain (; ). investigated the road performance of single-component geopolymer-stabilized recycled construction waste aggregate as a semi-rigid base material and identified an optimal formulatio ().
Current research on steel slag in semi-rigid bases applications predominantly prioritizes expansion inhibition, largely neglecting its potential to compensate for shrinkage, particularly in arid and cold climates. Similarly, literature regarding the application of coal gangue and construction waste in base courses remains fragmented, characterized by a paucity of comprehensive durability assessments. Crucially, there is a scarcity of systematic comparative studies evaluating these three solid waste streams under identical conditions. This absence of a unified evaluation framework hinders the optimal selection of materials for engineering practice.
Meanwhile, cracking represents a critical pathology in highway semi-rigid bases, particularly in arid and cold regions characterized by significant thermal fluctuations. Shrinkage cracking constitutes the primary failure mechanism in these structural layers. Consequently, evaluating the drying shrinkage performance of solid wastes utilized in highway subgrades is imperative. However, literature characterizing the shrinkage behavior of SS, CG, and CW in road engineering remains sparse. The existing shrinkage testing methods are often restricted in scope, lacking of long - term assessments of shrinkage performance and crack resistance, both of which significantly affect road service life.
Given the wide variety and diverse properties of industrial solid wastes, systematic comparison and classification of different types are essential for optimizing material selection and enhancing durability for semi - rigid bases. Accordingly,, three representative bulk industrial solid wastes—SS, CG, and CW are selected to investigate their effects on the drying shrinkage characteristics of cement - stabilized aggregate mixtures, utilizing a self - developed, high - precision drying shrinkage testing system. Furthermore, a shrinkage performance prediction model and a crack resistance evaluation index were established. Long - term mechanical evolution and freeze–thaw durability are also compared to elucidate the mechanisms by which these solid wastes influence the performance of cement - stabilized aggregate mixtures. Figure 1 illustrates the research flowchart of the paper.
FIGURE 1
2 Methodology
2.1 Materials
Steel slag (SS) was obtained from Jiuguan Iron and Steel Group Co., Ltd., coal gangue (CG) from Jingyuan Coal Industry Co., Ltd., and construction waste (CW) from a waste treatment facility in Lanzhou. Before use, each type of solid waste was crushed and processed into recycled aggregates suitable for highway base applications, as shown in Figure 2. Comprehensive characterization was carried out for each material, including tests for leaching toxicity, radioactivity, fundamental physical properties, and chemical composition. Then, a mix design was developed for cement - stabilized mixtures incorporating these recycled aggregates. Subsequently, a comparative analysis was conducted to evaluate the drying shrinkage characteristics, compressive strength, flexural strength, and frost resistance of the mixtures.
FIGURE 2
The cement used in this study was ordinary Portland cement (PO 42.5). Its main properties are as follows: initial setting time 180 min, final setting time 260 min, 28-day compressive strength 48.5 MPa, 28-day flexural strength 7.2 MPa.
2.1.1 Physical properties
The crushing value, density, and apparent density of the recycled solid waste aggregates were determined. Additionally, the volume stability of each solid waste was evaluated based on its specific characteristics, including the water resistance disintegration index for coal gangue (CG) and the soaking expansion rate for steel slag (SS). The test results are presented in Table 1.
TABLE 1
| Solid waste aggregate | Crushing value (%) | Water absorption (%) | Apparent density (g/cm3) | Water resistance to disintegration index (%) | Soaking expansion rate (%) |
|---|---|---|---|---|---|
| SS | 12.3 | 2.5 | 3.313 | — | 1.6 |
| CG | 23.1 | 2.8 | 2.582 | 75 | — |
| CW | 17.5 | 1.5 | 2.819 | — | — |
Basic physical properties of recycled solid waste aggregates.
The properties of CW are comparable to those of conventional aggregates, whereas SS and CG exhibit notable differences. SS is characterized by high density and low crushing value. However, its porous surface leads to relatively high water absorption. Furthermore, due to its chemical reactivity, SS shows expansibility that may affect volume stability. Therefore, in highway engineering applications, its soaking expansion rate is required to be ≤2%. In contrast, CG has a layered structure and a relatively soft texture, which results in a high crushing value, high water absorption, and low density. It is also prone to softening and disintegration when exposed to water. Thus, its slake durability index is specified to be ≥60%. According to the test results, all SS, CG, and CW aggregates used in this study meet the relevant application requirements.
2.1.2 XRF analysis
As can be seen from Figure 3, A comparative analysis of the chemical compositions of the three solid wastes reveals that steel slag (SS) has the highest calcium content, indicating its strong cementitious activity. As a result, the surface of coarse SS aggregates can form effective chemical bonds and adhere to cement (), while fine SS aggregates inherently possess cementitious properties. In contrast, coal gangue (CG) exhibits low reactivity, as reflected by its chemical composition. Construction waste (CW) contains relatively high calcium levels, mainly because of the residual cement or hydration products adhering to its surface. Since CW is mainly composed of fine aggregates, it shows a certain potential for secondary hydration reactions ().
FIGURE 3
2.2 Mix design
The C-B-1 gradation was selected for the cement-stabilized aggregate mixtures in this study (Figure 4). For each of the three types of solid waste, the replacement levels were set at 0%, 20%, 40%, 60%, and 80%, while the cement content was maintained at 4.5%. The detailed experimental program is presented in Table 2. The following abbreviations are used throughout this paper: cement - stabilized steel slag aggregate mixture (CSM), cement-stabilized coal gangue aggregate mixture (CCM), and cement - stabilized construction waste aggregate mixture (CCWM).
FIGURE 4
TABLE 2
| CSM | CCM | CCWM | |||
|---|---|---|---|---|---|
| Number | SS content (%) | Number | CG content (%) | Number | CW content (%) |
| SA-0 | 0 | CA-0 | 0 | CWA-0 | 0 |
| SA-1 | 20 | CA-1 | 20 | CWA-1 | 20 |
| SA-2 | 40 | CA-2 | 40 | CWA-2 | 40 |
| SA-3 | 60 | CA-3 | 60 | CWA-3 | 60 |
| SA-4 | 80 | CA-4 | 80 | CWA-4 | 80 |
Experimental design scheme.
2.3 Test methods
2.3.1 Drying shrinkage test
To evaluate the shrinkage characteristics of CS-AM, a dedicated shrinkage testing system was developed (Figure 5). The system comprises four main components: an environmental chamber for the precise control of temperature and humidity, testing fixtures to hold specimens, high - precision displacement sensors (accuracy: 0.01 mm), and a data acquisition unit. During testing, Three replicate specimens were prepared for each mixture, and the test results were averaged. One end of each specimen is fixed to restrict shrinkage, while the other end is connected to a displacement sensor to monitor deformation and calculate shrinkage strain. Specimens with dimensions of 400 mm × 100 mm × 100 mm were fabricated, demolded, and cured under standard conditions (temperature 20 °C ± 2 °C, relative humidity ≥95%) for 7 days. After curing, the specimens were placed in the testing system, where the environmental chamber was set to 30 °C and 45% relative humidity to simulate arid conditions. Shrinkage was continuously monitored for 180 days. The testing process is illustrated in Figure 5, and the shrinkage strain was calculated using Equation 1.
FIGURE 5
Where, εi represents the cumulative drying shrinkage strain up to the ith measurement (με), δi represents the cumulative drying shrinkage displacement (mm) and l represents the length of the standard specimen (mm).
2.3.2 Unconfined compressive strength test (UCS)
Cylindrical specimens (150 mm diameter × 150 mm height) were prepared and cured at 20 °C ± 3 °C and relative humidity above 95% for 7, 28, 90, 180, and 360 days. Unconfined compressive strength was measured at each curing age. Six cylindrical specimens were tested for each curing age and each mixture, and the representative value was calculated from the test results.
2.3.3 Flexural strength test (FS)
Rectangular specimens (400 mm × 100 mm × 100 mm) were prepared and cured for 360 days at 20 °C ± 2 °C and relative humidity above 95%. Finally, the flexural strength of the cured specimens was tested. Three replicate specimens were tested for each mixture, and the test results were averaged.
2.3.4 Freeze - Thaw resistance test (FTR)
After 180 days of curing, specimens were subjected to 10 freeze–thaw cycles (freezing at −18 °C for 12 h, thawing at 20 °C for 12 h). Frost resistance was evaluated by comparing compressive strength before and after cycling. Six specimens were tested before and after freeze-thaw cycles, and the representative value was calculated from the test results. The compressive strength retention rate after n cycles (BDRn) is calculated as shown in Equation 2:
Where BDRn is the compressive strength retention rate after n freeze - thaw cycles (%), RDC is the compressive strength after n freeze - thaw cycles (MPa), and RC is the compressive strength before freeze–thaw (MPa).
All tests were conducted on at least three replicate specimens, and consistent trends were observed. The results are presented as mean values.
2.4 Data analysis methods
A comprehensive performance evaluation of the three types of CS-AM was conducted using the entropy weight fuzzy algorithm (; ), considering 360 - day unconfined compressive strength, 360 - day flexural tensile strength, 180 - day frost resistance index, and 180 - day crack resistance index. The analysis procedure is summarized as follows.
2.4.1 Evaluation index matrix
Twelve schemes (m) and four evaluation indicators (n) were used to construct the original data matrix. The form of the data matrix is shown in Equation 3:
2.4.2 Data standardization
To eliminate dimensional differences, all indicators were normalized as benefit-type indices (the larger, the better) using, refer to Equation 4:
Where, xij is the original value of the j - th indicator in the i - th scheme, and max (xj) andmin (xj) are the maximum and minimum values of the jth indicator in all schemes, respectively. rij∈ [0,1].
2.4.3 Entropy weight calculation
Calculate the proportion of the i - th option under the jth indicator, refer to Equation 5:
Calculate the entropy value of the j - th indicator, refer to Equation 6:
Calculate the coefficient of difference for the jth indicator, refer to Equation 7:
A lower entropy value (ej) indicates greater information content and thus a higher weight. Weight of each indicator, refer to Equation 8:
The final weight vector W = (w1, w2, w3, w4).
2.4.4 Comprehensive score calculation
The weighted fuzzy evaluation matrix is obtained by multiplying the standardized matrix R by the weight vector W. The comprehensive score calculation for each scheme is shown in Equation 9:
For comparison, the highest score is normalized to 100, and other scores are scaled proportionally, calculate as Equation 10:
3 Results and discussion
3.1 Analysis of unconfined compressive strength of CS-AM
3.1.1 Unconfined compressive strength of CSM
As shown in Figure 6, the incorporation of steel slag (SS) enhanced the unconfined compressive strength of cement-stabilized aggregate mixtures. Compared with the control group (0% SS), the 360-day compressive strength increased by 16.0%, 27.0%, and 36.2% at SS contents of 40%, 60%, and 80%, respectively, while no increase was observed at 20% SS. A higher SS content led to a greater improvement in strength. Unlike conventional mixtures, which typically reach stable compressive strength after 180 days, the mixtures containing SS continued to gain strength beyond this period. At 360 days, the compressive strength was 3.5%–10.3% higher than that at 180 days. This late-stage strength development was more pronounced with higher SS content, which can be attributed to the reactivity of fine SS aggregates (; ).
FIGURE 6
3.1.2 Unconfined compressive strength of CCM
As shown in Figure 7, coal gangue (CG) reduced the compressive strength of cement-stabilized mixtures. Compared with the control group, the 360-day strength decreased by 13.7%, 30.9%, and 46.9% at CG contents of 40%, 60%, and 80%, respectively (no decrease at 20% CG). This reduction is attributed to the low strength and poor water resistance of CG, as well as its breakage under compaction, which increased fine aggregates and decreased coarse aggregates (Figure 8). These changes hindered skeleton formation, created weak zones due to poor cement coating, and ultimately reduced strength. Therefore, strict control of CG content is necessary.
FIGURE 7
FIGURE 8
3.1.3 Unconfined compressive strength of CCWM
As shown in Figure 9, construction waste (CW) enhanced the long-term compressive strength of cement-stabilized mixtures. Compared with the control group, the 360-day strength increased by 1.9%, 4.4%, and 7.3% at CW contents of 40%, 60%, and 80%, respectively (no increase at 20% CW), while the difference before 90 days was negligible. This improvement is attributed to the fine recycled aggregates introduced at CW contents of 60% or higher, which contain residual cement hydration products with certain reactivity, contributing to strength gain at later ages ().
FIGURE 9
3.1.4 Comparison of unconfined compressive strength of CS-AM
According to Figures 10A,B, The results indicate that steel slag (SS) has the most pronounced effect on enhancing compressive strength. The improvement becomes more significant as the SS content increases. In contrast, construction waste (CW) has little impact on early - age strength. The compressive strength remains relatively consistent across different CW contents before 90 days. However, CW does provide a modest improvement in long - term strength, though the effect is less substantial than that of SS. Coal gangue (CG), on the other hand, markedly reduces compressive strength, and higher CG contents lead to a greater decline.
FIGURE 10
In summary, both SS and CW can be incorporated into cement - stabilized aggregate mixtures in varying proportions to enhance performance, while the use of CG should be carefully controlled due to its detrimental impact on compressive strength.
3.2 Analysis of flexural strength of CS-AM
3.2.1 Flexural strength of CSM
According to Figure 11, The incorporation of steel slag (SS) significantly enhances the flexural strength of CS-AM. At SS contents of 20%, 40%, 60%, and 80%, the flexural strength increases by 15%, 39.2%, 42.1%, and 52.9%, respectively, compared to the control. Higher SS content consistently leads to greater flexural strength.
FIGURE 11
This improvement is attributed to the high hardness, angularity, and roughness of SS particles, which optimize aggregate gradation and promote the formation of a denser and more stable skeleton structure. Additionally, SS contains active mineral components such as dicalcium silicate (C2S), tricalcium silicate (C3S), free calcium oxide (f - CaO), and magnesium oxide (MgO). These components participate in secondary hydration reactions, generating additional gel products (e.g., C - S - H) (), which increase matrix bonding strength and improve interfacial adhesion between SS particles and cement paste. As SS content increases, the addition of reactive SS fine aggregates further enhances flexural strength through the combined effects of cement hydration and secondary hydration reactions ().
3.2.2 Flexural strength of CCM
According to Figure 12, The incorporation of coal gangue (CG) has a detrimental impact on the flexural strength of CS-AM. As the CG content increases, the flexural strength decreases. At a 20% CG content, the reduction is minor, but at 80%, the flexural strength drops by 40%. This decline is primarily due to the lower strength, porous structure, and layered cleavage. These characteristics create weak points that are prone to fracture under stress and inhibit the improvement of flexural strength ().
FIGURE 12
3.2.3 Flexural strength of CCWM
According to Figure 13, When construction waste (CW) is incorporated, the flexural strength remains comparable to that of the control group at 20% and 40% CW content. However, at 60% and 80% CW content, the flexural strength increases by 17.9% and 20%, respectively. This improvement is attributed to the inclusion of fine CW aggregates, which contain partially hydrated cement particles, cement hydration products (e.g., C - S - H, CH), and other potentially active mineral components (). These components can undergo secondary hydration during mixing and curing, generating new cementitious materials and enhancing matrix bonding (). Additionally, fine CW particles fill micro - voids, optimize the pore structure, and increase the matrix density (). Both effects become more pronounced at higher CW contents, collectively enhancing the flexural strength.
FIGURE 13
3.2.4 Comparison of flexural strength of CS-AM
According to Figure 14, Among the three types of cement - stabilized solid waste mixtures, steel slag (SS) offers the most significant enhancement in flexural strength, with more significant improvement at higher contents. This is primarily attributed to its excellent physical skeleton effect and remarkable chemical activity, which strengthen the interface. Construction waste (CW) also brings about notable improvement (18%–20%) at contents of 60% or higher, due to the secondary hydration and micro - filling effects of active fine particles. In contrast, coal gangue (CG) consistently decreases the flexural strength. Therefore, to improve the flexural strength of cement - stabilized aggregate mixtures, SS is the optimal option, CW is effective at high contents, and the contribution of CG is limited.
FIGURE 14
3.3 Analysis of freeze - Thaw resistance of CS-AM
Experimental results (Figure 15) demonstrate that the addition of steel slag (SS) significantly enhances the freeze - thaw resistance of CS-AM. The improvement becomes more pronounced at higher SS contents. For example, at 80% SS, the freeze - thaw resistance increases by 17.3%. This enhancement is attributed to the formation of a stronger and less defective interfacial transition zone between SS and cement paste, which reduces the pathways for water ingress and ice - crystal damage. Additionally, the micro - aggregate filling effect of SS and the C - S - H gel generated by its secondary hydration help refine the pore structure, decrease the number of harmful large pores, and lower the freeze - thaw water content and internal stresses.
FIGURE 15
In contrast, the incorporation of coal gangue (CG) adversely affects frost resistance. While a 20% CG content has minimal impact, levels above 40% lead to a reduction in frost resistance exceeding 10%. This deterioration is primarily due to the porous structure of CG and its weak interface with cement paste, which increases internal connectivity and microcrack formation (). CG’s strong water absorption also raises the degree of saturation, increasing the frozen water content and the risk of frost heave during freeze - thaw cycles ().
Construction waste (CW) has little influence on frost resistance. The performance across different CW contents remains comparable to the control. Although high CW content can improve mechanical properties, the effect is insufficient to significantly alter the pore structure or enhance freeze - thaw resistance, as observed with SS. Importantly, CW does not introduce additional harmful pores or absorb water excessively, unlike CG, resulting in a neutral impact on frost resistance (; ).
3.4 Analysis of drying shrinkage performance of CS-AM
3.4.1 Drying shrinkage performance of CSM
According to Figure 16. Experimental results indicate that the addition of steel slag (SS) to CS - AM compensates for and diminishes drying shrinkage. Higher SS contents lead to lower drying shrinkage strains. When the SS contents are 20%, 40%, 60%, and 80%, the drying shrinkage strain is reduced by 11.9%, 29.5%, 37.7%, and 40.0% respectively, in comparison to the control. This notably improves the cracking resistance of semi - rigid base courses.
FIGURE 16
The reduction can be ascribed to the abundant iron phases (e.g., Fe2O3) and active CaO/MgO in SS. During hydration, these substances generate expansive products such as calcium hydroxide and ettringite, which partially counteract drying shrinkage (; ). The micro - expansion effect is especially prominent at higher SS contents.
3.4.2 Drying shrinkage performance of CCM
According to Figure 17. Coal gangue (CG) also reduces the drying shrinkage strain in CS-AM. However, this compensating effect does not show a clear trend with the variation of CG content. When compared to the control group, across different CG contents, the drying shrinkage strain is reduced by 25%–28%. This might be because of the relatively low strength of CG. After compaction, fresh surfaces and weak zones are formed, which do not participate in the hydration process or experience shrinkage. Additionally, the minor expansion stemming from the layered structure of CG can further compensate for the shrinkage (; ).
FIGURE 17
3.4.3 Drying shrinkage performance of CCWM
According to Figure 18. The effect of construction waste (CW) on drying shrinkage strain exhibits a content threshold. At CW contents of 20% and 40%, the drying shrinkage strain is similar to that of the control group. In contrast, when the CW content reaches 80%, the strain increases by 10.8%. This threshold effect is likely due to the fact that the recycled coarse aggregates from CW are similar to natural aggregates and have a minimal impact at low contents. Nevertheless, when the content exceeds 60%, CW fine aggregates, which contain residual hydration products (C - S - H, Ca(OH)2), are introduced. This introduction leads to secondary reactions and increased self - shrinkage, ultimately resulting in an elevation of the early drying shrinkage strain (; ).
FIGURE 18
3.4.4 Drying shrinkage strain prediction model
This study fitted the variation law of dry shrinkage strain and time of CS-AM, as shown in the following Tables 3–5. The EXPASSOC model follows the general form:where y is the drying shrinkage strain at time x, ε0 is the initial strain, A is the amplitude parameter representing the total achievable shrinkage, and τ is the time constant that governs the rate of shrinkage development. A smaller τ indicates a faster approach to the shrinkage saturation limit.
TABLE 3
| SS content (%) | Fitting curve | Correlation R2 |
|---|---|---|
| 0 | 0.99871 | |
| 20 | 0.99866 | |
| 40 | 0.99815 | |
| 60 | 0.99805 | |
| 80 | 0.99853 |
Fitting curve and correlation coefficient (R2) for drying shrinkage of CSM.
TABLE 4
| CG content (%) | Fitting curve | Correlation R2 |
|---|---|---|
| 0 | 0.99871 | |
| 20 | 0.99148 | |
| 40 | 0.99623 | |
| 60 | 0.99232 | |
| 80 | 0.99208 |
Fitting curve and correlation coefficient (R2) for drying shrinkage of CCM.
TABLE 5
| CW content (%) | Fitting curve | Correlation R2 |
|---|---|---|
| 0 | 0.99871 | |
| 20 | 0.99810 | |
| 40 | 0.99815 | |
| 60 | 0.99836 | |
| 80 | 0.99877 |
Fitting curve and correlation coefficient (R2) for drying shrinkage of CCWM.
The fitted data indicate that the relationship between drying shrinkage strain and time for all three CS-AM types adheres to the EXPASSOC model (single exponential approaching saturation), with all correlation coefficients (R2) surpassing 0.99. This demonstrates that the model can reliably depict the long - term drying shrinkage behavior of CS-AM, clearly revealing a saturation limit for shrinkage strain.
The drying shrinkage process displays a distinct three - stage characteristic: Phase 1, a rapid contraction phase within the first 60 days (more than 85% of the total strain). Phase 2, a deceleration phase from 60 to 100 days (5%–8% increase). Phase 3, stabilization after 100 days (increment <2%). These stages correspond to changes in water loss mechanisms: early rapid evaporation of capillary water causes a sharp contraction, while later slow diffusion of gel water results in strain stabilization (; ). The results confirm that the EXPASSOC model can accurately forecast the long - term shrinkage behavior of CS-AM.
3.4.5 Crack resistance index
Based on the EXPASSOC model, this study introduces the crack resistance index (CRI, Equation 11) to quantitatively assess the crack resistance of different solid waste mixtures:
Where, CRI is the crack resistance index,εS,T is the dry shrinkage strain (µε) of solid waste content (s) at age T, ε0, T is the dry shrinkage strain (µε) of the benchmark group (0% solid waste) at age T.
According to Figure 19. Analysis of the crack resistance index (CRI) across ages reveals that CSM demonstrates minimal attenuation and strong performance stability. Even as the age increases, the crack resistance index remains high, particularly at SS contents ≥40%. For CCM, the age - dependent response is content - sensitive: at 20% CG, the index decreases sharply by 34%, but the rate of attenuation slows considerably at medium to high contents (≥ 40%), indicating improved long - term stability with increased CG content. In contrast, CCWM shows a tendency toward long - term deterioration. When the CW content is ≥60%, the negative value of the crack resistance index becomes more pronounced at 180 days compared to 60 days. Even at 40% CW, the index returns to its initial level after early fluctuations, indicating ongoing shrinkage deformation with age.
FIGURE 19
Comparing the crack resistance index (CRI) as a function of solid waste content, SS exhibits a pronounced compensatory effect on drying shrinkage strain, significantly reducing the risk of shrinkage cracking, and the effect strengthens with higher SS content. CG also offers some compensation for drying shrinkage strain, with optimal crack resistance observed at contents between 40% and 60%; however, the benefit plateaus beyond 20%. In contrast, when the CW content reaches or exceeds 40%, the crack resistance index turns negative, suggesting that shrinkage deformation predominates. At high CW contents (≥ 60%), this negative trend intensifies with age, highlighting a risk of long - term deterioration. Therefore, to ensure satisfactory crack resistance, the CW content should be limited to ≤40%.
3.5 Comprehensive performance evaluation
Using the entropy weight method, objective weights were calculated for each performance indicator. These weights reflect each indicator’s contribution to the overall performance. Figure 20 shows that compressive strength and crack resistance are the most influential factors in the comprehensive performance evaluation, highlighting the importance of strength and durability for base course materials.
FIGURE 20
Figure 21 demonstrates that SS exhibits the most balanced and highest overall performance, especially in terms of compressive and flexural strength as well as freeze - thaw durability. This confirms its suitability as the optimal solid waste for highway base application, with a recommended content of 40%–80%. CG lags behind, particularly in freeze - thaw resistance. The content of CG should be strictly limited to ≤40% to avoid grading deterioration and a sudden drop in strength. While CW shows low crack resistance at high dosages, the allowable content of CW is ≤40% (fine aggregate is prohibited).
FIGURE 21
Based on the comprehensive evaluation, SS is recommended as the optimal solid waste aggregate. For practical engineering applications, the following guidance is provided: (1) For heavy-duty highways and cold regions, SS content of 60%–80% is recommended to maximize strength, frost resistance, and crack resistance; (2) For medium-traffic roads or temperate climates, SS content of 40%–60% provides a balance between performance and material cost; (3) For low-traffic roads or as a preliminary design, SS content of 40% is sufficient. Further optimization based on local material availability and economic factors is encouraged.
4 Conclusion
This study systematically compared the mechanical properties, drying shrinkage characteristics, and freeze–thaw resistance of cement-stabilized mixtures incorporating SS, CG, and CW through a comprehensive set of laboratory tests. A drying shrinkage prediction model was established, and a crack resistance index was proposed. Furthermore, the comprehensive road performance was quantitatively evaluated and optimized using the entropy weight fuzzy algorithm. The main findings are as follows.
At SS contents ranging from 40% to 80%, the 360 - day compressive strength increased by 16.0%–36.2%, the flexural strength increased by 39.2%–52.9%, and the frost resistance increased by 17.3%, while the drying shrinkage decreased by 29.5%–40.0%. The active components of SS promote continuous hydration and micro - expansion, making it a suitable option for heavy - duty and cold - region pavement applications.
For coal gangue (CG), the content should be strictly limited to ≤40% to prevent significant reductions in compressive strength (30.9%–46.9%) and frost resistance (>10%) caused by gradation disruption and water - induced softening. Similarly, the use of construction waste (CW) as coarse aggregate should be restricted to ≤40%. The introduction of fine CW aggregates increases drying shrinkage by 10.8% due to secondary reactions, while the mechanical and frost resistance properties remain comparable to those of conventional mixtures.
The drying shrinkage behavior of all three cement - stabilized solid waste mixtures conforms to the EXPASSOC model (R2 > 0.99), exhibiting a distinct three - stage process: 85% of the shrinkage occurs within the first 60 days and stabilizes after 100 days. The proposed crack resistance index confirms that mixtures with high SS content (≥40%) consistently demonstrate superior crack resistance.
Based on the entropy weight fuzzy algorithm, SS is identified as the optimal solid waste aggregate, with a recommended content of 40%–80%. CG achieves a moderate comprehensive performance score, while CW ranks lower. Both CG and CW contents should be limited to ≤40%. For CW, it is recommended to use only coarse aggregates and limit the use of fine aggregates.
5 Discussion
5.1 Key research findings and innovations
This study systematically compared the durability performance of cement-stabilized semi-rigid base materials using three bulk solid waste recycled aggregates: steel slag (SS), coal gangue (CG), and construction waste (CW). The main findings and innovations are as follows: It revealed the nonlinear effects of waste type and dosage on performance. When SS was used at 40%–80% dosage, the 360-day compressive strength increased by 16.0%–36.2%, flexural strength increased by 39.2%–52.9%, and frost-thaw strength retention rate improved by 17.3%, while dry shrinkage strain decreased by 29.5%–40.0%, exhibiting a synergistic effect of ‘strengthening-crack resistance-frost resistance.’ When CG dosage exceeded 40%, due to lamellar structure breakage and softening collapse, strength decreased by 30.9%–46.9%, and frost-thaw performance dropped by more than 10%. In CW, when fine aggregate dosage was ≥60%, secondary hydration caused dry shrinkage strain to increase by 10.8%, while coarse aggregates had a smaller effect. A high-precision dry shrinkage prediction model and crack resistance index were established. The EXPASSOC model was applied to describe the long-term drying shrinkage behavior of mixtures containing these three solid wastes, achieving a high fitting goodness of R2 > 0.99. The drying shrinkage process exhibited three distinct stages: rapid development within the first 60 days, during which approximately 85% of the total shrinkage occurred, followed by a deceleration phase between 60 and 100 days, and finally stabilization after 100 days. On this basis, the Crack Resistance Index (CRI) was proposed, quantitatively confirming that SS dosage ≥40% had the best crack resistance performance, while CW dosage ≥40% led to a negative crack resistance index, providing a quantitative basis for material selection. A multi-index comprehensive evaluation system based on the entropy weight fuzzy algorithm was established, taking into account strength, frost resistance, and crack resistance, objectively weighting indicators and concluding that SS (recommended dosage 40%–80%) was the optimal solid waste, while CG and CW should be limited to ≤40% (fine aggregate in CW prohibited). Unlike existing studies that focus on single solid waste or single performance, this study systematically compared and quantitatively selected three typical solid wastes under the same conditions, filling the theoretical gap in classified resource utilization of multi-source solid waste in semi-rigid base layers.
5.2 Limitations and shortcomings of the study
Although this study has obtained clear experimental conclusions, the following limitations still exist:
SS, CG, and CW were respectively obtained from specific enterprises in the Gansu region, and their chemical compositions (such as f-CaO content) and physical properties (such as crushing strength and water absorption) have regional characteristics. The performance of solid wastes from different sources varies greatly, and the universality of the conclusions needs to be further verified under more regional and material conditions.
Although the paper attributes performance changes to the active hydration of SS, the lamellar softening of CG, and the secondary reaction of CW, it lacks direct observations of the interfacial transition zone morphology, types and distribution of hydration products, and pore structure evolution using microscopic methods such as SEM, XRD, and MIP. In particular, the micro-expansion compensating shrinkage mechanism of SS and the interfacial weakening mechanism of CG still lack quantitative microscopic evidence.
5.3 Theoretical significance and research advancement
This study promotes the theoretical development of semi-rigid base material design from two dimensions: ‘classification and utilization of multi-source solid waste’ and ‘quantitative evaluation of durability.’ It proposes a classification and utilization principle based on the characteristics of solid waste, clarifying the differentiated roles: SS is positioned as an activity-enhancing component, CG as an inert component that must be strictly limited, and CW as a substitute for coarse aggregate. This provides a theoretical basis for transforming solid waste resource utilization from ‘single disposal’ to ‘classified value addition.’ Analytical models for dry shrinkage behavior and quantitative indicators for crack resistance were established. The application of the EXPASSOC model revealed the saturation characteristics and temporal evolution law of dry shrinkage in solid waste mixtures, and the proposed Crack Resistance Index (CRI) converts abstract shrinkage risks into computable and comparable numerical indicators, offering a simple and effective mathematical tool for selecting engineering materials. A multi-objective comprehensive evaluation framework of ‘strength-frost resistance-crack resistance’ was constructed, using the entropy weight fuzzy algorithm to integrate multiple performance indicators into a unified evaluation system, avoiding the one-sidedness of single-indicator guidance, and can be extended to other types of solid waste or new cementitious material systems. These achievements not only enrich the theoretical system of solid waste reuse in road engineering but also provide a scientific basis and integrated solutions for low-carbon design, performance optimization, and standardized application of green road bases under the ‘dual-carbon’ goals, with strong interdisciplinary and engineering guidance value.
5.4 Future research directions and theoretical outlook
Based on the achievements and limitations of this study, further exploration can be carried out in the following directions.
Multi-scale mechanism and hydration-damage evolution study: Combine microscopic techniques such as SEM and BSE-EDS to quantitatively characterize the hydration kinetics of SS active components and their regulatory mechanisms on the micro-mechanical properties of the interfacial transition zone; use X-CT three-dimensional reconstruction technology to track the initiation and propagation of micro-cracks inside CG/CW under freeze-thaw cycles, and establish a cross-scale correlation model of ‘microstructure-macro performance.'
Long-term durability and life-cycle assessment under complex environments: Conduct accelerated tests coupling dry-wet, freeze-thaw, and loading conditions, and establish a damage mechanics-based model to predict strength degradation and drying shrinkage cracking; simultaneously perform life-cycle cost analysis (LCCA) and carbon footprint accounting, constructing a multi-objective optimization framework of ‘performance-cost-carbon emission,’ providing an economically optimal range of solid waste incorporation for different traffic levels and climate zones.
Intelligent mix design and on-site standardized processes: Use machine learning (such as random forest and neural networks) to establish prediction models linking solid waste characteristics, dosage, and performance, and develop intelligent mix design systems for different solid waste sources; study on-site applicable processes such as integrated steel slag aging and mixing, coal gangue grading and crushing, and fine screening of construction waste, promoting the standardization and industrial application of solid waste recycled base layers.
Statements
Data availability statement
The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary Material.
Author contributions
RN: Writing – original draft, Writing – review and editing, Visualization, Conceptualization, Project administration. XW: Writing – original draft, Methodology, Writing – review and editing, Investigation. JiZ: Writing – original draft, Writing – review and editing, Investigation, Data curation, Conceptualization. BH: Investigation, Writing – review and editing, Supervision, Validation, Funding acquisition. HQ: Investigation, Writing – review and editing, Writing – original draft, Validation. JuZ: Writing – review and editing, Writing – original draft, Supervision, Validation.
Funding
The author(s) declared that financial support was received for this work and/or its publication. The Gansu Provincial Department of Transportation funded project ‘Research on the Regulation and Application Technology of Crack-Resistant Substrates under Temperature and Humidity Environment’ (2024–49).
Acknowledgments
During the course of this research, we received comprehensive research platform support and strong assistance from the High-Performance Materials Research Institute of Gansu Transportation Planning and Design Institute Co., Ltd. We sincerely extend our heartfelt gratitude for this. We appreciate the institute for providing advanced instruments and a good research environment for the experiments, laying a solid foundation for the smooth progress of the study. At the same time, we sincerely thank all colleagues for their valuable suggestions and selfless help in experimental design, data collection, and analysis. Your professional opinions and collaborative spirit provided important support for this research. In addition, we also express our gratitude for the funding support from the Gansu Provincial Department of Transportation for the project “Research on Regulation and Application Technology of Crack-Resistant Substrate under Temperature and Humidity Environments.” With this paper, we express our deepest appreciation to all colleagues and institutions who have contributed wisdom and hard work to this research.
Conflict of interest
Authors RN, BH were employed by Gansu Road & Bridge Highway Investment Co., Ltd. Lanzhou. Authors XW, JiZ, HQ, and JuZ were employed by Gansu Transportation Planning, Survey and Design Institute Co., Ltd.
Generative AI statement
The author(s) declared that generative AI was not used in the creation of this manuscript.
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Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2026.1894197/full#supplementary-material
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Summary
Keywords
drying shrinkage, fuzzy comprehensive evaluation, highway bases, industrial solid waste, mechanical properties, recycled aggregate
Citation
Niu R, Wu X, Zhao J, Hu B, Qin H and Zhang J (2026) Durability evaluation of cement-stabilized semi-rigid base courses incorporating recycled aggregates from steel slag, coal gangue, and construction waste. Front. Mater. 13:1894197. doi: 10.3389/fmats.2026.1894197
Received
29 May 2026
Revised
10 June 2026
Accepted
10 June 2026
Published
16 July 2026
Volume
13 - 2026
Edited by
Reza Taherdangkoo, Freiberg University of Mining and Technology, Germany
Reviewed by
Liujun Fan, North China University of Water Conservancy and Electric Power, China
Haidong Ji, Tarim University, China
Updates
Copyright
© 2026 Niu, Wu, Zhao, Hu, Qin and Zhang.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Xu Wu, wx18893105922@hotmail.com; Jingzhuo Zhao, 549992047@qq.com
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.