Trans. Nonferrous Met. Soc. China 26(2016) 849-858

Formation mechanism and crystal simulation of Na₂O-doped calcium aluminate compounds

Yong-pan TIAN, Xiao-lin PAN, Hai-yan YU, Gan-feng TU

School of Metallurgy, Northeastern University, Shenyang 110819, China

Received 23 April 2015; accepted 10 November 2015

Abstract: Calcium aluminate clinkers doped with Na₂O were synthesized using analytically pure reagents CaCO₃, Al₂O₃, SiO₂ and Na₂CO₃. The effects of Na₂O-doping on the formation mechanism of calcium aluminate compounds and the crystal property of 12CaO·7Al₂O₃ (C₁₂A₇) cell were studied. The results show that the minerals containing Na₂O mainly include 2Na₂O·3CaO·5Al₂O₃ and Na₂O·Al₂O₃, when the Na₂O content in clinkers is less than 4.26% (mass fraction). The rest of Na₂O is mainly doped in 12CaO·7Al₂O₃, which results in the decrease of the crystallinity of 12CaO·7Al₂O₃. The crystallinity of 2Na₂O·3CaO·5Al₂O₃ is also inversely proportional to the Na₂O content in clinkers. The formation processes of 2Na₂O·3CaO·5Al₂O₃ and 12CaO·7Al₂O₃ can be divided into two ways, which are the direct reactions of raw materials and the transformation of CaO·Al₂O₃, respectively. The simulation shows that the covalency of O—Na bond in Na₂O-doped 12CaO·7Al₂O₃ cell is weaker than those of O—Ca and O—Al bonds. The free energy of the unit cell increases because of Na₂O doping, which results in the improvement of chemical activity of 12CaO·7Al₂O₃. The leaching efficiency of Al₂O₃ in clinker is improved from 34.81% to 88.17% when the Na₂O content in clinkers increases from 0 to 4.26%.

Key words: calcium aluminate; Na₂O-doping; formation mechanism; crystal structure; sintering; computer simulation

1 Introduction

The low-grade bauxites and non-bauxite sources such as nepheline, clay and fly ash are widely distributed in China. It is suitable to extract alumina from these sources by the lime sintering process since dry materials can be used during the sintering process, reducing the energy consumption of the alumina production industry. However, the application of the lime sintering process is restricted by the defects including the high consumption of calcium, large amount of slag and poor leaching property of clinkers. The clinker of the lime sintering process mainly consists of 12CaO·7Al₂O₃ (Ca₁₂Al₁₄O₃₃, C₁₂A₇), 2CaO·SiO₂ (C₂S), CaO·Al₂O₃ (CaAl₂O₄, CA) and 2CaO·Al₂O₃·SiO₂ (C₂AS). The calcium aluminate compounds, which can decompose in the sodium carbonate solution, are mainly C₁₂A₇ and CA. The previous researches [1,2] showed that the doping of Na₂O in the sintering process could improve the leaching performance of clinkers and reduce the consumption of CaO. Moreover, the results showed that the lattice constants of C₁₂A₇ increase with the increase of the molar ratio of Na₂O to Al₂O₃ (N/A ratio) of clinker. However, the details of phase structures were not analyzed, and the mechanisms by which Na₂O affects the calcium aluminate compounds were unknown.

In the Na₂O-doped CaO-Al₂O₃ system, the amounts of C₁₂A₇ and 2Na₂O·3CaO·5Al₂O₃ (Na₄Ca₃(AlO₂)₁₀) reach the maximum as the sodium content in CA increases to 10% (mass fraction) [3]. Additionally, the crystallinity of C₁₂A₇ decreased with the doping of Na₂O while that of CA increased [4]. FUKUDA et al [5] reported that Al³⁺ ions were substituted by Si⁴⁺ ions and Ca²⁺ ions were substituted by Na⁺ ions, when 3CaO·Al₂O₆ (C₃A) was doped with Na⁺ and Si⁴⁺ ions. Many reports [6-11] showed that the properties of crystal structures could be analyzed by the Material Studio. BRIK et al [12] showed that the structural and electronic properties of the two Cr³⁺-bearing systems (NaCrSi₂O₆ and LiCrSi₂O₆) could be studied by the CASTEP module of Materials Studio package with two ab initio DFT-based calculations. WU et al [13] observed that the coordination number of Al atom as well as its amphoteric property in the melt of xCaO·(1-x)Al₂O₃ (x represents mole fraction) and the microstructure properties of xCaO·(1-x)Al₂O₃ could be studied by the molecular dynamics simulation using Materials Studio. PAN et al [14] reported that the free energy of the C₁₂A₇ cell calculated by the Materials Studio increased because of the absence of some Ca atoms in C₁₂A₇ cells. In this work, the compounds containing Na₂O, the doping mechanism of Na₂O in compounds and the chemical activity of the C₁₂A₇ crystal structure with or without Na₂O doping as well as leaching characteristics of clinkers were investigated. Meanwhile, the phase constitutions, the distribution characteristics of phases in micro-regions and the crystal structure of C₁₂A₇ doped with Na₂O were also investigated through Material Studio.

2 Experimental

2.1 Materials

Analytically pure reagents (CaCO₃, Al₂O₃, SiO₂ and Na₂CO₃) were used in the current work. In Na₂O-doped CaO-Al₂O₃-SiO₂ system, the mass ratio of Al₂O₃ to SiO₂ (A/S) was about 1.6 and the molar ratio of CaO to Al₂O₃ (C/A) was about 0.85. The C/A ratios of Na₂O-doped CaO-Al₂O₃ system were 0.6 and 1.0. The amounts of Na₂O in clinker were represented by the values of the molar ratios of Na₂O to Al₂O₃ (N/A). The mixed materials were sintered at 1200 °C or 1350 °C in a MoSi₂ resistance furnace (KSL-1700-A2). Then, the clinkers were ground to 100 μm.

2.2 Methods

The X-ray diffraction (XRD) data were collected on a Philips PW3040/60 with Cu K_α (λ=1.54056 ) radiation operated at 40 kV and 40 mA. The K_α ray included two kinds of wavelength, which were K_α1 and K_α2, respectively. The data were analyzed by the MDI Jade with the database PDF2-2004. The cubic spline method was used to subtract the background and the peaks generated by the K_α2 radiation were also subtracted as the ratio of K_α1/K_α2 was 2.0. The Pseudo-Voigt function was used during the fitting processes of spectra, and nearly 100 peaks were fitted for every spectrum. The microstructural analyses were carried out by scanning electron microscopy (SEM, SHIMADZU SSX-550) and energy-dispersive X-ray spectroscopy (EDS, DX-4). The samples were polished flat in water-free ethylene glycol using SiC papers, cleaned by ultra-sound in water-free ethylene glycol and coated with gold by sputtering.

The Visualizer module of Materials Studio was used to establish three-dimensional models of C₁₂A₇ cells with or without Na₂O doping, which provided the foundation models for the subsequent calculation. The lattice parameter, the unit cell volume and free energy of the crystal structure, as well as the population and length of chemical bond were analyzed through the geometry optimization of the crystals. The CASTEP module of Materials Studio was used during the optimization process. The generalized gradient approximation (GGA) and the corresponding exchange-correlation potential (PBE) were used during the geometry optimization of the crystals through the BFGS algorithm while the Ultrasoft was selected as pseudopotential. The energy cut-off value was 340.0 eV. The SCF tolerance was 1.0×10^-6 eV/atom. The results were obtained after large numbers of iterations. The original crystallographic data of the calculation were taken from the inorganic crystal structure database (ICSD).

2.3 Leaching conditions

The leaching experiments were carried out in a thermostatic water bath. The sintered clinkers were leached by the sodium carbonate and sodium hydroxide solution at 80 °C for 30 min. The liquid-to-solid ratio for leaching was 10. The sodium carbonate concentration of the solution was 80 g/L (in the form of Na₂O) and the sodium hydroxide concentration of the solution was 15 g/L (in the form of Na₂O). The leaching residues were washed to neutrality with hot water and dried under 90 °C. The chemical compositions of clinkers and residues were analyzed by X-ray fluorescence (XRF-1800). The alumina leaching efficiencies (η_AO) of Na₂O-doped CaO-Al₂O₃-SiO₂ system clinker and Na₂O-doped CaO-Al₂O₃ system clinker were calculated according to Eq. (1):

                (1)

where (A/C)_residue and (A/C)_clinker are the mass ratios of Al₂O₃ to CaO of leached residues and sintered clinkers, respectively.

3 Results and discussion

3.1 Phase species and formation mechanism

As shown in Fig. 1(a), pure Na₄Ca₃(AlO₂)₁₀ was synthesized after sintering at 1200 °C for 30 min. There is no Na₄Ca₃(AlO₂)₁₀ card in the PDF2-2004. But the XRD pattern of Na₄Ca₃(AlO₂)₁₀ is similar to that in Ref. [3]. CA was synthesized after sintering at 1600 °C for 60 min. CA and Na₂CO₃ were sintered at 1350 °C for 60 min when the N/A ratio is 0.33. The XRD patterns of clinkers are shown in Fig. 1(b). Most of CA transforms to C₁₂A₇ and Na₄Ca₃(AlO₂)₁₀. Some Na₂O·Al₂O₃ (NA) also exists in clinkers.

Fig. 1 XRD patterns of Na₂O-CaO-Al₂O₃ system clinkers

Table 1 Chemical compositions of Na₂O-doped clinkers (mass fraction, %)

Fig. 2 XRD patterns of clinkers with different N/A ratios

The compositions of Na₂O-doped CaO-Al₂O₃-SiO₂ system clinkers are shown in Table 1. The phases of clinkers with different N/A ratios are shown in Fig. 2. The phases containing Al₂O₃ consist of C₁₂A₇, CA, NA, CaO·2Al₂O₃ (CA₂), Na₄Ca₃(AlO₂)₁₀ and C₂AS. The phases containing SiO₂ consist of C₂AS, β-C₂S and γ-C₂S. The phases containing Na₂O consist of Na₄Ca₃(AlO₂)₁₀ and NA. In addition, trace amounts of Ca_8.5NaAl₆O₁₈ and Ca_8.25Na_1.5Al₆O₁₈ are also found. When the N/A ratio is less than 0.04, small amount of CA₂ and some un-reacted CaO and Al₂O₃ exist in clinkers. As the N/A ratio increases to 0.17, CaO and Al₂O₃ react completely. The amount of CA₂ is also inversely proportional to the amount of Na₂O in clinkers. No CA₂ exists in clinkers as the N/A ratio increases to 0.17, which indicates that Na₂CO₃ accelerates the transformation reactions of CA₂. The melting temperature of Na₂CO₃ is about 850 °C and the decomposition temperature is about 1750 °C, so Na₂CO₃ melts at the clinkering temperature of 1350 °C. The diffusion speeds of ions are accelerated by the melting of Na₂CO₃, which results in higher reaction velocities of CaO, Al₂O₃ and CA₂. The sintering temperature is higher than the formation temperature of Na₄Ca₃(AlO₂)₁₀. So, some Na₄Ca₃(AlO₂)₁₀ form before the sintering temperature reaches 1350 °C in the Na₂O-doped CaO-Al₂O₃-SiO₂ system clinkers. The formation processes of Na₄Ca₃(AlO₂)₁₀ and C₁₂A₇ can be divided into two ways, which are the transformation of CA (Eq. (2) [3]) and the direct reaction among Na₂CO₃, CaO and Al₂O₃, respectively.

39(CaO·Al₂O₃)+10Na₂CO₃=2(12CaO·7Al₂O₃)+5(2Na₂O·3CaO·5Al₂O₃)+10CO₂↑           (2)

The intensities of characteristic peaks of different phases are shown in Table 2. With the doping of Na₂O and the absolute reaction of CaO and Al₂O₃, the intensity of the characteristic peak of C₂AS at 2θ value of 31.47° increases firstly and then decreases as the N/A ratio increases from 0.04 to 0.24. The increase of the amount of C₂AS may be due to the higher reaction velocities of CaO, Al₂O₃ and SiO₂ accelerated by the melting of Na₂CO₃, and the decrease may be due to the lower forming velocity of C₂AS compared with other compounds. C₂S exists as the form of β-C₂S when the temperature is above 800 °C and then transforms to γ-C₂S with the decrease of temperature. The Na₂O-doped clinkers contain large amounts of β-C₂S, which indicates that Na₂O restrains the transformation of β-C₂S. Moreover, the decrease of the intensity of the characteristic peak of γ-C₂S also suggests the reducing of γ-C₂S content. The intensities of the characteristic peaks at 2θ values of 18.23° and 21.10° increase as the N/A ratio increases from 0 to 0.24. The results suggest the increase of the C₁₂A₇ and Na₄Ca₃(AlO₂)₁₀ contents. On the contrary, the content of CA decreases with the increase of Na₂O content in clinkers. CA disappears as the N/A ratio increases to 0.17. Moreover, the results are in accordance with the phase diagram, which suggests that the zone of C₂AS gradually decreases while the zone of Na₄Ca₃(AlO₂)₁₀ increases with the N/A ratio increasing from 0.17 (Fig. 3(a)) to 0.33 (Fig. 3(b) [15]). No CA exists in the system when the N/A ratio is equal to 0.33.

Table 2 Intensities of peaks of phases in Na₂O-doped CaO-Al₂O₃-SiO₂ system clinkers

Fig. 3 Na₂O-CaO-Al₂O₃-SiO₂ quarternary system phase diagram [15]

The characteristic peak at 2θ value of 33.33° is the peak of C₁₂A₇ when the N/A ratio is 0 and then becomes a mixed peak of C₁₂A₇, Na₄Ca₃(AlO₂)₁₀ and NA when the clinkers are doped with Na₂O. Small amount of NA exists in the clinkers, so the contribution of CA to the intensity of the mixed peak can be ignored. The crystal faces of C₁₂A₇ represented by the characteristic peaks at 2θ values of 18.23° and 33.33° are (211) and (420), respectively. The intensity of the characteristic peak of C₁₂A₇ at 2θ value of 33.33° is stronger than that of the characteristic peak of C₁₂A₇ at 2θ value of 18.23° when the N/A ratio is 0. The intensity of the characteristic peak at 2θ value of 33.33° decreases from 21817 to 16862, while the intensity of the characteristic peak at 2θ value of 18.23° increases from 14726 to 15468 when the N/A ratio increases from 0.17 to 0.24. The results suggest that the C₁₂A₇ formed by the transformation of CA prefers to growing in the crystal face (211).

3.2 Microstructures of clinkers

As shown in Fig. 4, the representative microstructure of the clinker, when the N/A ratio is 0.04, displays three gray scales: the dark gray, the medium gray, and the light gray. The compositions of the micro-regions of Points A, B and C are listed in Table 3. The molar ratio of CaO to SiO₂ in the micro-region of Point A is close to 2, indicating that the light gray regions stand for C₂S. The molar ratio of CaO to Al₂O₃ at Point B is close to 1.71. So, the dark area is the distribution area of C₁₂A₇. The Na₂O content in C₁₂A₇ is 4.33% (mass fraction). The chemical formula can be written as 0.93Na₂O·10CaO·7Al₂O₃. At Point C where CaO is divided into two parts, the molar ratios of CaO to Al₂O₃ and CaO to SiO₂ are close to 1 and 2, respectively. Therefore, the region of Point C consists of CA and C₂S and the medium gray region stands for CA. Na₂O is mainly doped in C₁₂A₇ since the Na₂O content in the region of C₁₂A₇ is the highest. The analysis results of Ca and O elements in different areas do not show significant differences as the Na, Al and Si elements do.

Fig. 4 BSE images of sintered clinkers at N/A ratio of 0.04 and C/A ratio of 0.88

Table 3 Mole fractions of elements in clinkers determined by EDS (%)

Map scanning results are shown in Fig. 5. The high light areas in Figs. 5(b)-(f) are the main distribution areas of Na, Al, Si, Ca and O elements, respectively. The high light area of the picture is the main distribution area of the element. As shown in Fig. 5(b), it is obvious that Na₂O mainly exists in the dark gray area, which is the distribution area of C₁₂A₇. Figure 5(d) shows the scopes of C₂S and C₁₂A₇ clearly. According to Figs. 5(e) and (f), the distributions of O and Ca do not show significant differences, which is consistent with the analysis results of EDS.

Fig. 5 SEM image (a) and scanning maps (b-f) of clinkers at N/A ratio of 0.04 and C/A ratio of 0.88

Fig. 6 BSE images of sintered clinkers at N/A ratio of 0.24 and C/A ratio of 0.87 in different zones

Table 4 Mole fractions of elements in clinkers determined by EDS

BSE images of clinkers at N/A ratio of 0.24 are shown in Fig. 6. The analysis results of EDS are given in Table 4. It can be clearly seen that the phases distribute in different regions by mole fraction of elements. The lightest area is the distribution area of C₂S. It can be calculated that the Na₂O content doped in C₂S is about 1.97%. The regions represented by Points E and F are the distribution areas of C₁₂A₇ and the mass fractions of Na₂O doped in C₁₂A₇ at Points E and F are about 4.40% and 4.45%, respectively. So, the chemical formulae can be written as 0.92Na₂O·9.37CaO·7Al₂O₃ and 0.94Na₂O·9.61CaO·7Al₂O₃, respectively. The Na₂O content doped in C₁₂A₇ increases slightly from 4.33% to 4.45% when the N/A ratio is increased from 0.04 to 0.24. According to analysis results of Point G, the darker area at the edge of C₂S represents the region of CA. Trace amount of CA is formed when the N/A ratio of clinker is 0.24 and the content of Na₂O in the CA is about 1.43%. The results of element analysis of Points H and I confirm the existence of NA and Na₄Ca₃(AlO₂)₁₀. Moreover, NA and Na₄Ca₃(AlO₂)₁₀ are distributed in the same micro-regions.

The crystal system of CA is monoclinic and the space group is P21/n. The crystal system of C₁₂A₇ is cubic and the space group is I-43d. MIZUKAMI et al [16], SUSHKO et al [17] and HOSONO et al [18] reported that the C₁₂A₇ cell was constituted by sub-nanometer size cages, which was approximately 0.4 nm in diameter. Each C₁₂A₇ cell consisted of two molecules with twelve cages. Two free oxygen ions existed in the twelve cages. The free oxygen ion can be substituted by the hydroxide ion, chloride ion and fluoride ion without changing the crystal structure [19-23]. The radii of Ca²⁺ and Na⁺ ions are 0.130 and 0.102 nm, respectively. The radii of OH^-, Cl^- and F^- ions are 0.137, 181 and 133 nm, respectively. They are all larger than the radius of Na⁺ ion. Therefore, Na⁺ ion may replace Ca²⁺ ion or the free oxygen ion of C₁₂A₇ without changing the crystal system and space group. The Ca²⁺ and O^2- ions of C₁₂A₇ cells are not distributed symmetrically, while those of CA cells are distributed symmetrically. Therefore, it is easier for polar molecules such as H₂O and Na₂O to penetrate the cells or react with the cells.

The crystallinities of phases are calculated through fitting the spectra using MDI Jade, which are shown in Fig. 7. Due to the doping of Na₂O in the crystal structures, the crystallinities of C₁₂A₇ and β-C₂S decrease with the rise of Na₂O content in clinkers. The crystallinity of Na₄Ca₃(AlO₂)₁₀ decreases with the increase of Na₂O content in clinkers. The reason may be the coexistence of NA and Na₄Ca₃(AlO₂)₁₀ and the unbalanced distribution of Na₂O between NA and Na₄Ca₃(AlO₂)₁₀. On the contrary, the crystallinity of CA increases slightly from 96.87% to 97.72% when the N/A ratio increases from 0 to 0.04. This is due to the fact that the ions in the defects of CA crystal structure have higher activity. Thus, the ions are easier to react with Na₂CO₃, which reduces the defects of CA grains. The increase of the crystallinity of CA also shows that it is difficult for Na₂O to insert the cells of CA. With the doping of Na₂O in clinkers, the crystallinity of γ-C₂S slightly decreases. Yet, the crystallinity of γ-C₂S rises slightly with the increase of Na₂O content in clinkers. However, the crystallinity of γ-C₂S in different clinkers is around 96.5%.

Fig. 7 Crystallinities of phases in clinkers at C/A ratio of about 0.85

3.3 Simulation of Na₂O-doped C₁₂A₇ crystal

The analysis results of EDS show that Na₂O mainly coexists with C₁₂A₇ compared with other compounds. Material Studio was used to analyze the effect of Na₂O addition on the C₁₂A₇ cell. The setting values of the parameters during the optimization process of the crystal structures of C₁₂A₇ cells with or without Na₂O doping are the same to ensure the comparability of the results. As shown in Table 5, the convergence standards during the calculation process are also the same with each other. The crystal structure data illustrated in Table 6 are derived from the ICSD.

Table 5 Parameter settings for optimization of crystal structure of C₁₂A₇ cells during calculation

The primitive cell of C₁₂A₇, simulated by MS using the crystal structure data in Table 6, is shown in Fig. 8. The crystal structure of C₁₂A₇ has a high symmetry. Any Ca atom can be chosen to be replaced by Na atom. The results will be the same, since only one kind of Ca site exists in the structure. The cell, one Ca atom of which is substituted by one Na atom, is shown in Fig. 9.

Table 6 Crystal structure data and atomic coordinates of C₁₂A₇

Fig. 8 Model of C₁₂A₇ unit cell

Fig. 9 Model of C₁₂A₇ unit cell when one Ca atom is replaced by one Na atom

The crystal structure of C₁₂A₇ mainly contains O—Al, O—Ca and O—O bonds. The bond length of O—Al is about 0.17 nm with the 3s and 3p orbital electrons of Al atom and the 2s, 2p orbital electrons of O atom forming the hybrid orbit. The bond length of O—Ca is about 0.24 nm with the 3s, 3p and 4s orbital electrons of Ca atom and the 2s, 2p orbital electrons of O atom forming the hybrid orbit. The bond population and length of C₁₂A₇ are presented in Table 7. The bond population is useful for evaluating the bonding character. A high value of the population of one chemical bond suggests that the bond is a covalent bond, whereas a low value suggests that the bond is an ionic bond [24,25]. As shown in Table 7, the population of the O—Al bond is much higher than that of the O—Ca bond, suggesting that the covalent character of the O—Al bond is stronger than that of the O—Ca bond. The O—O bonds are separated into two kinds, which show the covalent character and the ionic character, respectively. The bond of O—O with larger population is composed of the two free oxygen atoms in the cage-like structures. The computer cannot simulate the free oxygen atoms, so the free oxygen atoms may not form chemical bonds in the real materials.

Table 7 Bond population and length of C₁₂A₇

Table 8 Bond population and length of C₁₂A₇ doped with one Na atom

The bond population and length of C₁₂A₇ doped by one Na atom are shown in Table 8. The positions of the atoms in the crystals are changed in order to minimize the free energy of the cells and improve the stability of the cells during the simulation. The data in Table 8 suggest that the doping of Na atom does not significantly change the atomic distances and populations of O—Al and O—O bonds. Due to the replacement of Na atom, the species of O—Ca bond decrease from the original 4 to 3. As shown in Table 8, the population of O—Na bond is 0.03, which is lower than that of O—Ca bond. In addition, the O—Na bond has a larger atomic distance than O—Ca bond. Consequently, the covalency of the O—Na bond is weaker than that of the O—Ca bond.

The lattice parameter, the unit cell volume and the free energy of C₁₂A₇ unit cell and Na₂O-doped C₁₂A₇ cell are illustrated in Table 9. The doping of the Na atom has no significant effect on the lattice parameters and cell volumes. The free energy of the unit cell is increased because of the Na₂O addition. The free energy is the internal energy converted into other kind of energy during reactions. The simulation processes have been done under the hypothesis that the external environment conditions such as temperature and pressure are stable. Therefore, the free energy is the characteristic of the stability of the crystal structure. According to the principle of minimum energy, a lower free energy value corresponds to a higher activity. The increase of the free energy shows that the doping of Na atom can increase the chemical activity of C₁₂A₇, which benefits the leaching process of C₁₂A₇ in the sodium carbonate solution.

Table 9 Structural parameters of C₁₂A₇ and Na₂O-doped C₁₂A₇

3.4 Leaching properties of sintered clinkers

CA, C₁₂A₇ and Na₄Ca₃(AlO₂)₁₀ can decompose in sodium carbonate solution. The processes are shown in Eqs. (3)-(5). Sodium hydroxide is also needed during the leaching process. The function of sodium hydroxide is to keep the stability of sodium aluminate in solution.

CaAl₂O₄+Na₂CO₃+4H₂O=CaCO₃↓+2NaAl(OH)₄  (3)

Ca₁₂Al₁₄O₃₃+12Na₂CO₃+33H₂O=12CaCO₃↓+14NaAl(OH)₄+10NaOH         (4)

Na₄Ca₃(AlO₂)₁₀+3Na₂CO₃+20H₂O=3CaCO₃↓+10NaAl(OH)₄               (5)

The leaching results of clinkers are listed in Tables 10 and 11. As given in Table 10, the leaching efficiency of CA clinkers containing small quantity of C₁₂A₇ is 86.28%. The leaching efficiency increases to 96.79% when the N/A ratio increases from 0 to 0.33. The leaching efficiency of pure Na₄Ca₃(AlO₂)₁₀ is 97.15%, which suggests that Na₄Ca₃(AlO₂)₁₀ has a better leaching performance than CA. Na₄Ca₃(AlO₂)₁₀ is easier to react with carbon sodium solution. Thus, the molecular polarity of Na₄Ca₃(AlO₂)₁₀ is higher than that of CA. As given in Table 11, the leaching efficiency of Al₂O₃ of the clinkers increases from 34.81% to 88.17% as the N/A ratio increases from 0 to 0.24. The C/A ratio of raw materials can be decreased to 0.85 when mixed with Na₂O and the clinkers can also have proper leaching performance.

The leaching residues are almost washed to neutrality. The contents of Na₂O in residues, as shown in Tables 10 and 11, are almost the same. Moreover, the Na₂O content in residues of clinkers without Na₂O is higher than that of clinkers doped with Na₂O. Thus, most of Na₂O in residues derives from the sodium carbonate or sodium hydroxide. Na₂O in compounds almost all dissolves in solution. It can be concluded that the improvement of leaching property of clinkers is attributed to the increase of Na₄Ca₃(AlO₂)₁₀ and C₁₂A₇ contents in clinkers and the improvement of the chemical activity of Na₂O-doped C₁₂A₇.

Table 10 Leaching results of clinkers of Na₂O-doped CaO-Al₂O₃ system

Table 11 Leaching results of clinkers of Na₂O-doped CaO-Al₂O₃-SiO₂ system

4 Conclusions

1) The phases containing Na₂O in clinkers of Na₂O-doped CaO-Al₂O₃-SiO₂ system mainly include Na₄Ca₃(AlO₂)₁₀ and Na₂O·Al₂O₃. The rest of Na₂O is mainly doped in C₁₂A₇. It is easier for Na₂O to insert into the C₁₂A₇ cell than to insert into CA cell. The crystallinities of Na₄Ca₃(AlO₂)₁₀ and C₁₂A₇ decrease gradually with the increase of Na₂O content in clinkers while that of CA increases slightly.

2) The doping of Na₂O promotes the formation of C₁₂A₇ and Na₄Ca₃(AlO₂)₁₀ and prohibits the formation of C₂AS. The formation processes of Na₄Ca₃(AlO₂)₁₀ and C₁₂A₇ can be divided into two ways, which are the direct reactions among raw materials and the transformation of CA, respectively.

3) The covalency of O—Al is the strongest in the C₁₂A₇ cells. The covalency of the O—Na bond is weaker than that of the O—Ca bond, which results in the increase of the free energy of C₁₂A₇ crystal structure and the decrease of the structural stability of C₁₂A₇.

4) C₁₂A₇ and Na₄Ca₃(AlO₂)₁₀ have better leaching properties than CA. The leachability of clinker is increased because of the formation of Na₄Ca₃(AlO₂)₁₀, the increasing amount of C₁₂A₇ and the doping of Na₂O in C₁₂A₇ cells.

References

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Na₂O掺杂铝酸钙化合物的形成机理与晶体模拟

田勇攀，潘晓林，于海燕，涂赣峰

东北大学 冶金学院，沈阳 110819

摘  要：采用分析纯试剂CaCO₃、Al₂O₃、SiO₂和Na₂CO₃合成Na₂O掺杂铝酸钙熟料，研究Na₂O掺杂对铝酸钙化合物形成机理及12CaO·7Al₂O₃(C₁₂A₇)晶体结构的影响。结果表明：当熟料中Na₂O含量低于4.26%(质量分数)时，含Na₂O物相主要为2Na₂O·3CaO·5Al₂O₃和Na₂O·Al₂O₃；其余Na₂O主要掺杂在12CaO·7Al₂O₃内，并且使其结晶度降低。2Na₂O·3CaO·5Al₂O₃的结晶度也随着熟料中Na₂O含量升高而降低。2Na₂O·3CaO·5Al₂O₃和12CaO·7Al₂O₃的生成途径有两种方式：一是由初始反应物的直接反应生成，二是由反应中间产物CaO·Al₂O₃的进一步转化反应生成。晶体结构模拟结果表明，在Na₂O掺杂的12CaO·7Al₂O₃晶格内，其O—Na键的共价性弱于O—Ca键和O—Al键的共价性，使12CaO·7Al₂O₃晶胞自由能升高，化学活性提高。当熟料中Na₂O含量由0增加至4.26%时，Al₂O₃的浸出率由34.81%增大至88.17%。

关键词：铝酸钙；Na₂O掺杂；形成机理；晶体结构；烧结；计算机模拟

(Edited by Wei-ping CHEN)

Foundation item: Projects (51174054, 51104041, 51374065) supported by the National Natural Science Foundation of China; Project (N130402010) supported by the Fundamental Research Funds for the Central Universities of China

Corresponding author: Xiao-lin PAN; Tel: +86-24-83686460; E-mail: panxl@smm.neu.edu.cn

DOI: 10.1016/S1003-6326(16)64176-6