Optimal Silica Fume Addition For Maximum Performance Of Alumina-Silica Castables

Silica micropowder has a good effect on water reduction and flow promotion, and it has been widely used in magnesium-based castables, bauxite-based castables, and other types of castables. By adding silica fume, a mullite phase can be generated with Al₂O₃ in alumina castables under high-temperature conditions, thereby improving high-temperature strength. However, in cement-bonded bauxite castables, especially in the production of prefabricated parts that require room temperature strength for demoulding, the introduction of additional Ca²⁺ ions leads to the formation of calcium feldspar, calcium yellow feldspar, and other low-melting substances at high temperatures, which can reduce high-temperature strength. For this reason, the effect of silica fume addition on the performance of castables using cement as a binding agent was investigated.

01 test

1.1 Raw Materials

The raw materials used in the test are as follows: 88 high-alumina bauxite (8~1 mm, ≤1 mm, ≤0.045 mm), white corundum (≤0.045 mm), 951U silica fume, S71 cement, and a water-reducing agent. The main chemical compositions of some of the raw materials are shown in Table 1.

1.2 Experimental Procedure

Table 2 shows the proportion of the specimen. According to Table 2, the ingredients were prepared and dry-mixed thoroughly, then stirred with water in a mixer. The mixture was vibration-molded into a specimen with dimensions of 40 mm × 40 mm × 160 mm. The specimen was removed from the mold after being kept at room temperature for 24 hours. It was then dried at 110°C for 24 hours and heat-treated at 110°C for 24 hours, 1000°C for 3 hours, and 1500°C for 3 hours, respectively.

1.3 Performance Testing

The flexural and compressive strength of the dried and fired samples were measured in accordance with YB/T 5201-1993. The apparent porosity and bulk density of the dried and fired samples were measured according to YB/T 5200-1993. The high-temperature flexural strength of the dried samples at 110°C for 24 hours was measured in accordance with GB/T 3002-1982 (1450°C heat preservation for 1 hour in an air atmosphere).

02 Results and Discussion

2.1 Water Addition

Figure 1 shows the trend of water addition with increasing silica fume content. As the amount of silica fume increases, the amount of water added initially decreases and then increases. When the silica fume content increased from 2% to 6%, the viscosity of the slurry gradually decreased. Silica fume reacts with water to form the following colloidal structure of H₂SiO₃: {[SiO₂]ₘ,ₙSiO₂₋₃₋₂(n₋��)H⁺}₂��H⁺. Under the action of the dispersant, the colloidal particles are dispersed on the surface of the double layer of electrostatic repulsive force, preventing inter-particle adsorption and agglomeration. On the other hand, the ultra-micro-powder in the castables helps fill the voids, and these two factors together enhance the fluidity of the slurry, thereby reducing the amount of water required. When the amount of silica fume added exceeds 6%, the voids in the casting material are filled, and the excess silica fume increases the viscosity of the slurry. This is because silica fume reacts with water to generate hydration products, which further polymerize, increasing the resistance to flow. As a result, more water is needed to ensure proper molding.

2.2 Bulk Density and Apparent Porosity

As the amount of silica fume increases, the bulk density of the specimen after heat treatment at 110°C for 24 hours, 1000°C for 3 hours, and 1500°C for 3 hours first increases and then decreases. The porosity decreases and then increases (see Fig. 2). This is because the degree of void filling increases with the addition of silica fume, while the amount of water added decreases and then increases. During heat treatment, water escapes from the specimen, and the porosity left behind first increases and then decreases, causing the bulk density to increase and then decrease. After heat treatment, the water escapes, leaving pores that initially decrease and then increase, resulting in the body density first increasing and then decreasing.

During the 1500°C for 3 hours heat treatment, silica fume and alumina micropowder reacted with mullite, causing expansion and reducing the porosity by squeezing the pores. However, when the silica fume content exceeded 6%, the effect of porosity caused by the volatilization of water became greater than the reduction in porosity due to expansion and pore compression.

2.3 Line Change Rate

Figure 3 shows the trend of the line change rate of the specimens with varying silica fume content after heat treatment at 1000°C for 3 hours and 1500°C for 3 hours. The line change rate of the specimen gradually increases after treatment at 1000°C for 3 hours. This is due to the reaction with Al₂O₃ in the powder or cement once the temperature exceeds 900°C, which gradually forms mullite and produces a volumetric expansion that offsets the contraction caused by water volatilization, among other factors. The more silica fume that is added, the more pronounced this effect becomes.

After heat treatment at 1500°C for 3 hours, the specimens initially expand and then contract, reaching the maximum change rate when the silica fume content is 6%. During the 1500°C for 3 hours treatment, in addition to the expansion effect caused by silica fume and mullite in alumina, alumina ultrafine powder (CL370) can induce the formation of more calcium hexa-aluminate from calcium aluminate under high temperature conditions, which prevents volumetric contraction. However, when the silica fume content exceeds 6%, more low-melting materials, such as calcium feldspar, are generated at high temperatures, leading to contraction of the sample.

2.4 Room Temperature Strength

The effect of silica fume addition on the room temperature strength of the specimens is shown in Fig. 4.

As shown in Figs. 4(a) and (b), the flexural and compressive strengths of the specimens after heat treatment at 110°C for 24 hours and 1000°C for 3 hours both increase and then decrease with increasing silica fume content, reaching a maximum at 6%. Under the 110°C treatment condition, the strength of the castables results from the combined action of cement hydration and cohesive bonding. Cement hydration forms a crystalline network structure, while silica fume forms silanol groups on the surface of SiO₂ when it reacts with water. After drying, dehydration, and bridging, it forms a siloxane network structure, leading to hardening. As a result, the more silica fume is added, the higher the strength.

During the 1000°C for 3 hours treatment, the specimen matrix generates acicular mullite. The more silica fume is added, the greater the amount of mullite formed, and consequently, the higher the strength. However, when the silica fume content exceeds 6%, the increase in water content leads to a decrease in body density and an increase in porosity, resulting in reduced strength after both 110°C for 24 hours and 1000°C for 3 hours treatments.

During the 1500°C for 3 hours treatment, the SiO₂ mesh structure reacts with Al₂O₃ to form needle-like mullite, while CA₆ and calcium feldspar are also produced. CA₆ has a columnar and needle-like form, and calcium feldspar appears as fine columnar structures. Together, these materials form a cross-linked skeleton of columnar and needle-like shapes, making the structure more solid and dense, which increases the strength.

2.5 High-Temperature Flexural Strength

Figure 5 shows the variation in high-temperature flexural strength of the specimens with increasing silica fume content during heat treatment at 1450°C for 1 hour.

The high-temperature flexural strength of the specimen gradually increases with the addition of silica fume. This is because, at 1450°C, mullite, calcium yellow feldspar (C₂AS), calcium feldspar (CaS₂), CA₆, and other phases are generated. When the silica fume content is less than 5%, the main minerals formed are calcium feldspar, calcium yellow feldspar, and other low-melting materials, which result in lower high-temperature flexural strength. However, when the silica fume content exceeds 6%, high-temperature phases such as acicular mullite, columnar CA₆, and other high-temperature phases are formed. These phases create a cross-linked skeletal structure, which improves the high-temperature strength.

03. Conclusion

As the amount of silica fume increases, the porosity of the specimen initially decreases and then increases, while the body density first increases and then decreases. The flexural and compressive strengths gradually increase after heat treatment at 1500°C for 3 hours. However, the high-temperature flexural strength at 1450°C for 1 hour initially increases and then decreases. The specimen achieves the best overall performance when the silica fume content is 6%.