Study on the Micro Mechanism of Failure Evolution of Desulfurization Gypsum–Fly Ash Fluidized Lightweight Soil Based on Discrete Element Method

Abstract

To investigate the macroscopic mechanical properties and failure evolution mechanism of desulfurization gypsum–fly ash fluid lightweight soil, a microscale numerical model using PFC2D (Particle Flow Code) was constructed. Uniaxial compression tests were conducted to determine the microscopic parameters of the model, extracting information on the discrete fracture network type, quantity, age, and particle displacement trend. The crack morphology and propagation evolution of desulfurization gypsum–fly ash fluid lightweight soil were explored, and the destructive properties of desulfurization gypsum–fly ash fluid lightweight soil material were evaluated through energy indicators. The research findings suggest that the discrete element numerical model effectively simulates the stress–strain curve and failure characteristics of materials. Under uniaxial compression conditions, microcracks dominated by shear failure occur in the initial loading stage of desulfurization gypsum–fly ash fluid lightweight soil, with a through crack dominated by tensile failure appearing once the load exceeds the peak stress. The dissipated energy evolution in the flow state of desulfurization gypsum–fly ash fluid lightweight soil is relatively gentle, leading to delayed cracking after surpassing the peak stress point.

1. Introduction

Desulfurization gypsum–fly ash flowable lightweight soil is a composite material comprising industrial components like fly ash, desulfurization gypsum, cement, and soil. It exhibits high fluidity and density, effectively addressing uneven settlement issues caused by varying strength and stiffness of materials in roadbed construction. This material offers clear advantages for future roadbed development. A stable, robust roadbed is crucial for ensuring road surface quality. There has always been a phenomenon of “heavy road surface, light roadbed”, which has led to frequent pavement problems caused by roadbed diseases [1]. Therefore, evaluating the macroscopic strength and material damage evolution of desulfurization gypsum–fly ash fluid lightweight soil is of significant importance.

In recent years, numerous scholars have extensively researched cement–fly ash materials. Yuan Xiaoya et al. [2] found that fly ash has the characteristics of promoting the flocculation structure and particle diffusion of cement particles, which can improve the internal microstructure of cement-based materials and enhance their mechanical properties. Jia Yan’s research [3] shows that cement–fly ash slurry has the characteristics of strong fluidity and high strength after setting, and can be used as a material for backfill of abutments. Huang Zhiqin [4] discovered that incorporating fly ash as an additive into red bed mudstone can effectively enhance soil particle agglomeration and significantly improve the soil’s deformation resistance. Due to the excellent crack resistance of fly ash concrete, many experts and scholars have recently shifted their focus towards increasing the use of fly ash in concrete [5,6,7,8]. Research by Ma Chengchang et al. [9] shows that under the same loading age, the tensile deformation of concrete increases with the increase in fly ash content. On a microscopic level, Liu Jun et al. [10] utilized scanning electron microscopy, X-ray diffraction analysis, and other microscopic methods to investigate the mechanical properties of lightweight aggregates under varying curing conditions. Wang Jiabin et al. [11] analyzed the correlation between the mechanical performance indicators of the multi-component gel material in the fly ash–cement–slag system. Their study revealed an exponential relationship between splitting compressive strength and compressive strength, establishing a mechanical performance model that considers the aggregate replacement rate.

Numerical simulation methods are a crucial tool for theoretical analysis in road engineering, allowing for the evaluation of macroscopic mechanics and mechanical response laws of structures. Zhang Yang et al. [12] introduced a constitutive relationship model for fly ash concrete based on uniaxial compression tests, which was validated through a finite element method. Petcherdchoo et al. [13] utilized the finite element method to develop a differentiated constitutive model, revealing that the initial shear modulus and unconfined compressive strength of Bangkok soft clay, enhanced by ordinary Portland cement and fly ash, increased with curing time. Cracks are identified as the primary cause of material performance deterioration [14,15,16,17]. In terms of crack evolution laws, Liang Dongxu [18] conducted crack propagation experiments on prefabricated cross-fractured rock samples, demonstrating that the initiation and aggregation stress of rock cracks escalate with the angle between the main crack and the axial load.

The material structure of desulfurization gypsum–fly ash fluidized lightweight soil comprises interacting particles, although the microstructure exhibits discontinuity. Existing continuous element constitutive models based on the finite element method struggle to accurately depict the failure development of granular materials, leading to an unclear understanding of the material’s failure mechanism [19]. This study presents a case analysis utilizing the discrete element method (DEM) numerical simulation to investigate the mechanical behavior of desulfurization gypsum–fly ash fluidized lightweight soil under uniaxial compression testing. The research aims to elucidate the failure mechanism of the material, assess crack evolution in conjunction with energy variations, and explore particle motion characteristics during testing. A two-dimensional discrete element numerical model was established based on a specified gradation, followed by the determination of micromechanical parameters for the contact model. The study analyzed strain and stress from numerical simulations and indoor experiments, focusing on crack evolution and its impact on the fracture surface. Cracks in the material were represented by a discrete fracture network (DFN) during numerical simulations of the failure process, enabling the identification of the failure mode of desulfurization gypsum–fly ash fluidized lightweight soil. The evaluation of damage in the material was conducted using energy indicators.

2. Materials and Methods

2.1. Material Parameters

The material utilized in this experiment is secondary fly ash (FA) from Shandong Huaneng Jinan Huangtai Power Generation Co., Ltd. (Jinan, China), with pertinent parameters detailed in

Table 1. The main chemical composition of fly ash.

Chemical Composition	Fe2O3	CaO	SO3	Al2O3	SiO2
Mass fraction/%	7.50	4.63	1.50	23.67	56.96

and

Table 2. Performance indicators of fly ash.

Density/(kg·m−3)	Specific Surface Area/(m2·kg−1)	Fineness/%	Loss on Ignition/%
2389	467	16.8	7.5

. The cement used is P·O 42.5 ordinary Portland cement, taken from Jinan, China, with related parameters presented in

Table 3. Performance indicators of cement.

Water Requirement of Normal Consistency/%	Setting Time/min		Flexural Strength/MPa			Compressive Strength/MPa
	Initial	Final	3 d	7 d	28 d	3 d	7 d	28 d
26.9	205	265	5.6	7.5	9.6	30.2	36.7	48.4

. Desulfurization gypsum (DG), a byproduct of flue gas desulfurization at Shandong Huaneng Jinan Huangtai Power Generation Co., Ltd. (Jinan, China), was employed. The silt was sourced from the trial section of the Beijing–Taiwan Expressway reconstruction and expansion project(Jinan, China), with related parameters presented in

Table 4. The grading criteria of silt.

Constrained Diameter d60	Effective Size d10	d30	Non-Uniformity Coefficient Cu	Curvature Coefficient Cc
0.092 mm	0.013 mm	0.087 mm	7.1	6.3

(the data in Table 1, Table 2, Table 3 and Table 4 are provided by the manufacturer).

2.2. Uniaxial Compressive Strength Test

Through SEM (Shandong Wusheng Information Technology Co., Ltd., Jinan, China) analysis, the particle sizes of each material were obtained as follows: the particle size of fly ash is between 0.92 µm and 31 µm; the particle size of desulfurization gypsum is between 90 µm and 200 µm; the particle size of silt is between 0.002 mm and 1 mm. Because the particle size of each raw material is small, it is easy to agglomerate in the preparation process, but it is difficult to mix water into the solid material and it is difficult to stir the mixture evenly. Following extensive preliminary trial mixing experiments, the subsequent mixing steps were employed in this study, as illustrated in Figure 1.

Firstly, accurately weigh raw materials according to the design mix ratio. The weighed fly ash, desulfurized gypsum, cement and silt are poured into the mixer for dry mixing for about 1 min. Next, pour half of the weighed water and stir for 30 s, followed by adding the remaining water and mixing for about 3 min to create a desulfurization gypsum–fly ash fluidized lightweight soil slurry. Pour the slurry into a test mold for casting. By adding water in stages, the risk of layering solid materials and water (making it difficult to mix) is minimized, and it also prevents splashing during mixing. For small batches, a cement paste mixer is used, while a vertical concrete mixer is employed for larger quantities. The mix ratio test plan for desulfurization gypsum–fly ash fluidized lightweight soil is detailed in

Table 5. Mix ratio test plan.

Sample No.	m(FA):m(DG)	Mass Fraction of Cement/%	Mass Fraction of Silt/%
A1	1:1	6	20
A2	1:1	8	20
A3	1:1	10	20

, with 3 samples in each group.

Cylindrical specimens measuring 50 mm in diameter and 100 mm in length were utilized to generate the stress–strain curve through uniaxial compression. These samples were cured in a standard environment for 28 days before testing. Following a specific mix ratio, cylindrical specimens of the same dimensions were prepared for the compression tests. The tests were carried out using an MTS810 electro-hydraulic servo testing machine, with a constant loading rate of 1 mm/min. The computer recorded the axial load and displacement during the tests. Loading was halted once the strain rate reached 5%, at which point the stress–strain curve of the specimen was determined.

2.3. Establishment of Discrete Element Test Piece Model

In this study, a 50 mm × 100 mm discrete element particle model is created using circular shapes in PFC. To ensure the proper functioning of PFC2D, material gradation is first imported using the table command. Subsequently, the discrete element particle model with the corresponding gradation is generated using the ball distribute command. The upper and lower loading plates are created with the clump command, with the upper loading plate applying axial force. Displacement loading is employed to simulate the uniaxial compression process of desulfurization gypsum–fly ash fluidized lightweight soil, with the loading plates being vertically loaded at a rate of 1 mm/min.

The parallel bonding model is used to represent the contact model of desulfurization gypsum–fly ash fluidized lightweight soil, as shown in Figure 2. This contact model consists of a bond model and a linear elastic model. When the bond force exceeds the bond strength, the bond model is fractured, and the stiffness related to the bond force and moment is removed from the model. However, the linear contact component always exists, and when all bonds are fractured, it can be considered as an unbonded linear elastic material.

Previous studies have demonstrated the challenges in directly determining the mesoscopic contact mechanical parameters of discrete element models in laboratory settings. Typically, a ‘trial and error method’ is employed to calibrate material parameters to ensure consistent simulation results with laboratory measurements. Building on this prior research, initial mesoscopic parameters of the contact model were established [20], followed by a calibration of material parameters based on uniaxial compression test results. The final mesoscopic parameters of the contact model for desulfurization gypsum–fly ash fluidized lightweight soil are detailed in

Table 6. Calibration results of micromechanical parameters.

Parameter	Silt	Cementitious	Silt–Silt	Silt–Cementitious	Cementitious–Cementitious
Effective modulus E∗/Pa	2.3 × 108	1.0 × 108	-	-	-
Normal-to-shear stiffness ratio k	1.6	1.5	-	-	-
Bond effective modulus $\overline{E}$∗/Pa	-	-	2.0 × 108	2.0 × 108	2.0 × 108
Bond normal-to-shear stiffness ratio k*	-	-	1.5	1.5	1.5
Tensile strength $\overline{\sigma }$c/Pa	-	-	1.6 × 107	1.6 × 107	1.6 × 107
Cohesion $\overline{c}$/Pa	-	-	2.3 × 107	2.3 × 107	2.3 × 107
Friction coefficient μ	0.7	0.5	0.5	0.5	0.5

The “*” indicates that the contact method in numerical simulation is adhesive contact.

. Given that the contact between silts mirrors the interaction of cementitious bonding on the silt surface, with all adhesive forces stemming from the cementitious material, the contact parameters for all particle interactions (silt–silt, silt–cementitious material, cementitious material–cementitious material) in this study remain constant.

During the discrete element simulation, the velocity of the upper loading plate was set to be consistent with the laboratory test of the uniaxial compression test model of desulfurization gypsum–fly ash fluidized lightweight soil. The contact force and displacement of the loading plate can be converted into the stress and strain of the specimen during the loading process. In the discrete element model of the specimen, all particles are applied with parallel bond contacts. The adhesion state of the bonded particles is monitored during the loading process. When the adhesion force is greater than the tensile strength or shear strength, the bond breaks. At this time, a DFN is inserted at the contact surface between the particles to represent a crack, and the direction of the DFN is parallel to the contact surface. During the loading process, the number of cracks is recorded in real time through the DFN, and the properties of particles at both ends of the broken bond contact are determined, and the type of crack is analyzed to evaluate the damage situation.

3. Results

3.1. Formation Mechanism of Strength of Desulfurization Gypsum–Fly Ash Fluidized Lightweight Soil

The mechanism behind the strength development of desulfurized gypsum and fly ash fluidized lightweight soil is intrinsically linked to the formation of the microstructure within the system composed of composite cementitious materials and silt. Desulfurized gypsum primarily consists of CaSO₄·2H₂O. On the one hand, the sulfate ions (SO₄²⁻) can promote the breaking of Si-O and Al-O chemical bonds on the surface of fly ash ball particles, and on the other hand, it can react with AlO^2- and Ca²⁺ to form ettringite, thereby reducing the AlO²⁻ content within the structural system and further aiding in the disintegration of the fly ash particles. Additionally, desulfurized gypsum provides ample Ca²⁺ to the reaction system, enhancing the formation of additional cementitious materials. The primary reactions involving desulfurized gypsum are outlined below:

$${{\mathrm{Ca}}^{2+}+\mathrm{Al}}_2{\mathrm{O}}_3+{\mathrm{OH}}^{-}+{\mathrm{SO}}_4^{2-}\to 3\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·3\mathrm{Ca}{\mathrm{SO}}_4·32{\mathrm{H}}_2\mathrm{O}$$

(1)

To explore in depth the intrinsic mechanism and process of strength formation of fluidized lightweight soil under the combined action of desulfurization gypsum and fly ash, samples from group A2 specimens were tested using SEM (Scanning Electron Microscope) at 28 days of curing, as illustrated in Figure 3.

As depicted in Figure 3, at a curing age of 28 days, a large amount of flocculent material appeared in the lightweight soil structure of desulfurization gypsum–fly ash flow state. The fly ash, desulfurized gypsum, and silt particles have been completely wrapped in a continuous mesh structure. Moreover, the presence of needle-like ettringite within the structure confirmed that the desulfurized gypsum has also participated in the reaction. Different particles are linked and overlapped by flocculent substances to form a whole, thereby increasing the bond strength and enhancing the density of the structure.

3.2. Stress–Strain Curve

The stress–strain curve is the most basic indicator for describing the mechanical properties of materials, which can well reflect the strength and deformation characteristics of desulfurized gypsum–fly ash lightweight soil flow. Therefore, exploring the stress–strain relationship and failure mode of desulfurized gypsum–fly ash fluidized lightweight soil during loading is of great significance for fully understanding the mechanical properties of desulfurized gypsum–fly ash fluidized lightweight soil.

Figure 4 depicts the stress–strain curve of desulfurization gypsum–fly ash fluidized lightweight soil across varying cement contents. The evolution process can be broadly categorized into four distinct stages, as indicated by the segmentation points on the A3 curve.

For clarity, only the A3 curve is segmented and annotated. Here is a breakdown of the specific stages:

(1)

Linear Elastic Stage: This corresponds to the OA segment in the figure. During the loading process of desulfurized gypsum–fly ash fluidized lightweight soil, the initial stage of its stress–strain relationship curve shows an approximately linear characteristic, indicating that the material is undergoing a significant hardening process during this stage. As the strain increases, the stress rapidly rises, showing a faster growth rate. Point A represents the proportional limit, where there is a good linear relationship between stress and strain.

(2)

Plastic Yield Stage: This stage is represented by the AB segment in the figure. At this stage, the slope of the curve begins to decrease, the line segments tend to flatten, the growth rate of strain begins to accelerate, and the growth rate of stress decreases. At this point, cracks and defects appear inside the specimen, and its stress level is close to the threshold, with point B being the peak stress.

(3)

Failure Stage: This phase is characterized by the BC segment in the figure. The stress–strain curve begins to decrease, and as the strain increases, the stress continuously decays. At this stage, through-cracks appear inside the specimen and are gradually destroyed.

(4)

Residual Strength Maintenance Stage: The CD segment in the figure represents this final stage, where the stress–strain curve approximates a horizontal line. During this phase, the stress remains at a relatively low and constant level, while the strain continues to increase. The stress at point D corresponds to the residual strength of the soil.

As shown in Figure 3, in the rising section of the stress–strain curve of desulfurized gypsum–fly ash fluidized lightweight soil, the effect of cement content is not significant. However, with the continuous increase in cement content, the falling section of the stress–strain curve begins to slow down significantly, and the downward opening of the curve gradually increases. This indicates that with the increase in cement content, the ductility of desulfurized gypsum–fly ash fluidized lightweight soil has been significantly improved. In addition, the compressive strength of desulfurized gypsum–fly ash fluidized lightweight soil fully meets the requirements of backfill strength, which can effectively avoid the problem of excessive differential settlement between the roadbed and the structure.

3.3. Comparison of Experimental Results

Figure 5 presents a comparison between stress–strain curves from uniaxial compression tests and numerical simulations, showing consistent failure modes. Prior to reaching peak stress, both laboratory tests and simulations exhibit similar trends. However, post-peak stress, the strain in numerical simulations is slightly smaller than in laboratory tests. Previous research [21] attributes this difference to the simulation of cementation in discrete element specimens using parallel bonding parameters. Upon surpassing peak stress, the internal parallel bonds in the specimen break and vanish. In laboratory tests, despite cracked cement paste losing some adhesion, residual connections remain, maintaining engagement of the uneven fracture surface for skeleton support and friction. Conversely, in the discrete element simulation’s stress–strain failure stage, only aggregate skeleton support and inter-aggregate friction exist, resulting in a rapid strength decline and lower ductility compared to reality. In conclusion, the discrete element model and parameters in this study accurately simulate the stress–strain curve’s rising segment, enabling the precise prediction of uniaxial compressive strength and peak strain.

Figure 6 illustrates the final propagation patterns of typical cracks under varying cement contents. A comparison between laboratory test images and discrete element numerical simulations reveals a basic similarity in the final propagation patterns, thus confirming the reliability of the discrete element simulation. Observing Figure 6, it is evident that cracks resulting from uniaxial compression tests predominantly initiate, evolve, and spread from the upper and middle sections. The deformation and failure modes of the specimens under different cement content conditions exhibit a relatively complex nature. Various degrees of microcracks emerge on the specimen surfaces. At a 6% cement content (A1), minimal microcracks are present on the surface, primarily comprising fully penetrating ‘Y’-shaped expansion cracks, with the primary crack propagation direction forming an angle of approximately 50° with the horizontal plane. With an 8% cement content (A2), the quantity of surface microcracks increases, and the angle of the penetrating cracks is around 60°. At a 10% cement content (A3), the situation is similar to the A2 scenario, but with a higher occurrence of microcracks, and the penetrating cracks take the form of vertical cracks, aligning at a 90° angle with the horizontal direction.

The increase in cement content improves the bonding strength of the ternary cementitious system consisting of Portland cement, fly ash, and desulfurization gypsum. The combination of fly ash and desulfurization gypsum in the composite cementitious material enhances the microstructure of the system when mixed with silt, leading to a more even stress distribution across the specimen under loading. This helps mitigate potential chain reactions resulting from particle bonding failure, thereby reducing stress concentration and damage issues arising from uneven stress distribution and internal bonding defects at a macroscopic level.

4. Discussion

4.1. Evolution and Morphological Characteristics of Crack Propagation

To further understand the crack propagation and evolution mechanism of desulfurization gypsum–fly ash fluidized lightweight soil, the relative age distribution of tensile and shear crack distribution in specimens with different gradations was extracted and presented in Figure 7. The failure of desulfurization gypsum–fly ash fluidized lightweight soil is primarily due to tensile fracture, with some shear fracture as well. By comparing Figure 7 with Figure 6, it was observed that shear failure is closely associated with microcracks, whereas tensile failure aligns with through-crack trajectories. This suggests that microcrack failure is linked to shear failure, while through-failure is connected to tensile failure. In terms of age progression, at the initial loading stage, the number of cracks is relatively low, and shear failure is predominant, with microcracks appearing on the specimen’s surface. As loading continues, tensile failure starts to increase and eventually becomes the dominant mode. During this phase, cracks in the specimen grow rapidly. In the later stages of loading, the specimen experiences crushing but still retains some load-bearing capacity, primarily resisting through friction between the crushed zones.

Figure 8 illustrates the relationship between stress–strain and crack number, providing insights into the evolution of crack propagation in the specimen. Initially, when the strain is below 0.5% in the three specimens, no microcracks are observed. Subsequently, as the strain increases, the number of microcracks in all three gradations follows a similar pattern over time. The crack evolution can be categorized into four stages based on the number and growth rate of the different types of cracks: calm, gradual rise, sudden increase, and stable [22]. The calm stage is characterized by the absence of cracks and intact internal bonding bonds in the specimen, corresponding to the elastic stage of the stress–strain curve. The gradual rise stage, occurring early in the plastic loading stage, involves slight damage to the specimen and the appearance of microcracks dominated by shear cracks. The steep increase stage follows, marked by a rapid rise in the number of cracks, corresponding to the peak stress stage. Subsequently, in the destruction phase, numerous through-cracks emerge until they are fully connected and penetrated, transitioning the crack development into a stable stage.

At the beginning of loading, the contact force between particles rarely reaches the stress limit. The trend of particle movement is a vertical dislocation, resulting in shear cracks. However, as the loading increases, the internal particles begin to be subjected to the upper pressure and lower resistance. Particles are squeezed against each other, showing a trend of horizontal movement. This leads to the destruction of parallel bonding bonds between particles. At this time, the number of cracks increases rapidly until a through fracture occurs.

Although the actual process of fracture evolution is far more complex than imagined, it can be roughly simplified by extracting the displacement direction of particles. Figure 9 shows the particle displacement trend diagrams of the three schemes. The final angle of the particle displacement trend is related to the peak stress. When the horizontal component of the particle displacement force exceeds the bond strength, the bond force and the particle displacement vector jointly form a fracture surface. Therefore, it can be concluded that as the bond strength increases, the angle of the particle displacement trend with respect to the vertical direction increases. The horizontal component force continues to increase until the horizontal force exceeds the bond strength. At this time, the joint action surface of the particle displacement force and the bond force vector can be approximately considered as the angle of the fracture surface.

4.2. Analysis of Energy Evolution Mechanism

Macroscopic damage and failure of materials are visible signs of underlying microstructural damage and instability phenomena driven by energy [23]. The PFC2D6.0 software is used to track and monitor strain energy and cementation energy. It is assumed that, during the compression process of desulfurization gypsum–fly ash fluidized lightweight soil, the system does not exchange heat with the surroundings and disregards any energy release like thermal radiation [24]. The total input energy U of the system is entirely converted into elastic strain energy $U_e$ and damage dissipation energy $U_d$ of the desulfurization gypsum–fly ash fluidized lightweight soil. Thus, the sum of $U_e$ and $U_d$ equals the total energy $U$, with energy density measured in J/m². Since only axial stress performs work in uniaxial loading, the energy calculation formula for uniaxial compression tests can be represented as follows [25]:

$$U_d=U-U_e={\int }_0^{\epsilon }\sigma d\epsilon -{\displaystyle \frac{1}{2}}\sigma \epsilon$$

(2)

where the total energy is $U={\int }_0^{\epsilon }\sigma d\epsilon$; the elastic strain energy is $U_e={\displaystyle \frac{1}{2}}\sigma \epsilon$; and $\sigma$ and $\epsilon$ represent the stress and strain values on the stress–strain curve, respectively.

Figure 10 shows the energy evolution curve of desulfurization gypsum–fly ash fluidized lightweight soil. The total peak energy of desulfurization gypsum–fly ash fluidized lightweight soil in schemes A1, A2, and A3 is 579, 1000, and 1750 J/m², respectively. During the loading process, the external input boundary energy gradually increases, and in the initial stage of loading, the sample does not initiate cracks. At this time, the material boundary energy is fully converted into strain energy and bonding energy, and the curve is relatively straight. In the early stage of plastic compression, the energy consumed by crack propagation gradually increases, and microcracks begin to gradually appear. After entering the pre peak plastic stage, the dissipation of bonding energy accelerates and the number of microcracks increases rapidly. When the bonding energy and strain energy reach their peak values, the specimen begins to fail and break. After the peak, the crack propagation of the sample accelerates, the total strain energy is rapidly released, and the system dissipation energy continues to increase. From an energy perspective, compared to the phenomenon of “sudden changes” in dissipated energy such as concrete and rock, there is a significant release of energy [25,26]. The evolution of dissipated energy in the desulfurization gypsum–fly ash fluidized lightweight soil is relatively smooth, and its corresponding macroscopic manifestation is that there is a certain delay in cracking of the desulfurization gypsum–fly ash fluidized lightweight soil after exceeding the peak stress point. Meanwhile, according to the principle of minimum energy, the total energy of desulfurization gypsum–fly ash fluidized lightweight soil is relatively low. Therefore, the energy released during destruction is also very limited, which allows it to effectively avoid the release of a large amount of energy during destruction.

5. Conclusions

(1)

As a material primarily providing adhesive strength, the amount of cement added has a significant impact on the performance of desulfurization gypsum–fly ash fluidized lightweight soil. Increasing the amount of cement can enhance the strength of desulfurized gypsum–fly ash fluid lightweight soil and delay the stage of crack penetration. However, under the action of axial force, the increase in cement content will lead to a significant increase in microcracks. Therefore, selecting the appropriate amount of cement is beneficial to improving the crack resistance and stability of desulfurization gypsum–fly ash fluidized lightweight soil.

(2)

The numerical simulation results show that under uniaxial compression conditions, the desulfurization gypsum–fly ash fluidized lightweight soil is affected by particle movement, and microcracks dominated by shear failure occur at the beginning of loading, followed by through-cracks dominated by tensile failure in the later stage.

(3)

According to the principle of minimum energy, the total energy of desulfurization gypsum–fly ash fluidized lightweight soil is low, and the cementation energy and strain energy account for a large proportion, resulting in higher overall material stability. Its dissipation energy release does not have a “sudden change stage”, and the release curve is relatively smooth, which can effectively avoid the road damage caused by the dissipation, release, and transfer of energy when disturbed by external factors.