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Article

Study on Synthesis of CSH Gel and Its Immobilization of Heavy Metals

by
Kunqian Zhu
,
Lijuan Wang
*,
Libing Liao
,
Yunlong Bai
and
Jing Hu
Engineering Research Center of Ministry of Education for Geological Carbon Storage and Low Carbon Utilization of Resources, Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, China
*
Author to whom correspondence should be addressed.
Crystals 2024, 14(10), 864; https://doi.org/10.3390/cryst14100864
Submission received: 31 August 2024 / Revised: 22 September 2024 / Accepted: 24 September 2024 / Published: 30 September 2024
(This article belongs to the Topic Advances in Inorganic Synthesis)
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Abstract

:
Calcium silicate hydrate (CSH) gel is the most important hydration product of cement. It influences the mechanical properties of resulting materials and plays an important role in the adsorption and immobilization of heavy metal ions. Research in the structure of CSH gel and its ability for heavy metal immobilization enables the development of tailored cement-based materials, a feature that holds significant future potential. In this study, CSH gel was synthesized under different pH and Ca/Si conditions. Structural and morphological changes in CSH gel were investigated using modern technologies. The results revealed that both pH and Ca/Si ratios were important factors influencing the structure of CSH gel. During the formation of CSH, both Cr3+ and Pb2+ can be incorporated into CSH gel, and promoting the formation of calcium hydroxide Cr3+ can also replace Si4+ in the Si-O bond.

1. Introduction

As hydration reactions proceed, ordinary Portland cement produces hydration products such as hydrated calcium silicate (CSH) and ettringite (AFt, 3CaO·Al2O3·3CaSO4·32H2O). CSH, which makes up approximately 60–70% of the total hydration products, is the key determinant of the performance of cementitious materials [1]. The structure of CSH gel is influenced by the duration and space available for the hydration reaction. Typically, CSH gel in cementitious materials consists of mixtures with varying Ca/Si ratios. Even with similar structures, CSH gel can exhibit significant differences in their Ca/Si ratios and chemically bound water content, ultimately influencing the mechanical properties of the materials. It allows for optimizing their mechanical properties or even heavy metal immobilization.
Currently, common synthesis methods for CSH gel include mechanical, sol-gel, precipitation, and hydrothermal techniques [2,3,4,5,6,7,8]. The formation process of CSH gel via the precipitation method involves non-classical nucleation. It is regulated by the reaction pH, which plays a crucial role in determining the actual Ca/Si ratio and the morphology of the product [9,10,11,12,13,14,15]. Research indicates that the actual Ca/Si in the synthesized CSH gel approaches the theoretical value only when the pH exceeds 12. At higher pH, CSH gel with a high Ca/Si can be formed [6,16,17]. However, CSH gel with a high Ca/Si ratio often includes impurities like Ca(OH)2 and CaCO3, leading some researchers to suggest that it is a two-phase mixture, with no such thing as a pure CSH gel with a high Ca/Si [18,19,20,21,22]. As such, the synthesis and structure of CSH gel still require further investigation.
The CSH gel employs distinct mechanisms for immobilizing various heavy metal ions, broadly categorized into adsorption, ion or ion cluster substitution, formation of insoluble substances, and encapsulation. For instance, arsenic (As) is typically distributed within the CSH gel through both precipitation and adsorption, where it replaces silicon atoms, becoming effectively immobilized. It was noted that the formation of Al-As compounds does not enhance the immobilization of As [23]. Teramoto [24] investigated the role of Cr(VI) in the formation of Portland cement, revealing that the presence of CrO42− facilitates the formation of the silicon tetrahedral structure in CSH, thereby enhancing the mechanical strength of the cement. CrO42− primarily exists as soluble Ca2CrO5∙3H2O and can also be chemically bound in small amounts within the CSH gel. Serclerat [25] noted that Cr(VI) can also integrate into the structure of ettringite, tightly bound within its columnar framework, besides entering the CSH through adsorption and precipitation. Cr(VI) in cementitious materials primarily undergoes adsorption and salt formation, which makes effective immobilization challenging [26,27]. Cadmium (Cd) can be immobilized in the layers of CSH as Cd(OH)2. Some Cd(OH)2 precipitates may reside in the pores of cementitious materials [28]. Additionally, Cd2+ may form Cd(OH)2 during the early hydration process of CSH [29]. Under certain conditions, CdCO3 is also produced, potentially hindering the hydration process. Lead (Pb) is uniformly distributed within CSH through adsorption and precipitation mechanisms. Experimental evidence showed that during the cement hydration process, a complex mixture resulting from the precipitation of Pb-containing hydrate phases may accumulate and form an impermeable coating [30]. Specifically, free hydroxide ions in the initial stage of cement hydration rapidly interact with soluble Pb, forming lead hydroxide precipitate, which ultimately dissolves into soluble Pb(OH)3− [31,32].
In this study, CSH gel was successfully synthesized with the precipitation reaction method, and its structural changes were then examined under varying pH and Ca/Si conditions. The study also evaluated the impact of Pb2+ and Cr3+ on the structure of CSH gel. The results indicate that the structure of the synthesized CSH gel is influenced by the Ca/Si ratio and the pH. When pH = 12, the crystal basal spacing of CSH gel decreased with the increase in Ca/Si, accompanied by the extension of the silicon chain and the removal of Ca2+. The immobilization mechanism of Pb2+ and Cr3+ in the CSH gel reaction system involves ion substitution. Both Cr3+ and Pb2+ can replace Ca2+ in CSH gel, and Cr3+ can additionally replace Si4+ in CSH.

2. Materials and Methods

2.1. Synthesis Methods

The experiment utilized reagents including Na2SiO3·9H2O, Ca(NO3)2·4H2O, NaOH, Pb(NO3)2, and Cr(NO3)3·9H2O. All of the reagents have analytical purity. Deionized water was boiled and then rapidly cooled to room temperature using an ice water bath for solution preparation. Na2SiO3 solution, Ca(NO3)2 solution, and 1.2 mol/L NaOH solution with different pH values were prepared. The mixing ratios are detailed in Table 1. The reaction apparatus is depicted in Figure 1. A high-purity N2 gas with a flow rate of 0.2 L/min was introduced into the system for 1 h to purge the air from the apparatus. The prepared solution was placed in a conical flask with a stopper and the flow of the solution was controlled solely through a peristaltic pump. The NaOH solution was quickly pumped into a three-neck flask using the peristaltic pump. Then Ca(NO3)2 and Na2SiO3 solutions were dripped into a three-necked flask containing the NaOH solution at a rate of 0.83 mL/min using a peristaltic pump, with stirring at 700–800 r/min, and titration completed at 25 ± 0.5 °C over 3 h. Then, two additional reaction hours followed. The entire reaction process was conducted under the N2 atmosphere. Solid–liquid separation was achieved through vacuum filtration, and a gel-like substance was obtained after three successive washes with deionized water and absolute alcohol. The samples were freeze-dried in a vacuum freeze dryer at −40 °C for 21 h to produce a fluffy powder, which was preserved in a sample bottle under vacuum.
Experiments were conducted on the immobilization of heavy metal ions during the formation of CSH gel, using Pb(NO3)2 and Cr(NO3)3 solutions at various pH values. Refer to Table 2 for detailed ratio parameters. The Pb(NO3)2 and Cr(NO3)3 solutions were placed in a three-necked flask that had been pre-filled with N2, followed by the introduction of Ca(NO3)2 and Na2SiO3 solutions. Subsequent procedures were as previously described.

2.2. Analytical Methods

X-ray diffraction (XRD) analyses were performed on D8 Advance (Bruker, Bremen, Germany) with CuKα radiation under 60 kV and 80 mA and a scanning speed of 1°/min. Scanning Electron Microscopy and Energy-Dispersive X-ray (SEM-EDS) observations were conducted on JSM-7610FPlus (Nihon Kohden, Tokyo, Japan). The SS29Si NMR of the CSH gel was measured using a Nuclear Magnetic Resonance Spectrometer (NMR) (JEOL ECZ600R/S3, Japan Electronics, Tokyo, Japan), employing the cross-polarization method. The Ca/Si in the prepared CSH gel was determined using an Inductively Coupled Plasma Spectrometer (ICP) (ICAP7000, Thermo Fisher, Lenexa, KS, USA). The materials’ chemical structure was analyzed using a Raman Spectrometer (Raman) (LabRAM HR Evolution, HORIBA, Kyoto, Japan). A 785 nm laser was employed, operating at its maximum power. Fourier Transform Infrared spectroscopy (FT-IR) (NICOLET iS20, Thermo Fisher, Lenexa, KS, USA) was conducted using diamond as a substrate and the ATR method. The specific surface area of the material was measured using an Automatic BET Surface and Pore Distributor (BET) (Autosorb-iQ, Quantachrome, Boynton, FL, USA). N2 was used as the experimental atmosphere, with the sample subjected to vacuum desorption at 80 °C and pressure < 6.7 Pa for 2 h, followed by N2 adsorption at 77 K. The XRF analysis was conducted using an X-ray fluorescence heavy metal analyzer (PW4400, Malvern Panalytical, Nottingham, UK).

3. Results and Discussion

3.1. Study on CSH Gel Formation at Different pH Values

CSH gel samples with varying synthetic pH levels and Ca/Si were prepared in order to investigate the correlation between the formation of CSH gel and pH value. The XRD patterns of A-14, B-14, and C-14 are presented in Figure 2a. When the pH of the synthetic solution is 14, the XRD patterns of the CSH gel (B-14, C-14) exhibit diffraction peaks characteristic of portlandite, and the intensity of these peaks increases with an increase in Ca/Si.
The XRD patterns of A-13, B-13, and C-13 are depicted in Figure 2b. At pH 13, the CSH gel is A-13, and only the phase of CSH (PDF#00-034-0002) is observed. At this point, the characteristic peak at 2θ = 7.06° corresponds to the crystal plane spacing of 12.59 Å. As the Ca/Si ratio increases, the characteristic peak of A-13 shifts to higher angles, resulting in a decrease in crystal plane spacing [7,33]. The XRD patterns of B-13 and C-13 reveal the formation of a new phase, portlandite (PDF#00-044-1481), and its intensity increases with an increase in the Ca/Si. In summary, at pH values of 14 and 13, the increase in the Ca/Si leads to a decreasing trend in the crystal plane spacing of the CSH gel. It also causes the production of portlandite.
The XRD patterns of A-12, B-12, and C-12 are shown in Figure 2c. At a synthetic solution pH of 12, no portlandite phase was formed in the CSH gels with different Ca/Si. There was a shift in the main peak position. Thus, pH exerts a significant influence on the synthesis of CSH gel. It was noticed that at lower pH, the charge on silicate tetrahedra in the solution is largely compensated by H+, while at higher pH, there are more negative charges, increasing the charge repulsion between Si-O [7]. Consequently, the polymerization degree of CSH gel decreases with increasing pH. Moreover, the OH− ions in the solution not only react with free Ca2+ ions but also tend to attack silicate chains, leading to depolymerization. The residual cations of A-12, B-12, and C-12 were tested using XRF, with the Na content being 0.142%, 0.057%, and 0.05%, respectively. CSH gels synthesized under these conditions can be free of other impurities, making them suitable for further characterization of the properties.

3.2. Effect of Ca/Si on the Composition and Structure of CSH Gel

3.2.1. ICP Analyses of CSH Gel

A volume of 10 mg of A-12, B-12, and C-12 was used to measure the concentration of Ca and Si in the solution using ICP to characterize the Ca/Si of the prepared CSH gel. As depicted in Table 3, a notable deviation exists between the Ca/Si measured by ICP and those intended in the experimental design. The test data reveal that pH is a crucial factor in preparing CSH gel via the precipitation method. Notably, at a pH of 12, elevating the Ca/Si fails to facilitate the synthesis of purer CSH gel with a higher Ca/Si. This finding was consistent with the researchers’ consensus that the structural model of CSH may encompass nanoscale Ca(OH)2 and that it was hard to prepare pure CSH gel with Ca/Si above 1.5 at a pH of 12 [3,21,34].

3.2.2. SSNMR

The SS29Si NMR of A-12, B-12, and C-12 was measured. As shown in Figure 3, the Q1 bond is located near −80 ppm, and the Q2 bond is located near −85 ppm. The peak intensity of the Q1 bond shows a trend of increasing with the increase in Ca/Si, while the Q2 bond shows a trend of decreasing first and then increasing with Ca/Si. The Si-O bond’s mean chain length (MCL) was calculated by Formula (1). The relative proportion of Qn can be obtained from the deconvolution of spectra performed by a mixed Lorentz–Gauss peak profile [35].
M C L = 2 ( 1 + Q 2 / Q 1 )
The data in Table 4 indicate that at a synthetic pH of 12, as the Ca/Si increases, the CSH gel may not undergo a structural transition at 1.03 but continues to maintain its original structure, extending the chain length of the Si-O bond. The structural transition occurs only when the Ca/Si ratio is greater than 1.34. The increase in MCL after the Ca/Si increases from 1.03 to 1.34 may involve both the removal of calcium ions and the rearrangement of low-polymerization silicon chains. It is consistent with the absence of the phase of portlandite in high Ca/Si CSH gel. After the Ca/Si increases from 1.34 to 1.38, the silicon–oxygen bond is difficult to extend, resulting in a decrease in the silicon chain polymerization degree.

3.2.3. Raman

To further investigate the impact of Ca/Si on the structural variations of CSH gel, the Raman spectrum was conducted on A-12, B-12, and C-12. The peaks were observed at 675, 869, 1010, and 1071 cm−1, representing the Q2 symmetric bending vibration, Q1 symmetric stretching vibration, Q2 symmetric stretching vibration, and carbonate symmetric stretch (Figure 4) [36,37,38]. The peaks of Q1 and Q2 are observed at the same positions, indicating similar CSH gel structures for A-12, B-12, and C-12. With the increase in Ca/Si, the peak intensity of the Q2 at 675 and 1010 cm−1 gradually increases, while the band intensity of the Q1 at 869 cm−1 gradually decreases. The peak intensity of the carbonate symmetric stretch at 1071 cm−1 for A-12 and B-12 was similar and higher than that of C-12. The decrease in the number of silicon-oxygen bonds at the termini of the silicon chains in B-12 and C-12, resulting in longer silicon chains, is indicated by the decrease in the peak intensity of Q1 and the increase in the peak intensity of Q2. This observation is similar to the trends observed in SSNMR regarding the variation of Q1 and Q2 with Ca/Si. The peak of the carbonate symmetric stretch may be caused by the contact of the prepared CSH gel with air, resulting in the formation of CaCO3.

3.2.4. BET

The BET method was used to measure the specific surface areas (SSA) and pore size distributions of A-12, B-12, and C-12. The SSA and peak pore size were 173.8 m2/g, 186.3 m2/g, and 162.4 m2/g and 15.6 nm, 29.3 nm, and 15.7 nm for A-12, B-12, and C-12. The SSA of CSH gel is influenced by factors such as pressure, reaction time, and Ca/Si. Chen [39] and Wu [40] separately prepared CSH gels with Ca/Si ratios of 0.65 and 0.7 by precipitation methods, resulting in SSAs of 637.2 m2/g and 505 m2/g. In contrast, the SSA of the CSH gel synthesized by Dambrauskas [41] based on the hydrothermal method with a Ca/Si of 1.5 was only 46.13 m2/g. The SSA of CSH gel can be effectively controlled by selecting appropriate experimental methods and reaction conditions, and it can be applied in catalytic and adsorption fields.
With the increase in Ca/Si, the SSA of the CSH gel synthesized at pH 12 initially increased and then decreased. As observed in the N2–adsorption-desorption curves (Figure 5), the adsorption isotherms of the three samples exhibit a significant increase in adsorption within the lower P/P0 range (0–0.1). It can be attributed to the filling adsorption in the micropores of the CSH gel. Within the range of P/P0 from 0.2 to 0.8, multi-layer adsorption occurs, resulting in a slow increase in adsorption. At higher P/P0 ratios, the adsorption isotherms rise rapidly, conforming to the characteristics of type IV adsorption isotherms, indicating the presence of mesopores in the CSH gel. The hysteresis loops exhibited in the adsorption-desorption isotherms (Figure 5) are classified as type H3, commonly seen in sheet-like particulate materials with flat slit structures. The pore size distribution diagram reveals that A-12, B-12, and C-12 have similar pore sizes, with the main pore sizes ranging between 20 and 40 nm.

3.2.5. FT-IR Analyses

The FT-IR spectra of A-12, B-12, and C-12 showed strong bands at 1410, 960, and 670 cm−1 [42], corresponding to the C-O bond and the Si-O bond Q2 and Q1 of the CSH gel silicon tetrahedra, respectively (Figure 6). According to the literature, the band of Ca(OH)2 is located at 3750 cm−1, and the band of the H-O bond is at 3490 cm−1 [43,44]. In CSH gels with different Ca/Si (A-12, B-12, C-12), the bands of Ca(OH)2 did not appear, indicating that the synthesized CSH gel did not contain portlandite, corroborated with the XRD results (Figure 2). In Figure 6, the intensity of the Q2 band for A-12, B-12, and C-12 is higher than that of Q1. With the increase in Ca/Si, the band of Q1 shows a trend of first decreasing and then increasing, with the Q2 band at 962 cm−1 for A-12 initially shifting to 965 cm−1 for B-12 and then to 963 cm−1 for C-12. Combined with Raman and SSNMR, it is concluded that the phenomenon of Si-O bonds moved first to higher wavenumbers and then to lower wavenumbers. It indicates that the degree of polymerization of the silicon chain in the CSH gel first increases and then decreases.

3.2.6. SEM Observations

The synthesized CSH gel exhibits a porous network structure formed by the lamellar stacking of fibrous connections. As the Ca/Si ratio increases, the CSH gel structure exhibits a relatively loose configuration, followed by aggregation, corroborating the conclusion from the BET observation that SSA first increases and then decreases with the Ca/Si. During the hydration process of cement, the CSH gel can be described as fibrillar or needle-like, tubular, foil, lace-like, plate-like, and spherulitic, varying depending on the influencing factors [45]. The CSH prepared via precipitation method tends to be spherical or lamellar in shape [3,39,40]. From the EDS images of A-12 and B-12 shown in Figure 7, it is evident that the CSH gel with uniformly distributed constituent elements Ca and Si can be synthesized through the precipitation method.

3.3. Immobilization of Heavy Metal Ions in CSH Gel Reaction Systems

3.3.1. Immobilization of Heavy Metal Ions in CSH Gel Reaction System at Different pH Values

According to the proportions listed in Table 2, CSH gel with immobilized heavy metal ions was prepared. The existence of heavy metal ions Pb2+ and Cr3+ within the CSH gel was confirmed through XRF analysis. Specifically, the proportion of CSH gel immobilization with Pb ions relative to the total elements stands at A-0.1Pb-12/A-0.5Pb-12/B-0.5Pb-12/C-0.5Pb-12 = 0.96%/0.12%/0.55%/0.12%, whereas the proportion of CSH gel immobilization with Cr ions comprises A-0.5Cr-12/A-1Cr-12/B-1Cr-12/C-1Cr-12 = 1.74%/1.12%/1.90%/1.85%. XRF data indicate that the immobilization effect of CSH gel on Cr3+ is greater than Pb2+. With the increase in concentration of heavy metal ions in the solution, the immobilization ability of CSH gel for both Cr3+ and Pb2+ decreases. Qiao et al. [46] demonstrated through simulation calculations that the immobilization effect of CSH gel on heavy metal ions was Pb > Zn > Cu, which was related to the shrinkage of CSH gel. It has been shown in the literature that CSH gel has a higher immobilization capacity for Pb2+ than Cr3+ [47,48]. However, some researchers believe that during the early hydration process, a low concentration of Cr promotes the formation of CSH gel [49]. The immobilization of heavy metals mainly depends on the complex pore structure and is affected by various factors such as curing time, the role of metal cations, and internal pH value. Therefore, it is not accurate to evaluate the immobilization ability of CSH gel to heavy metal ions through XRF data, which can only confirm the successful immobilization of heavy metal ions by CSH gel. The obtained samples were subjected to X-ray diffraction analyses (Figure 8). Peaks of portlandite appeared in A-0.5Pb-13, A-0.5Cr-13, and A-1Cr-13, indicating that Ca2+ in the CSH gel was substituted. The absence of portlandite peaks in A-0.1Pb-13 may be due to a lower immobilization amount. In the samples A-0.1Pb-13, A-0.5Pb-13, A-0.5Cr-13, and A-1Cr-13, where Pb2+ and Cr3+ were immobilized, the peak at 2θ = 7.14° shifted to 2θ = 7.08°, 2θ = 8.34°, and 2θ = 8.60°, respectively. The interplanar spacings were found to be 12.371 Å, 12.476 Å, 10.593 Å, and 10.274 Å. The trend of interplanar spacing exhibited an initial increase followed by a decrease. In contrast, the peak for A-13 in Figure 2b is located at 2θ = 7.20°. This peak shows a slight shift towards lower angles after Pb immobilization and a shift towards higher angles after Cr immobilization, both with increasing immobilization amounts. The ionic radii of Pb2+, Cr3+, and Ca2+ are in the order of Pb2+ > Ca2+ > Cr3+, indicating that the substitution of Ca2+ by Pb2+ leads to an increase in interlayer spacing, while the substitution by Cr3+ leads to a decrease, aligning with the peak shifts in A-0.1Pb-13, A-0.5Pb-13, A-0.5Cr-13, and A-1Cr-13. Therefore, the formation of new phases, as identified in XRD analyses, is attributed to the entry of heavy metal ions into the interlayer structure of CSH, replacing Ca2+, which in turn reacts with NaOH.

3.3.2. Immobilization of Heavy Metal Ions in CSH Gel Reaction Systems with Different Ca/Si

At a pH of 12, CSH gels were synthesized and immobilized with heavy metal ions. As shown in Figure 9a, no peaks of portlandite were observed after the immobilization of the heavy metal ions, and the characteristic peak at 2θ = 7.2° was not distinct. A-0.1Pb-12, A-0.5Pb-12, A-0.5Cr-12, and A-1Cr-12 did not exhibit significant peak shapes at 2θ = 7.2°. In contrast, Figure 9b reveals the presence of weaker portlandite peaks in the XRD patterns of CSH gels after the immobilization of heavy metal ions. Peaks B-0.5Pb-12 and C-0.5Pb-12 exhibited a leftward shift at 2θ = 7.2°, while peaks C-0.5Pb-12 and C-1Cr-12 showed a rightward shift. It confirms that substituting Pb2+ for interlayer Ca2+ leads to an increase in interlayer spacing, and substituting Cr3+ for interlayer Ca2+ results in a decrease in interlayer spacing [12,50]. The XRD patterns indicate that both Cr and Pb can replace Ca2+ in the CSH gel and generate portlandite. However, it was less evident at pH = 12 and Ca/Si = 1.03, whereas the phenomena are more pronounced at Ca/Si of 1.34 and 1.38. In Figure 9b, the primary peak of B-1Cr-12 and C-1Cr-12 XRD tends to shift towards the left compared to B-0.5Pb-12 and C-0.5Pb-12. This shift could potentially be ascribed to the partial substitution of Si4+ by Cr3+.
The synthesized CSH gel was subjected to FT-IR and Raman spectroscopic analysis to further investigate the structural alterations in CSH gel upon the immobilization of heavy metals. As shown in Figure 10, the CSH gel immobilized with heavy metals exhibits a minor band at 3750 cm−1, which is not significant. This result indicates that adding Pb2+ and Cr3+ leads to Ca(OH)2 formation. The Q2 band of A-12 and A-0.1Pb-12 remains at 963 cm−1 with no significant change. However, the Q2 bands of A-0.5Pb-12, A-0.5Cr-12, and A-1Cr-12 shift to a lower wavenumber at 962 cm−1, suggesting that the immobilization of heavy metals reduces the degree of polymerization of the silicon chains in CSH. Analysis of the Q1 bond band shifts in A-0.5Pb-12, A-1Cr-12, B-0.5Pb-12, B-1Cr-12, C-0.5Pb-12, and C-1Cr-12 reveals that the bands for A-0.5Pb-12, A-1Cr-12, and B-0.5Pb-12 remain at 962 cm−1, while those for C-0.5Pb-12 and C-1Cr-12 shift to 961 cm−1 on the right. The solidification of heavy metal ions in high Ca/Si CSH gel reduces the degree of polymerization of the silicon chains, ultimately leading to structural changes.
Raman spectroscopy has arrived at comparable conclusions, as depicted in Figure 11. The image shows no appreciable displacement in the Q1 and Q2 bonds. The Q3 bond of A-0.5Pb-12 and A-1Cr-12 is situated at 1056 cm−1, whereas B-0.5Pb-12 exhibits a shift to 1051 cm−1, and C-1Cr-12 demonstrates a shift to 1048 cm−1. Furthermore, a notable enhancement in peak intensity is evident in B-1Cr-12 and C-1Cr-12. This phenomenon suggests that as the Ca/Si increases, the proportion of the Q3 bond decreases in the CSH that has immobilized heavy metals, resulting in a decrease in the polymerization degree of the silicon chain. Notably, the CSH gel immobilized with Cr3+ (B-1Cr-12, C-1Cr-12) exhibits a higher intensity of Q3 bonds compared to the CSH gel immobilized with Pb2+ (B-0.5Pb-12, C-0.5Pb-12). It is inferred from the literature that Cr3+ can not only replace Ca2+ but also exhibits a certain degree of substitution for Si4+ [49,51].

4. Conclusions

In this paper, CSH gel was successfully synthesized, and the influence of varying Ca/Si and pH values on the properties of the synthesized CSH gel was examined. The main research findings are as follows:
(1)
Synthetic experiments indicate that the structure of CSH gel synthesized at pH = 14 changes by varying Ca/Si, accompanied by a decreasing trend in interplanar spacing and the concurrent formation of calcium hydroxide impurities. At pH = 12, the CSH gel exhibited an initial increase in silicon chain length, followed by a decrease as the Ca/Si ratio increased, without the formation of any impurity phases. As the Ca/Si increases, the interplanar spacing of the CSH gel tends to decrease, accompanied by the extension of silicon chains and the removal of Ca2+ ions. The released Ca2+ ions react with the NaOH present in the solution, forming calcium hydroxide.
(2)
The study delved into the immobilization mechanism of heavy metal ions within the CSH gel reaction system. Characterization results from XRD, FT-IR, and Raman spectroscopy indicate that both Cr3+ and Pb2+ can substitute for Ca2+ in the CSH gel and generate calcium hydroxide, with Cr3+ having the additional capability of substituting for Si4+ within the CSH structure. The findings of this study reveal that the immobilization mechanism of lead and chromium within the CSH gel reaction system involves ion substitution.

Author Contributions

Writing—original draft preparation, K.Z.; writing—review and editing, L.W.; methodology, L.L.; data curation, Y.B. and J.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by National Key Research and Development Program of China (Grant No. 2022YFE0129200).

Data Availability Statement

Dataset available on request from the authors.

Acknowledgments

The authors would like to acknowledge the financial support from National Key Research and Development Program of China (Grant No. 2022YFE0129200).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Cuesta, A.; Zea-Garcia, J.D.; Londono-Zuluaga, D.; De La Torre, A.G.; Santacruz, I.; Vallcorba, O.; Dapiaggi, M.; Sanfélix, S.G.; Aranda, M.A.G. Multiscale Understanding of Tricalcium Silicate Hydration Reactions. Sci. Rep. 2018, 8, 8544. [Google Scholar] [CrossRef] [PubMed]
  2. Land, G.; Stephan, D. The Effect of Synthesis Conditions on the Efficiency of C-S-H Seeds to Accelerate Cement Hydration. Cem. Concr. Compos. 2018, 87, 73–78. [Google Scholar] [CrossRef]
  3. Kumar, A.; Walder, B.J.; Kunhi Mohamed, A.; Hofstetter, A.; Srinivasan, B.; Rossini, A.J.; Scrivener, K.; Emsley, L.; Bowen, P. The Atomic-Level Structure of Cementitious Calcium Silicate Hydrate. J. Phys. Chem. C 2017, 121, 17188–17196. [Google Scholar] [CrossRef]
  4. Goracci, G.; Monasterio, M.; Jansson, H.; Cerveny, S. Dynamics of Nano-Confined Water in Portland Cement—Comparison with Synthetic C-S-H Gel and Other Silicate Materials. Sci. Rep. 2017, 7, 8258. [Google Scholar] [CrossRef]
  5. Yan, Y.; Bernard, E.; Miron, G.D.; Rentsch, D.; Ma, B.; Scrivener, K.; Lothenbach, B. Kinetics of Al Uptake in Synthetic Calcium Silicate Hydrate (C-S-H). Cem. Concr. Res. 2023, 172, 107250. [Google Scholar] [CrossRef]
  6. Xiang-peng, F.; Li-ping, G.; Jian-dong, W.; Bang-cheng, L.; Ying-jie, C.; Xu-yan, S. Impact of Temperature, pH Value and Multiple Ions on the Physisorption of Chloride Ion on C-S-H Gel Surface. Constr. Build. Mater. 2023, 392, 131967. [Google Scholar] [CrossRef]
  7. Maddalena, R.; Li, K.; Chater, P.A.; Michalik, S.; Hamilton, A. Direct Synthesis of a Solid Calcium-Silicate-Hydrate (C-S-H). Constr. Build. Mater. 2019, 223, 554–565. [Google Scholar] [CrossRef]
  8. Nithurshan, M.; Elakneswaran, Y.; Yoda, Y.; Yano, K.; Kitagaki, R.; Hiroyoshi, N. Exploring the Reinforcing Mechanism of Graphene Oxide in Cementitious Materials through Microstructural Analysis of Synthesised Calcium Silicate Hydrate. Cem. Concr. Compos. 2024, 153, 105717. [Google Scholar] [CrossRef]
  9. Yoshida, S.; Elakneswaran, Y.; Nawa, T. Electrostatic Properties of C–S–H and C-A-S-H for Predicting Calcium and Chloride Adsorption. Cem. Concr. Compos. 2021, 121, 104109. [Google Scholar] [CrossRef]
  10. Labbez, C.; Jönsson, B.; Pochard, I.; Nonat, A.; Cabane, B. Surface Charge Density and Electrokinetic Potential of Highly Charged Minerals: Experiments and Monte Carlo Simulations on Calcium Silicate Hydrate. J. Phys. Chem. B 2006, 110, 9219–9230. [Google Scholar] [CrossRef]
  11. Wang, Z.; Chen, Y.; Xu, L.; Zhu, Z.; Zhou, Y.; Pan, F.; Wu, K. Insight into the Local C-S-H Structure and Its Evolution Mechanism Controlled by Curing Regime and Ca/Si Ratio. Constr. Build. Mater. 2022, 333, 127388. [Google Scholar] [CrossRef]
  12. Bernard, E.; Yan, Y.; Lothenbach, B. Effective Cation Exchange Capacity of Calcium Silicate Hydrates (C-S-H). Cem. Concr. Res. 2021, 143, 106393. [Google Scholar] [CrossRef]
  13. Yan, Y.; Yang, S.-Y.; Miron, G.D.; Collings, I.E.; L’Hôpital, E.; Skibsted, J.; Winnefeld, F.; Scrivener, K.; Lothenbach, B. Effect of Alkali Hydroxide on Calcium Silicate Hydrate (C-S-H). Cem. Concr. Res. 2022, 151, 106636. [Google Scholar] [CrossRef]
  14. Yang, Z.; Kang, D.; Zhang, D.; Yan, C.; Zhang, J. Crystal Transformation of Calcium Silicate Minerals Synthesized by Calcium Silicate Slag and Silica Fume with Increase of C/S Molar Ratio. J. Mater. Res. Technol. 2021, 15, 4185–4192. [Google Scholar] [CrossRef]
  15. John, E.; Epping, J.D.; Stephan, D. The Influence of the Chemical and Physical Properties of C-S-H Seeds on Their Potential to Accelerate Cement Hydration. Constr. Build. Mater. 2019, 228, 116723. [Google Scholar] [CrossRef]
  16. Sun, G.K.; Young, J.F.; Kirkpatrick, R.J. The Role of Al in C–S–H: NMR, XRD, and Compositional Results for Precipitated Samples. Cem. Concr. Res. 2006, 36, 18–29. [Google Scholar] [CrossRef]
  17. Shen, X.; Feng, P.; Zhang, Q.; Lu, J.; Liu, X.; Ma, Y.; Jin, P.; Wang, W.; Ran, Q.; Hong, J. Toward the Formation Mechanism of Synthetic Calcium Silicate Hydrate (C-S-H)—pH and Kinetic Considerations. Cem. Concr. Res. 2023, 172, 107248. [Google Scholar] [CrossRef]
  18. Harris, M.; Simpson, G.; Scrivener, K.; Bowen, P. A Method for the Reliable and Reproducible Precipitation of Phase Pure High Ca/Si Ratio (>1.5) Synthetic Calcium Silicate Hydrates (C S H). Cem. Concr. Res. 2022, 151, 106623. [Google Scholar] [CrossRef]
  19. Chen, J.J.; Thomas, J.J.; Taylor, H.F.W.; Jennings, H.M. Solubility and Structure of Calcium Silicate Hydrate. Cem. Concr. Res. 2004, 34, 1499–1519. [Google Scholar] [CrossRef]
  20. Lothenbach, B.; Nonat, A. Calcium Silicate Hydrates: Solid and Liquid Phase Composition. Cem. Concr. Res. 2015, 78, 57–70. [Google Scholar] [CrossRef]
  21. Richardson, I.G. Model Structures for C-(A)-S-H(I). Acta Crystallogr. Sect. B Struct. Sci. Cryst. Eng. Mater. 2014, 70, 903–923. [Google Scholar] [CrossRef] [PubMed]
  22. García Lodeiro, I.; Fernández-Jimenez, A.; Palomo, A.; Macphee, D.E. Effect on Fresh C-S-H Gels of the Simultaneous Addition of Alkali and Aluminium. Cem. Concr. Res. 2010, 40, 27–32. [Google Scholar] [CrossRef]
  23. Pantuzzo, F.L.; Santos, L.R.G.; Ciminelli, V.S.T. Solubility-Product Constant of an Amorphous Aluminum-Arsenate Phase (AlAsO4·3.5H2O) AT 25 °C. Hydrometallurgy 2014, 144–145, 63–68. [Google Scholar] [CrossRef]
  24. Teramoto, H.; Koie, S. Early Hydration of a Superhigh-Early-Strength Portland Cement Containing Chromium. J. Am. Ceram. Soc. 1976, 59, 522–525. [Google Scholar] [CrossRef]
  25. Serclérat, I.; Moszkowicz, P.; Pollet, B. Retention Mechanisms in Mortars of the Trace Metals Contained in Portland Cement Clinkers. Waste Manag. 2000, 20, 259–264. [Google Scholar] [CrossRef]
  26. Wang, D.; Wang, Q.; Cui, Y.; Pang, L. Assessing Heavy-Metal Leaching Risk in Cement-Industrial Slag Composite Pastes Using a Mechanism-Based Thermodynamic Approach. Cem. Concr. Compos. 2023, 140, 105074. [Google Scholar] [CrossRef]
  27. Wang, D.; Wang, Q.; Huang, Z. Reuse of Copper Slag as a Supplementary Cementitious Material: Reactivity and Safety. Resour. Conserv. Recycl. 2020, 162, 105037. [Google Scholar] [CrossRef]
  28. Wang, H.; Ju, C.; Zhou, M.; Chen, J.; Dong, Y.; Hou, H. Sustainable and Efficient Stabilization/Solidification of Pb, Cr, and Cd in Lead-Zinc Tailings by Using Highly Reactive Pozzolanic Solid Waste. J. Environ. Manage. 2022, 306, 114473. [Google Scholar] [CrossRef]
  29. Hekal, E.E.; Kishar, E.A.; Mohamed, M.R.; Mohamed, B.A. Inertisation of Cd2+ Ions by Using Blended Cement Pastes. Adv. Cem. Res. 2013, 25, 80–85. [Google Scholar] [CrossRef]
  30. Wang, Y.-S.; Dai, J.-G.; Wang, L.; Tsang, D.C.W.; Poon, C.S. Influence of Lead on Stabilization/Solidification by Ordinary Portland Cement and Magnesium Phosphate Cement. Chemosphere 2018, 190, 90–96. [Google Scholar] [CrossRef]
  31. Ren, Z.; Wang, L.; Wang, H.; Liu, S.; Ren, J. Stabilization and Solidification Mechanism of Pb in Phosphogypsum Slag-Based Cementitious Materials. Constr. Build. Mater. 2023, 368, 130427. [Google Scholar] [CrossRef]
  32. Wang, F.; Shen, Z.; Liu, R.; Zhang, Y.; Xu, J.; Al-Tabbaa, A. GMCs Stabilized/Solidified Pb/Zn Contaminated Soil under Different Curing Temperature: Physical and Microstructural Properties. Chemosphere 2020, 239, 124738. [Google Scholar] [CrossRef] [PubMed]
  33. Tajuelo Rodriguez, E.; Garbev, K.; Merz, D.; Black, L.; Richardson, I.G. Thermal Stability of C-S-H Phases and Applicability of Richardson and Groves’ and Richardson C-(A)-S-H(I) Models to Synthetic C-S-H. Cem. Concr. Res. 2017, 93, 45–56. [Google Scholar] [CrossRef]
  34. Cuesta, A.; Santacruz, I.; De La Torre, A.G.; Dapiaggi, M.; Zea-Garcia, J.D.; Aranda, M.A.G. Local Structure and Ca/Si Ratio in C-S-H Gels from Hydration of Blends of Tricalcium Silicate and Silica Fume. Cem. Concr. Res. 2021, 143, 106405. [Google Scholar] [CrossRef]
  35. Foley, E.M.; Kim, J.J.; Reda Taha, M.M. Synthesis and Nano-Mechanical Characterization of Calcium-Silicate-Hydrate (C-S-H) Made with 1.5 CaO/SiO2 Mixture. Cem. Concr. Res. 2012, 42, 1225–1232. [Google Scholar] [CrossRef]
  36. Leukel, S.; Panthöfer, M.; Mondeshki, M.; Kieslich, G.; Wu, Y.; Krautwurst, N.; Tremel, W. Mechanochemical Access to Defect-Stabilized Amorphous Calcium Carbonate. Chem. Mater. 2018, 30, 6040–6052. [Google Scholar] [CrossRef]
  37. Gasharova, B.; Garbev, K.; Bornefeld, M.; Black, L.; Stemmermann, P. Raman and IR Spectroscopic Study of Nano-Crystalline Calcium Silicate Hydrates of Type C-S-H(I). Acta Crystallogr. A 2006, 62, s208. [Google Scholar] [CrossRef]
  38. Vetter, M.; Gonzalez-Rodriguez, J.; Nauha, E.; Kerr, T. The Use of Raman Spectroscopy to Monitor Phase Changes in Concrete Following High Temperature Exposure. Constr. Build. Mater. 2019, 204, 450–457. [Google Scholar] [CrossRef]
  39. Chen, L.; Wang, X.; Chen, Y.; Zhuang, Z.; Chen, F.-F.; Zhu, Y.-J.; Yu, Y. Recycling Heavy Metals from Wastewater for Photocatalytic CO2 Reduction. Chem. Eng. J. 2020, 402, 125922. [Google Scholar] [CrossRef]
  40. Wu, J.; Zhu, Y.; Chen, F. Ultrathin Calcium Silicate Hydrate Nanosheets with Large Specific Surface Areas: Synthesis, Crystallization, Layered Self-Assembly and Applications as Excellent Adsorbents for Drug, Protein, and Metal Ions. Small 2013, 9, 2911–2925. [Google Scholar] [CrossRef]
  41. Dambrauskas, T.; Davidoviciene, D.; Baltakys, K.; Eisinas, A.; Jaskunas, A.; Siler, P.; Rudelis, V.; Svedaite, E. Effect of Intercalated Metal Ions on the Specific Surface Area and Porosity of Dibasic Calcium Silicate Hydrate. Surf. Interfaces 2022, 33, 102243. [Google Scholar] [CrossRef]
  42. Liu, X.; Feng, P.; Cai, Y.; Yu, X.; Yu, C.; Ran, Q. Carbonation Behavior of Calcium Silicate Hydrate (C-S-H): Its Potential for CO2 Capture. Chem. Eng. J. 2022, 431, 134243. [Google Scholar] [CrossRef]
  43. Wang, D.; Xiong, C.; Li, W.; Chang, J. Growth of Calcium Carbonate Induced by Accelerated Carbonation of Tricalcium Silicate. ACS Sustain. Chem. Eng. 2020, 8, 14718–14731. [Google Scholar] [CrossRef]
  44. Zhang, L.; Zhao, C.; Jiang, Y.; Wang, Y.; Yang, W.; Cheng, T.; Zhou, G. Effect of Sodium Dodecyl Benzene Sulfonate on Morphology and Structure of Calcium Silicate Hydrate Prepared via Precipitation Method. Colloids Surf. Physicochem. Eng. Asp. 2018, 540, 249–255. [Google Scholar] [CrossRef]
  45. Basquiroto De Souza, F.; Sagoe-Crentsil, K.; Duan, W. A Century of Research on Calcium Silicate Hydrate (C–S–H): Leaping from Structural Characterization to Nanoengineering. J. Am. Ceram. Soc. 2022, 105, 3081–3099. [Google Scholar] [CrossRef]
  46. Qiao, G.; Hou, D.; Li, W.; Yin, B.; Zhang, Y.; Wang, P. Molecular Insights into Migration of Heavy Metal Ion in Calcium Silicate Hydrate (CSH) Surface and Intra-CSH (Ca/Si = 1.3). Constr. Build. Mater. 2023, 365, 130097. [Google Scholar] [CrossRef]
  47. Chen, L.; Nakamura, K.; Hama, T. Review on Stabilization/Solidification Methods and Mechanism of Heavy Metals Based on OPC-Based Binders. J. Environ. Manage. 2023, 332, 117362. [Google Scholar] [CrossRef]
  48. Król, M.; Florek, P.; Marzec, M.; Wójcik, S.; Dziża, K.; Mozgawa, W. Structural Studies of Calcium Silicate Hydrate Modified with Heavy Metal Cations. Spectrochim. Acta. A. Mol. Biomol. Spectrosc. 2024, 321, 124681. [Google Scholar] [CrossRef]
  49. Gedeike, I.; Baltakys, K.; Eisinas, A. Effect of Cr3+ Ions on the Crystallization of Calcium Silicate Hydrates (CaO/SiO2 = 1.5) under Hydrothermal Conditions and Their Thermal Stability. J. Therm. Anal. Calorim. 2023, 148, 1501–1510. [Google Scholar] [CrossRef]
  50. Li, G.; Li, M.; Zhang, X.; Cao, P.; Jiang, H.; Luo, J.; Jiang, T. Hydrothermal Synthesis of Zeolites-Calcium Silicate Hydrate Composite from Coal Fly Ash with Co-Activation of Ca(OH)2-NaOH for Aqueous Heavy Metals Removal. Int. J. Min. Sci. Technol. 2022, 32, 563–573. [Google Scholar] [CrossRef]
  51. Niuniavaite, D.; Baltakys, K.; Dambrauskas, T.; Eisinas, A.; Rubinaite, D.; Jaskunas, A. Microstructure, Thermal Stability, and Catalytic Activity of Compounds Formed in CaO-SiO2-Cr(NO3)3-H2O System. Nanomaterials 2020, 10, 1299. [Google Scholar] [CrossRef]
Crystals 14 00864 g001
Figure 1. CSH gel synthesis experimental device.
Figure 1. CSH gel synthesis experimental device.
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Crystals 14 00864 g002
Figure 2. XRD patterns of CSH gel prepared at (a) pH 14; (b) pH 13; (c) pH 12.
Figure 2. XRD patterns of CSH gel prepared at (a) pH 14; (b) pH 13; (c) pH 12.
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Crystals 14 00864 g004
Figure 4. Raman spectra of different Ca/Si CSH gels at pH 12.
Figure 4. Raman spectra of different Ca/Si CSH gels at pH 12.
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Crystals 14 00864 g003
Figure 3. SSNMR 29Si spectrum of CSH gel.
Figure 3. SSNMR 29Si spectrum of CSH gel.
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Crystals 14 00864 g005
Figure 5. Adsorption–desorption isotherms of different Ca/Si CSH gels at pH 12 for (a) A-12, (b) B-12, and (c) C-12.
Figure 5. Adsorption–desorption isotherms of different Ca/Si CSH gels at pH 12 for (a) A-12, (b) B-12, and (c) C-12.
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Crystals 14 00864 g006
Figure 6. FT-IR spectra of different Ca/Si CSH gels formed at pH 12.
Figure 6. FT-IR spectra of different Ca/Si CSH gels formed at pH 12.
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Crystals 14 00864 g007
Figure 7. SEM-EDS images of CSH gel (a) A-12; (b) B-12; (c) C-12; (d) A-12 EDS-Ca; (e) A-12 EDS-Si; (f) B-12 EDS-Ca; (g) B-12 EDS-Si.
Figure 7. SEM-EDS images of CSH gel (a) A-12; (b) B-12; (c) C-12; (d) A-12 EDS-Ca; (e) A-12 EDS-Si; (f) B-12 EDS-Ca; (g) B-12 EDS-Si.
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Crystals 14 00864 g008
Figure 8. XRD patterns of CSH gel synthesized and immobilized with different heavy metal ions.
Figure 8. XRD patterns of CSH gel synthesized and immobilized with different heavy metal ions.
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Crystals 14 00864 g009
Figure 9. XRD patterns of CSH gel immobilized metal ions with different calcium–silicon ratios (a) A-0.1Pb-12 and A-0.5Pb-12 for immobilized Pb, and A-0.5Cr-12 and A-1Cr-12 for Cr; (b) B-0.5Pb-12 and C-0.5Pb-12 for immobilized Pb, and B-1Cr-12 and C-1Cr-12 for immobilized Cr.
Figure 9. XRD patterns of CSH gel immobilized metal ions with different calcium–silicon ratios (a) A-0.1Pb-12 and A-0.5Pb-12 for immobilized Pb, and A-0.5Cr-12 and A-1Cr-12 for Cr; (b) B-0.5Pb-12 and C-0.5Pb-12 for immobilized Pb, and B-1Cr-12 and C-1Cr-12 for immobilized Cr.
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Crystals 14 00864 g010
Figure 10. FTIR spectra of CSH immobilized heavy metals (a) A-12, A-0.1Pb-12, A-0.5Pb-12, A-0.5Cr-12, A-1Cr-12; (b) A-0.5Pb-12, A-1Cr-12, B-0.5Pb-12, B-1Cr-12, C-0.5Pb-12, C-1Cr-12.
Figure 10. FTIR spectra of CSH immobilized heavy metals (a) A-12, A-0.1Pb-12, A-0.5Pb-12, A-0.5Cr-12, A-1Cr-12; (b) A-0.5Pb-12, A-1Cr-12, B-0.5Pb-12, B-1Cr-12, C-0.5Pb-12, C-1Cr-12.
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Crystals 14 00864 g011
Figure 11. Raman spectra of CSH with immobilized heavy metals.
Figure 11. Raman spectra of CSH with immobilized heavy metals.
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Table 1. Parameters for the preparation of CSH gel.
Table 1. Parameters for the preparation of CSH gel.
CompositionCa(NO3)2 Solution (mol/L)Na2SiO3 Solution (mol/L)Ca/Si*pH
Samples
A-140.150.11.514
B-140.180.11.814
C-140.20.12.014
A-130.150.11.513
B-130.180.11.813
C-130.20.12.013
A-120.150.11.512
B-120.180.11.812
C-120.20.12.012
Ca/Si* represents the theoretical Ca/Si stoichiometric composition.
Table 2. Experimental parameters of CSH gel synthesis and solidification of heavy metals.
Table 2. Experimental parameters of CSH gel synthesis and solidification of heavy metals.
CompositionCa(NO3)2 Solution (mol/L)Na2SiO3 Solution (mol/L)Cr(NO3)3 (mmol)Pb(NO3)2 (mmol)Ca/Si*pH
Samples
A-0.1Pb-130.150.100.11.513
A-0.5Pb-130.150.100.51.513
A-0.5Cr-130.150.10.501.513
A-1Cr-130.150.1101.513
A-0.1Pb-120.150.100.11.512
A-0.5Pb-120.150.100.51.512
A-0.5Cr-120.150.10.501.512
A-1Cr-120.150.1101.512
B-0.5Pb-120.180.100.51.812
B-1Cr-120.180.1101.812
C-0.5Pb-120.20.100.52.012
C-1Cr-120.20.1102.012
Ca/Si* represents the theoretical Ca/Si stoichiometric composition.
Table 3. ICP test data of CSH gel.
Table 3. ICP test data of CSH gel.
Ion ConcentrationCa (mg/L)Si (mg/L)Ca/Si
Samples
A-12243.2162.41.03
B-12223.2120.51.34
C-12202.7103.31.38
Table 4. Calculation of polymerization degree of silicon chain of CSH gel.
Table 4. Calculation of polymerization degree of silicon chain of CSH gel.
SamplesA-12B-12C-12
MCL454.8
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Zhu, K.; Wang, L.; Liao, L.; Bai, Y.; Hu, J. Study on Synthesis of CSH Gel and Its Immobilization of Heavy Metals. Crystals 2024, 14, 864. https://doi.org/10.3390/cryst14100864

AMA Style

Zhu K, Wang L, Liao L, Bai Y, Hu J. Study on Synthesis of CSH Gel and Its Immobilization of Heavy Metals. Crystals. 2024; 14(10):864. https://doi.org/10.3390/cryst14100864

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Zhu, Kunqian, Lijuan Wang, Libing Liao, Yunlong Bai, and Jing Hu. 2024. "Study on Synthesis of CSH Gel and Its Immobilization of Heavy Metals" Crystals 14, no. 10: 864. https://doi.org/10.3390/cryst14100864

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