Performance Evaluation and Mechanism Study of Solid Waste-Based Cementitious Materials for Solidifying Marine Soft Soil under Seawater Mixing and Erosion Action

Abstract

The main purpose of this research is to develop a solid waste-based cementitious material (SWC) instead of cement for solidifying a large amount of marine soft soil with high water content and low bearing capacity in coastal areas. This aims to solve the problems encountered in the practical application of cement soil, such as slow strength growth and poor durability. The SWC includes ground granulated blast furnace slag (GGBS), dust ash (DA), and activated cinder powder (ACP), with admixtures of naphthalene sulfonate formaldehyde condensate (NS) and compound salt early strength agent (SA). Both the 7 d and 28 d compressive strength values of the SWC formulations G4 and G7 are about twice as strong as those of cement soil (GC), even when mixed with seawater. Immersion tests revealed that stabilized soil had superior resistance to seawater corrosion compared to cement soil. X-ray diffraction, scanning electron microscopy, infrared spectroscopy, and thermogravimetric analysis explained that the main hydration products in cement soil are C-S-H and CH, while in stabilized soil, SWC generates a large amount of C-A-S-H with gelling properties and AFt with filling properties. These hydration products have better effects on strength and seawater erosion resistance.

1. Introduction

Soft soil is characterized by high water content, a high liquid limit, and complex soil quality [1]. Its large pore ratio and compressibility, as well as its high salt content, result in low carrying capacity, weak structure, and easy settlement [2]. Coastal cities are economically developed areas in China. In the past 40 years, China’s coastal areas have undergone significant modernization. Currently, more than 70% of China’s large and medium-sized cities are located in coastal areas. The process of modernization has been confronted with challenges related to shortages of resources and land [3]. The soft land foundation in the coastal areas has seriously hindered the process of urban development [4]. During the treatment process, a large amount of construction materials and human and material resources are consumed. Additionally, subsequent environmental problems and construction project quality are also worrying.

Existing soft soil reinforcement methods in China mainly involve driving into cement mixing piles [5]. Additionally, there has been a great deal of research internationally on the gravel replacement method, the electrokinetic treatment method, the deep vibration method, and so on. Heo et al. [6] conducted a study using the gravel replacement method for road construction in a city in South Korea as an example, and found that it reduced construction costs by 29% and improved foundation stability. Hassan et al. [7] improved the characteristics of soft soil through electrokinetic treatment, which significantly increased undrained shear strength near the foundation model and raised both liquid and plastic limits. Cristovao et al. [8] applied the deep vibration method to Lome Container Terminal in Togo. Cement mixing piles are a common method for strengthening saturated soft soil foundations. Compared with other construction methods, cement mixing piles have advantages such as simple equipment and convenient construction, leading to better economic and social benefits. However, due to the complexity of the geological formation environment, the unpredictable rheological characteristics of soft soil, the uncertainty in construction technology, and imperfections in the cement soil reinforcement mechanism, controlling the energy efficiency of cement mixing piles is difficult. This is especially true when used to solidify soft soil foundations along the coast where traditional cement soils face issues such as low early strength, slow construction progress, and high material costs [9]. In this context, using solid waste-based cementitious materials instead of cement as a new environmentally friendly soft soil stabilizer has emerged.

Soft soil stabilizer is an engineering material used to strengthen soft soil foundations. These materials can quickly and significantly improve the physical and mechanical properties of soft soil. After being mixed with soil, soft-soil-solidifying materials can reduce the porosity of the soil and improve the compaction degree through a series of physical and chemical reactions, so that the soil becomes a dense cementitious material. According to the selection and mechanism of different materials, soil stabilizers can be divided into four categories: inorganic soil stabilizers, polymer soil stabilizers, biological enzyme soil stabilizers, and compound soil stabilizers, among which inorganic soil stabilizers are commonly used [10]. Traditional inorganic soil stabilizers are mainly cement, lime, fly ash, etc. As more importance is attached to environmental protection and resource conservation by our country, traditional raw materials such as cement and lime are in short supply with rising prices. Therefore, it has become a research hotspot to find bulk industrial solid waste to replace traditional raw materials for preparing a new type of environmentally friendly soil stabilizers.

The use of cement to strengthen soft soil, both domestically and internationally, has a history of more than 60 years. After mixing cement with soil, the minerals in the cement undergo strong hydrolysis and hydration reactions with water in the soil. At the same time, calcium hydroxide (Ca(OH)₂) decomposes and forms other hydration compounds. The reinforcement of soil with cement is the result of the combined action of the skeleton effect of cement and the physicochemical action of Ca(OH)₂. The latter causes clay particles and microaggregates to form a stable aggregate structure; cement covers these particles and binds them together into a strong whole [11]. Scholars have conducted systematic research on fundamental engineering properties, constitutive relations, field test studies, and other aspects related to cement soil. They have also summarized empirical relationships between mechanical properties such as strength and basic physical properties including cement content, water content, and compaction degree. Additionally, experimental studies have been carried out on microscopic mechanism analysis for understanding how strength is formed in cement soil through scanning electron microscope observation and X-ray diffraction analysis [12].

However, after long-term practice, we have gradually recognized the limitations of cement and other materials in their effectiveness for curing treatment. These shortcomings primarily include the high cost of cement soil; restrictions imposed by soil types, resulting in bad reinforcement effects for soft clay with a high plasticity index, as well as soils with high organic matter or salt content; and susceptibility to drying shrinkage and cracking [13]. Currently, a variety of soil stabilizers have emerged, and the corresponding mechanisms of action have been widely studied. In recent years, a large number of new soil stabilizers have emerged in the international market such as TS-100 polymer soil stabilizer and EN-1 soil stabilizer, produced in the United States; Roadpacker biological soil stabilizer, produced in Australia; ATST series soil stabilizers, produced by the UKC company of Shinagawa, Japan); ISS soil stabilizer, produced in South Africa, etc. [14].

However, in actual applications, due to the lack of sufficient construction cases and experience summaries of non-inorganic solidified materials, construction units do not know much about their properties and mechanisms. There are concerns about long-term stability and environmental impact. Therefore, in actual construction, the construction unit is often still more inclined to use cement, lime, and other traditional soil stabilizers. With the increase in our state’s promotion of solid waste-based cementitious materials in the field of construction, more and more people are beginning to accept new inorganic soil stabilizers such as solid waste-based cementitious materials as a feasible scheme for soft soil reinforcement.

Murmu et al. [15] utilized an alkali-activated fly ash geopolymer to improve the engineering characteristics of soft soil. After curing, the compressive strength of the soft soil was significantly enhanced, while its expansion and contraction characteristics were weakened. Microscopic analysis also revealed that the soft soil cured using this cementable material had a tighter and denser microstructure. Wang et al. [16] employed slag, desulphurized gypsum, and calcium carbide slag to produce solid waste-based cementitious materials for solidifying marine soft soil. The experimental results indicate that the soft soil solidified using solid waste-based cementitious materials has better durability than when using cement alone. Microscopic analysis showed that C-S-H gel and ettringite formed by the solid waste-based cementitious materials in soft soil made the soil structure more dense through bonding and filling, thus improving the strength of the stabilized soil. Liu et al. [17] used industrial solid wastes such as slag, desulphurized gypsum, and fly ash to prepare cemented soil. Microscopic analysis revealed that the main reason for the strength improvement of stabilized soft soil was due to the formation of C-S-H, C-A-S-H, and N-A-S-H gels, as well as ettringite and zeolite hydration products from solid waste-based inorganic cementitious materials. Yao et al. [18] found that SiO₂ and Al₂O₃ in the soil could chemically react with a small amount of Ca(OH)₂ in the geopolymer to form a gel, which fills the space between the soil particles and connects them. Additionally, the alkali activator in the geopolymer undergoes substitution reactions with exchangeable alkali metal cations in the soil, resulting in the precipitation of alkali metal hydroxide near the surface of the soil particles. After the replacement reaction, the residual alkali activator reacts with SiO₂ and Al₂O₃ in the soil to form a thin film on their surface, consisting mainly of sodium silicate and sodium aluminate.

Extensive in-depth research has been recently carried out on the durability of soft soil solidified by solid waste-based cementitious materials. Huang et al. [19] used alkali-activated slag instead of cement as a soil stabilizer, and the results showed that soft soil solidified with alkali-activated slag exhibited higher compressive strength, lower water permeability and chloride ion mobility coefficient, better sulfate resistance, and lower porosity. Jiang et al. [20] compared the durability of slag-based and cement-stabilized soil under sulfate attack, and the results showed that after full immersion in Na₂SO₄ solution for 120 days, the surface of geopolymer-stabilized soil remained intact with almost no cracks. This is because the higher pH value and Na⁺ content in the Na₂SO₄ solution help maintain the uniformity of the internal pore structure in the geopolymer. Significant lateral and longitudinal cracks appeared on the surface of cement soil. The compressive strength of cement soil increased before 28 days of immersion but decreased sharply at later stages, while the compressive strength of geopolymer-stabilized soil decreased slowly and steadily with increasing immersion time.

In summary, large amounts of marine soft soil are found in coastal areas as a result of long-term deposition by rivers. When traditional cement is used as the stabilization material for mixing piles, there are problems such as low early strength, slow strength growth, and poor resistance to seawater erosion. On the other hand, the high-value utilization of general industrial solid waste is an important opportunity to help the cement industry reduce carbon emissions. With the increasing promotion of solid waste-based cementitious materials by our state, steel slag, granulated blast furnace slag, desulfurization gypsum, fly ash, etc., are also used for soil stabilization. However, further verification of the mechanical properties, stability, and environmental impact of stabilized soil is urgently needed along with studying the corresponding mechanisms of action.

This study aims to investigate the use of general industrial wastes, such as ground granulated blast furnace slag (GGBS), dust ash (DA), and activated cinder powder (ACP), as well as additives like naphthalene sulfonate formaldehyde condensate (NS) and compound salt early strength agent (SA), to produce solid waste-based cementitious materials (SWC) for marine soft soil stabilization. The incorporation of GGBS and ACP promotes the generation of hydraulic cementitious products, thereby enhancing the structural bonding between soil particles. Meanwhile, the addition of DA encourages the production of filling hydraulic products, which fill the numerous pores present in the soft soil and convert the free water within the soil into crystalline water, thus reducing the soil’s free water content. Furthermore, the alkaline components in DA can stimulate the active components in the solid waste raw materials and the minerals within the soil, thereby enhancing the solidification effect. The objective of this study is to employ SWC as a replacement for cement in order to address the issues of low strength and poor resistance to seawater erosion encountered in the traditional cement consolidation of soft soils. Additionally, this research offers a novel approach for the effective utilization of general industrial wastes, aligning with the concept of green development.

2. Materials and Methods

2.1. Raw Materials

2.1.1. Trial Soil

The soft soil used in the experiment (Figure 1) was sourced from Daxideng in Xiamen, China (Figure 2). It was extracted from the original mudflats and consisted mainly of fine-grained clay, mud, or muddy clay. The salt content was high, the texture was sticky and heavy, the structural integrity was poor, the specific gravity and bulk density were large, and it had characteristics of high compressibility, low bearing capacity, and large settlement. Jiangsu Zhugang Construction Group Co., Ltd. (Lianyungang, China) analyzed the natural specific gravity (γ), liquid limit (LL), plastic limit (PL), initial moisture content (W), salt content (Sc), organic matter content (Oc), alkalinity (ESP), porosity ratio (e), and compression coefficient (a) of the soft soil. The physical and chemical properties of the test soil are shown in

Table 1. Physical and chemical properties of marine soft soil.

γ (g/cm3)	LL (%)	PL (%)	W (%)	Sc (g/kg)	Oc (g/kg)	ESP (%)	e (--)	a (MPa−1)
16.40	49.70	31.20	60.60	30.23	1.84	28.80	1.56	1.28

, while its chemical composition is shown in

Table 2. Main chemical composition of marine soft soil (wt. %).

Composition	SiO2	Al2O3	Fe2O3	CaO	K2O2	MgO	Na2O	TiO2	Other
Soft soil	58.06	15.75	5.83	4.33	2.52	2.27	0.86	0.86	8.44

. The XRD analysis is presented in Figure 3, and the microscopic morphology is depicted in Figure 4.

2.1.2. Solid Waste-Based Cementitious Materials

The solid waste-based cementitious material (SWC) is mainly composed of three raw materials: ground granulated blast furnace slag (GGBS), dust ash (DA), and activated cinder powder (ACP). GGBS was sourced from Yonggang Group Co., Ltd. in Suzhou, Jiangsu Province, China, with a level of S95; ACP is a modified coal-based solid waste active powder from Huainan, China; and DA was sourced from Nanjing Yangzi Power Engineering Limited Co., Ltd. (Nanjing, China) Ordinary Portland cement (OPC 42.5 R) used in cement soil is taken from Anhui Conch Cement Co., Ltd. (Wuhu, China). The admixtures used are naphthalene sulfonate formaldehyde condensate (NS) and compound salt early strength agent (SA), both of which are commercially available products. The role of NS is to regulate the flow performance of slurry during the actual construction of mixing piles, while SA functions to adjust the solidification time of slurry in the same process. To ensure experimental consistency, the dosage of both admixtures remains constant. The chemical compositions of the main cementitious materials are shown in

Table 3. Chemical composition of main materials (%).

Composition	SiO2	CaO	Al2O3	SO3	MgO	Fe2O3	Na2O	TiO2	LOI
GGBS	29.37	40.82	14.12	0.20	1.32	8.02	0.46	0.36	1.12
ACP	21.35	57.03	6.45	3.69	3.10	4.01	0.32	0.75	0.28
DA	8.03	42.94	4.80	18.43	0.22	0.49	0.06	0.25	16.56
OPC	27.61	48.98	4.93	1.85	2.32	3.39	0.21	0.47	3.66

.

2.2. Proportions

To ensure the consistency of test methods and conditions, the test process for cement soil and stabilized soil was identical, except for the different material formulations added to the soft soil. SWC was prepared using GGBS, ACP, and DA, with the combined content of the three components totaling 100%. Considering the differences in fluidity and cost among the components, the ACP content was restricted to 0–25%, GGBS to 50–75%, and DA to 0–25%. A simplex lattice design was used for the mixture design experiment, selecting proportions represented by the vertices, midpoints of edges, and the center of the triangle within the defined ranges for testing.

The dosage of NS was fixed at 0.05%, the dosage of SA was fixed at 0.5%, the water-to-cementitious material ratio (W/C) was fixed at 0.6, and the cementitious material-to-marine soft soil ratio (C/S) was fixed at 0.2. The formulation is shown in

Table 4. Relative percentages of each component (%).

Sample	Cementitious Materials				Admixtures		W/C	C/S
	ACP	GGBS	DA	OPC	NS	SA
G1	12.5	62.5	25.0	/	0.5	0.05	0.6	0.2
G2	25.0	62.5	12.5	/	0.5	0.05	0.6	0.2
G3	0	75.0	25.0	/	0.5	0.05	0.6	0.2
G4	25.0	75.0	0	/	0.5	0.05	0.6	0.2
G5	12.5	75.0	12.5	/	0.5	0.05	0.6	0.2
G6	25.0	50.0	25.0	/	0.5	0.05	0.6	0.2
G7	16.7	66.6	16.7	/	0.5	0.05	0.6	0.2
GC	/	/	/	100	0.5	0.05	0.6	0.2

.

2.3. Experimental Methodology

2.3.1. Preparation of Seawater Solutions

Based on seawater sampling results from the coastal area surrounding the test soil, sea crystals were used to simulate the salinity of actual seawater in the laboratory. The required amount of sea crystals was weighed and dissolved in a beaker, then cooled to room temperature. The beaker and glass stirring rod were rinsed 2–3 times with distilled water, and the rinsing liquid was transferred to a container and mixed by shaking. When adding water to the volumetric flask to within 1–2 cm below the meniscus, a pipette was used to adjust the water level precisely to the meniscus. The bottle was then tightly sealed and gently shaken to mix the solution evenly.

2.3.2. Preparation and Basic Performance Testing of Stabilized Soft Soil Samples

(1)

Preparation of stabilized soil and cement soil samples

A specified amount of soft soil was weighed and added to the mixing pot. This was mixed with the prepared SWC slurry or cement slurry. The mixer was started and stirred at low speed for 120 s, followed by a 15 s stop to scrape down the mixing blade and pot walls. Mixing was then resumed at high speed for another 120 s, completing the preparation of the stabilized soil/cement soil mixture. The mixture was poured into a 40 mm × 40 mm × 40 mm square mold. After numbering, the test mold was placed in a constant temperature curing box and sealed in a bag. The sample was demolded 24 h after molding, taking care to avoid damage. The specimen was numbered and sealed in a bag until it reached the required curing age for the next test.

(2)

Compressive strength test

The compression test must be performed using a compression-testing machine. The compression surfaces are the two sides of the specimen formed during molding, each with an area of 40 mm × 40 mm. The loading speed of the press should be set to 1 mm/min. The compressive strength was tested using a CDT1305-2 (MTS, Minneapolis, MN, USA) electronic pressure testing machine, following Equation (1).

$$G_c=\frac{F_c\times 1000}{A}$$

(1)

where G_c is the compressive strength (MPa); F_c is the press reading (kN); and A is the compression area (mm²).

(3)

Water resistance test

Water resistance coefficient is an index to measure the water resistance of stabilized soil and cement soil samples. After curing for 28 d, the compressive strength (T₀) of the sample was obtained by testing. After soaking the cured samples for 1, 3, 7, 14, 28, and 60 days, the compressive strength (T_n) of the sample was also obtained by testing. The calculation formula of the water resistance coefficient (W) is shown in Equation (2).

$$W=\frac{T_n}{T_0}\times 100$$

(2)

where W is the water stability coefficient (%); T_n is the unconfined compressive strength of the specimen soaked in water for n days (MPa); and T₀ is the unconfined compressive strength of the specimen without soaking in water (MPa).

(4)

Seawater erosion resistance test

Seawater erosion resistance is an index to measure the resistance of solidified soil to seawater salt ions. Similar to the sample method in Section 2.3.2 (3), only the soaking environment of the sample was changed from fresh water to seawater with a salinity of 30‰. The compressive strength of the specimen after curing for 28 d (H₀) and the strength after soaking in seawater for n days (H_n) were also measured. The calculation method for the corrosion resistance coefficient (S) is shown in Equation (3).

$$S=\frac{H_n}{H_0}\times 100$$

(3)

where $S$ is the water stability coefficient (%); $H_n$ is the unconfined compressive strength of the sample soaked in water for n days (MPa); and $H_0$ is the unconfined compressive strength of the specimen not soaked in water (MPa).

2.3.3. Micro Analysis Test

(1)

X-ray Diffraction (XRD)

The stabilized soil and cement soil samples cured for 28 days were processed into powder, and these particles were dried in the oven, and then taken out and tested. The test instrument was a Rigaku-2500 X-ray (Rigaku, Tokyo, Japan) diffractometer. CuKα radiation was used with a scan rate of 10°/min at 30 kV and 60 mA for testing.

(2)

Scanning Electron Microscopy (SEM)

The morphologies of stable soil and cement soil samples cured for 28 days were analyzed using a FEI Scios 2 HiVac (Thermo Fisher Scientific, Waltham, MA, USA) scanning electron microscope. The sample was processed into thin slices and dried. Gold was sprayed on the surface of the sheet, and then it was tested in high vacuum mode; the acceleration voltage was 10 kV and the working distance was about 10 mm.

(3)

Thermogravimetric Analysis (TG)

The pre-treatment of the sample was the same as described in Section 2.3.2 (1). In total, 25 mg of the powder was weighed and a HCT-1 (HENVEN, Beijing, China) thermal analyzer was used. The powder was heated from 50 °C to 1000 °C at a rate of 10 °C/min in a nitrogen environment.

(4)

Fourier-Transform Infrared Spectroscopy (FTIR)

The pre-treatment of the sample was the same as described in Section 2.3.2 (1). In total, 1~2 mg of the sample powder was mixed with 100–200 mg of spectrally pure KBr powder, ground to about 2 μm. The mixture was pressed into a transparent disk with a thickness of about 1 mm and a diameter of about 10 mm at a pressure of 6 MPa. A Thermo Nicolet Nexus 670 (Thermo Fisher Scientific, Waltham, MA, USA) FTIR Spectrometer was used for analysis with a resolution of 4 cm⁻¹, 32 scans, and a range of 4000–400 cm⁻¹.

3. Results and Discussion

3.1. Optimization of Soft Soil Stabilization Material Mix Proportions

This study, based on the requirements of stabilized soft soil foundations using mixing piles, used SWC to replace cement and formulated the optimal ratio of soft soil stabilization materials. As described in Section 2.2, the proportions of the main components of the cementitious materials are variable, while the proportions of the additives, water-to-cement ratio, and C/S are constant.

Table 5. Compressive strength test results of the samples.

Sample	Compressive Strength
	3 d/MPa	7 d/MPa	28 d/MPa
G1	1.07	3.06	4.18
G2	2.01	4.03	5.75
G3	0.48	2.93	5.50
G4	3.96	4.92	5.91
G5	2.68	4.38	4.79
G6	1.25	2.85	4.22
G7	2.71	4.61	5.97
GC	0.89	1.68	2.64

lists the compressive strengths of the stabilized soil (G1~G7) and cement soil (GC).

As seen from Table 5, the early and late compressive strengths of stabilized soil G4 and G7 were greater than those of other samples and cement soil GC. The early strength of G4 increased rapidly, with the 7 d compressive strength reaching 4.92 MPa. Although the early strength of G7 was lower than that of G4, the late strength was higher, reaching 5.97 MPa at 28 d. The experimental results show that the prepared SWC can stabilize soft soil well and is much stronger than cement soil in both the early and late stages. The 7 d compressive strength of G4 and G7 exceeded that of GC by 192.9% and 174.4%, respectively, while the 28 d compressive strength of G4 and G7 surpassed that of GC by 123.9% and 126.1%.

3.2. Influence of Seawater Mixing on Stabilized Soft Soil

According to the previous investigation, the salinity of the seawater around the source of the soft soil used was about 30‰. To simulate the requirements of actual coastal construction, the influence of SWC after seawater mixing on the compressive strength of soft soil was evaluated. Cement and SWC with salinities of 0‰ (pure water), 30‰ (salinity of seawater), and 60‰ (twice the salinity of seawater) were used to solidify soft soil, and the curing ages were 3 d, 7 d, and 28 d.

As shown in Figure 5, with the increase in seawater salinity, the compressive strength of cement soil and stabilized soil also increased, and the compressive strength of stabilized soil was much higher than that of cement soil. In the early curing stage, when the salinity increased from 0‰ to 30‰, the 7 d compressive strength of cement soil, stabilized soil G4, and stabilized soil G7 increased from 1.68 MPa, 4.92 MPa, and 4.61 MPa to 1.90 MPa, 6.16 MPa, and 5.70 MPa, respectively, increasing by 0.22 MPa, 1.24 MPa, and 1.09 MPa. In the late curing period, the 28 d compressive strength of cement soil, stabilized soil G4, and stabilized soil G7 increased from 2.64 MPa, 5.91 MPa, and 5.97 MPa to 3.26 MPa, 7.03 MPa, and 7.52 MPa, respectively, increasing by 0.62 MPa, 1.12 MPa, and 1.55 MPa.

Whether in the early stage or the late stage, after seawater mixing, the compressive strength of the samples of cement soil and stabilized soil increased rather than decreased, and the compressive strength of the samples of stabilized soil increased more than that of cement soil. This indicates that the preferred SWC formulations have better adaptability to seawater.

3.3. Test on the Durability of Stabilized Soft Soil

3.3.1. Water Resistance Test

The water stability coefficient of stabilized soft soil measures the strength loss rate after soaking in water following curing to a certain age. A higher water stability coefficient indicates a better water resistance of the stabilized soil. In the construction of soft foundations, the water table is typically high year-round. Additionally, the permeability coefficient of soft soil is low, meaning that water takes a long time to discharge from the soil. As a result, pile foundations may be submerged by groundwater for an extended period after construction. If the water stability of the stabilized soft soil is poor upon contact with water, it will not only affect its compressive strength but also compromise the quality and lifespan of the foundation. The water stability properties of cement soil GC and stabilized soils G4 and G7 after curing for 28 days were tested using an immersion test. The ratio of the compressive strength of the sample after soaking to the compressive strength of the 28-day reference sample was used to determine the water resistance coefficient. Figure 6 shows the curve of the compressive strength of stabilized soft soil over time during immersion, and

Table 6. Water stability coefficient of stabilized soft soil.

Sample	Coefficient of Water Stability
	Soaking 14 d	Submerged for 28 d	Submerged for 60 d
GC	105.02%	103.88%	114.61%
G4	113.64%	107.32%	119.80%
G7	108.91%	101.72%	112.79%

presents the water resistance coefficient of the stabilized soft soil. The test used stabilized soft soil mixed with seawater, with a water content of 65% and a content ratio of 20%.

As shown in Figure 6, with the increase in soaking days, the compressive strength of cement soil GC and stabilized soils G4 and G7 slightly decreased during the early soaking stage but then steadily increased. After 14 days of soaking, the compressive strengths of GC, G4, and G7 were 4.05 MPa, 7.99 MPa, and 8.19 MPa, respectively, with water stability coefficients of 105.02%, 113.64%, and 108.91%. After 28 days of soaking, the compressive strengths of GC, G4, and G7 decreased to 4.01 MPa, 7.54 MPa, and 7.65 MPa, respectively. However, after 60 days of soaking, the compressive strengths of GC, G4, and G7 increased to 4.42 MPa, 8.42 MPa, and 8.48 MPa, respectively, with water stability coefficients of 114.61%, 119.80%, and 112.79%.

The samples demonstrated good water resistance, with both cement soil and stabilized soil (as shown in Figure 7) remaining intact and not disintegrating after long-term immersion.

3.3.2. Seawater Erosion Resistance Test

The seawater erosion resistance of stabilized soft soil refers to its ability to maintain strength without significant decline under seawater immersion after curing to a certain age. In coastal construction, the tidal effect often causes seawater to invade the soft soil foundation, and the salt content in the underground flow can be high. Prolonged exposure to seawater erosion may lead to problems such as cracking, which seriously affects durability.

Artificial seawater, prepared using sea crystals, was used in the test. Samples cured for 28 days were immersed in seawater to test the change in compressive strength. The ratio of the compressive strength of the sample after immersion to the compressive strength of the 28-day reference sample was used to determine the water resistance coefficient. Figure 8 shows the curve of compressive strength for cement GC and stabilized soils G4 and G7 over seawater soaking time, and

Table 7. Seawater erosion resistance coefficient of stabilized soft soil.

Sample	Coefficient of Resistance to Seawater Erosion
	Seawater Immersion for 14 d	Seawater Immersion for 28 d	Seawater Immersion for 60 d
GC	87.39%	86.79%	66.97%
G4	92.47%	92.01%	90.48%
G7	95.34%	94.76%	91.41%

presents the seawater erosion resistance coefficient of the stabilized soft soil. The test used stabilized soft soil mixed with seawater, with a water content of 65% and a content ratio of 20%.

It can be seen from Figure 8 that as the soaking days in seawater increased, the compressive strength of cement soil GC and stabilized soils G4 and G7 gradually decreased. However, the seawater erosion resistance of stabilized soil was better than that of cement soil.

After 14 days of seawater immersion, the compressive strengths of GC, G4, and G7 were 3.37 MPa, 6.50 MPa, and 7.17 MPa, respectively, with seawater erosion resistance coefficients of 87.39%, 92.47%, and 95.34%. After soaking for 28 days, the compressive strengths of GC, G4, and G7 decreased to 3.35 MPa, 6.47 MPa, and 7.13 MPa, respectively. After 60 days of soaking, the compressive strength of GC significantly decreased, while that of G4 and G7 remained relatively high, at 2.58 MPa, 6.36 MPa, and 6.87 MPa, respectively. The seawater erosion resistance coefficients for these samples were 66.97%, 90.48%, and 91.41%, respectively, with G7 exhibiting the best seawater erosion resistance.

As shown in Figure 9, the cement soil sample was more severely damaged by seawater erosion. A unique white substance appeared on the surface of the cement soil sample, accompanied by multiple cracks that grew rapidly with increasing seawater soaking time. In contrast, the stabilized soil samples G4 and G7 remained intact throughout, with no visible abnormalities on their surfaces. These observations are consistent with the compressive strength test results, demonstrating that stabilized soil has better resistance to seawater erosion.

This is primarily due to the large amount of SO₄²⁻ in seawater continuously entering the soil and absorbing a significant amount of water. Soluble salt crystals precipitate out, causing the soil to expand and leading to cracks in the GC [21]. The GGBS in the SWC has a higher SO₄²⁻ binding capacity, and the hydrated calcium aluminate formed during hydration reacts with SO₄²⁻ in seawater to produce Kuzel salt. This reaction reduces the soluble salt content and mitigates the erosive effects of seawater [22].

3.4. Analysis of the Mechanism of Curing Soft Soil

3.4.1. XRD Analysis

Figure 10 shows the XRD patterns of soft soil and seawater-mixed GC, G4, and G7 after curing for 28 days. The main crystalline phases of the undisturbed soft soil are quartz (SiO₂), kaolinite (Al₄[Si₄O₁₀](OH)₈), muscovite (KAl₂(AlSi₃O₁₀)(OH)₂), and montmorillonite ((Na,Ca)_0.33(Al,Mg)₂[Si₄O₁₀](OH)₂·nH₂O). The hydration products observed include C-S-H, C-A-S-H, and ettringite (AFt). The diffraction peaks for Friedel salt and other salts are also present in this figure.

By comparing the Friedel salt peak heights of cement soil GC and stabilized soil samples G4 and G7, it can be observed that the peak strength of GC is higher than that of G4 and G7. This indicates that cement produces more Friedel salt in the soft soil within the seawater system. Friedel salt is a hydration product with low gel strength that consumes a large amount of Ca(OH)₂, inhibits the formation of other gel-strengthening hydration products, and covers the surface of hydration products to prevent further reaction, resulting in the lower strength of cement soil. This figure shows that dispersion peaks for C-A-S-H related to Cl⁻ and SO₄²⁻ adsorption appear in stabilized soils G4 and G7. The reduction in Cl⁻ and SO₄²⁻ content in the solution can effectively mitigate their impact on the hydration reaction [23]. This figure also reveals that, due to the low content of solidified materials in soft soil and the high overlap of diffraction peaks between minerals and hydration products, phases with low crystallinity cannot be accurately identified [24]. Therefore, additional characterization methods are needed to study stabilized soft soil more comprehensively.

3.4.2. Thermogravimetric Analysis

To characterize the influence of cement and SWC on soft soil, TG/DTG analysis was conducted on samples with different formulations. Figure 10 shows the TG/DTG curves of seawater-mixed GC, G4, and G7 after curing for 28 days. The analysis was performed with a heating rate of 10 °C/min, and the test was completed as the temperature increased from 50 °C to 1000 °C.

The causes of weight loss in various temperature ranges in the thermogravimetric curves of soils and cementitious materials have been extensively studied. The weight loss of soil in the range of 60–120 °C is primarily due to the natural water content in the soft soil. The weight loss at 220–330 °C mainly results from the decomposition of organic matter. The weight loss at 330–600 °C is primarily due to the decomposition of kaolin [25]. The weight loss of hydration products in cementitious materials in the range of 50–200 °C mainly comes from the dehydration of natural water, Aft, and C-S-H [26]. The weight loss at 200–280 °C is mainly due to the decomposition of C-A-H or C-A-S-H [26]. The weight loss at 270–370 °C is primarily due to the decomposition of Friedel salt [27]. The weight loss in the range of 400–500 °C is mainly due to the decomposition of calcium hydroxide [28], and the weight loss at 500–700 °C is mainly due to the decomposition of carbonate minerals [28].

Referring to the existing research and combined with Figure 11, it can be observed that the total weight loss of soft soil, GC, and G7 is 3.6%, 12.2%, and 26.9%, respectively. After accounting for the removal of water and organic matter decomposed by heat in soft soil, the total weight loss of stabilized soil G7 is much higher than that of cement soil GC. In the range of 50–200 °C, which represents the decomposition and dehydration of C-S-H and AFt, the weight loss of stabilized soil G7 is 9.40%, significantly higher than the 4.75% observed for cement soil GC. However, G7 also shows a more pronounced peak in the range of 200–300 °C, which is attributed to the decomposition and dehydration of C-A-H or C-A-S-H, whereas cement soil exhibits less weight loss in this range. This indicates that SWC generates more cementitious and filler hydration products in stabilized soil. At the weight loss peak representing Ca(OH)₂ at 400–500 °C, G7 consistently shows higher weight loss than GC. The alkaline component in the DA provides a highly alkaline environment for the stabilized soft soil, stimulates the gelling activity of GGBS and ACP, and promotes the dissolution of active SiO₂ and Al₂O₃ in the soft soil, participating in the hydration reaction to produce more hydration products [18]. This may also explain why stabilized soil exhibits greater weight loss at 50–200 °C and 200–300 °C.

3.4.3. FTIR Analysis

Seawater-mixed GC, G4, and G7 samples after curing for 28 days were tested and analyzed to study the changes in functional groups when SWC stabilizes soft soil. This analysis aims to deepen the understanding of the stabilization mechanism in soft soil. The test results are shown in Figure 12.

As shown in Figure 11, the absorption peak of quartz appears near the wavenumber of 779 cm⁻¹. The absorption peaks of kaolinite and montmorillonite are observed near the wavenumber of 3619 cm⁻¹. The reduction in the kaolinite and montmorillonite peaks in the stabilized soil indicates that the high alkalinity of DA promotes the dissolution and consumption of active components in soft soil minerals, contributing to the formation of hydration products [24]. The peak at the wavenumber of 3440 cm⁻¹ is associated with the absorption of -OH groups [29]. The peak near 1630 cm⁻¹ is due to the bending vibration of -Si-OH bonds in the hardened product and the bending vibration of H-O-H in mineral crystalline water [30]. The decrease in the adsorption water peak in the stabilized soil suggests that some of the water is consumed in the reaction. Compared to GC and G4, the absorption peak at 464 cm⁻¹ in G7 is attributed to the bending vibration of Si-O-Si bonds. The bending vibration of Si-O-Al bonds can be observed at a wavenumber of 538 cm⁻¹. The absorption peak around 1000 cm⁻¹ is primarily due to the asymmetric stretching vibration of Si-O-X bonds (X = Si or Al), indicating the presence of more C-S-H and C-A-S-H gels in the hydration products [31].

3.4.4. SEM/EDS Analysis

Figure 13 shows the micromorphology of seawater-mixed GC, G4, and G7 after curing for 28 d.

As shown in Figure 13, after curing for 28 days, GC exhibits a large number of sheet-like products and a small number of network-like products. Based on the product shapes and related research [32], it can be concluded that GC contains a significant amount of calcium hydroxide (Ca(OH)₂) and a small amount of calcium silicate hydrate/calcium aluminum silicate hydrate (C-S-H/C-A-S-H), with a less intimate bond to the soil. Calcium hydroxide contributes minimally to the soil’s strength and tends to crystallize into coarse grains, weakening the bond at the interface. This makes it the weakest link in the cement soil. The weak interlayer connection may also be the origin of cracks in cement soil under stress and the first component to be eroded under erosion conditions [33]. In the G4 and G7 stabilized soils mixed with SWC, the number of flocculent C-S-H/C-A-S-H gels and acicular AFt is significantly higher compared to the cement soil samples. The C-S-H/C-A-S-H gels formed from the hydration of GGBS and ACP act as gelling agents. SO₄²⁻ in the slag reacts with C-S-H to adsorb free water into crystal water, generating AFt, which acts as a filler [34]. The abundance of hydration products in the stabilized soil creates a dense spatial network structure by cementing and filling the soft soil particles, enhancing the overall density and structural integrity.

Through the tests described in Section 3.3.2, it was observed that under sea erosion conditions, cement soil samples exhibited a unique white substance on their surfaces and developed cracks, while the stabilized soil samples remained intact. Figure 14 shows the microscopic morphology of the samples after 60 days of sea erosion. From Figure 14a, it can be seen that large amounts of calcium sulfate and calcium carbonate crystals were present in the cement soil according to the analysis of Energy Dispersive Spectroscopy (EDS). These crystals not only failed to provide a cementing effect but also caused excessive swelling in the cement soil, leading to cracking. In contrast, no such issues were observed in the stabilized soil samples G4 and G7.

By analyzing the microscopic morphology of the unique white substance on the surface of the cement soil sample (Figure 15), it was found that the surface was covered with large amounts of loose and porous calcium carbonate according to the analysis of Energy Dispersive Spectroscopy (EDS). These calcium carbonate crystals did not prevent sea erosion; rather, the seawater penetrated through the pores and continued to erode the sample from within. This is why large amounts of calcium sulfate and calcium carbonate crystals were observed in the cement soil sample.

4. Conclusions

This study investigates the use of SWC to stabilize marine soft soil. Using GGBS, DA, and ACP as cementitious materials, and NS and SA as admixtures, SWC were prepared for stabilizing marine soft soil. Compared to cement, the stabilized soil using SWC demonstrated improved performance. The experimental results indicate that the optimized stabilized soil formulations significantly outperform cement soil in terms of compressive strength. After being mixed with seawater, the compressive strength of the stabilized soil and the cement soil did not exhibit significant changes. However, during seawater immersion, the cement soil samples degraded more severely, while the stabilized soil experienced less deterioration. X-ray diffraction analysis, scanning electron microscopy, infrared spectroscopy, and thermogravimetric analysis were used to explain the effects of SWC on the performance of the stabilized soil. The specific experimental conclusions are as follows:

(1)

The optimized formulations G4 and G7 had higher early and late compressive strengths compared to GC. G4 showed rapid early strength, with a 7-day compressive strength of 4.92 MPa, while G7 had superior late strength, reaching 5.97 MPa at 28 days. At 30‰ salinity, the 28-day compressive strengths were 2.64 MPa for GC, 5.91 MPa for G4, and 5.97 MPa for G7. G4 and G7 consistently outperformed GC, indicating the feasibility of seawater mixing.

(2)

When GC, G4, and G7 were immersed in pure water and seawater, it was observed that the stability of cement soil and stabilized soil in pure water was good. However, the resistance of cement soil GC to seawater corrosion was significantly worse than that of G4 and G7, with corrosion resistance coefficients of 66.97%, 90.48%, and 91.41% after 60 days of immersion, respectively. The surface of GC was covered with white crystals and exhibited many cracks, while the stabilized soil samples remained intact. This indicates that the stabilized soil has better resistance to seawater corrosion.

(3)

The hydration products in cement soil with seawater as mixing water were primarily calcium hydroxide (CH) and a small amount of C-S-H gel, with some Friedel salts present, which have low gel-forming capability. Existing studies indicate that CH and Friedel salts contribute little to the strength of the soil, which explains the low strength of cement soil.

(4)

In the stabilized soil, a significant amount of C-A-S-H, which has gel-forming properties, and AFt, which has a filling effect, were observed. Compared to the small amount of C-S-H in cement soil, the stabilized soil contained a substantial amount of C-A-S-H. It is generally accepted that C-A-S-H has a higher adsorption capacity for Cl⁻ and SO₄²⁻ than C-S-H, which reduces the concentration of Cl⁻ and SO₄²⁻ in the reaction solution. This effectively slows down the inhibition of the hydration reaction and the structural damage caused by crystal precipitation.

(5)

The analysis of the cement soil and stabilized soil samples after seawater immersion confirmed the presence of calcium carbonate and calcium sulfate on the surface and inside the cement soil sample, leading to the expansion and cracking of the structure. In contrast, no such issues were observed in the stabilized soil samples.