Study on Mineral Compositions of Direct Carbonated Steel Slag by QXRD, TG, FTIR, and XPS

energies

Article Study on Mineral Compositions of Direct Carbonated Steel Slag by QXRD, TG, FTIR, and XPS

Xue Wang 1,2 , Wen Ni 1,2,*, Jiajie Li 1,2 , Siqi Zhang 1,2 and Keqing Li 1,2

1 School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (X.W.); [email protected] (J.L.); [email protected] (S.Z.); [email protected] (K.L.) 2 Key Laboratory of Resource-Oriented Treatment of Industrial Pollutants, Beijing 100083, China * Correspondence: [email protected]

Abstract: Steel slag CO2 sequestration helps mitigate global warming and decrease the stockpile of steel slag (SS). Through orthogonal design tests and single-factor tests, this paper evaluated the effects of the water/solid mass ratio (w/s), gypsum ratio (G/SS), molding pressure, and curing duration

on uniaxial compressive strength (UCS) and CO2 uptake of the compacts. The results indicated that high w/s enhanced both strength and CO2 capture ability. The proper addition of gypsum helps promote UCS increase and CO2 uptake of steel slag. In addition, increasing the molding pressure can signiﬁcantly improve UCS without reducing CO2 uptake. The optimum conditions in the study were a w/s of 0.20, G/SS of 1/16, and molding pressure of 27 MPa, under which conditions

1 d UCS and CO2 uptake were 55.30 MPa and 12.36%, respectively. Microanalyses showed that gypsum activates mainly mayenite in steel slag. An increase in water addition also increased the hydration and carbonation products greatly, and the strengthened molding pressure had a signiﬁcant densiﬁcation effect on micro-pore structures. The study gives guidance in the application of steel slag in CO2 capture and manufacturing green construction material. Citation: Wang, X.; Ni, W.; Li, J.; Zhang, S.; Li, K. Study on Mineral Keywords: carbon capture; orthogonal test design; gypsum; steel slag; UCS; XPS Compositions of Direct Carbonated Steel Slag by QXRD, TG, FTIR, and XPS. Energies 2021, 14, 4489. https:// doi.org/10.3390/en14154489 1. Introduction Carbon dioxide emissions contribute to global warming in ways that cannot be ig- Academic Editor: Rouhi Farajzadeh nored. Feasible carbon capture and storage (CCS) technologies mainly include geological, ocean, biological, and mineral sequestration [1], among which mineral sequestration is Received: 1 July 2021 considered the safest method. It sequesters carbon dioxide from emission sources, such as Accepted: 19 July 2021 Published: 24 July 2021 steel companies and power stations, with Ca- or Mg-bearing minerals to form thermodynamically stable carbonates [2–4].

Publisher’s Note: MDPI stays neutral Industrial solid wastes, including blast-furnace slag [5], coal fly ash [6,7], and steel with regard to jurisdictional claims in slag [8–14], were chosen to be suitable CCS feedstock because of their low cost, high carbon published maps and institutional affil- reactivity, and wide availability. Blast-furnace slag and coal fly ash are well studied and iations. utilized as supplementary cement materials, enhancing cement performance. On a related note, only 29.5% of steel slag is used in China [15]. Most of it is stockpiled, occupying land resources and polluting the environment [16,17]. A total of 1.8 billion tons of steel slag was stockpiled until 2018 in China, with 110 million tons generated each year. As the world’s largest emitter of carbon dioxide and the largest steel producer, China has promised to Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. become carbon neutral before 2060. Using steel slag to capture CO2 is one of the most This article is an open access article feasible ways of dealing with steel slag and CO2 emission at a low cost. distributed under the terms and The chemical composition of steel slag includes mainly CaO, 45–60%; SiO2, 10–15%; conditions of the Creative Commons Al2O3, 1–5%; Fe2O3, 3–9%; MgO, 3–13%; FeO, 7–20%; and P2O5, 1–4% [17]. The main Attribution (CC BY) license (https:// minerals in steel slag include tricalcium silicate (C3S), dicalcium silicate (C2S), RO phase (a creativecommons.org/licenses/by/ continuous solid solution formed by MgO, FeO, and MnO), f-CaO, f-MgO, calcium ferrite, 4.0/). and calcium aluminoferrite. Minerals such as C2S and C3S are hydraulic components,

Energies 2021, 14, 4489. https://doi.org/10.3390/en14154489 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 18

and calcium aluminoferrite. Minerals such as C2S and C3S are hydraulic components, making steel slag a potential supplement in the cement and concrete industry. However, f- CaO, f-MgO, and the relatively low-hydration-activity RO phase cause expansion after hydration reactions. In addition, the addition of steel slag in cement usually results in strength degradation because of low hydration activity [18–20]. Carbonation is considered an effective way to solve these problems. Steel slag has a significantly high carbon reactivity because of its high alkalinity caused by oxides. Liu et al. [21] found that carbonated steel slag can even enhance the mechanical properties of cementitious materials. Furthermore, through carbonation, the volume stability was improved considerably, as the carbonate minerals formed were thermodynamically stable [22–24]. Besides supplementing steel slag in cement, some researchers focused on their carbonation for building materials such as bricks or blocks to optimize their use. Before carbonation, fresh samples are mostly molded into specific shapes. The shaping methods include precasting and press shaping. The former molding method proceeds using the cementitious properties of steel slag. The latter method mainly uses a molding machine to press the samples into specific shapes for carbonation curing, as shown in Figure 1. For the second shaping method, three main factors influence the samples’ mechanical and CO2 capture performance: the raw material (steel slag types and activator), water/solid mass ratio (w/s), and molding pressure. In China, 70% of the steel slag produced is converter slag, the type we chose for this study. Gypsum is reportedly an effective activator for the hydration reaction of steel slag. Thus, this study also aims to determine the optimal amount of gypsum to be added to steel slag carbonation samples. To study factors such as the activator amount, w/s, and molding pressure on the samples, orthogonal ex- Energies 2021, 14, 4489 2 of 18 periments and single-factor experiments were performed to achieve optimized parameters. Microstructural analyses, such as Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP), and X-ray pho- makingtoelectron steel spectroscopy slag a potential (XPS), supplement were conducted in the to cement obtain and the concreteassociated industry. mechanisms. However, f-CaO,Gypsum f-MgO, could and the be relativelyused as an low-hydration-activity activator for steel slag hydration RO phase and cause carbonation expansion in after this hydrationstudy. To the reactions. best of our In addition, knowledge, the proportion addition of-optimization steel slag in cementdesign for usually the gypsum results-ac- in strengthtivated steel degradation slag carbonation because ofsamples low hydration has been activity rarely studied. [18–20]. This study optimized the proportionCarbonation for blocks is considered through orthogonal an effective experimental way to solve design these and problems. range analyses, Steel slag giving has aus significantly information high about carbon how reactivity w/s, gypsum because amount, of its high and alkalinity molding pressure caused by impact oxides.ed Liuthe etstrength al. [21] and found CO that2 capture carbonated capacity steel of the slag samples, can even providing enhance guidance the mechanical for industrial properties prac- oftical cementitious application. materials. In addition, Furthermore, the single-factor through experiments carbonation, and thethe volumecorresponding stability micro- was improvedanalysis methods considerably, helped as to the explore carbonate the chemical minerals and formed microstructural were thermodynamically properties of sam- sta- bleples, [22 as–24 well]. Besides as the reaction supplementing mechanisms. steel slag QXRD, in cement, especially, some can researchers clearly show focused the onamount their carbonationchanges of minerals for building in ca materialsrbonated suchsamples as bricks under or the blocks influence to optimize of carbonation their use. duration, Before carbonation,w/s, and molding fresh samplespressure. are MIP mostly can clearly molded show into specificthe pore shapes. structure The in shaping carbonated methods sam- includeples. The precasting results of andthe paper press shaping.offer reliable The guidance former molding for the efficient method proceedspreparation using of gyp- the cementitioussum-activated properties steel slag of carbonate steel slag. blocks. The latter method mainly uses a molding machine to press the samples into specific shapes for carbonation curing, as shown in Figure1.

Figure 1. Press-shaping methods and subsequent carbonation curing of steel slag.

For the second shaping method, three main factors influence the samples’ mechanical and CO2 capture performance: the raw material (steel slag types and activator), water/solid mass ratio (w/s), and molding pressure. In China, 70% of the steel slag produced is converter slag, the type we chose for this study. Gypsum is reportedly an effective activator for the hydration reaction of steel slag. Thus, this study also aims to determine the optimal amount of gypsum to be added to steel slag carbonation samples. To study factors such as the activator amount, w/s, and molding pressure on the samples, orthogonal experiments and single-factor experiments were performed to achieve optimized parameters. Microstructural analyses, such as Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP), and X-ray photoelectron spectroscopy (XPS), were conducted to obtain the associated mechanisms. Gypsum could be used as an activator for steel slag hydration and carbonation in this study. To the best of our knowledge, proportion-optimization design for the gypsum- activated steel slag carbonation samples has been rarely studied. This study optimized the proportion for blocks through orthogonal experimental design and range analyses, giving us information about how w/s, gypsum amount, and molding pressure impacted the strength and CO2 capture capacity of the samples, providing guidance for industrial practical application. In addition, the single-factor experiments and the corresponding microanalysis methods helped to explore the chemical and microstructural properties of samples, as well as the reaction mechanisms. QXRD, especially, can clearly show the amount changes of minerals in carbonated samples under the influence of carbonation duration, w/s, and molding pressure. MIP can clearly show the pore structure in carbonated samples. The results of the paper offer reliable guidance for the efficient preparation of gypsum-activated steel slag carbonate blocks. Energies 2021, 14, x FOR PEER REVIEW 3 of 18

Figure 1. Press-shaping methods and subsequent carbonation curing of steel slag. Energies 2021, 14, 4489 3 of 18 2. Materials and Methods 2.1. Raw Materials The steel slag used in this study, with a specific surface area of 460 m2/kg, was from 2. Materials and Methods Anshan Iron and Steel Co., Ltd. (Anshan, China). The desulfurization gypsum was from 2.1. Raw Materials Jintaicheng Environmental Resources Co., Ltd. (Hebei, China). The composition of gyp- The steel slag used in this study, with a specific surface area of 460 m2/kg, was sum was broadly CaSO4·2H2O. X-ray fluorescence and carbon/sulfur analyses were con- fromducted Anshan on the Ironsteel and slag Steel and Co.,gypsum Ltd. to (Anshan, obtain their China). chemical The desulfurization compositions. The gypsum samples’ was frommineral Jintaicheng compositions Environmental were characterized Resources using Co., Ltd. an (Hebei, X-ray diffractometer China). The composition (Rigaku Ulti- of · gypsummaIV) with was CuKα broadly radiation. CaSO4 Furthermore,2H2O. X-ray fluorescencequantitative X and-ray carbon/sulfur diffraction (QXRD) analyses analysis were conducted on the steel slag and gypsum to obtain their chemical compositions. The samples’ was conducted by Rietveld refinement using Highscore Plus software (with QXRD mineral compositions were characterized using an X-ray diffractometer (Rigaku UltimaIV) Rietveld method results shown in Figure S1 in Supplementary Material). Table 1 and Fig- with CuKα radiation. Furthermore, quantitative X-ray diffraction (QXRD) analysis was ure 2 show the chemical and mineral compositions of the raw materials, respectively. The conducted by Rietveld refinement using Highscore Plus software (with QXRD Rietveld main crystalline phases of the steel slag are larnite, alite, RO phase (magnesiowuestite in method results shown in Figure S1 in Supplementary Material). Table1 and Figure2 Table 1), brownmillerite, mayenite, and portlandite. Figure 3 shows the particle-size dis- show the chemical and mineral compositions of the raw materials, respectively. The main tributions of the steel slag powder and gypsum. crystalline phases of the steel slag are larnite, alite, RO phase (magnesiowuestite in Table1), brownmillerite, mayenite, and portlandite. Figure3 shows the particle-size distributions of Table 1. Chemical compositions of the steel slag and gypsum (wt %). the steel slag powder and gypsum. Chemical Chemical Steel Steel Slag Gypsum Components TableComposition 1. Chemical compositions of the steel slag and gypsum (wt %). Formula Slag CaO 44.78 48.06 Larnite Ca2SiO4 27.9 Chemical Chemical Steel Slag Gypsum Components Steel Slag CompositionAl2O3 5.70 1.18 Alite FormulaCa3SiO5 1.9 SiO2 15.28 2.95 Mayenite Ca12Al14O33 12.8 CaO 44.78 48.06 Larnite Ca2SiO4 27.9 Fe2O3 22.43 0.58 Portlandite Ca(OH)2 2.9 Al2O3 5.70 1.18 Alite Ca3SiO5 1.9 SOSiO32 0.3015.28 43.57 2.95 MayeniteCalcite Ca12AlCaCO14O333 12.82.9 MgOFe2O3 7.0422.43 1.48 0.58 PortlanditeQuartz Ca(OH)SiO22 2.92.2 MnOSO3 1.930.30 1.18 43.57 Magnesiowuestite Calcite (Fe, CaCO Mg,3 Mn)O 2.915.7 MgO 7.04 1.48 Quartz SiO2 2.2 PMnO2O5 1.00 1.93 0.03 1.18 MagnesiowuestiteMagnesioferrite (Fe,(Fe, Mg, Mg) Mn)O2O3 15.78.5 TiOP2O25 0.861.00 0.39 0.03 Brownmillerite Magnesioferrite (Fe,Ca Mg)2Fe2O2O3 5 8.515.7 VTiO2O25 0.200.86 -- 0.39 BrownmilleriteMagnesite Ca2MgCOFe2O5 3 15.76.3 V O 0.20 – Magnesite MgCO 6.3 Cr22O53 0.17 -- Periclase MgO3 3.0 Cr2O3 0.17 – Periclase MgO 3.0 OthersOthers 0.28 0.28 0.58 0.58 TotalTotal C 0.865 0.865 0.737 0.737

4000 : Alite 6: Brownmillerite 3500 2: Larnite 7: Magnesiowuestite 3: Portlandite 8: Periclase 3000 4: Calcite 9: Magnesioferrite 5 5: Mayenite Q: Quartz 3 2500 M: Magnesite

2M6 2000 12 5 7 6 6 Q 5 2 9 M

Intensity 1500 Q 4 3 2 9 Q 2 5 8 1 7 4 M 9 9 2 8 1 5 2 6 1000 6 3 9 7 3 6 2 2 2 22 9 2 2 500

0 10 20 30 40 50 60 70 2() Figure 2. X-ray diffraction pattern for the steel slag.

Energies 2021, 14, x FOR PEER REVIEW 4 of 18

Energies 2021, 14, 4489 4 of 18

Figure 2. X-ray diffraction pattern for the steel slag.

Figure 3. Particle-sizeParticle-size distributions of the steel slag powder and gypsum.

2.2.2.2. Experimental ProgramProgram and TestTest MethodsMethods A paste mixer was used to mix and stir the steel slag powder, gypsum, andand waterwater forfor 2 min after they were weighed according to the experimental proportion (Table2). The 2 min after they were weighed according to the experimental proportion (Table 2). The mixture was then weighed (8.00 g) and compacted into a cylindrical stainless steel mold mixture was then weighed (8.00 g) and compacted into a cylindrical stainless steel mold 20 mm in diameter under a certain molding pressure for 1 min. The compacts were then 20 mm in diameter under a certain molding pressure for 1 min. The compacts were then demolded and immediately placed into a carbonation chamber (20 ± 3 ◦C; 70 ± 2% relative demolded and immediately placed into a carbonation chamber (20 ± 3 °C; 70 ± 2% relative humidity; 20 ± 3 vol % CO2 concentration). humidity; 20 ± 3 vol % CO2 concentration). Table 2. Orthogonal experiment proportions. Table 2. Orthogonal experiment proportions. Experiment No. w/s (A) G/SS (B) Molding Pressure (MPa) (C) Gypsum (%) SS (%) Water (%) Experiment Molding Pres- Gypsum w/s (A) G/SS (B) SS (%) Water (%) 1 1:5(A1)No. 1:16(B1) 3(C1)sure (MPa) (C) 4.9 (%) 78.43 16.67 2 1:5(A1) 1:4(B2) 6(C2) 16.67 66.67 16.67 1 1:5(A1) 1:16(B1) 3(C1) 4.9 78.43 16.67 3 1:5(A1) 1:2(B3) 9(C3) 27.78 55.55 16.67 4 1:8(A2) 1:16(B1)2 1:5(A1) 1:4(B2) 6(C2) 6(C2) 5.2316.67 83.6666.67 11.1116.67 5 1:8(A2) 1:4(B2)3 1:5(A1) 1:2(B3) 9(C3) 9(C3) 17.7827.78 71.1155.55 11.1116.67 6 1:8(A2) 1:2(B3)4 1:8(A2) 1:16(B1) 3(C1) 6(C2) 29.635.23 59.2683.66 11.1111.11 7 1:10(A3) 1:16(B1)5 1:8(A2) 1:4(B2) 9(C3) 9(C3) 5.3517.78 85.5671.11 9.0911.11 8 1:10(A3) 1:4(B2) 3(C1) 18.18 72.73 9.09 9 1:10(A3) 1:2(B3)6 1:8(A2) 1:2(B3) 6(C2) 3(C1) 30.329.63 60.6159.26 9.0911.11 7 1:10(A3) 1:16(B1) 9(C3) 5.35 85.56 9.09 8 1:10(A3) 1:4(B2) 3(C1) 18.18 72.73 9.09 Uniaxial compressive strength (UCS) tests and carbon content tests on samples at 9 1:10(A3) 1:2(B3) 6(C2) 30.3 60.61 9.09 different ages (1, 3, 14, and 28 days) were conducted. The UCS at each curing age was determined by calculating the mean value of three tests. The carbon contents of samples wereUniaxial measured compressive using a EMIA-820 strength V (UCS) carbon/sulfur tests and combustion carbon content analyzer tests (Horiba). on samples If the at different ages (1, 3, 14, and 28 days) were conducted. The UCS at each curing age was CO2 uptake of the sample is x%, then according to the principle of carbon conservation, thedetermined following by equation calculating can the be obtained:mean value of three tests. The carbon contents of samples were measured using a EMIA-820 V carbon/sulfur combustion analyzer (Horiba). If the

CO2 uptake of the msamplebc × CO is2initial x%, then+ m bcaccording× CO2uptake to the= principlemdc × CO of2final carbon conservation,(1) the following equation can be obtained: where mbc is the weight of the compact before carbonation (8.00 g here); mdc is the weight mbc × CO2initial + mbc × CO2uptake = mdc × CO2final (1) of the dried carbonated compact (g); CO2initial is the CO2 content of the compact before carbonation, obtained using ωss × CO2ss; ωss is the steel slag content in the initial sample (wtwhere %); m andbc is CO the2final weightis the of COthe2 compactcontent ofbefore the dried carbonation carbonated (8.00 compact. g here); mdc is the weight of theEquation dried carbonated (1) can be rearrangedcompact (g); to obtainCO2initial the is equation:the CO2 content of the compact before carbonation, obtained using ωss × CO2ss; ωss is the steel slag content in the initial sample mdc × CO2final − mbc × CO2initial (wt %); and CO2final isCO the CO2 content= of the dried carbonated compact. (2) 2uptake mbc

Energies 2021, 14, 4489 5 of 18

The mass percentage of CO2 absorbed by the steel slag (i.e., the CO2 uptake of the steel slag) was calculated to evaluate the effect of gypsum on the carbon sequestration capacity of steel slag:

CO2uptake CO2 uptake of steel slag(%) = (3) ωss QXRD, TG–DTG, FTIR, MIP, and XPS were conducted to analyze the reaction mechanisms and microstructures of the samples. The XRD and QXRD analyses were conducted using the method described in Section 2.1. Note that the QXRD results are only for the crystalline components. The TG–DTG test was conducted using a NETZSCH STA 449F3 at 50 to 1200 ◦C with a heating rate of 10 ◦C/min. A NEXUS670 FTIR infrared spectrometer was operated for FTIR analysis, with a 400 to 4000 cm−1 wave number and a resolution of 3 cm−1. The equipment used for XPS analysis was a Thermo Scientiﬁc Escalab 250 Xi. A Shirley background was assumed in all cases. Spectra were calibrated using the adventi- tious hydrocarbon peak at 284.8 eV binding energy (BE) [21]. MIP analysis (Autopore IV 9500, Micromeritics) was conducted to determine the pore-size distributions of the samples.

2.3. Orthogonal Experimental Design The L9 (33) table was adopted in this test design. The three levels for the three factors were as follows: 1:5 (A1), 1:8 (A2), and 1:10 (A3) for the w/s ratio; 1:16 (B1), 1:4 (B2), and 1:2 (B3) for the mass ratio of gypsum and steel slag (G/SS); and 3 MPa (C1), 6 MPa (C2), and 9 MPa (C3) for the molding strength. The experimental scheme is shown in Table3. The inﬂuences of the w/s, G/SS, and molding strength on the uniaxial compressive strength (UCS) and CO2 uptake were analyzed using this approach.

Table 3. Orthogonal experimental results of CO2 uptake and UCS tests.

CO2 Uptake (%) CO2 Uptake of Steel Slag (%) UCS (MPa) Testing Number 1 d 3 d 14 d 28 d 1 d 3 d 14 d 28 d 1 d 3 d 14 d 28 d 1 11.93 11.72 12.62 12.95 15.21 14.94 16.09 16.51 18.20 17.90 25.19 15.16 2 9.87 10.38 10.98 11.36 14.80 15.57 16.47 17.04 17.72 19.49 22.42 15.96 3 8.65 9.36 10.12 10.47 15.57 16.85 18.22 18.85 19.36 15.70 26.94 22.80 4 10.33 10.07 11.65 12.32 12.35 12.04 13.93 14.73 13.66 10.73 17.96 13.47 5 9.23 9.93 10.63 11.14 12.98 13.96 14.95 15.67 18.44 23.31 19.03 21.40 6 8.64 8.70 9.12 9.34 14.58 14.68 15.39 15.76 8.17 8.09 7.61 8.28 7 9.10 9.66 10.05 10.29 10.64 11.29 11.75 12.03 13.44 8.22 16.46 17.55 8 7.78 8.66 9.16 9.48 10.70 11.91 12.59 13.03 4.87 5.92 5.70 4.55 9 6.19 8.22 8.09 8.21 10.21 13.56 13.35 13.55 7.01 8.66 8.85 12.52

3. Results and Discussion 3.1. Orthogonal Experimental Results

The effect curve of three factors on 1 d compressive strength and 1 d CO2 uptake capacity were shown in Figure4. The 1-day UCS values ranged from 4.87 to 19.36 MPa. The mean value of K (Kji) for different factors was at different levels in the range analysis. The best values for the three factors are as follows: the w/s value was 1:5 (A1), G/SS was 1:16 (B1), and the molding pressure was 9 MPa (C3). Meanwhile, Rj demonstrates the significance of the factor’s influence. A larger Rj value means that the factor has a more significant impact on the UCS. In Figure4, the decreasing order R A > RC > RB indicates that the level of significance of the factors was as follows: w/s (9.99) > molding pressure (6.67) > G/SS (3.59). Energies 2021, 14, x FOR PEER REVIEW 6 of 18

the level of significance of the factors was as follows: w/s (9.99) > molding pressure (6.67) > G/SS (3.59). The 1-day CO2 uptake values ranged from 6.19% to 11.93%. Figure 4 shows that the best values for the three factors were as follows: the w/s value was 1:5(A1), G/SS was 1:16 (B1), and the molding pressure was 3 MPa (C1). In Figure 4, the decreasing order RB > RA > RC indicates that the level of significance of the factors was as follows: G/SS (2.62) > w/s (2.46) > molding pressure (0.41). Table 3 shows that the CO2 uptake increased with the curing time, whereas the UCS value increased gradually until 14 days, after which the strength of some samples decreased. These results are attributed mainly to microstructure breaks due to excessive carbonation, which was also discussed by Wang [9]. Table 3 shows that the gaps between 14 d and 28 d strength sometimes were huge, so measurement error was not enough to explain the large strength drop during this period. The drop in strength could be attributed to microstructure breaks due to excessive carbonation. In other words, the formed hydration products, such as C-S-H gels and ettringite, could be consumed during the carbonation process, doing harm to strength development, especially in later stages. Table 3 indicates that strength degradation was the worst in samples with high w/s, proper amount of gypsum, and low molding pressure. The three above conditions all led to a high amount of hydration product and a loose pore structure. The UCS result indicated that the opti- Energies 2021, 14, 4489 6 of 18 mum curing duration was 14 days. Therefore, range analyses of UCS and CO2 uptake after short-term (1 day) and optimum (14 days) curing were conducted.

FigureFigure 4. 4. TheThe effect effect curve curve of of three three factors factors on on 1 1d dcompressive compressive strength strength and and 1 1dd CO CO2 2uptakeuptake capacity. capacity.

FigureThe 1-day 5 shows CO 2theuptake effect values curve rangedof three from factors 6.19% on 1 to4 d 11.93%. compressive Figure 4strength shows thatand the14 dbest CO2values uptake for capacity. the three The factors 14-day were UCS as values follows: ranged the w/sfrom value 5.70 to was 26.94 1:5(A1), MPa. G/SSThe best was values1:16 (B1), for the and three the factors molding were pressure as follows: was 3the MPa w/s (C1).value Inwas Figure 1:5 (A1),4, the G/SS decreasing was 1:16 order(B1), andRB >the R Amolding> RC indicates pressure that was the 9 MPa level (C3), of signiﬁcance which was of closely the factors in line was with as follows:the 1 d com- G/SS pr(2.62)essive > strength w/s (2.46) results. > molding In Figure pressure 5, the (0.41). decreasing order RA > RC > RB indicated that the level ofTable significance3 shows thatof the the factors CO 2 uptakewas as increasedfollows: w/s with (14.51) the curing > molding time, pressure whereas the(7.98) UCS > G/SSvalue (5.40). increased gradually until 14 days, after which the strength of some samples decreased. These results are attributed mainly to microstructure breaks due to excessive carbonation, which was also discussed by Wang [9]. Table3 shows that the gaps between 14 d and 28 d strength sometimes were huge, so measurement error was not enough to explain the large strength drop during this period. The drop in strength could be attributed to microstructure breaks due to excessive carbonation. In other words, the formed hydration products, such as C-S-H gels and ettringite, could be consumed during the carbonation process, doing harm to strength development, especially in later stages. Table3 indicates that strength degradation was the worst in samples with high w/s, proper amount of gypsum, and low molding pressure. The three above conditions all led to a high amount of hydration product and a loose pore structure. The UCS result indicated that the optimum curing duration was 14 days. Therefore, range analyses of UCS and CO2 uptake after short-term (1 day) and optimum (14 days) curing were conducted. Figure5 shows the effect curve of three factors on 14 d compressive strength and 14 d CO2 uptake capacity. The 14-day UCS values ranged from 5.70 to 26.94 MPa. The best values for the three factors were as follows: the w/s value was 1:5 (A1), G/SS was 1:16 (B1), and the molding pressure was 9 MPa (C3), which was closely in line with the 1 d compressive strength results. In Figure5, the decreasing order R A > RC > RB indicated that the level of signiﬁcance of the factors was as follows: w/s (14.51) > molding pressure (7.98) > G/SS (5.40). Energies 2021, 14, x FOR PEER REVIEW 7 of 18

EnergiesEnergies 20212021, ,1414, ,x 4489FOR PEER REVIEW 7 of7 of18 18

Figure 5. The effect curve of three factors on 14 d compressive strength and 14 d CO2 uptake capacity. FigureFigure 5. 5. TheThe effect effect curve curve of of three three factors factors on on 1 144 d compressive strengthstrength andand 1414d d CO CO22uptake uptake capacity. capacity. TheThe 14 14-day-day CO CO22 uptakeuptake values values ranged ranged from from 8.09% 8.09% to to 12.62%. 12.62%. The The best best values values for the three factors were as follows: the w/s value was 1:5 (A1), G/SS was 1:16 (B1), and the threeThe factors 14-day were CO as2 uptakefollows: values the w/s ranged value fromwas 1:5 8.09% (A1), to G/SS 12.62%. was The1:16 best(B1), values and the for mold- the molding pressure was 3 MPa (C1). The decreasing order R > R > R indicated that the threeing pressure factors were was as3 MPa follows: (C1). the Th w/se decreasing value was order 1:5 (A1), RB > G/SS RA B> wasRC indicateA 1:16 (B1),C d that and thethe levelmold- of level of signiﬁcance of the factors was as follows: G/SS (2.33) > w/s (2.14) > molding ingsignificance pressure wasof the 3 factorsMPa (C1). was Th ase follows: decreasing G/SS order (2.33) R >B >w/s RA (2.14) > RC indicate > moldingd that pressure the level (0.06). of pressure (0.06). significanceFigure of6 theshows factors the waseffect as curve follows: of threeG/SS (2.33)factors > w/son 1 (2.14) d and > molding14 d CO 2pressure uptake of(0.06). steel Figure6 shows2 the effect curve of three factors on 1 d and 14 d CO 2 uptake of steel slag.Figure The 1 -6day shows CO the uptake effect of curve steel ofslag three values factors ranged on 1 from d and 10.21% 14 d CO to 215.57%. uptake Theof steel best slag. The 1-day CO2 uptake of steel slag values ranged from 10.21% to 15.57%. The best slag.values The for 1 -theday three CO2 factorsuptake wereof steel as follows:slag values the w/sranged value from was 10.21% 1:5 (A1), to G/SS15.57%. was The 1:2 best(B3), values for the three factors were as follows: the w/s value was 1:5 (A1), G/SS was 1:2 (B3), valuesand the for molding the three pressure factors werewas 3as MPa follows: (C1). the The w/s decreasing value was order 1:5 (A1), RA >G/SS RC > was RB indicate1:2 (B3),d and the molding pressure was 3 MPa (C1). The decreasing order RA > RC > RB indicated andthat the the molding level of significancepressure was of 3 the MPa factors (C1). wasThe asdecreasing follows: w/sorder (4.68) RA >> RmoldingC > RB indicate pressured that the level of signiﬁcance of the factors was as follows: w/s (4.68) > molding pressure that(1.04 the) > levelG/SS of(0.72). significance of the factors was as follows: w/s (4.68) > molding pressure (1.04) > G/SS (0.72). (1.04) > G/SS (0.72).

18 1d 18 1d 14d 14d 16 16

12 12

CO2 uptake of steel slag (%) slag steel of CO2uptake 10

CO2 uptake of steel slag (%) slag steel of CO2uptake A1 A2 A3 B1 B2 B3 C1 C2 C3 A1 A2 A3 B1 B2 B3 C1 C2 C3 Factors

FigureFigure 6. 6. TheThe effect effect curve curve of of three three factors factorsFactors on on 1 d and 1 144 d CO 22 uptakeuptake of of steel steel slag. slag.

Figure 6. The effect curve of three factors on 1 d and 14 d CO2 uptake of steel slag. The 14-day CO2 uptake of steel slag values ranged from 12.03% to 18.85%. The best values for the three factors were as follows: the w/s value was 1:5 (A1), G/SS was 1:2

Energies 2021, 14, 4489 8 of 18

(B3), and the molding pressure was 9 MPa (C3). The decreasing order RA > RB > RC indicated that the level of significance of the factors was as follows: w/s (4.60) > G/SS (1.63) > molding pressure (0.41). From the range analyses results, w/s had a significant effect on both UCS and CO2 uptake values. The molding pressure impacted UCS greatly, but had very little influence on CO2 uptake. G/SS has the least impact on UCS, but affected CO2 uptake of steel slag greatly.

3.2. Effect of Gypsum Content From the orthogonal experiment results, when G/SS was 1:16, the compressive strength reached the highest values. The higher the gypsum content, the lower the strength and the CO2 uptake. However, regarding the CO2 uptake of steel slag (i.e., the capture ability of steel slag rather than that of the block), gypsum promoted their CO2 sequestration ability. The authors analyzed the effect of gypsum on steel slag carbonation previously [9], and found that gypsum acted as a catalyst as an aspect of CO2 capture. The main function of gypsum is to stimulate the activity of calcium aluminum minerals, such as mayenite (C12A7), to initiate a hydration reaction to form ettringite (C3A·3CaSO4·32H2O), which is carbonated easily to produce monocarboaluminate (C3A·CaCO3·11H2O) and gypsum. The ettringite formed at the beginning of curing helps to intertwine with C-S-H gels and enhance the strength. This is why a small amount of gypsum is required to reach optimum strength. The hydration and carbonation mechanism of main minerals in steel slag can be explained by the following chemical reactions:

C2S + H2O → C-S-H + Ca(OH)2 (4)

C3S + H2O → C-S-H + Ca(OH)2 (5)

Ca(OH)2 + CO2 → CaCO3 + H2O (6)

MgO + CO2 → MgCO3 (7)

C-S-H + CO2 → CaCO3 + SiO2·xH2O (8)

C12A7 + CaSO4·2H2O + Ca(OH)2 + H2O → C3A·3CaSO4·32H2O (9)

C3A·3CaSO4·32H2O + CO2 → CaCO3 + CaSO4·2H2O + Al(OH)3 + H2O (10)

C3A·3CaSO4·32H2O + CaCO3 → C3A·CaCO3·11H2O + CaSO4·2H2O + H2O (11)

3.3. Effect of Molding Pressure and Curing Durations

To determine the effect of molding pressure on the UCS and CO2 uptake of specimens, G/SS (1:16) and w/s (1:5) were controlled, but the molding pressure was varied from 3 to 27 MPa. The results are shown in Figure7a,b. As seen in Figure7a, UCS increased stably with increased molding pressure. From 1 to 14 days of curing, the UCS increased for most of the samples. In contrast, from 14 to 28 days of curing, the UCS of some samples, especially those with high molding pressure, dropped to some extent. This result was in agreement with the UCS result in the orthogonal experiment (Table3). What we need to notice is that the increase from 1 to 28 days was not signiﬁcant. This means that in some circumstances, to save time, 1 day of carbonation curing is enough. For samples with a 27 MPa molding pressure, the UCS was around 55 MPa after 1 day of carbonation curing. This strength is ideal in high-strength construction materials. Energies 2021, 14, x FOR PEER REVIEW 9 of 18

Energies 20212021,, 1414,, 4489x FOR PEER REVIEW 99 of of 18

14 70 (a) (b)14 70 60 (a) (b) 60 13 50 13

uptake / % / uptake 40 2 12

UCS / MPa / UCS 30

1d curing uptake / % / uptake CO 1d curing 2 12 UCS / MPa / UCS 30 3d curing 20 1d curing 3d curing 14d curing CO 14d curing 1d curing 20 28d curing 3d curing 3d curing 10 28d curing 14d curing 11 14d curing 0 5 10 15 20 25 30 0 5 10 15 20 25 30 28d curing 28d curing 10 Molding pressure / MPa 11 Molding pressure / MPa 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Molding pressure / MPa Molding pressure / MPa Figure 7. (a) UCS and (b) CO2 uptake results of samples with different molding pressures.

Figure 7. (a) UCS and (b) CO2 uptakeuptake results of samples with different molding pressures. The CO2 uptake of the samples cured for 1 day did not show a clear change with The CO2 uptake of the samples cured for 1 day did not show a clear change with molding pressure.The CO2 Thisuptake result of the proves samples that curedhigh molding for 1 day pressure did not (i.e., show high a clear density change and with molding pressure. This result proves that high molding pressure (i.e., high density and low low poremolding volume pressure. in the microThis resultstructure) proves almost that didhigh not molding block CO pressure2 from transferring(i.e., high density from and pore volume in the microstructure) almost did not block CO2 from transferring from the air the airlow into pore the volumeblock’s core.in the In micro the experiment,structure) almost when didthe moldingnot block pressure CO2 from was transferring 27 MPa or from into the block’s core. In the experiment, when the molding pressure was 27 MPa or greater, greater,the air water into began the block’s squeezing core. In out the of experiment, the block, when which the mean moldingt that pressure water saturationwas 27 MPa or water began squeezing out of the block, which meant that water saturation reached 100% reachedgreater, 100% waterat this beganpoint. squeezing out of the block, which meant that water saturation at this point. Sincereached 1 d 100% carbonation at this poi curingnt. almost reached the plateau, effects of short-term curing Since 1 d carbonation curing almost reached the plateau, effects of short-term curing time on UCSSince and 1 d CO carbonation2 uptake ofcuring 27 MPa almost molding reached pressure the plateau, samples effects were of investigatedshort-term curing, time on UCS and CO2 uptake of 27 MPa molding pressure samples were investigated, with with timeresults on shown UCS andin Figure CO2 uptake 8a. It is ofclear 27 thatMPa UCS molding increase pressured gradually samples with were curin investigatedg time, , results shown in Figure8a. It is clear that UCS increased gradually with curing time, while whilewith CO 2results uptake shown increase in dFigure significantly 8a. It is clearbefore that 6 hUCS curing, increase afterd whichgradually point with the curin valueg time, CO2 uptake increased signiﬁcantly before 6 h curing, after which point the value almost almostwhile remain COed2 uptake stable. increaseSemi-QXRDd significantly results in Figurebefore 8b6 halso curing, show after that thewhich amount point of the the value remained stable. Semi-QXRD results in Figure8b also show that the amount of the main mainalmost carbonation remain producted stable. calcite Semi in-QXRD samples results increase in Figured with 8b time also before show 6 that h, and the theamount value of the carbonation product calcite in samples increased with time before 6 h, and the value almost almostmain plateau carbonationed after product6 h, which calcite was inin samplesline with increase the COd2 uptakewith time results. before Figure 6 h, and 8b thealso value showsplateaued a fall in the after amount 6 h, which of larnite, was in mayenite, line with theportlandite, CO2 uptake alite, results. and gypsum, Figure8 bproving also shows almosta fall in plateau the amounted after of 6 larnite, h, which mayenite, was in line portlandite, with the CO alite,2 uptake and gypsum, results. Figure proving 8b theiralso their high carbonation activity, while brownmillerite, magnesiferrite, and magnesio- showshigh carbonation a fall in the activity, amount while of larnite, brownmillerite, mayenite, portlandite, magnesiferrite, alite, and and magnesiowuestite gypsum, proving wuestite remained stable in the mass percentage, corresponding to their low reactivity. theirremained high stable carbonation in the mass activity percentage,, while corresponding brownmillerite, to magn their lowesiferrite, reactivity. and Larnite, magnesio- the Larnite, the main carbonation feedstock, still existed after 24 h of carbonation curing. This wuestitemain carbonation remained feedstock, stable in the still mass existed percentage, after 24 h corresponding of carbonation to curing. their low This reactivity. might be might be attributed to the participation of carbonates at the surface of the calcium silicate Larnite,attributed the to main the participationcarbonation feedstock, of carbonates still atexist theed surface after 2 of4 h the of calciumcarbonation silicate curing. particles, This particles, blocking the CO2 diffusion. The QXRD Rietveld method results with different mightblocking be theattributed CO diffusion. to the participation The QXRD Rietveldof carbon methodates at the results surface with of different the calcium carbonation silicate carbonation durations2 are shown in Figure S2. The R-values for the Rietveld ranged from particles,durations blocking are shown the in CO Figure2 diffusion. S2. The The R-values QXRD for Rietveld the Rietveld method ranged results from with 5.00 different to 5.30, 5.00 to 5.30, indicating high reliability of the results. carbonationindicating high durations reliability are ofshown the results. in Figure S2. The R-values for the Rietveld ranged from 5.00 to 5.30, indicating high reliability of the results. 60 15

50 60 15 12 50 40 12 9 30 40 9

20 30 6

uptake (%) uptake 2

UCS (MPa) UCS 20 6 CO 10 (%) uptake

UCS 3 2 UCS (MPa) UCS

10 CO uptake CO 0 2 UCS 3 CO uptake0 0 0 5 10 15 20 2 25 0 0 5Curing time10 (h) 15 20 25 (a) Curing time (h) Figure 8. Cont. (a)

Energies 2021, 14, x FOR PEER REVIEW 10 of 18

100 90 Calcite 80 Magnesite Magnesiowuestite 70 Periclase 60 Brucite Magnesioferrite

50 Larnite

Portlandite 40 Wt. % Wt. Alite 30 Quartz Energies 2021, 14, 4489Energies 2021, 14, x FOR PEER REVIEW Mayenite 10 of 18 10 of 18 20 Gypsum 10 Monocarboaluminate Brownmillerite 0 0 1005 10 15 20 90 Carbonation time (h) Calcite 80 (b) Magnesite Magnesiowuestite Figure 8. UCS and CO2 uptake results70 (a) and semi-QXRD analyses results (b) of 27 MPa molding pressure Periclase samples with different curing times. 60 Brucite Magnesioferrite

50 Larnite

Portlandite 3.3.1. FTIR40 Analyses on Samples with Different Molding Pressures and Curing Dura- tions% Wt. Alite 30 Quartz Figure 9 shows the FTIR analyses of samples with different molding Mayenite pressures and 20 Gypsum curing times. From the figure, no clear differences were found between the samples with 10 Monocarboaluminate different molding pressures carbonated for 24 h. This indicated that molding Brownmillerite pressure had 0 little influence0 on carbonation.5 10 15 20 −1 The absorption bandsCarbonation from 3405 totime 3548 (h) cm were the stretching vibration bands of O–H in water molecules, and the 1621 cm(−1b )band was due to the bending vibration of it. The appearance of the broad stretching band resulted from the existence of hydrogen Figure 8. UCS and CO2 uptake results (a) and semi-QXRD analyses results (b) of 27 MPa molding pressure samples with bondsFigure with 8. aUCS wide and range CO2 ofuptake strengths results [9]. (a) The and intensity semi-QXRD of analysesthe O–H results bond (absorptionb) of 27 MPa bands different curing times. was moldinghigher in pressure low-molding samples-pressure with different samples, curing indicating times. that high molding pressure may lead to fewer hydration products in this system. This is because when the molding pres- 3.3.1. FTIR Analyses3.3.1. FTIR on Samples Analyses with on DifferentSamples Moldingwith Different Pressures Molding and CuringPressures Durations and Curing Dura- sure was 27 MPa, some of the water was squeezed out, which reduced the amount of Figure9 showstions the FTIR analyses of samples with different molding pressures and water for hydration. For the 27 MPa molding pressure samples under different curing curing times. From the ﬁgure, no clear differences were found between the samples with times, the sample curedFigure for 9 shows3 h had the the FTIR highest analyses amount of samples of hydration with products.different moldingThis was pressures and different molding pressures carbonated for 24 h. This indicated that molding pressure had because somecuring of the hydrationtimes. From products the figure, were no carbonated clear differences in the laterwere stages. found between the samples with little inﬂuence on carbonation. different molding pressures carbonated for 24 h. This indicated that molding pressure had little influence on carbonation. The absorption bands from 3405 to 3548 cm−1 were the stretching vibration bands of 9MPa-24h O–H in water molecules, and the 1621 cm−1 band was due to the bending vibration of it. The appearance of the broad stretching band resulted from the existence of hydrogen 15MPa-24h bonds with a wide range of strengths [9]. The intensity of the O–H bond absorption bands 27MPa-24h was higher in low-molding-pressure samples, indicating that high molding pressure may lead to fewer hydration products in this system. This is because when the molding pres- 27MPa-6h

sure was 27 MPa, some of the water was squeezed out, which reduced the amount of

27MPa-3h water for hydration. For the 27 MPa molding pressure samples under different curing times, the sample cured for 3 h had the highest amount of hydration products. This was because some of the hydration products were carbonated in the later stages.

27MPa-1h

1621

712

669

1141 993

9MPa-24h 1041

574

3548

873

3405

1112

516

1481 1420 4000 3500 15MPa-24h3000 1500 1000 500 -1 27MPa-24hWavenumbers(cm )

FigureFigure 9. 9.FTIR FTIR analysesanalyses27MPa-6h of of the the samples samples with with different different molding molding pressures pressures and and curing curing times. times.

The absorption27MPa-3h bands from 3405 to 3548 cm−1 were the stretching vibration bands of O–H in water molecules, and the 1621 cm−1 band was due to the bending vibration of it. The appearance of the broad stretching band resulted from the existence of hydrogen bonds with a wide range of strengths [9]. The intensity of the O–H bond absorption bands was

higher in low-molding-pressure27MPa-1h samples, indicating that high molding pressure may lead 1621

to fewer hydration products in this system. This is because when712 the molding pressure

669

1141

993

1041

574 3548

was 27 MPa, some of the water was squeezed out, which reduced873 the amount of water for

3405

1112

516 1481 hydration. For the 27 MPa molding pressure samples1420 under different curing times, the 4000 3500 3000 1500 1000 500 sample cured for 3 h had the highest amount of hydration products. This was because Wavenumbers(cm-1) some of the hydration products were carbonated in the later stages. Figure 9. FTIR analyses of the samples with different molding pressures and curing times.

Energies 2021, 14, x FOR PEER REVIEW 11 of 18

Energies 2021, 14, 4489 11 of 18

The “shoulder” at 3548 cm−1 may correspond to the water molecules in the gypsum −1 [25]. TheThe “shoulder”1420–1480 cm at 3548 bands cm −were1 may the correspond symmetric tostretching the water vibration molecules band insthe of C gyp-–O. −1 2− sumThe peaks [25]. The at 873 1420–1480 and 712 cmcm−1 werebands due were to the bending symmetric vibration stretching of CO vibration3 . It can bands be seen of that samples with different molding−1 pressures had almost the same intensity 2in− this range C–O. The peaks at 873 and 712 cm were due to the bending vibration of CO3 . It can be seenof curves. that samples This indicate with differentd that the molding molding pressures pressure had had almost little effe thect same on the intensity formation in this of rangecarbonized of curves. products, This indicated which was that in theline molding with the pressure CO2 uptake had results. little effect Furthermore, on the formation Figure 9 shows that the amount of carbonation products formed increased gradually with curing of carbonized products, which was in line with the CO2 uptake results. Furthermore, Figuretime before9 shows 6 h that curing, the amountwhile the of amounts carbonation almost products remained formed unchanged increased after gradually 6 h. with curingThe time 900 before to 1200 6 h cm curing,−1 bands while were the from amounts the asymmetric almost remained stretching unchanged vibration after of 6 Si h.–O [26]. TheParticularly, 900 to 1200 the cm993− and1 bands 516 cm were−1 peaks from were the asymmetricthe asymmetric stretching stretching vibration and bending of Si– O[vibration26]. Particularly, peaks of Si the–O– 993Si in and the 516C-S cm-H −gel,1 peaks respectively. were the In asymmetric addition, the stretching out-of-plane and bendingskeletal vibration vibration of peaks the Si of–O Si–O–Si bond contribute in the C-S-Hd to gel,the broad respectively. bands at In around addition, 516 the cm out-−1. It of-planeis seen clearly skeletal that vibration these characteristic of the Si–O peaks bond appear contributeded only to in the 1 h broad and 3 bandsh samples. at around When 516the samples cm−1. It were is seen carbonated clearly that for thesea long characteristic duration (6 h peaks or more), appeared the peaks only of in the 1 hC and-S-H 3gel h samples.almost disappeared. When the samples These results were carbonated agreed with for the a longfact that duration C-S-H (6 gels h or have more), high the carbon peaks ofreactivity, the C-S-H which gel almost means disappeared.that they are Theseprone resultsto carbonation agreed with reac thetions. fact For that samples C-S-H with gels havecuring high times carbon of 6 reactivity,h or longer, which the peaks means at that 993 they and are516 prone cm−1 disappeared, to carbonation whereas reactions. an Forab- samplessorption withband curingat 1041 timescm−1 appeared. of 6 h or longer,This was the because peaks atof 993the formation and 516 cm of− silica1 disappeared,-like prod- whereasucts and anan absorptionincrease in bandthe polymerization at 1041 cm−1 appeared. degree [27 This]. was because of the formation of silica-likeThe 1112 products and 1141 and ancm increase−1 peaks were in the characteristic polymerization of SO degree42− [25 [27]. ]. −1 2− The 1112 and 1141 cm peaks were characteristic of SO4 [25]. 3.3.2. TG–DTG Analyses of Samples with Different Molding Pressures and Curing Dura- 3.3.2.tions TG–DTG Analyses of Samples with Different Molding Pressures and Curing Durations Thermogravimetry–derivative thermogravimetry (TG–DTG) analyses of samples Thermogravimetry–derivative thermogravimetry (TG–DTG) analyses of samples with with different molding pressures and curing times are shown in Figure 10. From the TG different molding pressures and curing times are shown in Figure 10. From the TG curves, curves, the mass loss of S5 was greater than that of S12. The figure also shows that the the mass loss of S5 was greater than that of S12. The ﬁgure also shows that the difference difference between S5 and S12 was mainly in the range of 300–500 °C, where S5 had subtle between S5 and S12 was mainly in the range of 300–500 ◦C, where S5 had subtle mass loss mass loss peaks, whereas S12 did not. At this temperature, the C-S-H gel began to decom- peaks, whereas S12 did not. At this temperature, the C-S-H gel began to decompose. From pose. From 3 to 24 h of carbonation curing, mass loss increased with curing time. The 3 to 24 h of carbonation curing, mass loss increased with curing time. The mass-loss peak mass-loss peak in the range of 500–800 °C corresponded to the decarbonation of car- in the range of 500–800 ◦C corresponded to the decarbonation of carbonates. The intensity bonates. The intensity of the peaks increased with curing time, indicating a continuing of the peaks increased with curing time, indicating a continuing process of carbonation reaction.process of The carbonation mass loss reaction. around 100 The◦C mass was loss attributed around to 100 water °C was loss attributed of hydration to water products, loss suchof hydration as the C-S-H products, gel. Fromsuch as the the DTG C-S curves-H gel. inFrom Figure the 10DTG, 9 MPa-24curves in h Figure and 27 10, MPa-24 9 MPa h- had24 h similarand 27 amountsMPa-24 hof had hydration similar amounts products, of and hydration these amounts products, were and greater these amounts than those were of samplesgreater than with those shorter of curingsamples times. with shorter curing times.

100 0.30 27 MPa-3 h 27 MPa-6 h 95 0.25 27 MPa-24 h 90 9 MPa-24 h 0.20

C)  80 0.15 C-S-H 75

Mass/% Carbonates 0.10 DTG(%/ 70 Gypsum 0.05 65

60 0.00 0 200 400 600 800 1000 1200 Temperature (C) FigureFigure 10.10. TG–DTGTG–DTG analysesanalyses ofof samplessamples withwith differentdifferent moldingmolding pressurespressures andand curingcuring times.times.

Energies 2021, 14, x FOR PEER REVIEW 12 of 18

Energies 2021, 14, 4489 12 of 18

3.3.3. Pore-Size Distribution Analyses of Samples with Different Molding Pressures 3.3.3.The Pore-Size cumulative Distribution pore volume Analyses curves of Samples and pore with-size Different distribution Molding curves Pressures of samples with 9 MPa and 27 MPa molding pressure after 1 day of carbonation curing are shown in The cumulative pore volume curves and pore-size distribution curves of samples Figurewith 9 MPa11. The and curves 27 MPa prove molding clearly pressure that samples after 1with day higher of carbonation molding curingpressure are had shown less porein Figure volume. 11. TheIt is curveseasy to proveunderstand clearly that that higher samples molding with higherpressures molding led to pressuredenser struc- had tures.less pore As shown volume. in ItFigure is easy 11b, to understandS12 had more that pores higher smaller molding than pressures10 nm and led fewer to denser pores betwestructures.en 316 As and shown 10,000 in nm. Figure Larger 11b, pores S12 had are moreharmful pores to strength. smaller than The 1027 nmMPa and molding fewer pressurepores between sample 316 correspond and 10,000ed nm.to not Larger only a pores lower are pore harmful volume to, but strength. also smaller The 27 pores MPa, whichmolding is why pressure it had sample a higher corresponded compressive to strength. not only a lower pore volume, but also smaller pores, which is why it had a higher compressive strength.

(a) (b)

Figure 11.11. ((aa)) CumulativeCumulative porepore volume volume and and (b ()b Pore-size) Pore-size distribution distribution of blocksof blocks with with different different molding molding pressures pressures (S5, 9 (S5, MPa; 9 S12,MPa; 27 S12, MPa) 27 MPa) after 1-dayafter 1 curing.-day curing.

3.4. Effects Effects of of w/s w/s

3.4.1. Effects Effects of w/s on on UCS UCS and and CO CO2 2UUptakeptake An experiment was conducted to investigate the effectseffects ofof w/sw/s on the strength and CO2 uptakeuptake of of samples. samples. In In this this experiment, experiment, G/SS and and molding molding pressure pressure were were controlled at 1:16 and 9 MPa, respectively. The results are shown in Figure 1212.. Samples with a higher amount of of water water (w/s) (w/s) tend tendeded to to have have higher higher UCS UCS and and higher higher CO CO2 uptake.2 uptake. Therefore, Therefore, the additionthe addition of water of water promoted promoted not notonly only strength strength enhancement enhancement,, but butalso also the thecarbonation carbonation re- actionreaction process. process. We We also also conclude concludedd that that compared compared with with molding molding strength, strength, w/s w/s seem seemeded to to have a greater effect on the UCS and CO uptake. The contribution of water had two have a greater effect on the UCS and CO2 uptake.2 The contribution of water had two as- aspects. On the one hand, water takes part in chemical reactions. When CO transfers into 2 pects. On the one hand, water takes2− part in− chemical reactions. When CO transfers into the water, it dissolves into CO3 or HCO3 so that carbonation can occur easily. Water the water, it dissolves into CO32− or HCO3− so that carbonation can occur easily. Water is is also a necessary component for hydration reactions. Larnite and alite in steel slag have also a necessary component for hydration reactions. Larnite and alite in steel slag have weak carbon reactivity, but hydration products such as the C-S-H gel and portlandite are weak carbon reactivity, but hydration products such as the C-S-H gel and portlandite are carbonated easily. On the other hand, water is an important medium in this system. It carbonated easily. On the other hand, water2+ is −an important2− medium in this system. It promotes the diffusion of ions such as Ca , OH , and CO3 . promotes the diffusion of ions such as Ca2+, OH−, and CO32−. 3.4.2. Semi-Quantitive XRD Analyses of Samples with Different w/s Values The results of the QXRD Rietveld method for samples with different w/s ratios are shown in Figure S3. The semi-QXRD analyses results given in Figure 13 showed that with an increasing w/s, there was a clear increase in calcite amount and a clear decrease in larnite, mayenite, and gypsum amounts. The DTG curves showed a similar temperature range of mass loss, proving that under different w/s, the samples’ hydration and carbonation products were generally similar. However, the amounts of the products changed with different w/s. Higher w/s corresponded to higher amounts of carbonates such as calcite and monocarboaluminate. It also corresponded to less amounts of larnite, mayenite, and gypsum, in general. The QXRD clearly proved that water played an important role in the carbonation of steel slag.

Energies 2021, 14, x FOR PEER REVIEW 13 of 18

35 1d 13 3d 30 14d 12 28d 25

10 uptake / % UCS/MPa 1d 15 2 3d

CO 9 Energies 2021, 1410, 4489 14d 13 of 18 Energies 2021, 14, x FOR PEER REVIEW 13 of 18 28d 5 8 1/5 1/6 1/7 1/8 1/9 1/10 1/5 1/6 1/7 1/8 1/9 1/10

w/s w/s 35 1d 13 (a) 3d (b) 30 14d Figure 12. (a) UCS and (b) CO2 uptake results for12 samples with different w/s values. 28d 25

3.4.2. Semi-Quantitive XRD Analyses11 of Samples with Different w/s Values

20 The results of the QXRD Rietveld method for samples with different w/s ratios are 10

shown in Figure S3. The semi-QXRD uptake / % analyses results given in Figure 13 showed that with UCS/MPa 1d 15 2 an increasing w/s, there was a clear increase 3d in calcite amount and a clear decrease in

CO 9 10 larnite, mayenite, and gypsum amounts. The 14dDTG curves showed a similar temperature range of mass loss, proving that under different 28d w/s, the samples’ hydration and carbon- 5 ation products were generally similar.8 However, the amounts of the products changed 1/5 1/6 1/7 1/8 1/9 1/10 1/5 1/6 1/7 1/8 1/9 1/10 with different w/s. Higher w/s corresponded to higher amountsw/s of carbonates such as calcite andw/s monocarboaluminate. It also corresponded to less amounts of larnite, mayenite, and(a )gypsum, in general. The QXRD clearly proved that( bwater) played an important role in the carbonation of steel slag. Figure 12. (a) UCS and (b) CO2 uptake results for samples with different w/s values values..

3.4.2.100 Semi-Quantitive XRD Analyses of Samples with Different w/s Values The90 results of the QXRD Rietveld method for samples with different w/s ratios are shown80 in Figure S3. The semi-QXRD analyses results given in Figure 13 showed that with an increasing w/s, there was a clear increase in calcite Alite amount and a clear decrease in larnite,70 mayenite, and gypsum amounts. The DTG curvesLarnite showed a similar temperature Mayenite range 60of mass loss, proving that under different w/s, the samples’ hydration and carbon- Gypsum ation products were generally similar. However, the amounts of the products changed 50 Monocarboaluminate with different w/s. Higher w/s corresponded to higher Quartz amounts of carbonates such as cal-

cite and40 monocarboaluminate. It also corresponded Wuestiteto less amounts of larnite, mayenite, Wt. % Wt. and gypsum, in general. The QXRD clearly proved that Brownmillerite water played an important role in 30 the carbonation of steel slag. Magnesioferrite 20 Magnesite Calcite 10010 900 L1-1d L2-1d L4-1d L6-1d 80 Samples Alite 70 Larnite Mayenite Figure60 13. Semi-quantitiveSemi-quantitive XRD analyses of samplessamples withwith differentdifferent w/sw/s values at 11 d.d. (L1:(L1: 1/5,1/5, L2: Gypsum 1/6, L4: L4: 1/8, 1/8, L6: L6: 1/10) 1/10).. 50 Monocarboaluminate 3.4.3. TG TG–DTG–DTG Analyses of Samples with Different w/sQuartz Value Valuess

40 Wuestite Wt. % Wt. The results of the TG–DTGTG–DTG analyses in Figure Brownmillerite 14 show the tendency of more mass loss with 30 increased w/s. The The DTG DTG curves curves show show a a similar similar Magnesioferrite temperature temperature range range of of mass mass loss, loss, proving20 that the samples’ hydration and carbonation Magnesite products with different w/s were generally similar. The L6-1 d sample, which had the leastCalcite water, also had the least mass loss at around10 100 ◦C, where the C-S-H gel lost water. For L1-1 d and L1-3 d, more mass loss

was seen0 at this temperature range. This proves that water promoted hydration reactions. Gypsum decomposesL1-1d L2-1d at 120 ◦L4-1dC. L6-1 dL6-1d had the most mass loss at this temperature, which means that the reactionSamples extent of gypsum was lower than those of other samples, which had more water in the matrix. From 500 ◦C to 800 ◦C, the carbonate decomposition intensity Figurehad a trend13. Semi similar-quantitive to that XRD of hydration analyses of products. samples with Therefore, different the w/s TG values results at prove 1 d. (L1: the 1/5, role L2: of 1/6,water L4: in1/8, promoting L6: 1/10). hydration and carbonation—the UCS and CO2 uptake of the samples were promoted with the addition of water. 3.4.3. TG–DTG Analyses of Samples with Different w/s Values The results of the TG–DTG analyses in Figure 14 show the tendency of more mass loss with increased w/s. The DTG curves show a similar temperature range of mass loss,

Energies 2021, 14, x FOR PEER REVIEW 14 of 18

proving that the samples’ hydration and carbonation products with different w/s were generally similar. The L6-1 d sample, which had the least water, also had the least mass loss at around 100 °C, where the C-S-H gel lost water. For L1-1 d and L1-3 d, more mass loss was seen at this temperature range. This proves that water promoted hydration reactions. Gypsum decomposes at 120 °C. L6-1 d had the most mass loss at this temperature, which means that the reaction extent of gypsum was lower than those of other samples, which had more water in the matrix. From 500 °C to 800 °C, the carbonate decomposition

Energies 2021, 14, 4489 intensity had a trend similar to that of hydration products. Therefore, the TG results14 prove of 18 the role of water in promoting hydration and carbonation—the UCS and CO2 uptake of the samples were promoted with the addition of water.

100 0.25 L1-1d L4-1d 90 L6-1d 0.20 L1-3d 80 0.15

Carbonates

70 C-S-H 0.10 Mass(%)

Gypsum DTG(%/°C) 60 0.05

50 0.00 0 200 400 600 800 1000 1200 Temperature (°C) Figure 14. TG–DTGTG–DTG analyses of samples with different w/sw/s values and curing times (L1, 1:5; L4, 1:8; L6,L6, 1:10).1:10).

3.4.4. XPS Analyses of Samples with Different w/sw/s Ratios Ratios Figure 1515 showsshows thethe XPS XPS test test results results for for samples samples L1, L1, L2, L2, L4, L4, and and L6 atL6 the at agethe ofage 1 day.of 1 Theday. main The main forms forms of silicon of silicon in steel in slag steel were slag C 2wereS and C C2S3 S.and According C3S. According to Black to et al.Black [28 ],et the al. Si[28 2p], the binding Si 2penergy binding peaks energy for peaks C2S and for C C3S2S were and 100.80C3S were and 100.80 100.57 and eV, respectively.100.57 eV, res Sipec- 2p bindingtively. Si energy2p binding is related energy to is the related silicate to the tetrahedral silicate tetrahedral polymerization polymerization extent [29– extent31]. Black [29– et31]. al. Black [29] also et al. reported [29] also that reported Si 2p binding that Si 2p energies binding showed energies a strong showed negative a strong correlation negative withcorrelation C/S in with the C-S-H C/S in phases. the C-S Carbonation-H phases. Carbonation of C-S-H resulted of C- inS-H a decreasedresulted in C/S a decreased and thus anC/S increased and thus Sian 2p increased binding Si energy. 2p binding The curves energy. in The Figure curves 12a indicatein Figure that 12a the indicate Si 2p bindingthat the energySi 2p binding had increased energy had after increased 1 day of after carbonation 1 day of carbonation curing. This curing. may be This because may be dicalcium because silicatedicalcium and silicate tricalcium and silicatetricalcium are nesosilicatesilicate are nesosilica mineralste and minerals have low and Si have 2p binding low Si energies,2p bind- whereasing energies, the Si whereas 2p binding the Si energies 2p binding of carbonate energies products, of carbonate such products, as low Ca/Si such C-S-Has low gelCa/Si or silicaC-S-H gels, gel or are silica higher gels, because are higher of the because loss of of nonbridging the loss of nonbridging oxygen atoms oxygen during atoms hydration during andhydration carbonation. and carbonation. It is clear that the sample with the higher w/s value value had had a a higher higher Si Si 2p 2p binding binding energy, energy, indicating that water promotedpromoted thethe hydrationhydration andand carbonationcarbonation ofof silicatesilicate minerals.minerals. Water plays aa vitalvital rolerole inin thethe following following two two aspects. aspects. First, First, it it acts acts as as a a hydration hydration reactant. reactant. Water Water is theis the most most essential essential component component in in hydration hydration reactions. reactions. Hydration Hydration products products such such as as C-S-H C-S- gels and ettringite are prone to carbonation reaction, so water also promotes carbonation H gels and ettringite are prone to carbonation reaction, so water also2+ promotes− carbona-2− reactions.tion reactions. Second, Second, water water acts as acts a transport as a transport medium. medium. The dissolved The dissolved Ca , OH Ca,2+ and, OH CO−, and3 are transported via water, accelerating hydration and carbonation reactions and carbonate CO32− are transported via water, accelerating hydration and carbonation reactions and car- product precipitation. bonate product precipitation. Figure 15b shows the Ca 2p3/2 binding energy of the samples. It was previously Figure 15b shows the Ca 2p3/2 binding energy of the samples. It was previously de- determined that the Ca 2p3/2 binding energies of dicalcium silicate (C2S) and tricalcium termined that the Ca 2p3/2 binding energies of dicalcium silicate (C2S) and tricalcium sil- silicate (C3S) in the raw material were 346.87 and 346.55 eV, respectively [28]. Previous icate (C3S) in the raw material were 346.87 and 346.55 eV, respectively [28]. Previous stud- studies also showed that the Ca 2p3/2 binding energy of gypsum was 347.70 eV [32], and ies also showed that the Ca 2p3/2 binding energy of gypsum was 347.70 eV [32], and that that of Ca/Al-LDH was 347.5 eV [33]. The Ca2p3/2 binding energy for an OPC dry clinker sample was observed to be about 347.1 eV. For CaO, Ca(OH)2, and CaCO3, the Ca2p3/2 peak was around 347.0 eV [34]. It can be seen from Figure 15b that the Ca 2p3/2 binding energy of all the carbonated samples surpassed 347 eV. From L1 to L6, the w/s decreased, and the peaks at 347.9 and 347.1 eV increased. This may be attributed to the increasing amounts of ettringite, C-S-H gels, calcite, and monocarboaluminate during the curing period with the increasing addition of water. Energies 2021, 14, x FOR PEER REVIEW 15 of 18

of Ca/Al-LDH was 347.5 eV [33]. The Ca2p3/2 binding energy for an OPC dry clinker sample was observed to be about 347.1 eV. For CaO, Ca(OH)2, and CaCO3, the Ca2p3/2 peak was around 347.0 eV [34]. It can be seen from Figure 15b that the Ca 2p3/2 binding energy of all the carbonated samples surpassed 347 eV. From L1 to L6, the w/s decreased, and the peaks at 347.9 and 347.1 eV increased. This may be attributed to the increasing amounts of ettringite, C-S-H gels, calcite, and monocarboaluminate during the curing period with the increasing addition of water. Figure 15c shows the Al 2p binding energy results for samples L1 to L6. In steel slag, C12A7 and C3A were the most important sources of aluminum. From Dubina’s study [35], the binding energy of alumina (73.47 eV) was typical for AlO4 tetrahedra. C-A-H phases and the monocarboaluminate had an Al 2p binding energy of around 74.30 eV. From Fig-

Energies 2021, 14, 4489 ure 15c, the Al 2p binding energy increased with w/s, indicating that more hydration15 and of 18 carbonation products were formed when more water was added. This result was consistent with the carbon content and TG–DTG results.

347.5 102.51 74.28 347.1 347.9 350.8 102.57 351.7 74.35

102.63 74.42

102.83

74.46

Si-L6-1d Ca-L6-1d Al-L6-1d Si-L4-1d Ca-L4-1d Al-L4-1d Si-L2-1d Ca-L2-1d Al-L2-1d Ca-L1-1d Si-L1-1d Al-L1-1d 346 348 350 352 354 100 102 104 106 70 72 74 76 78 Binding energy (eV) Binding energy (eV) Binding energy (eV) (a) (b) (c)

Figure 15. XPS analyses of samples with different w/sw/s values values at at the the age age of of 1 1 day: day: ( (aa)) Si Si 2p; 2p; ( (bb)) Ca Ca 2p; 2p; ( (cc)) Al Al 2p 2p..

4. ConclusionsFigure 15c shows the Al 2p binding energy results for samples L1 to L6. In steel slag, C12ATo7 and determine C3A were the the carbonation most important potential sources of steel of aluminum.slag, through From orthogonal Dubina’s design study tests [35], andthe bindingsingle-factor energy tests, of aluminathis paper (73.47 evaluated eV) was the typical effects forof the AlO water/solid4 tetrahedra. mass C-A-H ratio phases (w/s), gypsumand the ratio monocarboaluminate (G/SS), and molding had pressu an Alre 2p on binding uniaxial energy compressive of around strength 74.30 (UCS) eV. From and Figure 15c, the Al 2p binding energy increased with w/s, indicating that more hydration CO2 uptake of the compacts. and carbonation products were formed when more water was added. This result was From the range analyses results, w/s had a significant effect on both UCS and CO2 uptakeconsistent values. with The the molding carbon content pressure and impact TG–DTGed UCS results. greatly, but had very little influence on CO2 uptake. G/SS had the least impact on UCS, but improved the CO2 uptake of steel 4. Conclusions slag greatly. The optimum conditions in the study were a w/s of 0.20, G/SS of 1/16, and To determine the carbonation potential of steel slag, through orthogonal design tests molding pressure of 27 MPa, under which conditions 1 d UCS and CO2 uptake were 55.30 MPaand single-factorand 12.36%, tests,respectively. this paper evaluated the effects of the water/solid mass ratio (w/s), gypsum ratio (G/SS), and molding pressure on uniaxial compressive strength (UCS) and In our study, CO2 uptake of steel slag in the optimal proportion was 16.32%. So, per CO2 uptake of the compacts. Mt of steel slag can capture 0.1632 Mt CO2. The steel slag from the Anshan Iron and Steel From the range analyses results, w/s had a signiﬁcant effect on both UCS and CO2 uptake values. The molding pressure impacted UCS greatly, but had very little inﬂuence on CO2 uptake. G/SS had the least impact on UCS, but improved the CO2 uptake of steel slag greatly. The optimum conditions in the study were a w/s of 0.20, G/SS of 1/16, and molding pressure of 27 MPa, under which conditions 1 d UCS and CO2 uptake were 55.30 MPa and 12.36%, respectively. In our study, CO2 uptake of steel slag in the optimal proportion was 16.32%. So, per Mt of steel slag can capture 0.1632 Mt CO2. The steel slag from the Anshan Iron and Steel Co. plant is a kind of typical BOF slag in China, which accounts for 70% of the steel slag in China. The stockpiled amount of steel slag in China is around 1800 Mt. Therefore, the amount of CO2 that can be captured by this method is 293.76 Mt in China, which is a huge amount and can contribute to mitigating the greenhouse effect to a great extent. In addition, the high-strength construction materials could substitute cement. For every ton of cement produced, 0.78 tons of CO2 is emitted. So, the technique could save 1800 Mt cement and decrease CO2 emissions by 1404 Mt. Energies 2021, 14, 4489 16 of 18

The gypsum single-factor experiment indicated that gypsum acted as a catalyst for calcium aluminate during the hydration and carbonation process of steel slag. Increasing curing duration helped to improve both UCS and CO2 uptake at less than 14 d, while excessive carbonation could do harm to the strength development. Samples at 1 d curing age had 70~100% UCS compared to that of 14 d, and 85~100% CO2 uptake compared to that of 14 d. To save time and energy, 1 d curing, and even 6 h curing, is long enough for the samples. An increase in water addition significantly enhanced the UCS and CO2 uptake. Increasing w/s from 0.1 to 0.2 could contribute to an increase in UCS from 7.45 MPa to 31.97 MPa, and an increase in CO2 uptake from 8.59% to 11.31%, increased by 329% and 32%, respectively. From micro-analyses, hydration and carbonation processes were significantly improved when enough water was provided. Water played an indispensable role in the hydration reactions. It also promoted the diffusion of ions such as 2+ − 2− Ca , OH , and CO3 . Strengthening molding pressure from 3 MPa to 27MPa led to an increased UCS from 22.90 MPa to 55.30 MPa, with no obvious effects on CO2 uptake value. Although the strengthened molding pressure almost did not impact the amounts of hydration and carbonation products in compacts, it had a significant densification effect on micro-pore structures from MIP results. Increasing molding pressure was an effective way to enhance UCS. From the analyses, the labile minerals in steel slag included larnite, alite, mayenite, portlandite, and periclase. If all of above-mentioned minerals were carbonated, from the raw material analyses (especially QXRD), theoretical CO2 uptake of steel slag should be 25.25%. However, the highest value we found in this study was 18.85%, only 74.65% of the theoretical value. The difference between practical and theoretical results could be attributed to the prohibited CO2 diffusion by the formed carbonates on the outside of the particles. Actually, some pure material carbonation research has reported that it is very difficult for the minerals to be completely carbonated. Wang and Chang [36] investigated carbonation of β-C2S, Ca(OH)2, and C4AF, finding that the carbonation degree of the minerals were 52%, 87%, and 25%, respectively. Zhao [37] explored carbonation degree of γ-C2S through biomineralization, with around 30% γ-C2S left in the carbonated samples. Zhang [38] studied the carbonation reactivity of C3S and β-C2S, finding a conversion rate of 65% and 70%, respectively. The most plentiful carbonation product in their studies was calcite, with some existence of aragonite, vaterite, and monocarboaluminate. The results in this study were consistent with their studies. Wang and Chang [36] revealed that the carbonation degree was not only controlled by the dissolution properties of minerals, but also influenced by the distribution of CaCO3, which was similar to the above explanation regarding the blocking effects of carbonates on CO2 diffusion.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/en14154489/s1, Figure S1: QXRD analysis result of steel slag, Figure S2: QXRD analysis results of steel slag after (a) 1 h, (b) 3 h, (c) 6 h, (d)12 h, and (e) 24 h carbonation. (molding pressure: 27 MPa, G/SS: 1/16, w/s: 0.2), Figure S3: QXRD analysis results of steel slag with different w/s ratio of (a) 1/5, (b) 1/6, (c) 1/8, and (d) 1/10. (molding pressure: 9 MPa, G/SS: 1/16, carbonation duration: 1d). Author Contributions: Conceptualization, methodology, formal analysis, writing—original draft, X.W.; supervision, resources, project administration, W.N.; writing—review and editing, project administration, validation, S.Z.; writing—review and editing, project administration, validation, J.L.; funding acquisition, K.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key Research and Development Program of China (2018YFC1900603), the China Scholarship Council (No. 201906460050), and the National Natural Science Foundation of China (52004021). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Energies 2021, 14, 4489 17 of 18

Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request. Acknowledgments: We especially want to thank Mei Zhang for the support in carbon content measurement. Conflicts of Interest: The authors declare no conflict of interest. The funders have no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the result.

References

1. Ren, S.; Aldahri, T.; Liu, W.; Liang, B. CO2 mineral sequestration by using blast furnace slag: From batch to continuous experiments. Energy 2021, 214, 118975. [CrossRef] 2. Wu, H.; Jayne, R.S.; Bodnar, R.J.; Pollyea, R.M. Simulation of CO2 mineral trapping and permeability alteration in fractured basalt: Implications for geologic carbon sequestration in mafic reservoirs. Int. J. Greenh. Gas Control 2021, 109, 103383. [CrossRef] 3. Zhang, W.; Zhang, F.; Ma, L.; Ning, P.; Yang, J.; Wei, Y. An efficient methodology to use hydrolysate of phosphogypsum decomposition products for CO2 mineral sequestration and calcium carbonate production. J. Clean. Prod. 2020, 259, 120826. [CrossRef] 4. Wang, B.; Pan, Z.; Cheng, H.; Zhang, Z.; Cheng, F. A review of carbon dioxide sequestration by mineral carbonation of industrial byproduct gypsum. J. Clean. Prod. 2021, 302, 126930. [CrossRef] 5. Xiong, Y.; Aldahri, T.; Liu, W.; Chu, G.; Zhang, G.; Luo, D.; Yue, H.; Liang, B.; Li, C. Simultaneous preparation of TiO2 and ammonium alum, and microporous SiO2 during the mineral carbonation of titanium-bearing blast furnace slag. Chin. J. Chem. Eng. 2020, 28, 2256–2266. [CrossRef] 6. Monasterio-Guillot, L.; Alvarez-Lloret, P.; Ibañez-Velasco, A.; Fernandez-Martinez, A.; Ruiz-Agudo, E.; Rodriguez-Navarro, C. CO2 sequestration and simultaneous zeolite production by carbonation of coal fly ash: Impact on the trapping of toxic elements. J. CO2 Util. 2020, 40, 101263. [CrossRef] 7. Gupta, S.; Kashani, A.; Mahmood, A.H.; Han, T. Carbon sequestration in cementitious composites using biochar and fly ash–Effect on mechanical and durability properties. Constr. Build. Mater. 2021, 291, 123363. [CrossRef] 8. Chen, Z.; Li, R.; Zheng, X.; Liu, J. Carbon sequestration of steel slag and carbonation for activating RO phase. Cem. Concr. Res. 2021, 139, 106271. [CrossRef] 9. Wang, X.; Ni, W.; Li, J.; Zhang, S.; Hitch, M.; Pascual, R. Carbonation of steel slag and gypsum for building materials and associated reaction mechanisms. Cem. Concr. Res. 2019, 125, 105893. [CrossRef] 10. Li, L.; Jiang, Y.; Pan, S.Y.; Ling, T.C. Comparative life cycle assessment to maximize CO2 sequestration of steel slag products. Constr. Build. Mater. 2021, 298, 123876. [CrossRef] 11. Zhong, X.; Li, L.; Jiang, Y.; Ling, T.-C. Elucidating the dominant and interaction effects of temperature, CO2 pressure and carbonation time in carbonating steel slag blocks. Constr. Build. Mater. 2021, 302, 124158. [CrossRef] 12. Jiang, Y.; Ling, T.C. Production of artificial aggregates from steel-making slag: Influences of accelerated carbonation during granulation and/or post-curing. J. CO2 Util. 2020, 36, 135–144. [CrossRef] 13. Hou, G.; Yan, Z.; Sun, J.; Naguib, H.M.; Lu, B.; Zhang, Z. Microstructure and mechanical properties of CO2-cured steel slag brick in pilot-scale. Constr. Build. Mater. 2021, 271, 121581. [CrossRef] 14. Wang, Q.; Yan, P. Hydration properties of basic oxygen furnace steel slag. Constr. Build. Mater. 2010, 24, 1134–1140. [CrossRef] 15. Brand, A.S.; Roesler, J.R. Interfacial transition zone of cement composites with steel furnace slag aggregates. Build. Cem. Concr. Compos. 2018, 86, 117–129. [CrossRef] 16. Wang, L.; Sarkar, B.; Sonne, C.; Ok, Y.S.; Tsang, D.C.W. Soil and geologic formations as antidotes for CO2 sequestration? Soil Use Manag. 2020, 36, 355–357. [CrossRef] 17. Said, A.; Laukkanen, T.; Järvinen, M. Pilot-scale experimental work on carbon dioxide sequestration using steelmaking slag. Appl. Energy 2016.[CrossRef] 18. Liu, J.; Lin, C.; Liu, T.; Han, L.; Shen, X.; Li, C.; Lu, A. An eco-friendly permeable brick with excellent permeability and high strength derived from steel slag wastes. Int. J. Appl. Ceram. Technol. 2020, 17, 584–597. [CrossRef] 19. Fang, Y.; Su, W.; Zhang, Y.; Zhang, M.; Ding, X.; Wang, Q. Effect of accelerated precarbonation on hydration activity and volume stability of steel slag as a supplementary cementitious material. J. Therm. Anal. Calorim. 2021, 1–11. [CrossRef] 20. Sun, J.; Zhang, Z.; Zhuang, S.; He, W. Hydration properties and microstructure characteristics of alkali–activated steel slag. Constr. Build. Mater. 2020, 241, 118141. [CrossRef] 21. Liu, Q.; Liu, J.; Qi, L. Effects of temperature and carbonation curing on the mechanical properties of steel slag-cement binding materials. Constr. Build. Mater. 2016, 124, 999–1006. [CrossRef] 22. Qian, C.; Yi, H.; Du, W. Bacteria fixing CO2 to enhance the volume stability of ground steel slag powder as a component of cement-based materials aiming at clean production. J. Clean. Prod. 2021, 314, 127821. [CrossRef] 23. Song, Q.; Guo, M.-Z.; Wang, L.; Ling, T.-C. Use of steel slag as sustainable construction materials: A review of accelerated carbonation treatment. Resour. Conserv. Recycl. 2021, 173, 105740. [CrossRef] Energies 2021, 14, 4489 18 of 18

24. Liu, Z.; Zhang, D.W.; Li, L.; Wang, J.X.; Shao, N.N.; Wang, D.M. Microstructure and phase evolution of alkali-activated steel slag during early age. Constr. Build. Mater. 2019, 204, 158–165. [CrossRef] 25. Hussein, A.M.; Madkour, F.S.; Afifi, H.M.; Abdel-Ghani, M.; Abd Elfatah, M. Comprehensive study of an ancient Egyptian foot case cartonnage using Raman, ESEM-EDS, XRD and FTIR. Vib. Spectrosc. 2020, 106, 102987. [CrossRef] 26. Chen, H.; Chen, Z.; Zhao, G.; Zhang, Z.; Xu, C.; Liu, Y.; Chen, J.; Zhuang, L.; Haya, T.; Wang, X. Enhanced adsorption of U(VI) and 241Am(III) from wastewater using Ca/Al layered double hydroxide@ carbon nanotube composites. J. Hazard. Mater. 2018, 347, 67–77. [CrossRef] 27. Ahmad, S.Z.N.; Hamdan, R.; Al-Gheethi, A.; Alkhadher, S.; Othman, N. Removal of phosphate from wastewater by steel slag with high calcium oxide column filter system; efficiencies and mechanisms study. J. Chem. Technol. Biotechnol. 2020, 95, 3232–3240. [CrossRef] 28. Black, L.; Garbev, K.; Beuchle, G.; Stemmermann, P.; Schild, D. X-ray photoelectron spectroscopic investigation of nanocrystalline calcium silicate hydrates synthesised by reactive milling. Cem. Concr. Res. 2006, 36, 1023–1031. [CrossRef] 29. Black, L.; Garbev, K.; Stemmermann, P.; Hallam, K.R.; Allen, G.C. Characterisation of crystalline C-S-H phases by X-ray photoelectron spectroscopy. Cem. Concr. Res. 2003, 33, 899–911. [CrossRef] 30. Black, L.; Breen, C.; Yarwood, J.; Garbev, K.; Stemmermann, P.; Gasharova, B. Structural features of C-S-H(I) and its carbonation in air-A Raman spectroscopic study. Part II: Carbonated phases. J. Am. Ceram. Soc. 2007, 90, 908–917. [CrossRef] 31. Black, L.; Stumm, A.; Garbev, K.; Stemmermann, P.; Hallam, K.R.; Allen, G.C. X-ray photoelectron spectroscopy of the cement clinker phases tricalcium silicate and β-dicalcium silicate. Cem. Concr. Res. 2003, 33, 1561–1565. [CrossRef] 32. Zeng, Q.; Luo, M.; Pang, X.; Li, L.; Li, K. Surface fractal dimension: An indicator to characterize the microstructure of cement-based porous materials. Appl. Surf. Sci. 2013, 282, 302–307. [CrossRef] 33. García Lodeiro, I.; Macphee, D.E.; Palomo, A.; Fernández-Jiménez, A. Effect of alkalis on fresh C-S-H gels. FTIR analysis. Cem. Concr. Res. 2009, 39, 147–153. [CrossRef] 34. Pane, I.; Hansen, W. Investigation of blended cement hydration by isothermal calorimetry and thermal analysis. Cem. Concr. Res. 2005, 35, 1155–1164. [CrossRef] 35. La Russa, M.F.; Fermo, P.; Comite, V.; Belfiore, C.M.; Barca, D.; Cerioni, A.; De Santis, M.; Barbagallo, L.F.; Ricca, M.; Ruffolo, S.A. The Oceanus statue of the Fontana di Trevi (Rome): The analysis of black crust as a tool to investigate the urban air pollution and its impact on the stone degradation. Sci. Total Environ. 2017, 593, 297–309. [CrossRef][PubMed] 36. Wang, D.; Chang, J. Comparison on accelerated carbonation of β-C2S, Ca(OH)2, and C4AF: Reaction degree, multi-properties, and products. Constr. Build. Mater. 2019, 224, 336–347. [CrossRef] 37. Zhao, S.; Liu, Z.; Wang, F. Carbonation reactivity enhancement of γ-C2S through biomineralization. J. CO2 Util. 2020, 39, 101183. [CrossRef] 38. Zhang, X.; Qian, C.; Yi, H.; Ma, Z. Study on carbonation reactivity of silicates in steel slag accelerated by Bacillus mucilaginosus. Constr. Build. Mater. 2021, 292, 123433. [CrossRef]