Experimental Study on the Synergistic Solidification of Soft Soil with Ceramic Powder–Slag–Phosphorus Slag

Abstract

The bearing capacity of silt soft soil is poor, making it difficult for it to be used as a subgrade material in foundation engineering, and the use of traditional Portland cement curing agents causes environmental pollution. In this study, a new soft soil curing agent, CSP (ceramic powder–slag–phosphorus slag), was prepared using ceramic powder, slag, and phosphorus slag. The unconfined compressive strength of 7-day was determined via an orthogonal test, and the optimal ratio of the curing agent was determined. The effects of the initial water content, curing agent content, admixture type, and admixture content on the mechanical properties of solidified soil were investigated via a uniaxial compression test. The microstructure characteristics of the solidified soil were analyzed via XRD and SEM-EDS, and the mechanism by which ceramic powder–slag–phosphorus slag acted as a curing agent to increase the strength of the soft soil was explored. The results show that the optimal ratio of the curing agent for the inorganic binder is ceramic powder/slag/phosphorus slag = 3:2:1, the best water glass modulus is 1 mold, the best water glass content is 26%, and the 7-day compressive strength can reach 2.382 MPa; the strength of the solidified soil decreases with an increase in the water content and increases with an increase in the curing agent content. When the water content is 35% and the curing agent content is 14%, the strength of the solidified soil can meet the requirements of relevant specifications. When the content of triisopropanolamine was 2.0% and 1.5%, the compressive strength of the 7-day and 28-day solidified soil specimens increased most significantly. The ceramic powder–slag–phosphorus slag can promote the formation of aggregates and amorphous hydration products (C-S-H, C-A-H), be distributed on the surface of the soil and fill the pores, and enhance the cementation between the particles, improving the compactness of the soil structure. In terms of the macroscopic performance, the mechanical properties of the solidified soil were significantly improved. Therefore, CSP curing agents can be promoted and applied as green, economical, environmentally friendly, and low-carbon curing materials in soft soil roadbed engineering.

1. Introduction

In recent years, China’s economy and the scale of transportation infrastructure construction have developed rapidly. It is often necessary to address the impact of soft soil and other poor foundations on the project [1]. The main characteristics of mucky soft soil are high water content, strong compressibility, low strength, high porosity, and poor permeability. The poor bearing capacity of mucky soft soil leads to uneven settlement when it is subjected to external loads. The disposal and utilization of mucky soft soil are difficult problems for the sustainable development of China’s transportation industry. The commonly used methods for treating soft soil in engineering include the soil replacement cushion method, deep compaction pile method, reinforced soil method, solidification agent reinforcement method, etc. [2]. The chemical reinforcement method mainly involves evenly mixing the curing agent with the soil and utilizing the chemical reaction between the curing agent and the soil to generate hydrated cementitious substances. These cementitious substances wrap around the surface of soil particles and play a bonding role, filling some pores, thereby achieving the effect of reinforcing the soil [3]. At present, the research at home and abroad mainly uses cement and lime as curing agents to reinforce the soil [4,5,6,7,8]. Liang Shihua et al. [9] used slag and cement as solidification agents to reinforce soft soil and conducted mechanical properties research. It was found that when the slag accounts for less than 60% of the solidification agent, the unconfined compressive strength of the solidified soil increases with the increase in the slag proportion. Wu et al. [10] studied the use effect of cement composite additives on stabilizing silt, analyzed the effects of additive types and amounts on the mechanical properties and microstructure of stabilized soil, and found that the optimal mixing ratios of fly ash, clay, and gypsum were 4%, 4%, and 2%. Dhar et al. [11] conducted clay solidification improvement experiments using plastic fibers and lime and conducted mechanical tests on solidified soil with different lime and fiber contents. The study showed that fibers help to increase the elastic modulus and transform the failure mode from brittleness to toughness. However, firstly, the production processes of cement and lime as curing agents cause environmental pollution, which is not in line with green sustainable development. Secondly, it is too expensive. At the same time, because the muddy soil contains a large amount of organic matter, the use of cement as a curing agent is high, the reinforcement effect is not good, and the strength of the solidified soil makes it difficult to achieve the desired effect [12,13].

As people have started to pay more and more attention to environmental protection and energy conservation, the use of solid waste has attracted more and more attention from the public. Phosphorus slag is an industrial waste produced during the preparation of yellow phosphorus. It has an annual output of about 8 million tons in China, but its comprehensive utilization rate is less than 30%. The phosphorus contained in phosphorus slag infiltrates the land due to rainwater erosion, causing groundwater pollution and soil pollution [14,15]. The main components of phosphorus slag are CaO and SiO₂, which may potentially be used to prepare cementitious materials [16,17,18]. Slag is an industrial waste residue formed in the process of iron making. Its chemical composition is similar to that of Portland cement clinker. It has potential hydraulicity. It is possible to form a cementitious material similar to cement by mixing slag with volcanic ash material. Therefore, it is feasible to use this cementitious material to stabilize soil [9,19,20,21]. In the process of preparing ceramic materials, a large amount of ceramic waste, which is difficult to use, is produced. Most ceramic waste is transported to remote areas and treated via leakage stacking and backfilling [22]. Ceramic products are mixed with clay, quartz, and feldspar and then calcined at high temperatures. They are mainly composed of chemical components such as silica and aluminum oxide. Jia Zhilong et al. [23] and L.G.L. et al. [24] added ceramic powder to concrete. The ceramic powder had a pozzolanic effect and micro-aggregate filling effect, thereby improving the strength of the concrete. In summary, if solid waste materials are used to develop an economical and efficient curing agent, this will not only save costs but will also be beneficial to environmental protection and will definitely generate significant social and economic benefits.

At present, most research is based on preparing curing agents using cement [25,26]. There are relatively few studies on using solid waste materials as curing agents. In this study, based on all uses of solid waste materials, a ceramic powder–slag–phosphorus slag multi-system was used to reinforce muddy soft soil. Firstly, the optimal ratio of inorganic binder was determined using an orthogonal test in which the 7-day unconfined compressive strength was determined. The effects of the initial water content, curing agent content, admixture type, and admixture content on the mechanical properties of solidified soil were investigated. Finally, the mechanism by which ceramic powder–slag–phosphorus slag acts as a curing agent to strengthen soft soil was analyzed using SEM and XRD, which can provide a theoretical basis for practical engineering applications of the new soft soil curing agent CSP. The research flowchart of this article is shown in Figure 1.

2. Test

2.1. Test Raw Materials

2.1.1. Soft Soil Sample

The soil sample used in the test was selected from the silt soil of a pond in Henan Province. The morphology of the test soil sample is shown in Figure 2, and its chemical composition is shown in

Table 1. Chemical composition of soil (%).

Chemical Composition	SiO2	Al2O3	CaO	MgO	Fe2O3
Soft soil	66.84	16.17	1.67	1.16	8.4

below. The soil samples were put into the oven to dry after grinding through a 2 mm sieve spare, using a Mastersizer 2000 laser (Marvin Instruments Limited, Worcestershire, UK) particle size analyzer to determine the particle size composition of soil samples. The particle size distribution of soil samples is shown in Figure 3. The plastic limit and liquid limit of the soil sample were measured using a digital liquid plastic limit tester. The liquid limit w_L = 40.3%, the plastic limit w_P = 27.1%, the plastic index I_P = 13.2, and the specific gravity was 2.60.

2.1.2. Curing Agent

The main chemical components of ceramic powder, slag, and phosphorus slag selected in this study are shown in

Table 2. Main chemical components of curing agent (%).

Chemical Composition	SiO2	Al2O3	CaO	MgO	Fe2O3	SO3
Ceramic powder	66.34	24.1	2.51	0.56	1.57	0.06
Slag	25.62	12.07	50.22	5.18	0.31	2.41
Phosphorus slag	30.09	4.6	49.37	1.71	1.05	4.06

. The water glass modulus was 2, which is soluble in water, and the solution was weakly alkaline. The purity of sodium hydroxide ≥ 96% was used to adjust the water glass modulus to the modulus required for the test.

2.2. Test Proportioning Design

The orthogonal test design was adopted to study the influence of the inorganic binder ratio (ceramic powder/slag/phosphorus slag), activator modulus, and activator content on the strength of solidified soil. The test adopted L9 (33), i.e., the test scheme was nine, three factors, and three levels orthogonal test. Among them, the content of the inorganic binder was 16% (the content of inorganic binder was mixed with the quality of dry silt muddy soil), and the content of the water glass was the proportion of the quality of inorganic binder. The orthogonal test table design is shown in

Table 3. Orthogonal test level factor table.

Level	Factor
	Ceramic Powder/Slag/Phosphorus Slag	Module of Water Glass	Water Glass Content
1	4:3:1	1.0	20%
2	3:2:1	1.3	23%
3	2:2:1	1.6	26%

.

2.3. Test Methods

2.3.1. Sample Preparation and Maintenance

The soil sample was put into the oven to dry the soil sample. After the soil sample was dried, it was crushed with a rubber hammer and through a 2 mm sieve, and the curing agent and the soil sample were mixed evenly according to the ratio. The water glass was dissolved in water and added to the mixed soil and stirred evenly. The stirred mixture powder was added to the mold with the inner wall coated with lubricating oil three times. The YE-200 A hydraulic pressure testing machine was used to compact the test block. After standing for 2 h, the sample was demolded and sampled, and the soil sample was wrapped with a fresh-keeping film. The process is shown in Figure 4. The solidified soil test block was placed in a standard curing room and cured to a specified age, and the test block was taken out to determine the unconfined compressive strength.

2.3.2. Unconfined Compression Strength Test

The unconfined compressive strength of the solidified soil was measured according to the “Standard for Soil Test Methods” (GB/T50123-2019) [27]. The CMT-5105 microcomputer-controlled electronic universal testing machine was used to carry out the unconfined compressive strength test. The specimen size was Φ50 mm × 130 mm. The test block was placed in the middle of the bearing plate, and the loading rate was set to 1.0 mm/min. The test block was compressed until it was destroyed. The maximum reading was automatically collected via the computer to record the ultimate load value when the test block was destroyed. The average strength of three parallel specimens in each group was taken as the final test result.

2.3.3. Microscopic Experiment

The mechanical properties test specimens were dried in an oven at 50 °C to form a powdered dry sample. According to the test requirements, the sample was treated into a test block with a size of 10 mm × 10 mm × 5 mm, and the sample was sprayed with gold for the SEM test. The dried samples were ground in a mortar and passed through a 0.075 mm sieve, and sufficient samples were taken for the XRD diffraction analysis.

3. Test Results and Analysis

3.1. Orthogonal Test Result

In this paper, the orthogonal test takes the unconfined compressive strength of the solidified soil specimen for 7-day as the evaluation index, carries out the range analysis, and discusses the best matching scheme of each component. The content of the inorganic binder is 16% (the content of inorganic binder is mixed with the quality of dry silt muddy soil), and the content of water glass is the proportion of the quality inorganic binder.

Table 4. Orthogonal test result.

Number	Ceramic Powder/Slag/Phosphorus Slag	Module of Water Glass	Water Glass Content	Unconfined Compression Strength (MPa)
T-L-1	4:3:1	1.0	20%	1.000
T-L-2	4:3:1	1.3	26%	0.766
T-L-3	4:3:1	1.6	23%	0.713
T-L-4	3:2:1	1.0	26%	2.382
T-L-5	3:2:1	1.3	23%	1.003
T-L-6	3:2:1	1.6	20%	0.605
T-L-7	2:2:1	1.0	23%	1.892
T-L-8	2:2:1	1.3	20%	0.835
T-L-9	2:2:1	1.6	26%	0.806

shows the test results of 7-day unconfined compressive strength of solidified soil under different mixing schemes of ceramic powder/slag/phosphorus slag, water glass modulus, and water glass content in the orthogonal test.

It can be seen from

Table 5. Range of 7-day analysis table.

	Ceramic Powder/Slag/Phosphorus Slag	Module of Water Glass	Water Glass Content
k1	0.826	1.758	0.813
k2	1.330	0.868	1.203
k3	1.178	0.708	1.318
R	0.504	1.050	0.505
Influencing primary and secondary factors	B > C > A

that ki is the average effect value at each factor level, and the R value is the range. The size of the R value is used to measure the influence of various factors on the strength of the solidified soil. A large R value indicates that the factor has a great influence on the strength. The primary and secondary relationship between the factors in the test and the test results is water glass modulus (B) > water glass content (C) > ceramic powder/slag/phosphorus slag (A). According to the average effect value data of each factor level in the range analysis table, we can conclude that each group of large values is taken, that is, the unconfined compressive strength corresponding to A₂ is 1.33 MPa, the unconfined compressive strength corresponding to B₁ is 1.758 MPa, and the unconfined compressive strength corresponding to C₃ is 1.318 MPa. Therefore, the optimal ratio of the three factors at 7-day age is the T-L-4 group (A₂B₁C₃), so the formula optimization result is an inorganic binder (ceramic powder/slag/phosphorus slag = 3:2:1), the water glass modulus is 1, and the water glass content is 26%. The following tests in this paper are based on this formula.

3.2. The Effect of Initial Water Content on the Mechanical Properties of Solidified Soil

According to the optimal ratio determined above, the influence of water content on the strength of solidified soil was analyzed. The water content was 30%, 35%, 40%, and 45%, respectively, and the test numbers were T1-1~T1-4. The unconfined compression test was carried out on the solidified soil with curing ages of 7-day and 28-day, and the test results are shown in

Table 6. Strength of solidified soil with different moisture content.

Number	Water Content	7-Day (MPa)	28-Day (MPa)
T1-1	30%	2.832	3.727
T1-2	35%	2.267	3.161
T1-3	40%	0.928	2.174
T1-4	45%	0.340	1.246

.

Figure 5 shows that with the increase in water content, the unconfined compressive strength of solidified soil decreases gradually. When the water content increased to 35%, 40%, and 45%, the 7-day compressive strength decreased by 4.82%, 61.04%, and 85.73%, respectively. When the water content increased to 45%, the 7-day unconfined compressive strength was only 0.34 MPa. When the water content of 28-day solidified soil specimens increased from 30% to 35%, 40%, and 45%, respectively, the compressive strength of 28-day decreased by 15.19%, 41.67%, and 66.57%, respectively, with the compressive strength of the 30% water content taken as the reference value. This is because with the increase in water content, the concentration of the alkali environment in solidified soil samples decreased, the Si⁴⁺ and Al³⁺ dissolved from aluminosilicate decreased, and the amount of hydration products decreased, resulting in a decreased strength of the solidified soil.

3.3. The Effect of Solidifying Agent Content on Mechanical Properties of Solidified Soil

The optimum ratio determined above is used to test the effect of curing agent content on the strength of the solidified soil. The water content was 30%, the dosages of the curing agent were 14%, 16%, 18%, and 20%, respectively, and the numbers were T1-5~T1-8. The test results are shown in

Table 7. Strength of solidified soil with different curing agent content.

Number	Dosage	7-Day (MPa)	28-Day (MPa)
T1-5	14%	1.380	3.539
T1-6	16%	2.382	3.727
T1-7	18%	3.400	4.439
T1-8	20%	3.660	4.985

.

Figure 6 shows that the compressive strength of 7-day and 28-day is positively correlated with the content of the curing agent. Taking 7-day as an example, when the content of the curing agent increases from 14% to 16% and 18%, respectively, the strength of the solidified soil specimens increases by 72.61% and 42.74%, respectively. The compressive strength is obviously enhanced when the content of the curing agent is 14% to 18%, while the compressive strength only increases by 7.65% when the content of the curing agent is 18% to 20%. This is mainly because as the content of the curing agent increases, the content of active SiO₂ and Al₂O₃ in the soil also increases. Under alkali excitation, the Al-O bond and Si-O bond in the vitreous body are destroyed and react with the dissolved Ca²⁺ to form hydrated calcium silicate (C-S-H) and hydrated calcium aluminate (C-A-H), which are cemented and filled between the loose particles of the soil. The compactness of the soil is enhanced, and the macroscopic performance is that the mechanical properties of the soil are improved. Upon further increasing the dosage of the curing agent, the strength improvement is not significant, indicating an optimal dosage of the curing agent. The strength of the solidified soil will not always increase rapidly with an increase in the dosage of the curing agent.

3.4. The Effect of Solidifying Agent Content on the Strength of Solidified Soil

Based on the optimal ratio, triisopropanolamine, anhydrous sodium sulfate, and polyaluminum chloride were used as admixtures. The dosages were 1%, 1.5%, 2%, and 2.5%, respectively, and the numbers were T1-9~T1-20, respectively. The effects of the admixture dosage and type on the strength of solidified soil were studied. The specific test plan and test results are shown in

Table 8. Different admixture curing schemes and compressive strength.

Number	Ceramic Powder: Slag: Phosphorus Slag	Module of Water Glass	Water Glass Content	Water Content	Admixture	7-Day/MPa	28-Day/Mpa
T1-9	3:2:1	1	26%	30%	1% Triisopropanolamine	2.262	4.163
T1-10	3:2:1	1	26%	30%	1.5% Triisopropanolamine	2.561	4.953
T1-11	3:2:1	1	26%	30%	2% Triisopropanolamine	3.137	3.844
T1-12	3:2:1	1	26%	30%	2.5% Triisopropanolamine	2.419	3.581
T1-13	3:2:1	1	26%	30%	1% Anhydrous sodium sulfate	2.221	3.136
T1-14	3:2:1	1	26%	30%	1.5% Anhydrous sodium sulfate	2.556	3.601
T1-15	3:2:1	1	26%	30%	2% Anhydrous sodium sulfate	1.935	3.442
T1-16	3:2:1	1	26%	30%	2.5% Anhydrous sodium sulfate	1.762	3.433
T1-17	3:2:1	1	26%	30%	1% Pac	1.700	3.614
T1-18	3:2:1	1	26%	30%	1.5% Pac	2.591	3.655
T1-19	3:2:1	1	26%	30%	2% Pac	1.731	3.775
T1-20	3:2:1	1	26%	30%	2.5% Pac	1.598	3.227

below.

Figure 7 shows that different admixtures and admixtures have different effects on the strength of solidified soil. At the age of 7-day, when the admixture content is 1%, the three admixtures have no reinforcing effects on the strength of solidified soil; when the dosage of the admixture is 1.5%, the strength of triisopropanolamine solidified soil and anhydrous sodium sulfate solidified soil increase by 7.5% and 7.3%, respectively. The strength of polyaluminum chloride solidified soil increases by 8.8%. When the admixture content is 2% and 2.5%, only triisopropanolamine as an admixture has a reinforcing effect on the solidified soil, and the strength of the solidified soil increases by 31.7% and 1.6%, respectively. At the age of 28-day, when triisopropanolamine is used as an admixture, the strength of solidified soil increases first and then decreases with the increase in dosage. When the dosage is 1.5%, the strength of solidified soil is the highest at 4.953 MPa, which is increased by 32.8%. Anhydrous sodium sulfate does not play a role in promoting the strength of solidified soil. When polyaluminum chloride is used as an admixture, the strength of solidified soil is promoted when the dosage is 2% and the strength increases by 0.8%. This is because the triisopropanolamine mainly accelerates the hydration of the silicate phase in the solidified soil sample. In an alkaline environment, it can promote the disintegration and hydration of the high-alumina glass phase and promote the pozzolanic effect. Enhancing its strength gradually manifests as mineral dissolution and forming hydration products. Anhydrous sodium sulfate as an additive has no ideal effect on the compressive strength of solidified soil. This is because after adding anhydrous sodium sulfate, the concentration of Na⁺ in the solidified soil sample increases, and the hydration radius of Na⁺ increases. After the negatively charged soil particles adsorb a large amount of Na⁺ on the surface, the double-layered thickness of the soil particles becomes thicker, and the cohesion between the soil particles is weakened. Therefore, the structure of the solidified soil sample becomes loose, and the strength decreases. When the soil is mixed with polyaluminum chloride, because after the hydrolysis of polyaluminum chloride, Al³⁺ can replace Si⁴⁺ in the silicon oxygen tetrahedron, and the degree of polymerization of the silicon oxygen tetrahedron and the amount of hydration products are increased. Filling some of the pores makes the solidified soil sample structure denser and improves the strength of the solidified soil. At the same time, adding polyaluminum chloride as an admixture will introduce a certain amount of chloride ions, which will help the condensation of the aluminum silicon oxygen tetrahedron to achieve the reinforcing effect on the soil.

3.5. CSP Solidification Mechanism of the Multi-Component Composite System

3.5.1. X-ray Diffraction Test

An XRD diffraction test was carried out to qualitatively analyze the mineral composition of solidified soil in the ceramic powder–slag–phosphorus slag multi-component composite system, and Jade (Jade 9.0) software was used for phase detection to identify the category of hydration products to which the characteristic peaks of XRD pattern belong. The peak shape and position of each curve did not change much, indicating that the type of hydration products of solidified soil did not change with the change in water content, curing agent content, and the addition of admixtures.

Figure 8a shows the XRD diffraction pattern of solidified soil and plain soil with different water contents. It can be seen from the diagram that the XRD diffraction pattern of plain soil is mainly composed of the characteristic diffraction peaks of non-clay minerals, such as quartz and albite, and the characteristic diffraction peaks of clay minerals, such as mica. After adding the curing agent, the characteristic diffraction peak of quartz decreased significantly, indicating that the quartz in the soil participated in the curing reaction. The characteristic diffraction peak of quartz decreased with the decrease in initial water content, indicating that with the increase in the initial water content, the alkali environment concentration of the solidified soil sample decreased, and the curing agent reaction weakened. There are no obvious diffraction peaks of calcium silicate hydrate (C-S-H), calcium aluminosilicate hydrate (C-A-S-H), and sodium aluminosilicate hydrate (N-A-S-H) in the XRD pattern. But there are diffuse diffraction peaks between 25° and 30°. This is because the hydration products generated by the curing agent exist in an amorphous form.

Figure 8b shows the XRD diffraction patterns of solidified soil and plain soil with different curing agent contents. It can be seen from the figure that with the increase in the curing agent content, the content of the hydration phase increases significantly, which indicates that the increase in the curing agent content accelerates the chemical reaction rate of active components. With the increase in the curing agent content, the characteristic diffraction peak of quartz decreases, indicating that quartz participates in the curing reaction and the active SiO₂ in the soil completely reacts. The increase in the curing agent content makes the curing reaction more sufficient.

Figure 8c shows the XRD diffraction pattern of solidified soil with different admixtures. It can be seen from the diagram that the characteristic diffraction peaks of quartz in the T1-16 group are significantly higher than those in other groups, indicating that adding anhydrous sodium sulfate as an admixture does not promote the reaction of solidified soil, which is consistent with the macroscopic test results of solidified soil samples.

3.5.2. SEM-EDS Test

• Micromorphology analysis

The SEM-EDS experimental analysis method was used to analyze the solidified soil samples at the age of 28-day to observe the microstructure changes in the solidified soil in the ceramic powder–slag–phosphorus slag multiple systems more intuitively. As shown in the SEM diagram, the new curing agent produces different chemical products, mainly some flocculent flaky C-A-H and C-S-H gel, in the solidified soil samples. These hydrated cementitious substances cover the surface of soil particles to connect the soil particles, densifying the soil structure, and the macroscopic performance is that the mechanical properties of the solidified soil samples are significantly enhanced.

Comparing Figure 9a,b, as the initial moisture content of the solidified soil increases, the pores of the solidified soil sample increase, and the generation of flocculent hydration and cementitious products decreases. There are more voids between soil particles, which are in a dispersed state. This is because, with the increase in the initial moisture content, the alkali environment concentration required for the hydration reaction of the solidified soil is reduced, leading to a weakened reaction of the solidified soil and a decrease in the generation of hydration products. The weakening of the bonding effect between soil particles leads to a decrease in the strength of solidified soil, which is consistent with the conclusion in the previous macroscopic experiment that the moisture content affects the mechanical properties of solidified soil.

Comparing Figure 9a,c, with the increase in the amount of curing agent added, the generation of hydration and cementation products increases and fills the pores of the solidified soil sample. The soil particles are cemented into a whole structure, and some larger pores are also filled with flocs, thus improving the overall structure of the solidified soil. This is due to the increase in the amount of curing agent, which increases the content of active SiO₂ and Al₂O₃ in the solidified soil sample. Under alkali excitation, the active Si-O and Al-O bonds break, leading to the volcanic ash reaction and the formation of hydrated silicate cementitious substances.

As shown in Figure 9d–f, with the addition of additives, the crystals generated by the curing agent will adsorb onto the surface of particles and between particles, causing them to bond into larger agglomerates. At the same time, the hydration and cementation products will fill the larger pores between soil particles, and the type of soil structure will change from loose point-to-point and point-to-surface contacts to a dense flocculation structure. When the dosage of the admixture is the same, and triisopropanolamine is used as the admixture, the number of hydration products generated is the highest, and the structure is the densest, which is consistent with the macroscopic mechanical indicators of the solidified soil mentioned earlier.

2.

EDS spectrum analysis

EDS point energy spectrum analysis was carried out on CSP solidified soil. The specimen T1-1 was taken at the 28-day curing age, and four different points were selected for energy spectrum analysis. The hydration reaction product and the microscopic composition of the solidified soil sample were determined according to the content and proportion of each point element. The results are shown in

Table 9. T1-1 element distribution table.

Element	Mass Percentage (%)				Atomic Percentage (%)
	S1	S2	S3	S4	S1	S2	S3	S4
C	6.23	5.08	3.13	3.07	11.25	9.25	6.45	6.21
O	36.07	37.07	25.02	26.84	48.91	50.62	38.62	40.71
Na	2.44	2.08	1.57	1.55	2.31	1.98	1.68	1.64
Mg	0.92	0.91	0.99	0.92	0.82	0.82	1.00	0.92
Al	11.66	11.70	14.26	13.90	9.38	9.48	13.05	12.50
Si	24.25	24.59	28.85	29.11	18.73	19.13	25.37	25.15
K	3.54	3.67	6.34	6.01	1.96	2.05	4.00	3.73
Ca	5.45	5.40	5.75	6.02	2.95	2.94	3.54	3.64
Ti	0.37	0.32	0.53	0.43	0.17	0.15	0.27	0.22
Fe	9.08	9.18	13.58	12.15	3.53	3.59	6.00	5.28

. The elements of the hydration product are O, Al, and Si and a small amount of C, Na, K, Ca, Fe, and other elements. The Ca/Si ratio is small, and the Ca/Si ratio in the C-S-H gel is generally between 0.8 and 1.5. This is because the amount of CaO in the inorganic binder is less than Al₂O₃ and SiO₂, so the amount of C-S-H gel formed via the reaction is limited, and the Al element content at these points is higher. It is speculated that part of the Si on the molecular chain is replaced with Al to form a (C, N)-A-S-H gel. There is no C element in the test raw materials, but there is a small amount in the EDS test of the test block, which may be due to the absorption of CO₂ in the air during the alkali excitation reaction, so a small amount of C is shown in the EDS.

4. Microscopic Strengthening Mechanism of Soft Soil Reinforced by CSP

Figure 10 is the meso-mechanism model of the CSP-solidified soil. It can be seen from the figure that the performance of solidified soil depends on the degree of pozzolanic reaction, micro-aggregate effect, and ion exchange reaction. Controlling the water content and increasing the amount of curing agent is conducive to forming more hydration products, playing the role of the skeleton and filling. At the same time, hydration products can promote ion exchange and enhance the agglomeration effect of soil particles, thus improving the strength.

• Volcanic ash reaction: The addition of alkaline agents provides a good alkaline environment for exciting potential active tissues in soft soil and curing agents. Ceramic powder contains a large amount of active SiO₂ and Al₂O₃, and three-dimensional network structures such as silica and aluminum oxide dissolve in an alkaline environment, generating [Al(OH)₄]⁻ and [SiO(OH)₃]⁻ plasma monomers. Subsequently, ionic monomers are attracted and connected via hydroxyl groups to form intermediate complexes. Then, they are dehydrated and condensed to form an oligomeric aluminosilicate vitreous sol. Under the excitation of an alkaline agent, the slag and phosphorous slag release a large amount of Ca²⁺ and further react with aluminosilicate vitreous sol to generate a gel such as calcium silicate hydrate (C-S-H) and calcium aluminosilicate hydrate (C-A-S-H). At the same time, the surplus Ca²⁺ will combine with OH⁻ to generate Ca(OH)₂ tabular crystals, which will absorb CO₂ in the air in an alkaline environment to carbonize and generate CaCO₃ whiskers. The Si-O-Si and other glass bodies in the soft soil structure also dissolve in the alkali environment, forming silicate series gel with the help of the curing agent. The soil system becomes more compact under the action of the generated series of polymers, hardening the rigidity of the solidified soil and strengthening the stability of the solidified soil.
• Micro aggregate effect: The incompletely reacted ceramic powder, slag, and phosphorus slag particles further refine the soil pore structure via the micro aggregate effect. The good intercalation between coarse particles increases the internal friction force of the soil, improves the compactness of the pore structure and interface structure, and provides a good spatial environment for various hydration reactions.
• Ion exchange reaction: Under the action of OH⁻, many low valent cations such as K⁺ and Na⁺ adsorbed on the surface of soft soil capture the high valent cations such as Ca²⁺ released from the hydration of slag and phosphorus slag for ion exchange. With the addition of these high valent cations, the double layer structure of particles is reduced, the electrokinetic potential is lowered, and the bound water is released, reducing the thickness of the bound water film on the surface of particles, leading to the flocculation and agglomeration of soil particles. The greater the binding force between each other, the greater the cohesion between soil particles, and the more significant the strength improvement of solidified soil.

5. Conclusions and Recommendations

• Through the orthogonal test, the main factors influencing the unconfined compressive strength were found in the following descending order: water glass modulus > water glass content > inorganic binder ratio. The ratio of ceramic powder/slag/phosphorus slag was 3:2:1, the water glass modulus was 1, and the water glass content was 26%. With these parameters, the unconfined compressive strength was 2.382 MPa.
• As the moisture content increases from 30% to 45%, the 28-day strength decreases by 66.56%. As the curing agent increased from 14% to 20%, the 28-day strength increased by 40.85%. The additive triisopropanolamine has the best effect on promoting. When the content of triisopropanolamine was 2.0% and 1.5%, the compressive strength of the 7-day and 28-day solidified soil specimens increased the most significantly by 31.7% and 32.8%, respectively.
• Through the analysis of the microscopic test results, it was found that, under the action of the alkaline agent, the constantly generated hydration products tightly bonded with the soil particles to form a dense gel crystal network structure, the soil structure was more compact, and the mechanical properties of the solidified soil were significantly improved.

The CSP soil stabilizer is a green, economical, environmentally friendly, and low-carbon curing material. It has achieved good results in curing soft soil, and its applicability to other soft soil and special soil needs to be further discussed. In the actual project, according to different soil and construction requirements, it is necessary to carry out the ratio allocation and the optimization of the content, so as to be better used in engineering practice.