Method for curing loess by using coal-based solid waste

Abstract

The invention belongs to the technical field of soil stabilization, and particularly discloses a loess solidification method using coal-based solid waste, which comprises the following steps: S1, material pretreatment: crushing loess, cement, fly ash and desulfurized gypsum, screening and drying to obtain loess, cement, fly ash particles and desulfurized gypsum particles with the particle size of not more than 0.075 mm; and S2, uniformly mixing loess, cement and the fly ash particles and the desulfurized gypsum particles obtained in the step S1 according to a ratio, and compacting to obtain the loess curing material. According to the method for curing the loess by using the coal-based solid waste, the effectiveness of stabilizing the loess by using the coal-based solid waste as a sustainable substitute of traditional cement is verified through mechanical property evaluation, microstructure characterization and environmental influence evaluation, and the key effect of microstructure densification in the aspect of enhancing UCS is revealed.

Description

Technical Field

[0001] The invention relates to the technical field of soil stabilization, and in particular to a loess solidification method utilizing coal-based solid waste. Background Art

[0002] Loess is a unique sediment with the typical characteristics of loose structure and metastable state, showing significant water sensitivity. When exposed to water, loess will rapidly settle significantly due to its own weight and external pressure, resulting in varying degrees of deformation, and even structural instability in severe cases. This instability poses a major threat to economic development and public safety.

[0003] To alleviate these problems, mechanical and chemical reinforcement technologies are generally used. Among chemical reinforcement materials, cement has become the preferred material for loess reinforcement due to its low cost and technical efficiency. Cement forms calcium silicate hydrate (CSH) gel through gelling reaction, thereby strengthening the soil structure, optimizing pore distribution, and reducing the voids between particles. However, the durability of cement-reinforced soil is significantly affected by environmental factors, which may lead to a decrease in the flexural strength of the material and an increased risk of cracking and deformation, thereby limiting its practical application. In addition, cement production is an important source of greenhouse gas emissions, which conflicts with modern environmental sustainable development 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 relatively limited and scattered, which is manifested as "single utilization mode, low synergy, narrow application field, low scale, chaotic stockpiling, low resource recovery rate, many industry barriers, and weak industrial integration". This scattered utilization method is difficult to meet the goals of harmless disposal and large-scale resource utilization.

[0005] In this context, proper treatment of fly ash (FA) and desulfurized gypsum (DG), two major byproducts of coal combustion, is particularly important. In the existing technology, coal-based solid waste is mainly used in the production of building materials and gypsum products. Although the environmental benefits of using fly ash and desulfurized gypsum to partially replace cement to reinforce loess have been proven, research on its specific emission reduction effect is still insufficient. There is a lack of a loess stabilization method that quantifies the mechanical property improvements and environmental benefits brought about by the stabilization process of FA and DG. Summary of the invention

[0006] The purpose of this paper is to provide a loess solidification method using coal-based solid waste. Through mechanical property evaluation, microstructural characterization and environmental impact assessment, the effectiveness of coal-based solid waste as a sustainable alternative to traditional cement to stabilize loess is verified, revealing the key role of microstructural densification in enhancing UCS.

[0007] To achieve the above object, the present invention provides a loess solidification method using coal-based solid waste, comprising the following steps:

[0008] S1. Material pretreatment: crush loess, cement, fly ash and desulfurized gypsum, and then screen and dry them to obtain loess, cement, fly ash particles and desulfurized gypsum particles with a particle size not exceeding 0.075 mm;

[0009] S2. Mix loess, cement, the fly ash particles obtained in S1 and the desulfurized gypsum particles uniformly and compact them according to proportion to obtain loess solidification material.

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

[0011] Dry at 100-110°C for 7-10 hours.

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

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

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

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

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

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

[0018] Therefore, the present invention adopts the above-mentioned loess solidification method using coal-based solid waste, and the beneficial effects are as follows:

[0019] The present invention verifies the effectiveness of coal-based solid waste as a sustainable alternative to traditional cement to stabilize loess through mechanical property evaluation, microstructural characterization and environmental impact assessment, and reveals the key role of microstructural densification in enhancing UCS. The loess-solidified material containing 5% fly ash, 10% desulfurized gypsum and 5% cement obtains the highest UCS, which is better than 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 particle bonding and structural integrity within the loess matrix. FTIR and SEM analysis confirm the formation of key phases such as CSH and calcium sulfonite, which reduce porosity and increase cohesion.

[0020] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0022] Figure 2 It is a statistical graph of unrestricted compressive strength of an embodiment of a loess solidification method using coal-based solid waste according to the present invention;

[0023] Figure 3 This is an FTIR spectrum diagram of an embodiment of a loess solidification method using coal-based solid waste according to the present invention;

[0024] Figure 4 1 is a microscopic morphology diagram of an embodiment of a loess solidification method using coal-based solid waste according to the present invention, wherein (a) is embodiment 1, (b) is embodiment 2, (c) is embodiment 3, (d) is embodiment 4, (e) is embodiment 5, and (f) is embodiment 6;

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

[0026] Figure 6 This is a life cycle assessment diagram of an embodiment of the loess solidification method using coal-based solid waste of the present invention, wherein (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 generation potential (POFP) value, and (f) is the energy consumption (PED) value. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0028] Unless otherwise defined, technical or scientific terms used in the present invention shall have the common meanings understood by one having ordinary skills in the field to which the present invention belongs.

[0029] Example 1

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

[0031] Table 1 Physical properties of loess

[0032] sample loess <![CDATA[Initial dry density (Mg / m 3 )]]> 1.55 Natural water content (%) 10 proportion 2.69 <![CDATA[Plastic limit (w P / %)]]> 12.81 <![CDATA[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. Mix 800kg loess and 200kg cement evenly and compact them to obtain a dry density of 1.55Mg / m 3 Loess solidification material.

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

[0037] C. Place in a humidity-controlled room with a temperature of 22°C and a relative humidity of 80%, and remove the mold after curing for 7 days.

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

[0041] B. Mix 150kg fly ash particles, 800kg loess and 50kg cement evenly and compact them to obtain a dry density of 1.55Mg / m 3 Loess solidification material.

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

[0043] D. Place in a humidity-controlled room with a temperature of 22°C and a relative humidity of 80%, and remove the mold after curing for 7 days.

[0044] Example 4

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

[0046] The loess, cement, fly ash and desulfurized gypsum were 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 desulfurized gypsum particles.

[0047] Step B is:

[0048] 100kg fly ash particles, 50kg desulfurized gypsum particles, 800kg loess and 50kg cement were mixed and compacted to obtain a dry density of 1.55Mg / m 3 Loess solidification material.

[0049] Example 5

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

[0051] The loess, cement, fly ash and desulfurized gypsum were 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 desulfurized gypsum particles.

[0052] Step B is:

[0053] 50kg fly ash particles, 100kg desulfurized gypsum particles, 800kg loess and 50kg cement were mixed and compacted to obtain a dry density of 1.55Mg / m 3 Loess solidification material.

[0054] Example 6

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

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

[0057] Step B is:

[0058] 150kg of desulfurized gypsum particles, 800kg of loess and 50kg of cement were mixed evenly and compacted to obtain a dry density of 1.55Mg / m 3 Loess solidification material.

[0059] Experimental Testing

[0060] The materials of Example 1 to Example 6 were used to carry out unconfined compression tests. The results are as follows: Figure 1 and Figure 2 shown.

[0061] Depend on Figure 1 and Figure 2 It can be seen that Example 1, which consists only of compacted loess, has the lowest UCS and failure strain values, highlighting its limited structural cohesion. In contrast, after 20% cement was added to Example 2, the strength was significantly improved, indicating that cement plays an important role in enhancing the structural stability of soil. Example 3 consists of 15% fly ash and 5% cement, and its strength is improved compared with Example 1, but slightly lower than Example 2, which shows that although fly ash contributes to matrix stability, its effect is not as significant as cement alone. The results of Example 4, Example 5 and Example 6 are particularly prominent, and these three groups gradually replace part of the fly ash with desulfurized gypsum. In particular, the UCS of Example 5 (5% fly ash, 10% desulfurized gypsum and 5% cement) is the highest among all groups, even exceeding Example 2, which only uses cement. This result reveals the synergistic effect of desulfurized gypsum and fly ash, which enhances the binding force and structural integrity of particles in the loess matrix due to sulfate activation, indicating that the effect of traditional cement stabilization can be exceeded by the reasonable addition of desulfurized gypsum.

[0062] The materials of Examples 1 to 6 were used for microstructural characterization. Figure 3 As shown in Figure 1, the transmission intensity spectra of each sample in different wave number ranges are displayed. Figure 4 As shown in Figure 2, the surface microstructure of each sample is shown. Figure 5 As shown, the pore angle distribution is analyzed to evaluate the directionality of the loess microstructure.

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

[0064] When fly ash (FA), desulfurized gypsum (DG) and cement were added to loess in different proportions, the strength of the functional groups was significantly enhanced, with Example 5 showing the most significant improvement, followed by Example 2. This trend is consistent with Figure 2 The UCS results are highly consistent with those of -1 A new absorption peak also appeared at 1642cm -1 The peaks of 800 to 1400 cm-1 are related to hydroxyl groups, reflecting the large amount of -OH groups introduced into the loess matrix by fly ash, desulfurized gypsum and cement. -1 In the region of 918 cm-1, the spectrum is complex and requires careful analysis due to the superposition of silicate and sulfate vibrations. -1 Move to 1009cm -1 , reflecting the polymerization of silicate units. The displacement of this Si–O asymmetric stretching band and the change in the relative intensity of the Si–O bending mode indicate the polymerization process of silicate units (SiO4), which is a clear sign of cement hydration and CSH formation. Therefore, 1024cm -1 The enhanced functional group intensity at 1110 cm-1 can be attributed to the hydration of cement and the pozzolanic reaction of fly ash, which generate additional CSH phases. In addition, the absorption peaks related to sulfate increase significantly, including 1110 cm-1. -1 The obvious S–O vibration at indicates the process of gypsum reacting with tricalcium aluminate to form ettringite.

[0065] Depend on Figure 4 As shown in (a), the C1 position represents face-to-face connection, and the P1 position represents the pores between aggregates, indicating that the network structure of compacted loess is relatively loose, the pores between aggregates are obvious, and the interlocking between particles is less, mainly point contact, which indicates that its cohesion is limited, resulting in higher porosity and lower unconfined compressive strength (UCS).

[0066] Depend on Figure 4 As shown in (b), the C2 position indicates a face-to-face connection. The addition of 20% cement significantly changes the microstructure, promotes a tighter arrangement of particles, increases the contact area, and transforms from point contact to surface contact. The calcium silicate hydrate (CSH) gel generated by cement hydration acts as a binder between particles, enhancing the cohesion of particles and reducing porosity. This structural densification improves load distribution, thereby increasing UCS.

[0067] Depend on Figure 4As can be seen from (c) in the figure, the mixing of fly ash (FA) and cement further improves the compaction of the material. The spherical morphology of FA particles helps to fill the pores and enhance the compactness of the structure. Under alkaline conditions, FA reacts with calcium hydroxide produced by cement hydration to produce secondary CSH and CAH gels, which further promote the bonding of particles. The image is expected to show a reduction in pores between agglomerates, but due to the high ratio of FA to cement, the connectivity and compactness between its particles may be slightly lower than that of Example 2, resulting in a UCS that is significantly improved compared to Example 1 but slightly lower than that of Example 2.

[0068] Depend on Figure 4 As shown in (d), the combined effect of FA, desulfurized gypsum (DG) and cement makes the microstructure more compact than that of Examples 1 and 3. DG promotes the formation of calcium sulfonate and additional CSH phases, and enhances the cohesion of particles. The SEM image may show that the pores between agglomerates are reduced, while the pores inside the agglomerates are increased. This structure is more stable and can reduce deformation under load, corresponding to the higher UCS results.

[0069] Depend on Figure 4 As shown in (e), the P2 position represents the pores within the aggregates, showing a denser structure and higher bonding strength between particles, which is due to the higher DG content. The enhanced sulfate activation of DG produces more calcium aluminate, which effectively bonds the loess particles together, resulting in the highest UCS of this group among all examples. SEM analysis is expected to show that there are very few pores between aggregates, while the pores within the aggregates form a highly compact structure, which minimizes pore connectivity and improves carrying capacity.

[0070] Depend on Figure 4 As can be seen in (f), due to the highest DG content, extensive particle bonding and ettringite formation were observed. However, the lower FA content may result in slightly more inter-agglomerate pores than in Example 5.

[0071] Depend on Figure 5 It can be seen that in Examples 3 to 6, including the combination of fly ash (FA), desulfurized gypsum (DG) and cement, the rose diagrams show similar orientation dispersion trends, 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 the interaction between particles, forms a denser microstructure and improves cementation. This balanced pore orientation distribution contributes to the improvement of UCS because it supports efficient load transfer and enhances structural integrity under stress.

[0072] In summary, the strength enhancement mechanism of fly ash (FA), desulfurized gypsum (DG) and cement in stabilized loess involves a series of interrelated physical and chemical processes that together promote a denser structure and higher unconfined compressive strength (UCS). This performance improvement is mainly attributed to the pozzolanic reaction between fly ash and cement hydration products, and the formation of calcium sulfonate induced by the sulfate content in desulfurized gypsum, which leads to significant microstructural and compositional changes. The cement hydration process first produces calcium silicate hydrate (CSH) and calcium hydroxide (CH), which provide the basis for the bonding structure. Subsequently, the silica (SiO2) in the fly ash undergoes a pozzolanic reaction with CH to produce additional CSH, which further enhances the bonding capacity of the system. The relevant reactions can be expressed as:

[0073] Ca(OH)2+SiO2→CSH

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

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

[0076] In conclusion, the combined stabilization of loess by FA, ​​DG and cement produced a synergistic effect, significantly improving UCS through both physical and chemical modifications. The pozzolanic activity of FA combined with the cementing and filling functions of CSH and ettringite reduced porosity and enhanced particle cohesion. The stabilized loess was transformed into a low-porosity, high-density structure with uniform pore distribution and smaller pore size.

[0077] The materials of Example 1 to Example 6 were used to carry out environmental benefit tests. The results are as follows: Figure 6 shown.

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

[0079] Depend on Figure 6As shown in (b), the acidification potential values ​​show a similar trend, with Example 2 showing the highest impact (about 0.6 kg SO2eq) due to sulfur dioxide emissions during cement production. As the cement ratio of Examples 3 to 6 decreases, the AP value steadily decreases, with Example 6 again showing the lowest AP value, highlighting the environmental advantages of adding desulfurized gypsum.

[0080] Depend on Figure 6 From (c) in the figure, we can see that the eutrophication potential (EP) value is between 0.05 and 0.09 kg PO4 3- eq fluctuates among them, among which Example 3 is slightly higher than the other groups, which may be attributed to the rich nutrients in fly ash, which may promote eutrophication under certain conditions. Figure 6 As shown in (d), Example 2 has the highest photochemical ozone generation potential (POFP, about 0.45 kg NMVOC eq), which is mainly caused by the volatile organic compounds released during cement production. By introducing supplementary materials such as FA and DG, the POFP values ​​of Examples 3 to 6 are significantly reduced. Example 6 shows the lowest value (about 0.25 kg NMVOC eq), which once again proves the environmental advantage of reducing cement use.

[0081] Depend on Figure 6 As can be seen from (e) in the figure, the energy consumption (PED) shows the expected pattern, with Example 3 showing the highest total primary energy demand (about 2200MJ), which is due to the high energy consumption of fly ash production. As the cement content of Examples 4 to 6 is gradually reduced, the PED value steadily decreases. Example 6 achieves the lowest energy demand (about 900MJ), reflecting the significant energy saving effect achieved by replacing cement with desulfurized gypsum.

[0082] The results show that replacing cement with supplementary materials like FA and DG can effectively reduce various environmental impacts. Example 6 proved to be the most environmentally friendly option, showing 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. It is worth noting that the UCS of Example 5 is significantly higher than that of Example 2 containing 20% ​​cement. From an environmental perspective, Example 5 also showed significant advantages, achieving a favorable balance between strength and reduced environmental impact.

[0083] Therefore, the present invention adopts the above-mentioned loess solidification method using coal-based solid waste, and verifies the effectiveness of coal-based solid waste as a sustainable alternative to traditional cement to stabilize loess through mechanical property evaluation, microstructure characterization and environmental impact assessment, revealing the key role of microstructure densification in enhancing UCS.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A loess solidification method using coal-based solid waste, characterized in that: The following steps are involved: S1. Material pretreatment: crush loess, cement, fly ash and desulfurized gypsum, and then screen and dry them to obtain loess, cement, fly ash particles and desulfurized gypsum particles with a particle size not exceeding 0.075 mm; S2. Mix loess, cement, the fly ash particles obtained in S1 and the desulfurized gypsum particles uniformly and compact them according to proportion to obtain loess solidification material.

2. The loess solidification method using coal-based solid waste according to claim 1, characterized in that: In S1, the drying process is specifically: Dry at 100-110°C for 7-10 hours.

3. The loess solidification method using coal-based solid waste according to claim 1, characterized in that: In S2, the mass ratio of the fly ash particles, desulfurized gypsum particles, cement and loess is 0-15:0-15:0-20:

80.

4. The loess solidification method using coal-based solid waste according to claim 3, characterized in that: In S2, the mass ratio of the fly ash particles, desulfurized gypsum particles, cement and loess is 5:10:5:

80.

5. The loess solidification method using coal-based solid waste according to claim 1, characterized in that: In S2, the moisture content of the loess is not less than 10%.

6. The loess solidification method using coal-based solid waste according to claim 1, characterized in that: In S2, the particle size distribution of the loess includes silt, sand and clay, wherein the content of silt is not less than 70%.

7. The loess solidification method using coal-based solid waste according to claim 1, characterized in that: In S2, the dry density of the loess solidification material is 1.50-1.70Mg / m 3 .

8. The loess solidification method using coal-based solid waste according to claim 1, characterized in that: In S2, the unrestricted compressive strength value of the loess solidification material is 1477.42 kPa.

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