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Article

Utilization of Marine-Dredged Sediment and Calcium Sulfoaluminate Cement for Preparing Non-Sintered Ceramsites: Properties and Microstructure

by
Jiuye Zhao
,
Zijian Wang
,
Mengying Xiao
,
Chunyi Cui
* and
Hailong Liu
College of Transportation Engineering, Dalian Maritime University, Dalian 116026, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2025, 13(5), 891; https://doi.org/10.3390/jmse13050891
Submission received: 24 March 2025 / Revised: 27 April 2025 / Accepted: 28 April 2025 / Published: 30 April 2025
(This article belongs to the Special Issue Nature-Based Solutions in Coastal Systems)
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Abstract

:
The resource utilization of marine-dredged sediment is considered a sustainable approach to its disposal. This paper investigates the preparation of non-sintered ceramsites from marine-dredged sediments and CSA cement via cold-bonded pelletization. The study examines the effects of various preparation conditions on the engineering properties, phase compositions and microstructures of non-sintered ceramsites. The results indicate that preparation conditions significantly influence the particle size distribution of non-sintered ceramsites. The early-strength development of non-sintered ceramsites prepared from CSA cement is remarkable, with the PCS achieving approximately 60% and 80% of the 28-day strength within 3 days and 7 days, respectively—a marked contrast to OPC. Response surface methodology analysis reveals significant interaction effects between the disc rotation angle, rotational speed, and duration of rotation on the PCS of non-sintered ceramsites. The open-ended porosity of non-sintered ceramsites exhibits greater sensitivity to changes in preparation parameters compared to closed-ended porosity and total porosity. The preparation conditions have negligible impact on the hydration process of CSA cement in non-sintered ceramsites. For both ellipsoidal and plate-like marine-dredged soil particles, ettringite and the AH3 phase provide effective pore-filling and binding effects in the microstructures of non-sintered ceramsites. These findings imply that low-carbon utilization of marine-dredged sediments through the preparation of non-sintered ceramsites offers a nature-based solution for sustainable management in coastal systems.

1. Introduction

Dredging is crucial for harbour maintenance; the pursuit of navigable waterways is estimated to result in an estimated annual dredging volume of billions cubic metres of marine sediment globally [1,2]. However, with the increasing accumulation of marine-dredged sediment, the complexity of its management challenge also escalates. Traditional disposal methods, primarily offshore dumping and landfilling, are becoming less sustainable due to the characteristics of marine-dredged sediment—high clay content, high thixotropy, and low strength—which can lead to marine environmental pollution and inefficient use of land resources [3,4,5,6,7,8]. In response, a multitude of studies have focused on the resourceful utilization of marine-dredged sediment as a sustainable management strategy [9,10,11]. To date, stabilized marine-dredged sediment has been successfully employed in various applications, such as the construction of pavement substrates, the production of pavement bricks, and the preparation of ceramsites [12,13,14,15].
Ceramsites characterized by low density, high porosity, and a large specific surface area have been extensively utilized in the fields of construction materials and water treatment filtration [16,17]. In the construction materials sector, ceramsites serve as a versatile filler material, predominantly utilized as a lightweight aggregate in concrete [18,19,20]. Traditionally, the production of ceramsites primarily relies on sintered clay. Nonetheless, to mitigate the depletion of natural clay resources, some solid wastes with properties close to those of natural clay, such as tailing sands and dredged sediments, have been tested for use in the preparation of sintered ceramsites [21,22,23,24]. Ceramsites derived from the sintering of solid waste exhibit superior mechanical properties, and the sintering technique efficiently eliminates organic content and stabilizes hazardous substances within silt waste [25,26]. However, the production of sintered forms requires temperature conditions in excess of several hundred degrees Celsius to ensure adequate sintering of minerals and the formation of a robust solid mass, which also raises concerns regarding the potential for high energy consumption and significant carbon emissions associated with the sintering process [27,28]. Producing foam concrete blocks using sludge-based non-sintering ceramsites as opposed to sintered ceramsites can significantly cut CO2 emissions from 305.57 kg to approximately 112.76 kg per unit volume [29]. Consequently, a series of studies have endeavoured to investigate low-carbon, non-sintered alternatives for the preparation of ceramsites, with a particular focus on the cold-bonded pelletization [30,31,32].
Ceramsites produced through the cold-bonded pelletization process are formed by mechanical granulation, leveraging the binding properties of the raw materials to ensure that the ceramsites acquire adequate strength and durability following a curing period [33,34]. Existing studies have successfully demonstrated that spherical aggregates, a potential use for ceramsites, with stable performance can be prepared from dredged sediments and cement through the cold-bonded pelletization process, and these aggregates can be utilized to manufacture building products of considerable market value, such as non-fired bricks [35,36]. Throughout the production process, the particle size and mechanical properties of the ceramsites can be finely tuned by modifying manufacturing parameters, including the disc rotation angle, rotational speed, and duration of rotation [37,38]. It is also worth noting that the binding component of the raw materials is crucial for ceramsites prepared by cold-bonded pelletization, especially when dredged marine sediments are utilized as one of the raw materials. The stabilizing effect of the binding component on marine sediments largely determines the mechanical properties and durability of ceramsites. Meanwhile, the energy consumption and carbon emissions associated with the production of the binding component substantially impact the sustainability of the non-sintered ceramics manufacturing process [38,39].
Calcium sulphoaluminate (CSA) cement is gaining increasing recognition as a low-carbon alternative to Ordinary Portland Cement (OPC), offering advantages such as lower raw material carbon content, reduced sintering temperatures for clinker production, and improved clinker grindability [40,41,42]. Moreover, CSA cement is also characterized by rapid hardening, accelerated strength development, and adjustable expansion properties, which present significant advantages in various applications [43,44]. In recent studies, the effectiveness of CSA cement in stabilizing dredged marine sediments has been demonstrated [45,46,47]. Additionally, a multitude of problematic soils, analogous to marine-dredged sediments, have also been effectively stabilized by CSA cement, exhibiting remarkable mechanical performance [48,49,50]. According to these studies, it can be inferred that the preparation of non-sintered ceramsites using CSA cement as a binding component in cold-bonded pelletization with dredged marine sediments tends to be feasible. A series of previous studies have explored the use of various low-carbon binders for the production of non-sintered ceramsites, such as waste materials including steel slag, ground granulated blast-furnace slag, red mud, etc. [51,52]. However, in current research on the preparation of non-sintered ceramsites using dredged sediments as the main raw material, only one study has attempted to use CSA cement as a minor binding material [53]. Systematic research on utilizing CSA cement as the primary binding component, as well as the impact of preparation conditions on the engineering properties and microscopic characteristics, has yet to be conducted.
In this study, aiming to address the environmental challenges posed by marine-dredged sediment disposal in coastal environments and provide a nature-based solution, marine-dredged sediments and CSA cement were employed as raw materials for the preparation of non-sintered ceramsites through a cold-bonded pelletization process. After comparing the appearance and particle size distribution characteristics of the prepared ceramsites, the interaction effects of the preparation conditions on the parallel Plate Crush Strength (abbreviated as PCS) of the non-sintered ceramsites were investigated. Furthermore, the pore characteristics of the non-sintered ceramsites under different preparation conditions were compared. Additionally, the phase composition and microstructure of the non-sintered ceramsites were investigated through microscopic experiments. These findings imply that low-carbon utilization of marine-dredged sediments through the preparation of non-sintered ceramsites offers a nature-based solution for sustainable management in coastal systems.

2. Materials and Methods

2.1. Raw Materials

XRD patterns (Figure 1) of all raw materials, including CSA clinker, gypsum and dredged marine sediment, were obtained using a Bruker D8 Advance X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany), with detailed testing procedures described in the Section 2.3. According to Figure 1, the main mineral in dredged marine sediments is quartz, accompanied by a certain amount of albite, labradorite and muscovite. The main phases in CSA clinker are Ye’elimite (chemical formula 4CaO·3Al2O3·SO3, abbreviated as C4A3$) and Belite (chemical formula 2CaO·SiO2, abbreviated as C2S), in which ye’elimite dominates the hydration reaction. Subsequently, the conventional abbreviated nomenclature for cement was applied, where C denotes CaO, A denotes Al2O3, $ denotes SO3, S denotes SiO2, and H denotes H2O. The gypsum samples exhibited a remarkable purity, with no visible impurities detected. The dredged marine sediments used for the preparation of non-sintered ceramsites were taken from the regional waterway of Dalian, a coastal city, and the basic physical indexes are shown in Table 1. It should be noted that marine-dredged sediments may contain considerable amounts of soluble salts, such as chloride, which could potentially affect the performance of non-sintered ceramsites. However, due to the complexity of the effects of chloride and the current study’s focus on preparation conditions, their specific impacts on strength are not addressed in this paper and will be explored in future research. In this study, the CSA cement utilized was formulated from a commercial CSA clinker blended with gypsum. The gypsum content was set at 28 wt.% of the stabilizer composition. This proportion was specifically determined based on the hydration of main phases in CSA clinker, ensuring the optimal strength of the stabilized marine-dredged sediments [54]. Particle size distributions of the materials were measured using a Mastersizer 2000 laser particle sizer (Malvern Instruments Ltd., Worcestershire, UK) and are presented in Figure 2.

2.2. Experimental Arrangements and Preparation of Ceramsite

In order to investigate the interaction effects of preparation conditions on the PCS of non-sintered ceramsites in subsequent sections, experimental arrangements in this paper were designed following the principles of response surface methodology (RSM). In particular, the disc rotation angle, rotational speed and duration of rotation were set as independent factors, respectively represented by x1, x2 and x3 in the following equations for RSM. Values for different levels of each independent factor can be seen in Table A1 in Appendix A. Experimental arrangements based on RSM in this paper are listed in Table 2.
The preparation process for non-sintered ceramsites is illustrated in Figure 3. During preliminary experiments, various water amounts and curing conditions were compared to identify the optimal conditions that promote the strength development of non-sintered ceramsites. In this study, to prepare a batch of non-sintered ceramsites, 480 g of dredged marine sediments underwent pretreatment procedures (labelled as ①), including drying at 80 °C to remove free moisture, homogenization using a mixing machine, and sieving with a 2 mm soil screen to eliminate large particles. These pretreated sediments were then thoroughly mixed with 120 g of CSA cement (labelled as ②). Subsequently, this well-mixed composite was carefully placed into a standard customized pelletizing turntable, with an inner diameter of 50 cm and a depth of 20 cm. After adjusting the disc to a preset angle, the process was carried out at a preset rotational speed, and 102 g of water was uniformly sprayed during the rotation until a predetermined pelletizing durability was achieved (labelled as ③). After moulding, the ceramsites were transferred to a curing box maintained at 20 ± 1 °C with 90% relative humidity and cured under these conditions until specified ages (1d, 3d, 7d, and 28d), labelled as ④.

2.3. Testing Methods

The sieving method was employed to assess the particle size distribution of non-sintered ceramsites. For this purpose, a series of standard sieve meshes were selected, with mesh sizes of 4.75 mm, 7.00 mm, 10.00 mm, 13.00 mm, and 15.00 mm, respectively. The test results were subsequently utilized to calculate the mass percentage of non-sintered ceramsites within each particle size range. The parallel plate crush test (PCS) was employed to evaluate the mechanical strength of the prepared non-sintered ceramsites with a WDW-50 universal pressure testing machine manufactured by Xinte Testing Machine Co., Ltd. in Changchun City, China. In accordance with the methodology outlined by [55,56], the loading rate was set at 1 mm/min. And the PCS was calculated using Equation (1).
σ = 2.8 F π D 2
In Equation (1), σ represents the PCS of the non-sintered ceramsite particles, F denotes the ultimate load endured by the non-sintered ceramsites, and D represents the diameter of the non-sintered ceramsites and also corresponds to the plate distance at the moment of effective loading in the parallel plate test. It is important to emphasize that non-sintered ceramsites may exhibit ellipsoidal or irregular morphologies, which can introduce measurement errors when determining the particle diameter and, consequently, affect the accuracy of PCS calculations. To mitigate this limitation, this study prioritized the use of spherical aggregates with nearly spherical morphologies for testing, thereby minimizing unavoidable measurement errors associated with non-spherical particles. Meanwhile, to ensure a representative sample, ceramsites within different specific ranges of particle sizes were selected for strength testing. The final strength was characterized by the mean value calculated from eight groups of parallel specimens (data within 30% of the mean are valid). After obtaining the experimental test values for the parallel PCS of non-sintered ceramsites, a second-order response function model was employed to fit the strength values of the non-sintered ceramsites at 28 days. This model was used to establish the relationship between the predicted response values of the PCS and the independent variables associated with each preparation parameter. The second-order response function model used for fitting, as well as the results of its fitting, can be found in Equation (A1) and Equation (A2) in Appendix A, respectively. The fitted correlation coefficients are presented in Table A2. The accuracy of the fit is discussed in Section 3.2.
A JHY-600 electronic solid density meter, manufactured by Jinheyuan in Xiamen City, China, was employed to measure the apparent density ρa and the drained volume Vc (the volume of the grains excluding open pores) of the non-sintered ceramsites. Additionally, a true density tester was utilized to assess the finely ground non-sintered ceramsites powders, thereby obtaining their true density ρt. Subsequently, the open-ended porosity (Po) and closed-ended porosity (Pc) of the non-sintered ceramsites were calculated using Equations (2) and (3), respectively. In these equations, m1 represents the weight of the non-sintered ceramsites after drying, αT is the temperature correction coefficient, and ρw denotes the density of water. It should be noted that in this study, the true densities (ρt) under different preparation conditions, as determined by the test results, are uniformly taken as 2.504 g/cm3. This consistency is attributed to the fact that the true density is primarily determined by the phase composition of the non-sintered ceramsites. The compositions of the non-sintered ceramsites prepared under various conditions in this study are highly similar, as detailed in Section 3.4. This similarity results in a standard deviation of 0.008 g/cm3 in true density measurements among groups, a variation that can primarily be attributed to system errors and the potential randomness of the samples.
P o = m 1 α T ρ w V c × 100 %
P c = ( 1 − ρ a ρ t ) × 100 %
Non-sintered ceramsites, maintained to a predetermined age, were subjected to terminal hydration using ethanols following crushing, in preparation for microscopic testing. The finely ground powdered samples were used for X-ray diffraction (XRD) tests and thermogravimetric analysis (TGA) tests. XRD tests were conducted using a Bruker D8 Advance X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany), operated at 1600 W (40 kV, 40 mA). The test range was set from 5° to 70° (2θ) with a step size of 0.02°. The resulting XRD patterns were analyzed for phase identification by referencing the Powder Diffraction File (PDF) and the Crystallography Open Database (COD) crystallographic databases. The TGA tests were performed using a Hitachi TDTA 7300 Integrated Thermal Analyzer, manufactured by Hitachi, Ltd., whose headquarters is located in Tokyo city, Japan. The temperature range was from room temperature to 1000 °C, with a heating rate of 10 °C/min. Nitrogen was employed as a protective gas to prevent oxidation during the analysis. A ZEISS Gemini scanning electron microscope (SEM), manufactured by Carl Zeiss AG with its headquarters in Oberkochen, Germany, was employed to examine the cross-section of non-sintered ceramsites after crushing. To enhance electrical conductivity, the sample surfaces were coated with gold. The accelerating voltage during observation was set to 5 kV.

3. Results and Discussions

3.1. Appearance and Particle Size Distribution

According to Figure 4, it can be observed that, under all preparation conditions, the ceramsites prepared are composed of particles with significant differences in size. Specifically, the smaller particles have diameters of less than or approaching 5 mm, while the larger particles can exceed 15 mm. Furthermore, it is noteworthy that the preparation conditions significantly influence the appearance and shape of the ceramsites. Under most of the preparation conditions used in this study, the non-sintered ceramsites, stabilized with CSA cement and prepared through cold-bonded pelletization, exhibited either an ellipsoidal or a spherical shape. However, under specific preparation conditions, non-sintered ceramsites, which are prone to forming irregular shapes resembling natural aggregates (e.g., A45-S42-T10 and A45-S42-T20), were observed. It should be noted that for these non-sintered ceramsites, which are prone to forming irregular shapes, the selection of ceramsites with an approximate spherical shape for testing in the subsequent parallel plate crush test was prioritized. This approach was taken to ensure the comparability of the data across different groups.
Particle distributions of prepared non-sintered ceramsites are shown in Figure 5. It is evident that the preparation conditions have a significant impact on the particle size distribution of non-sintered ceramsites. Notably, larger turntable angles tend to produce larger ceramsite particles. For all groups prepared under a disc angle of 30°, ceramsites smaller than 4.75 mm constituted a substantial proportion. In contrast, when the disc angle was set to 45°, the proportion of ceramsites smaller than 4.75 mm exceeded 10% only in groups A45-S22-T10 and A45-S42-T20. Furthermore, under the preparation condition with a 60° disc angle, the percentage of ceramsites smaller than 4.75 mm reached a maximum of approximately 9%. Unlike the disc rotation angle, the rotational speed and duration of rotation did not exhibit a deterministic pattern of influence on the particle size of non-sintered ceramsites. This lack of a clear pattern may be attributed to the complex interplay of factors: faster rotational speeds and longer durations of rotation could potentially promote the agglomeration of marine-dredged sediment and CSA cement particles, leading to the formation of larger ceramsites. Conversely, these conditions might also cause the fragmentation of existing larger ceramsites, resulting in the formation of smaller-sized ceramsites, a phenomenon that has actually been observed during the preparation process. It should be noted that, due to the limited practical application of ceramsites smaller than 4.75 mm as lightweight aggregates in actual projects, as recommended by the specification, subsequent strength tests were focused on ceramsites with particle sizes exceeding 4.75 mm.

3.2. Mechanical Properties

The PCS of non-sintered ceramsites prepared under various conditions at different curing ages is compared in Figure 6. It is evident that the PCS of non-sintered ceramsites prepared under any of the tested conditions can exceed 1.0 MPa at 3d and 1.4 MPa at 28d, respectively. This demonstrates the feasibility of utilizing marine-dredged sediments combined with CSA cement to prepare non-sintered ceramsites with superior mechanical properties. Concurrently, it is observed that the preparation conditions significantly influence the mechanical properties of non-sintered ceramsites. Intuitively, among the conditions tested, a larger disc rotation angle, a faster rotational speed, or a longer duration of rotation could potentially enhance the strength of non-sintered ceramsites. However, the combined effect of angle, rotational speed, and rotation duration appears to have a more pronounced impact on the strength of non-sintered ceramsites. This synergistic effect will be further analyzed in the subsequent sections using RSM. On the other hand, regarding the development of strength, for non-sintered ceramsites prepared under various conditions, the 3-day and 7-day strengths can reach approximately 60% and 80% of the 28-day strength, respectively. This characteristic of rapid strength development is not significantly influenced by the preparation conditions but is a direct reflection of the rapid hydration and rapid strength gain of CSA cement.
RSM is well suited for multivariate systematic analysis and has been effectively utilized to optimize the performance of various building material products [57,58,59]. In this study, RSM has been employed to investigate the interaction effects of disc rotation angle, rotational speed and rotation duration on the PCS of non-sintered ceramsites at 28 days. In the Appendix A, the second-order response regression model used for the RSM analysis is presented in Equation (A2). The reliability of this model fit can be assessed using various correlation coefficients, which are detailed in Table A2. Meanwhile, the predictive accuracy of the second-order response regression model can be verified in Figure 7, in which scatter points determined by the predicted and experimental PCS values fluctuate around the line of equality within a limited range, and no significant deviations are observed.
As shown in Figure 8, for a given rotation duration of 15 min, individually increasing the rotational speed or the disc rotation angle does not lead to a significant and sustained enhancement in the strength of non-sintered ceramsites. This is due to the non-linear interactive effect of rotational speed and disc rotation angle on PCS. In particular, for rotational speeds that do not exceed approximately 40 r/min, an excessively large disc rotation angle may instead lead to a slight reduction in the PCS of the non-sintered ceramsites. When the rotational speed and disc rotation angle are increased in tandem, the PCS of the non-sintered ceramsites can experience a rapid and significant increase. Maintaining a rotational speed of at least 39 r/min and a disc rotation angle of at least 50° can enable the 28-day compressive strength (PCS) of non-sintered ceramsites to potentially reach or exceed 2.0 MPa. This value is closely approaching the optimal strength of non-sintered ceramsites achieved through the wrap-shell optimization process [35]. While a synergistic increase in rotational speed and disc rotation angle can enhance the PCS of non-sintered ceramsites, excessively high values may cause raw material splashing during pelletization due to excessive speed or motion within the disc, thereby reducing raw material utilization and hindering operational consistency during actual manufacture. During the exploratory experiments conducted for this paper, raw material splashing was observed at rotational speeds exceeding 42 r/min or disc rotation angles greater than 60°. As a result, the maximum rotational speed and disc rotation angle were set at 42 r/min and 60°, respectively.
Regarding the duration of rotation coupled with the disc rotation angle or rotational speed, the non-linear interactive effect can also be observed in Figure 9 and Figure 10. Combined with the results in Figure 8, any combination of two factors among duration of rotation, disc rotation angle, and rotational speed exhibits a similar non-linear interaction effect on the PCS of non-sintered ceramsites. However, the direct outcomes resulting from different combinations still vary significantly, which can be clearly observed from the markedly different contour line diagrams for each combination. In Figure 9, for a rotational speed set at 32 r/min, the duration of rotation should be prolonged beyond approximately 16 min to achieve a PCS of more than approximately 2.0 MPa when the disc rotation angle exceeds 50°. Figure 10 indicates that to achieve a PCS of around 2.0 MPa or greater at 28 days, with the disc rotation angle set at 45°, the rotational speed should be above approximately 40 r/min, and the duration of rotation should be extended beyond 16 min.

3.3. Porosity

The porosity test results for non-sintered ceramsites prepared under various conditions, along with their statistical data, are presented in Table 3. Overall, larger disc rotation angles, faster rotational speeds, and longer durations of rotation generally result in lower total porosity of non-sintered ceramsites in the tested groups, which is consistent with the observed effects of preparation conditions on PCS. Meanwhile, comparisons between individual groups reveal that the effects of different preparation parameters on the open and closed porosity of non-sintered ceramsites appear to differ. Specifically, a larger disc rotation angle (e.g., comparing A45-S22-T10 with A45-S42-T10) and a faster rotational speed (e.g., comparing A30-S22-T15 with A60-S22-T15) both tend to simultaneously reduce the number of open and closed pores in the non-sintered ceramsites. However, when the duration of rotation is extended (e.g., comparing A45-S22-T10 with A45-S22-T20), the open-ended porosity of the non-sintered ceramsites tends to decrease, while the closed-ended porosity typically increases. In terms of pore statistics, the mean value of closed porosity is significantly larger than that of open porosity. However, the range of open porosity is larger than that of closed porosity. It should be noted that the coefficient of variation (CV), a standard statistical metric used to characterize data dispersion, exhibits significantly higher values for open-ended porosity compared to closed-ended and total porosity. This indicates that open-ended porosity demonstrates the highest sensitivity to preparation conditions among the three porosity types.
The PCS of non-sintered ceramsites at 28 days is correlated with total porosity, as shown in Figure 11. It is evident that within the tested range, there is a tendency towards a non-linear correlation between the PCS and total porosity of non-sintered ceramsites. A typical exponential function has been employed for fitting to quantitatively describe this non-linear relationship. The fitting results yields a high correlation coefficient (R2 = 0.957), with the majority of the experimental data points falling within the 95% confidence interval and all experimental data points within the 95% prediction interval.

3.4. Phase Composition and Microstructures

To further investigate the phase composition and microstructure of non-sintered ceramsites, a series of microscopic tests, including XRD, TGA, and SEM, have been conducted. Figure 12 displays a selected range of XRD patterns for samples A45-S32-T15 and A60-S42-T15 to highlight the variations in intensities of diffraction peaks, highlighted with a light blue background in the figure, corresponding to the reactants and hydration products within the non-sintered ceramsites. According to previous studies, in the presence of gypsum, the strength of CSA cement-hardened pastes is attributed to the hydration reaction between ye’elimite (C4A3S) and gypsum (C$H2), as shown in reaction Equation (4) [60]. Based on the XRD patterns obtained at different hydration ages, it is evident that for both samples A45-S32-T15 and A60-S42-T15, a significant amount of ettringite formed after 1d hydration, as indicated by the high-intensity diffraction peak at around 9.1°. This aligns with the characteristic rapid hydration of CSA cements and correlates with the relatively high PCS observed after 1d preparation of the non-sintered ceramsites. Although the XRD patterns at 1d still show detectable amounts of ye’elimite (characterized by a peak at around 23.6°) and gypsum (characterized by a peak at around 11.7°), these remaining reactants are progressively consumed as the curing age increases, leading to the formation of additional hydration products. By 28d, ye’elimite and gypsum are almost or completely consumed, as evidenced by the XRD patterns. The diffraction peaks of ettringite in both A45-S32-T15 and A60-S42-T15 exhibited an approximate 40% increase in intensity from 1 day to 28 days of hydration age, suggesting that ettringite continues to precipitate from the pore solution of non-sintered ceramsites during this period. It is important to emphasize that the near-complete consumption of gypsum in the non-sintered ceramsites after 28 days of curing is attributable to the optimized gypsum content. This optimization was informed by prior research on CSA cement-stabilized soils [54]. The primary goal of this optimization was to attain the optimal strength of CSA cement-stabilized soils. By optimizing the gypsum content, the risk of either an excess of unreacted gypsum or the isolated hydration of ye’elimite with water, as depicted in reaction Equation (5), is minimized. Either scenario could potentially lead to a reduction in the strength of the CSA cement-stabilized soils. Given that the XRD patterns of samples A45-S32-T15 and A60-S42-T15 exhibit similar variations, it can also be inferred that the preparation conditions have minimal impact on the hydration process of CSA cements in non-sintered ceramsites. This observation is consistent with the phenomenon of convergence in the true densities of non-sintered ceramsites prepared under different conditions.
C4A3$ + 2C$H2 + 34H → C6A$3H32 + 2AH3
C4A3$ + 18H → C4A$H12 + 2AH3
TGA is also commonly employed to determine the phase composition of hydration products in cement-based materials. It is particularly well suited for analyzing gel products that are challenging to detect using XRD, such as the AH3 phase gel, which is a primary hydration product in CSA cement, as described in reaction Equations (4) and (5) [61]. Figure 13 presents the TGA results for samples of A45-S32-T15 at 3 days and 28 days of curing. The primary hydration products of CSA cement, ettringite and AH3 phase gel, are identified through their characteristic decomposition temperatures. Ettringite decomposes in a temperature range of approximately 40–140 °C, while the AH3 phase gel decomposes in a range of approximately 140–350 °C during the heating process [62]. At 3d of hydration, the TGA curves of the A45-S32-T15 sample show a significant amount of ettringite and AH3 phase decomposition, with a notable decomposition peak around 110 °C indicating a small amount of residual gypsum. By 28d of hydration, a larger weight loss due to ettringite decomposition is observed, and the decomposition peak of residual gypsum is almost undetectable, indicating the continuation of the hydration process. These TGA results are consistent with the XRD findings, which also demonstrate that CSA cements exhibit rapid hydration in non-sintered ceramsites.
With regard to the microstructure of non-sintered ceramsites, SEM images for samples of A45-S32-T15 and A60-S32-T20 are shown in Figure 14. As shown in Figure 14a, hydration products of CSA cements are observed to adhere to the surface of ellipsoidal sediment particles and fill the inter-particle pores. At a higher magnification (see Figure 14b), clusters of needle-and-rod-shaped ettringite and gelatinous AH3 phases can be identified. The irregular gel-state features of the AH3 phase are more evident in Figure 14c. In the microstructure of CSA cement-stabilized soils, the strength of the hardened body is enhanced by the combined presence of ettringite and AH3 phases. Intergrown ettringite provides skeletal support between soil particles, while the gel-like AH3 phase acts as both a filler for micropores between ettringite crystals and a binder that consolidates loose particles into a stable structure through its cohesive properties [54,63]. It is worth noting that, unlike the fine aggregates in concrete, marine-dredged sediments contain a certain amount of plate-like mineral particles (e.g., feldspars) in addition to quartz and other minerals that tend to be ellipsoidal. Figure 14d shows a typical area where CSA hydration products are interspersed with these plate-like particles. As can be seen in Figure 14e,f, although there appears to be a tendency for larger pores to form at the interfaces where the CSA cement hydration products interact with the plate minerals, these hydration products can still be used as attachments and pore-fillers for the plate minerals in marine-dredged sediments. Taken together, in the microstructures of both A45-S32-T15 and A60-S32-T20, the ettringite and AH3 phases within the CSA cement hydration products collectively contribute to strength formation.

4. Conclusions

This study explores the preparation of non-sintered ceramsites, which have potential applications in lightweight concrete, using marine-dredged sediments and CSA cement via cold-bonded pelletization. The following are the key conclusions drawn based on this study:
1. Non-sintered ceramsites exhibit ellipsoidal or spherical shapes under most preparation conditions, but irregular morphologies resembling natural aggregates could form under specific conditions. At a disc rotation angle of 60°, non-sintered ceramsites with particle sizes exceeding 4.75 mm are more likely to be prepared.
2. Non-sintered ceramsites prepared from CSA cement and marine-dredged sediments achieve ~60% and ~80% 28-day strength at 3d and 7d, respectively. RSM analysis reveals significant interaction effects among preparation conditions on the 28-day PCS. Optimization of preparation conditions results in a PCS exceeding 2.0 MPa at 28 days.
3. Preparation parameters effectively regulate the porosity of non-sintered ceramsites, with open-ended porosity (CV = 0.21) showing greater sensitivity to parameter changes compared to closed-ended (CV = 0.10) and total porosity (CV = 0.08). A non-linear (exponential) relationship is observed between total porosity and 28-day PCS.
4. In non-sintered ceramsites, significant ettringite formation was detected after 1d, and ye’elimite and gypsum were nearly fully consumed by 28d. In the microstructures, for both ellipsoidal and plate-like marine-dredged soil particles, the skeletal supportive role of ettringite and the pore-filling/binding role of the AH3 phase jointly contribute to the strength formation.

Author Contributions

Conceptualization, J.Z. and C.C.; methodology, J.Z. and Z.W.; validation, M.X.; investigation, J.Z., Z.W. and M.X.; data curation, Z.W. and M.X.; writing—original draft preparation, J.Z. and M.X.; writing—review and editing, C.C. and H.L.; funding acquisition, J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by National Natural Science Foundation of China, Grant No. 52208331 and China Postdoctoral Science Foundation, Grant No. 2022M720634.

Data Availability Statement

Data will be made available upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
CSACalcium sulfoaluminate.
OPCOrdinary Portland Cement.
PCSPlate Crush Strength.
RSMResponse surface methodology.

Appendix A

Table A1. Experimental factors and levels.
Table A1. Experimental factors and levels.
FactorsFactor Levels
−101
x1304560
x2223242
x3101520
y = b 0 + ∑ i = 1 k b i x i + ∑ i = 1 k b i i x i 2 + ∑ i = 1 k ∑ j > 1 k b i j x i x j + ε
In Equation (A1), y represents the predicted response value; b0 is the model constant; xi and xj are the independent variables; k is the number of independent variables optimized in the study, with k = 3; bi representing the coefficient of the linear term; bii represents the coefficient of the quadratic term; bij represents the coefficient of the interaction term; xixj represents the interaction term between independent variables; xi2 represents the quadratic term of the independent variable; ε is the random error in predicting the response.
y = − 0.487983 + 0.025133 x 1 + 0.038942 x 2 + 0.06735 x 3 + 0.0006 x 1 x 2 + 0.000533 x 1 x 3   − 0.00005 x 2 x 3 − 0.00048 x 1 2 − 0.000704 x 2 2 − 0.002417 x 3 2
Table A2. Correlation coefficients.
Table A2. Correlation coefficients.
Types of Correlation CoefficientValue
Multiple Correlation Coefficient (R2)0.9862
Adjusted Multiple Correlation Coefficient (RAdj2)0.9614
Predicted Multiple Correlation Coefficient (RPred2)0.8825

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Figure 1. XRD patterns for raw materials, (a) marine-dredged sediment, (b) CSA clinker, (c) gypsum.
Figure 1. XRD patterns for raw materials, (a) marine-dredged sediment, (b) CSA clinker, (c) gypsum.
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Figure 2. Particle size distribution of raw materials.
Figure 2. Particle size distribution of raw materials.
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Figure 3. Process for preparation of non-sintered ceramsite.
Figure 3. Process for preparation of non-sintered ceramsite.
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Figure 4. Appearance of non-sintered ceramsite.
Figure 4. Appearance of non-sintered ceramsite.
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Figure 5. Particle distribution of prepared non-sintered ceramsites.
Figure 5. Particle distribution of prepared non-sintered ceramsites.
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Figure 6. PCS of prepared non-sintered ceramsites.
Figure 6. PCS of prepared non-sintered ceramsites.
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Figure 7. Predicted PCS vs. experimental PCS.
Figure 7. Predicted PCS vs. experimental PCS.
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Figure 8. Response surface diagram (rotational speed vs. disc rotation angle, PCS at 28d, duration of rotation set as 15 min); (a) 3D illustration; (b) contour line diagram.
Figure 8. Response surface diagram (rotational speed vs. disc rotation angle, PCS at 28d, duration of rotation set as 15 min); (a) 3D illustration; (b) contour line diagram.
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Figure 9. Response surface diagram (duration of rotation vs. disc rotation angle, PCS at 28d, rotational speed set as 32 r/min); (a) 3D illustration; (b) contour line diagram.
Figure 9. Response surface diagram (duration of rotation vs. disc rotation angle, PCS at 28d, rotational speed set as 32 r/min); (a) 3D illustration; (b) contour line diagram.
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Figure 10. Response surface diagram (duration of rotation vs. rotational speed, PCS at 28d, disc rotation angle set as 45°); (a) 3D illustration; (b) contour line diagram.
Figure 10. Response surface diagram (duration of rotation vs. rotational speed, PCS at 28d, disc rotation angle set as 45°); (a) 3D illustration; (b) contour line diagram.
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Figure 11. Relationship between PCS and total porosity at 28d.
Figure 11. Relationship between PCS and total porosity at 28d.
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Figure 12. Selected range of XRD patterns for non-sintered ceramsites, (a) A45-S32-T15, (b) A60-S42-T15.
Figure 12. Selected range of XRD patterns for non-sintered ceramsites, (a) A45-S32-T15, (b) A60-S42-T15.
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Figure 13. TGA curves for non-sintered ceramsites.
Figure 13. TGA curves for non-sintered ceramsites.
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Figure 14. SEM images for non-sintered ceramsites cured after 28d, (a) ×2000 magnification for A45-S32-T15, (b) ×5000 magnification for A45-S32-T15 (zoom from the dotted line in (a)), (c) ×10,000 magnification for A45-S32-T15 (zoom from the dotted line in (b)), (d) ×2000 magnification for A60-S32-T20, (e) ×5000 magnification for A60-S32-T20 (zoom from the dotted line in (d)), and (f) ×10,000 magnification for A60-S32-T20 (zoom from the dotted line in (e)).
Figure 14. SEM images for non-sintered ceramsites cured after 28d, (a) ×2000 magnification for A45-S32-T15, (b) ×5000 magnification for A45-S32-T15 (zoom from the dotted line in (a)), (c) ×10,000 magnification for A45-S32-T15 (zoom from the dotted line in (b)), (d) ×2000 magnification for A60-S32-T20, (e) ×5000 magnification for A60-S32-T20 (zoom from the dotted line in (d)), and (f) ×10,000 magnification for A60-S32-T20 (zoom from the dotted line in (e)).
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Table 1. Basic physical index of dredged sediment.
Table 1. Basic physical index of dredged sediment.
Specific GravityLiquid LimitPlastic LimitPlasticity IndexDry Density
2.7239.621.118.51.52 g/cm3
Table 2. Experimental arrangements based on RSM.
Table 2. Experimental arrangements based on RSM.
NomenclatureFactor
x1/°x2/(r/min)x3/min
A30-S22-T15302215
A30-S32-T10303210
A30-S32-T20303220
A30-S42-T15304215
A45-S22-T10452210
A45-S22-T20452220
A45-S32-T15(1) a453215
A45-S32-T15(2)453215
A45-S32-T15(3)453215
A45-S42-T10454210
A45-S42-T20454220
A60-S22-T15602215
A60-S32-T10603210
A60-S32-T20603220
A60-S42-T15604215
a A45-S32-T15(1), A45-S32-T15(2), and A45-S32-T15(3) are replicate experimental groups corresponding to the centre point of the experimental design, established based on the RSM-BBD (Box–Behnken Design) principle to enhance the predictive accuracy of the model.
Table 3. Porosity of non-sintered ceramsites.
Table 3. Porosity of non-sintered ceramsites.
Samples/StatisticClosed Porosity/%Open Porosity/%Total Porosity/%
A30-S22-T1515.6413.1128.76
A30-S32-T1018.598.7527.34
A30-S32-T2015.3010.5025.80
A30-S42-T1517.957.6225.56
A45-S22-T1015.3914.4429.83
A45-S22-T2018.267.4725.73
A45-S32-T1515.498.3023.80
A45-S42-T1013.1610.5323.69
A45-S42-T2015.248.0123.24
A60-S22-T1515.2810.6925.97
A60-S32-T1015.079.2124.28
A60-S32-T2015.358.2323.57
A60-S42-T1513.389.8223.21
Mean value15.709.7425.44
Standard deviation1.592.032.05
Range5.436.706.62
Coefficient of variation0.100.210.08
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MDPI and ACS Style

Zhao, J.; Wang, Z.; Xiao, M.; Cui, C.; Liu, H. Utilization of Marine-Dredged Sediment and Calcium Sulfoaluminate Cement for Preparing Non-Sintered Ceramsites: Properties and Microstructure. J. Mar. Sci. Eng. 2025, 13, 891. https://doi.org/10.3390/jmse13050891

AMA Style

Zhao J, Wang Z, Xiao M, Cui C, Liu H. Utilization of Marine-Dredged Sediment and Calcium Sulfoaluminate Cement for Preparing Non-Sintered Ceramsites: Properties and Microstructure. Journal of Marine Science and Engineering. 2025; 13(5):891. https://doi.org/10.3390/jmse13050891

Chicago/Turabian Style

Zhao, Jiuye, Zijian Wang, Mengying Xiao, Chunyi Cui, and Hailong Liu. 2025. "Utilization of Marine-Dredged Sediment and Calcium Sulfoaluminate Cement for Preparing Non-Sintered Ceramsites: Properties and Microstructure" Journal of Marine Science and Engineering 13, no. 5: 891. https://doi.org/10.3390/jmse13050891

APA Style

Zhao, J., Wang, Z., Xiao, M., Cui, C., & Liu, H. (2025). Utilization of Marine-Dredged Sediment and Calcium Sulfoaluminate Cement for Preparing Non-Sintered Ceramsites: Properties and Microstructure. Journal of Marine Science and Engineering, 13(5), 891. https://doi.org/10.3390/jmse13050891

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