Investigation into the Enhancement Characteristics of Fly Ash and Polypropylene Fibers on Calcium Carbide-Residue-Stabilized Soil

Abstract

The recycling and reuse of waste materials is an important part of promoting sustainable development. Encouraged by cleaner production and a circular economy, the introduction of calcium carbide residue (CCR) for the stabilization of soil foundations has become a hot topic in the road engineering industry. Aiming at the efficient application of CCR-stabilized soils, the optimization of the material composition was focused on in this work. Fly ash and polypropylene fibers were introduced into the preparation of CCR-stabilized soils, and their effects on the mechanical properties and water stability were tested. The findings highlight that the strength of fly-ash–carbide-residue-stabilized soil was higher than that of carbide-residue-stabilized soil at the same curing age. Furthermore, the unconfined compressive strength, splitting strength, and water stability of CCR–fly-ash-composite-stabilized soil initially increased and then decreased with a rise in polypropylene fiber content. The peak values of confining compressive and splitting strength were observed when the polypropylene fiber content was 1.2‰, while the water stability coefficient A reached its peak value at 0.8‰. From the standpoint of the comprehensive performance improvement and economy of composite-stabilized soil, it is advised that the dosage of polypropylene fibers falls within the range of 0.8–1.2‰. The engineering technical indexes of polypropylene-fiber–CCR-composite-stabilized soil fulfilled the requirements of the specification and had a satisfactory effect on delaying the cracking of the specimen. It is expected that this investigation will provide support for the resource utilization of CCR and the sustainable development of road construction.

1. Introduction

Calcium carbide residue (CCR) is a typical industrial waste, which has been facing serious problems in recycling, management, and resource utilization [1]. Disposal methods such as stockpiling or landfilling take up a large amount of space resources and pose soil pollution risks, which is not in line with the principles of a circular economy and sustainable development [2,3]. Promoting waste recycling and a lower carbon footprint is a necessity for sustainable development [4]. The aim of this study was to investigate the feasibility of utilizing waste CCR in road construction to reduce the cost of road building while minimizing waste emissions.

Calcium carbide residue (CCR) is the residual residue discharged when hydrolyzing calcium carbide to produce finished products (e.g., acetylene, polyvinyl chloride, polyvinyl alcohol), with its primary component being Ca(OH)₂ [5,6]. Equally important, its composition is comparable to that of lime, while its effective calcium and magnesium contents are high. With the rapid development of the chemical industry, a considerable amount of CCR waste is produced annually with a minimal resource utilization rate. Consequently, the traditional landfill treatment will continue to pollute the ecological environment. Meanwhile, in the domain of road engineering, lime-stabilized soil and lime–fly-ash-stabilized soil have been extensively used [7,8], and the related technology research is relatively mature. Nevertheless, due to the non-renewable nature of limestone resources, lime supplies are becoming increasingly strained. The main chemical composition of CCR is fundamentally the same as that of lime, which makes CCR-stabilized soil promising in road engineering [9]. The application of CCR-stabilized soil in road engineering shows promise, but some deficiencies regarding its strength and water stability still exist. The addition of fly ash is anticipated to enhance its performance. However, there is a lack of related research, and its material composition and performance-boosting effects are yet to be discussed at length.

In recent years, the role of fibers in enhancing the performance of road materials has garnered considerable attention among scholars. Adding a suitable amount of polypropylene fibers to concrete can effectively enhance the cracking resistance, impact resistance, freezing resistance, and fatigue resistance of concrete [10]. Upon using polypropylene fibers (PPFs) to improve the properties of a semi-rigid base material, the low-content PPF semi-rigid base material was observed to have a strong load-holding deformation ability. Additionally, its toughness and crack resistance were notably superior to those of the ordinary base material [11]. By adding fly ash and PPFs to concrete, composite-modified high-performance road concrete was prepared, thereby strengthening its abrasion resistance and extending the service life of the road [12]. Adding PPFs significantly enhanced the resistance to dry and temperature shrinkage cracking of cement-stabilized crushed stone [13]. Moreover, PPFs were integrated to upgrade fly ash soil, and the dynamic characteristics of the improved soil were analyzed. It was found that the dynamic strength of PPF-improved soil was greater than that of fly ash soil under the same confining pressure [14]. The road performance of cement lime soil stabilized sand material could be significantly improved by incorporating PPFs, with the evident advantages being in the crack resistance [15]. The cracking behavior and failure mode of concrete mixed with PPF are analyzed. It is observed that the incorporation of PPFs enhances the mechanical strength and durability of concrete, particularly its tensile strength; however, it does result in a reduction in the workability of the concrete [16]. By incorporating PPFs into geopolymer concrete, the durability and compressive strength of the material are significantly enhanced [17]. The addition of PPFs and fly ash to lime-improved soil has been found to significantly enhance the mechanical properties of subgrade soil [18]. The strength, ductility, and resistance to deterioration of the improved soil were significantly improved by using recycled PPFs to improve loess [19]. By adding calcium carbide slag and polypropylene fibers to black cotton soil, the optimal ratio suitable for roadbed construction is obtained [20].

In summary, there is a dearth of pertinent research on the enhancement of subgrade soil performance through the utilization of CCR, fly ash, and PPF composite curing technology, both domestically and internationally. In this investigation, fly ash and PPFs were successively added to CCR-stabilized soil. The effects of fly ash on the unconfined compressive strength, splitting strength, and water stability of CCR-stabilized soil and PPFs on the calcium–fly-ash-stabilized soil were determined. The performance improvement effects of the two modified materials on the CCR-stabilized soil were assessed, and the optimal content of PPFs was established. This study aimed to provide a reference for the refinement of properties and the popularization of calcium carbide-residue-stabilized soil.

2. Materials and Methodology

2.1. Materials

The soil used in the test was silty clay from northern Shaanxi, China.

Table 1. Properties of the soil sample.

Property	Liquid Limit/%	Plastic Limit/%	Plasticity Index	Specific Gravity	Maximum Dry Unit Weight/(g/cm3)	Optimal Moisture Content/%
Value	30.1	17.8	12.3	2.67	1.915	11.5

itemizes the core technical indicators. The gel materials, CCR, and fly ash were obtained from Shaanxi Beiyuan Chemical Industry Group Co., Ltd. (Xi’an, China).

Table 2. Chemical components of calcium carbide residue.

Chemical Composition	CaO	MgO	Fe2O3	Al2O3	SiO2	Loss on Ignition	Other Components
Content/%	64.93	0.18	0.72	1.40	3.97	22.79	6.01

and

Table 3. Base composition content of fly ash.

Chemical Composition	SiO2	Al2O3	Fe2O3	MgO + CaO	SO3	Loss on Ignition	Other Components
Content/%	50.2	22.2	3.0	10.1	4.0	9.4	1.1

show the main physical and chemical properties. Figure 1 illustrates the reinforcing PPFs, and

Table 4. Physical and mechanical parameters of polypropylene fibers.

	Average Diameter/mm	Average Length/mm	Tensile Strength/MPa	Elastic Modulus/MPa	Density/(g/cm3)	Melting Point/°C	Acid and Alkali Resistance
Bundled monofilament	0.045	25	560	3500	0.91	165	better

exhibits their physical and mechanical properties.

2.2. Preparation of Samples

To analyze the enhancement effect of fly ash and fibers on the performance of CCR-stabilized soil, three groups of specimens were designed for the performance test. The ratios for lime- and ash-stabilized soil were set with reference to the “Technical Guidelines for the Construction of Highway Roadbases” (JTG/T F20-2015, Chinese national specification) [21]. The first comprised three types of CCR-stabilized soil specimens: the CCR content was 9%, 11%, and 13%, which were recorded as CS-1, CS-2, and CS-3, respectively. The second encompassed three kinds of CCR–fly-ash-composite-stabilized soil specimens: (a) the weight ratio of CCR to fly ash was 1:1, and the glue (CCR and fly ash)-to-soil ratio was 20%; (b) the weight ratio of CCR to fly ash was 1:3, and the glue–soil ratio was 20%; (c) the weight ratio of CCR to fly ash was 1:3, and the glue–soil ratio was 30%, which were recorded as CFAS-1, CFAS-2, and CFAS-3. It must be noted that CS, CFAS, and LFAS represented CCR-, CCR–fly-ash-, and lime–fly-ash-stabilized soil, respectively. The third was the fiber-reinforced CCR–fly-ash-stabilized soil specimens, in which the optimal proportion of CCR and fly ash was determined through optimization.

After drying, crushing, and removing impurities through a 2.36 mm sieve, the test soil was weighed and mixed evenly prior to being simmered for 12 h according to the above test scheme. The 50 × 50 mm specimen, formed by the static pressure method, should be numbered after desiccation. The quality and dimensions of the specimens were measured, respectively, with a balance with an accuracy of 0.01 g and a vernier caliper. The compactness of the actual formed specimen was then calculated. Typically, the compactness of the specimen after formation does not exceed ±1% of the standard compactness. The compactness of the specimens used in the test was 97%. The specimens were sealed with plastic film and stored in a climate-controlled box (temperature 20 ± 2 °C, humidity ≥ 95%) for 6 days. On the 7th day, the specimens were removed and filled with water at 20 °C for 24 h (Figure 2).

2.3. Test Method

2.3.1. Strength Test

According to the “Test Methods of Materials Stabilized with Inorganic Binders for Highway Engineering” (JTG E51-2009, Chinese national specification) [22], unconfined compressive strength and splitting strength tests were conducted using the UTM-5000 material testing machine. The unconfined compressive strength of the specimen, R_c, was calculated using Equation (1).

$$R_c=\frac{P}{A}$$

(1)

where P is the maximum load when the specimen is damaged (N), and A is the cross-sectional area of the specimen (mm²).

The phase growth rate of the unconfined compressive strength is expressed as

$$r_c=\frac{R_{t,i}-R_{t,0}}{R_{t,0}}\times 100\%$$

(2)

where R_t,i and R_t,0 represent the unconfined compressive strength (MPa) of specimens and the unconfined compressive strength (MPa) of raw CCR specimens, respectively, for fly ash or PPF i dosing at the t age of regeneration.

The specimen splitting strength (indirect tensile strength) was obtained using

$$R_i=\frac{2P}{\pi dh}\left(\sin 2\alpha -\frac{a}{d}\right)$$

(3)

where R_i denotes the indirect tensile strength of the specimen (MPa), d is the diameter of the specimen (mm), a represents the width of the compression strip (mm), α is the circle center angle corresponding to the width of the half compression strip (°), P indicates the maximum load at the time of the specimen’s destruction (N), and h is the height of the specimen after recuperation (mm).

2.3.2. Water Stability Test

The unconfined compressive strength was measured in specimens with standard curing for 3 days and immersion for 4 days. The water stability of the specimen is characterized by the water stability coefficient A:

$$A=\frac{R_{c,w}}{R_c}$$

(4)

where R_c denotes the unconfined compressive strength (MPa) of a standardized corrosion test specimen, and R_c,w is the unconfined compressive strength (MPa) of an immersion corrosion test specimen.

3. Strengthening Effect of Fly Ash on the Stabilization Soil with CCR

3.1. Influence of Fly Ash on the Strength of CCR-Stabilized Soil

The differences in the unconfined compressive strength and splitting strength test results of CS and CFAS specimens at various ages (7, 14, 28, 60, 90, 180 d) were compared and examined. Figure 3 and Figure 4 display the test results.

Figure 3 and Figure 4 underline that with the increase in the age of the specimens, the unconfined compressive strength and splitting strength of CS and CFAS specimens increase. After adding fly ash, both strengths increased significantly. In contrast to the CS specimens, the unconfined compressive strength and splitting strength of CFAS specimens experienced a greater increase, and the gap between the two increased even more in the later stage, indicating that the addition of fly ash significantly enhanced the unconfined compressive strength of CCR-stabilized soil. In this case, at a rapid rate, the strength continued to increase along with the increase in age. This is due to the high contents of SiO₂ and Al₂O₃ in fly ash causing a more intense volcanic ash reaction, and the C-S-H gel generated by the reaction effectively filled the distributed internal voids. This increased the stability of the soil particles, thus improving the strength of CCR-stabilized soil.

The addition of fly ash can improve the reactivity of CCR and soil particles, but the pozzolanic effect was not evident during the early stage, so the initial strength did not change with the increase in fly ash content. With the growth in age, the final strength value rose significantly. For CFAS specimens with CCR–fly-ash-stabilized soil, despite the ratio of CCR to fly ash being the same, the unconfined compressive strength and cracking strength of CFAS-3 specimens with a 30% cement–soil ratio were significantly lower than those of CFAS-2 specimens with a 20% cement–soil ratio. In other words, a higher cement–soil ratio did not improve the strength. Coupled with that, the appropriate cement–soil ratio should be selected in engineering.

For CFAS-1 and CFAS-2 specimens with a 20% gum–soil ratio and 1:1 and 1:3 dosage ratios of CCR to fly ash, respectively, the variation in the growth rate of unconfined compressive strength with age is shown in

Table 5. Growth rate (r_c, calculated by Equation (2)) of strength of calcium carbide–fly-ash-stabilized soil.

Ratio of Calcium Carbide Residue to Fly Ash Mixing Ratio		rc (%)
		7–14 d	14–28 d	28–60 d	60–90 d	90–180 d
Rc	1:1	10.7	33.9	53.6	17.6	18.0
	1:3	16.0	48.6	66.7	16.7	16.5
Rf	1:1	34.2	49.1	71.3	17.8	16.9
	1:3	29.2	44.0	71.4	22.7	15.3

. The strength improvement rates of the dosage ratio 1:3 compared with the 1:1 dosage ratio of CCR to fly ash are also listed.

Table 5 reveals that, with the same clay–cement ratio, various amounts of fly ash feature different improvement effects on CCR-stabilized soil. The growth rates of compressive strength and splitting strength of the specimens throughout the maintenance period of 180 d exhibited a law of initially increasing and then decreasing. The r_c of the specimens reached the maximum in 28–60 d, and the growth rate gradually slowed down over the 60–90 d and 90–180 d periods. Meanwhile, the r_c growth of the specimens with a dosage ratio of 1:3 was greater than those with a dosage ratio of 1:1 in the initial and middle periods. In the later periods, the growth changes in the two specimens were small.

The addition of fly ash effectively filled the internal voids of the CCR-stabilized soil, but at the initial maintenance age, the strength composition of the specimen remained dominated by ion exchange flocculation and carbonization crystallization between lime or CCR and the soil particles. Incorporating fly ash raised the initial activation energy of the reaction, though the volcanic action was not discernible. With the growth in age, the volcanic action continued to absorb water, the reaction intensified, and the strength of the stabilized soil increased at an accelerated rate. With the continuous growth in age, the volcanic-action-generated crystals did not suffice in providing the strength corresponding to the age change. Simultaneously, the slowing rate of volcanic action and the long duration led to a significant decrease in the strength growth rate of the stabilized soil. Further extension of the maintenance age caused the formation of hydrated calcium silicate and ettringite crystals by volcanic action to become the primary source of strength of the CCR-stabilized soil. At the same time, a concentration difference in the CCR-stabilized soil was formed due to the consumption of chemical reactants, and its strength growth rate slightly increased.

3.2. Effect of Fly Ash on the Water Stability of CCR-Stabilized Soil

The strength attenuation changes of CS, LS, CFAS, and LFAS samples were investigated by changing the dry and wet conditions of health preservation. The water stability properties of CCR- and CCR–fly-ash-composite-stabilized soil were systematically evaluated according to the test results. Moreover, the effects of CCR and lime materials on the quality stability of loess were compared and analyzed.

Table 6. Water stability test results for CS and LS specimens.

Advanced Stage of Life	Type	A			B			C
		Standard	Immersion	W	Standard	Immersion	W	Standard	Immersion	W
7	CS	0.93	0.85	0.91	0.92	0.85	0.92	0.91	0.86	0.95
	LS	0.97	0.91	0.94	0.94	0.88	0.94	0.95	0.88	0.93
14	CS	1.19	1.04	0.87	1.09	0.98	0.90	1.19	1.09	0.92
	LS	1.23	1.09	0.89	1.24	1.11	0.90	1.21	1.09	0.90
28	CS	1.47	1.3	0.88	1.4	1.24	0.89	1.46	1.30	0.89
	LS	1.57	1.42	0.90	1.55	1.37	0.88	1.52	1.36	0.89
60	CS	1.6	1. 44	0.90	1.52	1.38	0.91	1.66	1.50	0.90
	LS	1.9	1.72	0.91	1.91	1.73	0.91	1.73	1.56	0.90

Notes: Letters A, B, and C represent the amounts of CCR or lime added to the stabilized soil, i.e., 9%, 11%, and 13%, respectively.

presents the test results.

According to Table 6, under the condition of immersion curing, the unconfined compressive strength of test CS and LS specimens still increased with the increase in curing age. However, the increase in range was smaller than that of standard curing to the same age. In this case, the growth rate of the unconfined compressive strength of CS and LS specimens under the condition of immersion was significantly lower than that of standard curing specimens. The strength of every group of CS specimens with standard curing for 4 d and immersion curing for 3 d (7 d immersion) was greater than R_c, _m = 0.7 MPa, fulfilling the design requirements for the structural strength of the secondary and secondary road base (Table 6). By evaluating the water stability coefficient, it was found that the water stability coefficients of CS-(A, B, C) and LS-(A, B, C) specimens under the test conditions varied within the ranges of [(0.87–0.91), (0.89–0.92), (0.89–0.95)] and [(0.89–0.94), (0.88–0.94), (0.89–0.93)], respectively. Generally, the water stability coefficient of CS and LS specimens was relatively stable, and the CCR-stabilized soil had water stability close to that of lime-stabilized soil.

Based on Figure 5, the change rules of unconfined compressive strength with the age of the test CFAS and LFAS specimens under the condition of water immersion were relatively similar. Also, the water stability coefficients of CFAS and LFAS specimens ranged between 0.92 and 0.97, which showed signs of improvement compared with those of CS and LS specimens from the quantitative point of view. At the same time, for CFAS and LFAS specimens, with the prolongation of age from 0 to 60 d, the coefficient of water stability constantly approached 1, implying that the CFAS and LFAS immersion strength was constantly reaching the standard strength, which is mainly due to the volcanic ash reaction with the prolongation of age. This progressively played a dominant role in enhancing the strength of the reaction, which generated C-S-H and calcium alumina Aft.

The reaction generated C-S-H and calcium alunite Aft, tightening the connection between the particles of the specimen. Along with that, the voids were gradually filled by crystals, and the water absorption was substantially reduced. Therefore, the inhibitory effect of immersion on the strength development of the specimen was steadily weakened. Both the CFAS and LFAS specimens could be immersed for a longer time compared with the CS and LS specimens.

During the test, it was observed that the CS, LS, CFAS, and LFAS specimens remained intact, and no collapse or break occurred throughout the immersion maintenance. According to the data, the immersion condition showed a certain inhibitory effect on the strength growth of stabilized soil, mainly because the immersion condition diluted the internal Ca²⁺ concentration of the specimen, the ion exchange rate declined, and the hardening process decelerated. Simultaneously, when the specimen was under the immersion condition, the CO₂ concentration required for the internal reaction decreased, and the carbonation process weakened. On the other hand, it was demonstrated that CCR, lime, CCR–fly ash, and lime–fly ash could react with the soil particles to form gel polymers during the maintenance of the test specimen. At the same time, the strength growth rate of the specimen under the immersion condition was lower than that under the standard maintenance condition. However, with the extension of age, the reaction-generated condensate continued to fill the gaps between the binders, and the inhibitory effect of the specimen’s moisture absorption on the strength development was diminished. Generally, under the premise of having initial strength, the CCR-stabilized and the CCR–fly-ash-stabilized soils were placed under full immersion, and their strength development still occurred, with their stability comparable to that of lime-stabilized soil. Moreover, in contrast to the CCR–fly-ash-stabilized soil, when the CCR-stabilized soil is utilized for the road base and bottom base filling, long-term immersion conditions should be avoided as far as possible.

4. Strengthening Effect of Fibers on CCR–Fly-Ash-Composite-Stabilized Soil

4.1. Effect of Fiber Dosage on Unconfined Compressive Strength

Figure 6 illustrates the influence of the incorporation amount of PPFs on the unconfined compressive strength of CCR–fly-ash stabilized soil.

As depicted in Figure 6, the addition of PPFs could effectively improve the unconfined compressive strength of fiber–carbide-residue-stabilized soil. At 0.12% fiber content, the percentage of fibers increasing the unconfined compressive strength of the specimen reached the peak. Under the condition of 7% content of carbide residue, the unconfined compressive strength of fiber-stabilized soil specimens at 7, 14, and 28 d ages increased by 15.3, 15.0, and 14.9%, respectively, compared with that of un-doped fiber specimens. For the carbide-residue-stabilized soil specimens with 9% content, the percentages of fibers fortifying the strength of the specimen at 7, 14, and 28 d ages were 6.5, 11.2, and 11.0%, respectively.

Incorporating PPFs effectively filled the pores in the mixture, thereby increasing the density of the CCR–fly ash soil specimen. Concurrently, with the cementation reaction of CCR and soil particles, the crystals generated in the soil continued to increase, and the pores in the mixture were squeezed even further. Given the relatively large surface area of the fibers, having the appropriate amount of fibers in the mixture forms a grid anchoring structure with soil particles, CCR particles, fly ash, and the generated gel. This intensifies the friction and cementation between particles, restricts the deformation of soil particles, and effectively increases the specimen’s strength. Under the action of an external load, the contribution of fibers to CCR soil is essentially to enhance the friction resistance between particles. When the fiber content is lacking, the establishment of crosslinking and overlapping between fibers is not feasible. On the contrary, with an excessively high fiber content, the uniformity of the fiber distribution in the mixture becomes extremely difficult to control, and agglomeration easily forms between fibers. This reduces the crosslinking and embedding effects of fibers on the mixture. Therefore, only when the fiber content in the mixture is at an optimal level can both effects on the mixture particles be maximized.

4.2. Effect of Fiber Dosage on Splitting Strength

The splitting strength reflects the material’s tensile properties. Figure 7 displays the changing regularity of the splitting strength of CCR-stabilized soil with the PPF dosage.

As exhibited by the test curve in Figure 7, adding PPFs also improved the splitting strength of stabilized soil specimens. The data chart emphasizes that when the PPF dosage was 0.12%, the splitting strength of CCR–fly-ash-stabilized soil specimens reached the peak, and its splitting strength at 7, 14, and 28 d was 1.08, 1.21, and 1.18 times that of CCR-stabilized soil without fiber addition, respectively. This case is similar to the change rule of the unconfined compressive strength of specimens after fiber addition. When the dosage of fibers exceeded the optimal dosage, the splitting strength of fiber–calcium carbide-residue-stabilized soil decreased accordingly.

The doped fibers significantly reduced the porosity of the stabilized soil specimen. Simultaneously, the doped PPFs delayed the failure time of the specimen in the indirect tensile test.

Table 7. The displacement results corresponding to the point at which maximum force appears.

	CCR Content, 7%					CCR Content, 9%
Fiber content/%	0	0.04	0.08	0.12	0.15	0	0.04	0.08	0.12	0.15
Displacement at the onset of maximum force (mm)	0.239	0.383	0.462	0.528	0.541	0.231	0.365	0.470	0.524	0.531

details the displacement results corresponding to the point where the maximum force appeared in the splitting strength test. With the increase in the PPF dosage, the time at which the maximum force appeared under the load was gradually prolonged. Despite the strength decreasing after the fiber dosage exceeded the optimal dosage of 1.2‰, the time and displacement corresponding to the maximum force continued to increase, indicating that the doped PPFs significantly improved the load failure time of the stabilized soil and enhanced the toughness of the stabilized soil with CCR. Figure 8 shows the split failure graphs of the stabilized soil with and without doped fibers, respectively. From the failure graphs and test data analysis, the doped PPFs distributed in the soil also played a role in the increasing friction. In this case, when the specimen is in a unidirectional tensile state, the external doped fibers distributed in the soil form a friction anchoring system with the soil, which can take part in buffering the failure of the specimen in the tensile failure process. Therefore, appropriately doped PPFs can also delay the failure time of the specimen.

4.3. Effect of Fiber Dosage on Water Stability

In this test, health-preserving 7 d specimens were selected as the research object, and the unconfined compressive strength of the 7 d specimens under standard health-preserving and immersed water-preserving conditions was assessed (Figure 9).

The data showed that the water stability coefficient of the specimens increased with the addition of PPFs. When the CCR content was 9%, the water stability coefficient of the soil without fibers was 1.28, and when 0.08% fiber content was added, the water stability coefficient could reach 0.94. The findings underscore that adding fibers can improve the water stability of CCR because it effectively reduces the pores in the mixture, thereby increasing the density of the stabilized soil. While immersed, the relative water absorption rate of the specimen declined, and the influence of external water on the strength of the specimen weakened. As illustrated in Figure 7, when the fiber content was excessively high, the fibers were stacked and agglomerated with each other. Due to the lack of bonding and fixation of soil particles and cementitious materials in the fiber agglomeration area of the specimen, when immersed in water, it became overly easy for water to enter the weak layer of the fiber stack. Subsequently, a layer of lubricating water film was formed, reducing the anchoring and frictional effects between fibers and stabilized soil particles. Next, the water stability coefficient of the specimen was reduced. Therefore, fibers with the appropriate content significantly improved the water stability of CCR-stabilized soil.

5. Conclusions

Oriented toward the value-added conversion of waste and the sustainable development of road construction, this study investigated the effects of fly ash and polypropylene fibers on the performance improvement of CCR-stabilized soils, and the following conclusions were obtained.

The incorporation of fly ash enhances the volcanic ash reaction, further filling the pores of the soil and effectively boosting the strength of stabilized soil and the ability to resist freeze–thaw cycles of CCR. The enhancement effect was associated with the proportion of fly ash and CCR and the ratio of clay–clay.

The unconfined compressive and splitting strength of CCR–fly-ash-composite-stabilized soil were significantly enhanced by adding PPFs. The improvement effect was optimal when the fiber content was 0.12%, which can effectively augment the strength and toughness of stabilized soil.

PPFs can effectively curb the deformation of CCR soil particles and reduce the particle gaps in the mixtures. In this way, soil particles become crosslinked with and embedded in each other, and the water stability of stabilized soil is improved.