J. Cent. South Univ. (2020) 27: 3269-3277

DOI: https://doi.org/10.1007/s11771-020-4545-1

Formation kinetics and transition mechanism of CaO·SiO₂ in low-calcium system during high-temperature sintering

PAN Xiao-lin(潘晓林)^{1, 2}, CUI Wei-xue(崔维学)², ZHANG Can(张灿)², YU Hai-yan(于海燕)^{1, 2}

1. Key Laboratory for Ecological Metallurgy of Multimetallic Mineral, Northeastern University,Shenyang 110819, China;

2. School of Metallurgy, Northeastern University, Shenyang 110819, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: The crystal structure, formation kinetics and micro-morphology of CaO·SiO₂ during high-temperature sintering process were studied in low-calcium system by XRD, FT-IR, Raman and SEM-EDS methods. When the molar ratio of CaCO₃ to SiO₂ is 1.0, β-2CaO·SiO₂ forms firstly during the heating process, and then CaO·SiO₂ is generated by the transformation reaction of pre-formed 2CaO·SiO₂ with SiO₂. 3CaO·SiO₂ and 3CaO·2SiO₂ do not form either in the heating or sintering process. Rising the sintering temperature and prolonging the holding time promote the phase transition of 2CaO·SiO₂ to CaO·SiO₂, resulting in the sintered products a small blue shift and broadening in Raman spectra. The content of CS can reach 97.4% when sintered at 1400 °C for 1 h. The formation kinetics of CaO·SiO₂ follows the second-order chemical reaction model, and the corresponding apparent activation energy and pre-exponential factor are 505.82 kJ/mol and 2.16×10¹⁴ s^-1 respectively.

Key words: calcium silicate compounds; formation kinetics; crystal structure; microstructure; sinter process

Cite this article as: PAN Xiao-lin, CUI Wei-xue, ZHANG Can, YU Hai-yan. Formation kinetics and transition mechanism of CaO·SiO₂ in low-calcium system during high-temperature sintering [J]. Journal of Central South University, 2020, 27(11): 3269-3277. DOI: https://doi.org/10.1007/s11771-020-4545-1.

1 Introduction

The calcium metasilicate compound of CaO·SiO₂ (CS) is widely used in ceramics, cement, paint, coating, steel, rubber and plastics industries, due to its high thermal and chemical stability, good dielectric and mechanical properties as well as high whiteness [1]. In the ceramics industry, the addition of CS can reduce the sintering temperature and time, and enhance the mechanical properties [2]. For biomaterials, CS can be used as a potential matrix of bone tissue due to its good bioactivity in body fluids and the formation of bone apatite on its surface [3, 4]. For composite materials, it is very suitable to add as reinforcing filler of polymer- based composite materials due to its good stability, excellent mechanical and electrical properties [5, 6].

CS has two crystalline forms [7, 8]: α-CS is a high-temperature calcium metasilicate (wollastonite) with a ternary ring structure, while β-CS is a low-temperature calcium metasilicate with a chain structure. The transformation of β-CS to α-CS occurs at 1125 °C. The synthesis of CS mainly includes the hydro-chemical methods and solid- state reaction methods [9, 10]. The hydro-chemical methods are comprised of hydrothermal method, chemical precipitation and sol-gel method, which have the advantages of good chemical homogenization and small particles. However, they also have disadvantages: the precursors required by hydrothermal method are very expensive, which is adverse to industrial production; the particle size of samples synthesized by the chemical precipitation is difficult to control, and the addition of precipitant may result in agglomeration or inhomogeneous composition; the sol-gel process is strictly required. Therefore, the solid-state reaction is the main synthetic method in the manual preparation of CS at present, because of its wide application, simple process and easy industrialization.

Some scholars have studied the synthesis and stability of CS. ARLYUK [11] reported that 2CaO·SiO₂ (C₂S) is usually generated first in CaO-SiO₂ system, and it is difficult to form 3CaO·SiO₂ (C₃S) and 3CaO·2SiO₂ (C₃S₂). However, ABOUZEID et al [12] found that C₂S forms first at 1200 °C, reaches the maximum reaction degree after 0.5 h, and then C₃S₂ and CS are generated. LIU et al [13] reported that the stability of CS is higher than that of β-C₂S both in caustic solution and soda solution. CS is a promising target phase to replace β-C₂S in the soda-lime sinter process to produce alumina from low-grade aluminum resources, which can significantly reduce the lime addition and the corresponding residue amount. However, the formation mechanism of CS in low-calcium system (the molar ratio of CaO to SiO₂ is around 1.0) during the sintering process is still unclear at present. Meanwhile, the crystal transition and formation kinetics of CS have not been reported so far. In this study, the formation kinetics and transition mechanism of CS during high- temperature heating and sintering processes were systematically studied using CaCO₃ and SiO₂ as raw materials.

2 Experimental

The analytical reagents of CaCO₃ and SiO₂ with the molar ratio (C/S) of 1:1 were mixed in a polyethylene tank for 2 h, and pressed into a cylinder with a diameter of 2 cm. The samples were sintered in a MoSi₂ furnace (KSL-1700X). Some samples were heated to the preset temperatures without any duration, and the other samples were sintered at different temperatures for 1 h. All the samples were heated at a uniform rate of 5 °C/min, and cooled in the air after sintering. The sintered samples were treated by two ways for tests. The samples for SEM observation were ground with sandpaper step by step and polished carefully without breaking. The other samples were crushed and milled to below 74 mm in particle size for tests.

The phase compositions and crystal structure of sintered samples were analyzed using a Philips X’Pert PW3050/60 X-ray diffractometer with CuKα1 radiation. The Fourier transform infrared spectroscopy (FTIR, SHIMADZU IRPrestige-21) was performed to study the absorption spectra, and KBr was used as the matrix material. The Raman spectra of samples were determined by a laser focusing Raman analyzer (HR800) with the wavelength of 488 nm and the laser power of 0.8 W. The microstructure of sintered samples was obtained by a scanning electron microscope (SEM, SHIMADZU SSX-550) with an energy-dispersive X-ray spectroscope (DX-4).

The semi-quantitative analysis was carried out to calculate the phase compositions of sintered samples according to the XRD spectra [14]. MgO was added to the samples after sintering by 10 wt% as internal standard. The calculation formula is as follows:

              (1)

where I_i, RIR_i and ω_i represent the characteristic peak intensity, and the value of reference intensity ratio and mass percentage of phase i, respectively; I_MgO, RIR_MgO and ω_MgO represent the characteristic peak intensity, and the value of reference intensity ratio and mass percentage of MgO, respectively.

3 Results and discussion

3.1 Formation behavior of CS

According to Ref. [15], both CS and β-C₂S can form when the C/S molar ratio is 1.0. The XRD spectra of CaCO₃ and SiO₂ mixture heated to different temperatures without duration are shown in Figure 1, and the corresponding phase compositions calculated from the XRD data are presented in Table 1. The phases mainly include CaO, SiO₂, CS and β-C₂S. The CS (PDF card 31-0300#) belongs to a cyclic calcium silicate with P-1(2) space groups, and the crystallographic parameters are as follows: a=b=0.6820 nm,c=1.9650 nm, α=β=90.4°, γ=119.3°. The β-C₂S (PDF card 33-0302#) belongs to larnite with P21/n(14) space groups, and the crystallographic parameters are as follows: a=0.9310 nm, b=0.6756 nm, c=0.5506 nm, α=γ=90.0°, β=94.46°.

Figure 1 XRD spectra of samples when heated to different temperatures without duration:

Table 1 Phase compositions of samples heated to different temperatures without duration

As shown in Figure 1(a), the mixture of CaCO₃ and SiO₂ does not react when sintered to 1200 °C because of the low diffusion rate during the solid-state reaction. As the reaction temperature rises, the diffraction peak intensity of CaO and SiO₂ gradually decreases, while the peak intensity of CS increases significantly. As listed in Table 1, the contents of SiO₂ and CaO in the sample after heating to 1200 °C are 61.0% and 39.9%, respectively, but they decrease to 7.0% and 1.3%, respectively, when heated to 1400 °C. It indicates that the increase of sintering temperature is largely beneficial to the solid-state reaction of CaCO₃ and SiO₂. The phases of C₃S₂ and C₃S were not found in the sintered samples under various sintering temperatures, indicating that C₃S₂ and C₃S are not involved in the reaction process when the C/S ratio is 1.0. Figure 1(b) reflects the changes of peak intensity between CS and β-C₂S. The primary phase when heated to 1250 °C is C₂S. As the heating temperature rises, the peak intensity of C₂S reaches a maximum at 1300 °C, and then decreases. Therefore, the formation of C₂S is prior to CS when the C/S ratio is 1.0, and then it transforms to CS as the sintering temperature rises.

The XRD spectra of CaCO₃ and SiO₂ mixture sintered at different temperatures for 1 h are shown in Figure 2. The peak intensity of CS gradually increases as the sintering temperature rises, and it reaches the strongest peak intensity at 1400 °C, indicating that the sintered sample is mainly composed of CS. On the contrary, the diffraction peak intensity of C₂S decreases gradually, and it almost disappears at 1400 °C. As presented in Table 2, the content of CS increases from 6.3% to 97.4% when the sintering temperature rises from 1200 °C to 1400 °C, and the content of β-C₂S decreases from 41.0% to 2.5%. It also indicates that CS is generated by the conversion reaction of C₂S, which is consistent with the XRD results in Figure 1 and Table 1. Meanwhile, prolonging the holding time is beneficial to promote the transformation of C₂S to CS.

Figure 2 XRD spectra of samples sintered at different temperatures for 1 h

Table 2 Phase compositions of samples sintered at different temperatures for 1 h

The FT-IR spectra of samples heated to different temperatures without duration are shown in Figure 3. The main absorption peaks when heated to 1200 °C are centered at 1152, 1112, 1086, 879, 780, 695, 629 and 477 cm^-1. By comparing the infrared spectra of quartz, C₂S and CS [16], it can be seen that most of the characteristic peaks correspond to quartz. However, the absorption peaks at 879 and 418 cm^-1 were conformed to the typical characteristic peaks of β-C₂S, which belong to the asymmetric expansion vibration and the bending vibration of O-Si-O bonds in C₂S, respectively. It indicates that a low-symmetry island structure of isolated Si-O tetrahedron initially forms at 1200 °C. When the heating temperature rises to 1300 °C, SiO₂ only has the characteristic peaks at 1112 and 780 cm^-1, but the corresponding peak intensities decline significantly. Besides the absorption peaks at 879 and 418 cm^-1, the characteristic peaks of β-C₂S also appear at 998, 890, 858, 629 and 477 cm^-1, indicating that the island structure of C₂S forms at 1300 °C. Meanwhile, new absorption peaks at 939, 920, 721, 565 and 498 cm^-1 exist, which were assigned to the characteristic peaks of high-temperature wollastonite (α-CS). The absorption peak at 721 cm^-1 is the typical peak of α-CS with a ternary ring structure. When the heating temperature rises to 1400 ^oC, most of the characteristic peaks of β-C₂S disappear, and only the absorption peaks at 998, 858 and 629 cm^-1 exist. The absorption peaks of α-CS are located at 1075, 985, 940, 920, 715, 560, 480 and 431 cm^-1, which are consistent with the characteristic peaks of high-temperature wollastonite. As the heating temperature rises, the content of C₂S increases first and then decreases,while the content of CS increases gradually, which are consistent with the XRD results.

Figure 3 FT-IR spectra of samples heated to different temperatures without duration

Figure 4 shows the Raman spectra of samples sintered under different conditions. The Raman spectra of CS was reported as follows [17]: the bands lower than 400 cm^-1 belong to the external vibration; the bands in 400-700 cm^-1 mainly correspond to the bending vibration of Si-O tetrahedron; the bands above 700 cm^-1 refer to the stretching vibration of Si-O tetrahedron. The vibrational peaks centered at 139, 371, 576, 893, 983, 1072 cm^-1 belong to the characteristic peaks of wollastonite, among which 371, 576 and 983 cm^-1 are the typical characteristic peaks of α-CS [18]. It indicates that the crystal structure of α-CS formed at high temperature can be kept by the rapid cooling to room temperature. As the sintering temperature and holding time increase, the peak intensity of CS gradually increases, indicating that the content of CS increases gradually. It is consistent with the XRD and FT-IR results. The bands at 825 and 858 cm^-1 are the characteristic peaks of β-C₂S, which were appointed to the symmetric stretching vibration and asymmetric stretching vibration of Si-O tetrahedron, respectively. The intensity of β-C₂S gradually enhances at 1250-1300 °C and decreases at 1300-1400 °C, indicating that the content of β-C₂S increases firstly and then decreases as the sintering temperature rises. As presented in Figure 4, when the sintering temperature rises and the holding time prolongs, there is a small blue shift and broadening in the stretching vibration, bending vibration and lattice vibration, which is mainly due to the changes of bond length and bond angle caused by the deformation correction of Si-O tetrahedron around the bridging oxygen bonds [19].

Figure 4 Raman spectra of samples sintered under different conditions

3.2 Formation kinetics of CS

According to the above analyses, the formation process of CS using CaCO₃ and SiO₂ as raw materials can be concluded as Eqs. (2)-(4). It involves the decomposition reaction of calcium carbonate, the direct combination reaction of C₂S and the conversion reaction of C₂S to CS.

CaCO₃(s)=CaO(s)+CO₂(g)                  (2)

2CaO(s)+SiO₂(s)=2CaO·SiO₂(s)             (3)

2CaO·SiO₂(s)+SiO₂(s)=2(CaO·SiO₂)(s)       (4)

The Coats-Redfern method is one of the simplest and widely applied kinetic models for studying the solid-state reactions, especially for those with decomposition [20-22]. The authors successfully used the Coats-Redfern method to study the formation kinetics of calcium aluminate compounds when using CaCO₃, Al₂O₃ and Na₂CO₃ [23]. In this work, the Coats-Redfern method was also used to determine the reaction model and the corresponding equation of CS, as presented in Eq. (5). The data of CS formation from CaCO₃ and SiO₂ mixture when heated to different temperatures without duration were adopted. Considering the simple solid-state reactions of CS based on the CaO-SiO₂ binary system, only the nucleation model, diffusion model and geometric contraction model were selected to fit the kinetic model of CS formation, as shown in Table 3.

ln[g(α)/T²]=ln[RA/(βE)(1-2RT/E)]-E/(RT)    (5)

where α is the formation percentage of CS, %; g(α) is the function of α as listed in Table 3; T is the reaction temperature, K; R is the gas constant, J/(K·mol); A is the pre-exponential factor, s^-1; β is the heating rate, K/min; E is the apparent activation energy, kJ/mol.

According to the relevant research results in Ref. [24], the value of 2RT/E is very close to 0 during the high-temperature sintering reaction, thus the value of (1-2RT/E) is approximately equal to 1. Then the formula ln[(RA/(βE))(1-2RT/E)] can be regarded as a constant, and ln[g(α)/T²] is approximately linear to E/(RT). Finally, the reaction activation energy and the pre-exponential factor can be obtained by the slope and the intercept of corresponding curves. The calculated results of CS formation with different solid-state reaction rate models are presented in Table 3. The F2 model is the best to represent the formation of CS during high-temperature sintering, and the corresponding fitting r and R² are much higher than other models. The fitting curve according to the F2 model is shown in Figure 5. Meanwhile, the apparent activation energies and pre-exponential factors of different models with the fitting R² values higher than 0.95 were calculated as listed in Table 3. As the theoretical range of pre-exponential factor based on the solid-state reaction is usually between 10⁶ and 10¹⁸ [25], it proves that the formation of CS follows the F2 model, which is also in good agreement with the formation model of calcium aluminate compounds [23].

Table 3 Calculated results of CS formation with different solid-state reaction rate models

Figure 5 Linear relationship for formation of CS according to F2 model

Table 4 Calculation results of activation energy and pre-exponential factor of CS formation

The apparent activation energy and pre- exponential factor were calculated to be 505.82 kJ/mol and 2.16×10¹⁴ s^-1, respectively. The formation of CS belongs to the second-order reaction model, and the formation kinetic equation is:

(1-α)^-1-1=2.16×10¹⁴exp(-505820/(RT))t      (6)

3.3 Morphology of CS

The SEM morphology of sample heated to 1250 °C without duration and its map scanning results are shown in Figure 6, and the EDS results are presented in Table 5. It is mainly composed of blocky particles and pores with different sizes (Figure 6(a)). As shown in Figures 6(b)-(d), Si element is mainly concentrated in large-size blocky particles, Ca element is evenly distributed except the large-size blocky particles, and O element is mainly concentrated on the massive particles with large grain size. The large-size blocky particles in highlight regions of Figure 6(b) (A) correspond to SiO₂, and the small-size particles with a uniform distribution (B) correspond to CaO or undecomposed CaCO₃. The medium-size particles (C) correspond to C₂S, which are mostly distributed near the large-size particles. It can be concluded that C₂S is generated by the diffusion of CaO to blocky SiO₂ according to their locations.

Figure 6 SEM image of sample heated to 1250 °C without duration (a) and its map scanning results (b-d)

Table 5 EDS results of sample heated to 1250 °C without duration in mole fraction

As seen in Figure 7 and Table 5, some blocky particles (D) corresponding to CS form during the heating process, and the CaO to SiO₂ molar ratio is below 1 because of the low diffusion rate and short reaction time. Meanwhile, a large quantity of CaO (E) exists on the surface of CS.

Figure 7 Morphology of initial formed CS in sample heated to 1250 °C without duration (a) and amplification of rectangular area (b)

The SEM images of sample sintered at 1400 °C for 1 h are shown in Figure 8, and the EDS results are listed in Table 6. It melts partly at 1400 °C. The melting areas are distributed in grid morphology (Figure 8(b)) and the non-melting areas are distributed in dispersed particles with much smaller size (Figure 8(c)). The CaO to SiO₂ molar ratios in different regions are about 1.0, indicating that the formation of CS is uniform and complete.

Figure 8 SEM images of sample sintered at 1400 °C for 1 h (a) and amplification of melting area (b) and non-melting area (c)

Table 6 EDS results corresponding to Figure 8 in molar fraction

4 Conclusions

1) β-C₂S forms firstly during the heating process of CaCO₃ and SiO₂ mixture with the equal molar ratio, and then CS is generated by the transformation reaction of pre-formed β-C₂S with SiO₂.

2) Increasing the sintering temperature and prolonging the holding time are helpful to promote the phase transition of C₂S to CS, and the content of CS can reach 97.4% when sintered at 1400 °C for 1 h.

3) The formation process of CS follows the second-order chemical reaction model, and the corresponding apparent activation energy and pre-exponential factor are 505.82 kJ/mol and 2.16×10¹⁴ s^-1, respectively. The formation kinetic equation of CS is as follows: (1-α)^-1-1=2.16×10¹⁴ exp(-505820/(RT))t.

Contributors

The overarching research goals were developed by PAN Xiao-lin and YU Hai-yan. CUI Wei-xue and ZHANG Can provided and analyzed the measured data. The initial draft of the manuscript was written by PAN Xiao-lin and CUI Wei-xue. All authors replied to reviewers’ comments and revised the final version.

Conflict of interest

PAN Xiao-lin, CUI Wei-xue, ZHANG Can, and YU Hai-yan declare that they have no conflict of interest.

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(Edited by ZHENG Yu-tong)

中文导读

CaO·SiO₂在低钙高温烧结过程的生成动力学与转变机制

摘要：利用XRD、FT-IR、Raman、SEM-EDS等方法研究了CaO·SiO₂在低钙高温烧结过程的晶体结构、生成动力学和微观组织。当反应物CaCO₃和SiO₂摩尔比为1.0时，加热过程中优先生成2CaO·SiO₂，然后2CaO·SiO₂进一步与SiO₂反应生成CaO·SiO₂；烧结过程中不会生成3CaO·2SiO₂和3CaO·SiO₂。升高反应温度和延长保温时间有助于2CaO·SiO₂向CaO·SiO₂转化，并使烧结产物的拉曼光谱特征峰发生一定的蓝移和宽化。在1400 °C烧结1 h时CaO·SiO₂在烧结产物的含量能够达到97.4%。CaO·SiO₂的生成动力学遵循二级化学反应模型，相应的反应表观活化能和指前因子分别为505.82 kJ/mol和2.16×10¹⁴s^-1。

关键词：硅酸钙化合物；生成动力学；晶体结构；微观组织；烧结法

Foundation item: Projects(51674075, 51774079) supported by the National Natural Science Foundation of China; Project(2018YFC1901903) supported by the National Key R&D Program of China; Project(N182508026) supported by the Fundamental Research Funds for the Central Universities of China

Received date: 2020-03-25; Accepted date: 2020-09-08

Corresponding author: PAN Xiao-lin, PhD, Associate Professor; Tel: +86-24-83686460; E-mail: panxl@smm.neu.edu.cn; ORCID: https://orcid.org/0000-0002-9814-0882