Effect of B₂O₃ Addition and CaO/Al₂O₃ Ratios on Melt Structure and Viscosity of CaO–Al₂O₃-Based Slags

Abstract

The effect of the addition of B₂O₃ and the CaO/Al₂O₃ ratios on the CaO–Al₂O₃–B₂O₃ melts’ structure was fully analyzed by molecular dynamics (MD) simulations. The results show that, with the increase in CaO/Al₂O₃ ratios, the charge compensation for [AlO₄]⁵⁻ tetrahedron changed from the oxygen tricluster structure to the charge provided by Ca²⁺, and the structure of [AlO₄]⁵⁻ tetrahedron was stabilized. The addition of B₂O₃ reduced the degree of polymerization of the aluminate melt, which, in turn, lowered the melt viscosity; in the systems with CaO/Al₂O₃ mole ratio less than 1, the added B₂O₃ mainly existed in the form of [BO₃]³⁻, which reduced the symmetry and the strength of the melt network structures; in the higher CaO/Al₂O₃ ratio systems, the depolymerization ability of the addition of B₂O₃ on aluminate network structures was enhanced with the increase in CaO/Al₂O₃ ratios, and the overall degree of polymerization of the melt decreased. The viscosity measurement shows that B₂O₃ lowers the viscosity of CaO–Al₂O₃ melts, which was consistent with the results predicted by MD simulations.

1. Introduction

Recently, advanced high-strength steels with high Al content have attracted much attention, such as transformation induced plasticity (TRIP) steels [1] and twinning induced plasticity (TWIP) steels [2]. However, in the production process, the traditional CaO–SiO₂-based slags react strongly with the Al in the molten steel, as shown in Formula (1). Furthermore, the reaction results in significant changes in the physical and chemical properties of molten slags, which impact the lubrication and heat transfer during casting, and severely affects the antegrade of the production process [3,4]. To solve the problem, the CaO–Al₂O₃-based slags, which have the potential to reduce slag–metal reaction during casting of high-aluminum steels, have been widely developed and applied [5,6]. In the design of CaO–Al₂O₃-based slags, B₂O₃ is one of the promising fluxing agents and achieved a good fluxing effect in the application; thus, it is very necessary to fully explore the fluxing mechanism of B₂O₃ in CaO–Al₂O₃-based slags.

$$3({\mathrm{SiO}}_2)+4[\mathrm{Al}]=2({\mathrm{Al}}_2{\mathrm{O}}_3)+3[\mathrm{Si}]$$

(1)

The fluxing effect and fluxing mechanism of adding B₂O₃ to CaO–Al₂O₃ system were considered by several researchers. Wang et al. [7] suggested that, by adding B₂O₃ to CaO–Al₂O₃-based slag, it can lower the viscosity of the slag because B₂O₃ can combine with many kinds of oxides to form low-melting-point eutectics, which decrease the melt point of slag. Akberdin et al. [8] created a mathematical model to study the viscosity of the CaO–Al₂O₃–B₂O₃ melt system on the base of experimental data. It was predicted that the viscosity of CaO–Al₂O₃ melt lowered by adding B₂O₃. Huang et al. [9] also reported that the viscosity of CaO–Al₂O₃-based slag lowered with an increase in the content of additive B₂O₃. They interpreted from the melt structure change that the three-coordinated boron is dominated in these slags, and the networks are loosened with increasing B₂O₃ content. In fact, B₂O₃ exists in the molten slag in the form of 2D-plane [BO₃]³⁻ groups and 3D-tetrahedron [BO₄]⁵⁻ groups. B₂O₃ existing in the form of three-coordination, as a simple planar structure, can greatly reduce the symmetry and the strength of the networks, while [BO₄]⁵⁻ tetrahedron can be connected with other tetrahedrons to increase the degree of polymerization (DOP) of the network structure [10]. If B₂O₃ exists in the form of [BO₄]⁵⁻ tetrahedron, it needs to be charge compensated by the cations in the system, same as the formation of [AlO₄]⁵⁻ tetrahedron. MAS-NMR results [11,12,13] showed that the formation of [AlO₄]⁵⁻ tetrahedron is strongly favored over that of [BO₄]⁵⁻ tetrahedron. In the system with CaO/Al₂O₃ mole ratio >1, after all the [AlO₄]⁵⁻ tetrahedrons have been fully compensated by the sufficient charges, the excess charges are conducive to the formation of [BO₄]⁵⁻ tetrahedron. In other words, adding B₂O₃ to CaO–Al₂O₃-based slag with the CaO/Al₂O₃ mole ratio >1 increases the DOP of borate network structure because of the formation of more [BO₄]⁵⁻ tetrahedrons. However, we suspect that the addition of B₂O₃ may not only affect the borate network structure, but also affect the aluminate network that plays a dominant role in the melt. Up until now, whether the addition of B₂O₃ has a positive or negative impact on the DOP of aluminate network under different CaO/Al₂O₃ mole ratios remained to be determined. In addition, the aluminate networks in the CaO–Al₂O₃ slags gradually depolymerize into simpler structural units, with the ratio of CaO/Al₂O₃ increasing [14,15,16], which has a decisive influence on the properties of aluminate melts. Therefore, the overall change in the DOP of network caused by B₂O₃ addition and CaO/Al₂O₃ ratios is worthy of attention. This change in the DOP of network is also closely related to the change in slag viscosity.

In this study, the basic structural parameters of the CaO–Al₂O₃–B₂O₃ melt structure were obtained by molecular dynamics (MD) simulations. The viscosities of CaO–Al₂O₃–B₂O₃ melts were obtained by rotating cylinder method. Then, the effects of adding B₂O₃ and changing the CaO/Al₂O₃ ratio on the basic structural units and the DOP of the slag were discussed in detail, and the mechanism of the improved slag fluidity was revealed from the perspective of melt structure changes.

2. Materials and Methods

2.1. Simulation Details

In the MD simulations, the approximation of the Buckingham form [17] was used as the pair potential:

$$U_{ij}(r)=\frac{q_i{q}_j}{r_{ij}}+A_{ij}\exp (-\frac{r_{ij}}{{\rho }_{ij}})-\frac{C_{ij}}{r_{ij}^6}$$

(2)

where the terms on the right side of the equation represent long-range Coulomb potential term, short-range repulsive potential term, and van der Waals force term, respectively. The potential function is explained in detail in our previous works [18,19,20]. The values of the potential parameter set used in the MD simulations are obtained from [21], as shown in

Table 1. Buckingham potential parameter of particle pairs in this study.

Pair	Aij/[eV]	ρij/[Å]	Cij/[eV·Å6]
B-B	56.25	0.16	0.00
B-Ca	4306.25	0.16	0.00
B-O	10,687.5	0.165	0.00
B-Al	483.25	0.16	0.00
Ca-Ca	329,051.6	0.16	4.335
Ca-O	717,827.0	0.165	8.67
Ca-Al	36,918.57	0.16	0.00
O-O	1,497,049.0	0.17	17.34
O-Al	86,057.58	0.165	0.00
Al-Al	4142.149	0.16	0.00

.

In the setting process of the studied systems, we refer to the theory that [AlO₄]⁵⁻ tetrahedron has a higher charge compensation order than [BO₄]⁵⁻ tetrahedron [11,12,13]. By changing the ratio of CaO/Al₂O₃, we designed components with insufficient charge compensation for [AlO₄]⁵⁻ tetrahedron by Ca²⁺ (NO. 1–4), components with sufficient charge compensation for [AlO₄]⁵⁻ tetrahedron by Ca²⁺ in theory (NO. 5–6), and groups with excessive charge compensation for [AlO₄]⁵⁻ tetrahedron by Ca²⁺ (NO. 7–10). The compositions of the studied systems are shown in

Table 2. Compositions of the studied CaO–Al₂O₃ and CaO–Al₂O₃–B₂O₃ systems.

NO.	C/A Mole Ratio	C–A System	Composition [Mole%/Mass%]			Atomic Number
			CaO	Al2O3	B2O3	Ca	Al	O	B	Total
1	0.5	C·2A	33.33/21.54	66.67/78.46	0	417	1668	2919	-	5004
2			30.47/20.03	60.95/72.93	8.58/7	373	1492	2926	210	5001
3	0.75	3C·4A	42.86/29.17	57.14/70.83	0	579	1544	2895	-	5018
4			39.35/27.13	52.48/65.87	8.17/7	515	1374	2897	214	5000
5	1	C·A	50/35.44	50/64.56	0	715	1430	2860	-	5005
6			46.09/32.96	46.09/60.04	7.82/7	637	1274	2872	216	4999
7	1.71	12C·7A	63.16/48.48	36.84/51.52	0	1020	1190	2805	-	5015
8			58.56/45.09	34.18/47.91	7.26/7	903	1054	2820	224	5001
9	2	2C·A	66.67/52.34	33.33/47.66	0	1112	1112	2780	-	5004
10			61.93/48.68	30.97/44.32	7.10/7	986	986	2804	226	5002

C represents CaO, A represents Al₂O₃.

.

The MD simulations were run in the LAMMPS program (version 22 August 2018, Sandia National Labs, Albuquerque, and Temple University, Philadelphia, PA, USA), and the simulation conditions were set up referring to our previous work [18,19,20]. Based on the phase diagram of CaO–Al₂O₃ binary system [22], the equilibrium temperature of the MD simulation was set to 2150 K, which can ensure that all slag samples can be melted. During the simulation, the initial temperature of the simulation was set to 4273 K, and 22,000 steps were run at this temperature to make the particles in the system mix uniformly and eliminate the influence of the initial state distribution. Then, the temperature of the system was slowly cooled to 2150 K through 100,000 steps. Finally, it continued to run for 80,000 steps at 2150 K to allow the system to reach equilibrium, and samples were drawn during the process. VMD (version 1.9.1, University of Illinois at Urbana-Champaign, Urbana-Champaign, Champaign, IL, USA) and MATLAB (version R2018b, The MathWorks, Inc., Natick, MA, USA) were used to analyze the data of the sampled samples, which can obtain diverse melt structure information.

2.2. Viscosity Measurement

The slags’ composition selected in the viscosity measurement experiment is determined according to the low-temperature composition zone of the CaO–Al₂O₃ phase diagram [22], as shown in

Table 3. Compositions of the viscosity measurement samples (wt pct) with different CaO/Al₂O₃ mole ratios.

Sample	C/A	CaO/[%]	Al2O3/[%]	B2O3/[%]
a	1.49	45	55	0
b	1.82	50	50	0
c	2.22	55	45	0
d	1.49	41.85	51.15	7
e	1.82	46.5	46.5	7
f	2.22	51.15	41.85	7

C/A represents CaO/Al₂O₃ mole ratios.

. The maximum temperature of the experiment is 1823K, which is the high temperature limit of the experimental equipment. In this experiment, the rotating cylinder method was used to measure the viscosity of the molten slag. The experimental equipment was RTW-16 tester (Northeastern University, Shenyang, China), including a test system, a heating system, and a rotating system. The key device of the tester is shown in Figure 1. The slag samples were prepared with pure chemical reagents CaO, Al₂O₃, and B₂O₃ (manufacturer: DAMAO chemical reagent factor, Tianjin, China). About 150 g of reagents prepared in proportion was fully mixed and put into a graphite crucible, and the crucible was put into a heating furnace. The specifications of the graphite crucible were the outer diameter of 50 mm, the inner diameter of 40 mm, and the height of 80 mm. A graphite sleeve with the same inner diameter was placed above the graphite crucible to prevent the heating furnace tube and the temperature measuring probe from being oxidized under high temperature conditions. The heating element adopted 6 U-shaped MoSi₂ heating elements, and the S-type thermocouple was used to control and measure the furnace temperature. After the temperature calibration, the temperature difference was guaranteed to be within ±0.5 K. The slag sample was heated to 1823 K according to the preset heating program, and the whole process was protected by argon gas, and the slag sample was completely melted by holding the temperature at 1823 K for 30 min. The molybdenum probe and the boom were installed in the center of the furnace tube, and the height of the furnace body was adjusted so that the lowest end of the molybdenum probe was 10 mm from the bottom of the graphite crucible. Then, the measurement system was turned on to measure the viscosity–temperature curve. When the viscosity reached 5 Pa·s, the measurement was stopped and the data were saved. The temperature of the heating furnace was raised to the temperature where the slag reached a low viscosity, and the temperature measuring probe was taken out. The test system was shut down after the furnace temperature had cooled to 573 K, as programed.

3. Results and Discussion

3.1. Radial Distribution Function (RDF) and Co-Ordination Number (CN) Distribution

The RDFs and CNs of different particle pairs in CaO–Al₂O₃–B₂O₃ melt are shown in Figure 2. In the RDFs curve, the abscissa value corresponding to the first peak in the curve represents the average bond length of the particle pair. It can be seen from Figure 2a that the average bond length of Al-O, Ca-O, B-O, and O-O is 1.76, 2.33, 1.36, and 2.76 Å, respectively. L. Cormier et al. [23] proposed the Al-O and Ca-O bond length in calcium aluminosilicate glasses is 1.76 and 2.35 Å by neutron diffraction. The B-O bond length in soda-boric oxide glass obtained by X-ray diffraction patterns is 1.37 Å [24]. The bond length values in this study are in good agreement with the experimental data, indicating that the MD simulations are reliable and accurate. Further analysis showed that the bond length of each particle pair in this study was not affected by the change in the CaO/Al₂O₃ ratio and the addition of B₂O₃, as shown in

Table 4. Bond length of different particle pairs with different CaO/Al₂O₃ mole ratio systems in this study.

C/A	System	Al-O/[Å]	Ca-O/[Å]	B-O/[Å]	O-O/[Å]
0.5	CaO–Al2O3	1.76	2.34	-	2.76
	CaO–Al2O3–B2O3	1.76	2.34	1.36	2.76
0.75	CaO–Al2O3	1.76	2.34	-	2.76
	CaO–Al2O3–B2O3	1.76	2.33	1.36	2.76
1	CaO–Al2O3	1.76	2.34	-	2.77
	CaO–Al2O3–B2O3	1.76	2.33	1.36	2.77
1.17	CaO–Al2O3	1.76	2.33	-	2.78
	CaO–Al2O3–B2O3	1.76	2.32	1.36	2.77
2	CaO–Al2O3	1.76	2.33	-	2.78
	CaO–Al2O3–B2O3	1.75	2.32	1.36	2.78

.

In the CNs curve, the ordinate value corresponding to the flat platform in the curve represents the co-ordination number of the particle pair. The co-ordination numbers of different particle pairs have changed significantly with the change in CaO/Al₂O₃ ratios and the addition of B₂O₃, as shown in

Table 5. Co-ordination numbers of different particle pairs with different CaO/Al₂O₃ mole ratio systems in this study.

C/A	System	Al-O	Ca-O	B-O	O-O	Al-Al
0.5	CaO–Al2O3	4.42	6.07	-	8.32	6.47
	CaO–Al2O3–B2O3	4.37	6.10	3.17	8.06	5.76
0.75	CaO–Al2O3	4.34	5.97	-	7.81	5.64
	CaO–Al2O3–B2O3	4.31	6.01	3.20	7.64	5.14
1	CaO–Al2O3	4.25	5.91	-	7.36	4.97
	CaO–Al2O3–B2O3	4.24	6.02	3.21	7.29	4.52
1.17	CaO–Al2O3	4.19	5.83	-	6.70	3.81
	CaO–Al2O3–B2O3	4.19	5.90	3.32	6.70	3.58
2	CaO–Al2O3	4.19	5.82	-	6.53	3.49
	CaO–Al2O3–B2O3	4.17	5.88	3.32	6.53	3.30

. The co-ordination numbers of Al-O gradually decrease with the increase in CaO/Al₂O₃ ratios and the addition of B₂O₃, and is closer to 4, which indicates the proportion of the [AlO₄]⁵⁻ structure in the system is increasing. The higher CaO/Al₂O₃ ratios in the system provide sufficient charge compensation, and the Ca²⁺ preferentially charge compensates the [AlO₄]⁵⁻ prior to charge compensating [BO₄]⁵⁻ [11,12,13]. Moreover, the addition of B₂O₃ introduces more nonbridging oxygen (NBO) or bridging oxygen (BO) that may participate in the aluminate network into the system. The above factors all contribute to the formation of [AlO₄]⁵⁻ structure. The co-ordination numbers of Ca-O decrease with the increase in CaO/Al₂O₃ ratios, while they increase with the addition of B₂O₃. This opposite trend is related to the different roles played by Ca²⁺ in the melt structure: modifying the network structure as a network modifier or providing charge compensation for [AlO₄]⁵⁻ and [BO₄]⁵⁻ structure. When Ca²⁺ plays a role in modifying the network, a Ca-NBO structure is formed in the system. Hannon and Parker [25] reported that the co-ordination number of Ca-NBO is 4. When Ca²⁺ plays the role of charge compensation, one Ca²⁺ simultaneously charges two tetrahedrons, and the co-ordination number of Ca-BO is 8. When the CaO/Al₂O₃ ratios increases, CaO introduces more free oxygen (FO) to depolymerize the network structure, and more Ca²⁺ ions are needed to modify the network structure. At this time, the systems form more Ca-NBO structures with lower co-ordination. The co-ordination numbers of Ca-O decrease. By the addition of B₂O₃ to the system, the more tetrahedrons ([AlO₄]⁵⁻ and [BO₄]⁵⁻) that need charge compensation are formed, resulting in an increase in the amount of Ca-BO with a higher co-ordination structure. The co-ordination numbers of Ca-O increase. The co-ordination numbers of B-O increase from 3.17 to 3.21 with the increase in CaO/Al₂O₃ ratios in the range where the CaO/Al₂O₃ ratio is less than 1; this slight increase indicates that the degree of conversion of [BO₃]³⁻ to [BO₄]⁵⁻ is very low, even if more cations have been added to these systems, and the priority of the structure of [BO₃]³⁻ in the system is still stable. The co-ordination numbers of B-O increase from 3.21 to 3.32 with the further increase in CaO/Al₂O₃ ratios. This is because excessive Ca²⁺ ions can charge compensate for the formation of [BO₄]⁵⁻ structure in this range of CaO/Al₂O₃ ratios, and the degree of conversion of [BO₃]³⁻ to [BO₄]⁵⁻ is enhanced.

Moreover, to investigate the effect of the B₂O₃ addition on the DOP of the entire network under different CaO/Al₂O₃ ratios, the changes in the co-ordination numbers of Al-Al and O-O are focused on. As the most important network former particles in the melt structure, the larger the co-ordination numbers of Al-Al and O-O are, the more complex the network structure of the melts is. Within the range of CaO/Al₂O₃ ratios set in this study, the co-ordination numbers of Al-Al and O-O decrease with the increase in CaO/Al₂O₃ ratios and the addition of B₂O₃. It shows that the aluminate network structure that plays a dominant role in the entire network structure is simplified with the increase in the CaO/Al₂O₃ ratios and the addition of B₂O₃, which becomes loose in spatial distribution. It is worth noting that, as the CaO/Al₂O₃ ratio increases, the effect of adding B₂O₃ to the CaO–Al₂O₃ melts to reduce the co-ordination number of O-O is weakened. This is because more [BO₄]⁵⁻ structures are formed in the system, which will increase the co-ordination number of O-O in borate network. Combining the changes in aluminate and borate network, the decrease trend of the overall co-ordination number of O-O is weakened. At high CaO/Al₂O₃ ratios, the co-ordination number of O-O before and after adding B₂O₃ is the same. Even so, the O-O structure of the CaO–Al₂O₃ melts and the CaO–Al₂O₃–B₂O₃ melts is different. In the CaO–Al₂O₃–B₂O₃ melts, there is a large amount of 2D-plane [BO₃]³⁻ structure in the overall network structure because of the addition of B₂O₃, and the DOP of the network structure is weaker than the CaO–Al₂O₃ melts. Therefore, the addition of B₂O₃ to the CaO–Al₂O₃ melts can reduce the DOP of the overall network structure in this study, which probably means that the viscosity lowers.

3.2. Distribution of Oxygen Types

It can be seen from Figure 3 and Figure 4 that there are free oxygen, non-bridging oxygen, bridging oxygen, and oxygen triclusters (TO) in the CaO–Al₂O₃ and CaO–Al₂O₃–B₂O₃ system. TO is the characteristic structure in Al₂O₃-contained systems. In the TO structure, it can be assumed that the third [AlO₄]⁵⁻ tetrahedron connecting the shared oxygen does not require charge compensation. In this way, when there are insufficient cations to charge compensation for the [AlO₄]⁵⁻ structure, three [AlO₄]⁵⁻ tetrahedrons share one apex oxygen that can balance the negative overcharge of the corresponding [AlO₄]⁵⁻ tetrahedrons [26,27]. With the increase in CaO/Al₂O₃ ratios, the content of cations increases and gradually replaces the TO structure as the main form of charge compensation for the [AlO₄]⁵⁻ structure. In theory, all the [AlO₄]⁵⁻ tetrahedrons can be compensated by the charge provided by Ca²⁺ at CaO/Al₂O₃ = 1. At this time, the system does not need deformed TO structure to balance the excess charge. Therefore, the change trend of TO appears to be a turning point at CaO/Al₂O₃ = 1 (see Figure 3b and Figure 4b).

The content of FO and NBO both increase with the increase in CaO/Al₂O₃ ratio, while the networked structural oxygen (BO and TO) decreases (see Figure 3a and Figure 4a). These results suggest that the network structure of the system is simplified with the CaO/Al₂O₃ ratio increases. Subdividing the networked structural oxygen into BO and TO, it can be found that the proportion of BO increases with the increase in CaO/Al₂O₃ ratio when the CaO/Al₂O₃ ratio is less than 1, as Figure 3b and Figure 4b show. As shown in Figure 5, with the addition of CaO within this range, there is a large amount of TO transform into BO, and this transition is stronger than the transition of BO to non-networked structural oxygen (NBO and FO), thus showing an increase in BO content. In addition, Figure 4c shows the detailed analysis of the distribution of BO and NBO types in the CaO–Al₂O₃–B₂O₃ system; it can be seen that the B-BO-B structure content is extremely small, and the added B₂O₃ mainly exists in the B-BO-Al structure, that is, the B-O structure participates in the Al-O network structure, and borate and aluminate do not exist separately.

3.3. Distribution of Qⁿ (Al) Species

Al³⁺ mainly exists in the form of tetrahedrons in this study. The [AlO₄]⁵⁻ tetrahedron structural units can be represented by Qⁿ (Al), where the superscript n represents the number of networked structural oxygen. As shown in Figure 6 and Figure 7, with the increase in CaO/Al₂O₃ ratios, the amount of Q⁴ decreases, while the amounts of Q² and Q³ increase accordingly, which is consistent with the report by Shu et al. [15]. It suggested that the Qⁿ (Al) units in both the CaO–Al₂O₃ system and the CaO–Al₂O₃–B₂O₃ system are simplified with the increase in the CaO/Al₂O₃ ratios, the aluminate network structure is depolymerized.

Table 6. The effect of the addition of B₂O₃ on the aluminate network under different CaO/Al₂O₃ mole ratios.

C/A	System	Q0 [%]	Q1 [%]	Q2 [%]	Q3 [%]	Q4 [%]
0.5	CaO–Al2O3	0	0.67	3.70	18.26	77.37
	CaO–Al2O3–B2O3	0.30	0.89	9.35	34.72	54.74
	difference	0.30	0.22	5.65	16.46	(22.63)
0.75	CaO–Al2O3	0	0.45	3.27	20.14	76.14
	CaO–Al2O3–B2O3	0.31	1.90	10.27	35.70	51.82
	difference	0.31	1.45	7.00	15.56	(24.32)
1	CaO–Al2O3	0	1.04	5.99	23.84	69.13
	CaO–Al2O3–B2O3	0	2.65	12.77	36.14	48.44
	difference	0	1.61	6.78	12.30	(20.69)
1.71	CaO–Al2O3	0.75	2.99	15.46	35.31	45.49
	CaO–Al2O3–B2O3	0.75	5.22	24.07	40.30	29.66
	difference	0	2.23	8.61	4.99	(15.83)
2	CaO–Al2O3	0.42	7.60	22.86	39.87	29.25
	CaO–Al2O3–B2O3	1.42	9.12	27.59	40.57	21.30
	difference	1	1.52	4.73	0.70	(7.95)

shows the effect of the addition of B₂O₃ on the aluminate network under different CaO/Al₂O₃ ratios. At each CaO/Al₂O₃ ratio, the addition of B₂O₃ decreases the content of Q⁴ and increases the content of other lower DOP structural units, which indicates that the addition of B₂O₃ to the CaO–Al₂O₃ melts cause the depolymerization of the aluminate network. Furthermore, we studied to what extent Q⁴ can be depolymerized by the addition of B₂O₃ under each CaO/Al₂O₃ ratio, as shown in Figure 8. With the increase in CaO/Al₂O₃ ratios, the proportions of lower DOP structural units are gradually increasing in the structural units transformed from Q⁴, which indicate that B₂O₃ has a stronger depolymerization effect on Qⁿ (Al) in the high CaO/Al₂O₃ ratio system. In the analysis of the co-ordination number of the network formers, we have pointed out that the overall DOP of the network structure decreases with the addition of B₂O₃. The depolymerization of the Qⁿ (Al) units, which play an absolutely dominant role in the entire network structure, also indicated that the DOP of the entire network structure decreases and the viscosity of the slags lowers.

3.4. Effect of B₂O₃ on Viscosity of CaO–Al₂O₃ Slag

Figure 9 shows the effect of B₂O₃ on the viscosity of CaO–Al₂O₃ slags from 1773 K to 1823 K. By comparing the measured viscosity value of the CaO–Al₂O₃–B₂O₃ system with the CaO–Al₂O₃ system, it can be found that the addition of B₂O₃ can effectively lower the viscosity of CaO–Al₂O₃ slag within the CaO/Al₂O₃ ratio range of the viscosity measurement. Although the temperature set in the experiment is different from the MD simulation, the magnitude relationship between the viscosity of the slag in the comparison group does not change above the turning point temperature, so that the magnitude relationship of viscosity at experimental temperature has reference significance for the magnitude relationship of viscosity at MD temperature. The effects of B₂O₃ addition on viscosity in measurement are consistent with our MD simulations.

4. Conclusions

In the present study, the effects of adding B₂O₃ and changing the CaO/Al₂O₃ ratio on the basic structural units and the DOP of the slag were discussed in detail, and the mechanism of the improved slag fluidity was revealed from the perspective of melt structure changes. The obtained results are summarized as follows:

(1) With the increase in the CaO/Al₂O₃ ratios, the proportion of nonstructural oxygen (free oxygen and nonbridging oxygen) increases; correspondingly, the proportion of networked structural oxygen (bridging oxygen and oxygen triclusters) decreases, the Qⁿ (Al) units become depolymerized, and the viscosity of the slag lowers.

(2) The main form of charge compensation for [AlO₄]⁵⁻ tetrahedron changes from the oxygen triclusters’ structure to the charge provided by Ca²⁺ with the increase in the CaO/Al₂O₃ ratios, and the deformed space structure is reduced, which can help stabilize the structure of [AlO₄]⁵⁻ tetrahedrons

(3) In the system where the CaO/Al₂O₃ ratio is less than 1, the added B₂O₃ mainly exists in the form of [BO₃]³⁻, which reduces the symmetry and the strength of the melt network’s structure and lowers the viscosity of the slag.

(4) In the system where the CaO/Al₂O₃ ratio is more than 1, the depolymerization ability of the addition of B₂O₃ on Qⁿ (Al) is enhanced with the increase in CaO/Al₂O₃ ratios, the depolymerization of the aluminate network structure is dominant in the overall change in the network structure, and the viscosity of the slag lowers.