Properties of Carbonated Steel Slag Admixture in the Cementitious System

Sun, Lie; Wang, Hui; Wang, Yali

doi:https://doi.org/10.1155/2023/5547591

Advances in Civil Engineering

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 5547591 | https://doi.org/10.1155/2023/5547591

Properties of Carbonated Steel Slag Admixture in the Cementitious System

Lie Sun,¹Hui Wang,²and Yali Wang¹

Academic Editor: Junqing Zuo

Received20 Jun 2023

Revised06 Oct 2023

Accepted08 Nov 2023

Published08 Dec 2023

Abstract

Global warming caused by carbon dioxide (CO₂) emissions has emerged as an undeniable environmental concern. While advocating for energy conservation and emissions reduction, the challenge of addressing the substantial CO₂ emissions cannot be underestimated. Currently, steel slag is utilized in carbon capture and storage technology due to its potential for carbonation. However, the carbonation of steel slag necessitates a stable and cost-effective carbon source. Industrial exhaust gases are considered a viable option, but they often have low CO₂ concentrations, resulting in sluggish carbonation rates. Therefore, this study focuses on directly converting steel slag powder into concrete mineral admixtures to enhance the carbonation rate at low CO₂ concentrations. Experimental results reveal that a carbonation time of 3–7 days, a liquid–solid ratio of 50%, and the selection of sodium silicate as the alkali activator yield the optimal carbonation conditions. Under these conditions, the CO₂ uptake can reach 15.3%–16.0%, and the f-CaO content can be reduced to 0.2%–0.3%. Mixing 30% carbonated steel slag powder with P·Ⅰ 42.5 cement in mortar samples yields a compressive strength of 32.1 MPa at 7 days and 47.5 MPa at 28 days, along with a flexural strength of 6.2 MPa at 7 days and 8.0 MPa at 28 days. The addition of carbonated steel slag powder not only enhances the mechanical properties but also reduces the pore diameter in the hardened cementitious system. In 7 days, the pore size decreases from being concentrated around 349 nm to approximately 282 nm, and in 28 days, the pore size decreases from being concentrated around 62 nm to roughly 55 nm. This transformation is primarily attributed to the role played by calcite grains in the carbonated steel slag powder, which facilitates nucleation and filling effects.

1. Introduction

Global warming is primarily caused by greenhouse gases such as carbon dioxide (CO₂), methane (CH₄), and nitrogen oxide (N₂O) [1]. Among these, CO₂ is the most significant. The environmental issue of global warming stemming from CO₂ emissions cannot be underestimated. Currently, various methods have been developed to mitigate CO₂ emissions into the atmosphere, with carbon capture and storage (CCS) technology being considered the most promising approach [2]. CCS technology is a critical means of addressing CO₂ emissions, and without it, the cost of achieving a 0.45‰ CO₂ equivalent reduction by 2,050 would increase by 138% [3]. Viable CCS technologies to date include geological storage, ocean storage, biological storage, and mineral storage. Mineral sequestration is regarded as the safest method for CO₂ sequestration, offering a high storage capacity (1 trillion tons of CO₂) and the potential to store all future emissions [4].

Numerous industrial waste slags are widely employed in CO₂ storage. In comparison to raw ores, industrial waste slag is not only readily available but also cost effective. Currently available industrial waste slag includes blast furnace slag [5], fly ash [6], and steel slag. Meanwhile, China, with its rapidly expanding steel industry, has become the world’s largest steel consumer, accounting for 50% of global crude steel production [7]. In 2019, China’s crude steel production reached 996.3 million tons, generating nearly 150 million tons of steel slag. However, the comprehensive utilization rate of steel slag in China stands at only 30%, significantly lower than the 85% rate achieved in many developed countries. A substantial amount of steel slag is improperly stored and landfilled, resulting in severe environmental and social hazards [8].

Steel slag shares chemical and mineral compositions similar to cement, making it a potential candidate for carbonation [9, 10]. The chemical composition of steel slag predominantly comprises CaO (45%–60%), SiO₂ (10%–15%), Al₂O₃ (1%–5%), Fe₂O₃ (3%–9%), MgO (3%–13%), FeO (7%–20%), and P₂O₅ (1%–4%). These oxides are primarily present in steel slag in mineral forms, including C₃S, C₂S, and the RO phase (a continuous solid solution composed of MgO, FeO, and MnO) [11]. Notably, f-CaO, f-MgO, C₃S, and C₂S can all react with CO₂ during carbonation.

Furthermore, China has set long-term goals of achieving a peak in carbon emissions by 2030 and carbon neutrality by 2060. To attain these objectives, industries are actively seeking emission reduction strategies. Among these, the cement industry contributes 8% to global CO₂ emissions, with China’s cement clinker production accounting for roughly 60% of the world’s output [12]. Consequently, there is a pressing need to identify new-generation low-carbon cementitious materials to replace conventional cement. Given the challenges associated with CO₂ emissions and steel slag reuse, the carbonation of steel slag to produce cementitious materials offers a viable solution.

Carbonating steel slag has several advantages, including the reduction of carbon emissions, resolution of CO₂ emission challenges, improved stability of steel slag in construction applications [13], and early strength enhancement [14]. Owing to its complex mineral composition, steel slag contains significant amounts of f-CaO and f-MgO in addition to its primary components such as C₃S and C₂S. These substances contribute to volume stability issues in cementitious systems, a critical concern in steel slag applications. The f-CaO in steel slag is not only abundant but is often encapsulated within C₂S [15], rendering it inaccessible. Carbonation of steel slag consumes C₂S, allowing for the utilization of f-CaO. Consequently, carbonation effectively removes f-CaO and f-MgO, facilitating the extensive use of steel slag in construction projects. As a cementitious material, steel slag typically exhibits low early stage strength due to the relatively high C₂S content and slow hydration rate. However, carbonation converts C₂S into calcium carbonate (CaCO₃), and the microhardness of CaCO₃ (calcite) is approximately 5–5.5 GPa [16], effectively improving early stage strength. In addition, in cementitious systems, carbonation of steel slag promotes a dense structure and enhances durability, including improvements in volume shrinkage [17] and freeze–thaw resistance [18]. In summary, the carbonation of steel slag holds significant application potential.

To date, scholars have explored various methods to carbonate steel slag and convert it into building materials such as mineral admixtures [19], bricks [20], and aggregates [21], yielding valuable results. However, several unresolved issues persist. First, most researchers have employed high-temperature, high-pressure, and high-concentration conditions for the carbonation of steel slag. While these conditions have produced promising test results, they are not conducive to industrial-scale implementation due to their high cost and impracticality. Table 1 illustrates the carbonation conditions for some steel slag studies.

In addition to the aforementioned challenges, carbonating steel slag requires a low-cost and readily available carbon source. Therefore, industrial exhaust gases containing CO₂ emerge as a favorable choice for carbonation of steel slag. Common industrial waste gases containing CO₂ include natural gas turbine flue gas (3%–4%), coal-fired boiler flue gas (12%–14%), and cement kiln tail gas (14%–33%) [26]. Among these, cement kiln tail flue gas offers the advantage of relatively high CO₂ concentration. However, variations in specific production processes can result in differences in concentration; for instance, Chen Yongbo used cement kiln tail gas with a CO₂ concentration of 22.2% [27], while Huang Lan’s study utilized flue gas with a concentration of only 8%–10% [28]. Nevertheless, cement kiln tail exhaust gas remains a high-quality carbon source for steel slag carbonation. Nonetheless, as previously mentioned, the limited CO₂ concentration in cement kiln tail gas presents a challenge. Thus, improving the efficiency and extent of carbonation at lower CO₂ concentrations becomes a critical issue to address. Consequently, this study aims to simulate cement kiln tail exhaust gas under laboratory conditions and investigate the carbonation mechanism of steel slag within cementitious systems at lower CO₂ concentrations, providing a foundation for the further application of steel slag carbonation.

2. Materials and Methods

2.1. Steel Slag Used in the Study

The steel slag used in this study is layer-poured steel slag from Hebei, China, and its chemical composition is presented in Table 2. Steel slag comprises several primary chemical components, with a significant mass fraction, including 37.8% CaO, 29.2% Fe₂O₃, 11.6% SiO₂, 10.1% MgO, 5.9% Al₂O₃, 1.5% P₂O₅, 1.6% MnO, 0.8% TiO₂, and 0.2% Cr₂O₃. These components collectively constitute the chemical composition of steel slag, influencing its properties and applications. A high CaO content in steel slag is beneficial for its reactivity with CO₂. This is because CaO is highly reactive with CO₂ [29]. During carbonation, CaO in steel slag reacts with CO₂ from the environment to form CaCO₃, a stable and environmentally benign compound. This reaction helps sequester CO₂ from the atmosphere, making steel slag with a high CaO content an effective carbon capture and utilization material.

The X-ray diffraction (XRD) diagram of the steel slag is displayed in Figure 1. The primary mineral components of steel slag include C₂S, RO phases, Ca₂Fe₂O₅, and Ca₁₂Al₁₄O₃₃, and it also contains a small amount of C₃S and f-CaO.

In this study, the steel slag powder was ground to a particle size of less than 150 μm. The steel slag powder was carbonated by changing different carbonation conditions to explore the effects of carbonation conditions on the properties of the carbonated steel slag admixture.

2.2. Preparation and Test Methods of Carbonated Steel Slag

As shown in Figure 2, the basic process of preparing an admixture from steel slag powder is as follows: the steel slag powder is thoroughly mixed with a portion of the water and then placed into a shallow dish for carbonation. The carbonation atmosphere is provided by a concrete carbonation box that regulates temperature, humidity, and CO₂ concentration. After carbonation, the carbonated steel slag powder is ground to a particle size of less than 150 μm to obtain a carbonated steel slag admixture. For the sake of clarity, this study refers to the ratio of water and steel slag powder as the liquid–solid ratio. The carbonation conditions provided by the carbonation box are as follows: 20% CO₂ concentration, temperature at 25°C, and humidity at 75%.

The influence of different carbonation conditions on the performance of the carbonated steel slag admixture was explored. The fixed liquid–solid ratio is 20%, and the carbonation times are 12 hr, 1, 3, 7, and 14 days, respectively. The samples under these conditions are recorded as T1–T5. The fixed carbonation time is 3 days, and the liquid–solid ratios are 0%, 10%, 20%, 30%, 50%, and 100%, respectively. These samples are recorded as W0–W5.

In addition to the two influencing factors of carbonation time and liquid–solid ratio, this study also investigated the influence of alkali activators on the carbonation of steel slag powder. Alkali activators can effectively enhance the reactivity of steel slag. Therefore, in this study, NaOH and Na₂SiO₃·9H₂O were selected as alkali activators and added to the samples in an amount equal to 0.75% of the mass of steel slag powder to increase the carbonation rate. When NaOH was chosen as the alkali activator, the fixed liquid–solid ratio was 50%, and the carbonation times were 3 and 7 days. These two samples were recorded as H1 and H2, respectively. Subsequently, when Na₂SiO₃·9H₂O was used as the alkali activator, these two samples were recorded as H3 and H4.

Samples including T1–T5, W0–W5, and H1–H4 were subjected to thermogravimetric analysis (TGA), XRD, and content of f-CaO tests. XRD measurements were conducted from 5° to 90° (2θ) with a step size of 0.01°. TGA involved heating approximately 30 mg of powder in an alumina crucible at a uniform rate of 10°C/min from ambient temperature to 1,000°C, in a nitrogen flow of 50 ml/min. The f-CaO content was determined according to the standard of GB/T176-2017. For the carbonated steel slag tests mentioned above, XRD was used to investigate the phase changes before and after carbonation of steel slag, TGA was employed to study the carbon sequestration capacity of steel slag, and the f-CaO content was used to evaluate the volume stability of carbonated steel slag.

To further explore the performance of carbonated steel slag powder as an admixture, samples such as T1–T5, W0–W5, and H1–H4 were used to create mortar specimens, and their compressive/flexural strengths were tested to assess the mechanical properties of carbonated steel slag. The specific test method involved replacing P·Ⅰ 42.5 cement with the admixture in a proportion of 30%, preparing the mortar according to the standard of GB/T17671-1999, and conducting compressive and flexural strength tests at 7 and 28 days. Steel slag without carbonation was also tested as a blank sample.

In addition, all samples were mixed at a ratio of 30% carbonated steel slag powder and 70% cement, with a liquid–solid ratio of 0.5, to create paste samples. These paste samples were then subjected to XRD, scanning electron microscopy—backscattered electron (SEM-BSE), and mercury intrusion porosimetry (MIP) tests to explore the effect of carbonated steel slag on the phase changes and microstructure of the cementitious system.

Specific changes in carbonation conditions can also be found in Table 3.

2.3. Method of Evaluating Carbonation Efficiency of Steel Slag

Currently, the evaluation of steel slag’s carbonation efficiency varies depending on its intended use as carbonated steel slag. In this study, CO₂ uptake and carbonation degree, determined through the mass loss method, were employed to assess the carbonation efficiency based on TGA [30]. The calculation of CO₂ uptake and carbonation degree is presented in Equations (1)–(4).

3. Results and Discussion

3.1. Properties of Carbonated Steel Slag

3.1.1. Phase Change of Carbonated Steel Slag

Figure 3 presents the XRD patterns of steel slag powder with varying carbonation times. As observed in the figure, the diffraction peaks of the original C₂S and RO phases in the steel slag decrease significantly during the carbonation reaction. In addition, it can be noted that the diffraction peak of CaCO₃ (Calcite) is rising, indicating continuous conversion of the carbonable phase in the steel slag to CaCO₃. Notably, when compared to the blank sample, the CaCO₃ diffraction peak in T1–T5 is quite distinct, signifying substantial CaCO₃ formation. For samples T1–T5, it is evident that as the carbonation time increases, the diffraction peaks of C₂S and RO gradually weaken, while the CaCO₃ diffraction peak gradually strengthens, suggesting an increasing CaCO₃ content in the samples. No diffraction peak of Ca(OH)₂ was observed in the XRD patterns, as Ca(OH)₂ produced by C₂S hydration was converted to CaCO₃. The carbonation reaction continues to progress over a certain carbonation time, and extending this time benefits the enhancement of the carbonation degree.

The XRD results of W0–W5 are depicted in Figure 4. The patterns of W0–W5 demonstrate that altering the liquid–solid ratio primarily affects the carbonation degree, with no generation of new phases. With the gradual increase in the liquid–solid ratio, the diffraction peak of CaCO₃ exhibits an initial increase, reaching its maximum at a liquid–solid ratio of 50%, after which it starts to decline. Consequently, the highest carbonation efficiency is achieved at a liquid–solid ratio of 50%.

The XRD results of H1–H4 are illustrated in Figure 5. Similar to the effect of the liquid–solid ratio, the addition of the alkali activator does not result in the generation of new phases during carbonation. However, it effectively enhances the carbonation efficiency, with a prominent CaCO₃ peak.

In conclusion, the XRD results can be summarized as follows: first, prolonging the carbonation time effectively enhances the degree of carbonation. Second, for carbonating steel slag powder, the highest efficiency is achieved with a liquid–solid ratio of 50%, corresponding to the highest degree of carbonation at the same age. Furthermore, after carbonating all the samples, there is a significant reduction in RO phases, including f-CaO, which effectively enhances the stability of steel slag.

The XRD results clearly indicate that the primary product of steel slag carbonation is CaCO₃, specifically in the form of calcite. To further confirm the crystal form of CaCO₃, scanning electron microscopy—energy dispersive X-ray spectroscopy (SEM-EDS) testing was conducted on the carbonated steel slag. Figure 6 and Table 4 displays the typical microscopic morphology of carbonated steel slag powder, with certain CaCO₃ grains marked. By combining the XRD results with the microscopic morphology observed through SEM-EDS, it can be confidently stated that the CaCO₃ formed after steel slag carbonation is indeed calcite. Calcite exhibits a small grain size, measuring only about 1 μm in diameter.

When these fine calcite particles are introduced into the cementitious system, they serve two crucial functions. First, they facilitate the nucleation of hydration products. Second, both the calcite particles themselves and the hydration products formed in their vicinity effectively fill the voids within the cementitious system, thereby enhancing its overall density.

3.1.2. Carbonation Efficiency and Stability of Carbonated Steel Slag

CO₂ uptake and carbonation degree/content of f-CaO for carbonated steel slag powder are shown in Figures 7–9.

The test data reveals that the CO₂ uptake of carbonated steel slag powder falls within the range of 5.0%–20.1%, and the carbonation degree ranges from 12.3% to 49.4% (with the theoretical maximum CO₂ uptake being 40.6%). This implies that every 1 ton of steel slag powder can capture between 50 and 201 kg of CO₂ (calculated based on the CO₂ uptake of the samples).

As depicted in Figure 7, both the CO₂ uptake and carbonation degree of the samples continue to increase as the carbonation time progresses. However, as the carbonation time extends, the rate of increase in CO₂ uptake and carbonation degree gradually slows down. While extending the carbonation time is beneficial for enhancing carbonation efficiency, the improvement in carbonation becomes progressively limited during longer carbonation periods. Furthermore, as the carbonation reaction proceeds, the content of f-CaO in the samples steadily decreases. This phenomenon indicates that f-CaO is partially or even completely consumed as a result of the carbonation reaction, effectively enhancing the stability of steel slag.

As demonstrated in Figure 8, the liquid–solid ratio significantly impacts both the CO₂ uptake and carbonation degree. With a continuous increase in the liquid–solid ratio, there is a noticeable trend of initially increasing and then decreasing CO₂ uptake and carbonation degree. The maximum values are reached when the liquid–solid ratio is 50%, after which they start to decline. This phenomenon is primarily due to the carbonation process of steel slag, which necessitates the presence of both water and CO₂ [31]. When the liquid–solid ratio is low, water is consumed as the reaction progresses, making it challenging for the carbonation reaction to occur, resulting in lower carbonation efficiency. Conversely, when the liquid–solid ratio is high, it leads to the steel slag particles being surrounded by water, making it difficult for CO₂ to contact the steel slag particles, again resulting in lower carbonation efficiency. Notably, the content of f-CaO in samples with a liquid-to-solid ratio of 100% is the lowest, at 0%. The overall trend is an initial increase followed by a decrease. In contrast to T1–T5, the main influencing factor for the f-CaO content of samples W0–W5 is the liquid–solid ratio, not the CO₂ uptake. Compared with W0 and W1–W5, the W0 samples had lower CO₂ uptake but higher f-CaO content. However, W0 samples remain in powdered form after carbonation, while W1–W5 samples agglomerate. In industrial applications, W1–W5 samples require additional energy for secondary grinding, making the carbonation conditions of W0 more suitable for industrial production.

As illustrated in Figure 9, the alkali activator also exerts a significant influence on the rate of the carbonation reaction. The addition of NaOH and Na₂SiO₃·9H₂O has led to a notable improvement in both CO₂ uptake and carbonation degree. Specifically, samples with Na₂SiO₃·9H₂O added at 3 and 7 days exhibited significant improvements compared to samples without an alkali activator. Samples with NaOH added showed a substantial improvement at 7 days. In the case of H1–H4, the f-CaO content decreased compared to W4. Nevertheless, the f-CaO content of H1–H4 remains higher than that of W5, reaffirming that the primary factor affecting f-CaO content is the liquid–solid ratio.

In conclusion, to enhance CO₂ uptake, extending the carbonation time should be considered as the first step, with CO₂ uptake increasing from 5.9% at 12 hr to 11.1% at 14 days. Second, selecting the appropriate liquid–solid ratio is crucial. The study’s experimental results indicate that a liquid–solid ratio of 50% results in the highest carbonation efficiency, with CO₂ uptake reaching 14.3% at 3 days. In addition, an alkali activator can effectively improve carbonation efficiency, particularly Na₂SiO₃·9H₂O, where CO₂ uptake can reach 20.1% at 7 days.

Regarding the influencing factors on f-CaO content, the most significant factor is the liquid–solid ratio used during the carbonation treatment in this study. When the liquid–solid ratio is 100%, all f-CaO can be consumed within 3 days. Furthermore, under constant liquid–solid ratio conditions, increasing CO₂ uptake can also lead to greater f-CaO consumption.

3.2. Carbonated Steel Slag as Admixtures in Cementitious System

3.2.1. Mechanical Properties

The influence of carbonated steel slag as a mineral admixture is reflected in the compressive and flexural strength of the mortar, and the test results are presented in Figures 10 and 11 and Table 5.

The addition of carbonated steel slag powder leads to a slight improvement in the compressive strength and flexural strength of the mortar at 7 days, but there is no significant improvement at 28 days. However, the flexural strength does show improvement at both 7 and 28 days. As indicated by T1–T5, with the extension of carbonation time and the increase in carbonation degree, the compressive and flexural strength do not exhibit significant increases, and the changes are minimal. It is noteworthy that the 28-day flexural strength of the mortar specimen made with W5 suggests that an excessively long carbonation time for steel slag powder may have a detrimental effect on the flexural strength of the cementitious system.

A similar trend can be observed in W0–W5, even though the overall carbonation degree of the W0–W5 samples is relatively higher compared to T1–T5. However, there is no significant improvement in the compressive and flexural strength, despite the increase in carbonation degree.

The results for H1–H4 show that the compressive and flexural strength of the NaOH samples align with the trends observed in the previous two groups of samples, with only slight improvements. However, when the sample was carbonated with Na₂SiO₃·9H₂O, its compressive strength exhibited significant improvement compared to the blank steel slag, with an increase of approximately 3.0–4.0 MPa at 7 days and 5.0 MPa at 28 days.

The increase in compressive and flexural strength is primarily attributed to the presence of a certain amount of CaCO₃ seeds in the carbonated steel slag powder. The addition of these CaCO₃ seeds accelerates the formation of hydration products, thereby enhancing both compressive and flexural strength, especially in the early stages [32]. It is evident that the compressive and flexural strength of all carbonated samples at 7 days exhibits improvement compared to the blank sample [33].

To sum up, the addition of carbonated steel slag powder can improve the mechanical properties of the cementitious system. When the carbonated steel slag powder content is 30%, the flexural strength can increase by about 0.5 MPa in 7 days and increase by about 0.5–1.0 MPa in 28 days. The compressive strength can increase by 2.0–3.0 MPa in 7 days, and compressive strength has no significant increase in 28 days. In addition, it should be noted that the carbonation time of carbonated steel slag powder should not be too long, and the flexural strength of W5 is 0. Moreover, the mechanical properties of the samples added with Na₂SiO₃·9H₂O during carbonation are better than NaOH, and the compressive strength is improved obviously both in 7 and 28 days.

3.2.2. Impact of Carbonated Steel Slag on the Cementitious System

XRD results for some representative paste samples are shown in Figures 12 and 13.

It can be seen from the figure that the main phases in the hydrated paste sample are C₂S, CaCO₃, Ca(OH)₂, Ca₂Fe₂O₅, RO phase, C₃S, Ca₅(SiO₄)₂(OH)₂, and Ca_1.5SiO_3.5·xH₂O. These phases can also be mainly divided into two categories, one is from the cement and carbonated steel slag itself including C₃S, C₂S, RO phase, Ca₂Fe₂O₅, and CaCO₃, and the other is the hydration products of cement and carbonated steel slag including Ca₅(SiO₄)₂(OH)₂, Ca_1.5SiO_3.5·xH₂O, and Ca(OH)₂. At the same time, it can also be seen that the sample added to the carbonated steel slag powder has a higher amount of hydration products at the same age, especially the C–S–H phase such as Ca₅(SiO₄)₂(OH)₂ and Ca_1.5SiO_3.5·xH₂O. The increase of hydration products should be the influence of CaCO₃ grains, which help the nucleation of hydration products and accelerate hydration to a certain extent.

The results of MIP are presented in Table 6 and Figures 14–17. First, it can be observed from Table 6 that there is not much difference in porosity between the blank sample and the W4 sample. However, the W4 sample containing carbonated steel slag powder exhibited a median pore diameter (volume) of 52.1 nm at 7 days, whereas the blank sample had a median pore diameter (volume) of 72.8 nm at 7 days. At 28 days, the W4 sample displayed a median pore diameter (volume) of 35.6 nm, while the blank sample had a median pore diameter (volume) of 42.0 nm at 28 days.

As evident from Figures 15 to 17, the cumulative porosity of the W4 sample and the blank sample does not show significant differences. However, as seen in Figures 14–16, the pore size in the sample containing carbonated steel slag powder is considerably reduced and finer. At 7 days, the pores in the W4 sample were concentrated around 282 nm, whereas the pores in the blank sample were concentrated at a much higher value, around 349 nm. At 28 days, the pores in the W4 sample were concentrated around 55 nm, while the pores in the blank sample were concentrated around 62 nm.

Based on the MIP results, it can be concluded that the addition of carbonated steel slag powder effectively reduces the pore diameter within the cementitious system, mitigating potential issues associated with larger pores. This is attributed to the role played by carbonated steel slag powder in nucleating and filling pores. Coupled with the previous findings on mechanical properties, all the mortar samples mentioned above displayed enhanced compressive and flexural strength at 7 days, likely due to the improved pore structure during this period. However, as the age reached 28 days, carbonated steel slag no longer contributes significantly to nucleation during the later stages of cementitious system hydration. At this point, hydration product formation is relatively dense, resulting in less reduction in median pore diameter (volume) at 28 days compared to the improvement observed at 7 days [34].

Figure 18 displays SEM-BSE photographs and elemental distribution maps of W4 samples (3 days). When examining the photograph and the elemental distribution map, it becomes apparent that the largest light-colored area in the photo represents the Ca–Si phase, while other small particles and pores are distributed around the Ca–Si phase. Notably, the distribution of the C element is primarily concentrated in the regions containing small particles and pores. In conjunction with the preceding information, it can be inferred that the CaCO₃ grains generated during carbonation are primarily distributed in these areas.

The results obtained from SEM-BSE corroborate the earlier conclusion that the addition of CaCO₃ grains contributes to nucleation, pore filling, pore diameter reduction within the cementitious system, overall density improvement, and enhancement of mechanical properties.

4. Conclusions

In this study, the impact of low CO₂ concentration on carbonated steel slag admixtures was investigated by subjecting steel slag powder to carbonation under various conditions. In addition, carbonated steel slag powder was incorporated into mortar and paste samples to assess its influence on the hydration and microstructure of the cementitious system. The following key conclusions have been derived:(1)The primary product of steel slag carbonation is CaCO₃, predominantly in the form of calcite, characterized by its small particle size, averaging around 1 μm. The introduction of these CaCO₃ particles alongside carbonated steel slag into the cementitious system facilitates nucleation and pore-filling, resulting in reduced pore diameter within the paste. This enhancement leads to improved compactness and strength.(2)Several factors, including carbonation time, liquid–solid ratio, and the addition of alkali activators, impact the CO₂ uptake of carbonated steel slag powder. Longer carbonation times and the use of alkali activators positively affect CO₂ uptake. Remarkably, a liquid–solid ratio of 50% yields higher CO₂ uptake. Among the alkali activators tested, Na₂SiO₃·9H₂O proves more effective, achieving a CO₂ uptake of up to 16.0%.(3)Carbonation leads to the conversion of f-CaO in steel slag to CaCO₃, enhancing stability. The most significant factor affecting f-CaO content is the liquid–solid ratio employed during carbonation. A ratio of 50% reduces f-CaO content to 0% in 7 days. However, at a ratio of 20%, it is reduced to only 1.5% in the same timeframe. Furthermore, under consistent liquid–solid ratios, increasing carbonation time and incorporating alkali activators can further reduce f-CaO content.(4)Incorporating carbonated steel slag powder generally enhances the mechanical properties of the cementitious system. Compressive strength increases at 7 days, and flexural strength improves at both 7 and 28 days. It is essential to note that excessive carbonation time negatively impacts flexural strength. The 14-day carbonation time of the T5 sample resulted in complete loss of flexural strength.(5)Considering comprehensive factors such as carbon sequestration capacity, stability, and mechanical properties, the study recommends a carbonation time of 3–7 days, a liquid–solid ratio of 50%, and the utilization of Na₂SiO₃·9H₂O as an alkali activator for steel slag powder carbonation.

Data Availability

Data available on request. If you need relevant data for research, you can contact Mr. Sun.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the Beijing Science and Technology Planning Project (Z221100007522002).

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