Introduction

In recent years, rapid industrialization has led to climate change, contributing to an increase in unexpected natural disasters worldwide. Among greenhouse gases, carbon dioxide (CO2) is a primary concern, with its atmospheric concentration nearing 400 ppm and an annual growth rate of approximately 2 ppm since 2000 [1]. The 2015 Paris Agreement underscored the urgency of addressing climate change, setting a global target to limit the average temperature rise to well below 2 °C above pre-industrial levels, and aiming for a cap of 1.5 °C. In response, achieving carbon neutrality has emerged as a universal objective, with nations and organizations around the globe, including the South Korean government, committing to carbon neutrality by 2050 [2].

One of the strategies to mitigate CO2 emissions involves post-treatment, its capture and conversion into valuable products, such as synthetic fuels [3], chemicals [4] and polymers [5] , or into thermodynamically stable forms like carbonate minerals for long-term sequestration [6]. Among these methods, mineral carbonization, which converts CO2 into solid minerals, stands out for its simplicity, stability, and the widespread availability of necessary minerals. This process involves the reaction of alkaline earth metal ions, such as calcium (Ca) and magnesium (Mg), with CO2 gas to form stable compounds like CaCO3 or MgCO3. Given the abundant presence of alkaline earth metals in industrial waste materials—including waste concrete [7, 8], steelmaking slags [9], fly ash [10], incineration ash [11] and cement kiln dust [12]—there has been significant interest in utilizing these materials for CO2 capture through mineral carbonization.

As of September 2023, 445,652 waste railway concrete ties have been left untreated and abandoned in South Korea. The disposal of these used railway ties poses a critical environmental challenge, necessitating efficient waste management strategies to address the disposal of this vast quantity of spent railway ties. Interestingly, these waste railway concrete ties contain approximately 10% alkaline earth metal ions, making them potential candidates for CO2 capture through mineral carbonation. Our prior research has demonstrated that, although railway ties have a lower content of Ca and Mg compared to regular concrete, they exhibit a CO2 capture capacity (4.31% CO2 loading ability) comparable to conventional concrete waste [13]. This is attributed to a high proportion of uncarbonized alkaline earth metals, underscoring their potential as a material for CO2 capture.

This study aims to augment the CO2 capture efficiency of waste concrete railway ties by treating them with a CaCl2 solution. Incorporating Ca or Mg sources into materials to enhance their CO2 capturing capabilities is a widely recognized method, typically involving the use of CaO or MgO) [14, 15]. However, the conventional production of CaO through the calcination of natural limestone inevitably releases a substantial amount of CO2 [16]. In contrast, CaCl2 can be sourced from seawater or brines, which are abundant natural mineral sources that do not contribute to CO2 production. Therefore, we explored a novel modification technique to improve the CO2 capturing ability of spent railway tie concretes, leveraging the environmental benefits of using CaCl2 derived from natural sources.

Initially, we developed a cube-shaped CO2 capturing material from waste railway tie concrete (RTC) and subsequently modified it with varying concentrations of CaCl2 solution to produce Ca-RTCs. We examined the characteristics of both RTC and Ca-RTCs to understand the modifications induced by the addition of CaCl2. Furthermore, the CO2 capturing performance of Ca-RTCs was assessed based on the varying amounts of CaCl2 incorporated. We also conducted studies on the kinetics of CO2 capture and the impact of CO2 pressure on the capturing efficiency of Ca-RTC, demonstrating the potential of Ca-RTCs as effective CO2 capturing materials. Finally, we explored the changes in the physicochemical properties of Ca-RTCs post-CO2 capture to elucidate the mechanism of CO2 capture by Ca-RTCs.

Materials and Methods

Synthetic Procedure of RTC and Ca-RTC

Railroad tie concrete sourced from Gyeonggi, South Korea, was utilized as the primary material for this study. This concrete was processed into a fine powder by ball milling with tungsten carbide (WC) balls in a stainless steel (SUS) milling jar, operating at 250 rpm for a duration of 12 h. A paste was then produced from the powdered railroad tie concrete by adding a calcium chloride (CaCl2, 97% purity, Sigma–Aldrich) solution in concentrations ranging from 0.0001 to 0.5 M, maintaining a waste-to-solution ratio of 0.45 (w/w). Specifically, 9 mL of the desired concentration of CaCl2 solution was mixed with 20 g of railroad tie concrete powder by hand mixing. The resultant slurry was injected into a 1 cm3 cubic mold using a 10 mL syringe and dried at 60 °C for 3 days in an oven, resulting in a material designated as Ca-RTC. Ten Ca-RTC cubes were prepared from the slurry. For the synthesis of Ca-RTC, a relatively high temperature of 60 °C was employed to expedite the synthesis of the CO2 capturing material. Despite this elevated temperature, the carbon (C) content in Ca-RTC was only 1.49%, indicating that the majority of the Ca (10.1%) remained in an uncarbonated form. The added amounts of Ca and Cl (mmol per Ca-RTC cube) are described in table s1.

For comparative purposes, CO2 capturing materials were also prepared using deionized water (DI water) and a 0.01 M sodium chloride (NaCl, 99% purity, SAMCHUN) solution, labeled as RTC and Na-RTC, respectively. These materials underwent a CO2 capture experiment following a 3-day hydration process. The preparation process for the CO2 capturing materials is detailed in Fig. 1.

Fig. 1
figure 1

Schematic draw of Ca-RTC preparation procedure

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Characterization of the Ca-RTC Before and After CO2 Capture

The crystal structures of RTC and Ca-RTC were analyzed using X-ray Diffractometry (XRD, Rigaku, Smartlab). X-ray Fluorescence (XRF, Rigaku, ZSX Primus-II) was employed to investigate their elemental composition and content. The surface area and pore size distribution were measured using Brunauer–Emmett–Teller analysis (BET, Micromeritics, ASAP2020). The BET model is used for determining surface area of Ca-RTC, and BJH (Barrett–Joyner–Halenda) model was used for analysis of pore size distribution. Field Emission Scanning Electron Microscopy (FESEM, Zeiss, Ultra Plus) was used to observe the surface morphology of RTC and Ca-RTC, both before and after the CO2 capture reaction. Additionally, the elemental distribution of Ca-RTC before and after the CO2 capture reaction was analyzed by SEM equipped with an Energy Dispersive X-ray Spectrometer (EDAX, Zeiss, Ultim Extreme). The zeta potential of Ca-RTC was measured using a zeta analyzer (Otsuka, ELS-Z) after grinding it into a powder. The alkalinity of Ca-RTC was determined using the following procedure: First, 5 g of ground Ca-RTC were thoroughly mixed with 50 ml of deionized water for 5 min using a vortex mixer. The mixture was then allowed to settle for 20 min. Finally, the pH of the supernatant was measured with a pH meter [17]. Phenolphthalein analysis was conducted to observe reaction kinetics after CO2 capture; a 1% Phenolphthalein solution (1% Purity, Daejung) was sprayed on freshly cut surfaces, indicating carbonated regions as colorless (pH < 9.2) and non-carbonated regions as purple (pH > 9.2) [18].

CO2 Capturing Experiments

To evaluate CO2 capturing abilities of prepared material, one piece (1 cm3) of CO2 capturing material was put it to the CO2 capturing reaction chamber (RAMT, maximum pressure 1 bar, diameter = 22 cm, depth = 50 cm) and evacuation to − 0.5 bar using vacuum pump to remove unwanted molecule bound on CO2 capturing materials. After 30 min evacuation, a 99.5% CO2 gas (99.5%, Sejong Industry Gas) atmosphere at a pressure of 0.5 bar for 7 days. To evaluate amount of captured CO2 on Ca-RTC, thermogravimetric Analysis (TGA, Scinco M&T, TGA N1000) was carried out. The weight change of Ca-RTC was monitored from 25 to 900 °C at a rate of 10 °C/min. Considering the decomposition of CaCO3 between 600 and 900 °C, the formula for calculating the CO2 absorption capacity is as follows [19]:

$$ {\text{CaCO}}_{3} \;{\text{formed}} {\text{during}}\;{\text{CO}}_{2} \;{\text{capture}}\;{\text{reaction}}\left( \% \right) = \frac{{\Delta w_{1} - \Delta w_{2} }}{{{\text{Weight of 900 }}\; \circ {\text{C}}}} \times \frac{100}{{43.66}}, $$
(1)

where Δw1 and Δw2 (g) is the weight difference of Ca-RTC at 25 and 900 ℃, after and before CO2 capturing reaction, respectively. We analyze the TGA of pure CaCO3 to confirm the reliability of CaCO3 decomposition at 900 ℃ and it is discovered that not all CaCO3 was decomposed at 900 ℃ and only 66.3% of weight loss occurred (Fig. S1, see supporting information). Considering this 100/43.66 was multiplied to compensate the actual CaCO3 (%). The CO2 loading on Ca-RTC was calculated by following equation, by considering molecular weight of CO2 (44.01 g/mol) and CaCO3(100.1 g/mol), respectively.

$$ {\text{CO}}_{{2}} {\text{ loading}} \left( \% \right) = {\text{CaCO}}_{3} \times \frac{44.01}{{100.1}}. $$
(2)

Results and Discussion

Characterization of RTC and Ca-RTC

Table 1 outlines the compositions of RTC and Ca-RTC. Both materials predominantly consist of silicon (Si), a fundamental component of concrete, with Si compositions of 26.6% and 25.3%, respectively. Following Si, calcium (Ca) is the next most abundant element, playing a crucial role in CO2 capture reactions, with both RTC and Ca-RTC containing approximately 10% Ca. The inclusion of CaCl2 solution marginally increased the Ca content in the CO2 capturing materials from 9.82 to 10.10%.

Table 1 Compositions of CO2 adsorbed prepared by RTC, Ca-RTC
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The carbon (C) content in both RTC and Ca-RTC was relatively low, recorded at 0.99% and 1.49% respectively, indicating a minimal presence of CO2 in their original chemical structures. This suggests that a significant portion of Ca and Mg within the materials remains uncarbonated and available for CO2 capture reactions. Additionally, other elements such as aluminum (Al), sodium (Na), and iron (Fe) experienced negligible changes, within 0.28%, even after the CaCl2 treatment. Based on the amounts of Ca and Mg, we can estimate the theoretical CO2 capturing ability of RTC and Ca-RTC. Alkaline earth metals such as Mg and Ca form insoluble mineral carbonates by reacting with CO2 through the following equations:

$$ {\text{Ca}}^{2 + } + {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}} \to {\text{CaCO}}_{3} \downarrow , $$
$$ {\text{Mg}}^{2 + } + {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}} \to {\text{MgCO}}_{3} \downarrow . $$

Considering that 1 mol of Ca or Mg reacts with 1 mol of CO2, the theoretical CO2 capturing ability of Ca-RTC is estimated to be 11.66%.

Figure 2a shows the X-ray diffractometry (XRD) results for RTC and Ca-RTC. Both materials displayed a dominant peak for SiO2, confirming the predominance of Si in the form of SiO2 crystals. Despite the low intensity of the CaCO3 peak, it verified the presence of trace crystalline CaCO3. The XRD patterns for RTC and Ca-RTC were similar, indicating that Ca addition did not alter the crystalline structure of the CO2 capture material. However, differences in morphology between RTC and Ca-RTC were evident in their scanning electron microscopy (SEM) images (Fig. 2e, f). The SEM image for RTC showed relatively smooth surfaces, whereas Ca-RTC exhibited large pores ranging from 10 to 100 μm and small aggregates, affecting the specific surface area as demonstrated in the nitrogen adsorption–desorption isotherms in Fig. 2b. Compared with raw railway tie concrete (before crushing), the crushing and reconstruction of Ca-RTC (or RTC) into a 1 cm3 cube structure significantly affects its morphology. The raw railway tie concrete exhibits a flat and non-porous surface, which hinders the reaction of alkaline earth metals (Ca or Mg) in concrete waste and results in relatively prolonged carbonation kinetics (Fig. S2). In contrast, both RTC and Ca-RTC exhibit hysteresis loops indicative of mesoporous materials. Notably, Ca-RTC has a higher specific surface area of 6.29 m2/g, approximately 1.52 m2/g greater than that of RTC (4.77 m2/g). The pore structure analysis indicated a reduction. In total pore volume but an Increase In average pore size from 36.9 to 42.7 nm after Ca addition, suggesting that the surface area increase is attributed to the formation of numerous particles rather than an increase in mesopore size (Fig. 2c).

Fig. 2
figure 2

XRD patterns (a), N2 adsorption–desorption isotherm (b), pore size distribution (c), zeta potential (d) of RTC and Ca-RTC, and SEM images of RTC (e) and Ca-RTC (f)

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The formation of these pores likely occurs during Ca-RTC fabrication, where Ca is introduced as a CaCl2 solution. This process involves the hydration and ionization of CaCl2, producing Ca2⁺ and Cl− ions that adhere to the RTC particle surfaces, making them more hydrophilic. Consequently, more H2O molecules are attracted to the surface of Ca-RTC due to its increased hydrophilicity compared to RTC. Upon oven drying, the water is removed, resulting in the formation of smaller aggregates on the surface of Ca-RTC than on RTC. This expanded surface area is anticipated to enhance the CO2 capturing capability of the material. We also analyzed the zeta potential of Ca-RTC as a function of solution pH (Fig. 2d). At pH levels between 2 and 4, Ca-RTC exhibited a slightly positive charge of approximately 5 mV. However, as the solution pH increased, the zeta potential decreased significantly, becoming negative. At pH 8, a markedly negative zeta potential of − 30 mV was observed. Considering that the pH of hydration water ranges from 5 to 6, Ca-RTC exhibited a negative zeta potential of − 20 mV within this range. During hydration, fewer Ca2⁺ ions are released from the Ca-RTC surface due to electrostatic interactions compared to Cl− ions. Additionally, we measured the alkalinity of Ca-RTC, which was found to be 12.87, indicating significant potential as a CO2 capturing material.

CO 2 Capturing Behavior by Ca-RTC

To investigate the effect of CaCl2 concentration on CO2 capture, experiments were conducted with Ca-RTCs prepared at various CaCl2 concentrations (Fig. 3a). After seven days of CO2 capture experiments, the pristine RTC showed a CO2 loading capacity of 4.35%. By adding 0.0001–0.5 M CaCl2 solution to preparation of Ca-RTC, the CO2 capturing ability is significantly enhanced. By increasing CaCl2 concentration from 0.0001 to 0.001 M, the CO2 capturing ability is proportionally increased and Ca-RTC with 0.001 M CaCl2 concentration shows 11.57%, which more than doubles the capacity compared to the untreated RTC. However, further increase of CaCl2 concentration caused decrease of CO2 capturing ability. Even though Ca content in Ca-RTC is increased by increasing CaCl2 concentration, the highest CO2 capturing performance was obtained with not 0.5 M CaCl2 concentration, but 0.001 M of lower CaCl2 concentration. If the enhancement were solely due to the Ca added in the Ca-RTCs, the CO2 loading capacity should increase proportionally with CaCl2 concentration. However, the 0.001 M CaCl2 solution, which is the middle range of CaCl2 concentration resulted in the highest CO2 loading capacity. Additionally, the Ca content in the Ca-RTC prepared with the 0.001 M CaCl2 solution is 10.1%, which is only a 0.28% increase compared to RTC. This result indicates that the extra Ca supplied by the CaCl2 solution is not the reason for the enhanced CO2 capturing ability. It is expected that the increase in CO2 capturing ability is due to the increased BET surface area, as discussed in Sect. 1.

Fig. 3
figure 3

CO2 capturing abilities (a) and Ca content (%) (b) of Ca-RTCs by CaCl2 solution concentration

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To further validate this point, Na-modified RTC (Na-RTC) was produced by adding Na ions, which do not participate in the mineral carbonation reaction, and its CO2 capture performance was evaluated (Fig. 3a). Na-RTC exhibited a CO2 loading capacity of 7.73%, which is 3.38% higher than that of RTC even though its alkaline earth metal (Ca and Mg) content is similar to pristine RTC. This result clearly proved that the enhanced CO2 loading capacity of Ca-RTC was not solely caused by addition of Ca into the material. Moreover, like Ca-RTC, Na-RTC showed an increased specific surface area of 5.78 m2/g compared to RTC, and its surface morphology became similar to that of Ca-RTC (refer to Fig. S3). The increase in CO2 capturing capacity due to the addition of Na ions is attributed to the expanded surface area, as Na ions do not participate in the CO2 loading reaction. In RTC, the Ca ions originally present were not completely reactive with CO2 within 7 days due to relatively low porosity and blocking. However, the expansion of surface area resulting from the addition of Na ions facilitates the reaction between Ca and CO2, thereby increasing the CO2 sorption capacity. These findings suggest that the surface area and pore structure of CO2 capture materials are significantly affected by ionic strength during the manufacturing process, with an increase in ion concentration in the composite leading to an enlarged specific surface area.

However, a further increase in CaCl2 concentration did not result in a higher CO2 capture ability (Fig. 3a). Analysis of the Ca content in Ca-RTCs prepared with different CaCl2 concentrations (Fig. 3b) showed a slight increase in Ca content from 9.22 to 9.58% with increased CaCl2 concentration. This indicates that the CO2 capture ability is not solely dependent on the Ca or Mg content within the Ca-RTCs, despite Ca being a critical ion for the mineral carbonation reaction. The decreased trend in CO2 capturing ability with increasing CaCl2 concentration is not fully understood, but it is thought to be due to the inhibitory effect of Cl− ions on the mineral carbonation reaction. As the concentration of CaCl2 increases, so does the concentration of Cl− ions, which has been reported to significantly reduce the rate of mineral carbonation on lime mud surfaces. Specifically, a Cl:Ca ion ratio greater than 2:100 has been shown to dramatically reduce CO2 capturing ability [20] . We also analyze the Cl:Ca ratio of Ca-RTC prepared at 0.5 M CaCl2 solution. The Cl:Ca ratio was 2.57:100, which can affect the CO2 capturing ability.

Following up, CO2 capture experiments were carried out with Ca-RTC treated with 0.001 M CaCl2 solution. Figure 4a illustrates the kinetics of CO2 capture by Ca-RTC. Upon injecting 99% CO2 at 0.5 bar, the CO2 loading rate surged to approximately 6% after just one day of capture, climbing to 10% after 3 days, and eventually reaching the saturation point of CO2 capture capacity. Given the initial Mg and Ca content in Ca-RTC (refer to Table 1), it is deduced that Ca-RTC achieved its theoretical maximum CO2 capture capacity. When examining the phenolphthalein response inside the CO2 capture material at various stages of capture, samples pre-capture and post one day of capture exhibited a pink color, signaling incomplete carbonate reactions due to the interaction with residual alkaline earth metals. However, samples taken after three days of reaction maintained their colorless state upon applying the same indicator, signifying the completion of the carbonate reaction. Although the carbonate reaction rate was anticipated to be influenced by CO2 diffusion into the Ca-RTC's interior macropores, the observed rapid penetration of CO2, attributable to the material's high porosity and extensive specific surface area, indicated a quicker internal reach than expected.

Fig. 4
figure 4

CO2 loading on Ca-RTC as a function of reaction time. Inset: phenolphthalein analysis of Ca-RTC (a) and CO2 loading on Ca-RTC under various pressure (b)

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We compared the CO2 capturing abilities of industrial waste-derived CO2 capture materials with Ca-RTC (Table 2). Except for magnesium slag, which contains more than 50% high CaO content, Ca-RTC demonstrates the highest CO2 capturing ability. Despite its relatively low Ca content of 15.0%, compared to other industrial wastes, Ca-RTC exhibits excellent CO2 capturing performance. This result indicates that Ca-RTC is a highly reliable and effective CO2 capturing material, emphasizing the reactivity of Ca for carbonation by increasing the surface area of the CO2 capturing material.

Table 2 CO2 capturing abilities of industrial waste and Ca-RTC
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To identify the rate-limiting step in CO2 capture, both the pseudo-second-order reaction kinetic model and the intraparticle diffusion model were employed (see supporting information). We expressed amount of CO2 captured on Ca-RTC as mg CO2 per g Ca-RTC in Fig. 4a and described kinetic model constants in table S2. While the intraparticle diffusion model yielded a linear regression coefficient of merely 0.60, the pseudo-second-order model demonstrated an excellent fit with a linear regression coefficient (R2) of 0.98. This suggests that CO2 diffusion into Ca-RTC was notably rapid, and the chemical reaction between CO2 and the alkaline earth metals within Ca-RTC constituted the rate-limiting step of the overall reaction. The maximum CO2 capturing amount expected from pseudo-second order model kinetic model was described as 142.85 mg/g (=CO2 loading 14.28%) which is similar and even higher than theoretical CO2 capturing capacity of Ca-RTC.

To further explore the impact of pressure on CO2 capture by Ca-RTC, experiments were conducted over a 1-day period at pressures of 0, 0.25, 0.75, and 1 bar, as shown in Fig. 4b. Given that CO2 capture reached saturation after 3 days at 0.5 bar, as demonstrated in the kinetics experiment, the efficiency of CO2 loading in Ca-RTC under varying pressures was assessed over a shorter duration of 1 day. At atmospheric pressure (0 bar), CO2 loading over the 1-day period was 10.5%. An increase in pressure from 0.25 to 0.75 bar led to higher CO2 capture rates of 11.17% and 12.28%, respectively. The CO2 capture at 0.75 bar aligned with the theoretical adsorption capacity of Ca-RTC, suggesting that a pressure increase up to 0.75 bar positively influences the mineral carbonation reaction. This enhancement in the mineral carbonation reaction at approximately 0.75 bar is attributed to the high pressure, which, according to previous kinetic experiments, does not significantly affect the diffusion rate of CO2 due to Ca-RTC's large surface area. Thus, the observed increase in CO2 capture rate with pressure is primarily due to an accelerated reaction rate of CO2 mineral carbonation, rather than an increase in diffusion rate. According to Le Chatelier's principle, an increase in pressure shifts the equilibrium to minimize the system's volume. [21] However, a further increase in CO2 pressure to 1 bar resulted in a sharp decline in CO2 loading. During the mineral carbonation reaction, water needs to be released from calcium or magnesium hydroxide, as represented by the reactions:

$$ {\text{Ca}}\left( {{\text{OH}}} \right)_{{2}} + {\text{ CO}}_{{2}} \left( {\text{g}} \right) \, \to {\text{ CaCO}}_{{3}} \left( {\text{g}} \right) \, + {\text{ H}}_{{2}} {\text{O }}\left( {\text{g}} \right), $$
(3)
$$ {\text{Mg}}\left( {{\text{OH}}} \right)_{{2}} + {\text{ CO}}_{{2}} \left( {\text{g}} \right) \, \to {\text{ MgCO}}_{{3}} \left( {\text{g}} \right) \, + {\text{ H}}_{{2}} {\text{O }}\left( {\text{g}} \right). $$
(4)

The increased pressure enhances the water retention capability, thereby reducing the mineral carbonation efficiency of Ca-RTC. Feng et al. reported similar findings, noting a decrease in CO2 capturing ability of Ca(OH)2 as a CO2 capturing material at pressures higher than 8 MPa [22].

CO 2 Sequestration Mechanism by Ca-RTC

We clarified the mechanism behind CO2 capture by analyzing the physicochemical properties of Ca-RTC both before and after the CO2 capture reactions. Notably, the crystallinity of Ca-RTC remained largely unchanged through the process of CO2 absorption. However, the post-capture X-ray Diffraction (XRD) analysis revealed a marked increase in the intensity of the CaCO3 peak, suggesting the formation of CaCO3 within Ca-RTC via the carbonation of calcium in the presence of CO2 as the precursor (Fig. 5a). This transformation involves gaseous CO2 converting into solid CaCO3, a change that was accompanied by a significant increase in the material's specific surface area, as shown in Fig. 5b. Specifically, the specific surface area of Ca-RTC more than doubled, from 6.29 m2/g before CO2 capture to 15.94 m2/g afterwards. Post-capture, there was a notable increase in the volume of macropores larger than 50 nm (Fig. 5c), while the volume of nano and mesopores remained similar to that of the untreated Ca-RTC. Given that CaCO3 is in a solid phase, in contrast to the gaseous phase of its precursor CO2, this suggests the creation of new surfaces during the carbonation reaction.

Fig. 5
figure 5

XRD patterns (a), N2 adsorption–desorption isotherms (b), pore size distributions (c), and FTIR spectrums (d) of Ca-RTC before and after CO2 capture

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We also analyzed the FTIR spectrum of Ca-RTC before and after the CO2 capturing reaction (Fig. 5d). The Ca-RTC consists of significant amounts of SiO, and peaks corresponding to Si–O–Si bonds were observed at 1010–1074 cm−1 [23] . Additionally, the peak at 3707 cm−1, representing the –OH group, was evident due to the hydration of Ca-RTC. A slight asymmetric CO₃ peak was also observed at 1404 cm−1 due to partial carbonation reaction. After the CO2 capturing reaction, the intensity of peaks at 1404, 870, and 700 cm−1, which represent the peaks of calcite (CaCO₃) [24], significantly increased. This result clearly supports that the CO2 capturing mechanism of Ca-RTC involves the mineral carbonation of Ca ions.

Scanning Electron Microscopy (SEM) analysis post-CO2 capture revealed the formation of numerous fine particles, less than 1 μm in size, on the surface of Ca-RTC. These particles, which filled the spaces between larger and mesopores, led to a denser structural arrangement (Fig. 6a,b). These newly formed particles are believed to be crystalline CaCO3, as corroborated by the XRD findings. Furthermore, SEM–energy dispersive x-ray analysis (EDAX) imaging of Ca-RTC post-CO2 capture (Fig. 6c) indicated that CaCO3 did not exist as isolated particles but rather, calcium and carbon were uniformly distributed across the surface of Ca-RTC. Silicon, the predominant component of Ca-RTC, was found to be co-distributed with aluminum, mostly in the form of aluminum silicate and crystalline SiO2. The areas with a high concentration of silicon and aluminum were distinct from those rich in calcium. The coinciding distributions of carbon with calcium, as previously identified in the XRD analysis, confirmed that CO2 was converted into CaCO3. This comprehensive evidence supports the conclusion that CO2 capture in Ca-RTC is directly associated with the formation of CaCO3, effectively elucidating the material's CO2 capture mechanism.

Fig. 6
figure 6

SEM images of Ca-RTC before (a) and after CO2 capturing (b), SEM-EDAX mapping of CO2 captured Ca-RTC (c)

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Conclusion

This study presents an innovative approach to carbon dioxide (CO2) capture by leveraging modified spent railway tie concrete (RTC), aiming to address the pressing need for efficient CO2 sequestration technologies. By incorporating a 0.001 M calcium chloride (CaCl2) solution in lieu of water, we developed a calcium-modified RTC (Ca-RTC) that exhibits a significantly enhanced CO2 capturing capability. The preparation process involved mixing the waste material with a water/waste ratio of 0.45 (w/w), followed by drying and hydration to form 1 cm3 cubes. Despite a marginal increase in calcium content (0.08%) compared to the pristine RTC, the Ca-RTC modified with 0.001 M CaCl2 solution demonstrated a substantial increase in BET surface area to 6.29 m2/g and a highly porous morphology. These changes are attributed to the altered surface properties induced by the ionic strength of the CaCl2 solution, resulting in a 2.5-fold increase in CO2 capture efficiency (11.57%) over the unmodified RTC. This enhancement is primarily due to the enlarged surface area, facilitating greater CO2 adsorption. However, increasing the concentration of CaCl2 beyond 0.001 M led to a decrease in CO2 capturing performance, likely due to the inhibitory effect of Cl− ions. The CO2 capture reaction with 0.001 M CaCl2-modified Ca-RTC was completed within three days under 0.5 bar pressure, with the chemical interaction between alkaline earth metal ions in Ca-RTC and CO2 identified as the rate-limiting step. Furthermore, elevating CO2 pressure to 0.75 bar enhanced the capture rate, whereas pressures above 1 bar adversely affected the CO2 capturing ability. Post-reaction analysis revealed the formation of crystalline calcium carbonate (CaCO3), contributing to an increased surface area of the material. This study not only elucidates the mechanisms underpinning the improved CO2 capture performance of Ca-RTC but also highlights its potential as a sustainable and efficient solution for CO2 sequestration.