Characteristics of Solid Mineral Phase Transitions During Sulfuric Acid Production from Gaseous-Sulphur-Reduced Gypsum

Abstract

The acid co-production of cement is a prominent research focus for the large-scale, high-value utilization of phosphogypsum in the context of dual-carbon strategies. This paper builds on extensive research conducted by its authors on the co-production of sulphoaluminate cement clinker through acid production from gaseous-sulphur-reduced phosphogypsum. The solid mineral phase transformations occurring in the kiln during this process are systematically studied, and the effects of various calcination regimes (temperature, time, and atmosphere) on the evolution of clinker mineral phases are elucidated. This paper provides basic data support for the gas-sulfur-reduced phosphogypsum-acid cogeneration of sulfoaluminate cement clinker processes, and promotes the realization of the large-scale high-value utilization of phosphogypsum resources. The generation of the clinker mineral phase anhydrous calcium sulphoaluminate (C₄A₃S̅) begins at 1100 °C. Increasing the calcination temperature and extending the calcination time promote C₄A₃S̅ formation. However, when the calcination temperature exceeds 1350 °C, C₄A₃S̅ decomposes, leading to the formation of low-activity C₂AS. In a CO atmosphere, the main mineral phases in the clinker transform into C₂AS and 12CaO·7Al₂O₃, owing to the decomposition of CaSO₄, which inhibits C₄A₃S̅ formation. At calcination temperatures exceeding 1300 °C, a significant amount of C₂AS appears in the calcined material, and 12CaO·7Al₂O₃ begins to form.

1. Introduction

Phosphogypsum is an industrial by-product generated during the wet production of phosphoric acid in the phosphorus chemical industry. For every ton of phosphoric acid produced, 4 to 5 tons of phosphogypsum are generated [1,2,3,4]. Reducing phosphogypsum through sulphur reduction for the co-production of sulphoaluminate cement clinker is an effective resource-utilization method, addressing the issue of large phosphogypsum accumulation [5]. The CaO produced by the partial decomposition of phosphogypsum serves as a calcium source in the clinker formation reaction, while the remaining portion is directly involved in the formation of anhydrous calcium sulphoaluminate (C₄A₃S̅) in the cement clinker [6,7,8,9].

To effectively reduce the environmental impact of phosphogypsum, and to ensure the high-value utilization of phosphogypsum resources, the use of coke reduction for the preparation of sulphoaluminate cement clinker has been initially proposed, and considerable research has been conducted on this approach [10,11,12,13]. However, coke reduction has several drawbacks, including a high decomposition temperature, low SO₂ concentration, and high energy consumption [14,15,16]. To address these limitations, sulphur was chosen as the reducing agent and an in-depth study was conducted on the process of sulphur reduction for both acid production and the co-production of sulphoaluminate cement clinker [17,18]. The Institute of Powder Engineering at Xi’an University of Architecture and Technology conducted a pilot test using dilute-phase suspension roasting technology for the reduction of phosphogypsum to produce acid and cement [19]. Relevant test results have proven the applicability of the gas sulfur reduction of phosphogypsum to the acid cogeneration of sulfoaluminate cement clinker, which can substantially increase the phosphogypsum consumption, and at the same time has a very considerable economic value. During this group’s research on this process, it was discovered that the clinker composition contained a significant amount of the inert mineral calcium–aluminum yellow feldspar (C₂AS). Therefore, understanding the formation mechanism of C₂AS is crucial. To further investigate the formation of the clinker mineral phases C₄A₃S̅ and C₂AS, raw material configurations were simulated and fed into a rotary kiln to study the solid-phase formation mechanisms and transformation characteristics.

This paper investigates the influence of various calcination regimes (temperature, time, and atmosphere) on the formation of clinker mineral phases. It examines the transformation patterns between different clinker mineral phases and identifies the main factors affecting these transformations. The goal is to determine the optimal control range for each process parameter in the co-production of sulphoaluminate cement clinker and acid from gaseous-sulphur-reduced phosphogypsum.

2. Materials and Methods

2.1. Materials and Experimental Methods

Gas sulfur reduction of phosphogypsum to make acid co-production of sulfoaluminate cement clinker is an effective method to solve the problem of large amounts of phosphogypsum stockpiling. The process flow diagram for the co-production of sulphoaluminate cement and acid from gaseous-sulphur-reduced phosphogypsum is illustrated in Figure 1a, with the pilot test site illustrated in Figure 1b. The process consists of two phases: (1) a preheating and pre-decomposition phase and (2) a secondary decomposition phase in the kiln. The material, composed of phosphogypsum and other silica–aluminum raw materials mixed in a specific proportion, is fed into the suspension preheater through a feeder. After heat exchange with high-temperature flue gas in the preheater, the material enters the reduction furnace, where it undergoes a redox reaction with high-temperature sulphur vapor, before entering the kiln for the clinker sintering reaction.

Analytically pure CaS, CaSO₄·2H₂O, SiO₂, and Al₂O₃ were used as raw materials (concentration greater than 99.7%). The aim was to exclude the influence of impurities in phosphogypsum on the experimental results. The raw materials were mixed with 10% anhydrous ethanol and then pressed into Φ12 mm × 10 mm specimens. The pressed specimens were placed in a drying oven at 120 °C and dried to a constant weight. The main components of the prepared raw materials, as measured via X-ray diffraction (XRD), are illustrated in Figure 2.

The theoretical clinker composition was determined as 60% C₄A₃S̅ and 40% C₂S, and the required amounts of raw materials were calculated to meet this composition. Some of the CaSO₄ and CaS initially underwent a complex decomposition reaction to form CaO, while the remaining CaSO₄ participated in the formation of the C₄A₃S̅ mineral phase of the cement clinker, acting as externally mixed gypsum. It is assumed that the clinker firing reaction at high temperatures follows Equations (1) and (2):

2CaO + SiO₂ = 2CaO·SiO₂

(1)

3CaO + CaSO₄ + 3Al₂O₃ = 3CaO·3Al₂O₃·CaSO₄

(2)

The raw material ratios and theoretical clinker compositions are shown in

Table 1. Raw material ratio and theoretical clinker composition.

	CaS	CaSO4	SiO2	Al2O3	C4A3S̅	C2S
Molar mass	72	136	60	102	610	172
Raw material ratio/wt%	9.2	61.2	9.4	20.2	60	40

.

The calcination regimes used for the tests included the following: calcination temperatures of 1000 °C–1400 °C, calcination times of 15 min–60 min, and various atmospheres (1% to 3% O₂, N₂, or 1% to 3% CO). After pressing, the test blocks were placed in a tube furnace where different calcination regimes were applied to study the evolution of the main mineral phases C₄A₃S̅, C₂AS, and C₂S in the clinker. The cement clinker was then removed from the tube furnace and allowed to cool. Figure 3 illustrates the flow chart of this experiment.

2.2. Material Characterization

The sulphide content was determined using the iodometric method specified in GB/T 176-2008 Methods of Chemical Analysis of Cement to determine the CaS content [20]. The mineral phase composition of the clinker was analyzed with a Bruker D8 Advance XRD instrument, using a test angle range of 20° to 60° (2θ), a scanning step of 0.02°, and a scanning speed of 2°/min. The structural composition and functional groups of the sulphoaluminate cement substances were examined with a Thermo Scientific Nicolet iS5 Fourier-transform infrared (FTIR) spectrometer, USA. The test range was 400–1400 cm⁻¹. A Zeiss Sigma 300 scanning electron microscope was used to observe the microscopic morphology of the cement clinker samples. Quantitative analysis of the clinker mineral phases C₄A₃S̅, C₂S, and C₂AS was conducted using the K-value method [21,22]. The strongest peaks of each mineral phase were selected for quantification, with K-values determined using Jade 6.5 software, as follows: K_CaSO4 = 1.75, K_CaS = 6.05, K_CaO = 2.54, K_C4A3S̅ = 1.02, K_C2S = 0.73, and K_C2AS = 2.44.

3. Results and Discussion

3.1. Effect of Calcination Temperature on the Evolution of Main Mineral Phases in Clinker

Figure 4 illustrates the XRD patterns of the materials calcined at different temperatures under a N₂ atmosphere and the corresponding trends in the content of each substance with respect to th calcination temperature. As illustrated in Figure 3, under a nitrogen atmosphere, increasing the calcination temperature causes CaSO₄ and CaS to initially undergo a complex decomposition reaction, producing CaO. This CaO then reacts further with CaSO₄ and Al₂O₃ to form C₄A₃S̅ [8]. With increasing calcination temperature, the content of C₂S in the calcined material also increases. The chemical reaction equations involved are shown in Equations (3)–(5).

3CaSO₄ + CaS = 4CaO + 4SO₂    ∆G₂₉₈._13K = 830.01 KJ·mol⁻¹

(3)

3CaO + CaSO₄ + 3Al₂O₃ = 3CaO·3Al₂O₃·CaSO₄    ∆G₂₉₈._13K = −8074.03 KJ·mol⁻¹

(4)

2CaO + SiO₂ = β-2CaO·SiO₂    ∆G₂₉₈._13K = −135.45 KJ·mol⁻¹

(5)

The characteristic diffraction peaks of C₄A₃S̅ begin to appear in the spectra of the material calcined at 1100 °C after 30 min. The intensity of these diffraction peaks increases with rising calcination temperature, reaching its maximum at 1350 °C. This indicates that C₄A₃S̅ starts to form when the calcination temperature reaches 1100 °C. Between 1100 °C and 1350 °C, the intensity of the diffraction peaks for C₄A₃S̅ increases with higher temperatures, suggesting that a higher calcination temperature is favorable for the formation of the C₄A₃S̅ mineral phase in the clinker. The intensity of the diffraction peak for C₄A₃S̅ reaches its maximum at 1350 °C. When the calcination temperature exceeds 1350 °C, the characteristic diffraction peak for C₂AS appears in the calcined material, indicating that C₄A₃S̅ begins to decompose into the less active C₂AS. Consequently, the intensity of the diffraction peak for C₄A₃S̅ decreases with further increases in calcination temperature [22]. The transformation of the clinker mineral phases with respect to the calcination temperature under a N₂ atmosphere is presented in

Table 2. The transformation of clinker mineral phases with respect to the calcination temperature under N₂ atmosphere.

Temperature	Clinker Mineral Phase
1000 °C	CaSO4, CaS, SiO2, Al2O3
1100 °C	C4A3S̅, CaSO4, CaS, SiO2, Al2O3, CaO
1200 °C	C4A3S̅, CaSO4, SiO2, CaO, C2S
1300 °C	C4A3S̅, CaSO4, SiO2, CaO, C2S
1350 °C	C4A3S̅, CaSO4, SiO2, CaO, C2S
1380 °C	C4A3S̅, C2S, C2AS, CaO
1400 °C	C4A3S̅, C2S, C2AS, CaO

, from which the generation and transformation temperatures of each clinker mineral phase can be seen.

Figure 5, Figure 6 and Figure 7 illustrate the XRD patterns of the materials calcined at different temperatures under 1% O₂, 2% O₂, and 3% O₂ atmospheres, respectively. The figures also include corresponding trend plots showing the changes in the content of each substance with calcination temperature. The characteristic diffraction peaks of C₄A₃S̅ begin to appear in the spectra of the material calcined at 1100 °C after 30 min, and the contents of both C₄A₃S̅ and C₂S increase with higher calcination temperatures. When the calcination temperature exceeds 1350 °C, the characteristic diffraction peaks of C₂AS appear in the calcined material, while the intensity of the diffraction peaks for C₄A₃S̅ gradually decreases with increasing temperature. This indicates that excessively high calcination temperatures lead to the decomposition of C₄A₃S̅, the primary mineral phase of clinker, and the formation of the less active C₂AS. The presence of C₂AS in clinker can adversely affect its quality and reduce its hydration activity [23].

In summary, under the oxidizing atmosphere, C₄A₃S̅ first appears in the material calcined at 1100 °C, and its content increases with rising calcination temperature. Once the calcination temperature reaches 1350 °C, the clinker mineral phase generation reaction is essentially complete. The transformation of clinker mineral phases with respect to the calcination temperature under an O₂ atmosphere is presented in

Table 3. The transformation of clinker mineral phases with respect to the calcination temperature under O₂ atmosphere.

Temperature	Clinker Mineral Phase
1000 °C	CaSO4, CaS, SiO2, Al2O3
1100 °C	C4A3S̅, CaSO4, CaS, SiO2, Al2O3, CaO
1200 °C	C4A3S̅, CaSO4, SiO2, CaO, C2S
1300 °C	C4A3S̅, CaSO4, SiO2, CaO, C2S
1350 °C	C4A3S̅, C2S, C2AS
1380 °C	C4A3S̅, C2S, C2AS
1400 °C	C4A3S̅, C2S, C2AS

, from which the generation and transformation temperatures of each clinker mineral phase can be seen. From the table, the generation and transformation temperatures of each clinker mineral phase can be visualized. When the calcination temperature reaches 1350 °C, the generation reaction of the clinker mineral phase occurs completely, and only the main mineral phase remains in the clinker, as C₄A₃S̅, C₂S, and C₂AS.

Figure 8, Figure 9 and Figure 10 illustrate the XRD patterns of materials calcined at different temperatures under 1% CO, 2% CO, and 3% CO atmospheres, respectively, along with the corresponding trend plots showing the content of each substance as a function of temperature. Similarly to the trends under nitrogen and oxidizing atmospheres, the intensity of the diffraction peaks for C₄A₃S̅ and C₂S in the calcined material increases with rising calcination temperature. The characteristic diffraction peaks of C₄A₃S̅ begin to appear in the material calcined at 1100 °C after 30 min, and the intensity of these peaks increases with rising calcination temperature. However, under a reducing atmosphere, after the calcination temperature exceeds 1300 °C, a significant amount of C₂AS appears in the calcined material, indicating a shift in the main mineral phase composition to C₂AS. This suggests that a reducing atmosphere promotes the formation of the inert mineral phase, C₂AS. Additionally, the characteristic diffraction peak of 12CaO·7Al₂O₃ appears in the calcined material, and its intensity increases with the strengthening of the CO atmosphere. The intensity of the diffraction peaks for C₂S is also higher than those of the peaks for C₂S subjected to nitrogen atmosphere and oxygen-poor conditions. This difference is due to the reaction between CaO, generated from the decomposition of C₄A₃S̅, and free SiO₂, which produces C₂S. Consequently, the C₂S content in the calcined material increases with higher calcination temperatures.

Under a reducing atmosphere, the content of CaO in the calcined material gradually increases, while the content of C₄A₃S̅ decreases with rising CO concentration at temperatures between 1100 °C and 1300 °C. This is due to the fact that under a CO atmosphere, CaSO₄ reacts with CO to form CaO, resulting in a significant increase in the CO content of the calcined material, as shown in the reaction equation in Equation (6). When the calcination temperature exceeds 1300 °C, a significant amount of 12CaO·7Al₂O₃ is formed in the calcined material.

In summary, C₄A₃S̅ begins to form at temperatures above 1100 °C. When the calcination temperature exceeds 1300 °C, C₂AS starts to appear in the calcined material. This occurs because C₄A₃S̅ decomposes to produce CaO, which then reacts with SiO₂ and Al₂O₃ to form C₂AS, as shown in Equation (7). Under an oxidizing atmosphere, the main mineral phases in the clinker are C₄A₃S̅ and C₂S, with a small amount of the intermediate mineral phase, C₂AS. In contrast, under a reducing atmosphere, the main mineral phases are C₂AS and C₂S, along with a certain amount of 12CaO·7Al₂O₃. This is due to the decomposition of C₄A₃S̅ to form 12CaO·7Al₂O₃ in the reducing atmosphere. The transformation of clinker mineral phases with respect to the calcination temperature under a CO atmosphere is presented in

Table 4. The transformation of clinker mineral phases with respect to the calcination temperature under CO atmosphere.

Temperature	Clinker Mineral Phase
1000 °C	CaSO4, CaS, SiO2, Al2O3
1100 °C	C4A3S̅, CaSO4, CaS, SiO2, Al2O3, CaO
1200 °C	C4A3S̅, CaSO4, SiO2, CaO, C2S
1300 °C	C4A3S̅, CaSO4, SiO2, CaO, C2S, 12CaO·7Al2O3
1350 °C	C4A3S̅, C2S, C2AS, 12CaO·7Al2O3, CaO
1400 °C	C4A3S̅, C2S, C2AS, 12CaO·7Al2O3

, from which the generation and transformation temperatures of each clinker mineral phase can be seen.

CaSO₄ +CO = CaO + SO₂ + CO₂    ∆G₂₉₈._13K = 164.6 KJ·mol⁻¹

(6)

2CaO + SiO₂ + Al₂O₃ = 2CaO·Al₂O₃·SiO₂    ∆G₂₉₈._13K = −141.84 KJ·mol⁻¹

(7)

The infrared spectra of sulphoaluminate cement clinker prepared at different calcination temperatures are illustrated in Figure 11. The absorption band at 411 cm⁻¹ is attributed to the bending vibration of (AlO₄) in C₄A₃S̅, while the absorption band at 881 cm⁻¹ is due to the asymmetric stretching vibration of (AlO₄⁵⁻) in C₄A₃S̅, where the absorption band at 1102 cm⁻¹ is caused by the asymmetric telescoping vibration of (SO₄). The symmetric stretching vibration of the Al-O bond occurs in the range of 500–800 cm⁻¹, while the bending vibration of (SO₄) is found in the range of 600–700 cm⁻¹. Consequently, vibrational coupling between (AlO₄) and (SO₄) in this wavenumber interval results in three absorption peaks at 610 cm⁻¹, 641 cm⁻¹, and 691 cm⁻¹, as illustrated in the figure. The absorption band at 520 cm⁻¹ is attributed to the bending vibration of (SiO₄), and the absorption band at 1000 cm⁻¹ is due to the asymmetric stretching vibration of (SiO₄) [24]. In summary, the characteristic absorption bands of C₄A₃S̅ are found at 1102 cm⁻¹, 881 cm⁻¹, 610 cm⁻¹, 641 cm⁻¹, 691 cm⁻¹, and 411 cm⁻¹. The characteristic absorption bands of C₂S are observed at 520 cm⁻¹ and 1000 cm⁻¹. When the calcination temperature reaches 1200 °C, the characteristic absorption band of C₂S appears on the spectrum, indicating the formation of C₂S. The intensity of the C₂S absorption band increases progressively with higher calcination temperatures. At the same time, as the calcination temperature increases, the peak intensity of the characteristic absorption band of C₄A₃S̅ also gradually strengthens. However, after the calcination temperature exceeds 1300 °C, the peak intensity begins to decrease. This indicates that a moderate increase in temperature promotes the formation of C₄A₃S̅, while excessively high temperatures lead to its decomposition. These findings are consistent with the XRD spectra analysis results.

The micro-morphological analysis of cement clinker at different calcination temperatures is illustrated in Figure 12. As the calcination temperature increases, the C₄A₃S̅ grains gradually aggregate and grow, forming polygonal plate-like structures with well-defined contours and clear grain boundaries, with a grain size of approximately 1–2 microns. At 1300 °C, the C₄A₃S̅ grains are generally large and concentrated, whereas at 1400 °C, the C₄A₃S̅ grains become more dispersed and smaller in size. These observations align with the changes in the mineral phase of C₄A₃S̅ at various calcination temperatures, indicating that excessively high temperatures lead to the decomposition of C₄A₃S̅ [9].

3.2. Effect of Calcination Time on the Evolution of the Main Mineral Phases of Clinker

Figure 13 illustrates the XRD pattern of the material calcined at 1300 °C under a N₂ atmosphere for varying times. The figure indicates that at a constant calcination temperature, the intensity of the diffraction peaks for the clinker mineral phases C₄A₃S̅ and C₂S increases with longer calcination times. This suggests that extending the calcination time enhances the formation of these mineral phases in the cement clinker.

Figure 14 illustrates the XRD patterns of the material calcined at 1300 °C for various durations in 1% O₂, 2% O₂, and 3% O₂ atmospheres. The figure reveals that under oxygen-poor conditions, the diffraction peak intensities of the clinker mineral phases C₄A₃S̅ and C₂S increase with longer calcination times. In contrast, under oxygen-rich conditions, CaSO₄ reacts with CaS to form CaO, leading to a rise in CaSO₄ content and a decrease in CaO content. This reduction in available CaO limits the formation of C₄A₃S̅, resulting in some CaSO₄ remaining in the clinker and not participating in the clinker mineral phase formation.

Figure 15 illustrates the XRD patterns of materials calcined at 1300 °C for different durations in 1% CO, 2% CO, and 3% CO atmospheres, along with the corresponding trends in the content of each substance with respect to calcination time. Under 2–3% CO atmospheres, the intensity of the diffraction peaks for C₄A₃S̅ decreases with increasing calcination time, while significant amounts of 12CaO·7Al₂O₃ and CaO appear in the calcined material. At 1300 °C, CaSO₄ undergoes decomposition to produce CaO, resulting in a gradual decrease in the intensity of the characteristic diffraction peaks for CaSO₄, and an increase in the content of CaO in the material calcined at this temperature. The results indicate that under a reducing atmosphere, the decomposition of CaSO₄ inhibits the formation of the C₄A₃S̅ phase, leading to a transformation of the main mineral phases in the clinker into C₂AS and 12CaO·7Al₂O₃.

In summary, the diffraction peak intensities of the clinker mineral phases C₄A₃S̅ and C₂S in the calcined material increase with extended calcination time under both N₂ and O₂ atmospheres. This indicates that prolonging the calcination time is beneficial for the formation of cement clinker mineral phases. In contrast, under a CO atmosphere, the diffraction peak intensity of the clinker mineral phase, C₄A₃S̅, decreases, while the quantities of 12CaO·7Al₂O₃ and CaO increase significantly in the calcined material. This shift indicates that under reducing conditions, the primary compositions of the calcined material become C₂AS, 12CaO·7Al₂O₃, and CaO, demonstrating that C₄A₃S̅ is prone to decomposition in a reducing atmosphere.

The mineral phases of sulphoaluminate cement clinker were qualitatively analyzed through backscattered scanning electron microscopy to examine the micro-morphology of the samples. Energy spectroscopy in a point-scan format was employed to analyze the clinker and verify the elements present in the three different mineral phases. The elemental compositions at each point of the clinker are summarized in

Table 5. Atomic proportions of mineral phase elements in clinker (%).

	O	Al	Si	Ca	S
a	55.44	17.26	7.74	19.56	/
b	53.49	0.66	14.01	31.84	/
c	58.57	23.83	0.68	13.62	3.3

. The energy spectrum analyses for the three different mineral phases are illustrated in Figure 16. Spectrum (a) shows a Ca:Al:Si:O ratio of 2.53:2.22:1:7.16, which is similar to the ratio of the number of atoms in C₂AS (molecular formula Ca₂Al₂SiO₇). Spectrum (b) shows a Ca:Si:O ratio of 2.27:1:3.82, which resembles the atomic ratio in C₂S (molecular formula Ca₂SiO₄). Spectrum (c) shows a Ca:Al:O:S ratio of 4.13:7.22:17.75:1, which aligns with the atomic ratio in C₄A₃S̅ (molecular formula Ca₄Al₆(SO₄)O₁₂).

3.3. Influence of O₂ Content in the Oxidizing Atmosphere on the Evolution of Main Mineral Phases in Clinker

Figure 17 illustrates the XRD patterns of the calcined materials obtained with different O₂ contents with a calcination time of 30 min and at calcination temperatures ranging from 1000 °C to 1400 °C. The figure shows that the intensity of the characteristic diffraction peaks of C₄A₃S̅ initially increases and then decreases with rising oxygen content under an oxidizing atmosphere. Moreover, the reaction between CaS and O₂ under an oxidizing atmosphere generates additional CaSO₄. This shifts the reaction equilibrium of 3CaO + CaSO₄ + 3Al₂O₃ = 3CaO·3Al₂O₃·CaSO₄ positively, leading to increased formation of C₄A₃S̅. However, as the reaction progresses, the decreasing content of CaO in the calcined material shifts the equilibrium negatively, causing the content of C₄A₃S̅ to gradually decrease. The decomposition of C₄A₃S̅ into C₂AS occurs at excessively high calcination temperatures, particularly after 1350 °C. At 1350 °C with 1% O₂, the intensity of the characteristic diffraction peaks of C₂AS is the lowest, indicating minimal formation of C₂AS under these conditions. The reason for this is that the amounts of CaSO₄ and CaO in the calcined material significantly influence the formation of the clinker mineral phases C₄A₃S̅ and C₂AS. Insufficient CaSO₄ or CaO content in the calcined material leads to inadequate formation of C₄A₃S̅ during the reaction. Consequently, the transitional mineral phase C₂AS cannot effectively react with CaO to form C₄A₃S̅, resulting in its persistence in the clinker and a reduction in clinker quality.

3.4. Effect of CO Content in a Reducing Atmosphere on the Evolution of the Main Mineral Phases in Clinker

Figure 18 compares the XRD patterns of the calcined materials with varying CO contents, obtained with a calcination time of 30 min and at temperatures of 1000 °C–1400 °C. The characteristic diffraction peaks of C₄A₃S̅ decrease as the CO content increases in the reducing atmosphere. This trend is due to the inhibiting effect of a reducing atmosphere on the reaction between CaO and CaSO₄ to form C₄A₃S̅. Additionally, C₂AS appears in the material calcined at 1300 °C under a 3% CO atmosphere, indicating that a reducing atmosphere favors the formation of the inert mineral phase, C₂AS. After the calcination temperature exceeds 1300 °C, the relative intensity of the characteristic diffraction peaks of CaSO₄ in the clinker significantly decreases, while the intensity of the diffraction peaks of CaO notably increases, because CaSO₄ reacts with CO to form CaO, SO₂, and CO₂. Furthermore, when the calcination temperature reaches 1300 °C, CaSO₄ decomposes to form CaO. This decomposition reaction inhibits the formation of the clinker mineral phase, C₄A₃S̅. At a calcination temperature of 1400 °C, as the CO content increases, the intensity of the characteristic diffraction peaks of C₂AS and 12CaO·7Al₂O₃ in the calcined material gradually increases, while the intensity of the characteristic diffraction peaks of C₄A₃S̅ decreases. Consequently, under a higher reducing atmosphere, the main mineral phases in the clinker are transformed into C₂AS and 12CaO·7Al₂O₃.

In summary, after the calcination temperature exceeds 1300 °C under a reducing atmosphere, a significant amount of C₂AS and 12CaO·7Al₂O₃ is formed in the clinker. This indicates that the formation of the clinker mineral phase, C₄A₃S̅, is inhibited by the decomposition of CaSO₄ in the reducing atmosphere, leading to a transformation of the main mineral phases in the clinker to C₂AS and 12CaO·7Al₂O₃.

4. Conclusions

This paper investigated the mechanism of mineral phase transformation in clinker during the cogeneration of sulphoaluminate cement clinker through the gas-phase sulphur reduction of phosphogypsum for acid production. The effects of different calcination regimes on the generation of mineral phases in cement clinker were designed and studied. The following conclusions can be drawn:

The formation of the main mineral phase, C₄A₃S̅, in clinker begins at 1100 °C. An increase in the calcination temperature or an extension of the calcination time enhances the formation of C₄A₃S̅. However, after the temperature reaches 1350 °C, C₂AS begins to appear in the clinker, indicating that C₄A₃S̅ decomposes at higher temperatures to form C₂AS.

Under N₂ and oxidizing atmospheres, the primary mineral phase in clinker is C₄A₃S̅, with some amounts of C₂S and C₂AS. The content of both C₄A₃S̅ and C₂S in the clinker increases with the extension of the calcination time.

In a CO atmosphere, the main mineral phases in clinker are C₂AS and 12CaO·7Al₂O₃. The decomposition of CaSO₄ inhibits the formation of the clinker mineral phase, C₄A₃S̅, resulting in a significant increase in C₂AS in the calcined material once the calcination temperature exceeds 1300 °C, and 12CaO·7Al₂O₃ begins to form. As the calcination time is extended, the content of C₄A₃S̅ in the clinker decreases, while the content of C₂AS and 12CaO·7Al₂O₃ increases.