Effect of CaO Sourced from CaCO₃ or CaSO₄ on Phase Formation and Mineral Composition of Iron-Rich Calcium Sulfoaluminate Clinker

Abstract

The performance of iron-rich calcium sulfoaluminate (IR-CSA) cement is greatly affected by mineral composition and mineral activity in the clinker. This study aims to identify the effect of CaO sources, either CaCO₃ or CaSO₄, on the phase formation and mineral composition of the IR-CSA clinker. Targeted samples were prepared with different proportions of CaCO₃ and CaSO₄ as CaO sources at 1300 °C for 45 min. Multiple methods were used to identify the mineralogical conditions. The results indicate that the mineral composition and performance of the IR-CSA clinker could be optimized by adjusting the CaO source. Both Al₂O₃ and Fe₂O₃ tend to incorporate into C₄A_3−xF_x$\overline{\mathrm{S}}$ with an increase in CaSO₄ as a CaO source, which leads to an increased content of C₄A_3−xF_x$\overline{\mathrm{S}}$ but a decreased ferrite phase. In addition, clinkers prepared with CaSO₄ as a CaO source showed much higher x value in C₄A_3−xF_x$\overline{\mathrm{S}}$ and higher compressive strength than clinker prepared with CaCO₃ as the sole CaO source. The crystal types of both C₄A_3−xF_x$\overline{\mathrm{S}}$ and C₂S were also affected, but showed different trends with the transition of the CaO source. The findings provide a possible method to produce IR-CSA cement at a low cost through cooperative utilization of waste gypsum and iron-bearing industrial solid wastes.

1. Introduction

Calcium sulfoaluminate (CSA) cement, which was first developed in China 40 years ago, has attracted increasing attention due to its lower energy consumption and CO₂ emissions than Portland cement (PC) [1,2,3]. CSA clinker consists mainly of calcium sulfoaluminate (C₄A₃$\overline{\mathrm{S}}$), dicalcium silicate (C₂S), and ferrite phases (C₂F~C₆A₂F). It can be produced at 1200 °C–1300 °C, which is about 150 °C lower than PC clinker production [4]. CSA cement exhibits excellent performance in engineering construction, such as precast products, 3D printing, marine-engineering concrete and cold environments, due to its characteristics of high early-age strength, rapid setting, low permeability, and low alkalinity [5,6,7,8]. Therefore, CSA cement will be a promising alternative to PC in future.

The high cost of raw meal has limited the massive scale application of CSA cement in engineering construction, especially expensive bauxite. To reduce production costs, CSA has been produced by using industrial solid wastes (ISWs) containing aluminum, such as aluminum ash and red mud, instead of high-grade bauxite [4,7,8,9]. However, aluminum-bearing ISWs such as red mud, steel slag, and tailings are rich in iron. The presence of iron has an impact on the formation, composition, and hydration activity of the key mineral phases in CSA clinker [10,11,12,13]. Iron is reported mainly to transform to the ferrite phase and ye’elimite [14] and that will make difference to the cement performance. The ferrite phase is beneficial to the early strength, but the ye’elimite is beneficial to both the early and late compressive strength [15,16]. More importantly, the incorporation of iron into ye’elimite makes it possible to decrease the aluminum content in raw materials and expand the aluminum sources. Therefore, researchers have tried to introduce more Fe₂O₃ into C₄A_3−xF_x$\overline{\mathrm{S}}$ in iron-rich calcium sulfoaluminate (IR-CSA) clinker.

Several researchers produced IR-CSA clinker in CaO-SiO₂-Al₂O₃-Fe₂O₃-SO₃-based systems and found that the x value in C₄A_3−xF_x$\overline{\mathrm{S}}$ is greatly affected by the Fe₂O₃, SO₃, and CaO proportions in the raw mixture [14,17,18,19]. Studies have confirmed that the x value in C₄A_3−xF_x$\overline{S}$ presents a rising trend with the increase in Fe₂O₃ content in the raw material; however, not all Fe₂O₃ can be substituted for Al₂O₃ to participate in the formation of C₄A_3−xF_x$\overline{S}$ [18]. Low CaO and rich SO₃ content in raw mixtures have also proven effective in promoting the incorporation of Fe₂O₃ into C₄A_3−xF_x$\overline{\mathrm{S}}$ in calcium sulpholuminate ferrite–based systems [17,19]. It is obvious that both CaO and SO₃ could be provided by CaSO₄. Thus, it is feasible to ensure sufficient SO₃ by batching excessive CaSO₄ as CaO sources in the raw material.

CaO is derived from both CaCO₃ and CaSO₄ during the sintering process. Some researchers have indicated that the content of CaSO₄ as a CaO source has an influence on the mineral composition of the clinker and further affects the performance of the cement. They have reported that a small amount of CaSO₄ as a CaO source makes no obvious difference to the mineral composition of the clinker and that the loss of SO₃ was compensated to promote adequate formation of ye’elimite; however, excess CaSO₄ as a CaO source was reported to lead to the formation of gehlenite, thereby decreasing the compressive strength of the cement [20,21]. Others have indicated that CSA clinker could be obtained with CaO as provided by the partial decomposition of CaSO₄, with both the key mineral phase composition of the clinker and the properties of the cement being similar to that produced by traditional methods (CaO source from CaCO₃) [22]. In particular, CSA clinker with CaSO₄ as the entire CaO source was produced with adequate formation of key mineral phases [23]. In our previous study, we found that batching the slight excess of CaSO₄ in the raw mixture is an effective method of promoting the utilization rate of Al₂O₃ (to form C₄A_3−xF_x$\overline{\mathrm{S}}$) [17,20]. In addition, the crystal structure of ye’elimite transformed from orthorhombic symmetry to cubic symmetry because of the incorporation of Fe₂O₃, and the x value of C₄A_3−xF_x$\overline{\mathrm{S}}$ increased with the increase in CaSO₄ content. We also prepared IR-CSA with gypsum as the entire CaO source and found that the added iron did not form the ferrite phase first but was incorporated into other phases such as ye’elimite [14]. From the above studies, we found that the phase formation, transformation, and composition of IR-CSA clinker is greatly affected by CaO sources. However, the conclusions of the above studies are slightly biased due to the different preparation conditions. Systematic research into the effect of CaO sources on the phase formation and mineralogic conditions of IR-CSA clinker are still limited, and the mechanism remains unclear.

$$Ⅰ 3\mathrm{CaO}+3{\mathrm{Al}}_2{\mathrm{O}}_3+{\mathrm{CaSO}}_4\stackrel{900~950 ℃}{\to } 3\mathrm{CaO}·3{\mathrm{Al}}_2{\mathrm{O}}_3·{\mathrm{CaSO}}_4$$

(1)

$$Ⅱ \mathrm{CaO}+{\mathrm{Al}}_2{\mathrm{O}}_3\stackrel{1050~1150 ℃}{\to } \mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3$$

(2)

$$3\left(\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3\right)+{\mathrm{CaSO}}_4\stackrel{1050~1150 ℃}{\to } 3\mathrm{CaO}·3{\mathrm{Al}}_2{\mathrm{O}}_3·{\mathrm{CaSO}}_4$$

(3)

$$Ⅲ 2\mathrm{CaO}+{\mathrm{Al}}_2{\mathrm{O}}_3+{ \mathrm{SiO}}_2\stackrel{850~900 ℃}{\to } 2\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·{\mathrm{SiO}}_2$$

(4)

$$3\mathrm{CaO}+3\left(2\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·{\mathrm{SiO}}_2\right)+3{\mathrm{CaSO}}_4\stackrel{1000~1100 ℃}{\to } 3\mathrm{CaO}·3{\mathrm{Al}}_2{\mathrm{O}}_3·{\mathrm{CaSO}}_4+3\left(2\mathrm{CaO}·{\mathrm{SiO}}_2\right)$$

(5)

$$2\mathrm{CaO}+{\mathrm{SiO}}_2\stackrel{1000~1100 ℃}{\to } 2\mathrm{CaO}·{\mathrm{SiO}}_2$$

(6)

$$2\mathrm{CaO}+{\mathrm{Fe}}_2{\mathrm{O}}_3\stackrel{1000~1100 ℃}{\to } 2\mathrm{CaO}·{\mathrm{Fe}}_2{\mathrm{O}}_3$$

(7)

$$2\left(2\mathrm{CaO}·{\mathrm{Fe}}_2{\mathrm{O}}_3\right)+\mathrm{CaO}+\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3\stackrel{1150~1200 ℃}{\to } 6\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·2{\mathrm{Fe}}_2{\mathrm{O}}_3$$

(8)

$$2(2\mathrm{CaO}·{\mathrm{SiO}}_2)+{ \mathrm{CaSO}}_4\stackrel{1150~1200 ℃}{\to }4\mathrm{CaO}·2{\mathrm{SiO}}_2·{\mathrm{CaSO}}_4$$

(9)

$$4\mathrm{CaO}·2{\mathrm{SiO}}_2·{\mathrm{CaSO}}_4\stackrel{1200~1250 ℃}{\to } 2\left(2\mathrm{CaO}·{\mathrm{SiO}}_2\right)+{ \mathrm{CaSO}}_4$$

(10)

$$6\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·2{\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{CaO}+\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3\stackrel{1250~1300 ℃}{\to } 2\left(4\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·{\mathrm{Fe}}_2{\mathrm{O}}_3\right)$$

(11)

$$4\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·{\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{CaO}+\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3\stackrel{1250~1300 ℃}{\to } 6\mathrm{CaO}·2{\mathrm{Al}}_2{\mathrm{O}}_3·{\mathrm{Fe}}_2{\mathrm{O}}_3$$

(12)

To make clear the effect of the CaO source, as either CaCO₃/limestone or CaSO₄/gypsum, on the mineralogic conditions of IR-CSA clinker, the targeted clinker was prepared with the mineral proportion of C₄A₃$\overline{\mathrm{S}}$:C₂S:C₄AF as 50:30:20 by mass. Five groups of raw materials with increasing proportions of CaSO₄ as a CaO source were designed; namely, whole CaCO₃ (the CaSO₄ was assumed to be undecomposed), 90 at.% CaCO₃ + 10 at.% CaSO₄ (at.% is the abbreviation form of the atomic ratio in the text), 80 at.% CaCO₃ + 20 at.% CaSO₄, 50 at.% CaCO₃ + 50 at.% CaSO₄, 20 at.% CaCO₃ + 80 at.% CaSO₄ and 100 at.% CaSO₄ were designed to produce the clinkers. To ensure that the mineral formation was complete, all clinker was prepared at 1300 °C for 45 min. The mineralogic conditions as well as the microstructure and the chemical composition of key minerals, which would be influenced by the CaO source, were identified by multiple methods. Finally, the compressive strength was tested as a supplemental validation to verify the feasibility of optimizing the mineralogic conditions of the IR-CSA clinker and further the performance of the IR-CSA cement by adjusting the content of CaSO₄ as the CaO source.

2. Materials and Methods

2.1. Raw Mixture

The raw materials (CaCO₃, Al₂O₃, CaSO₄·2H₂O, Fe₂O₃, and SiO₂) for preparing the IR clinker were analytical reagents provided by the Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China. CaCO₃ was used as the CaO source. CaSO₄ provided both CaO and SO₃ during the calcination process of the IR-CSA clinker.

The targeted mineral composition of all clinker and the contents of the raw materials in the preparation of 100 g clinker with chemical reagents are shown in

Table 1. Target mineral composition of the clinker and chemical reagent proportions in the raw materials to prepare 100 g clinker.

Sample	Targeted Minerals			Raw Material Proportion
	${\mathbf{C}}_4{\mathbf{A}}_3\overline{\mathbf{S}}$	C2S	C4AF	CaCO3	SiO2	Al2O3	Fe2O3	CaSO4·2H2O
S00	50	30	20	55.69	7.67	21.47	4.83	10.34
S10	50	30	20	48.19	7.37	20.64	4.64	19.15
S20	50	30	20	41.25	7.10	19.88	4.47	27.31
S50	50	30	20	23.20	6.39	17.89	4.02	48.51
S80	50	30	20	8.43	5.81	16.26	3.65	65.85
S100	50	30	20	0	5.47	15.33	3.44	75.76

. The Bogue method was used to obtain the corresponding raw mixture [17]. The clinkers were named as S00, S10, S20, S50, S80, and S100, based on the content of CaO in the form of CaSO₄. C_m indicates the degree of CaO in the raw material in order to satisfy that required for the formation of useful minerals in the corresponding clinker. The formula of C_m is defined as in Equation (13), and the value is fixed as 1.00 in this study. The Fe₂O₃/(Al₂O₃ + Fe₂O₃) ratio was the same for all the clinker in this study.

$${\mathrm{C}}_{\mathrm{m}}=\frac{\mathrm{w}\left(\mathrm{CaO}\right)-0.7\mathrm{w}\left({\mathrm{TiO}}_2\right)}{0.73\left[\mathrm{w}\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)-0.64\mathrm{w}\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)\right]+1.4\mathrm{w}\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)+1.87\mathrm{w}\left({\mathrm{SiO}}_2\right)}$$

(13)

2.2. Preparation Method

The reagents were dried at 110 °C for 2 h in a muffle furnace (LYL-16MA, LUO YANG LIYU KILN CO., LTD, Luoyang, China) and were then ground together in an agate bowl for 1 h to ensure the homogenization of the raw mixture; alcohol was added as a dispersant in the grind progress. After homogenization, 50 g per sample of raw mixture was added to the stainless steel mold (Tianjin Jingtuo Instrument Technology Co., LTD, Tianjin, China); a columnlike raw mixture with a 35 mm diameter was obtained under a pressure of 20 MPa. Thereafter, the obtained columnlike raw mixtureswere fired in an elevator-hearth furnace (LYL-17SJ, LUOYANG LIYU KILN CO., LTD, Luoyang, China) at 1300 ℃ for 45 min and then rapidly cooled at room temperature. Finally, the IR-CSA cement was obtained by milling the clinkers and gypsum. The gypsum added to the clinkers to react with ye’elimite would promote the formation of C₆A$\overline{S}$₃H₃₂ (AFt) according to Equation (14), but samples with higher M ratios hydrated too fast to form cracks between AFt [24] and thus decreased the mechanical properties of the cement. The optimal gypsum content mixed with clinkers were samples with 0.8 m ratios of gypsum- (the anhydrate in clinkers was involved) to-ye’elimite (M) suggested by [10] and [25]. The content of ye’elimite and anhydrate were determined by the Rietveld refinement method [26]. Subsequently, the cement was prepared as cubic specimens (20 mm × 20 mm × 20 mm) with the water-to-cement ratio of 0.28.

$${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}+2\mathrm{C}\overline{\mathrm{S}}{\mathrm{H}}_{\mathrm{x}}+{\mathrm{H}}_{38-2\mathrm{x}}\to {\mathrm{C}}_6\mathrm{A}{\overline{\mathrm{S}}}_3{\mathrm{H}}_{32}+2{\mathrm{AH}}_3-\mathrm{gel}$$

(14)

where C₄A₃$\overline{\mathrm{S}}$, C$\overline{\mathrm{S}}$H_x and C₆A$\overline{\mathrm{S}}$₃H₃₂ are the abbreviated form of 3CaO Al₂O₃ CaSO₄, CaSO₄ x H₂O and 6CaO Al₂O₃ SO₃ 32H₂O, respectively, and the superscript on S represents sulfate in the compound.

2.3. Testing Methods

The phenyl formic acid–ethyl alcohol titration method was adopted to determine the f-CaO content of the IR-CSA clinker according to the Chinese standard GB/T 176–2017.

The sulfate loss was calculated according to the difference in the SO₃ amount between the raw materials and the clinkers. The SO₃ contents were determined according to the barium sulfate precipitation in accordance with the Chinese standard GB/T176–2008 [23]. The gypsum decomposition rate is expressed as follows:

$$\mathrm{Decomposition} \mathrm{rate} (\%)=\frac{M_1\times S_1-M_2\times S_2}{M_{1 }\times S_1}$$

(15)

where M₁ and S₁ represent the mass and SO₃ content of a raw sample, respectively; M₂ and S₂ are the mass and SO₃ content of a clinker sample, respectively.

The mineral phase assemblages of all clinkers were determined by an X-ray diffractometer (Aeris, Malvern Panalytical, Malverin, UK) with Cu-Kα radiation. The voltage and the current were set to 40 KV and 15 mA, respectively. The detection range of the X-ray diffraction spectra was set to 10–60° with a scanning speed of 0.0027° per step. The mineral composition was quantified by Rietveld refinement using Topas-Academic V6 software. The crystallographic structures of the phases involved are listed in

Table 2. Crystallographic structures and ICSD codes of phases involved in the Rietveld refinement.

Scientific Names	Crystal System	Space Group	Abbrev. of Formula	Chemical Formula	ICSD
Ye’elimite-c	cubic	I-43m	C4A3$\overline{\mathrm{S}}$-c	Ca4Al6SO16	9560
Ye’elimite-o	orthorhombic	Pcc2	C4A3$\overline{\mathrm{S}}$-o	Ca4Al6SO16	80,361
Belite-beta	Monoclinic	P21/n	C2S-beta	Ca2SiO4	79,550
Belite-al’	orthorhombic	Pnma	C2S-al’	Ca2SiO4	81,097
Srebrodolskite	orthorhombic	Pnma	C2F	Ca2Fe2O5	15,059
Brownmillerite	orthorhombic	Ibm2	C4AF	Ca4Al2Fe2O10	9197
Gehlenite	Tetragonal	p-421m	C2AS	Ca2Al2SiO7	87,144
Anhydrite	orthorhombic	Amma	$\mathrm{C}\overline{\mathrm{S}}$	CaSO4	16,382

. The refined parameters included the background coefficients, instrument parameters, zero-shift error and unit cell parameters [14].

The microstructure of the pattern was acquired by scanning electron microscopy (SEM, Quattro S, Thermo Fisher Scientific, Waltham, MA, USA) with an acceleration voltage of 5 kV.

The chemical composition of clinkers was obtained by a field emission electron probe (FE-EPMA, JXA-8530F-Plus, Tokyo, Japan), and the backscattered electron (BSE) images and elemental distribution images of the patterns were detected with an acceleration voltage of 15 KV and an electron beam current of 5 × 10⁻⁸ A. The samples were first completely covered with epoxy resin and polished to a mirror state; the prepared patterns were then sprayed with carbon to improve their electrical conductivity for microanalysis.

The cement slurry was fully stirred with a water-to-cement ratio of 0.28 and was then poured into a 20 mm × 20 mm × 20 mm mold. The cement slurry specimens were demolded after curing with a temperature of 20 ± 1 ℃ and 99% relative humidity after 24 h. They were then cured for a standard time of 1, 3 and 28d to test the compressive strength [27].

3. Results and Discussion

3.1. Decomposition Rate of CaSO₄ to Free-CaO

The decomposition of CaSO₄ determines the actual CaO content participating in the reactions. In addition, the decomposition of CaSO₄ has a great influence on the phase formation, phase composition, and even the chemical composition of the mineral phases according to the study of [14,17]. However, the decomposition of CaSO₄ to CaO is affected by its proportion in the raw mixture [20]. Thus, it is necessary to confirm the decomposition rate after the calcination. Figure 1 shows the quantitative and theoretical sulfate loss of the clinkers with different CaO sources produced at 1300 ℃. As shown in Figure 1, although the same ye’elimite content was designed in clinkers, the actual SO₃ content differs in all clinkers. The SO₃ content increased gradually from clinkers S00 to S80, but then decreased in S100.

The gypsum decomposition rates of all clinker were lower than the theoretical value except for S00. In S00, all CaSO₄ was assumed to participate in the reactions to form ye’elimite. In fact, there was 14.7 wt.% sulfate loss in the clinker, which led to a higher CaO than the stoichiometric value. However, there was no f-CaO observed in the sample, which means that the extra CaO was involved in the reactions. Extra CaO induces phase equilibrium with a greater ferrite phase (with the CaO/(Al₂O₃ + Fe₂O₃) ratio of 2.0) formed rather than ye’elimite (with the CaO/(Al₂O₃ + Fe₂O₃) ratio of 1.3), because of the higher CaO/(Al₂O₃ + Fe₂O₃) ratio [28]. In clinkers S10−S100, the sulfate loss was less than the designed value and resulted in less CaO participating in the reactions, which led to more Fe₂O₃ being incorporated into the ye’elimite [17]. The CaO and SO₃ content of the clinkers and the differences between the quantitative and theoretical values are listed in

Table 3. CaO and SO₃ content of clinkers and their differences between quantitative and theoretical values.

$\mathbf{CaO}/{\mathbf{SO}}_3(\mathbf{\Delta })$ Content	Theo.	S00	S10	S20	S50	S80	S100
SO3	6.56	5.58	8.58	10.08	12.01	12.93	8.96
$\Delta \left({\mathrm{SO}}_3\right)=\mathrm{S}\left({\mathrm{SO}}_3\right)- \mathrm{Theo}.\left({\mathrm{SO}}_3\right)$	–	−0.98	2.02	3.52	5.46	6.37	2.40
CaO	42.53	43.22	41.12	40.07	38.71	38.07	40.85
$\Delta \left(\mathrm{CaO}\right)=\mathrm{S}\left(\mathrm{CaO}\right)- \mathrm{Theo}.\left(\mathrm{CaO}\right)$	–	0.69	−1.41	−2.46	−3.82	−4.46	−1.68

. The differences caused by different proportions of CaCO₃/CaSO₄ affected the formation of the mineral phases.

The actual CaO content involved in the reactions is calculated as in Equation (16). CaCO₃ was assumed to decompose completely above 900 °C and to participate in all reactions. CaO from CaSO₄ was calculated due to the SO₃ content.

$$\mathrm{CaO}={\mathrm{CaCO}}_3\times \frac{{\mathrm{M}}_{\mathrm{CaO}}}{{\mathrm{M}}_{\mathrm{CaCO}3}}+({\mathrm{CaSO}}_4\times \frac{{\mathrm{M}}_{\mathrm{CaO}}}{{\mathrm{M}}_{\mathrm{CaSO}4}}-{ \mathrm{SO}}_3\times \frac{{\mathrm{M}}_{\mathrm{CaSO}4}}{{\mathrm{M}}_{\mathrm{SO}3}}\times \frac{{\mathrm{M}}_{\mathrm{CaO}}}{{\mathrm{M}}_{\mathrm{CaSO}4}})$$

(16)

3.2. Mineralogical Characterization

3.2.1. Qualitative Phase Analysis

The XRD patterns of the IR-CSA clinker are shown in Figure 2. The key mineral phases, such as ye’elimite and belite existed in all the clinkers, while anhydrite and brownmillerite were partially present. The characteristic peak of anhydrate only presented in clinker S10–S80 and became stronger gradually, which is consistent with the SO₃ content as tested in Section 3.1. The characteristic peak of brownmillerite (around 12.1°) in S00 is the strongest and becomes weaker gradually with the increase in CaSO₄ as a CaO source. The characteristic peak of brownmillerite (around 12.1°) vanished in S50 and S80, but its intensity increased slightly in S100. On one hand, more CaSO₄ batching in the raw material is beneficial for Al₂O₃ to form ye’elimite rather than brownmillerite. With the increase in CaSO₄ as the CaO source, more CaO or CA is wrapped by CaSO₄ so that the formation paths of ye’elimite expressed as Equations (1)–(3) are prior to that of the ferrite phase expressed as Equations (7)–(11). On the other hand, the residual CaSO₄ in the clinker means the low CaO content participates in the reactions, and this induces more Fe₂O₃ to be incorporated into C₄A_3-xF_x$\overline{S}$ than to the form ferrite phase [17].

The characteristic peaks of C₄A₃$\overline{\mathrm{S}}$ and C₂S were also different between the clinkers. The characteristic peak of C₄A₃$\overline{\mathrm{S}}$-o (at about 18.1°) was obvious in S00 and became weaker, gradually vanishing when CaSO₄ provided more than 50% CaO. Moreover, the main characteristic peaks of ye’elimite (at approximately 23.7° and 27.4°) gradually shifted to lower angles with the increase in the CaO derived from CaSO₄ (Figure 3a,b). This was mainly because the interplanar spaces of ye’elimite (with the crystal plane exponent of (022) in C₄A₃$\overline{\mathrm{S}}$-o and (211) in C₄A₃$\overline{\mathrm{S}}$-c, respectively) were increased with the amount of Al³⁺ (0.535 Å) substituted by Fe³⁺(0.645 Å) [17]. Similarly, the crystal structure of belite was additionally affected by CaO sources and the crystal structure of α’-C₂S was more likely to occur when the CaO source of CaSO₄ was increased (Figure 3c). The most convenient condition for the formation of α’-C₂S is that CaSO₄ provides CaO with the proportion of 50–80 at.% and with anhydrite residue in the clinker.

3.2.2. Quantitative Phase Analysis

Quantitative analysis was conducted using the Rietveld refinement to better understand the formation and transformation of the key mineral phases affected by CaO sources. The results (Figure 4) indicate that the C₄A₃$\overline{\mathrm{S}}$-o decreased while the C₄A₃$\overline{\mathrm{S}}$-c increased with the increase in CaSO₄ as a CaO source, which indicates that more Fe₂O₃ is incorporated into C₄A_3-xF_x$\overline{\mathrm{S}}$ to stabilize C₄A₃$\overline{\mathrm{S}}$ as a pseudocubic crystal during the cooling process [29,30]. The crystal types of belite were also affected by CaO sources. The results indicate that C₂S only existed as a beta crystal type in S00; β-C₂S subsequently decreased gradually from S00 to S80, but increased in S100. However, the α’-C₂S phase showed the opposite trend. The literature has reported that the crystal structure of C₂S is correlated to its formation paths and that C₂S as obtained from Equations (5) and (6) exists as β-C₂S, while from Equation (10) as α’-C₂S [15]. It is clear that the increasing content of CaSO₄ induces a greater $4\mathrm{CaO}·2{\mathrm{SiO}}_2·{\mathrm{CaSO}}_4$ formation as the transition phase and finally decomposes as α’-C₂S.

The ferrite phase usually refers to a solid solution with the chemical composition of the formula Ca₂(Al_yFe_2-y)O₅, where y can vary from 0 to $4/3$. The ferrite phase, with a chemical composition of y = 0, is specified as srebrodolskite (C₂F), and the set of assemblages with 0 < y < $4/3$ usually have a chemical composition approximating to brownmillerite (C₄AF) [17]. The formation of brownmillerite is considered to comprise a continuous solid solution of calcium aluminate with srebrodolskite (C₂F) [31]. In this study, both srebrodolskite and brownmillerite existed in all clinker except for S00. There was no srebrodolskite in clinker S00, which shows that srebrodolskite reacts with calcium aluminate to form brownmillerite. In addition, the content of brownmillerite is slightly higher than the designed value; the main reason for this is that the CaSO₄ is insufficient to react with CA to form C₄A₃$\overline{\mathrm{S}}$ due to the partial decomposition, and more CA will be a solid soluble in brownmillerite according to Equations (11) and (12). The content of brownmillerite decreases gradually with the increase in CaSO_4. as the CaO source. In addition, the brownmillerite is much lower than the designed value in the clinker when CaSO₄ provides the CaO. Small amounts of srebrodolskite were present in clinker S10–S100, which shows that CA is more likely to act with CaSO₄ than C₂F when CaSO₄ batches in raw material as the CaO source.

3.3. Chemical Composition of Key Mineral Phases in Iron-Rich Sulfoaluminate Clinker

The crystal type of ye’elimite is determined by the incorporation level of Fe₂O₃, and be’lite is affected by the incorporation of SO₃ and Fe₂O₃, to inhibit crystal transformation during cooling. It is clear that the chemical composition of the mineral phase is affected by the CaO source, in accordance with the results specified in Section 3.2, and this could be determined by the elemental distribution obtained from the polished sections of the sample slices by EPMA; the results are shown in Figure 5 (back-scattered electron (BSE) micrograph Figure 5a—S00, Figure 5b—S50, Figure 5c–S80, Figure 5d—S100) and Figure 6 (a: with the back-scattered electron (BSE) micrograph; b element distribution map). There are 300 × 225 quantitative points of the polished area with elements as shown on the map.

3.3.1. Chemical Composition of Ye’Elimite

To obtain the chemical composition of ye’elimite, element contents were sorted and calculated from the ye’elimite phase area. The elemental composition of oxygen atoms was normalized to 16 for comparison. The results are presented in

Table 4. Mineralogical composition of ye’elimite phase of samples S00, S50, S80 and S100.

Sample	Mineralogical Composition	Fe/(Al + Fe)(wt.%)	Fe2O3 in Ye’elimite Phase(wt.%)
S00	Ca4.0Al5.82 Fe0.17Si0.07S0.96O16	5.71	2.19
S50	Ca3.94Al5.17 Fe0.45Si0.13S1.08O16	15.29	4.91
S80	Ca3.93Al5.24Fe0.53Si0.13S1.12O16	17.34	6.89
S100	Ca4.06Al5.09Fe0.46Si0.14S1.12O16	15.79	6.03

. The Al³⁺ is substituted by Fe³⁺ in all clinkers; however, the substitution rate was influenced by the CaO source. The content of Al³⁺ substituted by Fe³⁺ was much lower in clinker S00 than the other three. The maximum substitution rate of Fe³⁺ for Al³⁺ reached 17.34 wt.% in clinker S80, and the maximum proportion of Fe₂O₃ in the ye’elimite phase was 6.89 wt.%, expressed as C₄A_2.71F_0.29$\overline{\mathrm{S}}$.

3.3.2. Chemical Composition of Belite

The same method was used to study the chemical composition of the belite phase as was used for ye’elemite. The elemental composition of the belite phase area was calculated and oxygen atoms were normalized to four for comparison. The chemical composition of the belite phase is presented in

Table 5. Mineralogical composition of belite phase of samples S00, S50, S80 and S100.

Sample	Mineralogical Composition	Fe2O3 in Belite Phase (wt.%)	SO3 in Belite Phase (wt.%)
S00	Ca1.97Al0.08 Fe0.04Si0.85S0.05O4	1.86	2.32
S50	Ca1.96Al0.01 Fe0.02Si0.81S0.12O4	0.93	5.56
S80	Ca1.92Al0.01Fe0.01Si0.91S0.11O4	0.47	5.58
S100	Ca1.87Al0.05Fe0.02Si0.84S0.14O4	0.93	4.65

. The result indicates that the amount of Fe₂O₃ solidified into belite was much lower than that in C₄A₃$\overline{\mathrm{S}}$; the maximum value reached 1.86 wt.% in clinker S00. The main reason may be that Fe³⁺ is easier to substitute than Al³⁺ in C₄A₃$\overline{\mathrm{S}}$ due to the same valence state and similar ionic radius rather than entering the lattice gap of C₂S.

SO₃ was also incorporated into C₂S, and the incorporated amount increased with the increasing content of CaSO₄ as the CaO sources in the raw mixture until 80%, and then decreased slightly when CaO was entirely sourced from CaSO₄. The maximum incorporation content of SO₃ reached 5.58 wt.% in clinker S80 but was 4.65 wt.% in clinker S100. It is worth noting that the crystal type of belite is closely related to the SO₃ content incorporated into the C₂S. Usually, belite exists as α’-C₂S at 830–1470 °C and transforms to β-C₂S at temperatures between 520 °C and 670 °C. Finally, it transforms to γ-C₂S below 520 °C. However, the crystal transformation of C₂S can slow down or even not occur under the influence of CaSO₄. In addition, C₂S obtained from Equations (5) and (6) exists as β-C₂S but as α’-C₂S from Equation (10) according to [15]. In clinker S00, the CaSO₄ is exhausted after reacting with calcium aluminate or gehlenite to form ye’elimite that no more CaSO₄ exist and the reactions of Equations (9) and (10) will not happen. Thus, the belite in clinker S00 exists as β-C₂S. It is obvious that the increasing CaSO₄ content induces more $4\mathrm{CaO}·2{\mathrm{SiO}}_2·{\mathrm{CaSO}}_4$ formed as a transition phase and finally decomposed as α’-C₂S [15]. Therefore, more α’-C₂S is formed at the expense of β-C₂S with the increase in CaSO₄ as a CaO source. However, the decomposition of CaSO₄ in S100 may be faster than in the others and CaSO₄ rarely exists in the clinker. The reactions of Equations (9) and (10) are hampered, and the proportion of α’-C₂S decreases with the increased CaSO₄ in the raw mixture.

3.3.3. Chemical Composition of Ferrite Phase

The ferrite phase is a solid solution that mainly contains C₂F, C₆AF₂, C₄AF, and C₆A₂F. It is generally believed that iron is distributed in the CSA clinker in the form of C₄AF but mainly exists as C₆AF₂ in the IR-CSA clinker. Iron distributed in the ferrite phase is presented in Figure 7 and the EPMA confirms its inhomogeneous distribution. Iron decreased from the center to the edge in all clinkers, which is consistent with the results of [14]. The ferrite phase is uniformly dispersed around the ye’elimite in clinker S00 and S50, but is concentrated in several areas as small clumps in S80 and S100. The content of the ferrite phase decreased with the increase in CaSO₄ as the CaO source. To obtain the chemical composition of the ferrite phase, oxygen atoms were normalized to 15 for comparison. The results (presented in

Table 6. Chemical composition of ferrite phase of samples S00, S50, S80, and S100.

Sample	Chemical Composition	Fe/Al	Abbreviations
S00	Ca6.31Al2.98 Fe2.82O15	0.95	C4.23AF0.95
S50	Ca6.32Al2.68Fe2.98O15	1.12	C4.71AF1.12
S80	Ca6.46Al1.92Fe3.77O15	1.96	C6.73AF1.96
S100	Ca6.47Al1.82Fe3.87O15	2.12	C7.11AF2.12

) indicate that the chemical composition in the ferrite phase is close to C₄AF in S00 and S50 but close to C₆AF₂ in clinker S80 and S100. Although there is less iron in the form of the ferrite phase because of the incorporation into ye’elimite in clinkers S80 and S100 than in clinkers S00 and S50, CA is more likely to combine with CaSO₄ Equation (3) than C₂F Equation (8) or C₆AF₂ Equation (11) with the increasing CaSO₄ content in the raw mixture. Thus, the ferrite phase in clinkers S80 and S100 has a chemical composition with lower aluminum but higher iron content than in clinkers S00 and S50.

3.4. Microstructural Characterization

Figure 8 shows the scanning electron micrograph of the section of the clinker sintered at 1300 ℃ for 45 min. All samples are magnified 5000 times. The hexagonal platy structure or quadrilateral columnar structure can be seen, and the particle size of C₄A_3-xF_x$\overline{S}$ in S00 is about 10 μm, decreasing drastically in clinker S10 and then increasing gradually in S100. The iron-rich molten phase can be seen clearly around the ye’elimite in S00, which indicates a large amount of liquid ferrite phase during the process. However, the molten phase could hardly be observed in any other clinker with CaSO₄ as the CaO source.

3.5. Compressive Strength of the IR-CSA Cement

The compressive strength of IR-CSA cement clinkers S00–S100 after 1 d, 3 d, and 28 d of curing is shown in Figure 9. With the increase in CaO derived from CaSO₄, the 1 d and 3 d compressive strength exhibited slight strength advantages in cements S50 and S80, but disadvantages in S100. The 28d compressive strength was higher for cement S50–S100 than for S00–S20. To be specific, the cement in S50 showed the best early and late compressive strength. The main reasons may be as follows: (ⅰ) The anhydrate in cements S50 and S80 consumes more water for ye’elimite hydration than gypsum according to Equation (14); thus, the actual water/cement ratios of cements S50 and S80 were lower than other samples, affecting their mechanical properties; (ⅱ) The Al/Fe ratio of the ferrite phase of S50 (1:1.12) was higher than S80 (1:1.96) and S100 (1:2.12), which resulted in higher hydration activity of the ferrite phase and early compressive strength of the S50 than the S80 and S100. Furthermore, the C₄A_3-xF_x$\overline{S}$ particle size of S50 was smaller than S80 and S100, which was beneficial for the hydration rate and early compressive strength; (ⅲ) The late compressive strength was related to the hydration of C₄A_3-xF_x$\overline{S}$ and C₂S. With the increase in CaSO₄ as a CaO source, the content of C₄A_3-xF_x$\overline{S}$ in clinkers S50–S100 was higher than in S00–S20. In addition, the C₂S hydration activity in S50 and S80 is higher than in the other cements because of the crystal type (the crystal type of α’-C₂S exhibits higher hydration activity than β-C₂S at room temperature).

4. Conclusions

The effect of CaO sources, from either CaCO₃ or CaSO₄, on phase formation and mineral composition of iron-rich clinker was investigated by varying their proportions in raw materials. Compared with CaCO₃, CaO derived from CaSO₄ was more conducive for Al₂O₃ and Fe₂O₃ to form C₄A_3-xF_x$\overline{\mathrm{S}}$ rather than the ferrite phase which resulted in an increased content of ye’elimite but a decreased ferrite phase. More c-C₄A_3-xF_x$\overline{\mathrm{S}}$ formed at the expense of o-C₄A_3-xF_x$\overline{\mathrm{S}}$ in the clinker because of the incorporation of Fe₂O₃. In addition, the belite phase was more inclined to exhibit as α’-C₂S instead of β-C₂S when taking CaSO₄ as a CaO source, which is conducive to the hydration activity of the clinker. Finally, the crystal grain sizes decreased dramatically and then increased gradually with the increase in the proportion of CaO derived from CaSO₄. In view of the influence of the mineral composition, crystal structure and crystal grain sizes, cement with optimal mechanical properties was obtained with the proportion of 1:1 of CaCO₃ and CaSO₄ as a CaO source.

Under the specified conditions, the substitution of Fe³⁺ for Al³⁺ reached a maximum value of 17.34 wt.%, and the maximum proportion of Fe₂O₃ in the ye’elimite phase was 6.89 wt.%, expressed as C₄A_2.71F_0.29$\overline{\mathrm{S}}$. However, the incorporation amount of Fe₂O₃ into C₂S was no more than 1.86 wt.% and showed an irregular change trend with CaO sources. The chemical formula of the ferrite phase was calculated and found to be similar to C₄AF when the amount of CaO derived from CaSO₄ was less than 50% but similar to C₆AF₂ when the amount of CaO derived from CaSO₄ was more than 80%, which also indicates that Al₂O₃ is more inclined to be involved in C₄A_3-xF_x$\overline{\mathrm{S}}$ with the increase in CaSO₄ as a CaO source.

The findings provide a possible method to optimize the mineral composition of IR-CSA clinker by adjusting the content of CaSO₄ as a CaO source and to produce high-performance IR-CSA cement at a low cost through cooperative utilization of waste gypsum and iron-bearing industrial solid wastes.