Abstract
The disposal of peat soil poses an increasingly difficult problem for actual engineering projects of Dianchi Lake area. This study obtains peat soil from seven areas around Dianchi Lake, and the content of humic acid (HA) in peat soil is between 2.36% and 28.13%. Then, this study simulates the peat soil by adding HA into the cohesive soil and uses cement to solidify it. The effect of cement and HA on the strength development of samples is examined by the unconfined compressive strength (UCS) test. Additionally, the microstructures of typical mixes are analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP). The results showed that HA will significantly reduce the UCS of cement soil. The SEM, XRD, and MIP confirmed that cement hydration reaction increases with cement ratio. In addition to the cementitious soil particles, hydration products gradually fill the pores of the soil and effectively reduce the number of large-size (6000∼40,000 nm) pores in cement soil, which makes the soil particle framework stronger. When the cement ratio increases from 15% to 25%, the diffraction peak of CAH and CSH increases faster. Combined with the results of the UCS test, it could be proved that cement ratio greater than 20% weakens the influence of HA on the strength development of cement soil sample.
1. Introduction
Due to the special geographical location and plateau climate, Dianchi Lake area around Kunming in China belongs to the ancient large lakes and marshes, with deep Quaternary sediment and wide distribution of peat soil [1, 2]. The soil has high organic matter content, large void ratio, low natural weight, high water content, high compressibility, low bearing capacity, and so on [3, 4] With the rapid economic development and increasing social demands, more and more civil projects have to be located in areas where peat soil environment is distributed. Construction in progress includes highways, railways, residential buildings, factories, and docks. Therefore, how to strengthen peat soil foundation with insufficient bearing capacity has become an important research topic in the civil engineering industry.
In actual engineering projects, cement is usually used to strengthen the foundation, but in peat soil, the scanty geological environment of the cement soil will have a huge impact on its mechanical properties. The organic matter of peat soil contains a lot of humic acid (HA). In 1786, Achard [5] first isolated HA from peat soil. In 1826, Sprengel [6] comprehensively studied the origin and chemical properties of HA substances. He developed many methods of extracting and preparing HA, for example, the soil is pretreated with mineral acid before extraction with lye. In 2009, Haiyan et al. [7] used elemental analysis and Fourier transform infrared spectroscopy to study the chemical composition and structural characteristics of four different sources of HA. The results show that although the chemical composition and structure of HA from different sources have many similarities, they also have obvious differences in structure due to their different biological sources and diagenetic environments. In 2014, Yu [8] first studied the influence of the microstructure and material components of Dianchi peat soil on the mechanical strength through mechanical tests. HA is an amorphous, polydisperse organic polymer mixture formed by the decomposition and transformation of the remains of animals, plants, and microorganisms through a series of complex physical, chemical, and biological processes [9, 10]. Schmeide et al. [11–13] believe that fulvic acid and HA are the most representative humus in the peat soil. Among them, fulvic acid is soluble in both acidic liquid and alkaline liquid and exists in liquid form; HA is slightly soluble in alkaline liquid and insoluble in water and acidic liquid and exists in solid form.
Portland cement is most widely used as soft soil solidifying material because of its easy availability and reasonable price [14]. Calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) are formed after cement hydration in cement-solidified soil, which provides strength to cement-solidified soil [15–19]. Previous studies have confirmed that HA may seriously inhibit the cement hydration process [20]. This could be explained by the fact that the existence of HA decreases the pH of the pore solution and destroys the high alkaline environment needed for cement hydration [21]. Furthermore, the HA will also coat cement hydration product particles, delaying the hydration process [22]. Yufang et al. [12, 23, 24] studied the influence of organic matter on cement soil and proved that HA can affect the hydration reaction of cement, thereby reducing the strength of cement soil. Bertron et al. [25, 26] found that HA will react with the hydration products of cement, eroding the internal structure of cement soil, increasing its internal pores, and reducing its mechanical properties. Weidong et al. [21, 27–29] found that presence of organic matter in peat soil will make the soil have greater expansibility and lower permeability, and the solution will be acidic, which will destroy the alkaline conditions required for cement hydration and hinder the formation of cement soil strength.
Due to the particularity of the peat soil, many engineers and scholars have studied and proved that the peat soil will affect the strength of cement soil. However, most of them stay on the macrolevel and do not link the influence of peat soil on the strength development of cement soil with the change of microstructure. Therefore, this paper uses the method of adding HA to the alluvial cohesive soil with lower HA to simulate the peat soil of Dianchi Lake and adding cement with different mixing ratios in it. Under different curing ages, the indoor unconfined compressive strength (UCS) test is conducted to study the strength development. On this basis, through scanning electron microscopy (SEM), X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP), the mechanism of the influence of cement and HA on the strength of cement soil is revealed from the microscopic level. The research results provide a certain theoretical support for exploring the effect of cement strengthening peat soil foundation around Dianchi Lake and provide guidance for the actual project of surrounding Dianchi Lake.
2. Materials and Methods
2.1. Materials
In order to restore the real engineering conditions, seven representative groups of peat soil around Dianchi Lake are obtained by drilling. The thickness of peat soil in Dianchi Lake area is between 1 m and 6 m, and the average sampling depth is between 10 m and 20 m. The sampling site distribution is shown in Figure 1. Humic acid (HA) is a mixture of a series of compounds, and there is no unified molecular structure and molecular formula. This study measured the carbon content of different components in HA and calculated its content in combination with the corresponding carbon element content. Then, by combining the measured carbon content and carbon element content, the content of HA in the peat soil of Kunming Dianchi Lake is calculated (Table 1). It can be seen from Table 1 that although the peat soil samples are taken from the area around Dianchi Lake, the total amount of HA in samples varies greatly from site to site, with the content of HA ranging from 2.36% to 28.13%. Therefore, it can be determined that the amount of HA incorporated in the test is within 30%.
The study used the method of adding HA to the cohesive soil with lower natural HA to simulate peat soil. This method avoids the influence of natural HA on the accuracy of test results. The excavation position of the soil layer is shown in Figure 2. The basic physical and mechanical properties of the soil sample and the content of HA are measured through laboratory experiments. Among them, the content of HA is 0.32%, as shown in Table 2. The HA reagent is obtained from Tianjin Guangfu Chemical Reagent Factory. Ordinary Portland cement is selected as the test cement.
Figure 3 shows the particle distribution curve of the test material. Through the analysis of Figure 3, it is found that the cohesive soil used in the test is inhomogeneous soil (inhomogeneous coefficient Cu = 27.29 > 5). The particle diameter of humic acid particles is generally greater than 10 μm. The particle size of ordinary Portland cement is basically less than 100 μm, which meets the requirements for the fineness of ordinary Portland cement in “Common Portland Cement” (GB175-2007) [30].
2.2. Methods
The experiment used the method of adding HA to the cohesive soil of Dianchi Lake to prepare peat soil (Figures 4 and 5). The test is carried out according to the “Standards for Geotechnical Test Methods” (GB/T50123-2019) [30]. The test mold is a three-lobed mold with an inner diameter of d = 39.1 mm and a height of h = 80.0 mm. The water content of the sample (ω = 24%), void ratio (e = 0.8), water-cement ratio (c = 0.5), humic acid content (λ = 0%, 5%, 10%, 15%, 20%, 25%, 30%), and cement mixing ratio (β = 5%, 10%, 15%, 20%, 25%). Prepared cement soil samples are cured in a distilled water environment for 90 and 365 days. After that, unconfined compressive strength test, scanning electron microscopy test, X-ray diffraction test, and mercury intrusion porosimetry are performed on the cured samples. Scanning electron microscope test and X-ray diffraction test are carried out on the samples with 90 d curing age. Scanning electron microscope test uses Czech TESCAN-VEGA3 automatic tungsten filament scanning electron microscope; X-ray diffraction test uses Holland PANalytical X’Pert3 Powder multifunctional powder X-ray diffractometer.
3. Test Results and Analysis
3.1. Unconfined Compressive Strength Test Results and Analysis
Figure 6 shows the relationship between the strength and the amount of humic acid (HA) with 365 d curing. Figure 7 shows the relationship between the strength and the cement ratio with 365 d curing. Figure 8 shows stress-strain curve of cement soil cured for 90 days with different cement ratios.
From Figure 6, under different cement ratios, the strength of the cement soil samples gradually decreases with the increase of the HA mixing. From Figure 7, under different amounts of HA, the strength of the cement soil samples increases with the cement ratio. From Figure 8, the trend of the stress-strain curve of these samples is basically the same. The ultimate compressive strain range is 1.5% to 4.0%, and the ultimate compressive strain of these samples increases with cement ratio. When cement ratio is lower than 15%, the failure mode of these samples has a certainty of ductility, i.e., after these samples reached the peak stress, the strength will gradually decrease, but it still has a certain residual strength. When the cement ratio is higher than 15%, these samples show brittle failure.
From a chemical reaction point of view, this could be explained that the existence of HA decreases the pH of pore solution and then destroys the high alkaline environment needed for cement hydration [10]. Furthermore, a large amount of calcium hydroxide (Ca(OH)2) produced in the process of cement hydration will preferentially react with the HA, reducing the total amount of cement hydration products in the cement soil sample, ultimately resulting in a decrease in its strength.
However, with the gradual increase of cement ratio, cement hydration products increase. In addition to cementitious soil particles, hydration products gradually fill the pores of cement soil sample, so that the influence of HA on cement soil sample is gradually reduced.
3.2. Scanning Electron Microscope Test Results and Analysis
A 2000x SEM test is performed on the cement soil sample with 15% humic acid, 5%, 10%, 15%, 20%, and 25% cement ratio, and curing age of 90 days. The microstructure image is shown in Figure 9.
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From Figure 9(b), it can be clearly seen that the fibrous hydration product cements the soil particles together. It can be observed in Figure 9(d) that with the increase of cement ratio, cement hydration products continue to increase, fibrous hydration products gradually become flocculent, and flocculent hydration products gradually fill the cement soil sample structure. Figure 9(e) shows a more obvious grid structure. This phenomenon results from the fact that the hydration products of cement have strong cementing properties, which will cement the soil particles in the sample into a complete block. Its internal structure becomes denser.
In summary, from the SEM test, as the cement ratio gradually increases, the cement hydration products in cement soil sample gradually increase, changing from fibrous to flocculent. In addition to the cementitious soil particles, hydration products gradually fill the pores of the soil, which makes the soil particle framework stronger and reduces the influence of HA. The mechanical properties are manifested by the increase in the strength of the cement soil sample.
3.3. X-Ray Diffraction Test Results and Analysis
The XDR test is performed on the undisturbed soil and the cement soil sample with 15% humic acid (90 d curing). In order to improve the accuracy of the test, the particle size of the sample powder is controlled within 1 μm to 15 μm. The test results are shown in Figure 10.
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From Figure 10(a), it can be seen that the main components of undisturbed soil are silica, calcium carbonate, calcium oxide, and alumina (i.e., SiO2, CaCO3, CaO, and Al2O3). Figures 10(b)–10(f) show that the cement hydration products in samples are mainly calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH). Using Jade 5.0 software to analyze the XRD patterns of five samples, the total amount of diffraction peaks of the hydration products CSH and CAH in the cement soil sample is obtained and compared with the 90 d strength of the sample. The comparison result is shown in Figure 11.
It can be seen from Figure 11 that there is a strong positive correlation between the change of the peak total amount of CAH and CSH and strength growth of the cement soil sample, and it increases with the increase of the cement ratio. When the cement ratio increases from 10% to 15%, the strength of the sample and the diffraction peak of CAH and CSH both increase slowly, indicating that under these cement ratios, humic acid has a certain influence on the strength formation of each sample. The main reason is that HA can coat the cementitious material particles when treated with cement-based materials and then retard the hydration process [21].
When the cement ratio increases from 15% to 25%, the sample strength and the peak total of CAH and CSH increase faster, indicating that as the sample cement ratio increases, the cement weakens the influence of humic acid on the strength of the sample.
3.4. Mercury Intrusion Porosimetry Results and Analysis
In order to more intuitively study the change law of the internal pores of cement soil sample, mercury intrusion porosimetry (MIP) is carried out on the sample (the curing age is 90 days, the mixing amount of HA is 15%, and the cement ratio is 5%, 10%, 15%, 20%, and 25%). Figure 12 shows the cumulative curve of pore size of cement soil samples with different cement ratios. The abscissa of the curve is the pore diameter, and the ordinate is the cumulative percentage of mercury in the pores. The logarithmic coordinate is utilized. Figure 13 shows the relationship between pore size and pore percentage. Figure 14 shows the relationship curve between total pore volume per unit volume cement soil and cement ratio (15% HA, curing age of 90 days, and cement ratio).
Analyzing and comparing the five curves in Figure 12, it can be clearly seen that for the samples with cement ratios of 5% and 10%, the curve is gentle when the pore size is less than 10 nm or greater than 40,000 nm, indicating that there are very few pores in this pore size range; the curve is steep when the pore size is in the range of 10 nm∼60 nm, indicating that there are most pores in this pore size range. For samples with cement ratios of 15%, 20%, and 25%, the curve is gentle when the pore size is less than 10 nm or greater than 6000 nm, indicating that there are very few pores in this pore size range; the curve is steep when the pore size is in the range of 10 nm∼60 nm, indicating that there are most pores in this pore size range. It can be seen from Figure 13 that the pore size of each sample is concentrated in the range of 10 nm∼60 nm, and the proportion of this size range increases with the cement ratio. The test results show that as the cement ratio increases, the number of large-size (6000∼40,000 nm) pores in the sample is effectively reduced.
Figure 14 shows that under different cement ratios, the total volume of pores per unit volume of cement soil sample decreases as the cement ratio increases. When the cement ratio is increased from 5% to 25%, the total volume reduction value of pores per unit volume of cement soil sample is 0.028 ml/cm3.
Comparing Figures 12–14, it can be found that when the cement content is relatively low, the hydration products of cement are mainly cemented between the soil particles. Large-size pores (6000 nm∼40,000 nm) account for a larger percentage, and the total pore volume is greater. As the cement ratio increases, cement hydration products gradually increase. In addition to acting on the cementation between soil particles, they are also filled in the pores of the sample, effectively reducing the large-size pores (6000 nm∼40, 000 nm). Therefore, the influence of HA on each sample is gradually weakened.
4. Conclusion
This study obtains peat soil from seven areas around Dianchi Lake, and the content of humic acid (HA) in peat soil is between 2.36% and 28.13%. Then, this study simulates peat environment soil (PES) of Dianchi Lake area by adding humic acid (HA) into the cohesive soil and investigated the strength development of cement-solidified peat soil. Additionally, the microstructures of typical mixes are analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP). The following conclusions can be drawn:(1)The UCS test result shows that HA seriously affects the strength development of cement soil sample. This is attributed to the presence of HA which reduced the pH of pore water solution in cement soil sample; meanwhile, it adsorbed calcium ions (Ca2+) needed to produce cementitious substances and then hindered the hydration of cement. HA can also decompose the cementitious substances that have been produced, resulting in a reduction in the final amount of cementitious substances.(2)The SEM test result shows that as the cement ratio gradually increases, the cement hydration products in the sample gradually increase, changing from fibrous to flocculent. In addition to the cementitious soil particles, hydration products gradually fill the pores of the soil, which makes the soil particle framework stronger. The XRD test result shows that when the cement ratio increases from 10% to 15%, the diffraction peak of CAH and CSH increases slowly, indicating that HA has a certain influence on the strength formation of each sample. But when the cement ratio increases from 15% to 25%, the diffraction peak of CAH and CSH increases faster; combined with the results of the UCS test, it could be proved that the increase of the cement ratio weakens the influence of HA on the strength of the sample.(3)The MIP test shows that the total volume of pores per unit volume of cement soil sample decreases as the cement ratio increases. When the cement ratio is increased from 5% to 25%, the total volume reduction value of pores per unit volume of cement soil sample is 0.028 ml/cm3. When the cement content is relatively low, the hydration products of cement are mainly cemented between the soil particles. Large-size pores (6000 nm∼40,000 nm) account for a larger percentage, and the total pore volume is greater. As the cement ratio increases, cement hydration products gradually increase. In addition to acting on the cementation between soil particles, they are also filled in the pores of the sample, effectively reducing the large-size pores (6000 nm∼40,000 nm). Therefore, the influence of HA on each sample is gradually weakened.(4)Comprehensive test results demonstrate that the HA will significantly reduce the strength of cement soil sample, but the increase of cement ratio can effectively inhibit the effect of HA. The test results are consistent with expectations. Based on the previous research, this paper also explored the strength development law and microstructure changes of cement soil sample in the peat soil environment and proved that when the cement mixing ratio is 15% to 25%, the effect of HA on the cement soil sample is greatly weakened. Although the addition of cement can effectively inhibit the impact of HA in the soil, the presence of HA will still weaken the hydration reaction of cement, and the impact of HA on it cannot be completely ignored. Therefore, subsequent research will seek other admixture forms to solve the effect of peat soil environment on actual engineering projects, such as lime and nanomaterials.
Data Availability
The data used to support the findings of this study are included within the article.
Additional Points
Highlights. (1) The content of humic acid (HA) in peat soil of Dianchi Lake is between 2.36% and 28.13%. (2) Humic acid in peat soil environment of Dianchi Lake can significantly affect the strength development of cement soil. (3) Cement ratio greater than 20% weakens the influence of humic acid on the strength development of cement soil.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (41967035).