Refractory Blog Archives - Rongsheng Refractory Castable Manufacturer

Characteristics of Tabular Alumina (Tabular Corundum)

November 10, 2026November 8, 2026 by rsrefractorycastable

Once the lining (installed structure) made of refractory castable has hardened, it must undergo a drying process to remove internal moisture before it can be put into service. This process involves the characteristics inherent to tabular alumina (tabular corundum). In the case of cement-free, all-alumina (100% Al2O3) refractory castables, the mass decreases over time during the drying process as water is expelled. This results in an increase in the material’s apparent porosity and a rise in its gas permeability.

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Theoretically, different particle size distributions (PSDs) might be expected to result in varying levels of porosity. However, experimental results indicate that cement-free, all-alumina (100% Al2O3) refractory castables—prepared with different PSDs but the same water content—exhibit similar apparent porosity after drying. With a water content of 14% (by volume), the apparent porosity in each case is approximately 11.9%. These open pores are formed as moisture is expelled from the refractory castable during the drying process.

Given that the open porosity (apparent porosity) of these refractory castables is similar, differences in their gas permeability are likely primarily attributable to variations in pore distribution, which in turn stem from the distinct particle size distributions of each material.

How can high-performance refractory castables be obtained?

The performance of a castable depends, to a certain extent, on the appropriate selection of drying and heating conditions. High-alumina castables contain significant amounts of moisture—both free water and chemically bound (hydrated) water—which are released between 100°C and 550°C. During the initial heating phase, free water is expelled first, followed by the water released from hydrated phases. To ensure a successful installation, the following conditions must be maintained:

(1) During installation, ensure that the temperature of both the dry mix and the mixing water is approximately 20°C or higher.

(2) After installation, the curing temperature must not be lower than the temperature of the dry mix and mixing water at the time of preparation.

(3) The drying rate has a significant impact on the performance of the castable. Rapid heating causes a sharp rise in internal vapor pressure, which can lead to explosive spalling as the steam attempts to escape. Therefore, metal additives (such as aluminum) and organic fibers can be incorporated into dense castables to facilitate drying. The heating process should follow this schedule:

Heat from ambient temperature to 110°C at a rate of 15°C/h; hold at 110°C for 1 hour per 25 mm of thickness.
Heat from 110°C to 300°C at a rate of 15°C/h; hold at 300°C for 1 hour per 25 mm of thickness.
Heat from 300°C to 550°C at a rate of 25°C/h; hold at 550°C for 1 hour per 15 mm of thickness.
Heat from 550°C to the final operating temperature at a rate of 75°C/h.

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Application of Tabular Alumina as a Refractory Aggregate

Tabular alumina is used as a refractory aggregate in combination with refractory binders, such as high-purity calcium aluminate cement or phosphate-based binders. The inclusion of these binders significantly enhances the erosion resistance and wear resistance of tabular alumina-based refractory materials.

Characteristics of Tabular Alumina

Conventional sintered alumina is typically fired in rotary or tunnel kilns. Due to temperature limitations, small amounts of sintering aids—such as MgO or CaO—are usually added to promote densification. Consequently, conventional sintered alumina exhibits low porosity and a fine, dense crystalline structure, which results in relatively poor thermal shock resistance.

In contrast, tabular alumina is a recrystallized α-Al₂O₃ material, produced through rapid, thorough firing at temperatures exceeding 1900°C without the use of any additives. Its microstructure is characterized by the following features:

① It consists of well-developed α-Al₂O₃ crystals.
② The α-Al₂O₃ crystals are coarse, with a median grain size typically ranging from 40 to 200 μm; the crystals exhibit a plate-like, two-dimensional morphology and are interlocked with one another.
③ The α-Al₂O₃ crystals contain numerous closed, spherical pores (5–15 μm in size), while open porosity is relatively low, generally between 2% and 3%.
④ Due to the presence of impurities (approximately 0.16%), β-Al₂O₃ is also present within the structure, located exclusively at the grain boundaries of the α-Al₂O₃ crystals.

These structural characteristics endow tabular alumina with the following physical properties:

① A high melting point of approximately 2040°C.
② High grain hardness, with a Mohs hardness of 9 and a Knoop hardness of 2000.
③ Excellent resistance to chemical erosion; with the exception of hydrofluoric acid and phosphoric acid, most alkalis and mineral acids have no effect on tabular alumina.
④ Due to the absence of microcracks and large internal pores, it exhibits high strength. Furthermore, its strength does not decline significantly when subjected to thermal shock, resulting in excellent thermal shock stability.
⑤ It possesses high thermal conductivity and high electrical resistivity, ensuring good electrical properties even at high frequencies and high temperatures.

Tabular alumina is typically used as a refractory aggregate, though it can also be incorporated into the matrix as an additive. When combined with other refractory raw materials, it offers several advantages: high refractoriness, high strength under thermal load, low creep, high density, low gas permeability, good thermal shock stability, excellent wear resistance, low thermal shrinkage, and high purity—which minimizes the impact of impurities on the material’s high-temperature performance.

Tabular Alumina Castables

In recent years, tabular alumina has become a high-performance refractory material widely used in the steel, foundry, petrochemical, and ceramic industries. Tabular alumina consists of dense, fully shrunk, sintered α-Al₂O₃, with aggregates formed by grains ranging from 50 to 400 μm. It derives its name from the plate-like (tabular) shape of its grains. As a pure, sintered form of alumina—produced without the addition of agents such as MgO or B₂O₃—it features a well-developed, coarse-grained α-Al₂O₃ crystal structure. Its plate-like crystal morphology, combined with a high proportion of small, closed pores, results in a total porosity comparable to that of fused alumina. It offers high purity, excellent volume stability, and minimal permanent linear change (reheating shrinkage). Refractory products or castables formulated with tabular alumina exhibit superior thermal shock resistance and flexural strength after high-temperature treatment, though they are generally more expensive than other types of alumina.

Tabular alumina refractory castables are primarily composed of high-alumina materials, corundum, and mullite, bonded with high-temperature inorganic liquid binders. They are prepared through mixing and extrusion, and are typically applied via ramming. These materials possess good plasticity, exceptional spalling resistance, high refractoriness, high-temperature and mechanical strength, excellent thermal shock stability, good alkali resistance, and high wear resistance. They are primarily used for lining industrial furnaces, including high-wear areas such as the discharge ends (kiln hoods) of rotary kilns, and are particularly well-suited for the kiln inlets of large-scale cement kilns and coal-injection pipes.

Tabular alumina castables utilize a binder system consisting of calcium aluminate cement and silica fume. They are processed by subjecting the material—which serves as an alternative to bauxite-based sprayable refractories—to heat treatment at 1000°C, 1300°C, or 1500°C for a duration of three hours. Tabular alumina castables offer a wide range of superior properties and are utilized across a broad spectrum of industrial applications. Tabular alumina castables possess high flexural and compressive strength, as well as excellent wear resistance. These superior properties render tabular alumina castables indispensable.

Technological Fundamentals of Unshaped Refractories

November 10, 2026October 24, 2026 by rsrefractorycastable

Unshaped refractory materials—also known as monolithic refractories—consist of refractory aggregates, powders, binders, additives, and pores. They harden at either room temperature or elevated temperatures, exhibit limited shrinkage at service temperatures, and possess a refractoriness exceeding 1580°C. Some definitions specify a refractoriness of no less than 1500°C.

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Advantages of Using Unshaped Refractories

The development and production of unshaped refractories are of great significance to the national economy, as they enable the mechanization of construction and repair processes for various thermal-processing equipment across many industrial sectors. In developed capitalist nations, unshaped refractories account for 40% to 50%—or even more—of total refractory production. In many applications, the use of unshaped refractories offers clear economic advantages over the use of small, pre-formed refractory bricks or shapes.

Compared to fired refractory products, unshaped refractories (such as large precast blocks and monolithic linings) offer a number of fundamental technical advantages. For instance, the degradation of conventional refractory masonry typically begins at the brick joints; in contrast, monolithic linings made from unshaped refractories are entirely free of such joints.

Furthermore, when compared to fired products—even those composed of the same base material and possessing identical porosity—unshaped refractories exhibit superior thermal shock resistance. As temperature drops non-linearly across the thickness of a refractory lining, varying temperature gradients emerge in different sections, resulting in distinct thermal stresses. In a structure built from fired bricks, these thermal stresses act upon a rigid, uniform framework. In an unshaped, monolithic lining, however, the material develops a structure within each section that is better adapted to the local temperature gradient. Consequently, unshaped refractories possess a greater capacity for stress relaxation.

Another key advantage of unshaped refractories is their inherently low thermal conductivity. Conversely, their primary drawbacks include relatively poor abrasion resistance and issues such as structural collapse (loss of mechanical integrity) within certain temperature ranges. Therefore, it is neither accurate nor practical to view unshaped refractories and fired refractory products as mutually exclusive alternatives.

Selection of Components for Unshaped Refractories

The selection of components for dense unshaped refractories is guided by the principles of achieving minimum porosity and minimal shrinkage. To ensure that the binder effectively fills the spaces between the aggregate particles, the mixture must possess good packing characteristics. Materials that remain stable at high temperatures and do not form low-melting-point eutectic mixtures are chosen as aggregates. In principle, any non-shrinking refractory material can serve as an aggregate.

Maximum packing density is achieved by using a blend of two types of particulate fillers—typically 60%–70% coarse particles and 30%–40% fine particles—with the maximum particle size limited to 20–30 mm. To ensure dense packing, the average diameter of the fine particles should be 6–7 times smaller than that of the coarse particles. In the context of insulating (thermal-insulating) unshaped refractories, the objective is to achieve maximum porosity while maintaining sufficient mechanical strength.

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Binders for Unshaped Refractory Materials

Binders for unshaped refractory materials consist of a dispersed system comprising a dispersed phase (refractory raw materials with a particle size of less than 0.09 mm, such as cement) and a dispersion medium (chemical binders).

Based on their hardening characteristics, these binders are classified as follows:

(1) Hydration binders: These are dispersed systems in which the dispersed phase consists of materials such as high-alumina cement, alumina-based cement, barium-aluminate cement, periclase-based cement, hemihydrate gypsum, Portland cement, or other liquid binders, with water serving as the dispersion medium.
(2) Polymeric (polycondensation-type) and recrystallization-type chemical binders: These include orthophosphoric acid and its salts, soluble silicates (water glass), alkyl silicates, organoelement compounds, sols and gels of certain oxides, highly concentrated oxide suspensions, as well as magnesium-based, oxychloride, and oxysulfate compounds.
(3) Coagulation-type binders: These include refractory clay, bentonite, and organosilicon substances (up to their decomposition temperature; above this temperature, organosilicon substances function as polymeric binders).
(4) Organic binders: These include tar, pitch, starch, dextrin, and various thermal reaction products—such as aromatic or condensed aromatic structures combined with hexamethylenetetramine (acting as a hardener).
(5) Ceramic-type binders: In this category, additives that lower the sintering temperature—along with metal powders—are incorporated into the bulk refractory material. These components significantly reduce the temperature at which a liquid phase forms, promoting solid-liquid reactions at lower temperatures and thereby facilitating sintering-based bonding at low to intermediate temperatures. When a small amount of boric anhydride (B2O3) is added to corundum-based dry-vibratable mixes, it forms a viscous liquid phase between 450°C and 550°C. This liquid subsequently reacts with α-Al2O3 (solid-liquid reaction) to produce compounds with higher melting temperatures—specifically 2Al2O3 · B2O3 (incongruent melting point: 1035°C) and 9Al2O3 · 2B2O3 (incongruent melting point: 1950°C)—which serve to bond the corundum aggregates together.

Hardening processes in monolithic (unshaped) refractories include:

Chemical interaction between the refractory phase and the dispersion medium (chemical hardening). Example: MO + H3PO4 → M(HPO4) + H2O.
Recrystallization resulting in the formation of chemical compounds (recrystallization hardening). Example: n[Si(OH)4] → [SiO(OH)2]n + nH2O.
Formation of hydrates (hydration hardening). Example: 3(CaO · 2Al2O3) + aq → 3CaO · Al2O3 · 6H2O + 5Al(OH)3 + aq.
Adhesive bonding.

Depending on the composition of the binder, the hardening of monolithic refractories may involve processes such as polymerization, polycondensation, or the formation of hydrogen bonds. The selection of a binder type is ultimately based on two fundamental criteria:

1) Volumetric stability of the monolithic refractory.
2) Maintenance of required strength across the entire temperature range, from ambient temperature to the service temperature.

Volumetric stability is a primary requirement for monolithic refractories. Shrinkage poses a significant risk, as it can lead to the catastrophic failure of the refractory mass and contribute to cracking. It should be noted that the tensile strength of monolithic refractories is significantly lower than their compressive strength.

Regarding the shrinkage characteristics of monolithic refractories, laboratory testing involves heating the material for 5 hours at its intended service temperature. For dense monolithic refractories, shrinkage should not exceed 1%, while for insulating (lightweight) refractories, the limit is 2%. These criteria are used to determine the appropriate service temperature for the material. At operating temperatures, unshaped refractory materials are permitted to exhibit an expansion of less than 3%.

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Production Process of Unshaped Refractories

When designing the composition of unshaped refractories, it is essential to ensure that the material possesses sufficient strength across the entire range of operating temperatures.

The strength of unshaped refractories varies with temperature as follows:

(1) At relatively low temperatures (below 300°C), the material hardens, and its strength increases.
(2) In the 300–1000°C range, the binder undergoes dehydration—losing chemically bound water—and the polymerized/condensed structure may break down, leading to a reduction in strength (often referred to as a “strength dip” or “collapse in strength”).
(3) At temperatures above 1000°C, sintering occurs, resulting in an increase in strength.

In the production of unshaped refractories, the material generally exhibits adequate strength at room temperature, primarily due to the hardening of the binder. For precast shapes, a strength of 10–30 MPa is typically required to withstand subsequent transport and installation. While conventional castables may experience some softening, this generally does not lead to cracking or total structural failure. The overall strength of an unshaped refractory is derived from the strength of the aggregates and the strength of the binder (and the interfacial phases). Due to an uneven distribution of stress, the various components within the material exhibit different levels of strength and deformability; stress tends to concentrate on the components with a higher elastic modulus (the aggregates), while the load on components with a lower elastic modulus is reduced. Consequently, materials containing robust aggregates tend to exhibit higher overall strength.

The amount of binder used in an unshaped refractory composition often depends on the mixing and application requirements. While a higher binder content increases the material’s strength at room temperature, it also promotes the formation of a larger volume of liquid phase at high temperatures. Therefore, when determining the binder content, one must ensure that the amount of liquid phase formed at the intended operating temperature does not exceed 10%–15%.

Explosion Resistance and Thermal Shock Stability of Low-Cement Castables

November 10, 2026October 9, 2026 by rsrefractorycastable

Low-cement castables are widely used due to their advantages, such as low water requirement, high density, excellent volume stability, and high strength. However, their relatively poor gas permeability can lead to issues like explosive spalling, surface peeling, or flaking during the initial drying and firing processes. In severe cases, this can result in the complete structural failure of the castable lining, causing significant losses for the user. Consequently, improving the explosion-proof (anti-spalling) performance of low-cement castables is of great importance for their practical application and further development.

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Currently, the primary method for enhancing the explosion-proof performance of castables is the addition of anti-spalling fibers, which serve to increase the material’s gas permeability. This paper investigates the effects of anti-spalling fiber content and silicon carbide (SiC) content on the explosion-proof performance of low-cement castables. Furthermore, it examines the influence of anti-spalling fiber content on the porosity of these castables, as well as the impact of silicon carbide content on their thermal shock stability.

Raw Materials for Low-Cement Castables

The castables used in the experiments were low-cement castables. The primary raw materials included 85-grade homogeneous bauxite clinker, 97-grade black silicon carbide, α-Al₂O₃ micropowder, 95-grade silica fume, and Secar 71 cement. Polypropylene fibers were used as anti-explosion fibers.

Specimen Preparation and Performance Testing

The ingredients were proportioned according to the designed formulation. The mixture was dry-mixed in a mixer for 1 minute, followed by the addition of a specific amount of water and wet-mixing for 3 minutes. The resulting material was cast into specimens measuring 40 mm × 40 mm × 160 mm and 230 mm × 114 mm × 65 mm. After curing at 20°C for 24 hours, some specimens were placed directly into a preheated furnace; they were held at the target temperature for 15 minutes and then observed for signs of spalling (explosive spalling). Other specimens underwent heat treatment—at 110°C for 24 hours, 200°C for 24 hours, or 1400°C for 3 hours—and were subsequently tested for apparent porosity and thermal shock resistance. Apparent porosity was measured in accordance with the YB/T 5200 standard, while thermal shock resistance was evaluated using the water-quenching method (in accordance with the YB/T 376.1 standard).

Experimental Results and Discussion

Influence of Anti-Explosion Fiber Content on the Spalling Resistance and Porosity of Castables

The results of the spalling tests on low-cement castable specimens with varying anti-explosion fiber contents indicate that specimens without these fibers spalled at 600°C. As the content of anti-explosion fibers increased, the spalling temperature of the castable specimens rose to 700°C, 900°C, and above 1000°C, respectively. Furthermore, an increase in the anti-explosion fiber content led to a notable rise in the apparent porosity of the specimens after heat treatment at 200°C for 24 hours. Relevant literature indicates that when anti-explosion fibers are incorporated into castables, they shrink and melt during the heating and drying process. This creates slender, open-ended channels within the material, which increases its gas permeability and accelerates the escape of water vapor. Consequently, this enhances the castable’s resistance to explosive spalling and improves its overall anti-explosion performance.

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Effect of Silicon Carbide Content on the Spalling Temperature and Thermal Shock Stability of Castables

With the content of anti-explosion fibers held constant, the influence of varying silicon carbide (SiC) concentrations on the spalling temperature and thermal shock stability of low-cement castables was investigated.

Experimental results regarding the spalling resistance of castable specimens with different SiC contents indicate that as the SiC concentration increases from 0% to 13%, the spalling temperature of the specimens decreases from 900°C to 800°C, showing a downward trend.

This reduction is primarily attributed to the formulation adjustments: as the SiC content increases, the content of 85-grade homogeneous bauxite decreases (while the combined total of SiC and 85-grade homogeneous bauxite remains constant). Since the thermal conductivity of SiC is higher than that of 85-grade homogeneous bauxite, the overall thermal conductivity of the castable increases as the SiC content rises and the bauxite content falls. Consequently, higher transient steam pressure is generated within the castable at elevated temperatures; once this steam pressure exceeds the tensile strength of the material, spalling occurs. The experimental data show that while the spalling temperature of the low-cement castables exhibits a downward trend as the SiC content increases, the magnitude of this decrease is relatively small.

Regarding thermal shock stability, the test results for specimens heat-treated at 1400°C for 3 hours show a significant improvement: the number of thermal shock cycles the material can withstand increases from 4 to 25 as the SiC content rises. This trend is both pronounced and substantial. There are two primary reasons for this improvement.

First, there is a difference in the coefficients of thermal expansion (CTE) between the 97-grade SiC and the 85-grade homogeneous bauxite clinker; the 97-grade SiC has a lower CTE, whereas the 85-grade homogeneous bauxite clinker has a higher CTE. This mismatch in the coefficient of thermal expansion leads to the formation of microcracks within the castable. As the silicon carbide content increases—accompanied by a corresponding decrease in the content of the 85-grade homogenized bauxite—the overall coefficient of thermal expansion of the castable is reduced. This, in turn, lowers the thermal stress experienced by the material and improves its thermal shock stability.
Furthermore, increasing the silicon carbide content while reducing the 85-grade homogenized bauxite leads to a gradual rise in the overall thermal conductivity of the castable. This increase in thermal conductivity plays a significant role in enhancing the material’s thermal shock stability; specifically, higher thermal conductivity helps to reduce the internal thermal stress generated when the castable is subjected to thermal shock, thereby improving its overall thermal shock resistance.

In summary, the following conclusions can be drawn:

First, increasing the content of explosion-proof fibers significantly contributes to raising the apparent porosity of low-cement castables and improving their explosion-proof performance.
Second, reducing the silicon carbide content helps improve the spalling resistance of low-cement castables, though the effect is not very pronounced; conversely, increasing the silicon carbide content significantly enhances the thermal shock stability of low-cement castables after heat treatment at 1400°C.

Thermal-Shock-Resistant Low-Cement Castable

Definition: A thermal-shock-resistant low-cement castable is a high-performance refractory material. By optimizing the mix design—reducing the cement content to 4%–8%—it achieves significantly enhanced thermal-shock resistance (the ability to withstand rapid temperature fluctuations), while also offering high strength, low porosity, and excellent high-temperature stability.

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Outstanding Thermal-Shock Resistance

Low-Cement Matrix: Minimizes the formation of low-melting-point phases (such as anorthite) caused by CaO at high temperatures, thereby preventing material spalling or cracking during thermal cycling.

Micro-crack Mechanism: Ultrafine powder fillers create a dense structure, where localized micro-cracks effectively absorb thermal stress (with the critical micro-crack propagation depth being less than or equal to the depth of the thermal stress impact).

Optimized Aggregate Grading: Utilizes low-expansion aggregates—such as corundum or mullite (with a coefficient of thermal expansion ≤ 8×10⁻⁶/℃)—that are matched to the thermal expansion characteristics of the matrix, significantly reducing interfacial stress.

Key Aspects of Low-Cement Technology

Cement Substitution: Employs α-Al₂O₃ ultrafine powder (particle size < 1 μm) and silica fume (reactive SiO₂) to form a hydrated bonding network, effectively reducing the risk of high-temperature shrinkage and cracking.

Water Requirement: Requires only 5%–7% water (compared to 10%–15% for conventional castables), which lowers porosity to 13%–18% and substantially increases material density.

Typical Performance Indicators of Thermal-Shock-Resistant Low-Cement Castable

Items	Typical Value
Cement Content	4%-8%
Bulk density	≥2.8 g/cm³
Flexural Strength at 110℃	10-15 MPa
Flexural Strength at 1500℃	≥25 MPa
Thermal Shock Cycles (1100℃ Air-Cooled)	≥50 cycles without damage (standard materials only last 20–30 cycles)
Linear Change rate (1600℃×3h)	±0.2%

Thermal-Shock-Resistant Low-Cement Castable Application Scenarios

Environments subject to rapid thermal cycling: Ladle linings, RH vacuum refining vessel covers, firing zones of lime kilns, and linings for hazardous waste incinerators.

High-temperature, high-wear components: Cyclone separators in circulating fluidized bed (CFB) boilers and coal-injection pipes at the discharge ends of cement rotary kilns.

Performance Comparison: Offers a service life more than twice that of conventional high-cement castables (containing 12% CaO) and reduces high-temperature linear shrinkage by 40%.

Key Considerations for Thermal-Shock-Resistant Low-Cement Castable Installation and Curing

Moisture-proof Storage: Ultrafine powders are prone to moisture absorption and clumping; once opened, the material must be used within 3 days.

Water Quality Control: The chloride ion (Cl⁻) content in the mixing water must be <50 ppm to prevent high-temperature corrosion.

Drying/Heating Schedule: Maintain at 110°C for 24 hours → gradual temperature increase to 350°C (at a rate of ≤15°C/h) → final temperature of 600°C (total duration ≥96 hours). Strict adherence is required to prevent cracking caused by rapid dehydration.

Applications of Steel Fiber-Reinforced Refractory Castables in Kiln Components

September 24, 2026 by rsrefractorycastable

Steel fiber-reinforced castable is made using premium bauxite clinker as aggregate, high-quality bauxite clinker and corundum fine powder as matrix, and various composite materials such as ultrafine powder as binders and additives, plus stainless steel heat-resistant fibers. Furthermore, the addition of steel fibers significantly enhances the overall strength of the furnace wall after casting.

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Applications of Steel Fiber Reinforced Castables

Steel fiber-reinforced castables are mainly used in the following areas:

Linings for various thermal equipment.
Thermal insulation for flues and large-diameter metal chimneys.
Linings for industrial boilers, with particularly noticeable effects on furnace roofs.
Thermal insulation for low-temperature air (≤1200℃) pipelines.
KT-J impermeable carbon fiber castable for muffle-less carburizing furnace linings.
Various standard and custom-shaped precast components to meet specific user requirements.
Backing insulation layers for various low-temperature furnaces with interface temperatures ≤1050℃ (including furnace walls and roofs).
For industrial furnaces using flat-flame burners, providing sealing and insulation for the furnace roof, with superior sealing performance compared to any other material.
Linings for resistance furnaces with operating temperatures ≤1200℃; heating elements can be exposed or embedded.
This type of furnace lining is particularly suitable for applications with minimal mechanical vibration and large temperature variations, such as bell-type furnaces and movable lids of removable aluminum melting furnaces.
HS-J ultra-low temperature refractory fiber castable, ML-J polycrystalline mullite fiber castable, and L80-J alumina fiber castable can be used for industrial furnace linings with operating temperatures of 1300-1450℃.
This type of furnace lining, including furnace walls and roof, is suitable for various fuel furnaces with operating temperatures ≤1250℃. Composite linings made from different types of refractory fiber castable offer a stable structure, are economical and reasonable, and are easy to construct.

Performance of Steel Fiber Refractory Castables

The heat-resistant steel fibers used in common steel fiber-reinforced refractory castables should be selected according to the operating conditions of the thermal equipment, and the appropriate dosage should be determined. The preparation process is as follows: First, weigh the refractory aggregate, powder, binder, and admixtures, and dry mix for 1 minute. Next, add water and wet mix, evenly sprinkling the steel fibers into the mixture, ensuring no lumps form. Finally, mix for 1 minute before discharging; the total mixing time should not be less than 5 minutes. When using a vibratory rod for molding, the vibration direction should be staggered to ensure proper steel fiber orientation, which can improve performance.

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The performance of steel fiber-reinforced refractory castables and high-strength clay-bonded refractory castables uses refractory aggregates and powders made from grade III and grade I bauxite clinker, respectively. To improve matrix performance, fused brown corundum powder, active SiO2 ultrafine powder, and admixtures are added. CA-50 cement is used as an accelerator. Fused calcium aluminate cement-alumina-magnesia refractory castable uses dense fused corundum as aggregate, alumina and high-purity sintered magnesia powder as refractory powder, and also incorporates ultrafine powder and composite admixtures. Low-cement bauxite-mullite refractory castable uses first-grade bauxite clinker and mullite as refractory aggregate, high-grade bauxite clinker powder or corundum powder as refractory powder, and CA-70 cement as binder, while also incorporating ultrafine powder and composite admixtures. Low-cement special alumina corundum refractory castable also incorporates a small amount of admixtures. This type of high-quality refractory castable is used at higher temperatures; therefore, 330 heat-resistant steel fiber with an equivalent diameter of 0.5 mm and a length of 20 mm is selected, with a dosage of 2% and water dosage of 6%~7%. According to the selected mix proportions, all raw materials are weighed, mixed, and then molded. After molding, it is naturally cured and must not be wetted. After curing for 24 hours, remove the mold and continue natural curing for another 48 hours before testing the performance.

What role do steel fibers play in refractory castables?

When added to refractory castables in a certain proportion, steel fibers provide tensile, compressive, crack-resistant, and drop-resistant properties. Furthermore, the addition of steel fibers enhances the overall integrity of the refractory castable, making it less prone to deformation.

Steel fiber castables have a wide range of applications, showing significant effectiveness in industrial kilns with furnace lining temperatures below 1350℃. This is due to their long service life and high tensile strength. The steel fibers added to the refractory castable are arranged in a staggered pattern; the special shape of the fibers strengthens the bond strength of the refractory castable and also resists drop during temperature fluctuations.

Refractory castables with added steel fibers exhibit high strength, thermal shock resistance, and resistance to flue gas erosion. They not only withstand high temperatures and wear but also prevent the aggregate in the refractory castable from expanding at high temperatures and prevent stress damage caused by large temperature gradients during furnace start-up and shutdown.

Steel fibers can be added to any type of refractory castable, such as corundum refractory castable, high-alumina low-cement refractory castable, steel fiber reinforced wear-resistant castable, and high-strength castable. In short, adding steel fibers can enhance the tensile strength, crack resistance, and bond strength of any refractory castable.

Adding steel fibers to steel fiber castable increases the overall strength of the refractory castable. However, the proportion must be reasonable; too little will have no effect, while too much will have the opposite effect. The general proportion is 1-1.5%. Steel fiber castable can only be used at temperatures below 1350℃. Above 1350℃, even with added steel fibers, the temperature exceeds the critical point, and the effect is minimal.

Differences between Steel Fiber Castable and Steel Fiber Wear-Resistant Castable

Key Difference: Introduction of Silicon Carbide. Steel fiber wear-resistant castable, in addition to the above, introduces silicon carbide with a particle size of 0.5- 3 mm and a purity ≥90% (8-15% addition). Silicon carbide (SiC) has high hardness (Mohs 9.2) and a low coefficient of thermal expansion, forming a “hard core” skeleton within the material. This allows the SiC particles to bear the stress first when the surface layer is subjected to erosion from coal powder, incandescent ash, or clinker, thus significantly reducing the wear rate of the matrix.

Steel fiber castables and steel fiber wear-resistant castables both belong to the fiber-reinforced series and have the same construction methods. The only difference lies in whether silicon carbide is introduced. By adhering to the selection principle of “wear first, oxidation last” and strictly controlling the production and construction processes, safe, long-life, and economical furnace lining operation can be achieved in thermal equipment below 1350℃.

Brand Name and Logo Upgrade Announcement

September 15, 2026 by rsrefractorycastable

Dear Esteemed Customers and Partners,

We would like to express our sincere gratitude for your support and trust in Rongsheng Refractory over the years. In a strategic move to more effectively convey our brand concept and meet the evolving requirements of future development, Rongsheng Refractory will, as of now, officially adopt a new brand logo. Concurrently, the brand name “RONGSHENG” will be officially upgraded to “RISING“.

During the transition period of this brand logo upgrade, both the old and new logos will be synchronously used. We kindly request your understanding and patience.

This brand renewal signifies our commitment to persistently offering global customers high-efficiency, energy-saving, and innovative refractory products and services, all delivered with an even more professional demeanor. We are dedicated to upholding the highest standards of quality and service excellence.

Once again, we extend our heartfelt appreciation for your continuous companionship and support.

Rongsheng Refractory Manufacturer

Can Lightweight Castables be Used as Insulation Layers for Industrial Furnaces to Reduce Heat Consumption?

September 9, 2026 by rsrefractorycastable

Lightweight castables have lower refractoriness, compressive strength, and flexural strength than heavyweight castables, but they have lower thermal conductivity and better insulation. By selecting appropriate lightweight materials based on different temperatures and kiln linings, they can achieve insulation and energy savings.

Advantages of Lightweight Castables in Industrial Furnace Linings

Lightweight castables have lower thermal conductivity than heavyweight castables and significantly reduce the weight of the furnace lining. The rate of temperature diffusion during combustion in industrial kilns can be reduced by about 30% compared to heavyweight castables. Therefore, using lightweight insulating castables as an insulation layer can effectively reduce heat loss in industrial furnace linings.

Lightweight Castable Refractory for Furnaces

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Lightweight castables are typically the first layer of material in close contact with the furnace shell in the kiln lining. However, high-grade lightweight castables, such as alumina hollow sphere castables and lightweight mullite refractory castables, can also be used directly as a working layer in the linings of some lightweight kilns. This reduces the furnace weight and saves energy costs while reducing heat loss.

The thickness of lightweight castable refractory as an insulation layer should be between 80mm and 150mm. A thickness less than 50mm results in poor construction quality, and a thinner layer also offers insufficient insulation. Lightweight materials are an excellent choice for lining pipes and chimneys. With a specific gravity of around 1.5, the castable refractory can serve as both an insulation layer and a working layer, thus solving the weight issue while still meeting the requirements for chimney and pipe linings.

Lightweight castable refractory has different operating temperatures: low temperature 600–900℃, medium temperature 900–1200℃, and high temperature 1200℃. High-temperature insulation layers require high-grade lightweight materials, such as lightweight mullite castable refractory and alumina hollow sphere castable refractory. However, commonly used types include vermiculite, perlite, and ceramsite, which have an operating temperature of around 1000℃ and cannot be used at higher temperatures. Lightweight mullite castables and corundum-mullite lightweight castables can be used at temperatures of 1350-1500℃. Lightweight kiln linings can be used directly in the working layer.

Insulation Material Selection for Aluminum Melting Furnaces: Alumina Silicate Wool or Lightweight Castable Refractories?

Many aluminum plants struggle with the same question when modifying or repairing their aluminum melting furnaces and insulation furnaces: should they use alumina silicate wool or lightweight castable insulation for the furnace body insulation layer?

Many mistakenly believe that both are insulation materials with similar effects and that either can be chosen.

In reality, their upper limits for high-temperature resistance, structural strength, sealing performance, and service life are completely different. Choosing the wrong insulation material can lead to anything from high furnace shell temperature and soaring gas consumption to insulation layer collapse, gas leakage, damage to the working surface lining, and frequent shutdowns for repairs.

Rongsheng Refractory Materials Manufacturer, specializing in aluminum melting furnace material selection, uses firsthand practical experience to explain the suitable scenarios, advantages, disadvantages, and selection standards for both insulation materials, helping everyone choose materials accurately, eliminate waste, and achieve long-term energy-saving and stable production.

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Key Differences Between Two Insulation Materials

Alumina Silicate Wool

Alumina silicate wool is a flexible fiber insulation material. It is lightweight, has extremely low thermal conductivity, and is characterized by its lightweight nature, excellent insulation performance, and quick installation. It lacks structural strength, relying solely on fiber stacking to lock in heat. It has no load-bearing capacity or erosion resistance, making it purely an auxiliary insulation material.

Core Advantages: Excellent insulation effect, lightweight, does not increase furnace load, high cost-effectiveness.

Significant Disadvantages: Loose structure, susceptible to airflow erosion, susceptible to high temperatures and overheating, prone to collapse and pulverization under long-term vibration.

Lightweight Insulation Castable

Lightweight insulation castable is a rigid, monolithic refractory insulation material, made from lightweight refractory aggregates, powders, and additives. It is cast with water, and after curing, forms a seamless, rigid insulation layer. It balances thermal insulation with a certain structural strength, making it the mainstream insulation material for high-temperature zones in industrial furnaces.

Core advantages: Strong integrity, good sealing, resistant to airflow erosion, non-collapse, higher temperature resistance, and long service life.

Significant disadvantages: Greater self-weight than fiber cotton, requires pouring and curing, and has a slightly longer construction period.

Targeted Insulation Materials for Aluminum Melting Furnaces

Aluminum Silicate Wool

Aluminum silicate wool is suitable for various scenarios (low-temperature backing, static insulation). Specifically designed for the low-temperature insulation layer behind the furnace body, i.e., the back of the refractory material on the working surface, close to the furnace shell:

Static insulation on the back of the furnace shell of aluminum melting furnaces and holding furnaces, where there is no direct contact with high-temperature flames or airflow.

Insulation for the outer layer of flues and filling gaps at the edges and corners of the furnace body.

For parts of equipment with limited load-bearing capacity that require lightweight insulation.

Contraindications: Absolutely cannot be used directly on high-temperature working surfaces, cannot be directly exposed to airflow within the furnace, and cannot be used in areas with frequent high-temperature vibrations, as it is extremely prone to pulverization and collapse, losing its insulation effect.

Lightweight Insulation Castable

Lightweight insulation castable is suitable for various scenarios (medium-high temperature, dynamic operating conditions). Suitable for high-temperature areas, dynamic operating conditions, and locations requiring airtight insulation in aluminum furnaces:

The upper insulation structure of the furnace top and walls, requiring complete airtightness to prevent gas leakage and heat loss.

Areas with airflow erosion and large temperature fluctuations, such as around the furnace opening and the flue lining.

Furnace bodies subject to frequent start-ups and shutdowns and vibrations, where fiber cotton is prone to detachment and collapse.

Insulation layers requiring a smooth base layer that can be subsequently sprayed and repaired.

Seamless and integral after molding, with strong sealing properties, completely solving the common problems of hollowness, sinking, and heat leakage in traditional fiber cotton insulation.

Optimal Insulation Solution for Aluminum Furnaces

Based on years of experience in aluminum furnace construction, the most durable, energy-efficient, and cost-effective layered structure:

Inner working surface: Non-stick aluminum castable/wear-resistant refractory castable (resistant to molten aluminum, slag, and erosion).

Middle insulation layer: Lightweight insulating castable (completely airtight, resistant to airflow, and resistant to high-temperature fluctuations).

Back insulation layer: Alumina silicate fiber cotton (lightweight, heat-locking, reduces furnace shell heat dissipation).

Graded materials each serve a specific function, ensuring furnace structural stability, preventing air leaks and layer collapse, while minimizing gas consumption and extending the overall lifespan of the furnace lining.

One-stop Solution for Complete Aluminum Furnace Insulation Refractory Materials

For various smelting furnaces and insulation furnaces used for recycled and primary aluminum, Rongsheng Refractory Materials offers a full range of materials, including lightweight insulation castables, alumina silicate insulation cotton, and non-stick aluminum castables for working surfaces. We reject one-size-fits-all materials; instead, we develop layered material selection plans based on the customer’s furnace type, temperature range, equipment structure, and production rhythm, combining the most suitable materials. We also provide full-process services including on-site construction and furnace drying guidance, solving problems such as aluminum furnace insulation failure, furnace shell overheating, high energy consumption, and insulation layer collapse requiring repair, helping companies achieve long-term stable production and energy savings.

The Effect of Adding Six Kinds of Micro Powders on the Properties of High Alumina Refractory Castables

August 25, 2026 by rsrefractorycastable

Industrially, unshaped refractories with Al₂O₃ ≥ 45% are generally referred to as high-alumina unshaped refractories. Rongsheng Refractory Castable Manufacturer. High-alumina castables are also classified into three grades based on Al₂O₃ content: Grade 1 has an Al₂O₃ content greater than 75%; Grade 2 has an Al₂O₃ content of 60%–75%; and Grade 3 has an Al₂O₃ content of 48%–60%. Their main mineral components are corundum, mullite, and a glassy phase.

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Factors Affecting the Performance of High-Alumina Castables

Many factors affect the performance of high-alumina castables, primarily the composition of the raw materials and construction conditions. Regarding raw material composition, the performance of the castable can be optimized by controlling the structure, particle size, component content, or type of binder of the aggregates, and by adding appropriate amounts of fine powder and admixtures. In terms of construction, factors such as the amount of water added during construction and the baking conditions also affect the performance of the castable.

The Influence of Fine Powders in Castables on Their Performance

Castables typically incorporate various fine powders, which, through the combined action of binders and additives, form a matrix. The particle size and amount of fine powders affect the matrix structure. Since the matrix is most susceptible to erosion by the medium, it is often the first component to be damaged under stress. Therefore, it is necessary to optimize the state of the fine powders to improve their reaction with other components in the matrix, allowing them to bind aggregates and fill voids after high-temperature treatment, achieving close packing and promoting sintering. This, in turn, improves the room-temperature physical properties and performance of high-alumina castables. The fine powders added to high-alumina castables mainly include silicon powder, silicon carbide micropowder, Al powder, and Al₂O₃ micropowder.

Silica Micro-powder in Refractory Castable

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Effects of Silica Powder Addition on Castable Properties

High-alumina castables were prepared using bauxite particles as aggregate, with the addition of bauxite fine powder and silica powder, and calcium aluminate cement and p-Al₂O₃ micropowder as binders. The effects of silica powder addition (1%, 3%, and 5%) on various properties of the high-alumina castables were investigated. The study found that with increasing silica powder addition, the flowability of the castable decreased, and the linear shrinkage rate after firing increased. Appropriate silica powder addition can improve the creep resistance of the castable. This is because silica powder is oxidized during high-temperature heat treatment, and the oxidized silicon reacts with the matrix components in a mullitization reaction, causing the volume of the castable to expand and the linear shrinkage rate to increase. When the molar ratio of Al₂O₃ to SiO₂ in the matrix reaches a certain value, after the mullitization reaction is complete, the newly added SiO₂ glass phase will reduce the creep resistance of the castable.

The effect of elemental silica powder addition on ultra-low cement-bonded high-alumina castables was also investigated. The samples were subjected to high-temperature heat treatment in oxidizing, neutral, and reducing atmospheres. It was found that increasing the amount of elemental silicon powder gradually increased the high-temperature flexural strength of the samples after firing, especially after heat treatment in a neutral atmosphere where the change in flexural strength was most significant. The introduced silicon powder reacted in situ in an oxidizing atmosphere to form columnar mullite, in a reducing atmosphere to form silicon carbide whiskers, and in a nitrogen-neutral atmosphere to form fibrous silicon nitride. These in-situ formed phases significantly improved the strength of the castable.

Different amounts of silicon powder were introduced into traditional high-alumina castables, and heat treatment was performed under oxidizing, carbon-embedded, and nitriding atmospheres to investigate the effect of silicon powder on the strength properties of the castables. After heat treatment at 1450℃, the bulk density of the samples under the three atmospheres decreased slightly with increasing silicon powder content. With increasing silicon powder content, the room-temperature flexural strength of the samples gradually decreased under oxidizing and reducing atmospheres and after heat treatment at 1450℃, while the room-temperature flexural strength of the samples significantly increased after heat treatment in a nitriding atmosphere. The high-temperature flexural strength increased with increasing silica powder content under all three atmospheres, with the highest strength observed in the nitriding atmosphere and the lowest in the oxidizing atmosphere. With continuous introduction of silica powder, the sample under the oxidizing atmosphere gradually expanded, while the sample under the nitriding atmosphere initially contracted and then expanded, and the sample under the reducing atmosphere continued to contract. In summary, heat treatment under the nitriding atmosphere had the least impact on the permanent linear shrinkage rate of the castable. With increasing silica powder content, the sample heat-treated under the reducing atmosphere exhibited the best thermal shock resistance, followed by the sample heat-treated under the nitriding atmosphere.

Influence of SiC Micropowder Addition on Castable Properties

In preparing high-alumina castables, high-grade bauxite was used as aggregate, pure calcium aluminate cement as binder, and α-Al₂O₃ micropowder, SiO₂ micropowder, corundum fine powder, and SiC fine powder were added as main raw materials. The effect of varying SiC micropowder addition on the properties of the high-alumina castable was studied. The results showed that the water requirement of the castable increased with the addition of SiC micropowder. Therefore, excessive addition of SiC micropowder has an adverse effect on the low-temperature flexural strength of castables. At high temperatures, a liquid phase forms on the surface of the castable, reducing its strength and wear resistance. Appropriate addition of fine SiC powder is beneficial to improving the cold-state flexural strength of the castable. The amount of water added increases with decreasing SiC particle size; the easier the castable is to sinter, the higher its cold-state flexural strength, and the better its thermal shock resistance. This is because a gap structure is created around the SiC particles, which is more conducive to improving the thermal shock resistance of the castable.

In the preparation of high-alumina castables, using high-grade bauxite clinker, SiC, and silica micropowder as the main raw materials, and calcium aluminate cement as the binder, the effects of silicon carbide addition (1%, 2%, 3%, 4%, and 5%) on the room-temperature physical properties, thermal shock resistance, flexural strength, and slag resistance of the high-alumina castables were studied. The results show that the addition of SiC improves the strength of the castable, especially its thermal shock resistance. After various temperature treatments, the apparent porosity of the castable generally decreased while the bulk density increased with increasing SiC content. With the addition of SiC, the flexural strength of the samples after heat treatment at 1350℃ continuously increased, while the compressive strength reached its maximum at a SiC content of 3%, and then showed a downward trend. After heat treatment at 1500℃, the flexural strength of the samples first increased and then decreased; after the SiC content reached 3%, the compressive strength of the samples gradually decreased. The corrosion resistance of the samples with added SiC at high temperatures is unstable. This is because SiC can be oxidized at low temperatures, and a SiO₂ protective film forms on the surface of the samples at high temperatures, providing some protection. However, when the temperature continues to rise, the SiO₂ protective film is destroyed, causing damage to the material.

The Effect of Fly Ash Powder Addition on Castable Properties

With the development of the power industry, fly ash is currently one of the industrial wastes with the largest discharge volume. This study investigated the effects of introducing fly ash into high-alumina castables (0, 2%, 4%, 6%, 8%, and 10%) on the microstructure and various properties of the castables. The results showed that with increasing fly ash addition, the room-temperature flexural strength of the high-alumina castable continuously decreased, the bulk density significantly decreased, the apparent porosity increased, and the thermal conductivity decreased. This is because the addition of fly ash reduces the pore size of the samples. When a certain amount is added, the pore size is adjusted to the micrometer scale and uniformly distributed in the matrix, improving the material’s thermal insulation performance and effectively reducing heat loss.

The Effect of SiO₂ Micropowder Addition on Castable Properties

In the preparation of high-alumina castables, bauxite clinker and coke were used as aggregates, α-Al₂O₃ micropowder and SiO₂ micropowder were used as matrix materials, and high-alumina cement was used as binder. The effect of the amount of SiO₂ micropowder added on the properties of high-alumina castables was investigated. The study showed that the addition of an appropriate amount of SiO₂ micropowder reduced the water demand of the castable, achieving a certain water-reducing effect, and maintaining good flowability. Excessive addition led to powder agglomeration, making it difficult for the internal components of the castable to achieve a good bonding effect, and increasing the water demand, thus reducing the flowability. The cold flexural strength and compressive strength of the castable generally increased with the addition of SiO₂ micropowder, while the apparent porosity gradually increased, the bulk density decreased, and the linear shrinkage rate after firing remained stable.

The effect of adding SiO₂ micropowder of different specifications on the properties of high-alumina castables was also studied. Studies have found that the addition of SiO₂ micropowder of different specifications leads to significant differences in the rheological properties of castables. Larger SiO₂ micropowder particle sizes are more detrimental to the rheological properties of the castable, resulting in decreased flexural strength and compressive strength at room temperature. However, the specific surface area of SiO₂ micropowder has a relatively small impact on the rheological properties of the castable.

High-alumina castables were prepared using high-alumina homogeneous material, activated alumina micropowder, and SiO₂ micropowder as the main raw materials, and calcium aluminate cement as the binder. The effect of SiO₂ micropowder addition on the castable properties was investigated. Results showed that the flexural strength and compressive strength of the castable at room temperature first increased and then decreased with the addition of SiO₂ micropowder. After heat treatment at 1100℃, the apparent porosity of the castable first decreased and then increased with the increase of SiO₂ micropowder, the bulk density first increased and then decreased, the linear change rate increased, and the wear amount first decreased and then increased. The castable exhibits the best wear resistance when the SiO₂ powder content is 6%.

The Effect of Fly Ash Powder Addition on Castable Properties

The Effect of SiO₂ Micropowder Addition on Castable Properties

High-alumina castables were prepared using high-alumina homogeneous material, activated alumina micropowder, and SiO₂ micropowder as the main raw materials, and calcium aluminate cement as the binder. The effect of SiO₂ micropowder addition on the castable properties was investigated. Results showed that the flexural strength and compressive strength of the castable at room temperature first increased and then decreased with the addition of SiO₂ micropowder. After heat treatment at 1100℃, the apparent porosity of the castable first decreased and then increased with the increase of SiO₂ micropowder; the bulk density first increased and then decreased, the linear change rate increased, and the wear amount first decreased and then increased. The castable exhibits the best wear resistance when the SiO₂ powder content is 6%.

The Influence of the Addition of Five Aggregates on the Properties of High-Alumina Refractory Castables

August 13, 2026August 10, 2026 by rsrefractorycastable

Unshaped refractories offer advantages over shaped refractories in production, construction, energy conservation, and environmental protection. Their production volume and application proportion within the overall refractory materials industry reflect the technological development of the refractory materials sector.

Refractory castables are the most widely used type of unshaped refractories, primarily applied in industries such as petroleum, chemical, steel, building materials, and thermal equipment. They are classified by material into high-alumina castables, magnesia castables, and alumina-magnesia castables, typically composed of four parts: aggregate, fine powder, binder, and additives. Due to their advantages of high-efficiency production, simple construction, and ease of subsequent repair, castables are increasingly used in industry. They can be delivered dry and then poured on-site with water or other liquids, facilitating the casting of kilns with complex structural shapes.

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High-alumina Unshaped Refractories

Industrially, unshaped refractories with Al₂O₃ ≥ 45% are generally referred to as high-alumina unshaped refractories. High-alumina castables are also classified into three grades based on their Al₂O₃ content: Grade 1 has an Al₂O₃ content greater than 75%; Grade 2 has an Al₂O₃ content of 60%–75%; and Grade 3 has an Al₂O₃ content of 48%–60%. Their main mineral composition is corundum, mullite, and a glassy phase. Traditional high-alumina castables have certain limitations in application. Due to their high cement content, although they can achieve high mechanical strength under medium and low temperature conditions, the increased CaO content lowers the load softening temperature and worsens the thermal shock resistance. Furthermore, controlling the amount of water added is crucial for high-alumina castables; otherwise, the initial strength of the castable will be significantly affected.

Currently, high-alumina castables are trending towards low-cement and cement-free castables. Low-cement castables have a lower calcium content than traditional high-alumina castables, which reduces the formation of eutectic phases in the material. Low-cement castables improve the refractoriness, thermal shock stability, slag erosion resistance, and high-temperature strength of traditional high-alumina castables, resulting in a denser structure and reduced porosity.

Many factors influence the performance of high-alumina castables, primarily the composition of the raw materials and construction conditions. Regarding raw material composition, the performance of the castable can be optimized by controlling the structure, particle size, component content, and type of binder of the aggregates, and by adding appropriate amounts of fine powder and admixtures. In terms of construction, factors such as the amount of water added and the baking conditions also affect the performance of the castable.

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The Influence of Aggregates on the Performance of Castables

China has relatively abundant high-alumina bauxite resources, which are typically used as the main aggregate in high-alumina castables. With the vigorous development of energy conservation and environmental protection, high-alumina castables are also evolving towards high efficiency and energy saving. The most important aspect of this is the optimization of aggregates. This is achieved by replacing heavy aggregates with lightweight ones, optimizing low- and medium-grade high-alumina bauxite, or comprehensively utilizing industrial waste to better meet the production needs of high-alumina castables. This not only addresses the current shortage of high-grade bauxite reserves and improves the utilization rate of related resources, but also optimizes and improves the various properties of high-alumina castables.

The Effects of Electrofused Ceramsite Sand and High-Alumina Bauxite Composite Aggregates on Castable Properties

Using industrial waste electrofused ceramsite sand and high-alumina bauxite as composite aggregates, and premium bauxite powder, silica fume, and activated alumina powder as fine powders, and calcium aluminate cement as binder, this study investigated the effects of the addition amount of the substitute aggregate, electrofused ceramsite sand (0, 10%, 19%, and 28%), on the flowability and room-temperature physical properties of high-alumina castables. The results showed that with increasing electrofused ceramsite sand content, the water requirement of the castable decreased, flowability increased, and bulk density decreased. After heat treatment at 1100℃ and 1450℃, respectively, the room-temperature compressive strength and flexural strength of the castables tended to increase, while thermal shock resistance initially increased and then decreased.

The Influence of Alumina Homogenized Aggregate on Castable Properties

Alumina homogenized aggregate was prepared from low- to medium-grade bauxite raw materials through a homogenization and purification process. High-alumina castables were then prepared using both the homogenized aggregate and traditional bauxite clinker as aggregates, and their properties were compared. The study shows that the physicochemical properties of the homogenized aggregate are not affected by particle size, and it is more uniform and stable than bauxite clinker. Furthermore, the high-alumina castable prepared using homogenized aggregate as aggregate contains well-crystallized mullite and corundum as main mineral phases, resulting in better matrix bonding. Compared to the high-alumina castable prepared using bauxite clinker as aggregate, it exhibits better high-temperature mechanical properties and wear resistance.

The Influence of High-Strength Microporous Bauxite Aggregate on Castable Properties

The properties of high-alumina castables prepared from two types of high-strength microporous lightweight bauxite aggregates with different bulk densities were studied in practical applications and compared with those of high-alumina castables prepared from ordinary lightweight bauxite aggregates. The study found that the lower the density of the microporous lightweight bauxite aggregate, the lower the bulk density and the higher the apparent porosity of the prepared high-alumina castable. Compared with ordinary lightweight bauxite aggregate, microporous bauxite aggregate significantly improved the flexural strength and compressive strength of the castable after high-temperature treatment. However, it had little effect on the flexural strength under medium- and low-temperature treatment. The thermal conductivity of high-strength microporous bauxite aggregate at 500℃ was slightly higher than that of ordinary lightweight bauxite aggregate. However, as the temperature increased, the difference in thermal conductivity between the two became negligible, which is related to the change in porosity of the castable. High-strength microporous bauxite aggregate is more conducive to maintaining the volumetric stability of the castable and reducing the linear shrinkage rate. Taking all factors into consideration, increasing the Al₂O₃ content and micropore number of lightweight bauxite aggregate can significantly improve the performance of castables.

The Influence of Bauxite-Mullite-Hollow Sphere Aggregate on Castable Properties

The influence of bauxite-mullite-hollow sphere aggregate on the properties of high-alumina castables was studied. The results show that gradually replacing lightweight mullite with multiphase hollow spheres (0%, 10%, 20%, 30%, and 40%) reduces the water requirement of the castable as the replacement ratio increases. Simultaneously, it significantly improves the room-temperature compressive strength of high-alumina castables while reducing their bulk density and thermal conductivity.

The Influence of Alumina-Andalusite Aggregate on Castable Properties

The effect of adding andalusite to high-alumina castables on their properties was investigated. The results show that with increasing andalusite content, andalusite reacts in situ to form mullite, and the resulting volume expansion generates stress, causing microcracks within the matrix and reducing tensile strength. The formation of mullite also leads to a mismatch in thermal expansion between the surrounding aggregate and the matrix, reducing the flexural strength of the castable. However, it improves crack propagation resistance, which is beneficial for enhancing the thermal shock resistance of high-alumina castables.

In summary, in applications where temperature and castable strength requirements are not high, high-alumina castables are typically prepared using lightweight bauxite aggregate. Microporousization of high-strength bauxite aggregate can improve the refractoriness and service strength of the castable, indicating that reducing the pore size of the aggregate and ensuring its uniform distribution is beneficial for improving the strength and thermal insulation performance of the castable. Hollow spherical aggregate can also be used to optimize traditional high-alumina castables, creating uniformly distributed closed pores within the material to improve various performance characteristics.

How to Solve the Problem of High-Alumina Refractory Mortar Drying too Quickly on the Construction Site?

August 13, 2026July 26, 2026 by rsrefractorycastable

Refractory mortar, also known as refractory clay or fire mortar, is a material used in furnace construction to fill the joints between refractory bricks, bonding them together and providing the masonry with a certain degree of integrity, strength, and airtightness. Therefore, the use of refractory mortar has a significant impact on the quality of the masonry, the heat loss of the electric furnace, and the furnace shell temperature.

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How to solve the problem of high-alumina refractory mortar drying too quickly on the construction site?

If high-alumina refractory mortar dries too quickly on the construction site, it will prevent the refractory bricks from forming a bond, affecting the construction progress. There are several solutions.

First, check the ambient temperature at the construction site. Is the high-alumina refractory mortar not watered enough or not mixed evenly? If the mortar is too thick or has poor water retention, it will dry too quickly. Therefore, if the site temperature is too high, the mixed high-alumina refractory mortar should be covered with a damp cloth and not left exposed.
Secondly, if too much high-alumina refractory mortar is mixed in a single batch and exposed for too long, the surface will form a skin and lose water, also causing it to dry too quickly and affecting the construction progress. It is best to mix small batches at a time, using them within approximately 30 minutes.
If the temperature and mixing are normal, 0.2-0.5% hydroxypropyl methylcellulose (HPMC) can be added to slow down water loss. Alternatively, 0.5-1% dextrin or methylcellulose can be added to increase viscosity and delay drying, allowing sufficient time for refractory bricklaying. However, excessive binder should be strictly avoided to prevent decreased strength and cracking at the joints later on.
If the construction site is in a mountainous area with inconvenient transportation, or if the supplier adds excessively long transport times for the binder, add approximately 20% drinking water when mixing the high-alumina refractory mortar, let it stand for 2-3 minutes, and then add the remaining 5-10%. A small amount of household detergent can be added during mixing to increase the viscosity of the high-alumina refractory mortar and prevent delays in construction. (This method is only applicable when the construction site is in a remote mountainous area with inconvenient transportation.)

In summary, the above are all remedial measures. Ideally, the manufacturer should conduct proper process proportioning and testing during production to ensure that the high-alumina refractory mortar powder is above 180 mesh, has strong adhesion, and does not delay on-site construction time.

Can high-alumina refractory mortar still be used after long storage?

High-alumina refractory mortar is a jointing material. If it has been stored for a long time, its usability needs to be assessed. Individually packaged binders that have been stored for more than 6 months cannot be used and must be replaced. If there is no binder, you can judge by smell; if there is no odor, you can re-mix the binder and use it. If the binder was added directly and stored for more than 6 months, a test can be conducted. If it can bond two refractory bricks, it can be used; otherwise, add a reasonable proportion of binder and test again. If successful, it can be used.

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If there is no individually packaged binder, you can take a small amount and smell it. If there is an odor, the binder was already added to the refractory powder during production. If there is no odor, it proves that the binder was not added. If they were not mixed together, you can re-mix the binder and use it.

If the binder was added directly during the production of the high-alumina refractory mortar and the storage time has exceeded 6 months, a simple test should be conducted on-site. Weigh 1 kg of refractory powder, add 20-25% drinking water, and stir the refractory powder into a slurry. Take two refractory bricks of the same material, apply a portion of the refractory mortar to one brick, smooth it out, and then place the second brick on top. Rub the two bricks back and forth about fifteen times. If the two bricks stick together, the high-alumina refractory mortar is still usable.

If the high-alumina refractory mortar cannot bond the two bricks, add some binder in a proper ratio. Then conduct another on-site test using the above method. If the bricks bond together, it is ready for use.

The binder in high-alumina refractory mortar is for promoting adhesion of the slurry. After being baked at high temperatures, the binder essentially disappears. Therefore, the use of high-alumina refractory mortar that has been stored for a long time depends on its condition.

Requirements and Selection of Refractory Mortar for Refractory Brick Laying

Requirements for refractory mortar when laying refractory bricks:

(1) It should be able to be mixed with water to a certain consistency, filling uneven parts of the brick without causing cracks.

(2) It should have properties similar to the brick body (e.g., refractoriness, softening temperature under load).

(3) It should have good airtightness after drying and at high temperatures.

(4) It should have certain mechanical strength and good airtightness.

Selection of Refractory Mortar for Laying Refractory Bricks

Commonly used refractory mortars include clay-based, high-alumina, silica-based, and magnesia-based refractory mortars. Clay-based refractory mortar should be used when laying clay refractory bricks. Clay-based refractory mortar is composed of clay clinker and binding clay.

High-alumina refractory mortar should be used when laying high-alumina bricks. When laying high-alumina bricks containing 60%-70% Al₂O₃, the refractory mortar used consists of 75%-85% calcined alumina and 10%-20% raw clay. When laying high-alumina bricks containing 48%-55% Al₂O₃, high-refractory clay-based refractory mortar can also be used.

When laying silica bricks, silica refractory mortar with a SiO₂ content of 90%-93% should be used. Its particle size distribution should be: no more than 3% of particles remaining on a 1.0 mm sieve, and no less than 80% of particles passing through a 0.20 mm sieve.

When laying magnesia bricks, magnesia refractory mortar should be used. Its magnesium oxide content should be no less than 78%, its silicon oxide content no less than 6%, and its caustic soda content no more than 2%. The particle size distribution of the magnesia refractory mortar is: 100% smaller than 1 mm; no less than 97% smaller than 0.5 mm; and no less than 50% smaller than 0.125 mm.

Fine-grained refractory mortar is typically used for building electric furnaces.

Phosphate Castables are Used in High-Temperature Wear-Resistant Parts of Industrial Furnaces

August 13, 2026July 11, 2026 by rsrefractorycastable

Phosphate castables play an irreplaceable role in the construction of high-temperature kilns in many fields. Rongsheng Refractory Castable Manufacturer boasts an environmentally friendly and advanced monolithic refractory material production line, specializing in the production of integral refractory lining materials for high-temperature industrial furnaces. Contact Rongsheng for free samples and quotations.

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Characteristics and Applications of Phosphate Castables

Phosphate refractory castables are phosphoric acid-bonded wear-resistant castables, belonging to the acidic matrix, with a service temperature of 1400-1600℃. The phosphoric acid binder concentration in phosphate castables is 40%~60%, with an addition amount of 10%-14%. Aluminate cement is commonly used as a setting accelerator, with a dosage of 0.5%~3.0%. A certain proportion of fine magnesium oxide powder is also added.

At room temperature, phosphate refractory castables do not react with aluminosilicate materials except for iron. During construction, heating is required to dehydrate and condense the binder, adhering and cementing the aggregate powder together to achieve room temperature strength. When fine magnesium oxide powder is added, it reacts with phosphoric acid, causing the refractory castable to set and harden. When aluminate cement is added, it forms hydrated mono- and di-hydrogen phosphate. The physicochemical changes of phosphoric acid and phosphate refractory castables during heating are very complex. It reacts with refractory powder or additives to form phosphates, which are further dehydrated into metaphosphoric acid or metaphosphate. With continued temperature increases, polymerization, polycondensation polymerization, and primary cementation adhesion occur. Simultaneously, a ceramic bond is formed, giving the refractory castable better high-temperature performance.

Phosphate refractory castables use clay clinker or bauxite aggregate clinker as refractory aggregate and powder, with dosages of 65%-74% and 24%-35%, respectively. A common mix design is as follows: refractory aggregate with a particle size less than 15mm, with a particle size distribution of 15-5mm 50%, 5-2.5mm 24%, and less than 2.5mm 26%. The refractory powder fineness is greater than 80% less than 0.09mm. The accelerator is CA-50 high-alumina cement, the dosage of which depends on construction conditions and room temperature strength requirements. The standard dosage is around 2%. When the ambient temperature is low, the dosage should be increased appropriately. If room temperature strength is not required, CA-50 cement can be omitted or added in small amounts.

Phosphate castables are used in heating furnaces and soaking furnaces for heating metals, and can also be used in coking ovens, etc.

Performance Characteristics of Phosphate Castables

Phosphate castables are castables bonded to phosphates or other phosphates. Their hardening mechanism depends on the type of binder used and the method of accelerating hardening. The binder in phosphate castables can be phosphoric acid or a mixture of phosphoric acid and aluminum hydroxide to produce aluminum dihydrogen phosphate. At room temperature, the binder does not react with aluminosilicate materials (except iron), so it must be heated to dehydrate and shrink, thereby binding the aggregate powder and achieving room-temperature strength. When using a setting accelerator, heating is not required; fine magnesium oxide powder or high-alumina cement can be added to accelerate setting. After adding fine magnesium oxide powder, it reacts rapidly with phosphoric acid to form a hardened refractory material. Adding aluminate cement forms phosphates with good setting properties, such as hydrated phosphates like monocalcium phosphate and dicalcium phosphate, which then harden the material.

From the hardening mechanisms of phosphoric acid and phosphate refractory castables, it can be seen that only when the reaction rate of the binder, refractory aggregate, and powder is appropriate during heating can a high-quality refractory castable be formed. However, during the crushing, ball milling, and mixing processes, refractory raw materials are prone to introducing impurities such as metallic iron. These impurities react with the binder during mixing, releasing hydrogen gas, which causes the refractory castable to expand, resulting in a loose structure and reduced compressive strength. This process is detrimental to the production of ordinary phosphoric acid and phosphate refractory castables.

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Methods to Eliminate Expansion

Material Settling: High-temperature refractory aggregates and powders are thoroughly mixed, placed in a silo or storage area, covered with plastic sheeting to prevent water contact, and allowed to stand for a period of time. This allows the binder to fully react with the metals in the refractory aggregates and powders, releasing hydrogen.
Washing and Magnetic Separation: The powder is crushed using a ceramic ball mill or washed with a strong acid (such as hydrochloric acid), and impurities such as metals are removed using magnetic separation.
Inhibitors: The expansion of refractory castables can be reduced by decreasing the amount of binder used in the refractory material or by adding inhibitors.

Although the binder in phosphate castables does not react with the material itself at room temperature, it will slowly solidify and harden due to temperature changes or other environmental changes during storage or transportation. For long-term storage, retarders such as citric acid and tartaric acid can be added to extend the storage time.

Construction of Phosphate Castables

Formwork is required for the construction of any castable. Depending on the location within the industrial furnace, formwork is generally used for flat surfaces or curved arched surfaces, and based on material, it can be steel or wood. Formwork must be precisely dimensionally manufactured, and supports (or hanging) must be secure to prevent displacement during castable compaction. A layer of kraft paper or plastic sheeting should be laid on the side of the formwork that contacts the castable. Steel formwork should be coated with machine oil to remove surface roughness during demolding. During casting, a penetrating vibrator is generally used for compaction, stopping when the surface of the castable shows signs of slurry. Excessive compaction time will cause internal delamination, leading to a decrease in strength. Insufficient compaction time will result in a loose interior. This applies to phosphate castables as well.

Cultivation and Baking Process of Phosphate Castables

Phosphate castables are prone to absorbing moisture and deliquescence when exposed to humid atmospheres, which reduces their compressive strength. Therefore, they should be maintained in a dry environment. The ideal maintenance temperature is 20-25℃. If the ambient temperature is low, low-temperature baking is required. After a major overhaul of the hot blast stove, the ceramic burner (unfired) can be baked using an electric furnace. Phosphate castables contain a certain amount of water added during manufacturing (when diluting additives), and the castable also needs to undergo multiple dehydration processes at different temperatures to undergo crystal transformation and polymerization. Therefore, baking is necessary before using the furnace lining.

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Baking also allows the castable furnace lining to reach a certain sintering strength. Improper baking can cause damage to the castable furnace lining, such as cracking, detachment, or explosion. A scientific baking curve should be developed based on the castable construction area and thickness. For furnaces where the entire lining (including walls, top, or bottom) is constructed using phosphate castable, the baking time is generally 7-10 days. Specific holding times are performed at 150℃, 350℃, and 650℃, while the temperature can be continuously increased for the remaining time. For smaller construction areas, wood or an electric furnace can be used to bake the refractory to over 350℃. At this temperature, most of the free and crystalline water in the castable will be removed. Then, the baking process can be carried out according to the conditions of the refractory brick lining furnace.

Rongsheng Refractories’ phosphate castables are characterized by strong resistance to spalling, airflow erosion, and abrasion. The physical and chemical properties of the raw materials used to manufacture the phosphate castables meet the specifications, and the amounts of binders and accelerators are strictly controlled. Sufficient time must be allowed after the first mixing. Only by meeting these requirements can qualified phosphate castables be produced.

Precautions for Using Phosphate Refractory Castables

There are several important precautions to keep in mind when using phosphate refractory castables. First, if no setting accelerator was added during the mixing and molding process, the castable can be heated and baked after standing in a natural environment for two hours, followed by demolding. If a setting accelerator was added, the molded castable can be cured in a natural environment above 10°C.

When the ambient temperature is above 20°C, demolding can be done after 3 to 5 hours of curing. However, if the temperature conditions are not met, the curing time should be extended by 5 hours before demolding. Throughout the curing process, it is crucial to ensure that the castable is completely isolated from water to avoid any form of moisture contact. This is essential because the presence of moisture can adversely affect the performance of the castable.

Furthermore, the entire curing period is 3 days. During this period, a suitable curing environment should be maintained, and the demolding time and waterproofing requirements mentioned above must be strictly adhered to. By following these precautions, the excellent performance and stability of the phosphate refractory castable can be ensured during use.

Factors Affecting the Performance of Phosphate-Bound Castables

① Cementitious Agent Concentration and Dosage

The concentration and dosage of the cementitious agent play a crucial role in the performance of phosphate-bound castables. Only when the concentration and dosage of the cementitious agent are appropriate can the castable exhibit excellent performance. From a molding performance perspective, if the concentration of phosphate is moderate but the dosage is insufficient, the castable will be too dry and difficult to mold. Conversely, if the dosage is excessive, the castable will become too thin, and the hardening rate after molding will be relatively slow.

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Furthermore, the concentration and dosage of phosphate also have a significant impact on the room temperature compressive strength. With the main material composition and accelerator dosage remaining constant, the room temperature compressive strength tends to decrease with increasing phosphate concentration and dosage, and may even exhibit delayed setting or non-setting. It is worth noting that phosphate castables without added accelerators typically do not solidify at room temperature, which is a normal phenomenon.

The ideal concentration range for phosphoric acid and aluminum dihydrogen phosphate is approximately 40%-60%, while the recommended addition amount is controlled within the range of 11%-14%. It is worth noting that the optimal concentration and dosage of the binder will vary depending on the type of refractory aggregate powder used. For example, if the moisture content of the raw materials is too high, the concentration of the binder should be appropriately increased or its dosage reduced to ensure the castable performance reaches its optimal state.

② Refractory Aggregates and Fine Powders

Refractory aggregates and fine powders are the core components of phosphate castables, and their type, dosage, fineness, and particle size distribution have a profound impact on the performance of refractory castables. Therefore, in engineering applications, careful selection based on specific requirements is essential to ensure that the materials can achieve optimal performance.

Refractory aggregates play a skeletal role in refractory castables, typically accounting for about 70% of the total amount. The selection of their type has a significant impact on the performance of phosphate refractory castables. Different types and grades of aggregates exhibit varying performance characteristics; therefore, selecting suitable refractory aggregates is crucial for ensuring the performance of castables.

Simultaneously, refractory powders react with binders, undergoing a series of physicochemical changes that directly determine the final properties of the refractory castable. Experimental data shows that the post-firing compressive strength of refractory castables increases with increasing aluminum content in the powder. Particularly at around 1400℃, lower-grade powders are more conducive to sintering, exhibiting higher strength. Furthermore, the load softening temperature also slightly increases with increasing aluminum content in the powder, but its effect on parameters such as post-firing linear deformation, apparent porosity, and bulk density is not significant.

To ensure excellent performance of phosphate castables, a sufficient quantity of powder must uniformly coat the particle surface and fully react with the binder to form a strong bond. Therefore, the fineness and dosage requirements for powders used in phosphate-bonded castables are very strict: the fineness should be less than 0.088 mm, and the dosage should be approximately 30%-40%. Meanwhile, to avoid adverse effects on the performance of the castable, refractory mortar must not be used as powder, and no other impurities should be mixed into the powder.

③ Accelerator

Accelerators play a crucial role in the performance of phosphate-bonded castables. At room temperature, phosphate castables cannot set and harden on their own without the addition of an accelerator. To ensure that the castable has sufficient compressive strength to avoid damage during demolding, transportation, and hoisting, an appropriate amount of accelerator must be added. Currently, there are many types of accelerators on the market, and aluminate cement is one of the most commonly used and effective ones. Typically, adding only 2%-3% aluminate cement can significantly increase the setting and hardening speed of the castable, thereby enhancing its compressive strength and meeting various engineering requirements.

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