Effect of Iron Phase on the Formation of Barium Calcium Sulphoaluminate Clinker

Abstract

In this paper, analytically pure chemical reagents, as raw materials, were fired in barium calcium sulfoaluminate cement clinker. The effect of the iron phase on the calcination of barium calcium sulfoaluminate cement clinker was studied. The content of f-CaO in the sample was determined using the ethylene glycol method. The raw meal’s heat absorption and heat release were tested with an integrated thermal analyzer TG-DSC, and XRD and SEM measurements were used to characterize the composition and microstructure of the clinker. The results showed that the iron phase could lower the decomposition temperature of the calcium carbonate. When the calcination temperature increased, the lattice spacing of the mineral changed. The XRD pattern showed that a substitution reaction had occurred. Ba²⁺ replaced Ca²⁺ and formed a sulfoaluminate barium calcium mineral. The SEM images showed hexagonal plates or dodecahedral barium calcium sulfoaluminate minerals.

1. Introduction

As one of the most significant artificial materials in the world, concrete is widely used in construction [1]. As an essential component of concrete, cement is in increasing demand. Currently, the most commonly used cement on the market is Portland cement, but the production of Portland cement emits a lot of CO₂ [2,3,4,5,6], representing about 5% of anthropogenic carbon emissions [7,8]. Significant emissions of CO₂ cause global warming, sea level rises, and melting glaciers [9,10,11,12,13,14,15]. It is imperative to reduce CO₂ emissions and use low-carbon ecological concrete. In general, there are two main ways to reduce carbon emissions. One is to develop low-carbon materials from raw materials, and the other is to improve the durability of buildings. For example, monitoring concrete cracks and identifying damage in advance can effectively extend the lifespan of facilities [16,17,18]. As a low-carbon ecological cement, sulfoaluminate cement has fast setting and hardening, high early strength, and excellent durability. It is widely used in emergency repairs and construction, winter construction, and marine engineering [19,20,21,22,23,24,25]. Sulfoaluminate cement has a lower firing temperature than Portland cement and emits less CO₂. However, compared with Portland cement, the fundamental reasons for the limited application of SAC cement are the high cost of raw materials, the high price of bauxite, and the low output. Therefore, there is an urgent need to reduce the production cost of SAC.

With the acceleration of industrialization, a large amount of industrial waste is produced yearly, including red mud, garbage fly ash, and barium slag. It is urgent to dispose of these industrial wastes [26,27]. Long-term stacking of barium slag not only occupies a large amount of land but also pollutes the environment, especially at high temperatures; it can spontaneously ignite and release poisonous gases, which infiltrate water and produce sulfide. If it is not collected, this will flow into surface water and groundwater, polluting water bodies; at the same time, the barium sulfide and acid-soluble barium in the waste slag have a direct poisonous effect on the soil. Currently, the cumulative stockpiling of barium slag in China has exceeded 10 million tons, which demonstrates the necessity of the comprehensive utilization of barium slag. To solve these two problems, researchers have carried out a series of works on Ba²⁺ [28,29,30,31,32,33], to produce SAC clinker containing barium. Barium calcium sulfoaluminate mineral was thus produced [31].

Barium calcium sulfoaluminate minerals are produced by replacing the Ca²⁺ of calcium sulfoaluminate minerals with Ba²⁺, to produce barium calcium sulfoaluminate cement, thereby improving the cement performance [34]. Barium calcium sulfoaluminate cement has excellent mechanical properties and good durability. It is widely used in special projects, such as emergencies and construction, repair and reinforcement, marine engineering, and military engineering. Cheng Xin’s team have carried out a lot of research on barium calcium sulfoaluminate cement and used barium calcium sulfoaluminate cement as a repair material on the piers of the Yanwei Expressway that had become eroded by seawater, repaired and strengthened them, and found that barium calcium sulfoaluminate cement had a good repair performance and strong resistance to seawater erosion [34]. The formation kinetics of barium calcium sulfoaluminate minerals were systematically studied, and it was found that the diffusion process and interfacial chemical reaction were the main influencing factors. In addition, as an essential solvent mineral, the iron phase plays a vital role in the mineral form of Portland cement clinker. Currently, most research on the effect of iron on cement calcination and performance focuses on Portland cement and sulfoaluminate cement. At the same time, there has been little research on barium calcium sulfoaluminate cement [35]. Therefore, it is essential to study the effect of the iron phase on the calcination of barium calcium sulfoaluminate cement and its performance, which is beneficial for optimizing the production process of this type of cement and to reduce production costs. In this paper, analytically pure chemical reagents, as raw materials, were fired into barium calcium sulfoaluminate cement clinker. The effect of the iron phase on the calcination of barium calcium sulfoaluminate cement clinker was studied.

2. Raw Materials and Experimental Methods

2.1. Raw Materials

The experimental raw materials included calcium sulfate dihydrate, calcium carbonate, aluminum trioxide, iron trioxide, silicon dioxide, and barium carbonate (CaSO₄·2H₂O, CaCO₃, Al₂O₃, Fe₂O₃, SiO₂, BaCO₃); all raw materials were analytically pure chemical reagents produced by Tianjin Damão Chemical Reagent Co., Ltd. The mass of the designed cement clinker was 350 g, and the clinker’s mineral composition was 60% C_2.75B_1.25A₃$\overline{\mathrm{S}}$, 35% C₂S, and 5% iron phase. Four proportions of the iron phase were designed. The experimental proportions are shown in

Table 1. Chemical composition of samples.

Sample NO.	Iron Phase	Chemical Composition w/%
		CaO	Al2O3	Fe2O3	SiO2	BaO	SO3
F1	C2F	37.50	25.08	2.94	12.21	15.71	6.56
F2	C6AF2	37.65	25.75	2.11	12.21	15.71	6.56
F3	C4AF	37.74	26.13	1.64	12.21	15.71	6.56
F4	C6A2F	37.84	26.53	1.14	12.21	15.71	6.56

. The process was as follows: Mix the raw meal with a certain amount of water, pour it into a ball mill tank, stir with a planetary ball mill, and then dry the homogenized raw meal paste. Add 8% water to the raw meal, press it into a flat cylindrical sample with a diameter of 60 mm × 10 mm, and then place it in an oven at 100 °C for 24 h. The dried pieces were placed in a high-temperature furnace for calcination, and the calcination conditions were 5 °C/min to raise the temperature to the set temperature and then holding for 30 min. The samples were taken out and rapidly quenched to room temperature. Then the clinker at different firing temperatures was tested, to study iron’s effect on forming cement clinker.

Using a water/cement ratio of 0.3, pour the clinker and water into the stirring pot, stir at low speed for 2 min, stir at high speed for 2 min, then pour the slurry into a 20 mm × 20 mm × 20 mm test mold, place this in a standard curing box after vibrating and compacting (20 °C, 95% relative humidity), demold after curing for 24 h, put the sample into 20 °C water for curing to a specified time, and use a WHY-300 electronic pressure tester (Hualong, China) to measure its compressive strength.

2.2. Experimental Methods

According to the national standard GB/T 176-2017, the ethylene glycol method was used to determine the content of f-CaO in each sample. The clinker minerals were tested with XRD, using a Bruker D8 Advance X-ray diffractometer from Germany. The X-ray light source was a Cu target (CuKα12 radiation, λ₁ = 0.15406 nm, λ₂ = 0.15444 nm), the working voltage was 40 KV, and the operating current was 40 mA. The test range was 5–80°, the step size was 0.02°, and the XRD pattern was qualitatively analyzed using EVA software, to determine the mineral composition. The weight loss of the raw meal was tested with TG-DSC, using a Mettler Toledo TGA/DSC1 comprehensive thermal analyzer from Switzerland. The test temperature was 0–1400 °C, the heating rate was 10 °C/min, and the protective gas was nitrogen. The morphology of the clinker was observed with an FEI Quanta 450 scanning electron microscope, USA. Before the test, the sample was broken into small pieces and subjected to termination hydration treatment, and a small piece sample was sprayed with gold.

3. Results

3.1. Analysis of the Clinker Calcination Process

3.1.1. TG Analysis

Figure 1 and Figure 2 show the thermal analysis curves of four kinds of mixed raw meals. It can be seen from the figure that the weight loss temperature of calcium carbonate was 700 °C–800 °C. The iron phase could reduce the decomposition temperature of calcium carbonate. As the ratio of Al to Fe increased, the decomposition temperature of calcium carbonate decreased, in the proportions of C₆A₂F, C₄AF, C₆A_F2, and C₂F. However, the decomposition temperature did not change significantly.

3.1.2. f-CaO Analysis

According to GB/T, the 176-2017 Cement Chemical Analysis Method was used to determine the free calcium oxide content, and the free calcium oxide content in the samples at different temperatures is shown in

Table 2. f-CaO at different sintering temperatures.

Temperature (°C)	F1	F2	F3	F4
700	29.1	29.6	28.6	28.6
800	32.2	28.6	31.8	34.7
900	25.0	26.1	25.6	25.6
1000	16.9	13.8	15.2	16.6
1100	4.1	5.0	4.5	5.5
1200	0.1	0.1	0.1	0.1
1300	0.0	0.0	0.0	0.0
1350	0.0	0.0	0.0	0.0

and Figure 3.

It can be seen from Figure 3 that with an increase in temperature, the content of f-CaO first increased and then decreased. It can also be seen that calcium carbonate began to decompose at 700 °C, and when the temperature was increased to 800 °C, the amount decomposition of calcium carbonate was the highest. A similar trend can be seen in the thermal analysis curves. At this time, the content of f-CaO in the sample was the largest, and f-CaO showed a decrease after 800 °C, which indicates that calcium oxide had begun to react and form minerals. When the temperature was increased to 1200 °C, the content of free calcium oxide was almost zero, meaning that the calcium oxide was almost wholly involved in the reaction. At 1200 °C–1350 °C, the free calcium oxide content remained unchanged, which is an important stage for forming and growing the main minerals.

3.1.3. XRD Analysis

From Figure 4, we can see that the peak intensity of f-CaO was the highest at 800 °C, indicating that a decomposition reaction had occurred; that is, the decomposition of calcium carbonate produced CaO and CO₂, and the decomposition of calcium carbonate was the largest. The peak of f-CaO gradually decreased with the increase in temperature. When the temperature was increased to 1200 °C, the diffraction peak of f-CaO disappeared completely, indicating that f-CaO had completely participated in the reaction. When the temperature was increased to 900 °C, C₂AS was formed. At this time, the intensity of the diffraction peak was weak. When the temperature was increased to 1100 °C, the sharp diffraction peak of C₂AS could be seen. At this time, the diffraction of C₂S and C₄A₃$\overline{\mathrm{S}}$ could be seen. These peaks indicated the formation of these two minerals. When the temperature was increased to 1200 °C, the C₂AS began to decompose; and when the temperature was increased to 1300 °C, the C₂AS was wholly decomposed. Therefore, we can conclude that the initial stage of mineral formation temperature was 1100 °C, and the main formation stage was 1200–1300 °C. As the temperature continued to increase, the minerals hardly changed.

It can be seen from Figure 5 that the primary diffraction peak d value of barium calcium sulfoaluminate increased with the increase in temperature. At 1100 °C, the d value was 3.7667; at 1200 °C, the d value was 3.7799; at 1300 °C, the d value was 3.7855; and the d value at 1350 °C was 3.7925 because Ba²⁺ replaced Ca²⁺, and the radius of Ba²⁺ was more significant than that of Ca²⁺, resulting in a change of mineral lattice spacing. Therefore, as the temperature increased, conditions became more favorable for the substitution reaction to occur and barium calcium sulfoaluminate mineral formation.

3.2. Analysis of the Cement Clinker

3.2.1. Compressive Strength

It can be seen from

Table 3. The mechanical properties of different samples.

Number	Iron Phase Composition	Compressive Strength/MPa
		1 d	3 d	28 d
F1	C2F	64.0	78.4	107.9
F2	C6AF2	69.9	77.5	108.7
F3	C4AF	73.2	97.9	106.9
F4	C6A2F	72.1	74.4	111.9

that after 1 d of hydration, the compressive strength of each group was above 60 MPa, indicating that this series of cement was a fast-hardening and early-strength cement. Comparing the compressive strength of 1 d and 3 d, C₄AF had the most apparent effect on the compressive strength, which indicates that C₄AF is a mineral with good gelation. The early hydration rate was fast, which could quickly contribute to the mechanical properties. Compared with the compressive strength of 28 d, the difference between the four groups was not apparent.

3.2.2. XRD Analysis of Clinker

It can be seen from the XRD pattern that the main peak types were the peaks of C_2.75B_1.25A₃$\overline{\mathrm{S}}$ and β-C₂S. The peaks of these two minerals were sharp, and the impure peaks were relatively few, indicating that the sintered clinker had a high mineral content. From Figure 6a, it can be seen that there was no significant difference in the diffraction patterns of F1, F2, F3, and F4. To more clearly analyze the influence of the different iron phase compositions on the clinker minerals, we set the main peak shape of C_2.75B_1.25A₃$\overline{\mathrm{S}}$ zoom locally. As seen from Figure 6b, the C_2.75B_1.25A₃$\overline{\mathrm{S}}$ diffraction peak d values of samples F4 to F1 showed an increasing trend, because a substitution reaction occurred; that is, Ba²⁺ replaced Ca²⁺, which caused the lattice spacing to change. It can be concluded that with the increase of the Fe/Al molar ratio, the lattice spacing of the crystal increased gradually, indicating that increasing the amount of iron phase was conducive to the substitution reaction to generate barium calcium sulfoaluminate minerals.

3.2.3. SEM Observation

Figure 7 shows an SEM image of the clinker minerals at a temperature of 1350 °C. It can be seen from Figure 7a that there are hexagonal plate-shaped and dodecahedral particles in the F1 clinker; the particles are closely connected, and the crystal shape is regular. From the energy spectrum analysis at point 2 in Figure 7b, it can be seen that the mineral is a calcium sulfoaluminate mineral with a crystal size of 5 µm, as shown in Figure 7d. At the same time, it can be seen that there are many granular particles. The energy spectrum analysis at point 1 is β-C₂S, and the crystal size is 10 µm. It can be seen from Figure 7 that a liquid phase is encapsulated between the barium calcium sulfoaluminate mineral and β-C₂S. It can be seen from Figure 8 that the barium calcium sulfoaluminate mineral crystals of the F3 clinker are closely and regularly arranged, with regular shapes. Comparing the electron microscope photos of F1 clinker, we can see that the mineral crystallinity of F3 clinker is better, the grain boundary is clearer, and the particle distribution is more uniform; this can explain why the compressive strength of the F3 clinker was higher than the other groups.

Figure 9 displays surface scan photos of the F1 and F3 cement clinkers. A total of seven elements were tested, namely O, Si, Ca, Ba, Al, S, and Fe. It can be seen from the figure that the Si element was mainly distributed in the ovoid mineral region, which was due to the Si element participating in the reaction, to generate C₂S. The Ca element is uniformly dispersed in the whole area. The Ba element is mainly distributed on the hexagonal plate-shaped minerals, with Ba²⁺ substituting Ca²⁺ to generate the barium calcium sulfoaluminate mineral. It was also found that the Ba distribution in the C₂S region was due to the dissolution of Ba²⁺ into the minerals. The Ba²⁺ ion acts as an activated lattice, which can enhance the activity of C₂S, thereby improving the strength of the cement, which is also why the strength of this series of cement did not reduce. Most of the S element was distributed in the barium calcium sulfoaluminate mineral region, forming the mineral C_2.75B_1.25A₃$\overline{\mathrm{S}}$. Fe element was distributed in the whole area, with part of it distributed between the grains of C_2.75B_1.25A₃$\overline{\mathrm{S}}$ and C₂S, and a certain amount being dissolved in the minerals.

3.3. Influence of Calcination Temperature on the Clinker

3.3.1. Compressive Strength

According to previous research, we set the calcination temperature to 1300 °C, to study the effect of calcination temperature on the clinker. It can be seen that the compressive strength of each group increased with the curing time, as shown in Figure 10. For 1 d strength, C₆AF₂ had the lowest compressive strength compared to the other groups. Compared with the 1 d compressive strength, the 3 d compressive strength of each group was significantly improved, and the C₄AF group had the most improvement. At the same time, it can be seen that, with the increase of the Al/Fe ratio, the compressive strength of the specimen was higher. At a temperature of 1300 °C, the compressive strength of the specimen was higher than that at the temperature of 1350 °C, which was because, with the temperature increase, the minerals were not well developed, and the degree of crystallization was low.

3.3.2. XRD Analysis

Figure 11 shows the XRD patterns of the effect of 5% C₄AF on the clinker when the firing temperature was 1300 °C and 1350 °C. It can be seen from the XRD pattern that the prominent diffraction peaks were C_2.75B_1.25A₃$\overline{\mathrm{S}}$ and β-C₂S. It can be seen from the figure that the intensity of the C_2.75B_1.25A₃$\overline{\mathrm{S}}$ and β-C₂S diffraction peaks of the 1300 °C sample was higher than that of the 1350 °C sample. It was taller and sharper, which shows that at 1300 °C, the minerals of C_2.75B_1.25A₃$\overline{\mathrm{S}}$ and β-C₂S were well developed and had a high degree of crystallinity, which corresponded to the changing law of compressive strength. In addition, too high a temperature is not conducive to sintering minerals. Therefore, it is necessary to control the firing temperature when firing barium sulfoaluminate cement clinker.

3.3.3. SEM-EDS Analysis

The compressive strength and XRD pattern changes showed that the clinker minerals were well formed when the calcination temperature was 1300 °C. Therefore, we fixed the calcination temperature at 1300 °C and performed an SEM analysis on the samples F1 and F3. It can be seen from Figure 12 that the clinker minerals of the F1 sample were aggregated and stacked together. The crystals have different sizes and irregular shapes, indicating that the crystals were poorly developed and the sample structure was loose. To further determine the types of minerals, energy spectrum analysis was used to determine that the minerals were mainly calcium barium sulfoaluminate and β-C₂S, and that there was no apparent boundary between the two minerals. It can be seen from Figure 13 that the clinker mineral crystals of the F3 sample were regular in shape, uniform in size, and well-developed. The crystals were tightly bonded, the sample structure was dense, and there was a clear boundary between the calcium barium sulfoaluminate and β-C₂S, which indicates that the crystal of the F3 sample was well developed and could form a corresponding excellent relationship with the compressive strength.

4. Conclusions

In this paper, calcined barium calcium sulfoaluminate cement clinker was sintered with analytically pure chemical reagents, and the effect of iron on the calcination of the barium calcium sulfoaluminate cement clinker was studied. The main conclusions are as follows:

(1)

The iron phase reduces the decomposition temperature of calcium carbonate. As the ratio of Fe to Al in the iron phase decreases, the decomposition temperature of calcium carbonate increases, but the decomposition temperature range does not change significantly. With an increase in temperature, the content of f-CaO in the sample first increased and then decreased. At 800 °C, the content of f-CaO reached a maximum of 34.7%. When the temperature reached 1200 °C, the free calcium in each group was almost zero; the CaO produced by the decomposition reaction was almost entirely involved in the reaction.

(2)

With the increase in temperature, it was favorable for Ba²⁺ to replace Ca²⁺, and the substitution reaction occurred, to generate barium calcium sulfoaluminate minerals. From the SEM-EDS analysis, it can be seen that the egg-shaped clinker minerals are β-C₂S and the hexagonal flakes are C_2.75B_1.25A₃$\overline{\mathrm{S}}$ minerals. The Ba element was mainly distributed in the area of barium and calcium sulfoaluminate and was also partially dissolved in C₂S; The Fe element was distributed between C_2.75B_1.25A₃$\overline{\mathrm{S}}$ and C₂S grains in the form of an iron phase solid solution, which acts as a solvent, and its content increased with the increase in the content of iron phase.

Since the experiment was conducted under laboratory conditions, further research is needed on the influence of the iron phase as a solvent mineral in calculating clinker in rotary kiln operations, in actual production. The specific process parameters obtained under laboratory conditions need to be tested in practice; and further adjustments and optimizations are required for the production process.