Loess solidification method using coal-based solid waste

Abstract

The application belongs to the technical field of soil stabilization, and specifically discloses a loess solidification method using coal-based solid waste, which comprises the following steps: S1, material pretreatment: after the loess, cement, fly ash and desulfurization gypsum are crushed, screening and drying treatment are performed to obtain loess, cement, fly ash particles and desulfurization gypsum particles with a particle size of not more than 0.075 mm; S2, according to the proportion, the loess, cement and the fly ash particles and desulfurization gypsum particles obtained in S1 are uniformly mixed and compacted to obtain loess solidification material. The loess solidification method using coal-based solid waste is adopted, and through mechanical property evaluation, microstructure characterization and environmental impact assessment, the effectiveness of the coal-based solid waste as a sustainable alternative to traditional cement for stabilizing loess is verified, and the key role of microstructure densification in enhancing UCS is revealed.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of soil stabilization, and in particular to a loess solidification method using coal-based solid waste. BACKGROUND

[0002] Loess is a unique sediment characterized by loose structure and sub-stable state, showing significant water sensitivity. When exposed to water, loess will rapidly and significantly settle due to self-weight and external pressure, resulting in varying degrees of deformation, and even structural instability in severe cases. This instability poses a significant threat to economic development and public safety.

[0003] To alleviate these problems, mechanical and chemical reinforcement techniques are generally used. Among the chemical reinforcement materials, cement is the preferred material for loess reinforcement due to its low cost and high efficiency. Cement generates calcium silicate hydrate (C-S-H) gel through a gelation reaction, thereby enhancing soil structure, optimizing pore distribution, and reducing inter-particle voids. However, the durability of cement-reinforced soil is significantly affected by environmental factors, which can lead to a decrease in material flexural strength, an increased risk of cracking and deformation, thereby limiting its practical application. In addition, cement production is a significant source of greenhouse gas emissions, which conflicts with modern environmental sustainability goals. Therefore, there is an urgent need to explore alternative materials that can effectively reinforce loess while reducing environmental impact.

[0004] With the rapid development of coal mining, coal-fired power generation, and coal chemical industry, a large amount of coal-based solid waste is generated, and the utilization of coal-based solid waste is limited and scattered, characterized by "single utilization mode, low synergy, narrow application field, low scale, chaotic stacking, low resource recovery rate, many industry barriers, and weak industrial integration". This scattered utilization mode cannot meet the goals of harmless disposal and large-scale resource utilization.

[0005] Under this background, it is particularly important to properly handle fly ash (FA) and desulfurization gypsum (DG), two major by-products of coal combustion. In the prior art, coal-based solid waste is mainly used for the production of building materials and gypsum products, although the environmental benefits of using fly ash and desulfurization gypsum to partially replace cement for loess reinforcement have been proven, but the research on its specific emission reduction effect is still insufficient. There is a lack of a loess stabilization method that quantifies the improvement of mechanical properties and environmental benefits brought about by the stabilization process of FA and DG. SUMMARY

[0006] The application aims to provide a loess solidification method using coal-based solid waste, which verifies the effectiveness of coal-based solid waste as a sustainable alternative to traditional cement for stabilizing loess through mechanical property evaluation, microstructure characterization and environmental impact assessment, and reveals the key role of microstructure densification in enhancing UCS.

[0007] To achieve the above-mentioned purpose, the application provides a loess solidification method using coal-based solid waste, comprising the following steps:

[0008] S1, material pretreatment: crushing, screening and drying loess, cement, fly ash and desulfurization gypsum to obtain loess, cement, fly ash particles and desulfurization gypsum particles with a particle size of not more than 0.075 mm;

[0009] S2, mixing loess, cement and fly ash particles and desulfurization gypsum particles obtained in S1 in proportion, uniformly and compactly to obtain loess solidification material.

[0010] Preferably, in S1, the drying treatment is specifically:

[0011] drying at 100-110℃ for 7-10h.

[0012] Preferably, in S2, the mass ratio of fly ash particles, desulfurization gypsum particles, cement, loess is 0-15:0-15:0-20:80.

[0013] Preferably, in S2, the mass ratio of fly ash particles, desulfurization gypsum particles, cement, loess is 5:10:5:80.

[0014] Preferably, in S2, the water content of loess is not less than 10%.

[0015] Preferably, in S2, the particle size distribution of loess includes silt, sand and clay, and the content of silt is not less than 70%.

[0016] Preferably, in S2, the dry density of loess solidification material is 1.50-1.70Mg / m 3 .

[0017] Preferably, in S2, the unconfined compressive strength value of loess solidification material is 1477.42kPa.

[0018] Therefore, the application adopts the above-mentioned loess solidification method using coal-based solid waste, and has the following beneficial effects:

[0019] The present application verifies the effectiveness of coal-based solid waste as a sustainable alternative to traditional cement for stabilizing loess by mechanical property evaluation, microstructure characterization, and environmental impact assessment, reveals the key role of microstructure densification in enhancing UCS, and that the loess solidified material containing 5% fly ash, 10% desulfurization gypsum, and 5% cement obtains the highest UCS, which is superior to traditional cement stabilization (containing 20% cement). This improvement is attributed to the synergistic effect of DG and FA, including sulfate activation and pozzolanic reaction, which enhance the particle bonding and structural integrity within the loess matrix. FTIR and SEM analysis confirms the formation of key phases such as C-S-H and ettringite, which reduce porosity and increase cohesion.

[0020] The technical solutions of the present application are further described in detail below through the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a stress-strain curve of an embodiment of the loess solidification method using coal-based solid waste of the present application;

[0022] Figure 2 is a graph of unconfined compressive strength of an embodiment of the loess solidification method using coal-based solid waste of the present application;

[0023] Figure 3 is an FTIR spectrum of an embodiment of the loess solidification method using coal-based solid waste of the present application;

[0024] Figure 4 is a micro-morphology graph of an embodiment of the loess solidification method using coal-based solid waste of the present application, where (a) is Example 1, (b) is Example 2, (c) is Example 3, (d) is Example 4, (e) is Example 5, and (f) is Example 6;

[0025] Figure 5 is a pore angle distribution of an embodiment of the loess solidification method using coal-based solid waste of the present application, where (a) is Example 1, (b) is Example 2, (c) is Example 3, (d) is Example 4, (e) is Example 5, and (f) is Example 6;

[0026] Figure 6 is a life cycle assessment graph of an embodiment of the loess solidification method using coal-based solid waste of the present application, where (a) is the global warming potential (GWP) value, (b) is the acidification potential (AP) value, (c) is the eutrophication potential (EP) value, (d) is Example 4, (e) is the photochemical ozone formation potential (POFP) value, and (f) is the energy consumption (PED) value. DETAILED DESCRIPTION

[0027] The technical solutions of the present application are further described below with reference to the accompanying drawings and examples.

[0028] Unless otherwise defined, technical terms or scientific terms used in the present application shall have the usual meaning as understood by a person with ordinary skill in the art to which the present application pertains.

[0029] Example 1

[0030] A kind of loess, particle size distribution is mainly (about 71.5%) with silt, sand and clay are 25.9% and 2.6% respectively.Its physical properties are shown in Table 1.

[0031] Table 1 Physical properties of loess

[0032] Sample Loess Initial dry density (Mg / m 3 )]]> 1.55 Natural moisture content (%) 10 Specific gravity 2.69 plastic limit (w P / %)]]> 12.81 Liquid value (w L / %)]]> 23.56 Plasticity index (PI) 10.75

[0033] Example 2

[0034] A loess solidification method using cement, the steps are as follows:

[0035] A. 800 kg of loess and 200 kg of cement are mixed uniformly and compacted to obtain a loess solidification material with a dry density of 1.55 Mg / m 3 .

[0036] B. Distilled water is added to obtain a mixture with a moisture content of 10% and stored in a sealed polyethylene bag and placed in a humidity control box for 48 hours to achieve uniform moisture distribution.

[0037] C. It is placed in a humidity control room with a temperature of 22°C and a relative humidity of 80%, and after 7 days of curing, the mold is removed.

[0038] Example 3

[0039] A loess solidification method using coal-based solid waste, the steps are as follows:

[0040] A. The loess, cement and fly ash are crushed and sieved through a 0.075 mm sieve, and then dried at 105°C for 8h to obtain new loess, cement and fly ash particles.

[0041] B. 150 kg of fly ash particles, 800 kg of loess and 50 kg of cement are mixed uniformly and compacted to obtain a loess solidification material with a dry density of 1.55 Mg / m 3 .

[0042] C. Distilled water is added to obtain a mixture with a moisture content of 10% and stored in a sealed polyethylene bag and placed in a humidity control box for 48 hours to achieve uniform moisture distribution.

[0043] D. The formwork was removed after 7 days of curing in a humidity-controlled room at a temperature of 22°C and a relative humidity of 80%.

[0044] Example 4

[0045] The difference from Example 3 is that step A is:

[0046] The loess, cement, fly ash and desulfurization gypsum were crushed and sieved through a 0.075 mm screen, and then dried at 105°C for 8 h to obtain new loess, cement, fly ash particles and desulfurization gypsum particles.

[0047] Step B is:

[0048] The 100 kg of fly ash particles, 50 kg of desulfurization gypsum particles, 800 kg of loess and 50 kg of cement were mixed uniformly and compacted to obtain a loess solidified material with a dry density of 1.55 Mg / m 3 .

[0049] Example 5

[0050] The difference from Example 3 is that step A is:

[0051] The loess, cement, fly ash and desulfurization gypsum were crushed and sieved through a 0.075 mm screen, and then dried at 105°C for 8 h to obtain new loess, cement, fly ash particles and desulfurization gypsum particles.

[0052] Step B is:

[0053] The 50 kg of fly ash particles, 100 kg of desulfurization gypsum particles, 800 kg of loess and 50 kg of cement were mixed uniformly and compacted to obtain a loess solidified material with a dry density of 1.55 Mg / m 3 .

[0054] Example 6

[0055] The difference from Example 3 is that step A is:

[0056] The loess, cement and desulfurization gypsum were crushed and sieved through a 0.075 mm screen, and then dried at 105°C for 8 h to obtain new loess, cement and desulfurization gypsum particles.

[0057] Step B is:

[0058] The 150 kg of desulfurization gypsum particles, 800 kg of loess and 50 kg of cement were mixed uniformly and compacted to obtain a loess solidified material with a dry density of 1.55 Mg / m 3 .

[0059] Test Test

[0060] Unconfined compressive tests were performed on the materials of Examples 1-6, and the results are shown in Figure 1 and Figure 2 .

[0061] As can be seen from Figure 1 and Figure 2 , Example 1, which consists of compacted loess only, has the lowest UCS and failure strain values, highlighting its limited structural cohesion. In contrast, Example 2, which incorporates 20% cement, shows a significant increase in strength, indicating the important role of cement in enhancing the structural stability of the soil. Example 3, which consists of 15% fly ash and 5% cement, shows an improvement in strength compared to Example 1, but is slightly lower than Example 2, indicating that although fly ash contributes to the matrix stability, its effect is not as significant as using cement alone. This is particularly evident in the results of Examples 4, 5, and 6, which are groups of samples in which part of the fly ash is replaced by desulphurisation gypsum. In particular, Example 5 (5% fly ash, 10% desulphurisation gypsum, and 5% cement) has the highest UCS of all the groups, even exceeding Example 2 which uses cement alone. This result reveals a synergistic effect of desulphurisation gypsum and fly ash, which enhances the binding forces and structural integrity of the particles within the loess matrix due to sulphate activation, indicating that the effect of traditional cement stabilisation can be exceeded by the rational addition of desulphurisation gypsum.

[0062] Microstructural characterisation was performed on the materials of Examples 1-6, and the results are shown in Figure 3 , which displays the transmission intensity spectra of each sample over different wavenumber ranges. The surface microtopography of each sample is shown in Figure 4 . The directional nature of the loess microstructure was assessed by analysing the pore angle distribution, as shown in Figure 5 .

[0063] As can be seen from Figure 3 , for Example 1 (compacted loess), the FTIR spectrum shows four main absorption peaks at 3614, 1436, 1024, and 466 cm -1 . The peak at 3614 cm -1 is associated with the stretching vibration of the hydroxyl groups on the surface of the loess particles, the absorption peak at 1436 cm -1 corresponds to the bending vibration of the carbonate (–CO3) groups. The broad absorption band near 1024 cm -1 reflects the asymmetric stretching vibration of Si–O–Si, while the sharp peak at 466 cm -1 is associated with the symmetric stretching vibration of Si–O–Si, indicating the presence of silicate anions (SiO4 2- ) on the surface of the loess particles. These observations confirm the presence of hydroxyl, carbonate, and silicate groups on the surface of the compacted loess particles.

[0064] When different proportions of fly ash (FA), desulfurization gypsum (DG), and cement were added to loess, the intensity of functional groups was significantly enhanced, with Example 5 showing the most significant increase, followed by Example 2. This trend is highly consistent with the UCS results. Figure 2 In addition to the increase in functional group intensity, new absorption peaks appeared at 1642 and 1151 cm -1 , indicating an increase in the types of surface functional groups. The peak at 1642 cm -1 is associated with hydroxyl groups, reflecting the introduction of a large number of -OH groups from fly ash, desulfurization gypsum, and cement into the loess matrix. In the region from 800 to 1400 cm -1 , the spectrum is complex due to the superposition of silicate and sulfate vibrations and requires careful analysis. As cement hydrates, the Si–O peak moves from 918 cm -1 to 1009 cm -1 , reflecting the polymerization of silicate units. This shift in the Si–O asymmetric stretching band and the relative intensity change of Si–O bending modes indicate the polymerization process of silicate units (SiO4), which is a clear sign of cement hydration and C-S-H generation. Therefore, the enhanced functional group intensity at 1024 cm -1 can be attributed to the hydration of cement and the pozzolanic reaction of fly ash, which generates additional C-S-H phases. In addition, the intensity of absorption peaks associated with sulfates significantly increases, including the apparent S–O vibration at 1110 cm -1 , indicating the process of gypsum reacting with tricalcium aluminate phases to form ettringite.

[0065] From (a) in FIG. 1, Figure 4 the C1 position represents face-to-face connections, and the P1 position represents inter-aggregate pores, showing that the compacted loess network structure is relatively loose, with obvious inter-aggregate pores and less interlocking between particles, mainly in point contact, which indicates that its cohesion is limited, leading to high porosity and low unconfined compressive strength (UCS).

[0066] From (b) in FIG. 1, Figure 4 the C2 position represents face-to-face connections, and the addition of 20% cement significantly changes the microstructure, promoting a closer arrangement of particles with increased contact area, from point contact to face contact. The hydrated calcium silicate (C-S-H) gel generated by cement hydration acts as an inter-particle binder, enhancing the cohesion of particles and reducing porosity. This structural densification improves load distribution, thereby increasing UCS.

[0067] From (c) in FIG. 1, Figure 4As shown in (c), the mixing of fly ash (FA) and cement further improves the compaction of the material. The spherical morphology of FA particles helps fill pores and enhances structural density. Under alkaline conditions, FA reacts with calcium hydroxide produced during cement hydration to form secondary CSH and CAH gels, further promoting particle bonding. The image is expected to show reduced porosity between agglomerates, but due to the higher FA-to-cement ratio, the interparticle connectivity and compactness may be slightly lower than in Example 2, resulting in a significantly improved UCS compared to Example 1, but slightly lower than in Example 2.

[0068] Depend on Figure 4 As shown in (d), the combined effect of FA, desulfurized gypsum (DG), and cement results in a more compact microstructure compared to Examples 1 and 3. DG promotes the formation of ettringite and additional CSH phases, enhancing particle cohesion. SEM images may show reduced porosity between agglomerates and increased porosity within agglomerates, indicating a more stable structure that reduces deformation under load, corresponding to higher UCS results.

[0069] Depend on Figure 4 As shown in (e), position P2 represents the porosity within the aggregates, exhibiting a denser structure and higher interparticle bonding strength, thanks to the higher DG content. The enhanced sulfate activation by DG generates more ettringite, effectively binding the loess particles together, resulting in the highest UCS of this group among all examples. SEM analysis is expected to show minimal porosity between aggregates, while the internal pores form a highly compact structure, minimizing pore connectivity and enhancing load-bearing capacity.

[0070] Depend on Figure 4 As shown in (f), extensive particle bonding and ettringite formation were observed due to the highest DG content. However, the lower FA content may result in slightly more porosity between aggregates than in Example 5.

[0071] Depend on Figure 5 As can be seen, in Examples 3 to 6, the combinations of fly ash (FA), desulfurized gypsum (DG), and cement exhibit similar orientation dispersion trends in the rose diagrams, indicating a transition from the natural vertical arrangement in untreated loess to a more isotropic structural configuration. The combination of FA and DG with cement enhances interparticle interactions, resulting in a denser microstructure and improved cementitious properties. This balanced pore orientation distribution contributes to improved UCS because it supports efficient load transfer and enhances structural integrity under stress.

[0072] In summary, the strength enhancement mechanisms of fly ash (FA), desulfurization gypsum (DG), and cement in stabilizing loess involve a series of interrelated physical and chemical processes that collectively contribute to a denser structure and higher unconfined compressive strength (UCS). This performance improvement is primarily attributed to the pozzolanic reaction between fly ash and cement hydration products, and the ettringite formation induced by the sulfate content in desulfurization gypsum, which leads to significant microstructural and compositional changes. The cement hydration process first generates calcium silicate hydrate (C-S-H) and calcium hydroxide (CH), providing the foundation for the cohesive structure. Subsequently, the silica (SiO2) in fly ash undergoes a pozzolanic reaction with CH, producing additional C-S-H, which further enhances the cohesive ability of the system. The relevant reactions can be represented as:

[0073] Ca(OH)2+ SiO2→ C-S-H

[0074] Meanwhile, the addition of desulfurization gypsum promotes the formation of ettringite (C3A·CaSO4·32H2O), which grows in the pore space and acts as a structural binder, improving the cohesiveness of the particles and inducing slight volume expansion. This expansion leads to pore filling, further densifying the matrix and reducing the overall porosity. The formation reaction of ettringite is as follows:

[0075] C3A + 3CaSO4 + 32H2O → C3A·3CaSO4·32H2O (ettringite)

[0076] In summary, the combined stabilization of loess with FA, DG, and cement produces a synergistic effect, significantly enhancing the UCS through both physical and chemical modifications. The pozzolanic activity of FA, combined with the cementitious and filling functions of C-S-H and ettringite, reduces porosity and enhances particle cohesiveness. The stabilized loess transforms into a low-porosity, high-density structure with uniform pore distribution and smaller pore sizes.

[0077] Environmental benefit tests were conducted using the materials of Example 1-Example 6, and the results are shown in Figure 6 .

[0078] As can be seen from (a) in Figure 6 , Example 2 (containing 20% cement) shows the highest global warming potential (GWP, about 230 kg CO2eq), mainly due to the high energy consumption characteristics of the cement production process. By partially replacing cement with fly ash (FA) and desulfurization gypsum (DG) in Examples 3 to 6, CO2 emissions are effectively reduced. Example 6 (containing 15% desulfurization gypsum) achieves the lowest GWP (about 80 kg CO2eq), showing the significant emission reduction potential by replacing cement with desulfurization gypsum.

[0079] As can be seen from (b) in Figure 6As shown in (b), the acidification potential (AP) values ​​exhibit a similar trend, with Example 2 showing the highest impact (approximately 0.6 kg SO2eq) due to sulfur dioxide emissions during the cement production process. As the cement proportion decreased from Examples 3 to 6, the AP values ​​steadily declined, with Example 6 again showing the lowest AP value, highlighting the environmental advantages of incorporating desulfurized gypsum.

[0080] Depend on Figure 6 As shown in (c), the eutrophication potential (EP) value is between 0.05 and 0.09 kg PO4. 3- The values ​​fluctuated between eq values, with Example 3 showing slightly higher values ​​than the other groups. This may be attributed to the rich nutrient content in fly ash, which could promote eutrophication under certain conditions. Figure 6 As shown in (d), Example 2 exhibited the highest photochemical ozone generation potential (POFP, approximately 0.45 kg NMVOC eq), primarily due to the volatile organic compounds released during cement production. By introducing supplementary materials such as FA and DG, the POFP values ​​of Examples 3 through 6 were significantly reduced. Example 6 showed the lowest value (approximately 0.25 kg NMVOC eq), further demonstrating the environmental advantages of reducing cement usage.

[0081] Depend on Figure 6 As shown in (e), the energy consumption (PED) exhibits the expected pattern, with Example 3 showing the highest total primary energy demand (approximately 2200 MJ), due to the high energy consumption of fly ash production. The PED value steadily decreases as the cement content gradually decreases in Examples 4 through 6. Example 6 achieves the lowest energy demand (approximately 900 MJ), reflecting the significant energy savings achieved by replacing cement with desulfurized gypsum.

[0082] The results show that using supplementary materials like FA and DG to replace cement can effectively reduce various environmental impacts. Example 6 proved to be the most environmentally friendly option, exhibiting the lowest values ​​in GWP, AP, POFP, and PED, while maintaining a balanced EP level. However, although Example 6 demonstrated the best environmental performance, its mechanical properties were slightly lower than those of Example 5. Notably, the UCS of Example 5 was significantly higher than that of Example 2, which contained 20% cement. From an environmental perspective, Example 5 also showed significant advantages, achieving a favorable balance between strength and reduced environmental impact.

[0083] Therefore, this invention employs the aforementioned method for loess solidification using coal-based solid waste. Through mechanical performance evaluation, microstructure characterization, and environmental impact assessment, it verifies the effectiveness of coal-based solid waste as a sustainable alternative to traditional cement for stabilizing loess and reveals the key role of microstructure densification in enhancing UCS.

[0084] It should be pointed out that the above embodiments are only used to illustrate the technical solutions of the present application but not to limit the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still make modifications or equivalent replacements to the technical solutions of the present application, and these modifications or equivalent replacements should not make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present application.

Claims

1. A loess solidification method using coal-based solid waste, characterized by, The procedure is as follows: A. The loess, cement, fly ash and desulfurization gypsum are crushed and sieved through a 0.075 mm sieve, and then dried at 105°C for 8 h to obtain new loess, cement, fly ash particles and desulfurization gypsum particles; B. 50 kg of fly ash particles, 100 kg of desulfurization gypsum particles, 800 kg of loess and 50 kg of cement are mixed uniformly and compacted to obtain a loess solidified material with a dry density of 1.55 Mg / m 3 . C. Distilled water is added to obtain a mixture with a water content of 10%, which is stored in a sealed polyethylene bag and placed in a humidity control box for 48 h to achieve uniform moisture distribution; D. The mixture is placed in a humidity control room with a temperature of 22°C and a relative humidity of 80%, and cured for 7 days before demolding.

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