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

Authors: Shuiming Cheng; Huizhong Zhao; Yu Wang; Jianxiu Wei; Jinfeng Chen; Wei Cai; Han Zhang
Journal: Advances in Materials Science and Engineering
 (2021)

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## Abstract

Submicron materials are those with particle size diameters between 0.1 and 1μm. Submicron SiO2 generally refers to SiO2 powder with a D90 < 1 μm (D90 refers to the particle size distribution exhibited by the sample and corresponds, in this case, to 90% of the particles not exceeding a diameter of 1 μm). In this study, a new type of cement-free iron ditch castable was prepared using dense corundum and silicon carbide as the primary raw materials with submicron SiO2 powder as the binder. The effects of submicron SiO2 powder content on the bulk density, apparent porosity, linear rate of change, compressive strength, and bending strength were investigated. The mechanism of action of the submicron SiO2 powder was also investigated by analyzing its microstructure and particle size distribution. The results revealed that (1) the submicron SiO2 powder can be used as the sole bonding agent in the preparation of cement-free iron ditch castables; (2) in comparison to traditional castables, the cement-free castable developed in this study demonstrated strong service performance and high-temperature bending strength.


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## Body

## 1. Introduction

The iron ditch in a blast furnace forms a fundamental component in the ironmaking process. The vast majority of blast furnace casts worldwide use Al2O3–SiC–C iron ditch castables and pure calcium aluminate cement as the bonding agent [1–3]. However, the high-temperature performance of such castables declines due to the introduction of CaO in calcium aluminate cement [4–6]. This 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 Si3N4 [7] into the matrix, which is beneficial in improving the iron slag resistance properties and high-temperature performance. Thus, 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 [13–16].To meet the demands of intensified smelting operation in blast furnaces, there is an urgent need to develop a cement-free castable for iron ditch with excellent construction and service performance.In this study, a new type of high-purity submicron SiO2 powder was characterized, and its effects on the properties of Al2O3–SiC–C iron ditch castables were investigated. The use of a submicron SiO2 powder as the sole bonding agent in the preparation of iron ditch castables was examined. This provides a novel concept for the study of cement-free iron ditch castable bonding systems.


## 2. Characterization of SiO2 Powder

### 2.1. Particle Size and Morphology Analysis

Figure1 shows the particle size (Mastersizer 2000, Malvern, UK) and morphology (JEM-2100UHR STEM/EDS, JEOL, Japan) of the submicron SiO2 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 D50 = 0.242 μm. Figure 1(b) shows a transmission electron microscope image of the submicron SiO2 powder; the particle morphology was spherical, further confirmation that it was indeed submicron powder material.Figure 1
Particle size (a) and morphology (b) of SiO2 powder.
(a)(b)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. The 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:(1)S=4/3πR3−r3/a34/3πR3/a3,=1−1−aR3,=1−1−2aD3.In equation (1), R is the average particle radius, D is the particle size, and a is the lattice constant; r = R − a.The lattice constant (a) for SiO2 is 4.9133 Å (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 SiO2 powder adopts an amorphous morphology, and the calculation results given in equation (1) are used as reference rather than absolute calculation results.The silicon atoms, 3S and 3P, in SiO2 were hybridized with SP3. There are 4 mol of Si–O bonds in 1 mol of SiO2; 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. There 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. SiO2 is a three-dimensional network structure composed of a silicon atom and an oxygen atom at a ratio of 1 ∶ 2.The bulk density of the powder material was determined to be 0.31 g/cm3, and the specific surface area was 26.53 m2/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. This indicates that the high-purity submicron SiO2 powder used in this experiment has a certain solubility in water that allows it to form a true solution of SiO2 in its molecular dispersion state [17–19], i.e., monomolecular silicic acid (H4SiO4). H4SiO4 is unstable, and the two internal hydroxyl groups are dehydrated and decomposed into metasilicic acid (H2SiO3), 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 SiO2–H2O is acidic.(2)SiO2+2H2O⟶H4SiO4,(3)H4SiO4⟶H2SiO3+H2O,(4)H2SiO3⟶HSiO3−+H+.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. The resulting colloid is referred to as silicic acid sol, which is commonly known as silica sol. This is the theoretical basis for using a silicon micropower-water system as a binder for unshaped refractory materials.


### 2.2. Chemical and Phase Composition Analysis

The submicron SiO2 powder is a pure white powder; its chemical composition (ARL Perform’X, Thermo Scientific, USA) is described in Table 1, and phase composition is shown in Figure 2.Table 1
Chemical composition of submicron ultrahigh-purity SiO2 powder (wt.%).
SiO2Al2O3Fe2O3CaOMgONa2OK2OTiO2PC99.90.010.020.010.010.010.010.0030.0060.01Figure 2
X-ray diffraction patterns of submicron high-purity SiO2 powder.As can be seen in Table1, the purity of this submicron SiO2 was as high as 99.9%, indicating extremely low impurity content. Thus, it is a high-purity submicron powder.Figure2 shows the X-ray powder diffraction (X’Pert Pro, Philips, Netherlands, Cu target, 40 kV and 40 mA) of the submicron SiO2 powder. Within the diffraction angle range of 10–90°, only one diffuse scattering amorphous peak was observed near 2θ = 21.3°. This further confirms that the SiO2 powder used in this experiment was a high-purity amorphous submicron powder.


## 2.1. Particle Size and Morphology Analysis

Figure1 shows the particle size (Mastersizer 2000, Malvern, UK) and morphology (JEM-2100UHR STEM/EDS, JEOL, Japan) of the submicron SiO2 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 D50 = 0.242 μm. Figure 1(b) shows a transmission electron microscope image of the submicron SiO2 powder; the particle morphology was spherical, further confirmation that it was indeed submicron powder material.Figure 1
Particle size (a) and morphology (b) of SiO2 powder.
(a)(b)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. The 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:(1)S=4/3πR3−r3/a34/3πR3/a3,=1−1−aR3,=1−1−2aD3.In equation (1), R is the average particle radius, D is the particle size, and a is the lattice constant; r = R − a.The lattice constant (a) for SiO2 is 4.9133 Å (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 SiO2 powder adopts an amorphous morphology, and the calculation results given in equation (1) are used as reference rather than absolute calculation results.The silicon atoms, 3S and 3P, in SiO2 were hybridized with SP3. There are 4 mol of Si–O bonds in 1 mol of SiO2; 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. There 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. SiO2 is a three-dimensional network structure composed of a silicon atom and an oxygen atom at a ratio of 1 ∶ 2.The bulk density of the powder material was determined to be 0.31 g/cm3, and the specific surface area was 26.53 m2/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. This indicates that the high-purity submicron SiO2 powder used in this experiment has a certain solubility in water that allows it to form a true solution of SiO2 in its molecular dispersion state [17–19], i.e., monomolecular silicic acid (H4SiO4). H4SiO4 is unstable, and the two internal hydroxyl groups are dehydrated and decomposed into metasilicic acid (H2SiO3), 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 SiO2–H2O is acidic.(2)SiO2+2H2O⟶H4SiO4,(3)H4SiO4⟶H2SiO3+H2O,(4)H2SiO3⟶HSiO3−+H+.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. The resulting colloid is referred to as silicic acid sol, which is commonly known as silica sol. This is the theoretical basis for using a silicon micropower-water system as a binder for unshaped refractory materials.


## 2.2. Chemical and Phase Composition Analysis

The submicron SiO2 powder is a pure white powder; its chemical composition (ARL Perform’X, Thermo Scientific, USA) is described in Table 1, and phase composition is shown in Figure 2.Table 1
Chemical composition of submicron ultrahigh-purity SiO2 powder (wt.%).
SiO2Al2O3Fe2O3CaOMgONa2OK2OTiO2PC99.90.010.020.010.010.010.010.0030.0060.01Figure 2
X-ray diffraction patterns of submicron high-purity SiO2 powder.As can be seen in Table1, the purity of this submicron SiO2 was as high as 99.9%, indicating extremely low impurity content. Thus, it is a high-purity submicron powder.Figure2 shows the X-ray powder diffraction (X’Pert Pro, Philips, Netherlands, Cu target, 40 kV and 40 mA) of the submicron SiO2 powder. Within the diffraction angle range of 10–90°, only one diffuse scattering amorphous peak was observed near 2θ = 21.3°. This further confirms that the SiO2 powder used in this experiment was a high-purity amorphous submicron powder.


## 3. Application of Submicron SiO2 Powder in Cement-Free Iron Ditch Castable

### 3.1. Raw Materials and Sample Ratio

The main raw materials used in the experiment were tabular alumina,w (Al2O3) = 99.07%, SiC, w (SiC) = 98.12%, spherical asphalt, w (C) = 56.21%, dense corundum powder, w (Al2O3) = 99.56%, and w (Na2O) = 0.08%. The experimental formulations are presented in Table 2.Table 2
Formulations for the experiment.
Materials and content (wt)/(%)Particle size rangeABCDEFGTabular alumina0.1–8 mm60606060606060SiC0.5–1 mm18181818181818Spherical asphalt0.1–1 mm3333333Water-reducing agent—0.10.10.10.10.10.10.1Metal silicon≤0.05 mm1.51.51.51.51.51.51.5Submicron SiO2 powderD90 = 0.454 μm3456789Dense corundum powder≤0.06 mm14.413.412.411.410.49.48.4Water addition (flow value = 170 mm)—4.64.03.63.43.33.23.2


### 3.2. Sample Preparation and Performance Testing

The ingredients used in this study are listed in Table2. 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.The flow value of the mixture was tested according to YB/T 5202.1–2003 (Black Metallurgy Industry Standard of the People’s Republic of China). A shaking table was used to vibrate the mixture and obtain strip samples of 40 mm × 40 mm × 160 mm; then, the samples were cured under 25°C for 24 h to facilitate demolding. After demolding, the samples were naturally dried at 25°C for 24 h before being subjected to one of the following drying regimens: 110°C × 24 h, 1000°C × 3 h, and 1450°C × 3 h (high-temperature treatment).The bulk density and apparent porosity of the samples subjected to 1450°C for 3 h were tested in accordance with GB/T 2997–2015 (National Standards of the People’s Republic of China); the compressive strengths of the samples that were dried and heat-treated at different temperatures were tested in accordance with GB/T 5072–2008; the flexural strength of the samples that were dried and heat-treated at different temperatures was tested in accordance with GB/T 3001–2017. Additionally, the high-temperature (1400°C × 1 h) flexural strength of the samples dried in ambient air was tested in accordance with GB/T 3002–2017.


## 3.1. Raw Materials and Sample Ratio

The main raw materials used in the experiment were tabular alumina,w (Al2O3) = 99.07%, SiC, w (SiC) = 98.12%, spherical asphalt, w (C) = 56.21%, dense corundum powder, w (Al2O3) = 99.56%, and w (Na2O) = 0.08%. The experimental formulations are presented in Table 2.Table 2
Formulations for the experiment.
Materials and content (wt)/(%)Particle size rangeABCDEFGTabular alumina0.1–8 mm60606060606060SiC0.5–1 mm18181818181818Spherical asphalt0.1–1 mm3333333Water-reducing agent—0.10.10.10.10.10.10.1Metal silicon≤0.05 mm1.51.51.51.51.51.51.5Submicron SiO2 powderD90 = 0.454 μm3456789Dense corundum powder≤0.06 mm14.413.412.411.410.49.48.4Water addition (flow value = 170 mm)—4.64.03.63.43.33.23.2


## 3.2. Sample Preparation and Performance Testing

The ingredients used in this study are listed in Table2. 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.The flow value of the mixture was tested according to YB/T 5202.1–2003 (Black Metallurgy Industry Standard of the People’s Republic of China). A shaking table was used to vibrate the mixture and obtain strip samples of 40 mm × 40 mm × 160 mm; then, the samples were cured under 25°C for 24 h to facilitate demolding. After demolding, the samples were naturally dried at 25°C for 24 h before being subjected to one of the following drying regimens: 110°C × 24 h, 1000°C × 3 h, and 1450°C × 3 h (high-temperature treatment).The bulk density and apparent porosity of the samples subjected to 1450°C for 3 h were tested in accordance with GB/T 2997–2015 (National Standards of the People’s Republic of China); the compressive strengths of the samples that were dried and heat-treated at different temperatures were tested in accordance with GB/T 5072–2008; the flexural strength of the samples that were dried and heat-treated at different temperatures was tested in accordance with GB/T 3001–2017. Additionally, the high-temperature (1400°C × 1 h) flexural strength of the samples dried in ambient air was tested in accordance with GB/T 3002–2017.


## 4. Results and Discussion

### 4.1. Effects of Submicron SiO2 Powder Content on the Density of Samples

The physical properties of the samples treated at different temperatures are shown in Figure3. 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 SiO2 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 SiO2 powder content, shown in Figure 3(b).Figure 3
Physical properties of samples sintered at 1450°C for 3 h.
(a)(b)


### 4.2. Effects of Submicron SiO2 Powder Content on the Mechanical Properties

Figure4 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 SiO2 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 SiO2 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 SiO2 powder content was 5 wt%. This may be due to the highest density and the best direct bonding degree of the sample.Figure 4
Mechanical properties of specimens treated at different temperatures: (a) compressive and flexural strength of samples dried at 110°C for 24 h; (b) compressive and flexural strength of samples sintered at 1000°C for 3 h; (c) compressive and flexural strength of samples sintered at 1450°C for 3 h; (d) high-temperature flexural strength of samples sintered at 1400°C for 1 h.
(a)(b)(c)(d)


### 4.3. Scanning Electron Microscopy (SEM) Analysis

Figure5 shows the scanning electron microscopy (SEM, JSM-6610, JEOL, Japan) images of samples with different submicron SiO2 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 SiO2 powder had higher porosity and weak aggregate/matrix bonding. Increasing the submicron SiO2 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 SiO2 powder content to 7 wt% resulted in the occurrence of pores in the matrix. This may be due to the formation of Si–OH–Si bonds, which occurred as a result of the hydration of a greater volume of SiO2, that produced pores during the process of dehydration when the samples were sintered at high temperatures. Further increasing the submicron SiO2 powder to 9 wt% resulted in further weakened matrix bonding and more pores.Figure 5
SEM images of samples with different submicron SiO2 powder contents: (a) 3 wt%; (b) 5 wt%; (c) 7 wt%; (d) 9 wt%.
(a)(b)(c)(d)In addition, combined with the water addition amount (the flow value of all mixtures was controlled to approximately 170 mm shown in Table2), it is shown that the submicron SiO2 powder exhibits a strong micropowder filler function. At present, the D90 and D50 of most condensed SiO2 micropowders in China are approximately 7.599 μm and 0.416 μm, respectively, whereas the D90 and D50 of the submicron SiO2 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.The samples that were 4 wt% submicron SiO2 powder were found to have high demolding strength after being cured for 24 h. Additionally, increasing the content of submicron SiO2 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 SiO2 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 SiO2 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 stirring time was significantly lengthened, and there was no significant increase in the dried strength of the sample. Thus, the optimum submicron SiO2 powder content in an iron ditch castable was determined to be within the range between 4 and 6 wt%.Amorphous SiO2 has a short-range ordered structure, as shown in Figure 6. When water is added to the iron ditch castable, the amorphous SiO2 reacts with the OH− of the water on the surface, resulting in the hydroxylation of SiO2. Then, the hydroxylated SiO2 adsorbs water; during this time, the adsorbed water on the surface of the aggregate and matrix quenches the SiO2, forming a three-dimensional network bonding in the castable body. This structure is known to be associated with high strength; thus, the proposed castable has high strength. This process is illustrated in Figure 7.Figure 6
Unit cell of silica.Figure 7
Schematic of the processes of amorphous SiO2 hydroxylation and water adsorption [20].The experimental results show that the submicron SiO2 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.


## 4.1. Effects of Submicron SiO2 Powder Content on the Density of Samples

The physical properties of the samples treated at different temperatures are shown in Figure3. 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 SiO2 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 SiO2 powder content, shown in Figure 3(b).Figure 3
Physical properties of samples sintered at 1450°C for 3 h.
(a)(b)


## 4.2. Effects of Submicron SiO2 Powder Content on the Mechanical Properties

Figure4 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 SiO2 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 SiO2 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 SiO2 powder content was 5 wt%. This may be due to the highest density and the best direct bonding degree of the sample.Figure 4
Mechanical properties of specimens treated at different temperatures: (a) compressive and flexural strength of samples dried at 110°C for 24 h; (b) compressive and flexural strength of samples sintered at 1000°C for 3 h; (c) compressive and flexural strength of samples sintered at 1450°C for 3 h; (d) high-temperature flexural strength of samples sintered at 1400°C for 1 h.
(a)(b)(c)(d)


## 4.3. Scanning Electron Microscopy (SEM) Analysis

Figure5 shows the scanning electron microscopy (SEM, JSM-6610, JEOL, Japan) images of samples with different submicron SiO2 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 SiO2 powder had higher porosity and weak aggregate/matrix bonding. Increasing the submicron SiO2 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 SiO2 powder content to 7 wt% resulted in the occurrence of pores in the matrix. This may be due to the formation of Si–OH–Si bonds, which occurred as a result of the hydration of a greater volume of SiO2, that produced pores during the process of dehydration when the samples were sintered at high temperatures. Further increasing the submicron SiO2 powder to 9 wt% resulted in further weakened matrix bonding and more pores.Figure 5
SEM images of samples with different submicron SiO2 powder contents: (a) 3 wt%; (b) 5 wt%; (c) 7 wt%; (d) 9 wt%.
(a)(b)(c)(d)In addition, combined with the water addition amount (the flow value of all mixtures was controlled to approximately 170 mm shown in Table2), it is shown that the submicron SiO2 powder exhibits a strong micropowder filler function. At present, the D90 and D50 of most condensed SiO2 micropowders in China are approximately 7.599 μm and 0.416 μm, respectively, whereas the D90 and D50 of the submicron SiO2 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.The samples that were 4 wt% submicron SiO2 powder were found to have high demolding strength after being cured for 24 h. Additionally, increasing the content of submicron SiO2 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 SiO2 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 SiO2 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 stirring time was significantly lengthened, and there was no significant increase in the dried strength of the sample. Thus, the optimum submicron SiO2 powder content in an iron ditch castable was determined to be within the range between 4 and 6 wt%.Amorphous SiO2 has a short-range ordered structure, as shown in Figure 6. When water is added to the iron ditch castable, the amorphous SiO2 reacts with the OH− of the water on the surface, resulting in the hydroxylation of SiO2. Then, the hydroxylated SiO2 adsorbs water; during this time, the adsorbed water on the surface of the aggregate and matrix quenches the SiO2, forming a three-dimensional network bonding in the castable body. This structure is known to be associated with high strength; thus, the proposed castable has high strength. This process is illustrated in Figure 7.Figure 6
Unit cell of silica.Figure 7
Schematic of the processes of amorphous SiO2 hydroxylation and water adsorption [20].The experimental results show that the submicron SiO2 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.


## 5. Conclusions

(1)
The SiO2 powder used in this experiment was a high-purity submicron powder material.(2)
The submicron SiO2 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. The optimum submicron SiO2 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%.



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