The production of cement and concrete using carbonated steel slag as a supplementary cementitious material achieves the dual benefits of efficient steel slag utilization and CO2 fixation. In this study, a combination of microbial technology and a rotary kiln process was employed to expedite the carbonation of steel slag for CO2 fixation from cement kiln flue gas. This approach resulted in a significant increase in the CO2-fixation rate, with a CO2-fixation ratio of approximately 10% achieved within 1 h and consistent performance across different seasons throughout the year. Investigation revealed that both the CO2-fixation ratio and the particle fineness are pivotal for increasing the soundness and reactivity of steel slag. When the CO2-fixation ratio exceeds 8% and the specific surface area is at least 300 m2∙kg−1, the soundness issue of steel slag can be effectively addressed, facilitating the safe utilization of steel slag. Residual microbes present in the carbonated steel slag powder act as nucleating sites, increasing the hydration rate of the silicate phases in Portland cement to form more hydration products. Microbial regulation results in the biogenic calcium carbonate having smaller crystal sizes, which facilitates the formation of monocarboaluminate to increase the strength of hardened cement paste. At the same CO2-fixation ratio, microbial mineralized steel slag powder exhibits greater hydration activity than carbonated steel slag powder. With a CO2-fixation ratio of 10% and a specific surface area of 600 m2∙kg−1, replacing 30% of cement clinker with microbial mineralized steel slag powder yields an activity index of 87.7%. This study provides a sustainable solution for reducing carbon emissions and safely and efficiently utilizing steel slag in the construction materials sector, while expanding the application scope of microbial technology.
Chunxiang Qian, Yijin Fan, Yafeng Rui, Xiao Zhang, Yangfan Xu.
Microbial-Enhanced Steel Slag Fixation of CO2 from Cement Kiln Flue Gas for the Production of Supplementary Cementitious Material.
Engineering, 2025, 50(7): 143-156 DOI:10.1016/j.eng.2025.03.024
As a powdered hydraulic binding material, cement is widely employed in engineering construction due to its superior performance and wide applicability. However, with a carbon dioxide (CO2) emission coefficient exceeding 0.8 per metric ton of clinker, the cement industry accounts for approximately 8% of global CO2 emissions. Hence, seeking sustainable alternatives to cement is an effective approach to reduce its carbon footprint [1], [2]. Among various existing methods, accelerating the carbonation of alkaline solid waste and utilizing it as supplementary cementitious materials to replace a portion of cement clinker is a novel measure for reducing carbon emissions and increasing the value of waste utilization.
Steel slag is an alkaline solid waste generated during the steelmaking process. The global steel industry produces over 400 million metric tons of steel slag annually, of which China alone generates over 100 million metric tons per year. However, the comprehensive utilization rate is less than 30%, and the accumulated stockpile has exceeded 1 billion metric tons [3], [4], [5]. The substantial accumulation of steel slag not only encroaches upon land resources but also results in environmental pollution, posing potential hazards to human health and thus necessitating urgent and efficient utilization [6], [7]. Due to its high hardness and the presence of silicate minerals, steel slag can be utilized as aggregate in concrete or as a partial substitute for cement clinker. However, steel slag presents challenges regarding its soundness and exhibits low reactivity when applied, limiting its safe and extensive utilization in high dosages [8], [9], [10].
The presence of certain expansive components such as free calcium oxide (f-CaO) and free magnesium oxide (f-MgO) in steel slag is the main reason behind its poor soundness. Additionally, although the dicalcium silicate (C2S) and tricalcium silicate (C3S) in steel slag exhibit some degree of hydration activity, their mineral phase structure is dense and comprises less than 50% of the total content, resulting in lower hydration activity compared with ordinary Portland cement clinker [11]. Studies addressing these issues have shown that carbonation can increase the soundness of steel slag and improve its hydration activity to some extent [12]. The reason lies in the consumption of f-CaO, f-MgO, and Ca(OH)2 in steel slag during carbonation, whereby the formation of carbonates and amorphous silica gel facilitates the acceleration of cement hydration processes [13], [14]. Therefore, carbonated steel slag can serve as a supplementary cementitious material, replacing a portion of cement clinker and thereby achieving high-value utilization.
Methods of steel slag carbonation can be classified into indirect carbonation and direct carbonation [15]. Indirect carbonation typically employs ammonium salts and acids as leaching agents to accelerate the leaching of calcium and magnesium ions from steel slag through agitation [16]. Subsequently, CO2 is introduced into the system to precipitate calcium and magnesium ions, forming carbonates and achieving carbon sequestration. However, the application of indirect carbonation is limited due to the need for additional leaching agents and the complexity of the carbonation process. Direct carbonation is a three-phase reaction process that, under certain temperature, humidity, and pressure conditions, fixes CO2 without the need for leaching agents. In this process, steel slag and CO2 undergo carbonation reactions mediated by water, ultimately producing carbonate precipitates and other byproducts. Direct carbonation is a relatively simple process with lower costs and is conducive to widespread technological application. However, the rate of the direct carbonation of steel slag in its natural state is slow, and high temperatures and pressures are often required to accelerate the reaction process, which increases the energy consumption for the entire process [17], [18]. Moreover, the process often requires pressure vessels, which hinders its large-scale application and complicates its industrialization.
In recent years, our team has developed a microbial-assisted carbonation technology for steel slag. This technology accelerates the carbonation process of steel slag by promoting the hydration reaction of CO2 under the action of microbial enzymes [19], [20], [21], [22], [23], [24], [25]. In addition, the microorganisms regulate the morphology and size of the carbonate crystals, thereby increasing the hydration activity of steel slag. This microbial technology offers advantages such as mild reaction conditions, significant effects, and low energy consumption, making it relatively straightforward for scale-up production and widespread application. However, previous studies have mainly focused on laboratory-scale investigations. Further evaluation of the effectiveness of this microbial technology in accelerating steel slag carbonation and preparing supplementary cementitious materials under cement plant flue gas conditions is essential. Furthermore, in this study, a rotary reactor was employed for the entire carbonation process. Compared with traditional static reactions, a rotary carbonation device enhances the interaction between steel slag powder and gas, further increasing the carbonation rate of steel slag. The rotary device also facilitates the continuous operation of materials, promoting the large-scale production of steel slag powder.
This study aims to verify the feasibility of the microbial-accelerated steel slag carbon-fixation technology and rotary process under the flue gas conditions of a cement plant, study the stability of the microbial-accelerated steel slag carbon-fixation effect on different batches of steel slag and different seasonal temperatures, and explore this method’s capability for carbon fixation. The relationship and mechanism between the soundness of steel slag powder, activity of steel slag powder, and CO2-fixation ratio of steel slag are elucidated. The microorganisms accelerate the carbon fixation of steel slag and improve the hydration activity, thereby laying a foundation for the industrialization of steel slag carbon-fixation technology.
2. Materials and methods
2.1. Steel slag powder
The steel slag powder used in this study was sourced from Wuhan Iron and Steel Company Ltd. (China), with a total of four batches being selected and designated as SS1, SS2, SS3, and SS4. The chemical compositions, fineness, and specific surface areas of these batches are provided in Table S1 in Appendix A. The median particle size (D50) of the steel slag powder ranged from 19.56 to 25.47 μm, and the specific surface areas ranged from 338.5 to 391.1 m2∙kg−1.
2.2. Microorganism
The microorganism employed in this study is Bacillus mucilaginosus, a Gram-positive bacterium with a cell length of 5–10 μm (Fig. S1 in Appendix A). This microorganism catalyzes the hydration reaction of CO2, accelerating the formation of carbonate precipitates by combining with cations such as those of calcium and magnesium.
2.3. Cement kiln tail flue gas
Table S2 in Appendix A presents the temperature, flow rate, and composition of cement kiln flue gases at different periods. It can be observed that the temperature of the flue gases ranges from 48.0 to 66.9 °C, with a discharge flow rate of 3.84 × 105 to 5.43 × 105 m3∙h−1. The flue gases contain 22%–31% CO2, 5.8%–10.5% oxygen (O2), 3.6–18.8 mg∙m−3 particulate matter, 3–43 mg∙m−3 sulfur dioxide (SO2), 35–248 mg∙m−3 nitrogen oxides (NOx), and 0.51–1.37 mg∙m−3 fluorides. The flue gas emitted by the cement plant complies with the Chinese standard GB 4915–2013, “Emission standard of air pollutants for cement industry.” The presence of small amounts of sulfur/nitrogen compounds in the flue gases has a minor adverse impact on the microbial carbon-sequestration reactions. In fact, moderate levels of SO2 can even facilitate the absorption of CO2 by the slag.
2.4. Portland cement
The chemical composition and strength of the Portland cement used in this study are presented in Tables S3 and S4 in Appendix A, respectively.
2.5. The process of the microbial synergistic steel slag CO2-fixation reaction
Fig. 1 provides a schematic diagram of the carbonation reaction apparatus and the flue gas transmission pathway. Taking into account factors such as device sealing and pressure drop, the air flow rate introduced should exceed the theoretical demand for flue gas flow (Eq. (1)). Therefore, the concept of a surplus coefficient is introduced (Eq. (2)). With a flue gas surplus coefficient of 10%, the flue gas from the bottom of the cement kiln’s chimney is led by a fan into the carbonation reaction apparatus, which has the dimensions ϕ1.1 m × 3 m. The steel slag powder is mixed with a liquid containing microorganisms for 5 min using a vertical agitator to obtain the steel slag powder for carbonation, where the dosage of microbial additive is 0.6 weight percent (wt%) of the steel slag powder, and the liquid-to-solid ratio is 16%. A filling coefficient is introduced to describe the amount of steel slag powder added in a single operation within the device. According to the specifications outlined in the Chinese standard HG/T 20566–2011, “Design specification for chemical rotary kiln,” the filling coefficient is defined as the ratio of the area occupied by the material on the cross-section of the cylinder along the vertical axis to the cross-sectional area of the cylinder. The steel slag powder to be carbonated is placed into the carbonation reaction device at a filling coefficient of 10%, and the rotational speed of the carbonation reaction device is controlled at 5 r∙min−1, so that the steel slag powder is in contact with the flue gas in the device and remains for 20 min to 48 h. Throughout the entire reaction process, the flue gas flows from the inlet to the outlet of the apparatus and is subjected to dust removal in order to eliminate minute particles of steel slag powder carried by the airflow; ultimately, the flue gas is recirculated to the top of the chimney and discharged into the atmosphere (Fig. 1).
As illustrated in Table S5 in Appendix A, small batches of carbonated steel slag were prepared using four batches of steel slag powder during four periods from 2022 to 2023, and the temperature and humidity conditions within the apparatus during the reaction process were monitored.
where QT is the theoretical required flue gas flow (m3∙h−1); S is the cross-sectional area of the carbon fixation reaction device (m2); L is the axial length of the carbon fixation reaction device (m); f is the filling coefficient (%), 10%; ρs is the bulk density of the steel slag powder after the addition of additives (kg∙m−3), ranging from 1.4 to 1.8 kg∙m−3; CFT is the theoretical maximum CO2-fixation ratio of steel slag (%), 25% [26]; ρg is the flue gas density (kg∙m−3); Cc is the concentration of CO2 in the flue gas (%); ts is the contact time of steel slag powder with the flue gas in the device (h); SC is the surplus coefficient (%); and Qi is the induced gas flow (m3∙h−1).
2.6. Methods
2.6.1. CO2-fixation ratio of steel slag powder
The weight loss of the carbonated steel slag samples in the temperature range of 550–850 °C was measured using a simultaneous thermal analyzer (STA 449 F5, NETZSCH, Germany) to determine their CaCO3 content. The carbonation rate of the steel slag was calculated according to Eq. (3) [25].
where CFE is the experimental CO2-fixation ratio (%); and W550 and W850 are the masses of carbonated steel slag at 550 and 850 °C (mg), respectively.
2.6.2. Mineral phase of steel slag powder before and after the CO2-fixation reaction
The phase of the steel slag powder samples was determined using an X-ray diffractometer (D8-Discover, Bruker, Germany). Al2O3 was used as an internal standard, and the X-ray diffraction (XRD) data were analyzed using Topas software to calculate the content of each phase.
2.6.3. Microstructure of steel slag powder particles before and after the CO2-fixation reaction
The microstructure of the steel slag powder samples was observed using a field-emission scanning electron microscope (Sirion, FEI, USA). Elemental scanning analysis in multiple dimensions, including point, line, and area scans, was conducted on the observed regions using an energy dispersive spectrometer.
2.6.4. Soundness and activity of steel slag powder after carbonation
According to the provisions of standard GB/T1346–2011, “Test methods for water requirement of normal consistency, setting time and soundness of the Portland cement,” Le-Chatelier method was used to test the expansion value of the samples in order to characterize their soundness. After carbonation, the steel slag was used to replace cement clinker with a 30 wt% admixture, and the cement paste specimens were molded according to a water–cement ratio of 0.26. Next, the specimens were put into a standard curing room (20 °C ± 2 °C, relative humidity of 95% ± 5%) for 24 h and then demolded, and the distance between the tips of the Le Chatelier needles was measured using digital micrometer calipers (accuracy: 0.01 mm). Subsequently, the sample was boiled for 3 h, the distance between the tips of the Le Chatelier needles was measured again, and the difference between the before and after measurements was calculated as the expansion value of the sample.
The steel slag was ground after carbonation and then sieved through a 0.075 mm square hole sieve. Referring to the provisions of standard GB/T 20491–2017, “Steel slag powder used for cement and concrete,” the sand-to-binder ratio was controlled at 1:3, and the water-to-binder ratio was 0.5. The cement was partially replaced with 30% carbonated steel slag, and the mortar specimens were prepared according to the mix proportions shown in Table 1. After molding, the samples were demolded and cured in an environment with a relative humidity of 90% ± 5% and a temperature of 20 °C ± 2 °C for 24 h. Subsequently, the samples were further cured at a relative humidity of 95% ± 5% and a temperature of 20 °C ± 2 °C for 3, 7, and 28 days. Simultaneously, a control group of cement mortar without carbonated steel slag powder was prepared using ordinary Portland cement. The compressive strength of the mortar samples was tested using a universal testing machine (UTM7305, Shenzhen SUNS Technology Stock Co., Ltd., China), and the activity index of the samples was calculated according to Eq. (4).
where γ represents the compressive strength activity index of the samples at different ages (%), Cs represents the strength of the mortar with steel slag after carbonation (MPa), and C0 represents the strength of the control group mortar (MPa).
2.6.5. Analysis of hydration heat and hydration kinetics
A sample of 135 g of SS3 after carbonation (1 h reaction) was mixed and oscillated with 225 g of mixing water to extract the microorganisms from SS3 after carbonation; this resulted in an extraction solution containing microorganisms. The microorganism cells were separated from the extraction solution using centrifugation. Next, the microorganism cells were mixed with mixing water and then mixed with Portland cement for hydration. Mixing water without microorganisms was used as the control group to investigate the influence of the microorganisms on cement hydration. To investigate the influence of the microorganism carbon-sequestration products on cement hydration, calcium carbonate mineralized by microorganisms (Bio-CaCO3) and chemically formed calcium carbonate (Che-CaCO3) were obtained following the method of Rui and Qian [27]. The Che-CaCO3 and natural mineral limestone (LS) were used as control groups. Different calcium carbonates were added to Portland cement at 5 wt% for hydration. The hydration heat of each sample was measured using an isothermal calorimeter (TAM Air, TA Instruments, USA). In alignment with Refs. [28], [29], [30], the Krstulovic–Dabic model was used to fit the hydration heat data and calculate the hydration kinetics parameters.
2.6.6. Porosity
To investigate the influence of the microorganisms and carbon-sequestration products on the porosity of cementitious materials, the porosity of each sample in Section 2.6.5 was measured. The hardened cement paste was crushed to destroy both its open and closed pores. The volume density (ρb) and porosity (P) of the samples were calculated using Eqs. (5), (6).
where ρt is the true density of the sample (g∙cm−3), is the density of water (g∙cm−3), M1 is the dry weight of the sample (g), M2 is the buoyant weight of the sample after saturation (g), and M3 is the mass of the saturated sample (g).
3. Results and discussion
3.1. Stability of the CO2-fixation ratio of steel slag powder
CO2-fixation experiments were conducted using different batches of steel slag powder at different time periods. The 1 h CO2-fixation ratios of the prepared steel slag powder are shown in Fig. 2. Herein, “BSS1–BSS4” represents the different batches of microbially mineralized steel slag powder, while “CSS3” represents the carbonated steel slag powder without microorganisms. Twenty samples were randomly selected for measurement at each time period to evaluate the stability of the microbial-enhanced carbon-sequestration effect under different time periods for different batches of steel slag powder. To better represent the variation range of the CO2-fixation ratio, an X-bar control chart was plotted. The upper control limit (UCL) and lower control limit (LCL) of the samples were calculated based on the mean value (a) of the samples and the range of three standard deviations ([a – 3σ, a + 3σ], where σ is the standard deviation). From the graph, it can be observed that the mean CO2-fixation ratio of BSS1 is 9.37% (Fig. 2(a)), that of BSS2 is 8.65% (Fig. 2(b)), that of BSS3 is 10.12% (Fig. 2(c)), and that of BSS4 is 8.87% (Fig. 2(d)). Under the same time period and the same type of steel slag, the CO2-fixation ratios of all BSS samples are within the control limit of 3σ, indicating that the CO2-fixation ratio of steel slag under microbial action is stable. However, due to the different chemical compositions and fineness of the steel slag powder from different batches, there are differences in the CO2-fixation ratio among BSS1–BSS4. It is worth noting that, under the microbial-enhanced carbon-sequestration effect, BSS has a higher CO2-fixation ratio than CSS. For example, when using SS3 for 1 h carbon sequestration, the mean CO2-fixation ratio of CSS3 is 6.44%, while that of BSS3 is 10.12%, indicating an approximately twofold increase in the CO2-fixation rate (Fig. 2(c)).
3.2. Soundness and activity of CSS and/or BSS
3.2.1. Soundness of CSS and/or BSS
Since the SS3 steel slag powder had a higher CO2-fixation ratio than the other three batches of steel slag powder under the same carbon-sequestration reaction time, this batch of steel slag was selected for further investigation. By varying the carbon-sequestration time (from 20 min to 48 h), CSS3 and/or BSS3 with different CO2-fixation ratios (4%–18%) were obtained to analyze the effect of the CO2-fixation ratio on the soundness of the steel slag powder (Fig. 3). The results show that, as the CO2-fixation ratio increases, the expansion values of BSS3 and CSS3 initially decrease and then stabilize at around 0.5 mm, when the CO2-fixation ratio is approximately 10% (Fig. 3(a)). This finding indicates that carbon-sequestration reactions can improve the soundness of steel slag, and this effect increases with an increase in the CO2-fixation ratio. After reaching a CO2-fixation ratio of 10%, the expansion value remains relatively constant, indicating that the expansive components in the steel slag have been mostly consumed. From Fig. 3(b), it can be seen that, when the content of f-CaO is below 0.5%, the expansion value approaches 0 mm; moreover, when the content of f-CaO exceeds 2.5%, the expansion value exceeds 5 mm, indicating that the soundness is not qualified. In this study, the content of the MgO phase in the steel slag powder is 2.87% (≤ 5%). The requirements of the standard GB/T 20491–2017 indicate that it is not necessary to test the soundness of the autoclave. According to Fig. 3(c), when the CO2-fixation ratio of steel slag is greater than 8% and the D50 is less than 58 μm (corresponding to a specific surface area of approximately 300 m2∙kg−1), the steel slag’s soundness is qualified (a linear expansion value less than 1%), and the steel slag can be safely used.
3.2.2. Activity improvement and mechanisms of CSS and/or BSS
After grinding BSS3 and CSS3 to a similar fineness (D50 ≈ 21 μm, specific surface area ≈ 600 m2∙kg−1), the effect of the carbon-sequestration rate on the activity of the steel slag powder was analyzed. The compressive strengths of cementitious materials incorporating different CO2-fixation ratios of CSS3/BSS3 are shown in Fig. 4(a). A comparison of the different CO2-fixation ratios of CSS3/BSS3 indicates that the compressive strength increases with an increase in the CO2-fixation ratio at different hydration ages, and BSS3 exhibits superior hydration activity. When the CO2-fixation ratio of BSS3 increases from 5.12% to 18.31%, the compressive strength of the cementitious material at 28 days increases from 44.57 to 54.05 MPa, representing an increase of approximately 21.27%. Fig. 4(b) illustrates the influence of the CO2-fixation ratio on the activity index of carbonated steel slag powder at 7 and 28 days. It can be seen that the increase in the activity index is insignificant at lower CO2-fixation ratios. When the CO2-fixation ratio is below 5.0%, the activity index of CSS3/BSS3 does not differ significantly from that of SS3 (indicated by the red line). However, as the CO2-fixation ratio increases, the activity of CSS3/BSS3 shows a significant upward trend. When the CO2-fixation ratio exceeds 5%, both CSS3 and BSS3 reach the first-level standard specified in GB/T 20491–2017 at 7 days; when the CO2-fixation ratio exceeds 7%, the 28-day activity index of BSS3 meets the first-level standard; and, when the CO2-fixation ratio exceeds 8%, the 28-day activity index of CSS3 meets the first-level standard. When the CO2-fixation ratio exceeds 10%, the activity indexes of BSS3 at 7 and 28 days are higher than those of CSS3, with a 7-day activity index of 80.2%, representing an increase of 18.4%, and a 28-day activity index of 87.7%, representing an increase of 18.1%. When the CO2-fixation ratio of CSS3/BSS3 exceeds 12.0%, the rate of increase in the 28-day activity index slows down, and the 28-day activity index of BSS3 exceeds 90%, surpassing that of CSS3.
To elucidate the mechanism of the effect of different CO2-fixation ratios on the activity of steel slag, the hydration heat, hydration kinetics, hydration products, and porosity characteristics of cementitious materials incorporating different CO2-fixation ratios of steel slag powder were studied. The hydration heat results (Fig. 5) show that, as the CO2-fixation ratio increases (from 6.32% to 18.31%), the hydration heat evolution rate gradually increases, and the main peak shifts to the left. The peak value of the hydration heat evolution rate increases from 0.00181 to 0.00204 W∙g−1 (Fig. 5(a)), and the cumulative hydration heat increases from 146.55 to 160.09 J∙g−1 (Fig. 5(b)). This is because a higher CO2-fixation ratio indicates a higher degree of carbon-sequestration reaction in the steel slag, leading to more severe erosion on the surface of the mineralized steel slag powder and enhancing the hydration reaction. Based on the Krstulovic–Dabic hydration heat model, the hydration process of cementitious materials incorporating different CO2-fixation ratios of BSS3 follows this sequence: nucleation and crystal growth (NG)→interactions at phase boundaries (I)→diffusion (D) (Figs. 5(c)–(f)). With an increase in the CO2-fixation ratio, although the span of process I does not change significantly, the degree of hydration (α) at the phase-transition point gradually decreases. This finding indicates that increasing the CO2-fixation ratio can accelerate the phase-transition point of cementitious materials. The hydration kinetics results (Table 2) show that the reaction rate constant of the NG process is the highest, and the induction period time of cementitious materials incorporating different CO2-fixation ratios of BSS3 ranges from 2.3 to 2.5 h. With an increase in the CO2-fixation ratio, the crystal growth index first increases and then decreases, while the cumulative hydration heat gradually increases with an increase in the CO2-fixation ratio (from 234.2 to 266.7 J∙g−1).
Figs. 6(a) and (b) show the XRD patterns and infrared spectra of cementitious materials hydrated for 28 days. From Fig. 6(a), it can be observed that the hydration products of cementitious materials incorporating different CO2-fixation ratios of steel slag powder at 28 days contain Ca(OH)2, CaCO3, C2S, and C3S. The characteristic peak intensity of the (104) crystal plane of the CaCO3 mineral phase strengthens with an increase in the CO2-fixation ratio. At a lower CO2-fixation ratio, hemicarboaluminate (Hc) is mainly formed; with an increase in the CO2-fixation ratio, monocarboaluminate (Mc) appears. In the infrared spectra (Fig. 6(b)), the absorption peaks of crystalline water (1640 cm–1) and the O–H bond in Ca(OH)2 can be observed. In addition, the absorption peak of carbonate ions and the Q3-type Si–O bond in the C–S–H gel can be found at ∼970 cm–1. Considering the composite effect of calcium carbonate with cementitious materials and the amorphous-phase pozzolanic reaction, which are inherent mechanisms influencing the activity of mineralized steel slag powder, the influence of the contents of calcium carbonate and amorphous phase on the activity at different hydration ages was investigated, as shown in Figs. 6(c) and (d). It can be seen that an increase in the contents of calcium carbonate and amorphous phase in mineralized steel slag powder with different CO2-fixation ratios significantly increases the hydration activity. When the contents of calcium carbonate and amorphous phase exceed 20.0% and 7.5% respectively, the enhancement effect becomes less pronounced. The top activity at 7 days reaches 88%, while the top activity at 28 days reaches 91.6%. As increasing the CO2-fixation ratio of steel slag powder can accelerate the hydration heat evolution rate and promote the formation of more calcium carbonate and/or amorphous phase to facilitate the formation of Mc and/or the pozzolanic reaction effect, the porosity of cementitious materials also changes accordingly. From Fig. S2 in Appendix A, it can be observed that, at 28 days of hydration, as the CO2-fixation ratio increases (from 6.32% to 18.31%), the porosity decreases from 21.88% to 19.04%, indicating a decrease of 12.98%. This finding suggests that increasing the CO2-fixation ratio can make the microstructure of cementitious materials more compact.
3.3. Mechanism by which microorganisms accelerate the CO2 fixation of steel slag powder
3.3.1. Effect of microorganisms on the phase of steel slag powder
To investigate the effect of microorganisms on the phase composition of steel slag during CO2 fixation, the phases and phase contents of SS3, CSS3, and BSS3 after 48 h of CO2-fixation reaction were tested (Fig. 7). Fig. 7(a) shows that SS3 contains phases such as srebrodolskite (Ca2Fe2O5, C2F), mayenite (Ca12Al14O33, C12A7), β/γ-C2S, C3S, portlandite (Ca(OH)2), free oxides (f-MgO and f-CaO), wustite (FeO), and olivine ((Mg,Fe)2SiO4). CSS3 and BSS3 have the same phases; however, in comparison with SS3, they also have newly formed phases including CaCO3, MgCO3, and an amorphous phase. Fig. 7(b) and Table 3 show the content of each phase in SS3, CSS3, and BSS3, further demonstrating the effect of the microorganisms on the CO2 fixation of steel slag. It can be seen that the main phases in SS3 are β/γ-C2S, C3S, C2F, C12A7, and (Mg,Fe)2SiO4, with contents of 29.51%, 10.55%, 19.81%, 12.57%, and 14.52%, respectively. SS3 also contains 3.62% Ca(OH)2, 2.87% f-MgO, 2.54% f-CaO, 1.85% FeO, and 2.16% amorphous phase (denoted as “Amor” in Fig. 7(b)). Compared with SS3, the content of β/γ-C2S in CSS3 is 43.75% lower, that of C3S is 41.80% lower, that of C2F is 16.36% lower, that of C12A7 is 23.79% lower, and that of (Mg,Fe)2SiO4 is 25.55% lower. Ca(OH)2 and f-CaO are almost depleted in CSS3, and the absolute content of f-MgO is only 1.91%. This finding indicates that the main phases consumed during the CO2-fixation reaction of steel slag are C2S and C3S, and the improvement in the soundness of the steel slag after CO2 fixation is due to the consumption of f-CaO and f-MgO. Under the action of the microorganisms, the contents of C2S, C3S, C12A7, and C2F are further consumed in BSS3. Compared with SS3, the absolute content of β/γ-C2S in BSS3 is 8.44%, a decrease of 71.40%; the absolute content of C3S is 3.35%, a decrease of 68.25%; the absolute content of C2F is 14.24%, a decrease of 28.12%; the absolute content of C12A7 is 7.18%, a decrease of 42.88%; and the absolute content of (Mg,Fe)2SiO4 is 9.10%, a decrease of 37.33%. In addition, Ca(OH)2 and f-CaO are completely depleted, and the absolute content of f-MgO is 1.05% lower compared with that before reaction. This finding indicates that, the major phases consumed in BSS3 are C2S, C3S, and C12A7, and the degree of reaction of each phase is higher at the same reaction time. The silicon and aluminum phases in steel slag undergo carbonation reactions to form carbonates and amorphous silica gel. The main carbonation products in BSS3 and CSS3 differ significantly. The amount of CaCO3 generated in CSS3 is 24.84%, while that in BSS3 is 36.04%. The increase in BSS3 is more than 50%, which is in good agreement with the CO2-fixation ratios of CSS3 and BSS3 (12.10% and 20.12%, respectively). Furthermore, the microorganisms accelerate the CO2 fixation of the refractory phases. Apart from a small amount of calcium ferrite, the transformation of magnesium-containing minerals is promoted by the microorganisms. Compared with that in CSS3, the content of MgCO3 in BSS3 is 1.71% higher. Because of the consumption of more silicates, the absolute content of the amorphous phase in BSS3 is also higher, but this increase is slightly lower than that of the carbonates. This type of amorphous phase typically acts as an active component, participating in the pozzolanic reaction and providing strength, thereby contributing to the promotion of hydration in BSS3.
3.3.2. Effect of microorganisms on microstructure of steel slag powder particles
To investigate the influence of the microorganisms on the microstructure of the steel slag particles during CO2 fixation, the microscopic structure and elemental compositions of SS3, CSS3, and BSS3 after 48 h of CO2-fixation reaction were analyzed separately (Fig. 8). The backscattered electron images (Fig. 8(a)) show that the edges of the SS3 particles are relatively distinct and angular (Fig. 8(a-i), marked in red). Generally speaking, different mineral phases possess distinct optical properties. As shown in the figure, the dark gray matrix mainly consists of the calcium silicate mineral phase, the light gray matrix mainly consists of the calcium–iron mineral phase, and the dark black matrix mainly consists of the RO phase (FeO–MnO–MgO solid solution). After 48 h of CO2-fixation reaction, product layers appear on the edges of some particles in CSS3 and BSS3 (Figs. 8(a-ii) and (a-iii)). Since it is primarily the silicate mineral phase that is consumed during the CO2-fixation process (Fig. 7(b)), significant reaction transition zones (marked in red) appear at the silicate mineral particles of CSS3 and BSS3. This is because, during the CO2-fixation process, the phases of the steel slag gradually dissolve from the edges to form a porous structure, with carbonate gradually accumulating in the outermost layer, leading to the gradual filling of pores and the formation of a loose inner region and a dense outer region. However, the reaction transition zone in BSS3 is more pronounced than that in CSS3.
To further demonstrate the differences between CSS3 and BSS3, linear scans were conducted of the mineral phases at the red-marked regions in Figs. 8(a-ii) and (a-iii); the results are shown in Fig. 8(b). In CSS3, the intensities of calcium (Ca) and silicon (Si) gradually decrease along the scanning direction (indicated by the yellow arrow in Fig. 8(a-ii)), while the intensity of carbon (C) increases from the starting point at 5.0 to 10.8 μm (Fig. 8(b-i)). This range mainly corresponds to the solid carbonation reaction zone of the minerals, including the dissolution of the silicate mineral phases and the formation of carbonates, with a reaction depth of approximately 5.8 μm. In contrast, the reaction depth of BSS3 is deeper (approximately 9 μm), and a distinct silicon-rich region can be observed at a distance of 10 μm from the scanning starting point (Fig. 8(b-ii), the left yellow circle). This is due to the gradual diffusion of Ca2+ dissolved from the steel slag, leading to a significant decrease in the intensity of Ca, while the intensity of Si remains constant. With the increasing scanning distance, the intensity of Ca reaches its peak at a distance of 14 μm from the starting point (Fig. 8(b-ii), the right yellow circle), and the intensity of Si gradually weakens, indicating the presence of a large amount of calcium carbonate in this region. Therefore, it can be concluded that the microbial activity significantly increases the reactivity of the mineral phases in the steel slag, resulting in the formation of a composite SiO2–CaCO3 structure.
The distribution of the surface elements in BSS3 was scanned, and the results are shown in Fig. 8(c). It can be observed that the interior of the particle shown in the figure mainly consists of oxygen (O), Si, and Ca. Taking these findings together with the information from the backscattered electron image (Fig. 8(a-iii)), it can be inferred that the mineral phase composition is calcium silicate. The outer layer of the particle mainly contains C and Ca (Figs. 8(c-iv) and (c-v)), along with small amounts of magnesium (Mg) and iron (Fe) (Figs. 8(c-vi) and (c-vii)). An examination of the distribution of O, Si, C, and Ca reveals that there is a circle between the silicate phase and the outermost layer, which is rich in O and Si elements but lacks Ca element (Figs. 8(c-ii), (c-iii) and (c-v)). This finding indicates that the silicon-rich region mainly consists of SiO2, a byproduct of the silicate carbonation reaction. The outer layer is rich in O, C, and Ca, with calcium carbonate as the main phase, suggesting that dissolved calcium ions migrate from the silicate phase to the outer layer during the carbonation process, reacting with CO32– to form calcium carbonate, while SiO2 remains in the interior region surrounding the mineral phase in an amorphous form. Therefore, the carbonation of steel slag involves the leaching of mineral ions and their reaction with CO32– to generate calcium carbonate and SiO2. The calcium carbonate gradually deposits on the outer layer of the mineral particles, ultimately forming a layered structure of SiO2–CaCO3 composite material (Fig. 8(c-i)).
3.4. Mechanism by which microorganisms increase the activity of steel slag powder
3.4.1. Effect of microorganisms and carbonate products in BSS on the hydration of Portland cement
Figs. 9(a) and (b) illustrate the influence of the microorganisms on the heat evolution rate and the cumulative heat hydration of Portland cement. Compared with the cement without microorganisms, the peak heat evolution rate of the cement increases from 0.0028 W∙g−1 to approximately 0.0030 W∙g−1 when microorganisms are added, and the main hydration peak shifts to the right (Fig. 9(a)). This result indicates that, although microorganisms delay the heat evolution peak, they can increase the heat evolution rate. This is because the microorganisms act as nucleation sites to increase the hydration; moreover, their cell membrane surfaces with negative charges adsorb calcium ions, thereby increasing the number of ion collisions and thus increasing the hydration rate. From Fig. 9(b), it can be seen that the addition of microorganisms increases the cumulative hydration heat of the cement, with the heat release increasing from 205.56 to 216.27 J∙g−1. Therefore, the presence of microorganisms in BSS3 is an important factor in enhancing its hydration rate.
Figs. 9(c) and (d) depict the influence of calcium carbonate on the heat evolution rate and the cumulative hydration heat of Portland cement. It can be observed that, regardless of the type of calcium carbonate, its addition to cement significantly increases the heat evolution rate and leads to an earlier heat release peak (Fig. 9(c)). The peak heat evolution rates for cement with bio-synthesized Bio-CaCO3, chemically synthesized Che-CaCO3, and LS powder are 0.0035, 0.0031, and 0.0030 W∙g−1, respectively, representing increases of 25.0%, 10.7%, and 7.1% compared with the control group. These findings indicate that Bio-CaCO3 has a more significant enhancing effect on cement hydration than Che-CaCO3 or LS. Fig. 9(d) shows that, at 36 h of hydration, the cement containing Bio-CaCO3 exhibits the highest cumulative hydration heat of 223.31 J∙g−1, representing a 9.85% increase compared with the control group. The cumulative hydration heat of cement with Che-CaCO3 is comparable to that of the control group. Although the cumulative hydration heat of cement with LS is higher than that of the control group before 24 h, it decreases after 24 h of hydration. Therefore, the microbial mineralization of Bio-CaCO3 significantly promotes cement hydration.
Fig. S3 in Appendix A shows the fitting curves of the degree of hydration of cement with the addition of microorganisms and the carbonation product calcium carbonate. It can be observed that the hydration process of the cement with microorganisms and calcium carbonate follows the same NG→I→D hydration process. Compared with the control group, there is no significant difference in the degree of hydration corresponding to the microorganism transition point (α), but the transition points for cement with different types of calcium carbonate are all lowered. This is because calcium carbonate does not participate in the hydration reaction in the early stages but mainly has a dilution effect on cement and a nucleation effect on hydration products. Therefore, the degree of hydration is relatively lower when reaching the same stage compared with that of the control group. In addition, the transition points for cement with different types of calcium carbonate are similar, indicating that different types of carbonates have similar effects on the early hydration of cement, mainly contributing to dilution and nucleation.
Calculation of the hydration kinetics parameters (Table 4) reveals that the reaction rate constant for the NG process is the largest, approximately 4 times that of process I and 15 times that of process D. The hydration reaction rate constants for cement with the addition of microorganisms are similar to those of the control group, indicating that the microorganisms have a negligible effect on the reaction rate constants. However, the crystal growth index of the hydration products increases with the addition of microorganisms, indicating that the microorganisms affect the processes of crystallization nucleation and crystal growth of the hydration products. In addition, the microorganisms prolong the induction period of cement by approximately 0.32 h and increase the cumulative heat release at the termination of hydration by 27.6 J∙g−1, indicating that the microorganisms enhance the hydration of cementitious materials.
Regarding the influence of carbonate minerals on the hydration kinetics of cement, the addition of different types of calcium carbonate significantly affects the processes of crystallization nucleation and crystal growth of the hydration products, with higher crystal growth indices compared with the control group. The reaction rate constants for the NG and I processes are increased. From the acceleration period onward, the induction period of cement is shorter with the addition of calcium carbonate, being reduced by approximately 0.3 h. The maximum heat release of cement with the addition of Bio-CaCO3 is higher than that of the control group, while the maximum heat release of cement with the addition of Che-CaCO3 and LS is lower than that of the control group. These findings indicate that Bio-CaCO3 promotes hydration more than Che-CaCO3 and LS.
3.4.2. Effects of microorganisms and calcium carbonate in BSS on the porosity of hardened cement pastes
Fig. S4 in Appendix A shows the influence of microorganisms and calcium carbonate on the porosity of hardened cement pastes. As the hydration age increases, it can be seen that the hydration products gradually fill the pores of the cementing materials during the hydration process, resulting in a gradual decrease in the porosity of the hardened cement paste and a denser structure. Compared with the control group, the porosity of the cement with added microorganisms at 7 and 28 days is lower, at 15.55% and 13.31%, respectively, with a reduction in porosity of 11.50% and 11.38%. This finding indicates that the microorganisms can reduce the porosity of cement and make the structure more compact. A comparison of different types of calcium carbonate reveals that the porosity of the LS specimen with added calcium carbonate is similar to that of the control group. The porosity of cement paste with added Bio-CaCO3 and Che-CaCO3 at 7 and 28 days of hydration is lower than that of the control group. The porosity of cement with added Che-CaCO3 at 7 and 28 days is 16.33% and 14.21%, respectively, with a decrease of 7.06% and 5.40% compared with the control group. The porosity of cement with added Bio-CaCO3 at 7 and 28 days is 14.87% and 13.32%, respectively, with a decrease of 15.37% and 11.32% compared with the control group. These findings indicate that Bio-CaCO3 has a better filling effect on the porosity of cement, resulting in hardened cement paste with a denser structure.
To investigate the reasons why Bio-CaCO3 performs better in promoting cement hydration and filling hardened paste pores compared with other forms of calcium carbonate, a further analysis was conducted. Fig. S5 in Appendix A shows scanning electron microscopy images of the microstructures of Bio-CaCO3, Che-CaCO3, and LS at similar sizes (D50 ≈ 6.0 μm). It can be observed that Bio-CaCO3 mainly consists of spherical particles with a small number of attachments on the particle surface. Che-CaCO3 mainly consists of cubic particles with an interlocking structure and a relatively smooth surface. LS particles, on the other hand, have irregular morphology and are mostly in block form; moreover, compared with the Bio-CaCO3 and Che-CaCO3 particles, the surface of the LS particles is rougher and has small particles adsorbed onto it. The XRD spectra of the three types of calcium carbonate (Fig. S6 in Appendix A) show that the crystal phases of Bio-CaCO3 and Che-CaCO3 are mainly calcite and vaterite, with the calcite (104) crystal plane being the most prominent at 29°. The vaterite crystal planes (100), (112), (114), (300), and (200) appear at 24°, 27°, 32°, 43°, and 51°, respectively. LS is mainly composed of calcite, but there are also diffraction peaks of SiO2 at 21.0° and 26.6°. This is because LS is a naturally occurring mineral that contains some impurities [31], [32]. Because of the significant difference in crystal structure between LS and Bio-CaCO3 and/or Che-CaCO3, and the lower contribution of the crystal structure to promoting hydration and filling paste pores, the crystal content and size of Bio-CaCO3 and Che-CaCO3 were further calculated, as shown in Table 5. It can be seen that Bio-CaCO3 and Che-CaCO3 have similar calcite proportions, accounting for approximately 96.79% and 97.21%, respectively. However, the crystal size of Che-CaCO3 is 61.1 nm, while the crystal size of Bio-CaCO3 is 30.7 nm. This finding indicates that the calcium carbonate crystal size is smaller under microbial action. This difference in size may be an important factor in the faster hydration, larger heat release during hydration, and formation of denser hardened paste in cement with added Bio-CaCO3.
3.4.3. Explanation of mechanism
The mechanism by which the hydration activity of cementitious materials is increased by CSS and BSS is shown in Fig. 10. After CSS is incorporated into cementitious materials, the remaining C2S, C3S, C12A7, C2F, and new amorphous phases after carbonation in CSS undergo hydration reactions, resulting in the formation of a large amount of C–H and C–S–H gel. The Che-CaCO3 presented in the CSS reacts with aluminate phases to generate Mc, which enhances the strength of the cementitious materials. Compared with CSS, BSS exhibits higher hydration activity, primarily due to the following mechanisms: ① The remaining C2S, C3S, C12A7, C2F, and new amorphous phases after carbonation in BSS undergo hydration reactions, producing a significant amount of C–H and C–S–H gel, which contributes to the strength of the cementitious materials. ② Due to the higher carbonation rate of BSS under the same carbonation time, more Bio-CaCO3 reacts with the aluminate phases in the cementitious materials, leading to a greater increase in strength. ③ The smaller crystal size of Bio-CaCO3 compared with Che-CaCO3 results in a higher peak heat evolution rate and a greater cumulative heat release during hydration, leading to a better pore-filling effect in the hardened paste and enhancing the hydration activity of BSS. ④ The residual microbial biomass in BSS provides nucleation sites, significantly increasing the hydration reaction rate and cumulative heat release of the cementitious materials and thereby promoting a denser structure of the hardened paste.
4. Conclusions
This study utilized microbial-enhanced CO2-fixation technology under cement kiln flue gas conditions to prepare BSS and compare its effects with those of CSS without microbial assistance. The CO2-fixation ratios of four different batches of steel slag powder were measured under four different seasonal temperatures and flue gas conditions. The stability of this technology was verified, and the soundness and the activity of the CSS and/or BSS were investigated. The mechanisms by which the microbes enhanced the CO2 fixation of steel slag and the activity of BSS were explored. The following conclusions were drawn.
(1) Under microbial action, the CO2-fixation ratio of BSS3 was much greater than that of CSS3, with a CO2-fixation ratio of 10.12%–6.44% after 1 h of reaction. The CO2-fixation ratio of all the samples met the control limit (3σ) requirements, indicating the good stability of CO2 fixation for different batches of steel slag under different seasonal temperatures and fluctuations in flue gas composition.
(2) When the CO2-fixation ratio of BSS3 exceeded 8% and D50 was less than 58 μm (a specific surface area greater than 300 m2∙kg−1), the soundness problem of steel slag powder was solved. When the D50 of BSS3 was 21 μm (a specific surface area of approximately 600 m2∙kg−1), increasing the CO2-fixation ratio of BSS3 increased its activity. At a CO2-fixation ratio of 8.28%, the 28-day activity reached 80.5%; at a CO2-fixation ratio of 10.02%, the 28-day activity reached 87.7%; and at a CO2-fixation ratio of 11.73%, the 28-day activity reached 89.4%.
(3) Microorganisms enhance the degree of carbonation reaction undergone by mineral phases such as C2S and C3S in steel slag, with respective carbonation ratios of 71.40% and 68.25% after 48 h of reaction, representing respective increases of 63.2% and 63.3% in BSS3 compared with CSS3. Moreover, the microorganisms accelerate the carbonation of phases such as calcium ferrite and magnesium-containing minerals, leading to the formation of more carbonates, amorphous silica gel, and iron oxide gel. After carbon sequestration, a composite structure of SiO2–CaCO3 forms around the BSS particles, and the CO2-fixation reaction has a longer transition zone compared with that of CSS.
(4) Compared with CSS, BSS has more calcium carbonate, leading to the formation of more Mc during cement hydration, which has the benefit of improving the cement strength. The residual microbial cells in BSS act as nucleation sites, enhancing the hydration reaction of the steel slag. Additionally, the microbial formation of Bio-CaCO3 with a smaller grain size increases the heat evolution peak, the cumulative hydration heat, and the hydration kinetics constants. Under the effect of the microbes, the structure of the hydrated cementitious materials becomes denser, and the hydration activity is increased.
(5) Microbial-assisted CO2 fixation of steel slag can not only contribute to accelerating the carbonation process and reducing carbon emissions in the cement industry but also promote the high-value application of steel slag powder to replace a part of cement clinker. Thus, the proposed approach holds great potential for industrialization.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The research presented in this paper was sponsored by the National Key Research and Development Program of China (2021YFB3802000 and 2021YFB3802004) and the National Natural Science Foundation of China (52172016).
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