Factors Affecting Synthesized C-S-H CO₂ Uptake: Initial Alkalinity and Ca/Si

Abstract

The dynamic evolution of alkalinity during hydration/carbonation of CO₂-conditioned cements results in the formation of polymorphic hydrated calcium silicates (C-S-H), whose differences in carbon sequestration capacity have not been systematically investigated. However, the micro-nano structures and carbon sequestration capacities of C-S-H are controlled by the dynamic effects of pore solution alkalinity and Ca/Si. Accordingly, different alkalinity and Ca/Si were set to simulate the cement hydration environment for the synthesis of C-S-H, and tests such as thermogravimetric and ²⁹Si nuclear magnetic resonance (NMR) were used to investigate the effects and mechanisms of initial alkalinity and Ca/Si on the morphology of the synthesized C-S-H, the CO₂ uptake. The results showed that the C-S-H synthesized at pH 7.2–12.0 and Ca/Si ratio of 1.0–2.3 was in flocculated and acicular forms, which were well crystallized and dominated by Q², while its CO₂ uptake was positively correlated with Ca/Si. On the contrary, the synthesized C-S-H was poorly crystallized under the conditions of pH increasing to 13.5 and Ca/Si ratios of 1.0–2.3. With the increase in Ca/Si, the synthesized C-S-H evolved from Q²-dominated foil to Q¹-dominated porous structure, and its CO₂ uptake was non-positively correlated with Ca/Si. This was mainly related to the average pore diameter of C-S-H and its silica-oxygen tetrahedral structure. This was mainly related to the average pore diameter of C-S-H and its silica-oxygen tetrahedral structure.

1. Introduction

CO₂ mineralized concrete has been recognized as a promising approach to reducing carbon emissions in the cement industry [1,2]. Existing studies have found that the carbon sequestration capacity of CO₂-cured concrete can exceed 24% [3,4]. For hardened cement-based materials, the reaction with CO₂ mainly involves the hydration products of cement (such as CH, C-S-H, and AFt) [5], and the carbonation reaction formula is as follows: Equations (1)–(3) [6,7,8,9].

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(1)

xCaO·ySiO₂·zH₂O + xCO₂ → xCaCO₃ + ySiO₂·wH₂O + (z − yw)H₂O

(2)

3CaO·Al₂O₃·3CaSO₄·32H₂O + 3CO₂ → 3(CaSO₄·2H₂O) + 3CaCO₃ + Al₂O₃·xH₂O + (26 − x)H₂O

(3)

Among the cement hydration products, the carbonation reaction rate is highest for CH, followed by C-S-H [10]. In hydrated cement paste, C-S-H accounts for 50% to 70%, while mineral phases such as CH and AFt collectively account for 20% to 25% [11,12,13]. Therefore, C-S-H has great potential in CO₂ sequestration. Based on previous research findings, the CO₂ sequestration capacity of C-S-H depends on its pore structure and pore size distribution [14]. However, scholars have found that C-S-H synthesized in environments with a pH range of 10.0 to 13.5 and an initial Ca/Si ratio of 0.56 to 1.50 exhibit significant differences in their pore structures [14,15]. To this end, it is a fundamental task to increase the Ca/Si ratio and alkalinity, and systematically investigate the synergistic effects of both on the evolution of C-S-H pore structure and its mechanism of influencing C-S-H carbon sequestration.

It has been reported that C-S-H with different Ca/Si ratios exhibits various micro-nano morphologies, such as spherical, plate-like, and needle-like structures [16,17,18], which are primarily related to the content of Qⁿ in C-S-H synthesized with different Ca/Si ratios [19]. Meanwhile, some scholars have studied the effect of the initial Ca/Si ratio on the pore structure and specific surface area of synthesized C-S-H. They found that a higher initial Ca/Si ratio leads to larger pore structures, but there is no significant change in the specific surface area of C-S-H [14]. In addition to Ca/Si ratio, recent studies have found that the alkalinity during the synthesis of C-S-H can also significantly alter its pore structure and molecular structure [20]. For example, Duque-Redondo [21] and Chen et al. [22] found that C-S-H synthesized in environments with a pH range of 11.16 to 11.88 undergoes decalcification, while environments with a pH greater than 11 reduce the formation rate of C-S-H [23]. Shen et al. [24] found that pH primarily affects the surface charge of the silicon-oxygen tetrahedron and the ability to accommodate cations, which in turn further influences the Ca/Si ratio of C-S-H. Building on this, the effects of the initial Ca/Si ratio and alkalinity on the CO₂ sequestration capacity of synthesized C-S-H have been studied separately. Influenced by the initial Ca/Si ratio (0.67 to 1.60) as a single factor, the CO₂ uptake capacity of synthesized C-S-H shows a positive correlation with the initial Ca/Si ratio [25,26,27,28]. However, Bei, Sevelsted, Black et al. argued that for C-S-H synthesized in environments with a Ca/Si ratio ranging from 0.40 to 1.75, the CO₂ uptake capacity exhibits a negative correlation with the initial Ca/Si ratio after CO₂ curing [29,30].

Existing studies primarily focus on the effects of single factors such as Ca/Si (0.40 to 1.75) or pH (11.58 to 13.40) on the micro-nano structures of C-S-H. However, the alkalinity of cement hydration is generally in the range of pH = 11 to 13, and the Ca/Si ratio is between 1.0 and 2.0. Therefore, in order to more comprehensively and systematically simulate the influence of alkalinity and Ca/Si ratio on the micro-nano structure and carbon sequestration capacity of C-S-H in the process of cement hydration, it is necessary to set a wider range of Ca/Si ratios and alkalinity gradient and consider the synergistic effect of the two on the micro-nano structure of C-S-H. Although the effects and mechanisms of the single-factor Ca/Si (0.40 to 1.75) on the carbon sequestration capacity of synthesized C-S-H have been systematically studied, the influence and mechanisms of the synergistic effect of both Ca/Si and alkalinity, especially when expanding their ranges, on the carbon sequestration capacity of C-S-H remain unclear.

Therefore, in this study, C-S-H was synthesized in alkaline solutions with Ca/Si between 1.0 and 2.3 and pH between 12.0 and 13.5. The effects of alkalinity and Ca/Si on the crystallinity and microstructure of C-S-H were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Based on the above findings, thermogravimetric analysis (TGA) and a moisture analyzer were used to measure the weight loss of CO₂-cured C-S-H and calculate its CO₂ uptake capacity. Additionally, BET and NMR were employed to characterize the pore size distribution and decalcification degree of the synthesized C-S-H, thereby revealing the mechanisms by which alkalinity and Ca/Si influence the carbon uptake capacity of CO₂-cured C-S-H. Based on the optimal CO₂ uptake capacity of synthesized C-S-H, the best synthesis conditions were selected, providing a theoretical reference for the future use of synthesized C-S-H to partially replace cement and enhance the carbon sequestration capability of cement-based materials.

2. Materials and Methods

2.1. Materials

The experimental reagents used were analytically pure CaO, SiO₂ and NaOH. The purity of CaO (Guangzhou Metallurgy Co., Ltd., Guangzhou, China) was 99.99%, with a particle size of 10 μm and a specific surface area of 0.35 m²/g. The purity of SiO₂ (Hengyuan New Materials Co., Ltd., Zhengzhou, China) was greater than 98.1%, with a particle size of 3 μm and a specific surface area of 22.1 m²/g. The purity of NaOH (Tianjin Beilian Fine Chemicals Development Co., Ltd., Tianjin, China) was 96%.

2.2. Synthesis and Mineralization of C-S-H

Considering that the synthetic C-S-H will be mixed with cement as admixture in the follow-up study, the hydrothermal synthesis method with simple synthesis method and large scale is selected. At the same time, the water bath curing temperature is set to be 80 °C to promote the better crystallization of C-S-H and accelerate the formation rate of C-S-H [31,32,33].

Table 1. Synthesis ratio and testing methods of C-S-H.

Sample No.	Ca/Si	pH
1.0W	1.0	7.2
1.0L	1.0	12
1.0H	1.0	13.5
1.6W	1.6	7.2
1.6L	1.6	12
1.6H	1.6	13.5
2.3W	2.3	7.2
2.3L	2.3	12
2.3H	2.3	13.5
1.0LC	1.0	12
1.0HC	1.0	13.5
1.6WC	1.6	7.2
1.6LC	1.6	12
2.3WC	2.3	7.2
2.3HC	2.3	13.5

Notes: 1.0, 1.6 and 2.3 in the table represent the initial Ca/Si ratio for the synthesis of C-S-H; W represents the initial pH = 7.2; L represents the initial pH = 12; H represents the initial pH = 13.5; C represents the synthetic C-S-H cured by CO₂.

presents the experimental mix ratios for synthesizing C-S-H and the testing methods for commonly used indicators. First, NaOH and ultrapure water (pH = 7.2) were used to prepare solutions with pH values of 12.0 and 13.5. Next, the CaO and SiO₂ were weighed according to molar ratios of 1.0, 1.6, and 2.3, mixed thoroughly, and then combined with NaOH solution or ultrapure water in a 1:10 ratio. Two portions of slurry for each sample were transferred into sealed polypropylene boxes and placed in a constant-temperature (80 °C) water bath for curing up to 14 days, with periodic manual shaking. After curing for the specified age, the samples were repeatedly soaked, washed, and filtered with ultrapure water. One portion of the filtrate was immersed in anhydrous ethanol for 48 h to terminate the hydration process, then dried in an oven at 45 °C until a constant weight was reached. The dried filtrate was ground to 200 mesh and sealed for storage. The other portion of the filtrate was placed in a constant-temperature (20 ± 0.3 °C), constant-humidity (75% ± 0.5%) CO₂ curing box (with CO₂ concentration of 90%) for 6 h. After CO₂ curing, the sample preparation method for the C-S-H powder before and after mineralization remained the same.

2.3. Microstructural Characterization of C-S-H

The microstructure of C-S-H synthesized under different Ca/Si ratios and alkalinities was observed using scanning electron microscopy (SEM, SU8010, HITACHI, Tokyo, Japan). Prior to testing, the powder samples were uniformly dispersed on the surface of conductive films and gold-coated. The tests were conducted at an acceleration voltage of 15 kV.

2.4. Mineral Composition Analysis

The mineral composition of the synthesized C-S-H samples was analyzed using X-ray diffraction (XRD, ADVANCE D8, Bruker, Billerica, MA, USA). XRD measurements were performed in the 2θ range of 10° to 65° with a step size of 0.02° and a counting time of 0.24 s per step.

2.5. Structural Testing of C-S-H

The silicon-oxygen tetrahedron structure in the synthesized C-S-H was tested using ²⁹Si MAS nuclear magnetic resonance (NMR) spectrometer (AVANCE III 400 MHz, Bruker, Billerica, MA, USA). The magnetic field strength was set at 9.4 T, with a single π/2 pulse width of 4.97 μs, a spinning speed of 5 kHz, a relaxation delay of 10 s, and an RF power of 50 kHz. In this process, tetramethylsilane was used as a chemical shift reference. The relative proportion of Qⁿ was calculated through deconvolution of the Gaussian peak distribution spectrum. Based on this, the mean chain length (MCL) of the silicate chain was calculated using Equation (4) [34]. Q represents the silicon-oxygen tetrahedron in C-S-H, and n refers to the number of oxygen atoms attached to adjacent silicon-oxygen tetrahedron, ranging from 0 to 4.

$$\mathrm{MCL}={\displaystyle \frac{2({\mathrm{Q}}^1+{\mathrm{Q}}_{\mathrm{b}}^2+{\mathrm{Q}}_{\mathrm{p}}^2+{\mathrm{Q}}^3+{\mathrm{Q}}^4)}{{\mathrm{Q}}^1}}$$

(4)

2.6. Pore Size Distribution Testing of Synthesized C-S-H

A multipoint nitrogen physical adsorption device (ASAP 2460, Mack Instruments Inc., Westford, MA, USA) was used to measure the specific surface area and pore size distribution of the synthesized C-S-H samples. The samples were degassed at 40 °C for 24 h to remove physically adsorbed gases, followed by testing. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was calculated using the Barrett–Joyner–Halenda ‌(BJH) method.

2.7. Calculation of CO₂ Uptake

The CO₂ uptake during the curing of C-S-H was tested using a thermogravimetric-differential thermal analyzer (STA7300, HITACHI, Tokyo, Japan). The thermogravimetric test was conducted in an N₂ atmosphere, with a temperature ranging from 20 °C to 1000 °C and a heating rate of 10 °C/min. According to previous studies, the decomposition temperature of aragonite, vaterite, and amorphous CaCO₃ is about 500–750 °C [35,36], and the decomposition temperature of stable calcite is more than 750 °C [37]. The decomposition temperature of CH is about 420–510 °C [38]. In order to accurately calculate the mass of CaCO₃ and CH, the TG/DTG curve was tangent and extended by the epitaxial method [39]. The mass of CaCO₃ and CH was obtained by the difference between the beginning and the end of the decomposition of CaCO₃ and CH. The thermogravimetric curve and its first-order differential curve were used to calculate the mass of CaCO₃, as shown in Equation (5).

$${\mathrm{M}}_{\mathrm{CC}}(\mathrm{g}/100\mathrm{g})=100\cdot {\displaystyle \frac{100}{44}}({\mathrm{m}}_{\mathrm{CC}}^{\mathrm{start}}-{\mathrm{m}}_{\mathrm{CC}}^{\mathrm{end}})$$

(5)

M_CC represents the mass of CaCO₃ in the CO₂-cured synthesized C-S-H sample (g/100 g). $m_{CC}^{start}$ and $m_{CC}^{end}$ correspond to the masses at start and end points of CaCO₃ decomposition on the thermogravimetric curve, as illustrated in Figure 1.

Figure 2 shows the mineral composition of the synthesized C-S-H samples. From Figure 2, it can be observed that not all of the CaO and SiO₂ reacted to form C-S-H, with some CH crystals precipitating. This is mainly because when CaO and SiO₂ are used as raw materials for the synthesis of C-S-H, with a Ca/Si ratio greater than 1.5, CaO in the synthesized C-S-H sample does not react completely due to excess, thus existing in the state of CH. With the increase in the initial Ca/Si ratio, the content of CH in the synthesized C-S-H gradually increases [22]. Therefore, the CO₂ uptake of synthesized C-S-H (PDF: 00-034-002) should exclude the CO₂ uptake by CH (PDF: 96-100-8782) and include the CO₂ uptake corresponding to the residual CO₃²⁻ in the sample, as outlined in Equations (6)–(8).

$${\mathrm{M}}_{\mathrm{CH}}^{\mathrm{C}{\mathrm{O}}_2}(\mathrm{g}/100\mathrm{g})={\displaystyle \frac{44}{74}}\cdot 100\cdot {\displaystyle \frac{74}{18}}({\mathrm{m}}_{\mathrm{CH}}^{\mathrm{start}}-{\mathrm{m}}_{\mathrm{CH}}^{\mathrm{end}})$$

(6)

$M_{CH}^{C{O}_2}$ represents the CO₂ uptake of CH in the synthesized C-S-H sample. As shown in Figure 1, $m_{CH}^{start}$ and $m_{CH}^{end}$ correspond to the masses at start and end points of CH decomposition on the thermogravimetric curve.

The CO₃²⁻ solution was obtained through a solid–liquid extraction method. First, the sealed C-S-H samples were prepared by mixing with ultrapure water at a solid-to-liquid ratio of 1:15. The mixture was then stirred for 30 min at 350 rpm using a magnetic stirrer, followed by 2 h of standing, with the cup sealed using plastic wrap to prevent carbonation from air exposure. Finally, the concentration of CO₃²⁻ in the supernatant was measured using a moisture analyzer (DV1005, METTLER TOLEDO, Zurich, Switzerland).

The mass of CO₃²⁻ was calculated using Equation (7):

$${\mathrm{M}}_{\mathrm{C}{\mathrm{O}}_3^{2-}}(\mathrm{g}/100\mathrm{g})=\mathrm{C}\cdot {\displaystyle \frac{\mathrm{V}}{{\mathrm{M}}_{\mathrm{A}}}}$$

(7)

$M_{C{O}_3^{2-}}$ represents the mass (g) of CO₃²⁻ in the synthesized C-S-H sample, C represents the measured concentration (mg/mL) of CO₃²⁻, V is the volume of the C-S-H extract (mL), and M_A is the mass (g) of the synthesized C-S-H sample used in the test.

The CO₂ uptake in the C-S-H samples was calculated according to Equation (8):

$${\mathrm{M}}_{\mathrm{C}{\mathrm{O}}_2}^{\text{netC-S-H}}(\mathrm{g}/100\mathrm{g})={\mathrm{M}}_{\mathrm{C}{\mathrm{O}}_3^{2-}}\cdot {\displaystyle \frac{44}{60}}+{\mathrm{M}}_{\mathrm{CC}}\cdot {\displaystyle \frac{44}{100}}-{\mathrm{M}}_{\mathrm{CH}}^{\mathrm{C}{\mathrm{O}}_2}$$

(8)

$M_{C{O}_2}^{\mathit{\text{netC-S-H}}}$ represents the net mass of CO₂ absorbed by C-S-H.

3. Results

3.1. Microstructure of Synthesized C-S-H

Figure 3 shows the microstructure of the synthesized C-S-H. As can be seen from Figure 3a–c, when the Ca/Si ratio is 1.0 and the pH ranges from 7.2 to 12.0, the C-S-H is flocculent; at pH 13.5, the C-S-H evolves from a flocculent structure to a flaky structure. Previous studies have found that when the Ca/Si ratio is between 1.0 and 1.2 and the pH exceeds 13.0, C-S-H also crystallizes into a flaky form, which is consistent with the findings of this study. The difference is that when the pH ranges from 10.0 to 12.0, C-S-H exhibits a spherical shape [13,40,41]. This may be due to the differences in materials and synthesis methods for the synthesis of C-S-H. Figure 3d–f shows that when the Ca/Si ratio is 1.6, as the pH increases from 7.2 to 12.0, the C-S-H evolves from a honeycomb structure to a fibrous structure. When the pH increases to 13.5, the C-S-H becomes porous. For the Ca/Si ratio of 2.3, as the pH increases from 7.2 to 13.5, the C-S-H gradually evolves from a needle-like structure to a porous one, as shown in Figure 3g–i. Overall, as the Ca/Si ratio approaches 1.0 and the pH approaches 12.0, the morphology of C-S-H tends to become more flocculent. In contrast, as the Ca/Si ratio approaches 1.6 to 2.3 and the pH approaches 13.5, the morphology of C-S-H tends to become more porous. This is primarily due to the varying effects of alkalinity and Ca/Si on the crystallinity, silicon-oxygen tetrahedron bonding, and polymerization degree of C-S-H, which leads to significant morphological differences under different alkalinities and Ca/Si ratios.

3.2. CO₂ Uptake by Synthesized C-S-H

Figure 4 shows the CO₂ uptake of synthesized C-S-H samples cured in CO₂. As seen from Figure 4, when the pH is not greater than 12.0, the CO₂ uptake of the synthesized C-S-H and CH shows a positive correlation with the initial Ca/Si ratio. Although the presence of CH in the sample interferes with the net CO₂ uptake by C-S-H and the pH remains the same, the net CO₂ uptake still shows a positive correlation with the Ca/Si ratio. These findings are consistent with previous studies, which found that under ultrapure water conditions, the CO₂ uptake of synthesized C-S-H with a Ca/Si ratio of 0.2 to 1.5 increases with the Ca/Si ratio [26,28,42]. It shows that under the condition of low alkalinity, the CO₂ uptake of synthetic C-S-H and the higher initial Ca/Si ratio can provide sufficient calcium source for the carbonation reaction, so that the amount of CaCO₃ produced by the synthesis of C-S-H is greatly increased. In contrast, when the pH is 13.5, the CO₂ uptake and the net uptake of synthesized C-S-H show a negative correlation with Ca/Si ratios. Studies by Chang and Sevelsted found that when the pH is greater than 13.0, the CO₂ uptake of synthesized C-S-H increases with the Ca/Si ratio [14,29], indicating that the initial Ca/Si ratio and alkalinity are not the sole determinants of CO₂ uptake by C-S-H. The CO₂ uptake of C-S-H is also related to its own micro-nano structural characteristics.

4. Discussion

4.1. Effect of Alkalinity and Ca/Si on the Crystallinity and Polymerization Degree of C-S-H

Figure 5 is the crystallization index of the synthesized C-S-H phase on the 110 crystal plane in the XRD pattern. It can be seen from Figure 5 that when Ca/Si ratios are the same, increasing the alkalinity decreases the crystallinity of the synthesized C-S-H. This may be due to the higher OH⁻ concentration, which reduces the degree of crosslinking between [SiO₄]⁴⁻ [43,44], leading to the lower crystallinity of 1.0H and 2.3H. At the same time, the higher OH⁻ concentration also promoted the combination of calcium ions and [SiO₄]⁴⁻ in the solution, resulting in a significant decrease in the CH peak intensity with the increase in alkalinity in Figure 2. But the correlation between the crystallinity of C-S-H and its morphology has not been extensively reported in the literature. However, as shown in Figure 3 of this study, the crystallinity of the C-S-H synthesized under the Ca/Si ratios of 1.0 and 2.3, and a pH of 13.5, is found to be the lowest, corresponding to a relatively loose, flaky, and porous morphology. In order to reveal the effect mechanism of alkalinity and Ca/Si ratio on the crystallinity and microstructure of synthesized C-S-H, the C-S-H structure obtained from the ²⁹Si nuclear magnetic resonance (NMR) spectrometer was used for interpretation.

Figure 6 presents the ²⁹Si NMR spectra of C-S-H synthesized under different Ca/Si ratios and alkalinity conditions,

Table 2. Proportions of Qⁿ in C-S-H samples.

Sample No.	${\mathbf{Q}}^1$		${\mathbf{Q}}_{\mathbf{b}}^2$		${\mathbf{Q}}_{\mathbf{p}}^2$		${\mathbf{Q}}^3$		${\mathbf{Q}}^4$		MCL
	(ppm)	(%)	(ppm)	(%)	(ppm)	(%)	(ppm)	(%)	(ppm)	(%)
1.0 H	−78.8	40.2	−82.3	24.6	−84.5	35.2	/		/		5.0
2.3 H	−78.4	59.0	−81.8	14.2	−84.7	26.7	/		/		3.4
2.3 W	−79.2	50.2	−82.1	9.3	−84.9	40.5	/		/		4.0

shows the proportion of silicon-oxygen tetrahedron and the mean chain length, and Figure 7 illustrates the schematic diagram of the effect of alkalinity and Ca/Si ratio on the silicon-oxygen tetrahedral structure of C-S-H. According to Table 2 and Figure 6a,b, it is evident that, when the pH is 13.5 and the Ca/Si ratio increases from 1.0 to 2.3, the quantity ratio of Q¹ increases from 40.2% to 59.0%, and the MCL (mean chain length) decreases from 5.0 to 3.4. As shown in Figure 7, this is mainly due to the fact that, as the Ca/Si ratio increases from 1.0 to 2.3 under the same alkalinity conditions, the large amount of Ca²⁺ in the solution, carrying two units of positive charge, becomes more likely to bind with [SiO₄]⁴⁻, preventing significant crosslinking between [SiO₄]⁴⁻ during the formation.

As can be seen from Figure 6a,b and Table 2, when the Ca/Si ratio is 2.3 and the pH increases from 7.2 to 13.5, the number of [SiO₄]⁴⁻ monomers gradually increases, and the mean chain length gradually shortens, indicating the weakening of C-S-H polymerization. This is mainly due to the increase in OH⁻ concentration, which causes [SiO₄]⁴⁻ in the formation process to evolve from a protonated state (>Si−OH₂)⁺ to a neutral (>Si−OH)⁻, and finally to a deprotonated state (>Si−O−). As a result, there are more negative charges on C-S-H at higher pH values, and the charge repulsion between [SiO₄]⁴⁻ makes crosslinking between silicate chains more difficult [45] (Figure 7—Deprotonation process before chain formation), while the exposed negative charge sites on [SiO₄]⁴⁻ attract Ca²⁺ and bind with it. Moreover, as the OH⁻ concentration increases, OH⁻ will also react with groups in the already crosslinked chains, leading to pentacoordinate bonding of silicon atoms with OH⁻, and other OH⁻ will pull oxygen atoms from opposite directions in the Si−O−Si bond, causing the bond to dissociate as shown in the reaction of Equation (9) [46,47,48], thereby leading to the depolymerization of the silicon chains (Figure 7—Process of breaking silicate chains by OH⁻ attacks after chain formation). This leads to the manifestation of more Q¹ [SiO₄]⁴⁻ at higher pH levels. The increase in alkalinity and Ca/Si ratio causes the Q² groups in C-S-H to gradually depolymerize and convert into Q¹ groups (as shown in Figure 6 and Table 2), significantly decreasing the degree of polymerization and gradually shortening the silicon chains. The shorter chains prevent C-S-H from forming larger crystals [19], leading to a transformation in the morphology of C-S-H from a more compact flocculent form to a larger structure with pores (as shown in Figure 3 and Figure 8).

$$\mathrm{S}{\mathrm{i}}_2{\mathrm{O}}_7^{6-}+{\mathrm{OH}}^{-}\to \mathrm{Si}{\mathrm{O}}_4{\mathrm{H}}^{3-}+\mathrm{Si}{\mathrm{O}}_4^{4-}$$

(9)

4.2. Effect of Pore Structure of Synthesized C-S-H on CO₂ Uptake

Figure 8 shows the correlation between the specific surface area and pore volume of the synthesized C-S-H samples. As shown in Figure 8, the specific surface area and pore volume of the synthesized C-S-H samples follow a linear relationship. This suggests that the external surface area of synthesized C-S-H is relatively small, and most of the exposed surface area consists of internal pore surfaces. Therefore, the structure of synthesized C-S-H is loose and porous with a high proportion of pores. These finding aligns with the results described by Li [49], who characterized C-S-H as having a layered and porous structure. The diffusion and uptake of CO₂ within synthesized C-S-H primarily depend on the pore size. As shown in Figure 9, the average pore diameter of synthesized C-S-H increases with increasing alkalinity and Ca/Si ratio, which is similar to the findings of Chang J et al. [14], where an increase in the initial Ca/Si ratio (ranging from 0.56 to 1.5) led to an expansion in the pore diameter of C-S-H. This is mainly due to the fact that an increase in Ca/Si ratio and alkalinity results in a decrease in the crystallinity of C-S-H (as shown in Figure 5). The reduced crystallinity causes the internal pore structure of C-S-H to be larger, leading to significant differences in pore diameter under different alkalinities and Ca/Si ratios.

Figure 8. Linear fitting results of the specific surface area and pore volume of synthesized C-S-H.

Figure 8. Linear fitting results of the specific surface area and pore volume of synthesized C-S-H.

In addition, in Figure 9, the average pore diameter and the CO₂ uptake of the synthesized C-S-H are compared, and it is found that the trend correlation between the CO₂ uptake of the synthesized C-S-H and the average pore diameter is strong. In order to further clearly show the relationship between the average pore diameter and the CO₂ uptake of the synthesized C-S-H, the average pore diameter and the CO₂ uptake test results are linearly fitted (as shown in Figure 10). It can be seen from Figure 10 that the average pore diameter of the synthesized C-S-H is positively correlated with the CO₂ uptake, and the linear fitting result is good (R square is 0.92393), indicating that the diffusion degree and uptake amount of CO₂ in the synthesized C-S-H mainly depend on the pore diameter. Therefore, when the alkalinity is between 7.2 and 12.0, the average pore diameter of the synthesized C-S-H increases with the increasing Ca/Si ratio, which leads to an increasing trend in CO₂ uptake. When the alkalinity is 13.5, the average pore diameter of the synthesized C-S-H decreases with the increase in Ca/Si ratio. At the same time, the CO₂ uptake of the synthesized C-S-H decreases with the decrease in the average pore diameter, indicating that under the condition of high alkalinity, the CO₂ uptake of the synthesized C-S-H is less affected by the initial Ca/Si ratio, which mainly depends on the average pore diameter. The larger average pore diameter can promote the diffusion of CO₂ in the synthesized C-S-H, thereby improving the degree of carbonization reaction.

4.3. Effect of C-S-H Nanostructural Characteristics on CO₂ Uptake

A comparative discussion on the nano-structural characteristics of C-S-H in 1.0H and 2.3H before and after CO₂ curing was conducted since 1.0H and 2.3H samples showed higher CO₂ uptake after CO₂ curing. The NMR results are shown in Figure 11 and

Table 3. Qⁿ proportions of C-S-H samples after CO₂ curing.

Sample No.	${\mathbf{Q}}^1$		${\mathbf{Q}}_{\mathbf{b}}^2$		${\mathbf{Q}}_{\mathbf{p}}^2$		${\mathbf{Q}}^3$		${\mathbf{Q}}^4$		MCL
	(ppm)	(%)	(ppm)	(%)	(ppm)	(%)	(ppm)	(%)	(ppm)	(%)
1.0 H	−78.8	40.2	−82.3	24.6	−84.5	35.2	/		/		5.0
2.3 H	−78.4	59.0	−81.8	14.2	−84.7	26.7	/		/		3.4
1.0 HC	−79.1	1.5	−82.0	0.8	−85.0	18.9	−99.0	55.2	−109.7	23.6	135.2
2.3 HC	−79.3	7.8	−82.7	0.8	−84.9	37.4	−97.4	42.6	−108.9	11.4	25.5

. Compared to the results before CO₂ curing, the proportion of Q¹ groups in 1.0H and 2.3H samples decreased by 38.7% and 51.2%, respectively, while the proportion of Q² groups decreased by 40.1% and 2.7%, respectively. With reference to the schematic diagram of the demineralization process of silicon chains in C-S-H during CO₂ curing (Figure 12), CO₃²⁻ ions compete with Ca²⁺ for uptake after they enter the interlayer gaps of C-S-H, leading to carbonation reactions on some Q¹ and Q² groups, thereby forming CaCO₃. As shown in Figure 11 and Table 3, the conversion rates from Q¹ and Q² to Q³ and Q⁴ are higher in 1.0H than those in 2.3H, indicating that under the same CO₂ curing conditions, 1.0H is more prone to demineralization than 2.3H. This explains why the CO₂ uptake of 1.0H is higher than that of 2.3H in Figure 10, and further confirms that under 6 h of CO₂ curing at 99% concentration, the CO₂ uptake of 1.0H is stronger than that of 2.3H according to the demineralization degree of synthesized C-S-H.

In summary, alkalinity significantly affects the morphology and micro-nano structural characteristics of synthesized C-S-H. Under high alkalinity conditions, the depolymerization and difficulty in crosslinking of silicon chains reduce the polymerization degree of C-S-H, resulting in the gradual expansion of its pore structure. Additionally, from the perspective of the demineralization degree of synthesized C-S-H and the amount of carbonation products, it is shown that after CO₂ curing at a concentration of 99% for 6 h under high-alkalinity conditions, the CO₂ uptake of synthesized C-S-H does not increase with the increasing Ca/Si ratio. Instead, it depends on the internal pore structure of the synthesized C-S-H.

5. Conclusions

In this study, methods such as SEM, XRD, BET, TGA, and NMR were used to investigate the effects and mechanisms of the synergistic effect of Ca/Si (1.0 to 2.3) and alkalinity (7.2 to 13.5) on the morphology, structure, and CO₂ uptake of synthesized C-S-H. Key conclusions are as follows:

• C-S-H synthesized under conditions of pH 7.2 to 12.0 and Ca/Si ratios of 1.0 to 2.3 are flocculent, fibrous, and needle-like structures with high crystallinity and mainly linked by Q¹. At the same pH, increasing the Ca/Si ratio leads to an increase in the proportion of Q¹. The C-S-H synthesized under conditions of pH 12.0 to 13.5 and Ca/Si ratios of 1.0 to 2.3 is closer to a poor crystallinity structure with a predominant Q¹ group when the Ca/Si ratio approaches 1.6 to 2.3 and the pH approaches 13.5, resembling a short-chain structure of silicon-oxygen tetrahedron and exhibiting a porous structure. This is primarily attributed to the tendency of OH⁻ ions to protonate the silanol groups (>Si–OH), which facilitates the uptake of Ca²⁺ ions. This, in turn, weakens the extent of crosslinking between Si–OCa+ in Q¹ and thus reduces the formation of Q².
• Under the pH ranging from 7.2 to 13.5 and Ca/Si ratio from 1.0 to 2.3, the C-S-H synthesized after 6 h of CO₂ curing shows that an increase in Ca/Si ratio results in higher CO₂ uptake by the C-S-H when the pH is between 7.2 and 12.0. However, once the pH reaches 13.5, there is no positive correlation between the CO₂ uptake and Ca/Si ratio. Instead, C-S-H synthesized at a Ca/Si ratio of 1.0 exhibits the highest CO₂ uptake (18.393 g/100 g). This can be attributed to two factors: first, the average pore diameter of C-S-H synthesized at high pH (13.5) is larger; second, the silicon-oxygen tetrahedral chains primarily consisting of Q² units are more likely to release Ca²⁺ and participate in carbonation reactions.

The research results can provide a theoretical reference for optimizing the C-S-H seed crystal with excellent carbon fixation ability as an admixture to greatly improve the carbon fixation ability of cement-based materials while reducing the energy consumption and CO₂ emission of cement production.