Silica Fume Significantly Enhances These 4 Properties Of Refractory Castables!

In traditional refractory castables bonded with aluminate cement, the cement content generally ranges from 12% to 30%, and the water addition is 9% to 13%. A high water content results in high porosity and low strength in the castables. Although a high cement dosage can provide sufficient strength at room temperature, the strength at medium temperatures decreases significantly due to the crystal transformation of calcium aluminate. Meanwhile, CaO in the cement reacts with SiO₂ and Al₂O₃ in the castable mix to form low-melting-point phases such as anorthite or gehlenite, leading to a decline in high-temperature strength and corrosion resistance.

To overcome the above drawbacks of traditional castables, a new generation of refractory castables-the low-cement series-has been developed using micropowder technology. This series includes low-cement castables (LCC), ultra-low cement castables (ULCC), and cement-free castables. The combined use of micropowders and high-efficiency admixtures is the key technology for the low-cement series of castables. In LCC, with proper particle size distribution, micropowders exhibit good flowability and can fill almost all the voids between the main aggregate particles. As a result, the cement content can be reduced to below 8% and the water addition to 5%–7%, yielding castables with a dense microstructure, low porosity, high strength, good wear resistance, and excellent corrosion resistance.

SiO₂ micropowder, especially silica fume, is the most widely used material in the LCC series of castables. The role of silica fume in refractory castables mainly includes the following aspects:

01. Filling Effect

According to the patents of Prost and Lafarge, the cement content in refractory castables can be reduced to approximately 1% by using particles with carefully graded sizes down to the submicron scale. The application of micropowders is based on the assumption that, in castables with a standard particle size distribution, the density is limited by interparticle voids that are filled with excess water during construction. These voids are gradually filled by finer particles, which displace the water, and the remaining micropores are eventually filled with hydrated cement gel. Based on this principle, low-water-demand, high-density castable technology has been developed.

After firing at 1000 °C, the porosity of castables containing silica fume decreases from about 20%–30% to 8%–16%. In contrast, the decline in mechanical strength observed in conventional castables at medium temperatures is replaced by a steady increase. Figure 1 shows the change in bulk density of tabular alumina castables with the addition of silica fume after firing at 1000 °C for 24 h. It can be seen that the typical dosage of silica fume is 5%–7%.

02. Improving the Rheological Properties of Castables

The addition of a small amount of colloid-sized ultrafine powder to a suspension of coarse particles can significantly reduce the apparent viscosity of the suspension, thereby improving the flowability of the castable and lowering its water demand.

A comparison between activated alumina micropowder and silica fume with similar particle size distributions shows that, to achieve 100% flowability (measured by the ASTM cone test as the percentage increase in diameter after 15 seconds of vibration) at the same micropowder dosage, the water demand of castables containing activated alumina is much higher than that of castables containing silica fume, as shown in Figure 2.

03. Formation of a Network Chain Structure Bonded by Si–O–Si Linkages

A strong network chain structure bonded by Si–O–Si linkages is formed, providing the castable with excellent strength at both ambient and medium temperatures. As shown by the hydration weight gain curve of SiO₂ micropowder (Figure 3), the hydration weight gain rate of silica fume is much higher than that of crystalline SiO₂ micropowder. Infrared spectroscopy analysis confirms that Si–OH groups, similar to those in a silica gel structure, are formed on the surface of silica fume after hydration.

At approximately 40 °C, Si–OH groups begin to dehydrate and polymerize into long chains of microparticles strongly bonded by Si–O–Si linkages. This polymerization becomes most intense and nearly complete at 80 °C. These long chains form a network structure that remains stable up to 250 °C without degradation. In contrast, crystalline SiO₂ micropowder does not form this type of network structure. This network structure accounts for the high cold strength exhibited by silica fume-containing castables and products.

Moreover, castables or products containing silica fume also exhibit high strength after firing at medium temperatures, which is still attributed to the Si–O–Si network structure formed by silica fume and remaining stable above 1200 °C. Depending on the type of matrix fines in the castable, the bonding mechanism of silica fume can be classified into three categories:

① Fines containing Al₂O₃, such as various alumina powders and bauxite clinker fines. These powders generally do not undergo hydration reactions. At low temperatures, they adhere to the network structure formed by silica fume, resulting in relatively high low-temperature strength. Above 700 °C, they react with silica fume within the network to form non-stoichiometric compounds, and relatively large mullite crystals form at around 1200 °C. Due to the combined effect of interlocking acicular mullite crystals and the original network chains, the material achieves very high strength after firing at medium temperatures (approximately 1000 °C).

② Silica fume interacting with fines capable of forming hydrates, such as calcium aluminate cement, Portland cement, β-Al₂O₃, magnesia powder, etc. At low temperatures, silica fume modifies the original hydrates and forms new hydrates with them, which also develop a network structure. Except for calcium aluminate cement, the new hydrate networks formed by β-Al₂O₃ and magnesia powder with silica fume retain their morphology above 1200 °C, ensuring relatively high medium-temperature fired strength. The hydrate network formed by calcium aluminate cement and silica fume maintains its basic structure up to 1100 °C, after which ring-shaped crystals form around it, resulting in slightly lower compressive strength compared with the others.

③ Silica fume interacting with powders that undergo neither hydration nor chemical reactions with SiO₂, such as SiC and zircon (ZrSiO₄) fines. From low temperatures up to above 1200 °C, these particles adhere to the silica fume network. The stable morphology of this network in the medium-temperature range ensures considerably high strength after firing at medium temperatures.

04. Mineral Phase Transformations

A certain proportion of silica fume reacts with cement to form CASH phases, in addition to the CAH and AH phases typically observed during cement hydration. CASH phases exhibit zeolitic properties and transform into CAS₂ during heating, and may further convert into cristobalite or quartz. The amount of these hydration products is related to the purity of the silica fume. Furthermore, the addition of silica fume refines the pore size distribution of castables.

The main micropowders used in the refractory industry include SiO₂ powder, α-Al₂O₃ powder, magnesium aluminate spinel powder, mullite powder, zircon powder, zirconia powder, SiC powder, metallic Si powder, and metallic Al powder. Due to its unique morphology and its status as an industrial by-product, silica fume (a type of SiO₂ powder) is less constrained by processing equipment and cost factors. Therefore, it is the most widely used micropowder in monolithic refractories.