The taphole runner serves as a critical channel for channeling out high-temperature molten iron. During operation, it is subjected not only to severe scouring, erosion, and infiltration by flowing high-temperature molten iron and slag but also to intense thermal shock resulting from the intermittent nature of the tapping process. Intensified blast furnace smelting operations have led to higher tapping temperatures, increased flow rates of molten iron, larger volumes of tapped iron and slag, and prolonged tapping durations; consequently, the service conditions for the refractory materials used in taphole runners have become increasingly demanding. As a type of monolithic refractory material specifically designed for blast furnace taphole runners, taphole refractory castables account for over 70% of all refractory materials consumed in the blast furnace hearth area. Furthermore, the binder employed in these castables directly determines the tapping yield, the specific consumption of refractory materials per ton of iron produced, and the overall safety of hearth-side operations; thus, the selection and optimization of binders have emerged as a primary focus in the research and development of high-performance taphole castables.
1.1 Pure Calcium Aluminate Cement
Pure Calcium Aluminate Cement (CAC) is a hydraulic binder produced by the high-temperature sintering or electric-furnace electro-melting of high-purity calcium oxide and industrial alumina. Its chemical composition consists primarily of Al₂O₃ and CaO, with an Al₂O₃ mass fraction typically ranging between 70% and 80%. The principal crystalline phases responsible for CAC's hydraulic properties are monocalcium aluminate (CA) and dicalcium aluminate (CA₂)-though the dodecacalcium hepta-aluminate (C₁₂A₇) phase is also present in ultra-high-alumina CAC variants. This composition enables CAC to not only impart excellent mechanical properties to monolithic refractory materials but also endow them with superior high-temperature performance; consequently, it is currently a widely utilized binder in monolithic refractories such as taphole castables. Castables bonded with CAC require a proper curing process to ensure that the hydration products undergo complete setting and hardening. Notably, the specific hydration products formed by CAC vary depending on the curing temperature employed. When curing is conducted at temperatures below 20°C, the resulting hydration product consists of needle-like monocalcium aluminate decahydrate (CAH10). Between 21°C and 35°C, the primary hydration products are acicular-to-lamellar dicalcium aluminate octahydrate (C2AH8); above 36°C, the dominant product is granular tricalcium aluminate hexahydrate (C3AH6). These needle-like, acicular-to-lamellar, and granular hydration products overlap and interlace to form a network structure, thereby endowing the cast body with high green strength following the curing process. When selecting pure calcium aluminate cement, it is also essential to understand the hydration characteristics of its various constituent phases. Different phases exhibit vastly different heats of hydration and rates of setting and hardening. Generally, the lower the heat of hydration of a specific calcium aluminate mineral phase, the faster the setting and hardening rate of its hydration products.
1.2 Hydratable Alumina
Refractory materials for blast furnace tapholes typically utilize Al2O3-SiC-C based castables. Since the primary aluminum-bearing raw materials employed are various forms of corundum and high-grade bauxite, hydratable alumina (HA) serves as an effective binder for these refractory castables. HA is an amorphous, transitional alumina phase-specifically ρ-Al2O3-characterized by a high specific surface area (>150 m²/g). Chemically reactive, it undergoes a hydration-induced gelation reaction upon contact with water or water vapor, generating gels of pseudo-boehmite (AlOOH·nH2O, where n = 0.08–0.62) and boehmite (γ-AlOOH). This gelation process is accompanied by significant volume expansion, which helps reduce the porosity within the cast body's structure; ultimately, these gels transform into crystalline bayerite (Al(OH)3). The gelation process typically results in setting and hardening within a timeframe of less than one hour (at temperatures >25°C), thereby bonding the constituent particles together and generating green strength. Consequently, much like calcium aluminate cement (CAC), HA is classified as a hydraulic binder. It should be noted that the formation of crystalline bayerite is strongly dependent on particle size, hydration temperature, reaction time, and pH value.
1.3 Silica Sol Binders
The presence of CaO in cement-bonded castables leads to a problem wherein the volume of the liquid phase increases at high temperatures; consequently, many researchers in the field of refractory materials have considered utilizing various CaO-free binders. In addition to hydrated alumina (HA), other CaO-free binders attracting increasing attention include various types of colloidal binders or sols-such as SiO2 sol, Al2O3 sol, colloidal spinel, and colloidal mullite. Currently, the most widely applied and technologically mature calcium-free binder is silica sol. Unlike other binders, the inherent stability of the sol itself serves as the fundamental prerequisite for its binding capability. The surfaces of the solid colloidal particles within a silica sol acquire an electric charge through the dissociation of surface functional groups or the selective adsorption of specific ions from the surrounding solution. To maintain charge neutrality, the solution immediately adjacent to these charged particles must contain an equivalent amount of counter-ions bearing an opposite charge to that of the solid surface. The interface between the charged particle surface and the adjacent solution constitutes an "electrical double layer": one component is a stationary layer situated in direct contact with the solid particle surface-known as the "compact layer" or "Stern layer"-while the remaining portion (extending from the slip plane outward to the point in the bulk solution where the concentrations of positive and negative ions become equal) is termed the "diffuse layer."
Although refractory castables bonded with silica sol exhibit significantly enhanced mechanical strength following high-temperature sintering (>1000 °C) compared to those bonded with hydraulic cements (such as CAC and HA), the green strength of the unsintered (or low-temperature heat-treated) castable bodies is notably lower.
1.4 Alumina Sol Binders
In contrast to silica sol, there have been fewer studies conducted on alumina sol and its associated coagulation and hardening behaviors. Furthermore, the majority of published research findings pertain exclusively to sub-micron alumina suspensions rather than to nanoscale alumina sols. This suggests that the existing results may not be directly applicable to sols at the nanoscale. The surface charge of particles within an alumina suspension is determined by the pH value, the solid content, and the specific electrolytes employed to stabilize the suspension. In pure alumina suspensions, as the concentration of alumina increases, the surface charge shifts from positive to negative. With a further increase in suspension concentration, the magnitude of the net negative charge also increases. The "stabilization-flocculation" mechanism of alumina suspensions under ambient temperature conditions.
