Synergistic Control of Shrinkage and Mechanical Properties in Expansive Soil Slurry via Coupled Cement–Fiber Reinforcement

Abstract

This study elucidates the synergistic effects of polypropylene fiber and cement (physical–chemical) on stabilized expansive soil slurry. A comparative analysis was conducted on the fluidity, 28-day mechanical strength, and shrinkage properties (autogenous and drying) of slurries with different modifications. The underlying mechanisms were further investigated through Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) analysis. Results demonstrate that the cement addition substantially enhanced fluidity, mechanical strength, and early-age volume stability through hydration. However, it was insufficient to mitigate long-term drying shrinkage at low dosages. Conversely, incorporating 0.5% polypropylene fiber reduced slurry fluidity but markedly improved flexural strength. Crucially, a pronounced synergistic effect was observed in the co-modified slurry; the specimen with 20% cement and 0.5% fiber exhibited a 28-day drying shrinkage of only 0.57%, a performance comparable to the specimen with 60% cement and no fibers. Microstructural analysis revealed that cement hydration products created a robust fiber-matrix interfacial transition zone, evidenced by C-S-H gel enrichment. This enhanced interface enabled the fibers to effectively bridge microcracks and restrain both autogenous and drying shrinkage. This research validates that the combined cement–fiber approach is a highly effective strategy for improving expansive soil slurry, yielding critical enhancements in flexural performance and long-term dimensional stability while allowing for a significant reduction in cement content.

1. Introduction

The rapid pace of global economic integration and regional development has intensified the demand for large-scale infrastructure in transportation, water conservancy, energy, and urbanization, imposing increasingly stringent requirements on geological engineering conditions [1]. However, the prevalence of special geotechnical materials, such as expansive soils, presents formidable challenges to construction projects. Expansive soil, in particular, is a notoriously problematic material recognized for its acute sensitivity to water. It exhibits significant volumetric instability—characterized by swelling upon wetting and shrinking upon drying—which poses a severe threat to the safety, stability, and long-term performance of engineered structures [2,3,4].

Consequently, extensive engineering activities in regions rich in expansive soil, such as China’s Guangxi, Guizhou, and Yunnan provinces, generate vast quantities of spoil, slurry, and muck [5,6]. These byproducts exhibit undesirable properties, including low strength, high compressibility, and pronounced volumetric changes, rendering them unsuitable for direct use in applications like backfilling or foundation engineering. It has been observed that a rapid crack evolution in expansive soil from the Nanyang region during wet–dry cycles, eventually creating an interconnected network [7]. This crack development is accompanied by significant strength decay, resulting in extremely poor engineering properties for the soil. Similarly, Wang, W. [8] found that the fissure ratio of expansive soil from the Henan region reached 17.6% in just 3 days, indicating severe shrinkage. Mere stockpiling is not a viable solution, as it consumes valuable land and introduces potential geo-environmental hazards. Therefore, a thorough investigation into the deformation characteristics of expansive soil slurry and the development of effective mitigation strategies are imperative. Such research is crucial for ensuring engineering safety, enabling the resource utilization of these problematic soils, and promoting sustainable regional development.

To counteract the adverse properties of expansive soil byproducts, two avenues have primarily been explored by the engineering community: chemical stabilization and fiber reinforcement [9,10,11,12].

Chemical stabilization typically involves incorporating inorganic cementing agents—most commonly Portland cement and supplementary materials like fly ash or slag—to enhance soil strength and volumetric stability [13,14,15,16,17]. Through complex hydration and pozzolanic reactions, these agents form hydration products such as calcium silicate hydrate (C-S-H) gel and ettringite (AFt) crystals [18,19]. These products fill inter-particle voids and bind loose soil particles into a rigid skeletal structure. This stabilization significantly increases the soil’s strength and modulus; furthermore, by enhancing the soil’s overall stiffness and tensile strength, it effectively restrains the development of shrinkage cracks during dehydration, thus controlling volumetric changes [20,21].

However, this approach has significant drawbacks. Treating highly expansive soils often demands high cement dosages, leading to prohibitive material costs and a substantial carbon footprint, which conflicts with global green development goals [22,23]. Furthermore, cement-stabilized soil is inherently brittle and susceptible to cracking under stress, compromising its long-term durability [24,25]. This has spurred a search for alternative solutions that balance performance, cost, and environmental impact.

Fiber reinforcement is a physical method for strengthening soil that helps cooperatively control the swelling and shrinking of expansive soils due to moisture changes [26,27,28]. This method involves uniformly distributing short-cut fibers (or continuous filament fibers) with a certain tensile strength throughout the soil, creating a fiber-soil composite material [29]. For example, the fiber (bamboo/flax)-soil composite showed a significant improvement in cohesion and internal friction angle compared to the unreinforced soil, with an increase of 26.36% and 10.22%, respectively [30]. The fibers form a three-dimensional, randomly oriented network that enhances the soil’s mechanical properties through two primary mechanisms: interfacial interactions (such as friction and adhesion) with soil particles and the fibers’ own inherent tensile strength [31]. This reinforcement significantly improves the soil’s tensile capacity, toughness, and ductility, thereby restraining both swelling and shrinkage deformations.

The application of various fiber types has proven effective in improving the engineering properties of different soils [32,33,34]. For instance, research by Puppala, A. J. [35] found that fiber reinforcement enhanced the unconfined compressive strength of expansive soil while simultaneously reducing both volumetric shrinkage strains and swell pressures. Similarly, another study showed that PE fibers from waste fishing nets can effectively reinforce lightweight dredged soil, achieving a relatively high compressive strength at dosages between 0.25% and 0.75% [36]. In expansive soils specifically, fibers play a crucial role in crack mitigation. As the soil shrinks and tensile stresses develop, the fibers act as “bridges” across nascent fissures, inhibiting the initiation and propagation of microcracks and enhancing the soil’s overall crack resistance [37,38,39]. However, despite these benefits, the application of fiber reinforcement to control shrinkage in high-moisture expansive soil slurries remains underexplored, and a comprehensive understanding of the governing mechanisms is still lacking.

In summary, the existing modification methods present a clear trade-off. Chemical stabilization can substantially increase strength but often results in a brittle, costly, and environmentally burdensome material. In contrast, fiber reinforcement excels at improving toughness and crack resistance but provides only a modest increase in strength, with its application to slurries being insufficiently researched.

This study aims to address these limitations by investigating the synergistic effects of a combined cement–fiber reinforcement system. Focusing on expansive soil from the Guangxi region, a systematic experimental program was conducted to evaluate how cement and fibers, both individually and in combination, regulate shrinkage behavior. By elucidating the underlying physical–chemical mechanisms of their combined action, this work will provide a robust theoretical foundation and technical framework for the effective treatment and resource utilization of expansive soil byproducts in engineering applications.

2. Materials and Methods

2.1. Raw Materials

The binder used was a P.O. 42.5 ordinary Portland cement with a specific surface area of 0.398 m²/g, compliant with Chinese standard GB/T 175-2023 [40]. The expansive soil was sourced from the Guangxi region and had a liquid limit (W_L) of 53.7% and a plastic limit (W_P) of 26.7%. The chemical compositions and mineralogical phases of the cement and expansive soil, determined by X-ray Fluorescence (XRF) and X-ray Diffraction (XRD), respectively, are presented in

Table 1. Chemical composition of cement and expansive soil (wt%).

Raw Material	SiO2	Al2O3	Fe2O3	SO3	CaO	K2O	ZnO	TiO2	LOI.
Cement	19.47	3.83	4.00	2.73	68.10	1.14	0.12	0.30	0.31
Expansive soil	59.30	22.31	15.23	-	0.11	0.99	0.01	1.79	0.26

and Figure 1. The expansive soil contains 57.7% clay minerals, primarily composed of kaolinite, chlorite, and montmorillonite. Prior to slurry preparation, the raw soil was sieved to remove coarse particles (>2.5 mm) and organic debris such as grass roots.

Polypropylene fibers, with a length of 20 mm and a density of 0.91 g/cm³, were supplied by Changzhou Tianyi Engineering Fiber Co., Ltd. (Jiangsu, China). The appearance of the raw materials is shown in Figure 2.

2.2. Sample Preparation

The mix proportions for the slurry samples are detailed in

Table 2. Mix proportions of expansive soil slurry samples.

Sample	Expansive Soil (g)	Cement (g)	Polypropylene Fibers (g)	Water (g)
S	1470	0	0	450
SC-20	1176	240	0	504
SC-40	882	480	0	558
SC-60	588	720	0	612
SF	1470	0	6	450
SFC-20	1176	240	6	504
SFC-40	882	480	6	558
SFC-60	588	720	6	612

. A constant water-to-solid ratio of 0.60 was maintained for all mixtures. Cement was used to replace expansive soil at dosages of 0%, 20%, 40%, and 60% by mass of total solids. The mixing water was adjusted to account for the inherent moisture content (18.4%) of the expansive soil. Polypropylene fibers were added at a dosage of 0.50% by total mass of the solid components (expansive soil and cement), a typical dosage based on previous research [36,41].

2.3. Fluidity Test

Fluidity was measured in accordance with standard T/CECS 1175-2022 [42]. Freshly prepared slurry was poured into an acrylic cylinder (80 mm height, 80 mm inner diameter) resting on a smooth, level glass plate. After filling the cylinder and leveling the surface, it was lifted vertically at a uniform speed, allowing the slurry to spread freely. After 60 s, the final spread diameter was recorded as the average of two perpendicular measurements.

2.4. Flexural Strength and Compressive Strength

For each mix design, triplicate 40 × 40 × 160 mm prismatic specimens were cast for strength testing, following the GB/T 17671-2021 [43] standard. Specimens were cured in their molds for 3 days at 20 ± 2 °C and a relative humidity of 30–40%. After demolding, the specimens were cured under the same conditions for 28 days, at which point their flexural and compressive strengths were measured.

2.5. Drying Shrinkage

Drying shrinkage was measured on 40 × 40 × 160 mm specimens cast with embedded copper studs at both ends. The specimens were cured under the same conditions as the strength samples for 3 days before demolding. Immediately after demolding, the initial length (L₀) was measured using a vertical comparator (SP-175). Subsequent length measurements (L_t) were recorded at designated intervals under the same curing conditions to calculate the drying shrinkage rate using Equation (1).

$$S_t={\displaystyle \frac{(L_0-L_t)}{EF}}\times 100\%$$

(1)

where S_t is the drying shrinkage rate of the slurry specimen at time t (in d), L₀ (mm) is the initial length of the specimen measured after demolding, L_t (mm) is the length of the specimen measured at time t (in d), and EF is the effective length of the specimen (EF = 144 mm).

2.6. Autogenous Shrinkage

Autogenous shrinkage was monitored using the corrugated tube method in accordance with ASTM C 1698-09 (2014) [44]. Fresh slurry was poured into corrugated plastic tubes (420 ± 5 mm length, 29 ± 5 mm inner diameter), which were then sealed and placed horizontally in a measurement apparatus at 20 ± 2 °C. Length changes were automatically recorded every minute. Since specimens S and SF contained no cement and thus do not undergo hydration, their autogenous shrinkage was not measured.

2.7. X-Ray Diffraction (XRD)

Fragments were taken from the 28-day strength specimens and immediately immersed in ethanol for 24 h to arrest hydration. The samples were then removed, blotted dry, and placed in a vacuum oven at 40 °C until all ethanol had evaporated. A portion of the dried sample was ground to a fine powder (passing a 0.08 mm sieve) in an agate mortar. The X-ray Diffraction (XRD) analysis was performed on a Rigaku Ultima IV diffractometer with a scanning range of 5° to 90° at a rate of 5°/min.

2.8. Scanning Electron Microscope (SEM) Test

Small, fractured block samples, prepared using the same hydration arrest and drying procedure described in Section 2.7, were used for microstructural observation. The analysis was conducted using a ZEISS Gemini 300 (Oberkochen, Germany) Scanning Electron Microscope (SEM) equipped with an Oxford Xplore Energy (Oxford, England) Dispersive Spectroscopy (EDS) detector.

3. Results and Discussion

3.1. Fluidity of Expansive Slurry

The fluidity test results, presented in Figure 3, demonstrate that slurry fluidity is directly proportional to the cement dosage, while fiber inclusion has an adverse effect. The fluidity of cement-modified slurries (SC) increased with higher cement content. Conversely, the addition of polypropylene fibers consistently reduced the fluidity for all mix designs.

This phenomenon stems from the distinct surface properties and interactions of the materials. Unlike expansive soil particles, which have a high specific surface area and bind large volumes of water, cement particles absorb negligible water initially [2]. Instead, they act as fine, spherical aggregates, creating a “ball-bearing effect” that lubricates the mixture and enhances flow. In contrast, the fibers form an interlocking three-dimensional network within the slurry. This network physically obstructs the free movement of particles, leading to increased internal friction and a macroscopic reduction in flow.

3.2. Strength of Hardened Specimens

As shown in Figure 4, the plain expansive soil specimen (S) possesses negligible flexural strength, a consequence of weak inter-particle bonds. Its limited compressive strength is derived solely from inter-particle friction. The addition of cement (SC series) dramatically improves both strength properties. Cement hydration produces binding phases like C-S-H gel and ettringite (AFt), which transform the loose soil into a monolithic, reinforced matrix. Consequently, both flexural and compressive strengths increase proportionally with the cement dosage, as a higher binder content creates a denser and more robust hydration network.

The reinforcing effect of fibers (SF specimen) is primarily manifested in the material’s tensile behavior. While its compressive strength improvement is marginal, its flexural strength is significantly enhanced. This is attributed to the crack-bridging mechanism: when the matrix begins to fail under tension, the high-tensile-strength fibers bridge the nascent cracks, transferring stress across the fracture plane and effectively arresting crack propagation [31].

3.3. Drying Shrinkage

Figure 5 illustrates the appearance of specimens after 3 days of curing. A clear distinction in early-age volume stability is evident: the cement-stabilized specimen (SC-20) remained dimensionally stable, whereas the others experienced significant shrinkage. Taking the length of SC-20 as the 100% baseline, the fiber-only specimen (SF) and plain soil specimen (S) had shrunk to 92.8% and 83.6% of their original length, respectively. This profound initial shrinkage in the S and SF specimens caused the measurement probes to detach from their surfaces, precluding subsequent continuous data collection for these two groups. This initial result underscores the critical role of cement’s chemical stabilization in controlling early-age drying shrinkage, with fiber reinforcement appearing to play a secondary role at this stage.

The long-term drying shrinkage behavior of the cement-stabilized specimens is detailed in Figure 6. For specimens without fibers (Figure 6a), the shrinkage rate was low prior to 15 days. Subsequently, the SC-20 specimen exhibited a significant acceleration in its shrinkage rate, reaching 1.50% at 28 days. This increase was suppressed at higher cement dosages; for instance, the 28-day shrinkage of the SC-60 specimen was merely 0.53%. This late-stage acceleration in specimens with lower cement content is attributed to continuous water loss. As drying progresses, the accumulated internal shrinkage stress eventually exceeds the restraining capacity of the hydrated cementitious structure, triggering internal compaction or micro-crack development and leading to a sudden increase in the shrinkage rate [45]. A higher cement content creates a denser and more robust gel structure, enhancing its ability to restrain these internal stresses.

Furthermore, Figure 6b demonstrates that the inclusion of fibers effectively minimizes long-term drying shrinkage. The 28-day shrinkage rate of the SFC-20 specimen was only 0.57%, a reduction of 0.93% compared to SC-20, making its performance remarkably similar to that of the SC-60 specimen. However, this beneficial effect was less pronounced at higher cement dosages; the shrinkage reductions for the SFC-40 and SFC-60 specimens were only 0.56% and 0.19%, respectively. In these fiber-reinforced systems, a synergistic effect emerges where internal stresses from drying are resisted collectively by both the rigid cementitious network and the three-dimensional fiber network. Notably, this coupled effect is more significant at lower cement contents. This result is highly significant from a practical standpoint, suggesting that a minor fiber addition can potentially replace a substantial amount of cement for achieving target volume stability, leading to significant cost savings and a lower carbon footprint.

3.4. Autogenous Shrinkage

Figure 7 illustrates the evolution of autogenous shrinkage in the cement-containing specimens. This test excludes the S and SF groups, as autogenous shrinkage originates from processes inherent to cement hydration—namely, chemical shrinkage (the volume reduction of reactants) and self-desiccation (internal drying). Since the S and SF specimens contain no cement, they do not exhibit this phenomenon.

The test results show that the most significant increase in autogenous shrinkage occurs within the first 10 h of hydration, a period that directly corresponds to the primary phase of the cement’s hydration process. This suggests that the presence of the expansive soil has a minimal impact on the fundamental kinetics of cement hydration. As expected, a higher cement dosage leads to greater autogenous shrinkage, which can be attributed to the more pronounced chemical shrinkage and internal drying effects resulting from a larger volume of reacting cement. As a result, a relatively low dosage of cement is beneficial for controlling total shrinkage of cement-stabilized expansive soil, while the dry shrinkage can be further restrained by the addition of fibers.

Notably, at the same cement dosage, specimens containing fibers consistently exhibited significantly lower autogenous shrinkage than their counterparts without fibers. This indicates that the interlocking network of polypropylene fibers effectively restrains the internal volumetric changes caused by the hydration process. Therefore, fiber incorporation is an effective method for controlling not only drying shrinkage but also the autogenous shrinkage inherent to cementitious systems.

3.5. XRD Analysis

Figure 8 presents the XRD patterns for specimens with varying cement dosages, alongside a pure cement reference, after 28 days of hydration. As established in Figure 1, the primary crystalline phase in the raw expansive soil is quartz (SiO₂). With the addition of cement, the characteristic peak intensity of SiO₂ is progressively attenuated. Concurrently, the characteristic peaks of hydration products, such as Portlandite (Ca(OH)₂), become more pronounced with increasing cement content. This signifies a greater degree of hydration, which corresponds to the formation of a more extensive C-S-H gel network. This denser network is directly responsible for the observed enhancements in strength and reductions in drying shrinkage, as previously demonstrated in Figure 4 and Figure 6, respectively.

It is also noteworthy that while Portlandite is a key hydration product, its contribution to the structural network is secondary to that of C-S-H gel. This suggests a potential for further enhancement: introducing supplementary cementitious materials like fly ash or slag could trigger a pozzolanic reaction, consuming the Portlandite to form additional C-(A)-S-H, thereby further boosting the strength and volume stability of the system.

3.6. SEM and EDS Analysis

Since all cement-modified slurries form the same types of hydration products, a representative selection of specimens (S, SF, SC-20, and SFC-20) was chosen for Scanning Electron Microscopy (SEM) analysis, as shown in Figure 9.

Figure 9a reveals that the plain expansive soil (S) consists of loosely aggregated, flaky clay particles and larger sand grains. A poor interfacial transition zone (ITZ), characterized by noticeable pores and a lack of bonding, is evident between the sand grains and the surrounding clay matrix. This loose, weakly connected microstructure is the fundamental reason for the material’s low strength and severe drying shrinkage.

In the fiber-reinforced specimen (SF), shown in Figure 9b, the fibers exhibit almost no effective bond with the soil matrix. Clay particles are merely attached loosely to the fiber surfaces. This weak fiber-matrix interface means the fibers cannot provide sufficient restraint against shrinkage, an observation consistent with the significant drying shrinkage still seen in the SF specimens.

Figure 9c shows the microstructure of the cement-only specimen (SC-20). Here, cement hydration products, including C-S-H gel and Ca(OH)₂ crystals, have formed an interconnected, three-dimensional skeletal network. This rigid network encapsulates and binds the soil particles into a dense, monolithic matrix, which imparts considerable strength and effectively resists shrinkage.

The synergistic interplay between cement and fiber is evident in Figure 9d (SFC-20). Unlike in the SF specimen, the fibers in SFC-20 are intimately integrated into the cement-soil matrix. Hydration products, particularly C-S-H gel, act as a crucial bridging and filling agent, creating a robust fiber-matrix ITZ. This enhanced interfacial bond allows the fibers to more effectively transfer stress and restrain the matrix when shrinkage stresses develop, corroborating the superior volume stability of the composite system.

The EDS analysis in Figure 10 provides elemental evidence supporting the SEM observations. A comparison of the fiber surface chemistry in SF (Figure 10a) and SFC-20 (Figure 10b) shows a significantly higher concentration of calcium (Ca) on the fiber surface in the SFC-20 specimen. This directly confirms that Ca-rich hydration products, primarily C-S-H gel, have precipitated onto and adhered to the fiber surfaces.

This elemental mapping validates, from a compositional standpoint, the formation of a strong interfacial bond mediated by hydration products. This robust ITZ provides effective anchoring points for the fibers to resist matrix deformation. Simultaneously, the fiber network reinforces the continuity and integrity of the cementitious matrix itself. It is precisely this dual-action mechanism—the cementitious products enhancing the fiber-matrix bond, and the fibers reinforcing the cementitious network—that is the root cause of the observed synergistic improvement in the volume stability and mechanical performance of the hardened slurry.

3.7. The Physical–Chemical Coupling Mechanism

Based on the preceding results, it is evident that both cement stabilization (a chemical action) and fiber reinforcement (a physical action) individually influence the shrinkage behavior of expansive soil, yet each possesses inherent limitations. Cement hydration forms a rigid skeleton of C-S-H gel, which significantly enhances strength and suppresses early-age shrinkage [15]. Controlling the drying shrinkage of expansive soil with cement alone often requires large dosages. Because expansive soil absorbs a significant amount of water during mixing, substantial internal stress develops as the water evaporates over time. Consequently, a low dosage of cement is often insufficient to control long-term drying shrinkage. This limitation was evident in the SC-20 specimen, which showed a significant acceleration in its shrinkage rate after 15 days of curing. While a much higher cement dosage can effectively control this shrinkage, this approach leads to prohibitive material costs and a substantial carbon footprint.

Conversely, fibers improve toughness and inhibit crack propagation via their tensile strength. The randomly oriented fiber network can bond expansive soil particles, while the inner shrinkage stress can be restricted by bridging the nascent cracks and transferring stress across the fracture plane. However, fiber reinforcement alone is not sufficient to control shrinkage effectively. After 3 days of curing, the fiber-only (SF) specimen shrank to 92.8% of its original length, which was less than the plain soil (S) specimen (83.6%) but significantly more than the cement-stabilized SC-20 specimen. This limited performance is due to the weak physical bond between the fibers and uncemented soil particles. As SEM analysis revealed, the fiber-matrix interface is unable to transfer enough stress to restrain the significant internal shrinkage forces of the expansive soil matrix.

Research on chemical stabilization shows significant gains in compressive strength, but this approach often suffers from drawbacks such as brittleness or the requirement for high cement dosages that are costly and environmentally burdensome [17,22]. The results of this study demonstrate that the coupled approach can achieve comparable, if not superior, volumetric stability with substantially less cement, directly addressing these economic and environmental concerns. For example, the SFC-20 specimen achieved the same 28-day drying shrinkage control as the SC-60 specimen.

Furthermore, research on fiber-only reinforcement often highlights improved crack resistance, but the overall performance is constrained by the weak interfacial bond between the fibers and the uncemented soil particles [31]. A key advantage of the physical–chemical coupling is that cement hydration fundamentally enhances the fiber-matrix ITZ. As confirmed by SEM and EDS analysis, the C-S-H gel provides a robust anchoring medium for the fibers, allowing them to more effectively bridge microcracks and restrain macroscopic shrinkage. This synergy overcomes the primary limitation of using fibers alone.

Therefore, the superior performance of the composite system stems from this physical–chemical coupling effect, which enables the SFC specimens to exhibit the lowest drying and autogenous shrinkage rates. However, the current study is limited to one fiber type and a 28-day evaluation period. Future work should expand on these findings by investigating a wider range of fiber types and conducting long-term durability tests to fully validate their practical applicability.

4. Conclusions

This study systematically investigated the synergistic effect of a coupled cement–fiber reinforcement on expansive soil slurry, leading to the following conclusions:

The core finding of this study is the pronounced synergistic effect between cement and fibers, which overcomes the individual shortcomings of each modifier. While cement hydration forms a gel network that resists internal shrinkage stress, its effectiveness is highly dosage-dependent. For instance, the SC-20 specimen exhibited a high 28-day drying shrinkage of 1.50%. Achieving long-term volume stability required a much higher, 60% cement dosage, which reduced the shrinkage to 0.53%. On its own, fiber reinforcement was also insufficient, as the fiber-only (SF) specimen displayed notable shrinkage after just 3 days of curing.

The power of their combined action was evident in the results. By adding just 0.5% polypropylene fiber to the 20% cement mix, the 28-day drying shrinkage of the SFC-20 specimen decreased to 0.57%. This superior performance was comparable to the SC-60 specimen and demonstrates a level of shrinkage control far exceeding that of slurries modified by either agent alone.

The underlying mechanism for this synergy was identified through microstructural analysis as a dual-action physical–chemical coupling. Cement hydration not only creates a rigid, strength-providing matrix but also, more importantly, forms a robust interfacial bond that effectively anchors the fibers. This enhanced interface allows the well-anchored fiber network to act as a micro-reinforcement, efficiently bridging cracks and restraining both drying and autogenous shrinkage.

Ultimately, this research provides both a practical solution and a fundamental understanding for the treatment of problematic expansive soils. It validates the cement–fiber composite approach as a promising pathway for developing durable, cost-effective, and low-carbon construction materials from what is often considered engineering waste. The elucidated coupling mechanism offers a theoretical foundation for the future design and optimization of advanced, high-performance geotechnical composites.