With the continuous advancement of industrial technology, there is an ever-growing demand for high-quality micropowders. In the specific context of refractory materials, the drawbacks associated with traditional cement-bonded castables-such as high porosity, loose structure, and low strength-have necessitated the development of new types of refractory castables. The emergence of micropowders has undoubtedly propelled the development of refractory materials, and their application prospects in this field are vast.
Among the micropowders used in refractory castables, reactive SiO₂ powder and α-Al₂O₃ micropowder are the most prevalent, followed by SiC powder, high-alumina powder, white fused alumina powder, brown fused alumina powder, zircon powder, and spinel powder. The incorporation of micropowders significantly improves both the workability and service performance of castables, particularly regarding strength. Consequently, the properties of micropowders and their impact on the various characteristics of refractory materials have become a key area of focus for professionals in the industry in recent years.
Ultrafine powder technology is a critical component in the production of low-cement refractory castables. Typically, a particle size of 5 μm serves as the dividing line: powders with a size of 5 μm or less are classified as ultrafine powders, while those larger than 5 μm are termed micropowders. There is a wide variety of ultrafine powders and micropowders available, with silica fume and α-Al₂O₃ micropowder being the most commonly used.
Ultrafine powders significantly influence the properties of refractory materials. In the formulation of low-cement castables, the proper selection of ultrafine powder types and dosages is critical, as it directly determines the material's performance in service.
It is well known that traditional cement-bonded refractory castables rely on high cement content to achieve sufficient strength at room temperature. However, at intermediate temperatures, the crystalline phase transformation of the cement leads to a significant drop in strength. Furthermore, the cement introduces 3–10 wt% CaO, which reacts with SiO₂ and Al₂O₃ in the castable to form low-melting-point phases such as anorthite (CAS₂) or gehlenite (C₂AS), thereby compromising the material's high-temperature strength and corrosion resistance.
The incorporation of ultrafine powders and high-efficiency additives effectively addresses these issues, enabling the production of low-cement, ultra-low-cement, and cement-free castables with superior properties. These materials exhibit excellent thixotropy and maintain their strength at intermediate temperatures. They have been widely adopted in sectors such as metallurgy, building materials, petrochemicals, and power generation, delivering outstanding performance results.
While the mechanisms by which ultrafine powders function are complex, the fundamental principles are filling and lubrication. Ultrafine powders fill the voids between aggregates and matrix powders, thereby reducing the water requirement. Upon the removal of water during forming, fewer residual pores remain, leading to increased bulk density and reduced apparent porosity; this enhances the material's structural integrity and optimizes its properties. Additionally, the surfaces of ultrafine powder particles can adsorb dispersants to form a water film layer, which enhances lubrication and improves flowability, further optimizing material performance.
The following sections provide a detailed overview of the applications of SiO₂ ultrafine powder, reactive α-Al₂O₃ micropowder, α-Al₂O₃ nanopowder, and gel powder in the field of refractory materials.
1. Application of ultrafine SiO₂ powder in refractory castables
There are currently two main types of SiO₂ ultrafine powder used in refractory castables: one produced from high-purity silica, and the other obtained as a byproduct of silicon metal or ferrosilicon production. Both are amorphous materials. The former consists of inert, granular particles, whereas the latter consists of hollow, spherical particles that are reactive, resistant to agglomeration, and possess excellent filling properties. Upon incorporation into the castable and subsequent setting, silanol groups form on the SiO₂ surface; through drying, dehydration, and cross-linking, a siloxane network structure is established. This structure remains stable at elevated temperatures, thereby enhancing the intermediate-temperature strength of the castable. At high temperatures, the SiO₂ ultrafine powder reacts with Al₂O₃ to form mullite, further contributing to the material's strength. Consequently, SiO₂ ultrafine powder is widely used in low-cement, ultra-low-cement, and cement-free castables.
Ultrafine SiO₂ powder
When SiO₂ ultrafine powder is used in castables alongside an appropriate dispersant, its distinctively spherical particles easily penetrate the material's microscopic voids. Due to their small particle size, they not only provide an effective water-reducing effect but also enhance the density of the refractory castable; this reduces residual voids and porosity after drying, thereby improving both mechanical strength and high-temperature performance. Simultaneously, the reactive SiO₂ ultrafine powder forms a colloid in water; the colloidal particles adsorb the dispersant to create a solvation layer, which increases the castable's fluidity and improves its molding characteristics. Furthermore, the ultrafine nature of the SiO₂ powder-characterized by high surface free energy, numerous lattice defects, and high reactivity-facilitates solid-state sintering and mullitization reactions with the Al₂O₃ in high-alumina refractory materials at medium to high temperatures, thereby enhancing the fired strength of low-cement refractory castables.
However, if the dosage exceeds a certain threshold, the system's viscosity rises continuously as more SiO₂ ultrafine powder is added. This occurs because the SiO₂ powder reacts with water to form hydration products that undergo further polymerization; this increases molecular volume and laminar flow resistance within the slurry, leading to higher viscosity and adversely affecting the castable's workability.
Castables bonded with SiO₂ ultrafine powder have been successfully put into production. However, because these castables rely primarily on thixotropic forming and coagulation-and the filling effect of the ultrafine powder renders the material dense-there are strict requirements regarding their installation. First, water content must be strictly controlled. The spherical particles of the ultrafine powder provide a significant water-reducing effect, so the water requirement for casting is lower than that of conventional castables. For Grade I high-alumina bauxite castables, the added water content is typically around 5–6 wt%; excessive water leads to excessive fluidity, which compromises the material's strength after drying. Second, sufficient mixing time is essential to fully realize the thixotropic properties; generally, mixing for at least 5 minutes is required to fully develop this behavior. Finally, since the filling effect of the ultrafine powder largely eliminates porosity within the castable, the drying process should be conducted slowly.
MgO-based castables bonded with SiO₂ ultrafine powder exhibit excellent room-temperature physical properties. The SiO₂ ultrafine powder significantly inhibits the hydration of MgO particles, thereby greatly reducing the tendency for the castable to disintegrate into powder or crack during the firing process. Additionally, the incorporation of SiO₂ ultrafine powder imparts good rheological properties to the castable. Slag resistance is a critical factor determining the service life of MgO-based castables, and the matrix-formed by the interaction of SiO₂ ultrafine powder and magnesia powder-plays a pivotal role in this resistance. Researchers such as Wei Yaowu and Li Nan investigated the relationship between matrix composition and slag resistance in these castables; their findings indicated that the material's resistance to molten slag penetration improved as the content of SiO₂ ultrafine powder increased. However, neither excessively low nor excessively high additions were optimal for enhancing corrosion resistance; an addition level of approximately 3 wt% yielded the best results. At this level, the castable exhibited lower porosity after firing, a higher temperature for the onset of the liquid phase, and a reduced formation of low-melting-point compounds and liquid phases resulting from reactions with the slag.