Submicron SiO2 Powder: Characterization and Effects on Properties of Cement-Free Iron Ditch Castables

1 e State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science & Technology, Wuhan Research Institute of Metallurgy Construction Co. Ltd of MCC Group, Wuhan, China Wuhan University of Science and Technology, Wuhan, China 3 e State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, China Wuhan Research Institute of Metallurgy Construction Co. Ltd of MCC Group, Wuhan, China


Introduction
e iron ditch in a blast furnace forms a fundamental component in the ironmaking process. e vast majority of blast furnace casts worldwide use Al 2 O 3 -SiC-C iron ditch castables and pure calcium aluminate cement as the bonding agent [1][2][3]. However, the high-temperature performance of such castables declines due to the introduction of CaO in calcium aluminate cement [4][5][6]. is shortcoming cannot be optimized irrespective of the purity of the refractory raw materials. In addition, during the process of fabricating cement-bonded iron ditch castables and as a result of strong basic properties of cement, the comprising hydroxide ions can dissolve metal aluminum to produce aluminum hydroxide, which can cause metal aluminum powder to react with water to produce a large amount of hydrogen; this leads to porosity and even cracking of the material. Moreover, the bonding system containing cement also limits the introduction of additives such as Si 3 N 4 [7] into the matrix, which is beneficial in improving the iron slag resistance properties and high-temperature performance.
us, to solve this problem, refractory workers have carried out extensive research on cement-free combinations of iron ditch castables, and the general method of fabrication entails replacing pure calcium aluminate cement with silica sol or hydraulic hydrated alumina [8]. However, the applicability of silica sol is limited [9,10] due to the disadvantages such as the reduced early strength exhibited by iron ditch castables bonded using silica sol while the time available for construction is severely limited and there is the high susceptibility to freezing in winter. In addition, its application offers no clear advantage over cement-incorporated iron trench castables [11,12].
Iron ditch castables combined with hydraulic hydrated alumina exhibit not only low strength but also poor stability. Particularly, the speed of hydration is strongly dependent on temperature; as an example, more time and higher temperatures are required to eliminate water crystals, which can produce cracks and burst, thus affecting the structural density of the constructed body [13][14][15][16].
To meet the demands of intensified smelting operation in blast furnaces, there is an urgent need to develop a cementfree castable for iron ditch with excellent construction and service performance.
In this study, a new type of high-purity submicron SiO 2 powder was characterized, and its effects on the properties of Al 2 O 3 -SiC-C iron ditch castables were investigated. e use of a submicron SiO 2 powder as the sole bonding agent in the preparation of iron ditch castables was examined. is provides a novel concept for the study of cement-free iron ditch castable bonding systems. Figure 1 shows the particle size (Mastersizer 2000, Malvern, UK) and morphology (JEM-2100UHR STEM/EDS, JEOL, Japan) of the submicron SiO 2 powder used in the experiment. As can be seen in the particle size distribution diagram in Figure 1(a), the particle size distribution range was narrow, ranging from 0.1 to 1.0 μm, with D 50 � 0.242 μm. Figure 1(b) shows a transmission electron microscope image of the submicron SiO 2 powder; the particle morphology was spherical, further confirmation that it was indeed submicron powder material.

Particle Size and Morphology Analysis.
Smaller particle sizes of the powder material are associated with higher specific surface areas and higher proportion of surface atoms, which tends to increase the reactivity and surface energy of the powder material. e number of atoms on the surface of the powder material is equal to the total number of atoms in the powder, which can be calculated by solving the following equation: In equation (1), R is the average particle radius, D is the particle size, and a is the lattice constant; r � R − a. e lattice constant (a) for SiO 2 is 4.9133Å (0.49133 nm), and the interatomic distance was set to be 0.3 nm. For the calculation, a in equation (1) was set to be 0.79 nm, and the percentage of atoms on the surface of the powder material, i.e., S, was calculated to be 2.35%, indicating that although the powder has a certain degree of reactivity, the activity is not strong. It should be noted that the submicron SiO 2 powder adopts an amorphous morphology, and the calculation results given in equation (1) are used as reference rather than absolute calculation results. e silicon atoms, 3S and 3P, in SiO 2 were hybridized with SP3. ere are 4 mol of Si-O bonds in 1 mol of SiO 2 ; thus, the basic structural unit is a tetrahedron. Each silicon atom is bound to four oxygen atoms; there is a silicon atom in the center and an oxygen atom at the four vertices. ere are also six silicon atoms and six oxygen atoms in the smallest ring. Many of these tetrahedrons are connected by an oxygen atom at the top corners, and each oxygen atom is shared by two tetrahedrons, i.e., each oxygen atom is connected to two silicon atoms. SiO 2 is a three-dimensional network structure composed of a silicon atom and an oxygen atom at a ratio of 1 ∶ 2.
e bulk density of the powder material was determined to be 0.31 g/cm 3 , and the specific surface area was 26.53 m 2 /g. Ten grams of the powder was uniformly dispersed in 100 g of deionized water. Following this was a 30 min period of ultrasonic dispersion; the dispersion solution was determined to have an acidic pH value of 3.51 as measured by a pH meter (Mettler-Toledo FE-28-Standard, manufactured by Mettler-Toledo International Trading (Shanghai) Co., Ltd), whereas the pH value of the aqueous dispersion solution of general silicon powder was determined to be approximately 7. is indicates that the high-purity submicron SiO 2 powder used in this experiment has a certain solubility in water that allows it to form a true solution of SiO 2 in its molecular dispersion state [17][18][19], i.e., monomolecular silicic acid (H 4 SiO 4 ). H 4 SiO 4 is unstable, and the two internal hydroxyl groups are dehydrated and decomposed into metasilicic acid (H 2 SiO 3 ), as described by equations (2)-(4). Metasilicic acid is a weak acid with a steady ionization constant of 2 × 10 −10 (under 25°C), which can ionize H + ; thus, the dispersion system of submicron SiO 2 -H 2 O is acidic.
Monomolecular silicic acid is soluble in water, but it gradually associates into bimolecular and trimolecular units in the solution, eventually forming an insoluble multimolecule polymer. e resulting colloid is referred to as silicic acid sol, which is commonly known as silica sol. is is the theoretical basis for using a silicon micropower-water system as a binder for unshaped refractory materials.

Chemical and Phase Composition Analysis.
e submicron SiO 2 powder is a pure white powder; its chemical composition (ARL Perform'X, ermo Scientific, USA) is described in Table 1, and phase composition is shown in Figure 2.
As can be seen in Table 1, the purity of this submicron SiO 2 was as high as 99.9%, indicating extremely low impurity content. us, it is a high-purity submicron powder. Figure 2 shows the X-ray powder diffraction (X'Pert Pro, Philips, Netherlands, Cu target, 40 kV and 40 mA) of the submicron SiO 2 powder. Within the diffraction angle range of 10-90°, only one diffuse scattering amorphous peak was observed near 2θ � 21.3°. is further confirms that the SiO 2 powder used in this experiment was a high-purity amorphous submicron powder.  Table 2.

Sample Preparation and Performance Testing.
e ingredients used in this study are listed in Table 2. After the mixture was dry mixed for 60 s, a certain volume of water was added; the flow value of all mixtures was controlled to approximately 170 mm by measuring the flow value of the mixture several times. e flow value of the mixture was tested according to YB/T 5202.    Advances in Materials Science and Engineering 3

Effects of Submicron SiO 2 Powder Content on the Density of
Samples. e physical properties of the samples treated at different temperatures are shown in Figure 3. As shown in Figure 3(a), the samples that were sintered at 1450°C for 3 h were observed to have the highest bulk density and lowest apparent porosity when the submicron SiO 2 powder addition amount was 5%. Additionally, the linear rate of change for the samples treated at 1450°C for 3 h increased with increasing submicron SiO 2 powder content, shown in Figure 3(b). Figure 4 shows the mechanical properties exhibited by samples treated at various temperatures. As shown in Figure 4, the samples dried at 110°C for 24 h exhibited greater compressive and flexural strength when the submicron SiO 2 powder content was between 6 and 8 wt%. Samples sintered at 1000°C for 3 h exhibited greater compressive and flexural strength when the submicron SiO 2 powder content was between 5 and 8 wt%. Moreover, the compressive and flexural strength exhibited by the samples sintered at 1450°C for 3 h, as well as the high-temperature flexural strength exhibited by the samples sintered at 1400°C for 1 h; both reached their maximum values when the submicron SiO 2 powder content was 5 wt%. is may be due to the highest density and the best direct bonding degree of the sample. Figure 5 shows the scanning electron microscopy (SEM, JSM-6610, JEOL, Japan) images of samples with different submicron SiO 2 powder contents that have been sintered at 1450°C for 3 h. As can be seen in Figure 5, the sample with 3 wt% of submicron SiO 2 powder had higher porosity and weak aggregate/matrix bonding. Increasing the submicron SiO 2 powder content to 5 wt% resulted in stronger matrix bonding and a clear increase in the density of the sample. However, further increase in the submicron SiO 2 powder content to 7 wt% resulted in the occurrence of pores in the matrix.

Scanning Electron Microscopy (SEM) Analysis.
is may be due to the formation of Si-OH-Si bonds, which occurred as a result of the hydration of a greater volume of SiO 2 , that produced pores during the process of dehydration when the samples were sintered at  A  B  C  D  E  F  G  Tabular alumina  0.1-8 mm  60  60  60  60  60  60  60  SiC  0.5-1 mm  18  18  18  18  18  18  18  Spherical asphalt  0.1-1 mm  3  3  3  3  3  3  3  Water-reducing   high temperatures. Further increasing the submicron SiO 2 powder to 9 wt% resulted in further weakened matrix bonding and more pores.
In addition, combined with the water addition amount (the flow value of all mixtures was controlled to approximately 170 mm shown in Table 2), it is shown that the submicron SiO 2 powder exhibits a strong micropowder filler function. At present, the D 90 and D 50 of most condensed SiO 2 micropowders in China are approximately 7.599 μm and 0.416 μm, respectively, whereas the D 90 and D 50 of the submicron SiO 2 powders used in this study were 0.454 μm and 0.242 μm, respectively. As shown in Figure 1, having smaller-sized particles corresponds to an increased interspace filling of the powder, which leads to higher reactivity. e samples that were 4 wt% submicron SiO 2 powder were found to have high demolding strength after being cured for 24 h. Additionally, increasing the content of submicron SiO 2 powder to 5 wt% corresponded to an increase in the viscosity of the castable slurry; however, the medium-high temperature strength of the sample did not significantly increase. Further increase in the submicron SiO 2 powder content to 6 wt% did not coincide with a significant decrease in the water content of the castable; however, the speed of solidification was found to have accelerated. Finally, when the submicron SiO 2 powder content reached 7-8 wt%, the viscosity of the castable was found to improve, while the volume of water required to meet the construction condition also increased, the required  Advances in Materials Science and Engineering stirring time was significantly lengthened, and there was no significant increase in the dried strength of the sample. us, the optimum submicron SiO 2 powder content in an iron ditch castable was determined to be within the range between 4 and 6 wt%. Amorphous SiO 2 has a short-range ordered structure, as shown in Figure 6. When water is added to the iron ditch castable, the amorphous SiO 2 reacts with the OH − of the water on the surface, resulting in the hydroxylation of SiO 2 . en, the hydroxylated SiO 2 adsorbs water; during this time, the adsorbed water on the surface of the aggregate and matrix quenches the SiO 2 , forming a three-dimensional network bonding in the castable body.
is structure is known to be associated with high strength; thus, the proposed castable has high strength. is process is illustrated in Figure 7.
e experimental results show that the submicron SiO 2 powder can be used as a binder for cement-free iron ditch castables, indicating that the realization of cement-free iron ditch castables is possible.

Conclusions
(1) e SiO 2 powder used in this experiment was a highpurity submicron powder material. (2) e submicron SiO 2 powder can be used as the sole bonding agent for the production of iron ditch castables, thereby offering a cement-free option for iron ditch castables. (3) As compared with conventional castables, the cement-free iron ditch castable developed in this study had a significantly lower water content (minimum of 3.2%). Moreover, the high-temperature flexural strength was significantly higher, with the maximum exceeding 13 MPa. e optimum submicron SiO 2 powder content for the type of iron ditch castable developed in this study was determined to be within the range between 4 and 6 wt%.

Data Availability
e data used to support the findings of this study are included within the article.