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Article

Physical and Mechanical Properties of All-Solid-Waste-Based Binder-Modified Abandoned Marine Soft Soil

by
Qiang Liu
1,2,
Junjie Yang
1,2,
Yalei Wu
1,2,*,
Zimou Wang
1,2,
Xinyi Qiu
1,2 and
Lijun Yan
1,2
1
College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China
2
The Key Laboratory of Marine Environment and Ecology of the Ministry of Education, Ocean University of China, Qingdao 266100, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2024, 12(3), 393; https://doi.org/10.3390/jmse12030393
Submission received: 11 January 2024 / Revised: 22 February 2024 / Accepted: 22 February 2024 / Published: 24 February 2024
(This article belongs to the Section Ocean Engineering)
Browse Figures
Versions Notes

Abstract

:
Large quantities of abandoned marine soft soil are generated from coastal engineering which cannot be directly utilized for construction without modification. The utilization of traditional binders to modify abandoned marine soft soil yields materials with favorable mechanical properties and cost efficiency. However, the production of traditional binders like cement leads to environmental pollution. This study uses a CGF all-solid-waste binder (abbreviated as CGF) composed of industrial solid waste materials such as calcium carbide residue (CCR), ground granulated blast furnace slag (GGBS), and fly ash (FA), developed by our research team, for the modification of abandoned marine soft soil (referred to as modified soil). It is noteworthy that the marine soft soil utilized in this study was obtained from the coastal area of Jiaozhou Bay, Qingdao, China. Physical property tests, compaction tests, and unconfined compressive strength (UCS) tests were conducted on the modified soil. The investigation analyzed the effects of binder content, compaction delay time, and curing time on the physical, compaction, and mechanical properties of CGF-modified soil and cement-modified soil. Additionally, microscopic experimental results were integrated to elucidate the mechanical improvement mechanisms of CGF on abandoned marine soft soil. The results show that after modification with binders, the water content of abandoned marine soft soil significantly decreases due to both physical mixing and chemical reactions. With an increase in compaction delay time, the impact of chemical reactions on reducing water content gradually surpasses that of physical mixing, and the plasticity of the modified soil notably modifies. The addition of binders results in an increase in the optimum moisture content and a decrease in the maximum dry density of CGF-modified soil, while the optimum moisture content decreases and the maximum dry density increases for cement-modified soil. Moreover, with an increase in binder content, the compaction curve of CGF-modified soil gradually shifts downward and to the right, while for cement-modified soil, it shifts upward and to the left. Additionally, the maximum dry density of both CGF-modified and cement-modified soils shows a declining trend with the increase in compaction delay time, while the optimum moisture content of CGF-modified soil increases and that of cement-modified soil exhibits a slight decrease. The strength of compacted modified soil is determined by the initial moisture ratio, binder content, compaction delay time, and curing time. The process of CGF modification of marine soft soil in Jiaozhou Bay can be delineated into stages of modified soil formation, formation of compacted modified soil, and curing of compacted modified soil. The modification mechanisms primarily involve the alkali excitation reaction of CGF itself, pozzolanic reaction, ion-exchange reaction, and carbonization reaction. Through quantitative calculations, the carbon footprint and unit strength cost of CGF are both significantly lower than those of cement.

1. Introduction

A substantial volume of marine soft soil results from activities such as harbor dredging, channel dredging, and pit excavation in coastal regions [1,2]. Marine soft soil exhibits elevated water content, clay content, and organic content, rendering it susceptible to soil stability issues, including low strength and significant deformation, if utilized directly for filling works without appropriate treatment [3,4]. Conventional disposal methods, such as land disposal, not only consume valuable land resources but also contribute to environmental pollution. Similarly, marine disposal alternatives, involving offshore abandonment, incur considerable costs, while near-shore abandonment poses risks of marine environment pollution and depletion of marine resources [5]. On the other hand, construction projects like highways and railways demand substantial quantities of high-quality fill materials. A viable approach to address this demand involves improving abandoned marine soft soil by incorporating binders such as cement, lime, and others. This approach aims to improve the stability of soil, thereby aligning it with the specifications required for use as fill material. This strategy represents a crucial avenue for the resource utilization of abandoned marine soft soil [6,7,8]. Simultaneously, it offers a practical solution to the environmental challenges associated with abandoned soil disposal.
After the addition of a binder, the abandoned marine soft soil underwent discernible changes in its physical, compaction, and mechanical characteristics, influenced by the content of the binder [9], compaction delay time [10], and curing time [4]. The high water content in abandoned marine soft soil is significantly reduced after modification with binders such as cement and lime [11,12]. Simultaneously, with an increase in content of the binders, the modified soil exhibits elevated liquid limits and plastic limits, accompanied by a decrease in plasticity index [13,14,15,16,17]. The optimal moisture content decreased, and the maximum dry density increased with rising cement content [18,19,20]. Conversely, for the lime-modified soil and the solid-waste-based binder-modified soil, the optimal moisture content increased, while the maximum dry density decreased with higher lime contents [21,22,23,24,25]. Under a certain content of binder, the optimal moisture content of cement-modified soil decreased with an extended compaction delay time [20,26], while the optimal moisture content of lime-modified soil increased [27]. However, the maximum dry densities in both cases exhibited a decreasing trend, stabilizing subsequently [28,29,30,31]. The strength of the modified soil exhibited a gradual increase with an augmented binder content [9], followed by a tendency towards linear growth [32,33]. As the compaction delay time increased, the strength of the cement-modified soil gradually decreased due to a reduction in dry density [26], whereas the strength of lime- modified soil experienced a gradual increase [34]. Moreover, the strength of the modified soil displayed an ascending trend with prolonged curing time [35,36,37].
The modification of soil by cement is primarily achieved through the processes of cement hydration, ion exchange reactions, and pozzolanic reactions [19,38]. The most significant distinction between alkali-activated binders and traditional Portland cement lies in the fact that the reaction of Portland cement only requires mixing with water, whereas alkali-activated binders necessitate the addition of alkaline substances in aqueous form [39]. Therefore, alkali-activated binders typically consist of alkali activators, pozzolanic materials, and supplementary materials [40,41,42,43]. Essentially, alkali-activated binders involve the dissolution of silica and alumina-rich raw materials mixed with alkaline substances, resulting in the generation of hydration products such as Calcium Silicate Hydrate(C-S-H) and Calcium Aluminate Hydrate(C-A-H) [44,45,46]. The most commonly used alkali activators, such as alkali hydroxides (sodium hydroxide) [47] and alkali silicates, are widely utilized. Common pozzolanic materials include fly ash, etc. [48]. While extensive research has been conducted on the use and mechanisms of alkali-activated binders for soil modification, there remains limited investigation into the application of alkali-activated binders based on all-solid-waste.
This study explores the application of the CGF all-solid-waste binder (abbreviated as CGF) [42], a formulation developed by our research group that utilizes calcium carbide residue (CCR), ground granulated blast furnace slag (GGBS), and fly ash (FA) as components, to modify abandoned marine soft soil. Various tests were conducted, including basic physical properties tests on the modified soils, compaction tests, and unconfined compressive strength (UCS) tests on the compacted modified soils. These tests involved comparisons with soil modified using cement. The investigation primarily focused on the physical properties, compaction properties, and mechanical properties of the marine soft soil modified using CGF. Elucidation of the modified process of CGF on abandoned marine soft soil and its modification mechanisms was carried out by combining the results of microscopic tests. Moreover, the modification process of CGF on abandoned marine soft soil and its modification mechanisms was elucidated by combining the results of the microscopic test. The research results can provide a theoretical basis for both resource utilization of abandoned marine soft soil and engineering construction in coastal areas.

2. Materials and Methods

2.1. Test Materials

2.1.1. Test Soil

The test soil was extracted from the marine soft soil of Jiaozhou Bay in Hongdao Economic Zone, Qingdao City, China. The sampling site primarily consists of Quaternary artificial fill layers and Quaternary Holocene coastal swamp layers. The soil was collected from depths ranging from 1.3 m to 3.8 m below the ground surface, predominantly comprising illite, kaolinite, and quartz minerals. This soil underwent a drying process lasting 12 h, followed by crushing and sieving through a 0.075 mm mesh to prepare it for testing. The basic physical properties of the test soil were examined in accordance with the Chinese National Standard (GB/T50123) [49]. These properties are detailed in Table 1, while the cumulative grain size gradation curve is depicted in Figure 1. As per the current Chinese National Standard (GB50007) [50], the test soil is classified as clay soil.

2.1.2. CGF and Cement

The CGF utilized in the test was formulated using calcium carbide residue (CCR), ground granulated blast furnace slag (GGBS), and fly ash (FA) in a mass ratio of 4:4:2 [51]. The CCR was procured from Shihua Chemical Co. in Dezhou, China; GGBS was sourced from Qingdao Specialty Iron & Steel Co. in Qingdao, China; FA was obtained from Qingdao Dongyi Cogeneration Co. in Qingdao, China. The cumulative granular gradation curves for each CGF component are presented in Figure 1, while the chemical compositions are detailed in Table 2. The P.O 42.5 cement used as the control group was supplied by Shandong Cement Group Co. (Jinan, China).

2.2. Testing Program

2.2.1. Factors Affecting the Performance of Modified Soils

The performance of modified soil is influenced by various factors at different stages of field construction. To investigate the impact of these factors on modified soil, it is crucial to clarify the test contents and influential variables of indoor studies based on the field construction process. Figure 2 illustrates the correlation between the field construction process of modified soil and the steps of indoor simulation tests. The onsite construction process is segmented into stirring, storage, transportation, rolling, onsite curing, and performance monitor, while the corresponding indoor simulation test involves blending, compaction delay, compaction, indoor standard curing, and performance testing. As a result, the time of the modified soil is divided into two phases. Time t1 represents the duration from storage to transportation, occurring after the modified soil is mixed with the binder and before compaction, corresponding to the compaction delay time before compaction in the indoor simulation test; this is referred to as the compaction delay time. Time t2 denotes the curing period from the compaction of modified soil in the field to the performance test. This corresponds to the indoor simulation test’s curing time after compaction of the modified soil and is termed the curing time. Based on this, the indoor test of modified soil can be divided into two sections: Test I (involving mixing of soil and binder, compaction, which is the formation of modified soil) considers factors such as the soil’s basic physicochemical properties, binder components and binder proportions, and the content of binders; compaction delay method and the compaction delay time (t1) of modified soil. Test II (involving compaction tests and performance tests of modified soil after compaction) considers factors including the unmodified soil’s basic physicochemical properties, binder components and proportions, binder content, the compaction delay method, compaction delay time (t1), the modified soil’s basic physicochemical properties, compaction method, curing method, and curing time (t2).

2.2.2. Testing Program

This paper focuses on elucidating the physical and mechanical properties of CGF- modified soil through basic physical property tests between CGF-modified and cement- modified soil, compaction tests, and subsequent unconfined compressive strength tests. Considering influencing factors such as binder content, compaction delay time, and curing time, the designed test program is detailed in Table 3. The initial moisture content ratio (αwn) represents the ratio of water content in the unmodified soil to the liquid limit of unmodified soil, while the binder content (as) denotes the mass ratio of binders to the dry soil of the test specimens.

2.3. Specimen Preparation

Following the test program (Table 3), the test soil and binders were combined in a mixing bucket, creating a dry powder mixture. To simplify indoor testing, the experimental seawater was prepared by artificial formulation using seawater concentrate. The seawater concentrate was mixed with water at a ratio of 1:30, and thorough stirring was applied to achieve complete dissolution. The mixture was utilized after the water became clear and stable. Subsequently, the artificial seawater was added to the dry powder mixture, followed by additional stirring for 5–10 min. The well-mixed combination was sealed in a plastic bucket and placed under standard curing conditions for compaction delay (temperature 20 ± 2 °C, humidity ≥ 95%). Basic physical property tests and standard Proctor tests were conducted on the modified soil that had achieved the required compaction delay time. The compacted soil specimens were shaped into cylindrical forms with a diameter of 50 mm and a height of 100 mm. This process was accomplished using a steel wire saw, which delicately removed excess soil until the specimen reached the predetermined dimensions, aiming to minimize disturbance to the soil. Unconfined compressive strength testing was conducted at predetermined times under standard curing conditions (temperature 20 ± 2 °C, humidity ≥ 95%); after the USC test, the specimens were crushed, and fresh sections were selected for SEM analysis. The specific specimen preparation and testing procedure are illustrated in Figure 3. The Atterberg limit was determined using the LP-100D Soil Liquid and Plastic Limit Integrated Tester. The compaction test was the standard Proctor test, utilizing a 2.5 kg hammer, a drop height of 305 mm, and a cylindrical test specimen with a diameter of 102 mm and a height of 116 mm. The apparatus for unconfined compressive strength testing was the WCY-1 Unconfined Compressive Strength Testing Apparatus. It was strain-controlled, operating at a loading rate of 1 mm/min. The XRD test was performed utilizing X-ray diffractometer model X’Pert Powder (PANalytical B.V., The Netherlands). The XRD test involved drying the specimen at 105 °C until reaching constant weight, followed by grinding through a 0.075 mm sieve and utilizing a copper rotating anode target. The SEM test employed a benchtop scanning electron microscope GeminiSEM300 from ZEISS (Crossbeam, Germany). The SEM test necessitated pre-freeze-drying and subsequent surface coating with gold for the specimens. All the tests were conducted in accordance with the Standard for Geotechnical Test Methods (GB/T 50123-2019) [49].

3. Results

3.1. Physical Properties Tests

Figure 4 presents the experimentally measured water content of the modified soil at varying compaction delay times. The water content of the modified soil exhibited a declining trend as the compaction delay time increased. Notably, there was a substantial decrease in water content from 38% to a range between 37.13% and 34.41% within the initial one day of compaction delay. Subsequently, from one to seven days of compaction delay, the reduction in water content occurred gradually, correlating with the increased binder content, and the water content plateaued after the seventh day.
Figure 5 illustrates the shifting patterns in the Atterberg limit of modified soils in relation to binder content. In comparison to unmodified soil, the plastic limit (PL), liquid limit (LL), and plasticity index (PI) of the modified soil varied noticeably, particularly evident in conditions featuring 8% binder content in 0 d of compaction delay. In this scenario, the plasticity index for CGF-modified soil decreased from 22.5 to 21.4, while for cement-modified soil, it reduced to 21.3. The plasticity of the modified soil was significantly enhanced, and the binder demonstrated a pronounced improvement effect on the soil. Additionally, the liquid limit and plastic limit of CGF modified soil across different compaction delay times exceeded those of unmodified soil, while the plasticity index decreased. With the binder content increased, the liquid limit and plastic limit of CGF modified soil gradually increased, while the plasticity index decreased. The plastic limit, liquid limit, and plasticity index of marine soft soil reduced slightly with the addition of cement and further decreased with higher binder content. Moreover, the Atterberg limit of the modified soil showed no significant variations concerning compaction delay time (t2).

3.2. Standard Proctor Tests and Unconfined Compressive Strength Tests

In this study, the peak stress on the stress-strain curve represents the unconfined compressive strength (UCS), while the ratio of half the unconfined compressive strength to the corresponding strain defines the modulus of elasticity (E50), also known as the coefficient of deformation, indicating the strain at half the unconfined compressive strength [51,52,53]. The modified soil with compaction delay times of 0 d, 3 d, and 7 d underwent compaction and unconfined compressive strength testing immediately. Figure 6 and Figure 7 depict the results of the compaction tests along with the corresponding unconfined compressive strength and modulus of elasticity (E50). The dry densities of the modified soils increased with rising water content and decreased after reaching their peak values. This suggests that there exists an optimum moisture content and maximum dry density for all modified soils. Furthermore, the maximum strength of compacted modified soil was achieved prior to reaching the optimum moisture content. Subsequently, as the moisture content surpassed this optimum level, the strength gradually declined with increasing moisture content. Similarly, the deformation modulus exhibited a corresponding trend with variations in strength.
Stress-strain curves are effective in analyzing the deformation properties of CGF- modified and cement-modified soils after compaction. Figure 8 shows the stress-strain curves of the compacted modified soils. Despite the different types of binders used, as the axial strain increases, the stresses in the compacted soil with the same binder content initially increase linearly, reach a maximum value, and then gradually decrease. This behavior indicates the property of strain softening in the compacted soil.

3.3. XRD Analysis and SEM Analysis

The XRD patterns of the unmodified soil and the CGF-modified soil are shown in Figure 9. The mineral phase composition of the unmodified soil primarily comprises kaolinite, illite, quartz, and calcite. After the addition of CGF to the abandoned marine soft soil, under the alkaline-activated effect of CCR, the vitreous shell of active silica-alumina phase material in GGBS and FA dissolved, diffused, polymerized, and gelatinized. This led to a pozzolanic reaction [39], with the appearance of alkaline-excited products like Calcium Aluminate Hydrates (C-A-H), Calcium Silicate Hydrates (C-S-H), and Calcium Aluminate Silicate Hydrates (C-A-S-H), and hydrotalcite-like compounds (HT) within the diffraction range of 35° to 45°. This observation indicates that the modified soil underwent reactions akin to equations [42,52]:
x Ca 2 + + y SiO ( OH ) 3 - + ( z - x - y ) H 2 O + ( 2 x - y ) OH - → C x - S y - H z ( C - S - H   gel ) 4 Ca 2 + + 2 Al ( OH ) 4 - + 6 H 2 O + 6 OH - → C 4 - A - H 13 ( C - A - H   gel )
Figure 10 shows the SEM image of CGF modified soil. As shown in Figure 10a, the reaction products, excited by alkali, appeared in the form of flocs and flakes, distributed among the soil particles following the CGF modification. Based on the EDS results, alkali-activated reaction products Calcium Silicate Hydrates(C-S-H) and Calcium Aluminate Silicate Hydrates(C-A-S-H) adhere to point 1 in Figure 10b, while point 2 is associated with the attachment of alkali-activated reaction product Calcium Aluminate Silicate Hydrates(C-A-S-H). After the addition of CGF to the soil, the available free water was consumed. The resulting reaction products contributed to a cementing and filling effect, causing the soil particles to begin forming agglomerate structures (seen in Figure 10d). As the compaction delay ended, these agglomerates had gained a certain level of strength. The compaction of these agglomerates was observed as shown in Figure 10e. The consumption of compaction energy by the agglomerates led to a reduction in the compacted soil’s density, ultimately impacting its strength.

4. Discussion

4.1. Physical Properties of Modified Soil

The mechanical behavior and microstructure of modified soil are closely related to the water content within the modified soil. Therefore, studying the variations in water content can assist in understanding the mechanical behavior and reaction mechanisms of the modified soil. As shown in Figure 4, the modified soil, upon the addition of the binder, exhibited a reduction in water content compared to the unmodified soil. This phenomenon is consistent with the findings reported in reference [12]. This decrease can be attributed partly to the difference in water content between the binder and the unmodified soil, resulting from the physical blending of two solids with varying moisture levels. Additionally, another contributing factor is the chemical interaction between the binder and the water.
In order to analyze changes in water content, let the water content of the unmodified soil be represented as wn. Following physical mixing with the binder, the resulting water content of the modified soil is denoted as wmix, indicating the moisture content when no chemical reaction takes place. Subsequently, the reduction in the water content of the soil due to physical mixing, Δwphy, is calculated using Equation (2).
Δ w p h y = w n − w m i x ,
wherein the water content of the unmodified soil after physical mixing with the binder, wmix, is calculated using Equation (3).
w m i x = m n 1 + w n w n + m b 1 + w b w b m n 1 + w n + m b 1 + w b ,
where wmix is the water content of the unmodified soil when it is physically mixed with the binder without chemical reaction, %; mn is the mass of the unmodified soil, g; wn is the water content of the unmodified soil, %; mb is the mass of the binder, g; and wb is the water content of the binder, %.
The reduction in water content attributed to the chemical reaction between the binder and the water is represented as Δwche, calculated from Equation (4). This value is determined by the disparity between the water content wmix of the unmodified soil subsequent to physical mixing with the binder and the measured water content of the modified soil, wt.
Δ w c h e = w m i x − w t
Upon plotting the calculation results in Figure 11, the reduction in water content within the modified soil can be attributed to both physical mixing and chemical reactions. Specifically, the reduction in water content resulting from physical mixing increases proportionally with a higher content of binder, irrespective of the compaction delay period. Conversely, the reduction in water content attributed to chemical reactions increases as the compaction delay period extends. During the initial one day of compaction delay, the CGF and cement underwent alkali excitation and hydration reactions, respectively. These reactions consumed a significant amount of free water, thereby causing a decrease in the water content of the modified soil. As the compaction delay time extended, the pozzolanic reaction within the modified soil occurred gradually. This reaction produced calcium aluminum hydrates (C-A-H) and calcium silicate hydrates (C-S-H), converting the free water in the soil into chemically bonded water [12]. Subsequently, the water content exhibited a gradual decline and began stabilizing after 7 days. At 9 days of compaction delay, the reduction in water content for modified soil with 2%, 4%, 6%, and 8% binder content respectively reached 1.40% and 1.36% (Figure 11a,e); 3.37% and 2.87% (Figure 11b,f); 3.83% and 3.98% (Figure 11c,g); and 4.95% and 4.80% (Figure 11d,h).
In order to analyze the effect of physical mixing and chemical reaction on water content reduction, the ratio of water content reduction due to chemical reaction and physical mixing (Δwche/Δwphy) is attended versus compaction delay time in Figure 12. Δwche/Δwphy gradually increased with compaction delay time due to the fact that the reduction in water content due to physical mixing was able to be completed at the time of the mixing of the binder with the modified soil, while the chemical reaction occurred gradually with the increase in compaction delay time. The effect of the chemical reaction on the reduction in water content increased gradually and stabilized after 7 d. At 9 d of compaction delay, this ratio ranged from 0.71 to 1.31, i.e., the reduction in water content due to chemical reaction of the modified soil after compaction delay was able gradually to reach the level of the reduction in water content due to physical mixing.
The Atterberg limit is a straightforward and readily available indicator used to evaluate the modification performance of soil, and can help to explain the alterations in the mechanical, chemical, and mineral composition of the modified soil [54]. The phenomenon of increased liquid limit and plastic limit, coupled with a decrease in plasticity index in CGF-modified soil, is consistent with the findings reported in references [15,16]. The rise in liquid limit and plastic limit could be attributed to the increased specific surface area of the modified soil due to the addition of CGF, enhancing its water absorption capacity. The reduction in plasticity index was attributed to alterations in the arrangement of soil particles induced by calcium aluminate hydrates (C-A-H), calcium silicate hydrates (C-S-H), and calcium aluminate silicate hydrates (C-A-S-H) generated through the pozzolanic reaction, reducing inter-particle sliding and deformation and subsequently lowering soil plasticity. Chew et al. [14] suggested that the decrease in liquid limit and plasticity index could be due to hydration products adhering to agglomerate surfaces, decreasing their activity and bounding water content.

4.2. Compaction and Mechanical Properties of Modified Soils

4.2.1. Compaction Properties of Modified Soils

The data in Figure 6 and Figure 7 were used to plot the variation patterns of optimum moisture content and maximum dry density of the modified soil in Figure 13. Upon adding 2% CGF, the marine soft soil exhibited an increase in optimum moisture content from 18.56% to a range of 18.95% to 23.25%, accompanied by a decrease in dry density from 1.70 g/cm3 to a range of 1.57 g/cm3 to 1.66 g/cm3 during the compaction delay. This phenomenon is attributed to the alkali excitation reaction during the compaction delay, which consumed a substantial amount of water, consequently elevating the optimum moisture content. Simultaneously, the reaction products of the compaction delay enhanced particle resistance to dynamic loading, leading to the observed decrease in dry density [55]. As the content of CGF increased, the optimum moisture content gradually rose while the maximum dry density declined, consistent with findings on lime modified soil [22]. However, the hydration products from cement’s hydration reaction filled inter-particle pores, reducing the soil’s water holding capacity and resulting in a decrease in optimum moisture content [56]. With the addition of 2% cement, the dry density of the modified soil increased due to cement’s higher specific gravity compared to that of the unmodified soil. Notably, the modified soil exhibited a decreasing trend in optimum moisture content with increasing cement content, while the maximum dry density exhibited a gradual increase. Additionally, the variation in compaction delay has a negligible impact on both the optimal moisture content and the maximum dry density.
Based on the compaction curves depicted in Figure 8 and Figure 9, the variation in the modified soil’s optimum moisture content and maximum dry density concerning compaction delay time is illustrated in Figure 14. As the compaction delay time increased, the compaction curves for CGF-modified soil shifted towards the lower right, while those for cement-modified soil moved towards the lower left. The optimum moisture content of CGF-modified soil continued to rise, eventually stabilizing, whereas the optimum moisture content of cement-modified soil showed a slight decline. With prolonged compaction delay time, both CGF-modified soil and cement-modified soil exhibited a decreasing trend in dry density, eventually stabilizing. This trend is attributed to the sustained alkali excitation reaction of CGF and the ongoing hydration reaction of cement. These reactions led to an increase in calcium ion concentration, providing ample ions for cation exchange and flocculation-aggregation [57]. Consequently, highly cemented agglomerates formed within the modified soil, consuming a portion of the compaction energy and resulting in a decrease in the maximum dry density of the compacted modified soil [26].

4.2.2. Deformation Properties of Compacted Modified Soils

As an indicator of the compacted modified soil’s resistance to deformation, a higher modulus of elasticity indicates a stronger resistance to deformation. The modulus of elasticity and strength exhibited a similar trend during the compaction delay. To further analyze their correlation, the test results were plotted (Figure 15) with the unconfined compressive strength as the horizontal axis and the modulus of elasticity as the vertical axis. From the Figure 15, a similar linear relationship between the strength and modulus of elasticity is evident for both CGF compacted modified soil and cement compacted modified soil.

4.2.3. Strength Properties of Compacted Modified Soils

Figure 16 illustrates the variation trend in the unconfined compressive strength of CGF compacted modified soil and cement compacted modified soil (t2 = 0) following a seven-day compaction delay, concerning the binder content. The strength of both CGF- and cement-modified soils displayed an increase followed by a decrease with the rising binder content, indicating an optimal binder range between 4% and 6% for different moisture content ratios of the compacted modified soil. This trend is attributed to the rapid consumption of excess water in marine soft soil after the addition of binders, reducing the lubrication effect of water on soil particles, thereby improving strength. However, excessive binder causes the soil particles to form hardened agglomerates during compaction delay, consuming substantial compaction energy, resulting in increased pore in the compacted modified soil, reduced dry density, and subsequently decreased strength. Hence, the modified soil exhibits an optimal mixing range of 4% to 6%, indicating the transition towards solidified soil [28].
Given that the optimal binder content for different water content ratios in compacted modified soil ranges between 4% and 6%, the impact of compaction delay time on the strength of compacted modified soil was examined by maintaining the binder content at 4%. Figure 17 shows the trend in unconfined compressive strength variation in compacted modified soil with compaction delay time. The strength of CGF compacted modified soil at initial moisture content ratios of 0.58 and 0.64 decreases as the compaction delay time increases. In contrast, the strength of CGF compacted modified soil at an initial moisture content ratio of 0.69 initially increases and then decreases with rising compaction delay time. The strength of cement compacted modified soil at an initial moisture content ratio of 0.48 declines with increasing compaction delay time. Conversely, the strength of cement-compacted modified soil at initial moisture content ratios of 0.53 and 0.58 initially increases, followed by a decrease as compaction delay time extends. This trend is due to the continuous decrease in water content and dry density of both CGF- and cement- modified soils with prolonged compaction delay time, leading to reduced strength in compacted modified soils with lower initial moisture content ratios. However, in soils with relatively higher initial moisture content ratios, the chemical reaction between the binder and water during the compaction delay tends to consume excess water, diminishing the lubrication effect of the water on the soil particles and increasing the compacted modified soil’s strength. As compaction delay time continuously extends, the surplus water becomes depleted, resulting in a decrease in dry density in the modified soil, consequently impacting the strength of the compacted modified soil.
In this study, to avoid the impact of compaction delay, standard curing was conducted following compaction sampling of the modified soil without any compaction delay. Unconfined compressive strength tests were conducted on specimens that had reached the prescribed curing time. Figure 18 illustrates the variation in unconfined compressive strength with curing time (t2) for both CGF-modified soil and cement- modified soil after compaction. The strength of the compacted modified soil exhibited an upward trend with the increase in curing time. In the case of modified soil with 2% binder content, there was a marginal increment in strength with the duration of curing. However, for the compacted modified soil containing 4% binder content, a significantly more pronounced increase in strength was observed with extended curing time, followed by a slight plateau. Modified soil compositions containing 6% and 8% binder content demonstrated a substantial increase in strength as the curing time extended, indicating a delayed strength contribution from the pozzolanic reaction. With increasing curing time, the rate of strength enhancement in CGF-modified soil was slower than that in cement up to seven days. After seven days, the rate of strength improvement in CGF-modified soil exhibited no significant difference from that of cement.

4.3. Microscopic Modification Mechanism

The modification process and mechanisms of CGF on marine soft soil in Jiaozhou Bay were elucidated by examining the physical, compaction, and mechanical properties of CGF-modified soil. This understanding was complemented by delineating the correlation between compaction, strength, and deformation modulus of elasticity, and the microscopic attributes of CGF-modified soil (Figure 19).
The modification process of marine soft soil in Jiaozhou Bay can be divided into three stages: the formation of modified soil after the addition of the binders and compaction delay (Stage 1); the formation of compacted modified soil through compaction after the compaction delay (Stage 2); and the stage of compacted modified soil after standard curing (Stage 3).
The unmodified marine soft soil has a water content exceeding the liquid limit (a moisture ratio greater than 1), where the soil particles are surrounded by a considerable amount of free water. Because the state of the soil is controlled by the form of water in the soil and the degree of bonding between the soil and the water, excessive free water prevents the marine soft soil from undergoing compaction testing. At this stage, the unmodified soil mainly consists of soil particles, free water, and bound water.
Upon the addition of the binder to the marine soft soil (Stage 1), the alkali activator CCR ionizes and hydrolyzes, consuming a significant amount of water by generating OH− and Ca2+, resulting in a substantial reduction in free water content in the modified soil. In the alkaline environment provided by OH−, the Si-O-Si bonds and the Al-O-Al bonds in the active glass phase of the Al2O3 and SiO2 fracture, releasing [SiO4]4− and [AlO4]5− into the water. These ions, combined with Ca2+ and other ions, undergo a pozzolanic reaction to produce cementitious materials like calcium aluminate hydrates (C-A-H), calcium silicate hydrates (C-S-H), and calcium aluminate silicate hydrates (C-A-S-H). Some of these cementitious materials adhere to the surface of the soil particles, causing the soil particles to bind together, forming aggregates with a certain strength. Simultaneously, the Na+ and K+ adsorbed on the surface of clay minerals in the marine soft soil easily undergo ion exchange reactions with Ca2+ from the alkali activator. This ion exchange reaction disrupts the double-layer structure on the surface of the clay particles, leading to a thinner double layer, which may depress interparticle repulsion, consequently enhancing interparticle attraction and leading to a change in interparticle arrangement, ultimately forming a flocculation aggregation effect.
However, due to the relatively low content of binder in the modified soil, it fails to form a hardened structure similar to solidified soil, resulting in the formation of loosely modified soil from marine soft soil. Moreover, during reactions between hydrolyzed OH− and Ca2+ with other substances, they continuously absorb carbon dioxide from the air and carbonate ions from the water, resulting in a carbonization reaction that generates CaCO3, which fills the pores of the modified soil. At this stage, the modified soil mainly comprises cohesive aggregates bonded by alkali-induced products, incompletely reacted binder particles, soil particles, bound water, and a small amount of free water.
The water content of the soil in Stage 1 of modification undergoes a substantial decrease; however, due to the low dosage of the binders, it still fails to generate sufficient strength. The preliminary aggregates formed in this phase, under the compaction efforts (Stage 2), begin to allow relative movement between the soil particles, initiating the formation of a framework structure. Excess air within the soil starts to evacuate, leading to the creation of a denser soil mass. At this stage, the composition of particles and water in the modified soil remains similar to the preceding phase.
The modified soil, post-compaction, attains a certain level of strength. During the curing period (Stage 3), the pozzolanic reaction continues, further enhancing the strength of the compacted modified soil. The binder fully engages in the reaction during this phase, resulting in the modified soil primarily consisting of alkali-induced products, soil particles, and bound water.
In summary, the reactions of CGF modified Jiaozhou Bay marine soft soil during compaction delay mainly include (1) alkali excitation reaction of CGF itself, (2) pozzolanic reaction, (3) ion exchange reaction between CGF and clay minerals, and (4) carbonization reaction between the modified soil and the external environment. The reactions during curing time include (1) pozzolanic reaction and (2) carbonization reaction between the modified soil and the external environment.

4.4. Quantitative Evaluation for Environmental and Economic Benefits

This paper explores the sustainability and cost-effectiveness of the CGF binder through calculations of carbon footprint and cost, considering that the construction of modified soil is a mature construction technology, with conventional construction processes used when using CGF as the sole binder. In this analysis, a comparative assessment is made solely for the production stages of CGF and cement, introducing the carbon emissions per unit strength of the modified soil (kgCO2e/MPa) as the carbon footprint evaluation metric and the cost per unit strength of the modified soil (CNY/MPa) as the cost evaluation metric after curing for 0 days, 7 days, and 28 days [42,52].
As both CCR and FA require no processing, the GWP (global warming potential) for this portion of CGF is considered to be zero [42,58]. Meanwhile, the GGBS is obtained through crushing and grinding, with a calculated GWP of 44 kgCO2e/t based on electricity consumption, while the cement’s GWP is 940 kgCO2e/t. The analysis focuses on modified soil with a 6% binder content, assuming a mass of 1 ton for the untreated soil. The results of the carbon emissions per unit strength for the modified soil are depicted in Figure 20. It is evident from the graph that the carbon emissions per unit strength for CGF are consistently below 10 kgCO2e/MPa, whereas for cement, the values exceed 100 kgCO2e/MPa.
At the Chinese current market reference prices, the raw material prices for CCR, GGBS, and FA are 70 CNY/ton (9.73 USD/ton; 8.99 EUR/ton; 7.69 GBP/ton; 8.57 CHF/ton), 200 CNY/ton (27.81 USD/ton; 25.67 EUR/ton; 21.98 GBP/ton; 24.47 CHF/ton), and 180 CNY/ton (25.03 USD/ton; 23.11 EUR/ton; 19.78 GBP/ton; 22.03 CHF/ton), respectively, while the reference price for P.O 42.5 is 379 CNY/ton (52.70 USD/ton; 48.66 EUR/ton; 41.64 GBP/ton; 46.38 CHF/ton). This study considers modified soil with a 6% binder content, assuming a mass of 1 ton for the untreated soil. The cost per unit strength of the modified soil is calculated and presented in Figure 21. As observed from the graph, the cost per unit strength for CGF remains below 21 CNY/MPa, (2.92 USD/MPa; 2.70 EUR/MPa; 2.31 GBP/MPa; 2.57 CHF/MPa), while for cement, the values exceed 45 CNY/MPa (6.26 USD/ MPa; 5.78 EUR/MPa; 4.95 GBP/MPa; 5.51 CHF/MPa).

5. Conclusions

In this paper, the physical property test, compaction test, and unconfined compressive strength test were conducted on CGF-modified abandoned marine soft soil. The aim was to comprehensively analyze and compare the impacts of binder content, compaction delay time, and curing time on the physical, compaction, and mechanical properties of both CGF-modified and cement-modified soils. The research results indicate that certain properties of CGF-modified soil surpass those of cement-modified soil, thereby suggesting that CGF-modified marine soft soil may serve as a suitable fill material for foundation constructions. Furthermore, the investigation sought to elucidate the modification mechanisms of CGF on marine soft soil in Jiaozhou Bay. The main conclusions are as follows:
  • The water content of marine soft soil decreased from 38% to ranges between 37.13% and 34.41% within 1 d after the addition of the binder. This reduction in water content can be attributed to both physical mixing and chemical reactions. Specifically, the water content reduction stemming from physical mixing remained unchanged irrespective of the compaction delay time, whereas the reduction due to chemical reactions progressively increased with compaction delay time, tending to stabilize after 7 d. Under conditions of 8% binder content and 0 d compaction delay, the plasticity index of CGF-modified soil decreased from 22.5 to 21.4, while that of cement-modified soil decreased to 21.3. The liquid limit increased for CGF-modified soil and decreased for cement-modified soil. Additionally, as the binder content increased, the plasticity index of the modified soil gradually decreased. Meanwhile, the liquid limit and plastic limit of CGF-modified soil showed an increasing trend, whereas those of cement-modified soil decreased.
  • After the addition of 2% CGF, the optimum moisture content of marine soft soil increased from 18.56% to a range between 18.95% and 23.25%. Simultaneously, the dry density decreased from 1.70 g/cm3 to a range between 1.57 g/cm3 and 1.66 g/cm3 during the compaction delay. With increasing binder content, the optimum moisture content of the CGF-modified soil gradually rose while the maximum dry density decreased. Conversely, the cement-modified soil exhibited an opposing trend. Additionally, as the compaction delay time increased, the optimum moisture content of the CGF-modified soil gradually rose, while that of the cement-modified soil showed a slight decrease. Meanwhile, the maximum dry density of both modified soils displayed a decreasing trend with the increase in compaction delay time.
  • There exists a linear relationship between the strength and deformation modulus of CGF-modified and cement-modified soils. For the seven-day compaction delayed modified soils, the strength increases initially and then decreases with the rise in binder content, with the optimal content varying between 4% and 6% for different moisture content ratios of the compacted modified soils. Under constant binder content, the strength variation is influenced by the initial water content as the compaction delay time (t1) increases. Meanwhile, with an increase in the curing time (t2), there is a consistent upward trend in the strength of the modified soils.
  • The process of CGF modification on marine soft soil in Jiaozhou Bay can be segmented into three stages: the formation of modified soil, the formation of compacted modified soil, and the curing of compacted modified soil. The modification mechanisms primarily involve the alkali excitation reaction of CGF, the pozzolanic reaction, the ion-exchange reaction between CGF and clay minerals, and the carbonization reaction.
  • After calculation, both the carbon footprint and cost per unit strength of CGF are significantly lower than that of cement.
This investigation primarily focuses on the short-term behavior of the modified soil, lacking an in-depth exploration of long-term durability and performance in natural environmental conditions. While the alkaline leaching issue was considered during the initial design of the binder, its behavior in prolonged, adverse environmental conditions remains to be thoroughly investigated. In addition, the type of unmodified soil may also influence the experimental results. This study specifically focuses on marine soft soil near Jiaozhou Bay, which may limit the generalizability of the findings. Therefore, future efforts should primarily focus on the durability and performance studies of the modified soil under long-term conditions, concurrently addressing the alkaline leaching issue in adverse environments. These studies are imperative to assess the practical viability and sustained effectiveness of the proposed binder over an extended period. Simultaneously, corresponding studies should be conducted for modified soils of different soil textures to improve the universality of research outcomes.

Author Contributions

Conceptualization, Q.L. and X.Q.; methodology, Y.W. and J.Y.; validation, Y.W. and L.Y.; formal analysis, Q.L.; resources, J.Y.; data curation, Q.L. and Z.W.; writing—original draft preparation, Q.L.; writing—review and editing, J.Y. and Y.W.; visualization, Q.L.; supervision, J.Y. and Y.W.; funding acquisition, J.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 52378380), the National Natural Science Foundation of China (No. 52078474) and the National Natural Science Foundation of China (No. 51779235).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Some or all of the data, models, or codes that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

We gratefully acknowledge the financial support of the above funds and the researchers of all reports cited in our paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Accumulation curve of particle size gradation of unmodified soil and each component of CGF.
Figure 1. Accumulation curve of particle size gradation of unmodified soil and each component of CGF.
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Figure 2. Schematic diagram of onsite construction process and laboratory simulation test (Revised based on [28]).
Figure 2. Schematic diagram of onsite construction process and laboratory simulation test (Revised based on [28]).
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Figure 3. Test process.
Figure 3. Test process.
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Figure 4. Relation between water content and compaction delay time of modified soil.
Figure 4. Relation between water content and compaction delay time of modified soil.
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Figure 5. Variation in Atterberg limit with the content of binders (t1 = 0 d, 3 d, 7 d).
Figure 5. Variation in Atterberg limit with the content of binders (t1 = 0 d, 3 d, 7 d).
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Figure 6. Compaction-UCS-E50 curves of CGF modified soil.
Figure 6. Compaction-UCS-E50 curves of CGF modified soil.
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Figure 7. Compaction-UCS-E50 curves of cement modified soil.
Figure 7. Compaction-UCS-E50 curves of cement modified soil.
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Figure 8. Stress-strain curves of compacted modified soils (t1 = 0, t2 = 0).
Figure 8. Stress-strain curves of compacted modified soils (t1 = 0, t2 = 0).
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Figure 9. XRD pattern of unmodified soil and CGF modified soil.
Figure 9. XRD pattern of unmodified soil and CGF modified soil.
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Figure 10. SEM images of CGF modified soil.
Figure 10. SEM images of CGF modified soil.
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Figure 11. Changes in water content caused by physical mixing and chemical reactions.
Figure 11. Changes in water content caused by physical mixing and chemical reactions.
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Figure 12. Relation between Δwche/Δwphy and compaction delay time.
Figure 12. Relation between Δwche/Δwphy and compaction delay time.
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Figure 13. Variation in optimum moisture content and maximum dry density with binder content.
Figure 13. Variation in optimum moisture content and maximum dry density with binder content.
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Figure 14. Variation in optimum moisture content and maximum dry density with compaction delay time.
Figure 14. Variation in optimum moisture content and maximum dry density with compaction delay time.
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Figure 15. Relationship between the modulus of elasticity and UCS of compacted modified soils.
Figure 15. Relationship between the modulus of elasticity and UCS of compacted modified soils.
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Figure 16. Relation between binder content and the strength of compacted modified soils (t1 = 7, t2 = 0).
Figure 16. Relation between binder content and the strength of compacted modified soils (t1 = 7, t2 = 0).
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Figure 17. Relation between compaction delay time and the strength of modified soils (t2 = 0).
Figure 17. Relation between compaction delay time and the strength of modified soils (t2 = 0).
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Figure 18. Relation between curing time and the strength of modified soils (t1 = 0).
Figure 18. Relation between curing time and the strength of modified soils (t1 = 0).
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Figure 19. Modification process and mechanisms of Jiaozhou bay marine soft soil by CGF.
Figure 19. Modification process and mechanisms of Jiaozhou bay marine soft soil by CGF.
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Figure 20. GWP of 1 ton of unmodified soil per unit of strength at 0 d,7 d, and 28 d.
Figure 20. GWP of 1 ton of unmodified soil per unit of strength at 0 d,7 d, and 28 d.
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Figure 21. Cost of 1 ton of unmodified soil per unit of strength at 0 d, 7 d, and 28 d.
Figure 21. Cost of 1 ton of unmodified soil per unit of strength at 0 d, 7 d, and 28 d.
Jmse 12 00393 g021
Table 1. Physical properties of unmodified soil.
Table 1. Physical properties of unmodified soil.
Specific GravityPlastic Limit (%)Liquid Limit (%)Plasticity Index
2.7415.237.722.5
Table 2. Major chemical composition and content of solid waste [43].
Table 2. Major chemical composition and content of solid waste [43].
Chemicals (%)CaOAl2O3SiO2MgOFe2O3Na2OK2OSO3P2O5Others
CCR68.801.563.591.210.09/0.030.75/23.97
GGBS41.1713.6129.478.040.430.680.354.900.031.32
FA6.6012.6661.290.024.483.751.320.660.019.21
Table 3. Test program of CGF modified soil.
Table 3. Test program of CGF modified soil.
Test SoilInitial Moisture Content Ratio, αwnTypes of BinderContent of Binders, as (%) 2Compaction Delay Time, t1 (d)Curing Time, t2 (d)Tests
Jiaozhou bay marine soft soil1.0CGF,
P.O 42.5 1		2, 4, 6, 80, 3, 70, 7, 28Water content tests, Atterberg limit tests, compaction tests, unconfined compressive strength tests, scanning electron microscopy (SEM), X-ray diffraction (XRD)
1 P.O 42.5 refers to the Ordinary Portland Cement with the label 42.5. 2 In the subsequent figures, the CGF content of 2%, 4%, 6%, and 8% is abbreviated as CGF2, CGF4, CGF6, CGF8; while the cement content of 2%, 4%, 6%, and 8% is abbreviated as OPC2, OPC4, OPC6, OPC8.
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Liu, Q.; Yang, J.; Wu, Y.; Wang, Z.; Qiu, X.; Yan, L. Physical and Mechanical Properties of All-Solid-Waste-Based Binder-Modified Abandoned Marine Soft Soil. J. Mar. Sci. Eng. 2024, 12, 393. https://doi.org/10.3390/jmse12030393

AMA Style

Liu Q, Yang J, Wu Y, Wang Z, Qiu X, Yan L. Physical and Mechanical Properties of All-Solid-Waste-Based Binder-Modified Abandoned Marine Soft Soil. Journal of Marine Science and Engineering. 2024; 12(3):393. https://doi.org/10.3390/jmse12030393

Chicago/Turabian Style

Liu, Qiang, Junjie Yang, Yalei Wu, Zimou Wang, Xinyi Qiu, and Lijun Yan. 2024. "Physical and Mechanical Properties of All-Solid-Waste-Based Binder-Modified Abandoned Marine Soft Soil" Journal of Marine Science and Engineering 12, no. 3: 393. https://doi.org/10.3390/jmse12030393

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