Ca3.8Na0.2Al5.6Fe0.2Si0.2SO16): Experiments and Kinetic Analysis

applied sciences

Article Synthesis and Formation Process of a Typical Doped Solid-Solution Ye’elimite (Ca3.8Na0.2Al5.6Fe0.2Si0.2SO16): Experiments and Kinetic Analysis

Jiuye Zhao, Jiazhi Huang, Chunyang Yu and Chunyi Cui *

College of Transportation Engineering, Dalian Maritime University, Dalian 116026, China; [email protected] (J.Z.); [email protected] (J.H.); [email protected] (C.Y.) * Correspondence: [email protected]

Abstract: Ye’elimite is a dominant phase in calcium sulfoaluminate cement, which is a promising

alternative type of cementitious binder. Ca3.8Na0.2Al5.6Fe0.2Si0.2SO16 (abbreviated as ss-C4A3$) is a kind of typical doped solid-solution ye’elimite. In this study, the formation process of ss-C4A3$ was investigated. Clinkers of ss-C4A3$ were sintered at various temperatures for different holding times. X-ray diffraction tests and Rietveld quantitative phase analysis were conducted to determine

the phase compositions of the clinkers. Meanwhile, the formation process of ss-C4A3$ was analyzed by kinetic theory. The results show that solid reactions between intermediate phases (calcium

aluminate phases) and anhydrite mainly resulted in the formation of ss-C4A3$. In the conditions ◦ of 1150–1250 C, ss-C4A3$ tended to be formed and stable until 4 h. However, when the sintering ◦ temperature was 1300 C, the ss-C4A3$ decreased to generate calcium aluminate phases after 2 h. Compared to other kinetic models, the three-dimensional diffusion model mostly conformed with

the formation process of ss-C4A3$, and the ﬁtting results obtained by the Jander model exhibited the Citation: Zhao, J.; Huang, J.; Yu, C.; highest correlation coefﬁcients. The activation energy of ss-C A $ formation equaled 285.6 kJ/mol, Cui, C. Synthesis and Formation 4 3 Process of a Typical Doped which was smaller than that of stoichiometric ye’elimite. Solid-Solution Ye’elimite Keywords: calcium sulfoaluminate cement; ye’elimite; formation process; kinetic analysis; Rietveld (Ca3.8Na0.2Al5.6Fe0.2Si0.2SO16): Experiments and Kinetic Analysis. method Appl. Sci. 2021, 11, 8015. https:// doi.org/10.3390/app11178015

Academic Editor: Daniel Dias 1. Introduction As the production process of calcium sulfoaluminate (CSA) cement produces fewer Received: 4 August 2021 CO2 emissions and consumes less energy compared to Portland cement, CSA cement is Accepted: 27 August 2021 treated as a type of alternative and low-carbon binder [1,2]. Due to characteristics of early Published: 30 August 2021 strength and tailored expansion, CSA cement has also been widely used as a rapid repairing material since the 1970s and it has recently been promoted for application in numerous Publisher’s Note: MDPI stays neutral domains, such as soft soil stabilization and 3D printing [3–5]. Ye’elimite is a dominant with regard to jurisdictional claims in phase in CSA cement and generally accounts for over 50 wt.% in CSA clinkers [6,7]. published maps and institutional affil- Sintering conditions of CSA clinkers are determined by the forming conditions of ye’elimite; iations. meanwhile, the main hydration products of CSA cement result from the reactions between ye’elimite and gypsum [8–12]. The chemical formula of stoichiometric ye’elimite is Ca4Al6SO16, which can be abbreviated as st-C4A3$ (hereafter, nomenclatures are used—C: CaO, $: SO3, A: Al2O3, S: Copyright: © 2021 by the authors. SiO2). The crystallographic structure of st-C4A3$ has been identified to be orthorhombic at Licensee MDPI, Basel, Switzerland. room temperature, with a space group of Pcc2. Meanwhile, the formation and hydration This article is an open access article mechanisms of st-C4A3$ have also been intensively investigated by a number of stud- distributed under the terms and ies [13–15]. On the other hand, certain types of doping ions (such as iron ions and sodium conditions of the Creative Commons ions et al.) enter into the lattices of ye’elimite in the manufacturing process of CSA clinkers, Attribution (CC BY) license (https:// which leads to the formation of doped/solid-solution ye’elimite [16–18]. Specifically, when creativecommons.org/licenses/by/ solid waste is utilized as raw materials, various types of solid-solution ye’elimite tend 4.0/).

Appl. Sci. 2021, 11, 8015. https://doi.org/10.3390/app11178015 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 8015 2 of 10

to be formed more easily [19]. A series of recent studies focused on different types of doped ye’elimite, including strontium-bearing ye’elimite, barium-bearing ye’elimite, and iron-bearing ye’elimite. The consensus obtained by studies on doped ye’elimite is that dopants result in a crystallographic system of ye’elimite, partly or totally transforming from an orthorhombic system to a cubic system [16,17,20]. Particularly, Ca3.8Na0.2Al5.6Fe0.2Si0.2SO16 (abbreviated as ss-C4A3$), a typical kind of doped solid-solution ye’elimite, has attracted special atten- tion, as the crystallographic system of ss-C4A3$ has been proved to be purely cubic at room temperature [21]. Comparing it to st-C4A3$, ss-C4A3$ also exhibited obviously distinct hydration characters [22]. Additionally, elemental compositions of ss-C4A3$ approached sintered ye’elimite phases in CSA clinkers due to the existence of iron ions and silicon ions in raw materials [16]. In this study, the formation process of ss-C4A3$ was investigated. Clinkers of ss-C4A3$ were sintered at various temperatures for different holding times. X-ray diffraction (XRD) tests and Rietveld quantitative phase analysis (RQPA) were conducted to determine the phase compositions of the clinkers. According to the results of the RQPA, different models of solid-state reactions were employed to fit the isothermal formation process of ss-C4A3$. By comparing the obtained correlation coefficients, the formation mechanism of ss-C4A3$ was confirmed. Lastly, the value of activation energy for ss-C4A3$ was acquired by the Arrhenius equation.

2. Materials and Methods 2.1. Raw Materials and Sintering Conditions for Clinkers

According to the elemental molar ratios of st-C4A3$ and ss-C4A3$, analytical reagents of CaCO3, Na2CO3, Al2O3, Fe2O3, SiO2, and CaSO4 were weighed and mixed by a planetary ball mill for homogeneousness to prepare raw materials (purities of all analytical reagents were more than 99.99 wt.%). Afterwards, well-mixed raw materials were compressed into cylindroid samples (Φ5.0 cm × H1.0 cm) and placed in a high-temperature furnace ◦ for sintering. As for st-C4A3$, through a heating rate of 10 C/min, the raw materials ◦ were sintered at 1300 C for 240 min. With regard to ss-C4A3$, the raw materials were respectively sintered at 1150 ◦C, 1200 ◦C, 1250 ◦C, and 1300 ◦C (10 ◦C/min heating rate) for various holding times (0 min, 15 min, 30 min, 60 min, 120 min, 180 min, and 240 min).

2.2. Test Methods After sintering, synthesized clinkers were finely ground to pass a sieve of 45 µm for X-ray diffraction (XRD) testing. A Bruker diffractometer with CuKα1,2 radiation (without a monochromator, λ1 = 0.15406 nm, λ2 = 0.15444 nm) was employed at 40 kV and 40 mA. For quantitative phase analysis, the range of data collection was 5–80◦ (in this paper, all angles refer to a 2θ value) with a step size of 0.02◦, and the total measurement time for each sample was approximately 30 min. When the lattice parameters needed to be refined, the range of data collection was expanded to be 5–120◦, and the total measurement time for each sample was increased to approximately 120 min. XRD patterns obtained by direct tests were firstly analyzed qualitatively in Bruker Evolution software to determine the phases in the clinkers. Then, Topas 4.2 software was employed to perform RQPA for dealing with the obtained XRD patterns. Table1 exhibits the COD codes for all the phases involved in the RQPA.

Table 1. COD codes and references of phases involved in the RQPA.

Phases Formula (Abbreviation) COD Code Reference

Typical solid-solution ye’elimite Ca3.8Na0.2Al5.6Fe0.2Si0.2SO16 (ss-C4A3$) 45,119,60 [21] Monocalcium aluminate CaAl2O4 (CA) 43,080,75 [23] Anhydrite CaSO4 (C$) 50,000,40 [24] Mayenite Ca12Al14O33 (C12A7) 21,029,55 [25] Tricalcium aluminate Ca3Al2O6 (C3A) 90,159,66 [26] Calcium oxide CaO (C) 72,006,86 [27] Appl. Sci. 2021, 11, 8015 3 of 10

2.3. Kinetic Theory To characterize the process of chemical reactions for a certain type of product, reaction degree α is calculated according to Equation (1).

m − m α = t 0 (1) m∞ − m0

In Equation (1), t is the reaction time at the isothermal stage and m0 is the initial mass percentage of the product for the isothermal stage. The mass percentage of the product at holding time t is written as mt, and m∞ represents the (theoretical) ﬁnal mass percentage of the product for the isothermal stage. In isothermal conditions, the temperature is constant. Based on the kinetic reaction equation, the reaction degree α can be related to holding time t, as shown in Equation (2).

Z α dα g(α) = = kt (2) 0 f (α)

For Equation (2), g(α) and f (α) are the integral and differential forms of the kinetic models, respectively. The models used for the solid-state reactions in this study are exhibited in Table2.

Table 2. Kinetic models for solid-state reactions [28].

Symbol Models f(α) g(α) D1 One-dimensional diffusion 1/2α α2 D2 Two-dimensional diffusion −1/ln(1 − α) α + (1 − α)ln(1 − α) Jander D3 3(1 − α)2/3/{2[1 − (1 − α)1/3]} [1 − (1 − α)1/3]2 (Three-dimensional diffusion) Ginstling–Brounstein D4 3/{2[(1 − α)−1/3 − 1]} 1 − (2α/3) − (1 − α)2/3 (Three-dimensional) R3 Model for interface reaction 3(1 − α)2/3 1 − (1 − α)1/3 A3 Avrami–Erofeyev 3(1 − α)[− ln(1 − α)]2/3 [−ln(1 − α)]1/3 F1 Crystal nucleation growth model 1 − α −ln(1 − α)

Based on the results of linear ﬁtting for Equation (2), the slopes of the kinetic equation k for various temperatures were obtained. Furthermore, the values of the parameters in the Arrhenius Equation, expressed in the logarithmic form and exhibited in Equation (3), were conﬁrmed. k A ln = ln − Ea/RT (3) In Equation (3), A is the preexponential factor, k is the slope of the kinetic equation, Ea represents activation energy, and R is the gas constant (8.314 J/(mol·K)).

3. Results 3.1. Crystallographic Distinctions between st-C4A3$ and ss-C4A3$ To distinguish the crystallographic distinctions between orthorhombic and cubic ◦ ye’elimite, XRD patterns of st-C4A3$ (sintered at 1300 C for 240 min) and ss-C4A3$ (sintered at 1250 ◦C for 240 min) were compared, as shown in Figure1. As can be seen, except for some low-intensity peaks for monocalcium aluminate (CaAl2O4) and calcium oxide (CaO), all peaks belonged to orthorhombic and cubic ye’elimite. Meanwhile, the patterns for st-C4A3$ and ss-C4A3$ tended to be similar, as the major peaks of both patterns overlap at 23.7◦, 33.7◦, 41.6◦, (angles refer to 2θ in this paper) and so on. However, it should also be noted that, compared to ss-C4A3$, the pattern of st-C4A3$ exhibited some additional characteristic peaks (such as the peaks at 18.1◦, 20.6◦, 35.8◦, and 37.3◦), which can be observed in the zoomed-in images of Figure1. The additional characteristic peaks were used to distinguish the st-C4A3$ from the ss-C4A3$. Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 11

To distinguish the crystallographic distinctions between orthorhombic and cubic ye’elimite, XRD patterns of st-C4A3$ (sintered at 1300 °C for 240 min) and ss-C4A3$ (sintered at 1250 °C for 240 min) were compared, as shown in Figure 1. As can be seen, except for some low-intensity peaks for monocalcium aluminate (CaAl2O4) and calcium oxide (CaO), all peaks belonged to orthorhombic and cubic ye’elimite. Meanwhile, the patterns for st-C4A3$ and ss-C4A3$ tended to be similar, as the major peaks of both patterns overlap at 23.7°, 33.7°, 41.6°, (angles refer to 2θ in this paper) and so on. However, it should also be noted that, compared to ss-C4A3$, the pattern of st-C4A3$ exhibited some additional

Appl. Sci. 2021, 11, 8015 characteristic peaks (such as the peaks at 18.1°, 20.6°, 35.8°, and 37.3°), which can be4 of ob- 10 served in the zoomed-in images of Figure 1. The additional characteristic peaks were used to distinguish the st-C4A3$ from the ss-C4A3$.

FigureFigure 1.1. XRD patterns of solid-solution ye’elimite (sinte (sinteredred at at 1250 1250 °C◦C for for 4 4 h) h) and and stoichiometric stoichiometric ye’elimiteye’elimite (sintered(sintered atat 13001300 ◦°CC forfor 44 h).h).

ToTo furtherfurther confirmconfirm thethe lattice lattice parameters parameters of of synthesized synthesized st-C st-C4A4A3$3$ andand ss-C ss-C44AA33$,$, thethe RietveldRietveld method was employed employed to to refine refine the the pa patternstterns in in Figure Figure 1.1 .The The fitting fitting result result of ofss- ss-CC4A34$A can3$ canbe seen be seen in Figure in Figure 2, and2, andthe obtained the obtained values values of the of lattice the lattice parameters parameters are exhib- are exhibitedited in Table in Table 3. According3. According to the to the results results of of the the Rietveld Rietveld refinement, refinement, thethe crystallographiccrystallographic Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 11 characterscharacters ofof synthesizedsynthesized st-Cst-C44AA33$ and ss-Css-C44A3$$ in this paper were consistentconsistent withwith thosethose ofof previousprevious studies.studies.

Table 3. Lattice parameters of synthesized solid-solution and stoichiometric ye’elimite.

Types of Crystallographic System Lattice Parameters (Å) Sources Ye’elimite (Space Group) Orthorhombic a = 13.041, b = 13.036, c = 9172 This paper Stoichiometric (Pcc2) a=13.037, b = 13.035, c = 9168 [15] Typical solid-so- Cubic a = 9191 This paper lution (I-43 m) a = 9197 [21]

◦ Figure 2. Rietveld plot for XRD pattern of ss-C 4AA33$$ (sintered (sintered at at 1250 1250 °CC for for 4 4 h). h).

Table 3. Lattice parameters of synthesized solid-solution and stoichiometric ye’elimite. 3.2. Formation Process of ss-C4A3$

TheTypes formation of processCrystallographic of st-C4A System3$ can be divided into three stages, including the Lattice Parameters (Å) Sources stageYe’elimite for the decomposition(Space of Group)raw materials (mainly referring to the decomposition of CaCO3, as shown in EquationOrthorhombic (4)), the stage afor= 13.041, the formationb = 13.036, cof= 9172intermediateThis paper phases Stoichiometric (mainly referring to the formation(Pcc2) of CA, as showna = 13.037, in Equationb = 13.035, (5)),c = 9168and the formation[15] of ye’elimiteTypical (mainly referring toCubic the reaction between CA aand= 9191 C$, as shown in Equation This paper (6)). To determinesolid-solution the involved solid(I-43 m)reactions in the formationa = 9197 process of ss-C4A3$, [ 21the] XRD patterns of clinkers sintered at various temperatures for different holding times were compared,3.2. Formation as shown Process in Figure of ss-C 3.4 A3$ The formation process of st-C A $ can be divided into three stages, including the stage 4 CaCO3 3→CaO + CO2 (4) for the decomposition of raw materials (mainly referring to the decomposition of CaCO3, as CaO + Al2O3→CaAl2O4 (5)

3CaAl2O4 + CaSO4→Ca4Al6SO16 (6)

Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 11

Appl. Sci. 2021, 11, 8015 Figure 2. Rietveld plot for XRD pattern of ss-C4A3$ (sintered at 1250 °C for 4 h). 5 of 10

3.2. Formation Process of ss-C4A3$

The formation process of st-C4A3$ can be divided into three stages, including the shownstage for in the Equation decomposition (4)), the of stageraw materials for the formation(mainly referring of intermediate to the decomposition phases (mainly of referring toCaCO the3, formationas shown in of Equation CA, as shown(4)), the in stage Equation for the (5)), formation and the of formationintermediate of phases ye’elimite (mainly referring(mainly referring to the to reaction the formation between of CA, CA as and shown C$, in as Equation shown (5)), in Equationand the formation (6)). To of determine the ye’elimite (mainly referring to the reaction between CA and C$, as shown in Equation (6)). involved solid reactions in the formation process of ss-C4A3$, the XRD patterns of clinkers To determine the involved solid reactions in the formation process of ss-C4A3$, the XRD sintered at various temperatures for different holding times were compared, as shown in patterns of clinkers sintered at various temperatures for different holding times were com- Figurepared, as3. shown in Figure 3. CaCO3 → CaO + CO2 (4) CaCO3→CaO + CO2 (4) CaO + Al2O3 → CaAl2O4 (5) CaO + Al2O3→CaAl2O4 (5) → 3CaAl3CaAl2O24 O+ CaSO4 + CaSO4→Ca44Al6SOCa16 4Al6SO16 (6) (6)

Figure 3. XRD patterns of clinkers for ss-C4A3$, (a) sintered at various temperatures without holding times, (b) sintered in conditions of different holding times at 1150 ◦C, (c) sintered at 1300 ◦C for different holding times.

Comparing the XRD patterns for st-C4A3$ and ss-C4A3$ in Section 3.1, it was observed that the additional characteristic peaks of st-C4A3$ were not in any of the clinkers of ss- C4A3$ in Figure3, which means that the formation process of ss-C 4A3$ did not involve the formation of st-C4A3$. Clinkers sintered at various temperatures without holding times are exhibited in Figure3a. It can be seen that certain amounts of ss-C 4A3$ were ◦ formed at 1150 C, but CaAl2O4 and CaSO4 can obviously still be detected. With sintering ◦ temperatures increasing from 1150 to 1300 C, the intensity of the peaks of ss-C4A3$ steadily increased; meanwhile, the intensity of the peaks of CaAl2O4 and CaSO4 decreased. Similar variational tendencies for the peaks of ss-C4A3$, CaAl2O4, and CaSO4 can also be observed in the clinkers sintered at 1150 ◦C for different holding times (see Figure3b). ◦ However, for clinkers sintered at 1300 C (see Figure3c), CaSO 4 could not be detected after a 0.5 h holding time, and peaks of CaAl2O4 reappeared after sintering for over 3 h. To further determine the contents of the phases in clinkers sintered in different conditions, Rietveld quantitative phase analysis was conducted to identify the XRD patterns. Taking the analysis of the clinkers sintered at 1150 ◦C for 60 min as an example, the selected range and fitting results of the Rietveld quantitative phase analysis are shown in Figure4. Figure5 exhibits the contents of the phases in clinkers sintered at different temperatures as a function of holding time, of which three stages were identified. The first stage was the forming stage, which included 1150 ◦C (within 240 min), 1200 ◦C (within 240 min), 1250 ◦C (the first 120 min), and 1300 ◦C (the first 30 min). In the forming stage, the contents of ss-C4A3$ steadily increased with increasing holding time; meanwhile, the contents of calcium aluminate phases (in most instances, CA accounted for the overwhelming majority) Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 11

Figure 3. XRD patterns of clinkers for ss-C4A3$, (a) sintered at various temperatures without holding times, (b) sintered in conditions of different holding times at 1150 °C, (c) sintered at 1300 °C for different holding times.

Appl. Sci. 2021, 11, 8015 Comparing the XRD patterns for st-C4A3$ and ss-C4A3$ in Section 3.1, it was observed 6 of 10 that the additional characteristic peaks of st-C4A3$ were not in any of the clinkers of ss- C4A3$ in Figure 3, which means that the formation process of ss-C4A3$ did not involve the formation of st-C4A3$. Clinkers sintered at various temperatures without holding times areand exhibited anhydrite in Figure continually 3a. It can decreased. be seen that The certain variation amounts tendency of ss-C4A3 illustrates$ were formed that at ss-C4A3$ was 1150formed °C, but through CaAl2O the4 and reaction CaSO4 can between obviously the still calcium be detected. aluminate With sintering phases tempera- and anhydrite in the turesforming increasing stage, from which 1150 was to 1300 similar °C, the to intensity the reaction of the in peaks Equation of ss-C (6).4A3$ Aftersteadily the in- forming stage, creased; meanwhile, the intensity of the peaks of CaAl2O4 and CaSO4 decreased. Similar the stable stage was observed at 1250 ◦C (in the range of 120-240 min) and 1300 ◦C (in the variational tendencies for the peaks of ss-C4A3$, CaAl2O4, and CaSO4 can also be observed inrange the clinkers of 30–120 sintered min). at 1150 In the °C stablefor different stage, holding the contents times (see of Figure ss-C4 3b).A3$ However, remained stable and forgenerally clinkers remainedsintered at over1300 °C 90 (see wt.% Figure in the 3c), clinkers; CaSO4 could meanwhile, not be detected the calcium after a 0.5 aluminate h phases holdingand anhydrite time, and accounted peaks of CaAl for2O the4 reappeared absolute after minority sintering (less for than over 103 h. wt.%). To further It should also be determine the contents of the phases in clinkers sintered in different conditions, Rietveld noted that the contents of ss-C4A3$ tended to decrease and generate calcium aluminate quantitativephases when phase the analysis clinkers was were conducted sintered to identify over 120 the min XRD at patterns. 1300 ◦C, Taking which the can anal- be identiﬁed as ysis of the clinkers sintered at 1150 °C for 60 min as an example, the selected range and the decomposing stage. fitting results of the Rietveld quantitative phase analysis are shown in Figure 4.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 11

Figure 4. 4.SelectedSelectedalso range rangebe and noted fitting and that ﬁttingresults the contents of results the Rietve of of ss-C theld quantitative4A Rietveld3$ tended quantitativephase to decrease analysis and phaseof clinkers generate analysis calcium of clinkers alu- sintered at 1150 °Cminate for 60 min.phases when the clinkers were sintered over 120 min at 1300 °C, which can be sintered at 1150 ◦C for 60 min. identified as the decomposing stage. Figure 5 exhibits the contents of the phases in clinkers sintered at different temperatures as a function of holding time, of which three stages were identified. The first stage was the forming stage, which included 1150 °C (within 240 min), 1200 °C (within 240 min), 1250 °C (the first 120 min), and 1300 °C (the first 30 min). In the forming stage, the contents of ss-C4A3$ steadily increased with increasing holding time; meanwhile, the contents of calcium aluminate phases (in most instances, CA accounted for the overwhelming majority) and anhydrite continually decreased. The variation tendency illustrates that ss-C4A3$ was formed through the reaction between the calcium aluminate phases and anhydrite in the forming stage, which was similar to the reaction in Equation (6). After the forming stage, the stable stage was observed at 1250 °C (in the range of 120-240 min) and 1300 °C (in the range of 30–120 min). In the stable stage, the contents of ss-C4A3$ remained stable and generally remained over 90 wt.% in the clinkers; meanwhile, the calcium aluminate phases and anhydrite accounted for the absolute minority (less than 10 wt.%). It should

Figure 5. FigureContents 5. ofContents phases in clinkers of phases sintered in clinkers at differe sinterednt temperatures at different as a func temperaturestion of holding time: as a ( functiona) 1150 °C, of (b holding) 1200 °C, (c) 1250 °C, (d) 1300 °C. time: (a) 1150 ◦C, (b) 1200 ◦C, (c) 1250 ◦C, (d) 1300 ◦C.

3.3. Kinetic Analysis for the Formation Process of ss-C4A3$

The reaction degrees α of ss-C4A3$ in the forming stage were calculated according to the contents of the phases in the clinkers obtained by the RQPA. Afterwards, the linear relationship between the reaction degrees and the holding times were fitted by Equation (2) (the integral form of kinetic models), and the correlation coefficients (R2) were compared to determine the potential optimal models. According to the comparison in Table 4, all diffusion models exhibited obviously higher correlation coefficients than other types of models. It should be also noted that the correlation coefficients of the D3 (Jander) model and the D4 (Ginstling–Brounstein) model were very close and higher than the results yielded from the D1 and D2 models. The results illustrate that the formation reactions of ss-C4A3$ take place in a process of three- dimensional diffusion, which accords with st-C4A3$ and several types of solid-solution ye’elimite [29,30]. Considering the D3 model exhibited the most acceptable fitting results, the values for k in Equation (2) were determined by the D3 model, and the fitting results can be seen in Figure 6a. Afterwards, the values of Ea were obtained through a linear fitting for Equation (3) (Arrhenius equation), and they are shown in Figure 6b. According to the results of the Arrhenius fitting, the Ea of ss-C4A3$ equaled 285.6 kJ/mol in this study. Appl. Sci. 2021, 11, 8015 7 of 10

3.3. Kinetic Analysis for the Formation Process of ss-C4A3$

The reaction degrees α of ss-C4A3$ in the forming stage were calculated according to the contents of the phases in the clinkers obtained by the RQPA. Afterwards, the linear relationship between the reaction degrees and the holding times were fitted by Equation (2) (the integral form of kinetic models), and the correlation coefficients (R2) were compared to determine the potential optimal models. According to the comparison in Table4, all diffusion models exhibited obviously higher correlation coefficients than other types of models. It should be also noted that the correlation coefficients of the D3 (Jander) model and the D4 (Ginstling–Brounstein) model were very close and higher than the results yielded from the D1 and D2 models. The results illustrate that the formation reactions of ss-C4A3$ take place in a process of three- dimensional diffusion, which accords with st-C4A3$ and several types of solid-solution ye’elimite [29,30]. Considering the D3 model exhibited the most acceptable fitting results, the values for k in Equation (2) were determined by the D3 model, and the fitting results can be seen in Figure6a. Afterwards, the values of Ea were obtained through a linear fitting for Equation (3) (Arrhenius equation), and they are shown in Figure6b. According to the Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 11 results of the Arrhenius fitting, the Ea of ss-C4A3$ equaled 285.6 kJ/mol in this study.

Table 4. Correlation coefﬁcients (R2) ﬁtted by different reaction models. Table 4. Correlation coefficients (R2) fitted by different reaction models. Temperature Kinetic Models Temperature Kinetic Models (◦C) D1 D2 D3 D4 A3 R3 F1 (°C) D1 D2 D3 D4 A3 R3 F1 11501150 0.9608 0.9608 0.97910.9791 0.99420.9942 0.98480.9848 0.73570.7357 0.90310.9031 0.92930.9293 1200 0.9314 0.9590 0.9869 0.9693 0.7655 0.9106 0.9442 1200 0.9314 0.9590 0.9869 0.9693 0.7655 0.9106 0.9442 1250 0.9994 0.9886 0.9682 0.9800 0.8658 0.9888 0.9986 1250 0.9994 0.9886 0.9682 0.9800 0.8658 0.9888 0.9986 1300 0.9937 0.9998 0.9961 0.9996 0.9195 0.9807 0.9931 1300 0.9937 0.9998 0.9961 0.9996 0.9195 0.9807 0.9931 AverageAverage 0.97130.9713 0.98160.9816 0.98640.9864 0.98340.9834 0.82160.8216 0.94580.9458 0.96630.9663 Standard deviation 0.0274 0.0150 0.0110 0.0109 0.0743 0.0392 0.0301 Standard deviation 0.0274 0.0150 0.0110 0.0109 0.0743 0.0392 0.0301

Figure 6. Plot of kinetic analysis for for ss-C ss-C44AA3$,3$, (a ()a )Jander Jander fitting ﬁtting of of ss-C ss-C4A43A$ 3formation$ formation at atdifferent different temperatures, ((b)) ArrheniusArrhenius ﬁttingfitting forfor thethe formationformation ofof ss-Css-C44A3$.

3.4. Influence Inﬂuence of Sintering Temperature onon MicrostructuralMicrostructural MorphologiesMorphologies ofof ss-Css-C44AA33$ Microstructural morphologies morphologies of of four four types types of of clinkers clinkers containing containing over over 90 90wt.% wt.% of ss- of ss-CC4A34$A are3$ arecompared compared in Figure in Figure 7. As7. shown As shown in Figure in Figure 7a, 7thea, themorphology morphology features features of ss- of ss-C A $ sintered at 1200 ◦C for 240 min were identiﬁed as approximately 1–2 µm C4A3$ sintered4 3 at 1200 °C for 240 min were identified as approximately 1–2 μm polyhedral polyhedral granules with clear boundaries. For ss-C A $ sintered at 1250 ◦C for 240 min granules with clear boundaries. For ss-C4A3$ sintered4 3at 1250 °C for 240 min (shown in (shownFigure 7b), in Figure the majority7b), the of majority granules of granulesstill exhibited still exhibitedclear boundaries, clear boundaries, but a few but granules a few granules tended to fuse and combine with adjacent granules (shown by the arrow). When tended to fuse and combine with adjacent ◦granules (shown by the arrow). When pro- prolongedlonged times times extended extended to 240 to 240 min min at 1250 at 1250 °C (seeC (see Figure Figure 7c),7c), fusion fusion among among the the granules granules of ss-C4A3$ was more evident (shown in the circles). As for ss-C4A3$ sintered at 1300 °C for 120 min (see Figure 7d), partial granules fused to be larger than 2 μm; meanwhile, the boundaries of the granules tended to be more vague.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 11

Table 4. Correlation coefficients (R2) fitted by different reaction models.

Temperature Kinetic Models (°C) D1 D2 D3 D4 A3 R3 F1 1150 0.9608 0.9791 0.9942 0.9848 0.7357 0.9031 0.9293 1200 0.9314 0.9590 0.9869 0.9693 0.7655 0.9106 0.9442 1250 0.9994 0.9886 0.9682 0.9800 0.8658 0.9888 0.9986 1300 0.9937 0.9998 0.9961 0.9996 0.9195 0.9807 0.9931 Average 0.9713 0.9816 0.9864 0.9834 0.8216 0.9458 0.9663 Standard deviation 0.0274 0.0150 0.0110 0.0109 0.0743 0.0392 0.0301

Figure 6. Plot of kinetic analysis for ss-C4A3$, (a) Jander fitting of ss-C4A3$ formation at different temperatures, (b) Arrhenius fitting for the formation of ss-C4A3$.

3.4. Influence of Sintering Temperature on Microstructural Morphologies of ss-C4A3$ Microstructural morphologies of four types of clinkers containing over 90 wt.% of ss- C4A3$ are compared in Figure 7. As shown in Figure 7a, the morphology features of ss- C4A3$ sintered at 1200 °C for 240 min were identified as approximately 1–2 μm polyhedral Appl. Sci. 2021, 11, 8015 granules with clear boundaries. For ss-C4A3$ sintered at 1250 °C for 240 min (shown8 of 10in Figure 7b), the majority of granules still exhibited clear boundaries, but a few granules tended to fuse and combine with adjacent granules (shown by the arrow). When pro- longed times extended to 240 min at 1250 °C (see Figure 7c), fusion among the granules of of ss-C A $ was more evident (shown in the circles). As for ss-C A $ sintered at 1300 ◦C ss-C4A34$ was3 more evident (shown in the circles). As for ss-C4A34$ sintered3 at 1300 °C for µ for120 120min min (see (see Figure Figure 7d),7d), partial partial granules granules fused fused to to be be larger larger than 2 μm;m; meanwhile, the boundaries of the granules tended to be more vague.vague.

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◦ FigureFigure 7.7. MicrostructuralMicrostructural morphologies of ss-C44A33$ sinteredsintered atat variousvarious temperaturestemperatures forfor differentdifferent timestimes ((aa)) 12001200 °CC forfor 240240 min,min, ((bb)) 12501250 ◦°CC for 120 min, ((cc)) 12501250 ◦°CC for 240 min, (d)) 13001300 ◦°CC for 120 min.

4. Discussion Values of Ea for different types of ye’elimite were compared and are shown in Table Values of Ea for different types of ye’elimite were compared and are shown in Table5 . 5. It can be seen that the Ea of ss-C4A3$ was slightly smaller than that of st-C4A3$ and ob- It can be seen that the Ea of ss-C4A3$ was slightly smaller than that of st-C4A3$ and obviouslyviously smaller smaller than than that that of strontium/barium-bearing of strontium/barium-bearing ye’elimite. ye’elimite. From From the point the point of view of viewof physical of physical significance, signiﬁcance, the value the value of E ofa reflectsEa reﬂects the thebarrier barrier needed needed to tobe beovercome overcome for for a 4 3 areaction reaction [28]. [28 ].This This indicates indicates that that the the ss-C ss-CA4A$ 3tended$ tended to be to bemore more easily easily formed formed than than the the st- st-CC4A43$A and3$ and strontium/barium-bearing strontium/barium-bearing ye’elimite ye’elimite in the in thesame same sintering sintering conditions. conditions.

Table 5.5. ValuesValues ofof activationactivation energyenergy ((EEaa)) forfor differentdifferent typestypes of of ye’elimite ye’elimite formation. formation.

Target Minerals Dopants Model Ea(kJ/mol) Sources Target Minerals Dopants Model Ea (kJ/mol) Sources Ca3.8Na0.2Al5.6Fe0.2Si0.2SO16 Na+, Fe3+, Si4+ D3 285.6 This paper + 3+ 4+ Ca3.8Na0.2Al5.6Fe0.2Si0.2SO16 Na , Fe , Si D3 285.6 This paper Ca4Al6SO16 − D3 300.8 [30] Ca4Al6SO16 − D3 300.8 [30] Ca4Al6SO16 − D3 304.0 [29] Ca4Al6SO16 − D3 304.0 [29] Ca4−xBaxAl6SO16 1 2+Ba2+ D3 304 + 71.9x [29] Ca4−xBaxAl6SO16 Ba D3 304 + 71.9x [29] Ca3SrAl6SO16 2+Sr2+ D3 367.9 [30] Ca3SrAl6SO16 Sr D3 367.9 [30] 1 2+ 2+. 1 xx representsrepresents substituted substituted amounts amounts of Ba of2+ forBa Ca for2+. Ca

Less barrier for formingforming ss-Css-C44A3$$ can can be mainly attributed to the effect of differentdifferent types onon thethe formationformation processprocess ofof ye’elimite.ye’elimite. ForFor strontium/barium-bearingstrontium/barium-bearing ye’elimite,ye’elimite, strontic/baric ions ions introduce strontic/baric sulf sulfatesates or strontic/baric aluminates as distinct intermediate phases, which results in different solid reactions for forming strontium/barium-bearing ye’elimite compared to st-C4A3$ [29,30]. It should also be noted that strontic/baric sulfates are more stable than calcium sulfate at high temperatures, and thus, higher barriers need to be overcome for forming strontium/barium-bearing ye’elimite compared to st-C4A3$ [29]. However, for st-C4A3$, the formation process of ss-C4A3$ mainly resulted from the solid reactions between the calcium aluminate phases and anhydrite, which is in accordance with the st-C4A3$. Meanwhile, considering the mineralizing effect caused by the substitution of Fe3+ ions with Al3+ in the crystal structure of ye’elimite, ss-C4A3$ tended to be more easily formed than st-C4A3$ [31].

5. Conclusions Based on the Rietveld quantitative phase analysis for clinkers sintered at various temperatures for different holding times, the formation process of ss-C4A3$ was investigated through the lens of kinetic theory. The following conclusions can be made:

(1) XRD patterns of st-C4A3$ and ss-C4A3$ tended to be similar, but st-C4A3$ was distinguished from ss-C4A3$ by some additional characteristic peaks. The formation process of ss-C4A3$ mainly resulted from solid reactions between the intermediate Appl. Sci. 2021, 11, 8015 9 of 10

intermediate phases, which results in different solid reactions for forming strontium/barium-bearing ye’elimite compared to st-C4A3$ [29,30]. It should also be noted that strontic/baric sulfates are more stable than calcium sulfate at high temperatures, and thus, higher barriers need to be overcome for forming strontium/barium-bearing ye’elimite compared to st-C4A3$ [29]. However, for st-C4A3$, the formation process of ss-C4A3$ mainly resulted from the solid reactions between the calcium aluminate phases and anhydrite, which is in accordance with the st-C4A3$. Meanwhile, considering the mineralizing effect caused by the substitution of Fe3+ ions with Al3+ in the crystal structure of ye’elimite, ss-C4A3$ tended to be more easily formed than st-C4A3$ [31].

(1) XRD patterns of st-C4A3$ and ss-C4A3$ tended to be similar, but st-C4A3$ was distinguished from ss-C4A3$ by some additional characteristic peaks. The formation process of ss-C4A3$ mainly resulted from solid reactions between the intermediate phases (calcium aluminate phases) and anhydrite, which did not involve the formation ◦ of st-C4A3$. In the conditions of 1150–1250 C, ss-C4A3$ tended to be formed and stable for 4 h. However, when the sintering temperature increased to 1300 ◦C, ss- C4A3$ decomposed to generate calcium aluminate phases after 2 h. (2) Compared to other kinetic models, the three-dimensional diffusion model mostly conformed with the formation process of ss-C4A3$, and the fitting results obtained by the Jander model exhibited the highest correlation coefficients. The activation energy of ss-C4A3$ formation equaled 285.6 kJ/mol, which was lower than that of stoichiometric and strontium/barium-bearing ye’elimite. ◦ (3) The morphology features of ss-C4A3$ sintered at 1200 C for 240 min were identified as approximately 1–2 µm polyhedral granules with clear boundaries; higher sintering temperatures or longer holding times would lead to granules fusing together.

Author Contributions: J.Z.: Conceptualization, funding acquisition, writing—original draft; J.H.: data curation, investigation, visualization; C.Y.: data curation, investigation; C.C.: project administra- tion, supervision. All authors have read and agreed to the published version of the manuscript. Funding: This study was financially supported by the Fundamental Research Funds for the Central Universities (No. 3132020167). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Glasser, F.; Zhang, L. High-performance cement matrices based on calcium sulfoaluminate-belite compositions. Cem. Concr. Res. 2001, 31, 1881–1886. [CrossRef] 2. Zhou, Q.; Milestone, N.; Hayes, M. An alternative to Portland Cement for waste encapsulation—The calcium sulfoaluminate cement system. J. Hazard. Mater. 2006, 136, 120–129. [CrossRef] 3. Meng, K.; Cui, C.; Liang, Z.; Li, H.; Pei, H. A new approach for longitudinal vibration of a large-diameter floating pipe pile in visco-elastic soil considering the three-dimensional wave effects. Comput. Geotech. 2020, 128, 103840. [CrossRef] 4. Khalil, N.; Aouad, G.; El Cheikh, K.; Rémond, S. Use of calcium sulfoaluminate cements for setting control of 3D-printing mortars. Constr. Build. Mater. 2017, 157, 382–391. [CrossRef] 5. Cui, C.; Meng, K.; Xu, C.; Liang, Z.; Li, H.; Pei, H. Analytical solution for longitudinal vibration of a floating pile in saturated porous media based on a fictitious saturated soil pile model. Comput. Geotech. 2020, 131, 103942. [CrossRef] 6. Bullerjahn, F.; Zajac, M.; Ben Haha, M. CSA raw mix design: Effect on clinker formation and reactivity. Mater. Struc. 2015, 12, 3895–3911. [CrossRef] 7. Alvarez-Pinazo, G.; Cuesta, A.; Garcia-Mate, M.; Santacruz, I.; Losilla, E.R.; Torre, A.G.; Leon-Reina, L.; Aranda, M.A.G. Rietveld quantitative phase analysis of Yeelimite-containing cements. Cem. Concr. Res. 2012, 42, 960–971. [CrossRef] Appl. Sci. 2021, 11, 8015 10 of 10

8. Juenger, M.C.G.; Winnefeld, F.; Provis, J.L.; Ideker, J.H. Advances in alternative cementitious binders. Cem. Concr. Res. 2011, 12, 1232–1243. [CrossRef] 9. Shi, C.; Fernandez, A.; Palomo, A. New cements for the 21st century: The pursuit of an alternative to Portland cement. Cem. Concr. Res. 2011, 7, 750–763. [CrossRef] 10. Hargis, C.W.; Telesca, A.; Monteiro, P.J.M. Calcium sulfoaluminate (Ye’elimite) hydration in the presence of gypsum, calcite, and vaterite. Cem. Concr. Res. 2014, 65, 15–20. [CrossRef] 11. Winnefeld, F.; Martin, L.H.J.; Muller, C.J.B. Lothenbach, using gypsum to control hydration kinetics of CSA cements. Constr. Build. Mater. 2017, 155, 154–163. [CrossRef] 12. Bizzozero, J.; Gosselin, C.; Scrivener, K.L. Expansion mechanisms in calcium aluminate and sulfoaluminate systems with calcium sulfate. Cem. Concr. Res. 2014, 56, 190–202. [CrossRef] 13. Bullerjahn, F.; Zajac, M.; Ben Haha, M.; Scrivener, K.L. Factors influencing the hydration kinetics of Ye’elimite; effect of mayenite. Cem. Concr. Res. 2018, 116, 113–119. [CrossRef] 14. El, Y.; El, Y.; Smith, A. Examination of ye’elimite formation mechanisms. J. Eur. Ceram. Soc. 2019, 39, 5086–5095. 15. Cuesta, A.; De la Torre, A.G.; Losilla, E.R.; Peterson, V.K.; Rejmak, P.; Ayuela, A.; Frontera, C.; Aranda, M.A.G. Structure, Atomistic Simulations, and Phase Transition of Stoichiometric Yeelimite. Chem. Mater. 2013, 25, 1680–1687. [CrossRef] 16. Zea-Garcia, J.; Santacruz, I.; Aranda, M.; De, G. Alite-belite-ye’elimite cements: Effect of dopants on the clinker phase com-position and properties. Cem. Concr. Res. 2019, 115, 192–202. [CrossRef] 17. Li, C.; Wu, M.; Yao, W. Effect of coupled B/Na and B/Ba doping on hydraulic properties of belite-ye’elimite-ferrite cement. Constr. Build. Mater. 2019, 208, 23–35. [CrossRef] 18. Andaç, O.; Glasser, F.P. Polymorphism of calcium sulphoaluminate (Ca4Al6O16·SO3) and its solid solutions. Adv. Cem. Res. 1994, 6, 57–60. [CrossRef] 19. Chang, J.; Cheng, X.; Liu, F.; Lu, L.; Teng, B. Influence of fluorite on the Ba-bearing sulphoaluminate cement. Cem. Concr. Res. 2001, 31, 213–216. [CrossRef] 20. Zhao, J.; Chang, J. Crystallographic Analysis of Sr-Bearing Ye’elimite. J. Inorg. Organomet. Polym. Mater. 2017, 27, 1694–1702. [CrossRef] 21. Cuesta, A.; de la Torre, Á.G.; Losilla, E.; Santacruz, I.; Aranda, M. Pseudocubic crystal structure and phase transition in doped ye’elimite. Cryst. Growth Des. 2014, 14, 5158–5163. [CrossRef] 22. Cuesta, A.; Pinazo, G.; Sanfélix, S.; Peral, I.; Aranda, M.; De la Torre, A. Hydration mechanisms of two polymorphs of synthetic Ye’elimite. Cem. Concr. Res. 2014, 63, 127–136. [CrossRef] 23. Lazic, B.; Krüger, H.; Kahlenberg, V.; Konzett, J.; Kaindl, R. Incommensurate structure of Ca2Al2O5 at high temperatures— Structure investigation and Raman spectroscopy. ACS Catal. 2010, 64, 417–425. [CrossRef][PubMed] 24. Cheng, G.C.H.; Zussman, J. The crystal structure of anhydrite (CaSO4). Acta Crystallogr. 1963, 16, 767–769. [CrossRef] 25. Boysen, H.; Lerch, M.; Stys, A.; Senyshyn, A. Structure and oxygen mobility in mayenite (Ca12Al14O33): A high-temperature neutron powder diffraction study. Acta Crystallogr. Sect. B Struct. Sci. 2007, 63, 675–682. [CrossRef] 26. Mondal, P.; Jeffery, J.W. The crystal structure of tricalcium aluminate, Ca3Al2O6. Acta Crystallogr. Sect. B Struct. Crystallogr. Cryst. Chem. 1975, 31, 689–697. [CrossRef] 27. Verbraeken, M.; Suard, E.; Irvine, J. Structural and electrical properties of calcium and strontium hydrides. J. Chem. Educ. 2009, 19, 2766–2770. [CrossRef] 28. Laidler, K.J. The development of the Arrhenius equation. J. Chem. Educ. 1984, 61, 494. [CrossRef] 29. Bao, X.; Zhao, P.; Liang, C.; Li, Q.; Wang, S.; Cheng, X. Phase formation mechanism and kinetics in solid-state synthesis of 0 Ba-doped Ye elimite: The effect of Ba-doping concentration on C4−xBxA3$ systems. Ceramics-Silikáty 2020, 64, 338–347. [CrossRef] 30. Zhao, J.; Chang, J. Kinetic Analysis for Formation Process of Sr-Bearing Ye’elimite. J. Inorg. Organomet. Polym. Mater. 2017, 27, 1861–1869. [CrossRef] 31. Bullerjahn, F.; Scholten, T.; Scrivener, K.; Ben Haha, M.; Wolter, A. Formation, composition and stability of Ye’elimite and iron-bearing solid solutions. Cem. Concr. Res. 2020, 131, 106009. [CrossRef]