Influence of Peat Soil Environment on Mechanical Properties of Cement-Soil and Its Mechanism

Abstract

The influence of peat soil environment (PSE) on the mechanical properties of cement-soil in the area around Dianchi Lake and Erhai Lake in Yunnan Province has attracted much attention. This study explores the change law of cement-soil UCS in the PSE, and provides guidance for the development and sustained usage of peat soil foundation. The paper discusses the preparation of cement-soil samples by adding humic acid (HA) and cement to cohesive soil with low organic matter content (blending method) and soaking it in fulvic acid (FA) solution and deionized water (steeping method) to simulate the actual working environment of cement-soil. Unconfined compressive strength (UCS), acid consumption, ion leaching, scanning electron microscope (SEM), and X-ray diffraction (XRD) tests are carried out on cement-soil samples soaked for 90 days. The results show that HA can significantly reduce the UCS of cement-soil. FA can reduce the UCS of cement-soil when the content of HA is less than 18%. However, when the amount of HA is more than 18%, the UCS of cement-soil increases slightly. FA makes the deformation and failure type of cement-soil gradually change from brittle shear failure to plastic shear failure. FA reacts with the cement hydration products in the sample so that the cumulative acid consumption of the cement-soil sample continues to increase, and the dissolution of Ca²⁺, Mg²⁺, Al³⁺, and Fe³⁺ in the sample increases the ion concentration of the soaking solution. In addition, SEM and XRD show that HA can increase the macropores and connectivity of cement-soil, while FA fills part of the pores of the wetting layer. In the PSE, FA can strengthen the inner structure of HA particles and fill and cement the layers of cohesive particles, enhancing the construction of cement-soil with HA content greater than 18%, so that its UCS is relatively improved. However, when the amount of HA is less than 18%, there are more small pores in the cement-soil. The interaction between FA and HA in the cement-soil is weak. The influence of FA on cement-soil is mainly a weakening effect, and its UCS is relatively reduced.

1. Introduction

The area around Dianchi Lake and Erhai Lake in Yunnan belongs to the ancient lake and swamp area. The particular geographical location and climatic conditions give this area much peat soil. Peat soil has high organic matter content, a large void ratio, high compressibility, high water content, and low bearing capacity [1,2,3], all characteristics which belong to poor foundation soil. It can be reinforced with cement [4,5] to improve its bearing ability and to meet the needs of engineering construction. However, the cement-soil formed by cement-reinforced peat soil is still in a peat soil environment (PSE), and the influence of this environment on the mechanical properties of cement-soil is not yet evident. Therefore, studying the effect of PSE on the mechanical properties of cement-soil has specific theoretical and practical value for actual engineering construction and engineering safety.

As early as 1826, Sprengel [6] first systematically studied the humic group’s source and chemical nature and explored many preparation methods. This discovery laid a chemical foundation for exploring the corrosive mechanism of the humic group on cement-soil. After that, Schmeide et al. [7], and Xiongtian [8] specifically, distinguished the main components of the humic group as fulvic acid (FA) and humic acid (HA). FA is soluble in alkaline, water, and acidic solutions and exists in soil pores as a liquid. Humic acid is soluble in alkaline liquid but insoluble in water and acidic liquid and exists in the soil in the form of solid particles [9]. However, many scholars and engineers have mainly discussed the humic group in previous studies. For example, in 2000, Valls et al.s’ [10] studies found that soil and its surrounding solution are acidic, with high moisture capacity and poor perviousness, which changed its geomechanical properties. In 1989, Kamon et al. [11] used cement to solidify hedoro soils and found that its UCS decreased with the increase in humic group. In addition, when ettringite is present in the solidified liquid sludge, the UCS of the solidified sludge increases. In 2000, Xun [12] studied the effect of organic matter content on the UCS of cement-soil. His research specifically distinguished the representative components of organic matter as humic acid and fulvic acid, which provided new ideas for the later study of the effect of the humic group on cement-soil. In 2002, Tremblay et al. [13] studied the impact of organic compounds on the UCS of cement-soil, and found that humic group and other organic acids affect the development of the UCS by affecting the pH of pore solution. In 2002, Zeng et al.s’ [14] studies found that the organic matter in the soil is an acid body, which hinders the hydration reaction of cement and affects the UCS of the cement solidified body. In 2009, Zhu et al. [15] used dredged sludge to explore the effect of the humic group on hydration products. The results show that the humic group reacts with the alkali metal hydroxide produced by cement hydration, which affects the development of the UCS of the solidified product. In 2009, Zhang et al. [16] studied the effect of soil humic group components on the UCS of cement-soil through engineering site sampling. The study found that cement hydration products reacted with the humic group to form more complex compounds, which changed the microstructure of cement-soil and reduced its UCS. In 2017, Kang et al. [17] added humic group to dredged sea cohesive and conducted research using cement as a curing agent. The results show that the UCS of cement-soil is determined by the cement content and the humic group content. In 2019, Qi et al. [18] studied the effect of the humic group on the mechanical properties of cement-soil. They found that the UCS of cement-soil gradually decreased to a certain critical value with the increase in humic group content and then stabilized. Given this, Cao et al. [19] explored the corrosion law of fulvic acid on CFG piles through laboratory experiments in 2019. In 2021, Cao et al. [20,21] then researched humic acid, one of the main components of the humic group, but did not comprehensively discuss the joint action law of the two. In addition, Wang et al. [22] studied the relationship between mechanical properties such as cohesion, shear strength, and compression modulus of calcareous silt and physical parameters. Shen et al. [23] showed that the marine sedimentary soil environment is extremely complex, and calcareous silt compactness varies with the location. Therefore, they studied the influence of hydraulic filling on the water characteristics of calcareous silty sand. When studying the influence of peat soil environment on the mechanical properties of cement-soil, the two studies provided the paper with ideas.

In summary, many scholars and engineers have found that the humic group will adversely affect the engineering performance of cement-soil. However, they ignored the fact that the humic group is a complex organic mixture composed of humic acid, fulvic acid, and other substances. Different components’ effects on cement-soil’s engineering performance may have noticeable differences. Therefore, this paper proposes an original test method for the differences in the existing form and solubility of humic acid and fulvic acid in peat soil. The cement-soil samples are prepared by adding humic acid and cement to cohesive soil with a low organic matter content (blending method). They are immersed in fulvic acid solution and deionized water (steeping method) to simulate the actual working environment of cement-soil. When the soaking time reached 90 days, the UCS, acid consumption, and ion leaching tests are carried out on the cement-soil. Combined with the microscopic test results of SEM and XRD, we analyzed the influence of PSE on the mechanical properties of cement-soil and its mechanism. It lays a foundation for further exploring the influence of the PSE on the engineering properties of cement-soil working in this environment for a long time. Secondly, the research results can guide the development and sustained usage of peat soil foundation.

2. Materials and Methods

2.1. Materials

The apparent conditions of soil sample, cement, humic acid reagent, and fulvic acid reagent used in the test are shown in Figure 1.

The actual conditions of test materials and deionized water are as follows:

• The soil used in the experiment is cohesive soil from the north slope of Chenggong dormitory at the Kunming University of Technology. The soil sample has a low organic matter content and has little impact on the test results. The fundamental physical indicators are shown in

  Table 1. Physical and mechanical properties index of the soil used in the test.

  Test Soil	Natural Water Content (%)	Liquid Limit wL (%)	Plastic Limit wP (%)	Natural Density (g·cm−3)	Grain Specific Gravity Gs
  Cohesive soil	18.60	39.20	23.00	1.96	2.73

  . The test soil is naturally air-dried, ground, passed through a 2.00 mm geotechnical sieve, and boxed for later use. The chemical composition of the test soil and the mass fraction of each composition are tested using the X-ray fluorescence (XRF) test. As shown in

  Table 2. Chemical composition of the soil used for the test and the mass fraction of each composition.

  Test Soil	The Chemical Composition and Its Mass Fraction (%)
  	SiO2	Fe2O3	Al2O3	TiO2	K2O	MgO	CaO	Na2O	MnO	P2O5	LOI
  Cohesive soil	46.57	21.22	20.80	8.90	0.48	0.48	0.16	0.04	0.14	0.57	0.64

  , the main chemical components of the test soil are SiO₂, Fe₂O₃, and Al₂O₃. The X-ray diffraction (XRD) test is carried out on the soil for testing to determine its phase composition. As shown in Figure 2, the main phases of the soil sample are quartz, kaolinite, mica, goethite, and anatase. The results of the soil composition analysis show that the soil conforms to the general characteristics of cohesive soil, and the organic matter content is low. When studying the influence of peat soil environment on the mechanical properties of cement-soil, using peat soil can minimize the influence of raw materials on the test results.

2.

The cement used is Shilin brand 42.5^# ordinary Portland cement produced by Yunnan Huaxin Cement Co., Ltd. The cement company is located in Kunming, China.

3.

The humic acid produced by Tianjin Guangfu Chemical Reagent Factory is selected, which is sealed and stored through a 2.00 mm geotechnical sieve. The chemical reagent plant is located in Tianjin, China.

4.

Fulvic acid (FA) is selected from the biochemical FA reagent produced by Pingxiang Red Land Humic Group Co., Ltd. The humic group company is located in Pingxiang City, China.

5.

The test water is deionized water produced by Hugke Water treatment Equipment (Xi’ bei) Co., Ltd. The deionized water production company is located in Shenzhen, China.

2.2. Experimental Design and Sample Preparation

According to the previous research results, the total amount of humic group constituents in the peat soil of Dianchi Lake is between 7.15% and 50.06%. Among them, the content of HA is between 2.36% and 28.13%, and the content of FA is between 0.79% and 8.34% [24]. The results provide evidence for the sample’s selection of the amount of HA and FA. In addition, Shao believes that for general soil, the most economical cement incorporation ratio is about 12% [25]. However, peat soil is a special soft soil. We should comprehensively consider the reinforcement effect and economic and environmental factors and reasonably select the cement mixing ratio.

The water content of the test control sample is ω = 24%, the void ratio e = 0.8, and the water-cement ratio c = 0.5. The cement mixing ratios of the design samples are 15%, 20%, and 25%, and the HA content is 0%, 15%, 20%, 25%, and 30%. The soaking solution is deionized water and fulvic acid solution with a pH value of 6.0, respectively. When the soaking time reaches 90 days, the unconfined compressive strength (UCS) test, ion leaching test, XRD test, and SEM test are carried out on the sample. This is to determine acid consumption of samples during soaking. The specific test scheme is shown in

Table 3. Test scheme.

No.	Test Items	Cement Mixing Ratio (β)/%	Soaking Solution	HA Content (λ)/%	Soaking Time/d
1	UCS test	15, 0, 25	FA solution (pH = 6), Deionized water	0, 15, 20, 25, 30	90
2	Acid consumption test	20	FA solution (pH = 6)	0, 15, 20, 25, 30
3	Ion leaching test	20	FA solution (pH = 6), Deionized water	20
4	SEM, XRD	20	FA solution (pH = 6), Deionized water	20
			FA solution (pH = 6)	0, 20, 30

.

In accordance with “Standards for Geotechnical Test Methods” (GB/T50123-2019) [26], calculate the dosages of cohesive particles, HA particles, and cement, according to Formulas (1) and (2). Mix all ingredients well and divide evenly into three portions. Then, the weighted mixture is poured into the three-lobe mold (the inner diameter of the three-lobe mold was 39.10 mm, and the height was 80.00) and the layers compacted. After compacting each layer, shave and repeat three times to finish the sample. Then, the mold is demolded and sealed with plastic wrap. The samples were placed in a curing box for ten days. The temperature in the control box was 20 ± 3 °C during curing. After curing, it is soaked in FA solution and deionized water. In this test, three parallel samples are prepared for each ratio to ensure the accuracy of the test.

$$\mathsf{\beta }=\frac{{{\displaystyle \mathrm{m}}}_{\mathrm{s}(\mathrm{c}\mathrm{e})}}{{{\displaystyle \mathrm{m}}}_{\mathrm{s}(\mathrm{H}\mathrm{A})}+{{\displaystyle \mathrm{m}}}_{\mathrm{s}(\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l})}}\times 100\%$$

(1)

$$\mathsf{\lambda }=\frac{{{\displaystyle \mathrm{m}}}_{\mathrm{s}(\mathrm{H}\mathrm{A})}}{{{\displaystyle \mathrm{m}}}_{\mathrm{s}(\mathrm{H}\mathrm{A})}+{{\displaystyle \mathrm{m}}}_{\mathrm{s}(\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l})}}\times 100\%$$

(2)

where m_s(ce) is the mass of cement, g; m_s(HA) is the mass of the HA particle, and g; m_s(soil) is the mass of soil particles, g.

2.3. Experimental Procedure

In this paper, the UCS, acid consumption, ion leaching, SEM, and XRD tests are carried out on the samples. The specific test process is as follows:

• UCS test

The UCS test apparatus is the YSH-2 lime-soil electromotion unconfined compressive strength instrument produced by Nanjing Ningxi Soil Instrument Co., Ltd. The instrument measured the UCS of soaked cement-soil samples for 90 days. The axial compression rate of the test control instrument is 1.0 mm/min, and the sample is continuously pressed until it fails. Take the average value of the UCS of the tested samples as the UCS value of this group of samples.

2.

Acid consumption and ion leaching test

The cement-soil samples are placed in separate soaking boxes, and the liquid level of the soaking solution is 2.0~2.5 cm below the top of the samples. Continuous addition of FA reagent to the FA soaking box kept the pH of FA soaking solution stable and accurately recorded the quality of FA reagent added each time to determine its acid consumption. The contents of Ca²⁺, Mg²⁺, Al³⁺, and Fe³⁺ are determined in 150 mL of each sample soaked for 90 days.

3.

SEM and XRD

The SEM uses the Czech VEGA3-TESCAN automatic tungsten filament scanning electron microscope as the test instrument. Samples are taken from the strongly infiltrated reaction layer and the weakly infiltrated reaction layer of the cement-soil samples, and their microscopic morphology is measured. After grinding the cement-soil samples, use a geo-sieve to control the particle size of the samples to be less than or equal to 13 μm [27]. The phase identification is carried out using a multi-functional powder X-ray diffractometer of the PANalytical X’Pert3 Powder type from the Netherlands.

3. Results and Analysis

3.1. Influence of Peat Soil Environment on Cement-Soil Strength

Figure 3a–c shows the relationship between cement-soil’s unconfined compressive strength (UCS) and peat soil environment (PSE). The UCS of cement-soil samples immersed in deionized water decreased significantly with the increase in HA content, which indicates that HA particles greatly influence the UCS of cement-soil samples. This is because the special connection mode of HA molecules leads to the loose and porous sponge-like structure of HA particles [28], and its structure is poor. On the other hand, HA particles can be adsorbed on the surface of cohesive particles [29], which weakens the bonding effect of cement hydration products between cohesive particles. In addition, the hydrogen ions ionized by HA react with cement hydration products, thereby consuming part of the hydration products. Given the above reasons, the UCS of cement-soil samples mixed with HA particles is greatly reduced. When HA is more than 18%, the UCS of soaked cement-soil samples in FA solution is higher than that in deionized water. The results show that the immersion of FA can weaken the negative effect of HA on the UCS of cement-soil samples to some extent, and can increase the UCS of cement-soil samples with high HA content slightly. The porous structure of HA particles results in its strong water-holding and absorption capacity and can accommodate organics with smaller molecular weights. Therefore, when cement-soil samples are soaked in FA solution, HA particles are filled with FA with lower molecular weight, the structure of HA particles is enhanced, and the UCS of cement-soil samples with a higher amount of HA decreases. When the content of HA is less than 18%, the adsorption and accommodation of FA by HA particles is limited, and its structure has relatively little effect on the UCS of cement-soil samples. A large amount of FA directly consumes the hydration products of cement, and the adverse impact on the UCS development of cement-soil samples is dominant, which leads to the decrease in UCS. At the same time, the UCS of cement-soil samples is weakened by FA.

The comparison of (a), (b), and (c) in Figure 3 shows that when the cement incorporation ratio gradually increases, the decreasing trend of cement-soil UCS gradually slows down with the increase in HA incorporation. The results show that the increase in the cement mixing ratio can effectively weaken the influence of PSE on the UCS of cement-soil. However, high cement content is not conducive to environmental friendliness. Therefore, other curing agents should be combined with cement to reduce the amount of cement.

Select a cement-soil sample with a 20% of cement mixing ratio and 20% of the HA content for research. Grind the cement-soil sample and bake it in the oven at low temperature until the mass of the sample remains the same before and after twice. Get the total quality of the sample after being soaked.

Table 4. Increase in dry matter of sample.

Type of Soaking Solution	Mass of Each Material Before Soaking/(g)			Total Dry Matter Mass of the Sample Before Soaking/(g)	Quality of Water Required for Complete Curing of Cement/(g)	The Total Mass of the Drying Sample After Soaking/(g)	Increase in Sample Dry Substances/(g)
	Cohesive Soil	HA	Cement
Deionized water	83.07	20.82	20.82	124.71	5.00	125.03	−4.68
FA solution (pH = 6)						145.14	15.43

shows the increase in dry matter of the samples in the two soaking solutions calculated using Formula (3).

$$\Delta {{\displaystyle \mathrm{m}}}_{\mathrm{s}}={{\displaystyle \mathrm{m}}}_{\mathrm{s}(\mathrm{A}\mathrm{I})}-{{\displaystyle \mathrm{m}}}_{\mathrm{s}(\mathrm{B}\mathrm{I})}-{{\displaystyle \mathrm{m}}}_{\mathrm{s}(\mathrm{c}\mathrm{e})}\times 24\%$$

(3)

where Δm_s is the increase in dry matter, calculated value, g; m_s(BI) is the measured total mass of dry matter of the sample before immersion, g; m_s(AI) is the measured total mass of the dried sample after immersion, g; m_(ce) is the cement content, and g; 24% is the percentage of water demand for complete cement curing in cement mass [30,31].

This study found that after soaking the cement-soil samples in deionized water for 90 days, the pH value of the deionized water soaking solution rose to 10.85. At this time, the dry matter mass of the sample increases negatively. The reason is that Ca(OH)₂ produced by cement hydration increases the pH of deionized water soaking solution to alkalinity, and part of the HA particles forming the soil skeleton gradually dissolve out. The increase in dry matter mass decreases the structural weakening and the UCS of the sample. When the samples are immersed in FA solution, the dry matter mass increases and becomes larger. It shows that the FA converges in the cement-soil samples and interacts with the HA particles, significantly increasing dry substances in the cement-soil samples. Observing the FA’s colloidal effect, some cohesive particles are cemented and filled some pores of the cement-soil samples. Therefore, when the cement-soil samples with a doping volume greater than 18% are soaked in the FA solution, the structure is slightly enhanced, and the intensity increases somewhat.

Figure 4 shows the failure mode of samples soaked in different soaking solutions with a cement mixing ratio of 20%. The inner and outer layers of the samples soaked in deionized water are consistent, and the samples are subjected to destructive pressure to cause a long penetrating crack. When the HA content is low, the specimen will crack under pressure, showing brittle failure. With the increase in HA content, the hydration reaction of cement is delayed, and some hydration products are consumed, which shows the specimen’s ductility under compression. The sample can be divided into a strong wetting reaction layer (SWRL), wetting reaction transition layer (WRTL), and weak wetting reaction layer (WWRL) from outside to inside according to the phenomenon of sample damage, the color difference between the inner and outer layers, and the wetting depth. The thickness of the SWRL gradually thickens with the increase in HA content. According to the actual measurement, the thickness of the SWRL of samples with HA content of 0~30% is about 2.5 mm, 4.0 mm, 5.5 mm, 7.5 mm, and 10.0 mm, respectively. The WRTL is the middle layer between the SWRL and WWRL. The infiltration of the FA solution leads to its inhomogeneity, which makes the SWRL and the WWRL peel off when the sample is deformed under pressure.

In summary, although the FA infiltrates the cement-soil samples in the PSE, it is not completely soaked. FA has many carboxyl and phenol hydroxyl groups, making its aqueous solution acidic. Some cement hydraulic products are consumed after being immersed in cement-soil. However, the molecular configuration of the FA has changed under the influence of ionic strength, forming a rigid sphere with hydrophobic inside and outside hydrophilic with colloidal properties [32], and partially filling the pores of the cement-soil. Therefore, the failure mode of the specimen changes from brittle shear failure to plastic shear failure gradually.

3.2. Test Results and Analysis of Ion Leaching and Acid Consumption

Table 5. Contents of ions in FA solution with different pH values.

Type of Soaking Solution	Ion Content/(mg·L−1)
	Ca2+	Mg2+	Al3+	Fe3+
FA solution (pH = 6)	232.00	75.30	296.00	20.20
Deionized water	41.30	1.30	3.36	0.035

shows the contents of four ions of Ca²⁺, Mg²⁺, Al³⁺, and Fe³⁺ in different soaking solutions when the cement mixing ratio is 20%, and the HA content is 20%. The content of four ions in FA soaking solution is much higher than that in deionized water. FA easily dissolves in water, ionizes many hydrogen ions, and reacts with alkali metal hydroxides such as Ca(OH)₂ produced by cement hydration. The cement-soil sample’s surface and internal hydration products are dissolved, and each ion is dissolved into the soaking solution. Therefore, the contents of four ions in FA soaking solution increased significantly.

The results of ion leaching show that FA can efficiently react with alkali metal hydroxides such as Ca(OH)₂ produced by cement hydration, which leads to the continuous increase in pH value. FA reagent should be added continuously to keep the pH of the solution constant, so the cement-soil sample has a certain amount of acid consumption. The infiltration model is shown in Figure 5. In the acid consumption test, the cement-soil samples with 20% cement ratio are selected to study, and the relationship curve between the sample’s accumulative acid consumption and time in the FA solution (pH = 6) is obtained as shown in Figure 6. This test can reflect the speed of soaking in the PSE. The cumulative acid consumption of the samples soaked in FA solution continued to increase, but the increasing speed of the cumulative acid consumption gradually slowed down with time. It means that the acid consumption rate of cement-soil gradually slows down. In addition, the cumulative acid consumption of cement-soil without HA was higher than that of cement-soil mixed with HA.

It can be seen from Figure 5 and Figure 6 that when the cement-soil samples are soaked in the FA solution, the small molecules of FA and the hydrogen ions ionized by FA contact and react with the surface of the samples. After the concentration difference is formed at the solid-liquid contact surface, FA small molecules and hydrogen ions continuously infiltrate into the cement-soil samples and take effect. However, with the increase in soaking time, the hydration reaction of cement gradually progressed. Some pores of cement-soil samples are filled with cement hydration products, gradually weakening the FA solution’s interaction in cement-soil samples. In addition, FA small molecules aggregated in the cement-soil samples and filled some pores, which significantly slowed the infiltration rate of FA solution in the cement-soil samples. Therefore, the boundary of the SWRL of the cement-soil sample moves inward slowly with the increase in soaking time, and the cumulative acid consumption curve gradually slows down. On the other hand, HA is weakly acidic, which decreases the cumulative acid consumption with the increase in HA content.

3.3. SEM and XRD Results and Analysis

The cement-soil samples soaked for 90 days are sampled at the same position for SEM and XRD tests. The above tests can better observe the sample’s internal micro-domain morphology and micro-structural characteristics and clarify the sample’s main crystal types and changes.

3.3.1. SEM Results and Analysis

In SEM, selected samples with cement mixing ratio of 20%, pH value of the soaking solution of 6, and HA content of 0%, 20%, and 30% were chosen for research. The SEM image of the samples magnified by 1000 times is shown in Figure 7a–c. Figure 8 shows the HA particles’ SEM image after magnification of 10,000 times. Figure 7 shows that the soil particles and cement hydration products in the samples without HA are agglomerated, with a small amount of almost unconnected pores distributed. With the increase in HA content, the connection form between cohesive clay particles and cohesive clay particles, cohesive clay particles and HA particles, HA particles and HA particles gradually transformed into surface–surface connection, point–edge connection, and even point–point connection. The pores in samples gradually change from poorly interconnected to better interconnected pores, and the structure gradually weakens. The small particles of HA that cause this phenomenon are adsorbed on the surface of soil particles, which increases the dispersibility of the soil particles’ surface. HA also delayed the progress of the cement hydration reaction and filled the pores at a slower rate. In addition, as shown in Figure 8, the large HA particles formed by aggregation of HA small particles are still a porous structure. This is one of the reasons why the UCS of samples decreases with the increase in HA content.

Figure 9 shows the SEM image of cement-soil samples soaked in different solutions with a fixed cement mixing ratio of 20% and HA content of 20%, magnified by 500 times and some areas by 5000 times. The structure of cement-soil samples soaked in deionized water is dense. There is a small amount of developing needle-like, flocculent, and massive hydration products in its pores [33,34]. There are no visible fibrous and flocculent hydration products in the two wetted reaction layers of the cement-soil samples. However, massive amorphous hydration products can be observed in the SWRL of cement-soil samples, and hexagonal massive hydration products (Ca(OH)₂) can be observed in the WWRL. Due to the colloidal nature of FA [35,36,37], it fills part of the pores in the two wetting reaction layers. The results show that the fibrous or flocculent hydration products developed in the pores are easily eroded and dissolved by FA solution, thus enlarging the pores in cement-soil. However, the dense massive hydration products composed of many hydration products cannot be decomposed in a short time and continue to play their gelling role. In addition, FA fills part of pores after coalescence in cement-soil.

3.3.2. SEM Results and Analysis

XRD analysis is carried out on the cement-soil immersed in different solutions with a cement mixing ratio of 20% and a HA content of 20%. The results are shown in Figure 10. Figure 11 shows the cement-soil samples XRD diffractograms of different HA content (0%, 20%, and 30%) in FA solution (pH = 6) with a cement mixing ratio of 20%. Evidently, the main crystalline phases in cement-soil samples are silica, calcium silicate hydrate (CSH), and calcium hydroxide. Silica is the main component of cohesive minerals [38], while calcium silicate hydrate and calcium hydroxide are cement hydration products.

Figure 10 shows the cement-soil samples soaked in deionized water have higher CSH diffraction peaks at 2θ = 29.35° and 2θ = 50°, and a Ca(OH)₂ diffraction peak at 2θ = 36.53°. When the cement-soil samples are immersed in FA solution (pH = 6), the diffraction peaks of CSH at 2θ = 29.35° and 2θ = 50° decrease, and even CSH disappears at 2θ = 29.35°. The intensity of Ca(OH)₂ diffraction peaks at 2θ = 36.53° decreases gradually, indicating that FA can react with CSH and Ca(OH)₂, reducing or even completely consuming them. It can be seen from Figure 11 that when the cement-soil is soaked in FA solution (pH = 6), the intensity of the diffraction peak of CSH at 2θ = 50° produced by cement hydration also gradually decreases with the increase in HA content. The diffraction peak of CSH at 2θ = 29.35° disappears. The results further showed that the humic group reacted with cement hydration products, reducing the content of cement hydration products, weakening the structure, and then affecting the UCS of cement-soil.

4. Conclusions

The form and solubility of humic acid (HA) and fulvic acid (FA) in peat soil are different. Therefore, this paper uses blending and steeping methods to simulate the working environment of cement-soil. The unconfined compressive strength (UCS) test is carried out on the cement-soil samples, and the change law of the mechanical properties of the cement-soil in the peat soil environment (PSE) is studied. Combined with the acid consumption, ion leaching, scanning electron microscope (SEM), and X-ray diffraction (XRD), the PSE’s influence mechanism on cement-soil’s mechanical properties is further discussed. The conclusions are as follows:

• HA can significantly reduce the UCS of cement-soil. FA reduced the UCS of cement-soil containing less than 18% HA. However, when the content of HA is more than 18%, FA can weaken the negative influence of HA on the UCS of cement-soil, and make the UCS slightly increase. Under FA and HA, cement-soil samples’ deformation and failure type gradually changed from brittle shear failure to plastic shear failure.
• FA reacted with cement hydration products, which made Ca²⁺, Mg²⁺, Al^3+, and Fe³⁺ in the cement-soil gradually dissolve into the soaking solution, increasing the content of four ions. The accumulative acid consumption of cement-soil increased continuously, but the increasing rate gradually decreased with the soaking time, indicating that the infiltration rate of FA in cement-soil slowed down. In practical engineering, designers should take certain anti-corrosion measures. For example, the use of admixtures to cement-soil becomes dense. In addition, an anticorrosive coating can be applied to the outer surface to weaken the infiltration effect of FA on cement-soil.
• The SEM images indicated that HA increased the macropores of cement-soil and enhanced the connectivity. There were no prominent fibrous and flocculent hydration products in the SWRL of cement-soil, but massive amorphous hydration products can be observed, and hexagonal massive hydration products can be observed in the WWRL. Part of the pores of the two wetting reaction layers is filled with FA with colloidal properties. XRD showed the existence of CSH and Ca(OH)₂ hydration products in cement-soil. However, FA and HA reacted with hydration products, decreasing diffraction peak strength.
• The PSE has the twofold effect of infiltration enhancement and infiltration weakening on the UCS of cement-soil simultaneously. Fibrous and flocculent hydration products cannot be observed in cement-soil treated with FA and HA, but massive hydration products can still be observed. The structure of HA particles has a particularity and is easily strengthened by FA. In addition, FA has a filling and cementing effect on the interlayer of cohesive particles. The structure of cement-soil with HA content greater than 18% is enhanced, and its UCS is relatively improved. When HA content is less than 18%, there are more small pores in the cement-soil. The interaction between FA and HA is weak. Therefore, the effect of FA on the UCS of cement-soil is mainly a weakening effect, which leads to a relative decrease in its UCS.
• The increase in cement mixing ratio can weaken the influence of PSE on the UCS of cement-soil, and the effect is very significant at 20%. Fortunately, the cement mixing ratio in this study is not high for the peat soil foundation. Therefore, the pH value and HA content of peat soil can be measured in engineering practice. According to the test results of this paper, following the concept of economic saving and environmental friendliness, other curing agents are reasonably selected to be used in combination with cement, and the amount of cement is appropriately reduced. These measures can better realize the development and sustainable development of the peat soil foundation.