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Article

Preparation of Artificial Aggregates from Marine Dredged Material: CO2 Uptake and Performance Regulation

by
Chunyang Yu
1,
Chunyi Cui
1,*,
Jiuye Zhao
1,
Fang Liu
2,
Jian Su
3 and
Jia Yuan
1
1
Department of Civil Engineering, Dalian Maritime University, Dalian 116026, China
2
Shaanxi Key Laboratory of Safety and Durability of Concrete Structures, Xijing University, Xi’an 710123, China
3
College of Ocean and Civil Engineering, Dalian Ocean University, Dalian 116023, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2023, 11(5), 1054; https://doi.org/10.3390/jmse11051054
Submission received: 10 April 2023 / Revised: 10 May 2023 / Accepted: 12 May 2023 / Published: 15 May 2023
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Abstract

:
A continuous treatment process using steel slag powder including foam drying and carbonation processes (termed the SSFD-C method) is a novel technology previously developed in our laboratory. It has achieved the first application of carbonation solidification technology to recycle marine dredged material with high moisture content. The aim of this study is to investigate CO2 uptake and performance regulation in the preparation of carbonated eco-aggregates (CEAs) from dredged soils processed using the SSFD-C method. Steel slag and lime hydrate independently contribute to the strength of CEAs. However, the influence they exert on CO2 uptake, along with other properties, such as pH values and water absorption of CEAs, remains unclear. Furthermore, it is important to clarify whether the soluble silica in a CEA originates from dredged soil or steel slag, as a CEA has the potential to provide silica nutrients to plants. The findings indicated that within the initial three hours of carbonation, the strength of CEAs could approximate 65% of the ultimate stable strength. The moisture absorption for CEAs was noted to be in the 26–30% range. Carbonation over a 24 h period can lower the pH of the CEA to less than 10, and the carbonation reaction can penetrate the core of the 10–15 mm CEA pellets. Carbonation of the lime hydrate fraction was more favorable to increase the CO2 uptake of the CEA, and carbonation of the steel slag fraction was more favorable to decrease the pH value and water absorption of the CEA. The water-soluble silicon of the CEA was found to have been mainly derived from steel slag, while it was established that carbonation could increase the water-soluble silicon content of the CEA by 5–8 times. The result of this study could provide theoretical guidance for regulating the performance of CEAs.

1. Introduction

Annually, China dredges hundreds of millions of tons of slurry to maintain navigable waterways and preserve channel depths. This dredged slurry, characterized by fine-grained soil and copious amounts of water, has traditionally been regarded as waste material, primarily disposed of through offshore dumping [1,2]. However, recent studies have suggested that, when recycled and solidified, dredged slurry can serve as a promising alternative construction material [3,4,5,6].
Vegetation concrete, a blend of traditional rigid concrete techniques and biotechnology for grass planting, is extensively employed in the field of ecological slope protection [7,8]. Research on vegetation concrete has predominantly been centered on utilizing low-alkali cement, slag admixtures, and other alkalinity reducing procedures, such as washing, to mitigate alkalinity and ensure compatibility with vegetation [9,10].
Contrastingly, vegetation concrete is typically composed of sand-free components, with aggregates constituting over 80% of its volume. Recent studies have highlighted the substantial potential of artificial aggregates, boasting excellent quality control and performance stability, as alternative aggregates for concrete production [11,12,13]. These studies suggest that the adoption of artificial aggregates could not only curtail the usage of natural stone resources and the ensuing pollution from mining activities but also transform large quantities of waste into high-value-added products, aligning with the demands of a recycling-centric economy [14,15].
Conversely, eco-aggregates, also known as artificial aggregates with low alkalinity and abundant silica nutrients, have scarcely been explored. In the past decades, mixing cement-based materials with soil to improve the mechanical properties of soil products has been one of the commonly used solidification techniques, where the strength increase mainly arose from the hydration of the cement [4]. However, cement solidification techniques have become increasingly limited by the high energy consumption and high carbon footprint incidental to cement production [16]. In recent years, the development of carbonation, a new low-carbon solidification technique that uses CO2 gas and calcium- or magnesium-rich industrial waste residue as reactants to solidify soil products, has attracted significant attention and has been the subject of much research [17,18,19]. This process enables the solidified soil product to quickly gain strength and achieve low alkalinity within a few hours [20].
Steel slag, a calcium-rich byproduct of steel manufacturing, sees an annual discharge in China that exceeds 100 million tons [21]. Steel slag has been shown to provide silicon fertilizer for plants, which has been widely studied and applied in agriculture [22]. In addition, various studies on the carbonation of steel slag established that steel slag displayed a high carbonation activity and that its carbonated products presented a lower alkalinity and higher biocompatibility compared to conventional cement-based materials [23,24]. For example, Oyamada et al. [25] prepared carbonated steel slag blocks, which developed a faster surface algae coverage than ordinary cement blocks. The above findings, therefore, suggest a very promising potential for applying steel slag carbonation techniques to solidify dredged soil in order to produce eco-aggregates.
However, applying steel slag carbonation techniques to the field of recycling dredged slurry presents numerous challenges. This is because dredged slurry is a viscous compound with a high moisture content of around 80–200%, which is intrinsically difficult to dry [26,27]. The solid composition of dredged material is dominated by fine-grained soil, including a large number of clay minerals that are negatively charged and with a high capacity for water adsorption [28,29,30]. This results in poor air permeability of the dredged slurry, which falls short of meeting the conditions to implement carbonation. Therefore, the processes of drying and altering the air permeability of dredged slurry must imperatively be completed prior to carbonation solidification.
In the authors’ preceding study [31], they explored the foam drying process of dredged slurry pretreated with steel slag powder. It was found that the inclusion of steel slag could effectively decrease the drying duration by nearly 60%. Another compelling discovery was that, after the drying process, the resulting dried soil (DS) containing steel slag could be effortlessly crushed, remixed with water, and compacted for carbonation solidification. Remarkably, the strength of the carbonated DS could still be enhanced by a factor of 5–8 times. This continuous treatment process using steel slag powder, including foam drying and carbonation processes, is termed the SSFD-C method, which presents a promising avenue for achieving large-scale reuse of dredged materials in a cost-effective and environmentally sustainable manner.
In contrast to conventional carbonation solidification, the supplementary drying process in the SSFD-C method was observed to diminish the carbonation strength of DS by lowering its alkalinity. The primary carbonation minerals in steel slag, dicalcium silicate (2CaO·SiO2, C2S) and calcium hydroxide (Ca(OH)2, CH), exhibit differential behavior during the drying process. CH is notably prone to depletion through air carbonation or interaction with reactive clay minerals, whereas C2S is largely conserved owing to its minimal hydration activity [21]. Nevertheless, the carbonation activity of C2S minerals in DS is also significantly reduced due to the lack of CH, which can provide a highly alkaline environment. This observation aligns with studies reporting that the alkalinity of pore water profoundly impacts the efficacy of the carbonation process [32]. Hence, it is advisable to enrich DS with an alkali agent prior to the carbonation process in the SSFD-C method.
This study presents a comprehensive investigation of using the SSFD-C method to prepare carbonated eco-aggregate (CEA). In a preceding study investigating the carbonation properties of DS, where different steel slag components were supplemented with lime hydrate (LH), we observed that identical mechanical properties could be attained when either steel slag or LH were used as primary carbonation components. This finding was intriguing. Thus, when equivalent mechanical properties are achieved, how do steel slag and LH components uniquely influence CO2 uptake, pH values, and water absorption properties of CEAs? Understanding this differentiation is vital for the performance regulation of CEAs. Furthermore, it is important to clarify whether the soluble silica in a CEA originates from dredged soil or steel slag, as a CEA has the potential to provide silica nutrients to plants. As the present authors deemed finding answers to these fascinating questions and elucidating the above mechanisms as crucial for predicting and regulating the properties of CEAs, they prioritized these investigations as part of the latter.

2. Materials and Methods

2.1. Materials

The dredged slurry selected for the present study was sampled from the Port of Dalian, Liaoning Province, China, with a natural moisture content of 69.3% and a liquid limit, plastic limit, and plasticity index of 43.6%, 23.2%, and 20.4%, respectively. Its content was classified following the unified soil classification system prescribed by ASTM D 2487 as low plasticity clay (CL). Basic oxygen furnace steel slag from Hebei Jianye Iron and Steel Group, with a specific surface area of 117.3 m2/kg, was used for this study. The chemical and mineral compositions of the raw materials were determined using X-ray fluorescence and an X-ray diffractometer, as shown in Table 1 and Figure 1 below. In this study, the activator LH was Ca(OH)2 with a chemical purity exceeding 95% Compared to other chemical alkalies, LH not only provides alkali but also is a candidate for carbonation. Moreover, LH can be replaced by calcium carbide slag, given that calcium carbide slag is an alkaline industrial waste slag with LH as its main component [33].

2.2. Specimen Preparation

In this study, the specimen preparation process was divided into three parts: the preparation of mixture powder for pelletization, the pelletization process, and the carbonation process, as illustrated in Figure 2. The detailed process will be described as follows.

2.2.1. Preparation of Powder Materials for Pelletization

Any impurities, such as large stones and shells, were first removed from the fresh field dredged slurry, which was then dried under natural conditions and broken up in a 2 mm sieve. A total of 69.3% (weight to dredged soil) of tap water was then measured and mixed with the dredged soil to form the treatment slurry. Next, the steel slag powder with contents of 5% and 30% (weight to dredged soil), respectively, was mixed with the slurry and stirred for 5 min at an ambient temperature of 25 °C. Subsequently, foam with a density of 45 kg/m3 was mixed with the mixture slurry and stirred for another 5 min. Lastly, the foam–slurry mixtures were poured into a 25 cm long × 20 cm wide × 5 cm deep PVC box and placed in a convection oven. After being dried for 48 h at 40 °C, the dewatered soil samples were broken down and passed through a 2 mm sieve as DS. More details of the above DS preparation process can be found in [31]. The DS with different ratios of steel slag were marked as DS-SS5 and DS-SS30, respectively.
Findings from the authors’ previous studies revealed that the DS-SS5 sample groups mixed with 15% LH and DS-SS30 sample groups mixed with 5% LH attained similar carbonation strengths. In this study, DS-SS5 and DS-SS30 were then uniformly mixed with the corresponding LH ratios of 15% and 5% of the DS mass, respectively, as powder materials for pelletization, designated as DS-SS5-LH15 and DS-SS30-LH5, respectively. The material composition of DS-SS5-LH15 and DS-SS30-LH5 is tabulated in Table 2.

2.2.2. Pelletization

Pelletization is a common pressure-free agglomerating method that enlarges moisturized fine particles into spherical pellets through their colliding and coalescing as a result of their rolling motion from being placed in and spun by a pelletizer disk [34]. Numerous studies on the pelletization process found that the angle of the pelletized disk, rotational velocity, and moisture content all affected pelletization efficiency [35,36,37]. In this study, the angle and speed parameters recommended by Colangelo and Cioffi [38], namely an angle of 50° and a speed of 45 rpm, were adopted. Harikrishnan and Ramamurthy [39] reported that moisture content was the main factor affecting aggregate size. Other scholars proposed that the moisture content ratios required to produce pellets in a size range of 5–8 mm to 10–20 mm were 15 and 35 wt%, respectively. In this study, the pelleting moisture content was set to 30%. The pelleting time was controlled within a 5 min accuracy span. After the pelleting process, the mass of the unpelleted powder did not exceed 10% of the total pellet mass, and the volume of 10–15 mm pellets accounted for no less than 70% of the total.

2.2.3. Carbonation Process

Fresh pellets with a particle size range of 10–15 mm were sieved and placed in a ventilated laboratory for 18 h at room temperature. The objective of this standing time was, firstly, to homogenize the moisture content between the pellet layers and secondly, to reduce the moisture content of the fresh pellets from 30% to a degree suitable for carbonation, namely 16%, according to the results of previous experiments. The moisture content of the fresh pellets had to be reduced because the diffusion rate of CO2 in the solution was much lower than in the air, hence the moisture content would have been too high and prevented the diffusion of CO2 from the shell to the core of the pellets and thus reduced carbonation efficiency [40]. The carbonation of the fresh pellets was subsequently performed in a kettle, as illustrated in Figure 2. A total of 99.9% pure CO2 was introduced into the kettle, and the pressure was maintained at 0.2 MPa. The carbonation durations were set to 3, 6, 24, 48, and 72 h. The pellets prepared by DS-SS30-LH5 and DS-SS5-LH15 (five groups each) were carbonated with different carbonation durations to obtain carbonated pellets labeled as CS-SS30-LH5 and CS-SS5-LH15 (five groups each), respectively.

2.3. Testing Methods

CO2 uptake measurements are widely used to establish the extent of the carbonation reaction of materials [32,41]. In contrast to previous carbonation studies where the experimental components were often individual, two different ratios of steel slag and LH mixtures were prepared for the CEA for the present carbonation study. As the CO2 sequestration efficiency of steel slag and LH are known to be different, the contribution of steel slag and LH to CO2 uptake could not be directly distinguished. Therefore, the CO2 uptake value selected for the present investigation was the ratio of the mass of sequestrated CO2 to the mass of the total carbonation components of the pellets. The CO2 uptake of CS-SS30-LH5 and CS-SS5-LH15 was calculated by Equation (1):
CO 2 uptake = m d c − m d × 100 % m d × k
where md is the dry mass of pellets before carbonation (g); mdc is the dry mass of pellets after carbonation (g); and k is the proportion of carbonation components in CS-SS30-LH5 and CS-SS5-LH15, namely 33.33% and 17.39%, respectively.
A single particle crushing loading method was used to test the strength of the CEA pellets [42], whereby the pellets were placed between two parallel plates and crushed under a radial load of 20 grains per group, with the splitting load averaged at a loading rate of 0.5 mm/min. The strength was calculated by Equation (2):
σ = 2.8 × P π × X 2
where σ is the particle crushing strength (MPa); X is the distance between loading points (mm); and P is the failure load (N). After the crushing test, half of the broken pellets were used as carbonation depth test samples, and the other half were dried in a vacuum drying oven at a temperature of 60 °C for 24 h and were then ground and sieved through an 80 μm mesh to provide samples for pH testing, as well as silicon-molybdenum blue colorimetric and mineral composition analyses.
As using phenolphthalein indicator solutions to monitor the carbonation depth has been proven to be convenient and effective [43], the experiment for carbonation depth consisted of spraying the phenolphthalein reagent on the pellet profiles. The experiments for pH testing and silicon-molybdenum blue colorimetric were performed as follows: accurately weighed 2.0 g portions of the powder samples were placed in 50 mL plastic centrifugal tubes, to which 25 mL of distilled water was added; the tubes were then sealed, shaken vigorously, and incubated in an incubator at a constant temperature of 25 °C for 24 h before analysis. The sampling method consisted of centrifugation at a speed of 3000 rpm for 15 min, after which the extracted supernatant was divided into 15 mL and 5 mL specimen tubes for pH and silicon-molybdenum blue colorimetric tests, respectively (CJ/T141-2001).
The composition analysis of two types of CEAs before and after carbonation was measured using an X-ray diffractometer (XRD) and derivative thermogravimetric analysis (TG-DTG). The XRD tests were conducted by CuKα12 radiation (K = 0.154 nm). The operating voltage of the X-ray source was 40 kV, with a current of 40 mA. Lastly, VAE V3 software was used for mineral identification. TG-DTG analysis was conducted using a METTLER TOLEDO TGA/DSC1 at a heating rate of 10 °C/min from 50 °C to 1000 °C under a nitrogen atmosphere.

3. Results and Discussion

3.1. Mechanical Properties

Figure 3 below shows the particle crushing strength of the CS-SS30-LH5 and CS-SS5-LH15 specimens at different carbonation times. Results established that the strength of both CS-SS30-LH5 and CS-SS5-LH15 increased substantially during the first 3 h of carbonation, and eventually stabilized at about 1.1 MPa as the carbonation time was extended. The rapid increase in strength over a short period of time is consistent with many studies of carbonated solidification soils using activated magnesium oxide [44,45]. The strength increase in the pellets after carbonation was ascribed to the carbonation reaction of the carbonation components in pellets. In addition, the strongest growth rate of CS-SS30-LH5 by carbonation was found to be slower than that of CS-SS5-LH15, as reflected by the fact that the carbonation strength of CS-SS5-LH15 stabilized after just over 6 h of carbonation, while CS-SS30-LH5 only stabilized after over 48 h of carbonation.
Figure 4a below shows the calculated CO2 uptake of CS-SS30-LH5 and CS-SS5-LH15 at different carbonation times. It can be observed that as the carbonation time was extended, the CO2 uptake of both CS-SS30-LH5 and CS-SS5-LH15 increased rapidly and then stabilized, and the CO2 uptake of CS-SS30-LH5 was lower than CS-SS5-LH15 at any carbonation time. This disparity might be associated with the solubility of C2S and CH, the primary carbonation minerals in steel slag and LH, respectively. The associated dissolution equations are:
2CaO·SiO2 + 4H2O → 2Ca2+ + 4OH− + H4SiO4
Ca(OH)2 → Ca2+ + 2OH−
C2S dissolution and subsequent Ca2+ ion release prove more difficult than CH [32]. Consequently, this led to a diminished CO2 uptake of CS-SS30-LH5 compared to CS-SS5-LH15 for identical carbonation durations.
The fitted lines of CO2 uptake and the particle crushing strength for CS-SS30-LH5 and CS-SS5-LH15 are shown in Figure 4b below. The data used are the mean values. It can be found that the particle crushing strength of both CEA types presented a highly positive linear fitted correlation with CO2 uptake. One further interesting finding was that the gradient of the fitted line for CS-SS30-LH5 was sharper than that for CS-SS5-LH15, suggesting a greater contribution to strengthening the specimens from steel slag than LH carbonation. This phenomenon is associated with the distinct carbonation products of C2S and CH. The corresponding carbonation equations for C2S and CH are as follows:
2CaO·SiO2 + 2CO2 + nH2O → 2CaCO3 + SiO2·nH2O (gel)
Ca(OH)2 + CO2 + nH2O → CaCO3 + (n + 1) H2O
The carbonation of C2S yields SiO2 gel in addition to CaCO3, and this SiO2 gel contributes to CEA strength by further filling the pores. Consequently, at comparable CO2 uptake levels, the steel slag fraction is more beneficial for CEA strength than LH.

3.2. Moisture Absorption

Figure 5 below shows the moisture absorption rates of CS-SS30-LH5 and CS-SS5-LH15 at different carbonation times. It revealed that carbonation significantly reduced the moisture absorption of two types of CEAs as follows: after the first 3 h of carbonation treatment, the moisture absorption of CS-SS30-LH5 and CS-SS5-LH15 decreased by 15.3% and 16.9%, respectively. As the carbonation time extended subsequently, the decreasing rate of moisture absorption stabilized. Furthermore, it was noted that the moisture absorption of CS-SS30-LH5 was consistently lower than CS-SS5-LH15 at any carbonation time.
Many studies of cold-bonded pellets after sintering or cementing found that the main factor affecting pellet moisture absorption was the moisture content of fresh pellets, and fresh pellets with a high moisture content usually demonstrated a higher moisture absorption after sintering or cementing [14,36,46]. In this study, as both types of fresh pellets shared the same moisture content before carbonation, it was assumed that both types of pellets had been filled through their pores with the same carbonation product generated by their carbonation, namely CaCO3, thus reducing moisture absorption. In contrast, given that CS-SS30-LH5 contained a greater proportion of steel slag, its carbonation process produced a larger quantity of SiO2 gel. This gel filled the pores of the CEA, leading to a more compacted pellet structure and, consequently, a lower moisture absorption rate compared to CS-SS5-LH15.

3.3. Carbonation Depth and pH Value

The experimental results to establish the carbonation depth by spraying phenolphthalein onto the pellet profiles of both types of CEAs treated with different carbonation times are presented in Figure 6 below. As the phenolphthalein solution is a colorless acid-base indicator that turns purple at pH values greater than 10 [43], dark purple with uniform distribution at all depths can be clearly observed in the profiles of both types of CEA spheres before carbonation treatment. This indicated that the pellets without carbonation had high alkalinity and were unsuitable for plant growth. In contrast, it can be seen that after 3 h of carbonation, the CS-SS30-LH5 sample profiles had been restored to the natural color of pellets, with no purple being visible. This was possible because carbonation could neutralize the alkaline component of CEA pellets, namely steel slag and LH. It was speculated that the pH of CS-SS30-LH5 could be reduced to below 10 after 3 h of carbonation, which was to be verified in the following pH test in Figure 7. Additionally, it was noted that purple coloring could still be observed at all depths of the CS-SS5-LH15 sample profiles during the initial 6 h of carbonation. In contrast, the sample profiles of CS-SS5-LH15 all appeared in the natural color of pellets after 24 h of carbonation, indicating that sufficient carbonation time could allow the carbonation reaction to reach the center of 10–15 mm pellets.
Figure 7a below shows the pH values of CS-SS30-LH5 and CS-SS5-LH15 at different carbonation times. Firstly, the pH values of both types of CEA pellets can be observed to have decreased rapidly at first, and then become stabilized as the carbonation time was extended and secondly, whereas the rapid drop in the pH value of CS-SS30-LH5 occurred in the first 6 h of carbonation, CS-SS5-LH15 was delayed and occurred after 24 h of carbonation. As can be seen in Figure 7, the pH value of CS-SS30-LH5 at 3 h of carbonation was 9.22, that is, below 10, and the pH value of CS-SS5-LH15 at 6 h of carbonation was 11.25, above 10. These results proved consistent with those in Figure 6. Moreover, the pH value of CS-SS5-LH15 after carbonation was nearly one unit higher than CS-SS30-LH5. Figure 7b below shows the fitted lines of CO2 uptake versus those of pH values. The data used are the mean values. It is obvious that it is much more difficult to reduce the pH of CS-SS5-LH15 by carbonation than CS-SS30-LH5. This is attributed to the extremely slow hydration rate of C2S. The OH− produced by C2S hydration can hardly be effectively replenished after being carbonated, whereas CH can be rapidly replenished with OH−.

3.4. Water-Soluble Silicon

Figure 8a below shows the water-soluble silicon content of CS-SS30-LH5 and CS-SS5-LH15 at different carbonation times. As can be observed, the water-soluble silicon content of both specimen groups increased rapidly during the initial carbonation time. While the water-soluble Si content of CS-SS30-LH5 was augmented nearly eight times during the first 3 h of carbonation, CS-SS5-LH15 increased by a more sedate five times after 24 h of carbonation. As depicted in Figure 8a, the water-soluble silicon content of CS-SS30-LH5 consistently surpassed CS-SS5-LH15 across all carbonation durations. Figure 8b illustrates the fitted lines correlating CO2 uptake and Si content for both CS-SS30-LH5 and CS-SS5-LH15, using mean values. An increase in CO2 uptake corresponded to an increase in the water-soluble silicon content for both types of CEAs. However, the origin of the water-soluble silicon in the CEAs—whether it derives from the carbonated steel slag or the dredged soil fraction—warrants further exploration.
As previously outlined in this paper’s introduction, steel slag, rich in silicate minerals, can serve as a silicon fertilizer for plants—a concept widely studied and employed in agriculture [22]. Furthermore, Chang et al.’s [47] investigation into the carbonation of steel slag revealed a substantial increase in water-soluble silicon content in steel slag by 80% after 72 h of carbonation. The marked increase in water-soluble silicon content in the CEAs of CS-SS30-LH5 and CS-SS5-LH15 post-carbonation, as observed in this study, seems to stem from the carbonation of the steel slag fraction. However, we must not overlook the influence of pH changes prompted by carbonation on the leaching of water-soluble silica from soils.
The primary source of water-soluble silicon in soils typically is silicic acid, produced by the weathering of soil minerals. This silicic acid tends to be adsorbed by clay minerals in the soil, a process that intensifies with rising pH and peaks at a pH of 9 [48]. Given the pH results in Figure 7, the increased water-soluble silicon content in CS-SS30-LH5 with prolonged carbonation time (as pH decreased from 9.22 to 7.95) could be linked to a rise in the water-soluble silicon leaching from the dredged soil fraction in CS-SS30-LH5. Conversely, the pH of CS-SS5-LH15 dropped from 12.02 to 9.22 over the carbonation period. Under alkaline conditions, the leaching of water-soluble silica from the soil is challenging, and amorphous silica transforms into water-soluble silica as pH values rise [49]. If the dredged soil is the source of the water-soluble silicon in CEAs, the water-soluble silicon content of CS-SS5-LH15 should decrease with extended carbonation time, a trend inconsistent with the results found in this study.
Carbonation can transform the crystalline silicate minerals in steel slag into silica gel [47]. As carbonation time extends, more silica gel is produced, leading to an increase in water-soluble silica. Hence, we can posit that the primary source of water-soluble silicon in both CEAs is the carbonated steel slag fraction. The influence of carbonation in augmenting the water-soluble silicon content of CEAs is illustrated in Figure 9.

3.5. Composition Analysis

The composition analysis for two types of pellets before and after 72 h of carbonation was tested by XRD and TG-DTG, and the results are shown in Figure 10a,b. Figure 10a shows the XRD results for both types of CEA pellets before and after 72 h of carbonation. Quartz, calcite (CaCO3, CČ), muscovite, and portlandite (Ca(OH)2, CH) were detected in the mineral composition of both types of pellets before carbonation. It can be found that the CH diffraction peak is highly pronounced in DS-SS5-LH15, while it is nearly absent in DS-SS30-LH5. After 72 h of carbonation, the diffraction peaks of CČ in CS-SS30-LH5 and CS-SS5-LH15 were enhanced, and the diffraction peaks of CH in CS-SS5-LH15 disappeared. It was conjectured that the sources of the respective enhancements of the diffraction peaks of CČ in CS-SS30-LH5 and CS-SS5-LH15 were different in that after carbonation, the increase in CČ in the CS-SS30-LH5 sample arose from the carbonation of silicate minerals dominated by C2S in the steel slag, while the increased CČ in the CS-SS5-LH15 sample was mainly derived from the carbonation of CH. In addition, it was observed that the diffraction peak of CČ in CS-SS5-LH15 was higher than CS-SS30-LH5, which was consistent with the high CO2 uptake values of CS-SS5-LH15 shown in Figure 4 above. The TG-DTG analysis for two types of pellets before and after 72 h of carbonation is shown in Figure 10b. The decomposition of Mg(OH)2 (MH), CH, and CČ caused mass losses at 255 °C–340 °C, 450 °C–550 °C, and 600 °C–700 °C [32,50]. Before carbonation, the DS-SS30-HL5 and DS-SS5-HL15 samples showed a significant weight loss of both CH and CČ. Only the DS-SS30-LH5 sample showed a weight loss of MH, which was derived from steel slag. The weight loss of CH and MH in both DS-SS30-HL5 and DS-SS5-HL15 samples disappeared after the carbonation process, which was due to the conversion of CH and MH to carbonate by carbonation. In addition, a right shift in the decomposition temperature of CČ was observed in both CS-SS30-HL5 and CS-SS5-HL15 samples, indicating that the carbonation process enhanced the development of calcite crystals. The weight loss of CČ in the CS-SS5-LH15 sample was greater than CS-SS30-LH5, which was consistent with the XRD results. Unfortunately, neither the XRD results nor the TG-DTG analysis directly observed the presence of calcium silicate minerals or silica gel in the CS-SS-30-HL5 sample, which was related to its small content.

4. Conclusions and Perspectives

4.1. Summary and Relevance of Key Study Findings

This study presents a comprehensive investigation using the SSFD-C method to prepare CEAs. In this study, two types of CEAs with the same mechanical properties, consisting mainly of steel slag and lime hydrate for carbonation, were used as study subjects. Through the systematic test with the carbonation time as the independent variable, it was found that compared with lime hydrate, steel slag components could bring a lower pH value, moisture absorption rate, and greater silicon dissolution. This finding is crucial for predicting and regulating the properties of CEAs. The main conclusions are as follows:
  • Carbonation could greatly increase the particle crushing strength of both types of CEAs in the first 3 h. The carbonation reaction of both steel slag and LH was the source of CEA strengthening. The LH component proved instrumental in enhancing the efficiency of CO2 uptake by CEAs. The steel slag component was more beneficial for improving the CEA’s strength under equivalent CO2 uptake.
  • Carbonation could significantly reduce the moisture absorption of both types of CEAs. For instance, after the first 3 h of carbonation, the moisture absorption of CS-SS30-LH5 and CS-SS5-LH15 decreased by 15.3% and 16.9%, respectively. The steel slag component was the more favorable material for carbonation to further reduce the moisture absorption of CEAs.
  • Carbonation could effectively reduce the pH value of both types of CEAs, and the steel slag component was more conducive to carbonation, further reducing the pH value of CEAs. The experiment of spraying phenolphthalein indicators on the profiles of the CEA pellets established that 24 h of carbonation sufficed for the carbonation reaction to reach the center of the 10–15 mm CEA pellets.
  • The water-soluble silicon content of both types of CEAs was mainly derived from steel slag. While the water-soluble Si content of CS-SS30-LH5 increased by nearly eight times after 3 h of carbonation, that of CS-SS5-LH15 also increased by a more sedate five times after 24 h of carbonation.
  • The carbonation products for both types of CEAs predominantly consisted of calcium carbonate. However, the main sources of calcium carbonate produced by the carbonation of CS-SS30-LH5 and CS-SS5-LH15 were LH and steel slag silicate minerals, respectively. Compared with LH, the steel slag component of CEAs generated silica gel through carbonation, thus positively influencing the strength, moisture absorption, and water-soluble silicon content of CEAs.

4.2. Limitations and Perspectives

This study has some limitations that should be considered. Firstly, this study only discussed the influence of the carbonation of steel slag and lime hydrate on CEA performance using two mixtures. However, the findings of this study still offer valuable insights into the variations in the impact of carbonation of steel slag and lime hydrate on CO2 uptake, pH, moisture absorption, and silicon leaching from CEAs. Further systematic laboratory experiments are needed to establish mathematical formulas for optimizing CEA preparation, with more compositions of CEAs and their corresponding carbonation strength, pH value, moisture absorption, and silicon dissolution. Additionally, a life cycle assessment (LCA) for CEAs should be conducted. The environmental impact and cost of CEAs with different components should also be included in the optimization index of CEA preparation.

Author Contributions

Methodology, C.Y.; resources, C.C. and J.S.; supervision, C.C. and F.L.; writing—original draft, C.Y.; writing—review and editing, J.Z. and J.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research and Development Program of China (grant No. 2021YFB2601102), the National Natural Science Foundation of China (grant No. 51878109), and the Cultivation Program for the Excellent Doctoral Dissertation of Dalian Maritime University (0143210270).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data will be made available upon request.

Conflicts of Interest

The authors declare no conflict of interest.

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Jmse 11 01054 g001 550
Figure 1. XRD patterns of raw materials. (a) Dredged soil and (b) steel slag.
Figure 1. XRD patterns of raw materials. (a) Dredged soil and (b) steel slag.
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Figure 2. Flowchart of specimen preparation.
Figure 2. Flowchart of specimen preparation.
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Figure 3. Particle crushing strength of CS-SS30-LH5 and CS-SS5-LH15 at different carbonation times.
Figure 3. Particle crushing strength of CS-SS30-LH5 and CS-SS5-LH15 at different carbonation times.
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Figure 4. CO2 uptake (a) showing the fitted lines of CO2 uptake and (b) the particle crushing strength of CS-SS30-LH5 and CS-SS5-LH-15 at different carbonation times.
Figure 4. CO2 uptake (a) showing the fitted lines of CO2 uptake and (b) the particle crushing strength of CS-SS30-LH5 and CS-SS5-LH-15 at different carbonation times.
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Figure 5. Moisture absorption of CS-SS30-LH5 and CS-SS5-LH15 at different carbonation times.
Figure 5. Moisture absorption of CS-SS30-LH5 and CS-SS5-LH15 at different carbonation times.
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Figure 6. Magnified images of both types of CEA pellets sprayed with the phenolphthalein reagent.
Figure 6. Magnified images of both types of CEA pellets sprayed with the phenolphthalein reagent.
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Figure 7. pH values (a) as fitted lines of CO2 uptake and (b) pH values of CS-SS30-LH15 and CS-SS5-LH15 at different carbonation times.
Figure 7. pH values (a) as fitted lines of CO2 uptake and (b) pH values of CS-SS30-LH15 and CS-SS5-LH15 at different carbonation times.
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Figure 8. Si content: Si content of CS-SS30-LH5 and CS-SS5-LH15 at different carbonation times (a) and (b) fitted lines of CO2 uptake.
Figure 8. Si content: Si content of CS-SS30-LH5 and CS-SS5-LH15 at different carbonation times (a) and (b) fitted lines of CO2 uptake.
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Figure 9. Effects of carbonation on increasing the water-soluble silicon content of CEAs.
Figure 9. Effects of carbonation on increasing the water-soluble silicon content of CEAs.
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Figure 10. Composition analysis for two types of pellets before and after 72 h of carbonation ((a): XRD result and (b): TG-DTG analysis).
Figure 10. Composition analysis for two types of pellets before and after 72 h of carbonation ((a): XRD result and (b): TG-DTG analysis).
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Table
Table 1. Chemical composition of raw materials (w/w %).
Table 1. Chemical composition of raw materials (w/w %).
MaterialCaOSiO2Fe2O3Al2O3MgOMnOTiO2K2OClOthers
Dredged soil5.356.210.815.43.10.11.23.91.92.1
Steel slag39.014.523.43.87.37.31.4-0.13.2
Table
Table 2. The material composition of CEAs (w/w %).
Table 2. The material composition of CEAs (w/w %).
CEA TypeMaterial Composition
Dredged SoilSteel SlagLime Hydrate
DS-SS5-LH1582.64.413
DS-SS30-LH566.628.64.8
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Yu, C.; Cui, C.; Zhao, J.; Liu, F.; Su, J.; Yuan, J. Preparation of Artificial Aggregates from Marine Dredged Material: CO2 Uptake and Performance Regulation. J. Mar. Sci. Eng. 2023, 11, 1054. https://doi.org/10.3390/jmse11051054

AMA Style

Yu C, Cui C, Zhao J, Liu F, Su J, Yuan J. Preparation of Artificial Aggregates from Marine Dredged Material: CO2 Uptake and Performance Regulation. Journal of Marine Science and Engineering. 2023; 11(5):1054. https://doi.org/10.3390/jmse11051054

Chicago/Turabian Style

Yu, Chunyang, Chunyi Cui, Jiuye Zhao, Fang Liu, Jian Su, and Jia Yuan. 2023. "Preparation of Artificial Aggregates from Marine Dredged Material: CO2 Uptake and Performance Regulation" Journal of Marine Science and Engineering 11, no. 5: 1054. https://doi.org/10.3390/jmse11051054

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