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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.

Permeability Resistance of Basic Refractory Castables

June 26, 2026 by rsrefractorycastable

Basic refractory castables represent a significant advancement and evolution in refractory castable technology. Currently, the range of basic refractory castables includes magnesia, magnesia-chrome (MgO-Spinel), magnesia-alumina (MgO-Spinel), magnesia-zirconia, magnesia-alumina-zirconia (MgO-Spinel-ZrO₂), magnesia-alumina-titania [MgO-Spinel(Ti)], and MgO-CaO castables, among others. Typically, the formulations for basic refractory castables are designed with an ultra-low cement bond (utilizing ultrafine powders, with CAC serving as a setting accelerator) or a cement-free bond (utilizing ultrafine powders); designs incorporating a low-cement bond (CAC + ultrafine powders) are comparatively rare.

As we have previously discussed, there are several major challenges regarding basic refractory castables that require resolution:

Ensuring sufficient working time.
Preventing the hydration of fine basic powders during the drying process.
Maintaining excellent volume stability under conditions involving multiple thermal cycles.
Minimizing slag penetration during operational use.
Preventing structural spalling when in contact with basic ferrites (iron-oxide-bearing slags).

How to Improve the Slag Penetration Resistance of Basic Refractory Castables?

Apart from issues related to hydration, another significant challenge facing basic refractory castables is their low resistance to slag penetration.

To address this, studies have investigated the incorporation of additives such as SiO₂, Al₂O₃, Cr₂O₃, and ZrO₂·SiO₂ to enhance the material’s resistance to slag penetration. The results indicated that ZrO₂·SiO₂ offered the highest resistance to slag penetration; however, its use was accompanied by a significant issue: severe corrosion. Consequently, researchers explored the combined addition of Al₂O₃ and TiO₂ to MgO-rich alkali refractory castables. This approach aims to generate a [MgO-Spinel(Ti)ss] material system, utilizing a magnesium-titanium spinel solid solution [Spinel(Ti)ss] as the bonding phase. Upon firing, materials of this type fall under the category of spinel-bonded MgO-spinel refractory castables.

Many researchers have observed a crucial fact: the addition of fine spinel powder to refractory castables helps to improve their resistance to slag penetration. Furthermore, it has been found that the finer and more uniform the added spinel particles are, the more effectively they limit slag penetration. However, even extremely fine pre-synthesized spinel powders are significantly larger than the spinel formed via *in-situ* reactions. Refractory castables containing pre-synthesized fine spinel powder offer only limited protection against slag penetration into the matrix, whereas *in-situ* formed spinel effectively restricts such penetration. This conclusion has been consistently validated in practical applications involving basic refractory castables.

Additionally, controlling the penetration of slag into the matrix of basic refractory castables can be achieved by increasing the slag’s viscosity; both fine SiO₂ and Al₂O₃ powders are effective in increasing slag viscosity.

Given the reactivity of basic refractory castables with molten metals and their susceptibility to corrosion by basic slags, it is necessary to limit the amount of added SiO₂. This is particularly true for basic refractory castables utilizing ultra-fine SiO₂ (uf-SiO₂) as a binder; while the use of uf-SiO₂ reduces slag penetration, it simultaneously increases the material’s susceptibility to corrosion. In such instances, the combined use of ultra-fine Al₂O₃ (uf-Al₂O₃) can serve to strike a balance between the material’s corrosion resistance and its resistance to slag penetration. This figure indicates that when the addition of fused SiO₂ (medium-sized particles) is 4% and the addition of uf-Al₂O₃ is no less than 5%, the magnesia refractory castable exhibits superior erosion resistance and penetration resistance.

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Magnesia-Cement Refractory Castables

Magnesia cements primarily include periclase cement and periclase-spinel cement; these are produced by finely grinding highly recrystallized sintered magnesia.

Setting and Hardening Mechanism of Magnesia-Cement Refractory Castables

The mechanism responsible for their setting and hardening is primarily the hydration reaction of magnesium oxide and the crystallization of magnesium hydroxide.

However, the hydration rate of periclase cement is relatively slow; therefore, admixtures—such as magnesium chloride, magnesium sulfate, and magnesium nitrate—should be added to accelerate the hydration and crystallization processes. It is well known that magnesium hydroxide exhibits very low solubility in water, making it difficult for its colloidal gel to crystallize into large particles.

Upon the addition of these admixtures, the boiling point of the mixing water is elevated, and the solubility of magnesium hydroxide is increased, thereby accelerating the crystallization and growth of the magnesium hydroxide gel. These acicular (needle-like) brucite crystals intergrow, imparting strength to the magnesia-cement refractory castable.

Magnesium chloride admixtures generally yield superior results compared to other admixtures, such as magnesium sulfate. This is because magnesium chloride admixtures also facilitate the formation of magnesium oxychloride, which gradually crystallizes and further promotes the setting and hardening of the cement.

Enhancing the Intermediate-Temperature Strength of Periclase-Cement Refractory Castables

The mix design for magnesia-cement-bonded refractory castables typically consists of refractory aggregates (specifically metallurgical magnesia aggregates and chrome slag), cement, and iron powder. A solution containing magnesium chloride and magnesium sulfate is added as the mixing liquid, while the iron powder functions as a mineralizer or sintering aid.

The maximum service temperature for this class of refractory castables is 1600°C. Periclase-cement refractory castables exhibit high strength after drying. However, at temperatures around 400°C, the strength begins to decline due to the thermal decomposition of magnesium hydroxide. As the heating temperature increases further, the microstructure becomes more porous, and the strength continues to diminish. When the temperature reaches the 1000–1200°C range—prior to the onset of sintering—the strength drops to its lowest point, ranging from a mere 2.5 to 9.3 MPa. Once the temperature exceeds 1200°C, the strength recovers slightly due to the recrystallization of magnesium oxide. At approximately 1400°C, solid-phase reactions commence, resulting in an increase in strength to approximately 25% of the dried strength. The addition of iron powder and chrome slag promotes the sintering of periclase, thereby enhancing the medium-temperature strength.

Applications of High-Strength Wear-Resistant Al2O3-SiC-C Castables

June 11, 2026 by rsrefractorycastable

High-strength wear-resistant castables are suitable for use in areas of industrial kilns subject to severe abrasion. Compared to standard refractory castables, they possess higher density and superior wear resistance; furthermore, they are capable of withstanding impact forces at high temperatures and exhibiting strong shear resistance under heavy structural loads.

The defining characteristics of wear-resistant castables are their compressive strength, flexural strength, and abrasion resistance. These properties are directly linked to the specific process formulation and the raw materials utilized. The raw materials used in wear-resistant refractories typically feature a high alumina content and high bulk density; additionally, the aggregate particles within the process mix are generally larger than those found in standard refractory formulations. Incorporating a specific proportion of silicon carbide into the mix further enhances the castable’s flexural strength and wear resistance.

High-Strength Wear-Resistant Castables Refractory

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Performance Characteristics of Wear-Resistant Castables

When a suitable proportion of steel fibers is added to wear-resistant castables, their tensile strength is significantly increased, resulting in improved operational performance. During production, the addition of heat-resistant stainless steel fibers to the aggregate and powder mixture helps prevent thermal expansion-induced stresses at high temperatures and enables the material to withstand the thermal gradient stresses generated during furnace start-up and shut-down cycles. Thanks to the inclusion of steel fibers, the cast furnace lining exhibits enhanced structural integrity and tensile bonding strength. If nickel-plated stainless steel fibers are utilized, the material demonstrates even greater high-temperature stability, as well as enhanced resistance to oxidation and thermal fatigue.

High-strength, wear-resistant castables are available in various grades, distinguished by differing bulk densities and performance specifications. The specific grade selected for a given application is determined by the operating temperature and the atmospheric conditions within the furnace. Regardless of the specific quality grade, however, superior wear resistance remains a fundamental characteristic inherent to every variety of this castable material.

Where Are Wear-Resistant Castables Most Effectively Applied?

High-strength, wear-resistant castables are ideally suited for use in the front and rear arch walls of various industrial boilers, as well as in the furnace linings of waste incinerators. Additionally, composite-type wear-resistant castables are frequently employed in areas such as electric furnace roofs, heating furnace hearths, cement rotary kiln mouths, and material-retaining rings.

Al2O3-SiC-C Castable Refractory — Al2O3-SiC-C Refractory Castable

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Properties and Applications of Al2O3-SiC-C Refractory Castables

Alumina-Silicon Carbide-Carbon (Al2O3-SiC-C) castables are a type of castable refractory material composed of corundum or high-alumina clinker, silicon carbide, carbon binders, and various additives. Primarily utilized as the lining for blast furnace tapholes and iron troughs, they are also commonly referred to as “iron trough refractory castables.”

The high-velocity flow of molten iron and slag—reaching flow rates of several tons per minute—subjects the trough lining to severe mechanical erosion and abrasion. Given this extremely harsh operating environment, the castables are required to possess exceptional resistance to both chemical corrosion and mechanical abrasion. Furthermore, the operation of the iron trough is intermittent; temperatures drop sharply at the conclusion of each tapping cycle, subjecting the castable lining to severe thermal shock. Consequently, materials used for iron trough linings must exhibit excellent thermal shock resistance. The castable must maintain sufficient structural integrity at high temperatures to withstand the erosive forces of the molten iron and slag, as well as its own gravitational load.

Due to the inherently low permeability of this type of castable, rapid spalling caused by the sudden evaporation of moisture is a significant risk during the drying and baking process. Therefore, anti-explosive agents—such as metallic aluminum powder, aluminum lactate, azodicarbonamide, or anti-explosive fibers—are typically incorporated into the mixture. However, the dosage of these anti-explosive agents must be strictly controlled; excessive addition can lead to a reduction in bulk density and mechanical strength, as well as a deterioration in corrosion and abrasion resistance.

The specific composition of Al2O3-SiC-C castables varies depending on the specific operating environment and conditions. For instance, castables intended for the iron troughs of large-scale blast furnaces typically require electro-fused corundum as the primary aggregate, whereas medium-to-small blast furnaces may utilize high-alumina clinker as the aggregate material. The proportion of silicon carbide added also varies according to the specific location within the trough: concentrations typically range from 18% to 30% in the taphole and slag-line zones, while areas below the slag line generally contain 12% to 15% silicon carbide.

Alumina-Silicon Carbide-Carbon castables can be applied either by direct on-site casting or by utilizing pre-cast refractory shapes. The service life of these materials varies significantly, depending largely on the quality of the raw materials employed in their manufacture. In large blast furnaces, corundum-silicon carbide-carbon castables are employed to construct the lining of the main trough (450–500 mm thick); the typical iron throughput before maintenance is 100,000 to 150,000 tons of hot metal, which can be extended to over 300,000 tons following patch-casting or gunning repairs.

Changes in the Properties of Alkali-Resistant Castables for Cement Kilns Upon the Addition of Various Admixtures

June 4, 2026May 27, 2026 by rsrefractorycastable

Temperatures in various sections of a cement kiln—including the preheater system, precalciner, riser duct, and tertiary air duct—range between 800°C and 1200°C. In these areas, due to the enrichment of alkalis within the kiln gas, the refractory materials employed are susceptible to alkali corrosion; this process readily leads to the formation of expansive minerals—such as leucite (KAlSiO4) and kalsilite (KAlSi2O6)—within the refractory bricks. Consequently, the material structure becomes porous and prone to cracking, severely compromising kiln operations. For sections where shaped refractory bricks are unsuitable, high-performance alkali-resistant castables are typically utilized as the kiln lining. Alkali-resistant castables are hydraulic refractory materials formulated using alumina-silica aggregates as raw materials, calcium aluminate cement as a binder, and appropriate chemical admixtures to impart alkali resistance.

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When alkali-resistant castables come into contact with alkali vapors under high-temperature conditions, a dense glaze-like layer forms on the surface. This layer effectively inhibits further corrosive penetration, thereby extending the service life of the kiln lining. Cement and silica fume constitute the primary raw materials for refractory castables, and their respective addition levels exert a significant influence on both the ambient-temperature and high-temperature performance of the material. In this study, construction ceramics were employed as the principal raw material to systematically investigate the impact of varying cement and silica fume addition levels on the performance of alkali-resistant castables; furthermore, the microstructure of the test specimens was subjected to detailed analysis.

The Effect of Calcium Aluminate Cement Content on the Properties of Castables

This study investigates the impact of calcium aluminate cement content on the room-temperature properties of specimens subjected to heat treatments of 110°C for 24 hours and 1100°C for 3 hours. It is observed that, following the 110°C/24h treatment, the room-temperature strength of the specimens increases as the cement content rises. Conversely, following the 1100°C/3h heat treatment, the room-temperature strength of the specimens exhibits a trend of initially increasing and then decreasing as the cement content increases. Furthermore, after the 1100°C/3h heat treatment, the permanent linear change upon heating—specifically, the shrinkage rate—increases as the cement content increases. In the formulation design, the increase in calcium aluminate cement content was achieved by reducing the proportion of 200-mesh fine powder derived from construction ceramics. With the content of silica fume held constant, variations in specimen strength are primarily attributable to changes in the cement content. An increased cement content generates a greater quantity of hydration products, which serve a binding function while simultaneously filling pores within the specimen, thereby enhancing both the density and strength of the specimen.

Following the 1100°C/3h heat treatment, the hydration products formed by the calcium aluminate cement at room temperature are decomposed due to the removal of structural water, thereby losing their binding efficacy. When the cement content does not exceed 8%, the increase in the specimen’s room-temperature strength is primarily attributed to the fact that the cement particles are finer than the construction ceramic fine powder and possess higher reactivity; consequently, the majority of the strength is derived from sintering. Under the experimental conditions, when the cement content exceeds 8%, the CaO within the cement reacts with the Al2O3 and SiO2 present in the matrix to form low-melting eutectics. At high temperatures, the formation of a significant liquid phase—coupled with the dehydration shrinkage of the cement colloids, which induces cracking—leads to a reduction in strength. Therefore, the formation of these low-melting eutectics is the primary reason why the permanent linear change upon heating (shrinkage) of the specimens, following the 1100°C/3h heat treatment, increases in proportion to the increase in cement content.

Furthermore, the surface of the construction ceramic raw materials contains certain glaze components. These glaze components possess relatively low melting points and are therefore more prone to forming a liquid phase during the sintering process. On one hand, the formation of this liquid phase facilitates structural densification, acting as a sealant and enhancing alkali resistance. On the other hand, the liquid phase forms predominantly at the interfaces between the aggregate particles and within the matrix, thereby strengthening the bonding performance between the matrix and the aggregates. While this enhances the structural integration of the castable’s aggregates and matrix, it simultaneously leads to a reduction in the castable’s room-temperature toughness and an increase in its brittleness. This is particularly significant for castables, given their inherently heterogeneous internal structure; consequently, their strength becomes highly sensitive to various factors such as internal defects and cracks—factors that ultimately impact overall strength. Taking all these considerations into account, the optimal addition level for calcium aluminate cement is determined to be approximately 7% to 8%.

The Effect of Microsilica Content on the Properties of Castables

Under the condition that the calcium aluminate cement content is fixed at 7.5%, this study investigates the impact of varying microsilica content on the room-temperature properties of specimens following heat treatments at 110°C for 24 hours and 1100°C for 3 hours. It can be observed that as the microsilica content increases, both the flexural and compressive strengths of the specimens—after heat treatment at different temperatures—show an improvement; however, this trend is not particularly pronounced, and the linear shrinkage rate of the specimens increases concurrently. Microsilica is an ultrafine powder material formed through the rapid gas-phase reaction and subsequent condensation of SiO₂ and Si gases—generated during the smelting of ferrosilicon alloys and industrial silicon—with oxygen in the air. Possessing high surface activity, it functions similarly to silica sol as a binder upon hydration, thereby imparting a certain level of strength and enhancing the overall strength of the specimens following heat treatments at various temperatures. Furthermore, microsilica effectively fills the voids between the aggregates and the matrix, thereby reducing the water demand for mixing and increasing the bulk density of the alkali-resistant castable.

Microsilica serves both as a filler and a sintering promoter; when used in conjunction with appropriate dispersants, it enables the castable to exhibit superior rheological properties with minimal water addition, while simultaneously enhancing the high-temperature strength of the alkali-resistant castable. In the experimental design, the increase in microsilica content was achieved by correspondingly reducing the proportion of 200-mesh fine powder derived from construction ceramics. Since microsilica exhibits significantly higher reactivity at elevated temperatures compared to the fine powder from construction ceramics, an increased microsilica content results in a higher post-firing shrinkage rate for the specimens. Conversely, if the microsilica content is too low, it is insufficient to effectively enhance the strength of the alkali-resistant castable; conversely, if the content is excessively high, it tends to induce cracking, thereby compromising the high-temperature volume stability of the castable. Based on a comprehensive analysis, an optimal microsilica content of 5% to 6% is recommended.

Microstructural Analysis

The micrograph presented here reveals the cross-sectional microstructure of a specimen—prepared with a 7.5% addition of calcium aluminate cement and a 5% addition of silica fume—following a heat treatment at 1100°C for 3 hours. As observed, the specimen exhibits a distribution of residual micropores; however, the bonding between the aggregates and the matrix is robust. This specific structural configuration primarily facilitates transgranular fracture—a mode of failure in which cracks propagate directly through the aggregate particles—which is highly conducive to enhancing material strength. The aggregate particles are predominantly lamellar (flaky) in shape; this morphology is attributed to the source of the raw building ceramics used, their original forming method, and the subsequent crushing and processing procedures. Notably, this lamellar morphology is detrimental to improving the flowability of the castable material. Within the matrix, the introduced silica fume reacts with alumina powder to generate columnar or acicular mullite crystals. These crystals exhibit high inter-granular bonding strength and serve to intersperse within or fill the voids within the material’s skeletal framework. The network structure formed during the development of this mullite phase exerts a significant reinforcing and toughening effect on the material. Furthermore, the matrix contains a certain proportion of a glassy phase, which is beneficial for enhancing both the density and the alkali resistance of the castable. This glassy phase originates from two primary sources: the glaze components present in the raw building ceramics, and the *in-situ* formation of eutectic phases within the matrix itself.

High-Performance Alkali-Resistant Castables for Cement Kilns

(1) The change in room-temperature strength of specimens—following heat treatments at 110°C for 24 hours and 1100°C for 3 hours—is correlated with the addition amount of calcium aluminate cement. Increasing the cement content leads to an increase in the volume of eutectic phases within the specimens at high temperatures, while simultaneously resulting in a greater permanent linear change. Under the experimental conditions, an optimal addition amount of calcium aluminate cement is approximately 7%–8%.

(2) The incorporation of silica fume serves to reduce the required water content while enhancing the density and strength of the castable. Under the experimental conditions, an addition amount of 5%–6% silica fume is considered optimal.

(3) Following the heat treatment at 1100°C for 3 hours, the cross-sections of the specimens exhibit transgranular fracture; furthermore, the matrix reveals evidence of a dual reaction process involving the formation of both mullite and eutectic phases.

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Lightweight Alkali-Resistant Castables for Cement Kilns

Lightweight alkali-resistant castables are a type of hydraulic refractory castable formulated using lightweight alumino-silicate materials as the primary raw material, calcium aluminate cement as the binder, and an appropriate amount of additives to impart alkali resistance. These lightweight refractory castables are characterized by their low bulk density, excellent resistance to alkali corrosion, and strong thermal insulation properties. At high temperatures, they react with alkali metal oxides to generate a high-viscosity liquid phase, forming a dense, vitrified protective layer that effectively prevents the further penetration and corrosion of molten alkali substances. Lightweight alkali-resistant refractory castables are primarily utilized as alkali-resistant thermal insulation linings in areas of kilns and furnaces subject to alkali corrosion, such as the preheaters within cement kiln pre-calcination systems, the ductwork of decomposition furnaces, and the preheating and decomposition zones of dry-process rotary kilns.

Installation of Lightweight Alkali-Resistant Castables

Water: Clean potable water with a pH value between 6 and 8 must be used; the mixing water should be accurately weighed. Water usage: 16%–17% of the total dry material weight for Premium Grade products; 18%–20% for First Grade products.
Mixing: A forced-type mixer must be used. All mixing tools must be clean, and mixing should continue until the material is thoroughly uniform.

Installation Precautions

All molds intended for casting must be coated with a layer of machine oil.
All embedded metal components must be coated with a layer of asphalt paint prior to casting.
Lightweight castable mixtures must be used within 0.5 hours of mixing. When the casting thickness is 200 mm or less, it is advisable to cast the material to the full specified thickness in a single pour, vibrating it until it is completely compacted.
Curing: Maintain a relative humidity of 100%. The curing temperature should be kept between 10°C and 30°C. Molds may be removed 24 hours after casting; the total curing period is 3 to 7 days.
Lightweight alkali-resistant castables must not be mixed with, or placed in direct contact with, freshly mixed concrete.

Drying Schedule

Linings or structures constructed using lightweight alkali-resistant castables must undergo appropriate heat treatment prior to use.

Dry at approximately 150°C to eliminate adsorbed water; the duration depends on the thickness of the material.
Raise the temperature at a rate of 25°C/h up to approximately 500°C; this rate may be adjusted based on the specific section of the structure and practical feasibility.
Maintain a constant temperature of approximately 500°C for 2 days.
From approximately 500°C, raise the temperature at a rate of 30–50°C/h until the operating temperature is reached.
Maintain the operating temperature for a minimum of 2 days, adjusting the duration based on the specific requirements and practical constraints of the various sections of the structure.

Lightweight Castable Lining for Petrochemical Tubular Furnaces

June 4, 2026May 12, 2026 by rsrefractorycastable

Tubular furnaces in the petrochemical industry utilize lightweight castable linings bonded with high-alumina cement, featuring a bulk density ranging from 500 to 1300 kg/m³. The castables are accompanied by a Certificate of Conformity and a Performance Index Inspection Report issued by the manufacturer—Rongsheng Refractories—and are also supplied with detailed instructions regarding installation methods.

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Raw Materials for Lightweight Castables in Tube Furnaces

High-alumina cement.
Expanded shale lightweight aggregates and expanded clay lightweight aggregates.
Vermiculite: calcined at a high temperature of 900–950°C; its secondary expansion rate shall not exceed 0.5%, and it must be free of impurities.
Glazed perlite: subjected to high-temperature treatment at no less than 1250°C, followed by hydraulic classification and crushing; its refractoriness shall be no less than 1280°C, and its water absorption rate shall not exceed 17%.
Expanded perlite: subjected to rapid high-temperature treatment at no less than 1250°C; its refractoriness shall be no less than 1280°C.
High-temperature calcined bauxite grog.
High-alumina brick grog: produced by crushing and screening IZ-5 grade high-alumina bricks. Lightweight brick grog. Utilization of a lightweight, heat-resistant lining composed of high-alumina cement, lightweight aggregates, and vermiculite (in a ratio of 1:2:4); specific mix proportions and performance characteristics are detailed herein.

Packaging, Transport, and Storage of Lightweight Castables for Tube Furnaces

Water used for lining construction shall have a pH value between 6.5 and 7.5.
During transport, materials must be protected against moisture, properly packaged, and clearly marked.
During storage, materials shall be stacked in an orderly manner according to their category, specifications, and batch number; exposure to moisture or rain is strictly prohibited. High-alumina cement must not be stacked together with other types of cement.
If materials become contaminated or deteriorate due to moisture ingress resulting from damaged packaging or spillage, the affected packages must not be used.
Expired materials may be used only after passing a re-inspection; however, their reuse is generally not recommended.

Preparations for the Installation of Lightweight Castable Refractory in Tubular Furnaces

Personnel responsible for the installation of the castable lining must undergo training and pass a qualification assessment before participating in the construction work.
During castable installation, the ambient temperature must be above 5°C; otherwise, cold-weather protection measures must be implemented.
All containers and tools used for castable installation must be thoroughly cleaned to prevent contamination by residual lime, cement, clay, or other debris.
Prior to castable installation, all embedded components—such as openings in the furnace wall, refractory anchors, and sleeves—must be fully installed and verified as compliant through inspection. Any temporary fixtures that obstruct the lining installation process, or that cannot be removed after the lining is in place, must be completely removed before construction begins.
Refractory anchors must be positioned and welded in strict accordance with the design specifications; the weld beads must be full and free of undercut defects. Each anchor must be individually struck with a 0.5 kg hand hammer; a clear, ringing metallic sound should be produced upon impact. For cylindrical and Y-shaped anchors, a random sample check must be performed at a rate of one anchor per 4 square meters: the top of the welded anchor is struck with a hammer and bent to a 90-degree angle; it must not fracture during this process. If it does fracture, a replacement anchor must be welded immediately adjacent to the failed one. In the event of a fracture, the underlying cause must be investigated, and appropriate remedial measures must be formulated.
Before castable installation, the furnace wall must undergo thorough rust removal—either manually or using power tools—to completely eliminate oil stains, rust, and other surface contaminants from the interior surface. The metal surface, once derusted, must be protected from exposure to rain and moisture, and the refractory lining installation should commence as soon as possible thereafter.
The exterior surfaces of all pipe supports, sleeves, and other metal components (excluding refractory anchors) that are to be embedded within the castable lining must—following rust removal—be coated with a 0.5 to 1 mm thick layer of asphalt, or wrapped with a 0.5 to 1 mm thick layer of ceramic fiber paper or kraft paper.
If metal mesh reinforcement is required over the refractory anchors prior to castable installation, the mesh must be properly positioned and securely fastened to a flat plane to ensure its correct placement within the finished lining.
Prior to castable installation, appropriate protective measures must be implemented for any embedded pipes or tubes.
The surfaces of any hygroscopic masonry components that will come into contact with the castable refractory must be treated with waterproofing measures. 11 Prior to construction, the properties of the castable shall be tested; construction may proceed only after the tests have been successfully completed.

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Construction and Quality Inspection of Lightweight Castables for Tubular Furnaces

Mechanical Spraying Method

Prior to formal spraying operations, trials regarding the spraying process and tests on the performance of the finished product must be conducted. Finished product performance parameters shall include bulk density, compressive strength, flexural strength, and linear change after firing; formal construction may commence only after verification confirms compliance with the requirements specified in the design documents. During construction, operations must strictly adhere to the spraying process established during these trials.
During spraying, the moisture content of the lining must be strictly controlled in accordance with the requirements outlined in the construction method instructions for the specific material grade being used. The lining’s moisture content shall be determined as specified in Appendix AQ2.
When employing the mechanical spraying method, the lining shall be sprayed in sections, proceeding from bottom to top, and the process must be continuous until the required thickness is achieved within the designated area. If spraying is interrupted, the lining material must be immediately cut back to the surface of the wall panel; the cut face shall be perpendicular to the wall panel surface.
Rebound material generated during the construction process shall not be reused for the lining.
The volume of rebound material generated during the spraying process shall not exceed the limits specified in the construction method instructions for the specific material grade being used.

Manual Ramming Method

During mixing, while ensuring adequate workability, the water content should be minimized as much as possible, and the water-to-material ratio must be strictly controlled. The appropriate water-to-material ratio shall be provided by the manufacturer. Note: Water-to-material ratio = Water / (Cement + Aggregate).
During construction, the number of joints should be minimized as much as possible. If the area is large, or if other circumstances necessitate sectional construction, the joints shall be formed in a stepped configuration, as illustrated in Figure 4.2.2. For linings with a thickness not exceeding 75 mm, straight joints may be utilized. Prior to resuming construction on an adjacent section, the joint surface must be scored to create grooves, loose particles must be removed, and the surface must be moistened with water before ramming operations may continue.

Quality Inspection

Linings that have been applied via spraying or casting must be shaped to match the external dimensions specified in the design documents before initial setting occurs. During leveling and compaction, the application of water, cement slurry, or dry powder to the surface is strictly prohibited.
During the construction process, test specimens (test blocks) shall be sampled in accordance with the specified construction process parameters. For each specific grade or mix ratio within a single project, test specimens shall be retained for inspection in batches of 20 m³; quantities less than 20 m³ shall also constitute a single inspection batch. The inspection items shall include bulk density, compressive strength, flexural strength, and linear change after firing; these results shall be recorded in the project handover and acceptance documentation.
Upon completion of construction, the lining surface shall be flat and of uniform thickness; the allowable tolerance for thickness is +5 mm.
After the lining has been constructed and cured, the entire surface of the lining shall be tapped using a 0.5 kg hammer at grid points spaced according to the following specifications: Furnace Roof: 610 × 610 mm; Side Walls and Furnace Bottom: 920 × 920 mm. The tapping sound should be solid; hollow sounds (indicating voids) are not permitted.
After the furnace has undergone the drying-out process, the width of any cracks on the lining surface shall not exceed 5 mm, and the depth shall not exceed half of the lining thickness (1/2); furthermore, no through-cracks or interconnected network cracks are permitted.
Expansion joints shall be provided in accordance with the requirements specified in the design documents. In the absence of specific design requirements, for any lining with a thickness exceeding 75 mm, a grid-patterned (cross-hatch) expansion joint—2 to 3 mm in width and 20 to 30 mm in depth—shall be provided at longitudinal and transverse intervals of 800 to 1200 mm.

Lining Patching and Repair at Joints

The patching of lining at joints shall comply with the following provisions:

- (1) At the lining interfaces of components that were assembled and welded in sections, a margin of no less than 100 mm in width shall be left unlined on each side of the joint.
- (2) Lining patching at the joint interface may only proceed after the welding of the joint seam and the anchoring studs has been inspected and approved.

Any defects identified during the lining construction process that fail to meet the requirements of the design documents—and which would adversely affect the intended use of the lining—shall be repaired in accordance with the applicable regulations.
The lining at the repair site must be chipped away down to a sound surface or the steel shell, exposing at least two anchor studs. The chipped-out section of the lining should be shaped such that it is narrower at the outer surface and wider at the inner surface.
The area to be repaired must be thoroughly cleaned and moistened with water.
The raw materials, mix proportions, installation methods, and curing procedures used for patching the lining at the joint—as well as for the general repair—must be identical to those employed during the original installation of the castable lining.
For cracks that do not meet the specified criteria for standard repair, refractory fibers impregnated with a high-temperature bonding agent should be used as packing material, selected according to the operating temperature of the application.

Curing of Lightweight Castable Refractories in Tubular Furnaces After Installation

Appropriate curing must be performed after the installation of each layer of the castable lining. Curing procedures should strictly follow the requirements specified by the castable manufacturer. In the absence of specific requirements, water-spray curing should commence once the lining has reached its initial set—specifically, when the surface no longer adheres to the hand upon light manual pressure. The curing period must extend for a minimum of 24 hours, with water spraying performed approximately every 30 minutes; the frequency of spraying may be adjusted as appropriate based on prevailing climatic conditions.
Steam curing is strictly prohibited. During the water-spray curing period, the lining should not be covered with materials such as straw bags or similar items.
Upon completion of the lining curing process, an additional 48-hour period of natural air drying is required before the furnace unit may be moved or hoisted.

Baking-out of Lightweight Castable Refractories in Tubular Furnaces After Installation

Once the curing of the castable lining is complete, the ambient temperature must be maintained above 5°C. Furthermore, a minimum period of 5 days of natural air drying is required before the baking-out process may commence.
The following preparatory measures must be completed prior to baking out:

- (1) All construction work on the tubular furnace unit must be fully completed and have successfully passed final inspection.
- (2) All necessary utility lines (e.g., fuel, air), fire safety equipment, and related facilities required for the baking-out process must be inspected and verified as being in good working order.
- (3) All thermal instrumentation and control devices required for monitoring the baking-out process must be fully calibrated.

During the baking-out process, steam should first be introduced into the furnace tubes to pre-warm the furnace for a period of 1 to 2 days, after which the burners may be ignited. Gaseous fuel is the preferred choice for the baking-out process. Throughout the baking-out cycle, the temperature rise must be uniform; the rate of temperature increase should strictly adhere to the manufacturer’s specifications or follow the prescribed baking-out curve.
During the baking-out process, the steam temperature at the outlet of the furnace tubes must not exceed the following limits: 350°C for carbon steel tubes, and 450°C for chromium-molybdenum steel tubes.
Comprehensive records must be maintained throughout the baking-out process, and a graph depicting the actual temperature profile (baking-out curve) must be plotted.
Upon completion of the baking-out process, a thorough inspection of the refractory lining must be conducted, and detailed inspection records must be compiled. Should any damage be detected, the underlying cause must be analyzed immediately, and appropriate repairs must be executed promptly.

Zircon Sand Ramming Mix for Glass Melting Furnaces

June 4, 2026April 27, 2026 by rsrefractorycastable

In the glass industry, the selection of refractory materials for glass melting furnaces is a critical factor in determining the service life of the furnace, and it is a subject that receives widespread attention. Ramming mixes—which are extensively utilized for the furnace hearth—represent one such material. As a hearth lining, ramming mixes offer distinct advantages: strong resistance to slag corrosion, excellent structural integrity, and minimal shrinkage. By eliminating the presence of brick joints, they effectively prevent the leakage of molten glass; furthermore, ramming mixes are easy to install, cost-effective, and highly adaptable.

Zircon Sand Ramming Mix

Ramming mix fundamentally consists of three components: base material, sintering aid, and binder. The base material constitutes the primary component of the ramming mix; it forms the refractory crystalline matrix and determines the material’s key properties. Typically accounting for approximately 90% of the mix, it is often selected from high-quality refractory materials such as zircon sand, or sintered or electrofused zircon-corundum grog. The primary function of the sintering aid is to facilitate high-temperature sintering of the material and enhance the ramming mix’s high-temperature strength; plastic clay is frequently employed for this purpose. The binder serves primarily to render the mixture easily formable and to impart a certain degree of mechanical strength; currently, aluminum dihydrogen phosphate is widely utilized as the binder. Zircon sand ramming mix is a protective lining material widely used for the furnace bottoms of glass melting kilns.

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Corrosion Mechanisms and Mineral Phase Analysis of Zircon Sand Ramming Mixes

The causes of corrosion in ramming mixes stem from two primary factors:

(1) The resistance of the ramming mix material to glass melt corrosion: This requires that the primary crystalline phase of the ramming mix possess excellent resistance to attack by molten glass.
(2) The surface density of the ramming mix: Surface pores and microcracks constitute a significant factor leading to material degradation. Due to the capillary action of these pores and microcracks, the glass melt infiltrates the interior of the ramming mix, thereby accelerating the corrosion process. Consequently, the entire ramming mix structure is required to exhibit high volume stability and excellent density.

Based on the aforementioned corrosion mechanisms, the following analysis examines the crystalline phase composition of zircon sand ramming mixes and the conditions under which they are produced, with the aim of establishing the performance requirements for such mixes regarding their resistance to glass melt corrosion.

The primary constituent of zircon sand is zircon (ZrSiO4). The theoretical composition of zircon is ≥67.2% ZrO2 and ≥32.8% SiO2; it crystallizes in the tetragonal system and has a true density of 4.6–4.8 g/cm³. Analytical data indicate that zircon exhibits good chemical stability against acids; however, at certain temperatures, it undergoes decomposition upon contact with molten alkali metal oxides, hydroxides, carbonates, and similar substances. In the presence of Na2O, the decomposition of zircon commences as early as 900°C and proceeds rapidly at 1200°C. This decomposition yields baddeleyite (ZrO2) and cristobalite; the resulting baddeleyite manifests as fine agglomerates rather than forming a dense, consolidated structure. The liberated SiO2 reacts with the Na2O and the Al2O3 present in the ramming mix to form a liquid phase, thereby rendering the ramming mix susceptible to corrosion by the glass melt. Therefore, to fundamentally enhance the resistance of the ramming mix to glass melt corrosion, efforts must be made to minimize the proportion of the zircon phase within the material as much as possible. Based on the typical composition of zircon sand ramming mixes (approximately 90% zircon sand, 5% clay, and 5% binder), their chemical composition falls within the baddeleyite phase region of the Al2O3–ZrO2–SiO2 phase diagram. Dense baddeleyite exhibits excellent resistance to corrosion by molten glass; the formulation of zircon ramming mixes is specifically designed to generate this baddeleyite phase, thereby ensuring the material’s corrosion resistance.

For zircon sand-based ramming mixes to develop baddeleyite as their primary crystalline phase during service, specific firing conditions are required. If standard zircon sand is utilized, the sintering temperature must exceed 1550°C; higher temperatures accelerate the formation of the baddeleyite crystalline phase. Typically, sintering at high temperatures—specifically above 1600°C—is mandated. If the firing temperature is too low, the conditions necessary for baddeleyite formation are not met; consequently, the baddeleyite phase will either fail to appear or will form in such negligible quantities that it cannot establish itself as the primary crystalline phase. Furthermore, due to the presence of clay within the ramming mix, sintering during firing may instead result in the formation of a mullite phase; in such cases, the original zircon phase—having not participated in the sintering reaction—remains the dominant crystalline phase. Should such a ramming mix come into contact with molten glass, it will inevitably suffer from the severe corrosion described previously. In practical application, the sintering reactions within the ramming mix are completed during the kiln’s initial heat-up (or “firing-in”) process; therefore, the peak temperature attained during this heat-up phase—as well as the duration for which this temperature is sustained—are critical factors determining whether the baddeleyite primary crystalline phase successfully forms within the ramming mix, directly impacting the material’s ultimate performance in service.

Application of Zircon Sand Ramming Mixes in Glass Furnaces

1. Currently, when ramming mixes are utilized, they typically do not come into direct contact with the molten glass; instead, a layer of high-quality refractory bricks—known as paving bricks—is laid over the ramming mix to serve as a protective surface layer with superior corrosion resistance. This method of application effectively reflects a lack of confidence in the corrosion resistance of the ramming mix itself, and it appears to negate the true intended purpose of using a ramming mix. In reality, under this specific application method, the ramming mix serves merely as a sealing and protective barrier. Consequently, some end-users do not place significant emphasis on the careful selection of the ramming mix.
2. Observations regarding the performance of ramming mixes covered by paving bricks indicate that, due to the insulating effect of the overlying brick layer, the maximum temperature reached by the ramming mix during the furnace heat-up (firing) process rarely exceeds 1400°C. Based on the preceding analysis, it is evident that zircon sand ramming mixes sintered at this temperature will not develop the desired baddeleyite (monoclinic zirconia) as their primary crystalline phase; consequently, their performance falls far short of the anticipated requirements.
3. It is essential to select a high-quality zircon sand ramming mix and formulate its composition specifically to meet the requirements for generating the baddeleyite phase. If the solid-phase reaction temperature requirements of the ramming mix are duly considered during the furnace heat-up process, the paving bricks at the furnace bottom can be eliminated from the furnace design, thereby allowing the ramming mix—now containing the generated baddeleyite primary phase—to come into direct contact with the molten glass. Such a material indeed possesses excellent resistance to corrosion by molten glass; this method constitutes the true, authentic application of a ramming mix. Furthermore, by eliminating the need for paving bricks, furnace construction costs can be significantly reduced.
4. Achieving furnace heat-up temperatures exceeding 1600°C is often an unrealistic objective for many glass furnaces; therefore, it becomes necessary to improve the zircon sand ramming mix itself. This can be achieved by incorporating appropriate mineralizers to lower the reaction temperature required for the formation of the baddeleyite primary phase. However, implementing this improvement requires extensive research and development to identify a specific formulation—including the type and quantity of mineralizers—that effectively lowers the desired reaction temperature while simultaneously minimizing the generation of undesirable glassy phases.

Performance Advantages of Zircon Sand Ramming Mixes

(1) In terms of material composition, zircon sand ramming mixes inherently possess excellent resistance to corrosion by molten glass.
(2) To ensure that zircon sand ramming mixes deliver optimal performance results, the specific conditions under which they are utilized—namely, the application environment and operational parameters—are often of even greater significance. Typically, the kiln temperature must reach 1600°C for the sintering reaction of the main baddeleyite crystal phase to occur. However, this temperature threshold can be further adjusted and optimized by modifying the material composition.

What Factors Determine the Performance of Lightweight Castables?

April 12, 2026 by rsrefractorycastable

Lightweight castable, also known as insulating castable refractory, is a lining material used in the insulation layer of industrial kilns. However, the performance of lightweight castable is determined by raw materials, process proportions, production process control, construction site specifications, and proper drying and baking.

Factors Determining the Performance of Lightweight Castables

First, the fundamental factor determining the performance of lightweight castables is the careful selection of lightweight aggregates and powders, and the rationality of particle size distribution. Water usage should be minimized to maximize the density and strength of the insulating castable refractory.

Secondly, the binder is crucial in determining the initial strength and high-temperature performance of lightweight castables. Generally, lightweight insulating castable refractory with high cement or water glass content requires the use of micro-powders to enhance strength and improve performance.

Chemically bonded lightweight castables typically employ composite bonding to optimize performance across different temperature ranges. However, the binder ratio must be carefully controlled; too much binder reduces strength and refractoriness, while too little results in insufficient strength at room temperature. Accelerators and retarders should be avoided unless used to adjust the setting rate during construction; their addition should be minimized based on actual conditions. Appropriate amounts of explosion-proof fibers can be added to create venting channels during baking, preventing material cracking under steam pressure.

By controlling the production, process proportions, and construction stages, the performance of lightweight insulating castable refractory is essentially controlled. The selection of the type of lightweight castable depends on the properties, temperature, and size of the industrial furnace lining. For example, lightweight castables suitable for acidic media are used for acidic furnace linings, while neutral materials are chosen for neutral lightweight insulation. The bulk density also depends on the temperature of the furnace lining, determining whether a lightweight castable with a bulk density of 1.0, 0.8, or 0.6 is used.

Insulating Castables Can Be Applied To Kilns To Save Energy — Lightweight Insulating Castable Refractory

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Design and Performance Testing of a Lightweight Insulating Refractory Castable

By selecting lightweight mullite aggregate with a porous structure, alumina hollow spheres, and zirconium-containing high-alumina refractory fibers with low thermal conductivity, the thermal insulation performance of the furnace roll is improved, reducing heat loss carried away by cooling water. Simultaneously, the bulk density is reduced, decreasing the weight of the furnace roll and the power consumption for rotation. The addition of 0.1~1 mm slag bauxite compensates for the performance degradation of lightweight aggregates smaller than 1 mm. Furthermore, the excellent thermal shock stability and high-temperature performance of slag bauxite improve the overall performance of the insulating castable refractory. Cement, silica, and -Al₂O₃ micro powder are used as composite binders. Both hydration and coagulation bonding mechanisms are introduced to improve the low, medium, and high-temperature strength of the material.

The addition of a small amount of spodumene powder promotes sintering, improving the sintering condition of the castable under operating conditions and promoting the formation of a high-temperature ceramic bonding phase in the furnace roll castable insulation lining, thus enhancing the insulation lining’s resistance to breakage.

By adding zirconium-containing high-alumina refractory fibers, the reinforcing and toughening effects produced by their pull-out action within the matrix are utilized to improve the mechanical vibration resistance and impact toughness of the furnace roll castable.

By adding kyanite powder as an expanding agent, the volume expansion caused by the irreversible decomposition of kyanite into mullite and free SiO₂ at high temperatures offsets the volume shrinkage of the insulating castable refractory at high temperatures. This prevents cracking and detachment of the castable layer caused by thermal expansion mismatch between the castable and the metal roll body. The use of admixtures such as water-reducing agents, organic defoamers, and silane coupling agents improves the workability of the castable and reduces the amount of water required, thus improving the construction quality and overall performance of the furnace roll castable insulation lining.

Through laboratory formulation design and optimization, the optimal formulation was finally determined.

Lightweight Castable Refractory for Furnaces

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Performance Testing of Lightweight Insulating Refractory Castable

Materials were weighed according to the optimized formula, mixed evenly, and then water was added for stirring. The mixture was then cast into 40mm × 40mm × 160mm samples, cured naturally at room temperature for 24 hours, demolded, and dried at 110℃ for 24 hours. Some samples were further heat-treated at 1100℃ and 1300℃ for 3 hours each. The bulk density, flexural and compressive strength, thermal conductivity, and other physical properties of the samples after different temperature treatments were then tested according to relevant standards.

Compared to conventional furnace roll heavy castable (bulk density 2.2 g·cm⁻³, thermal conductivity 0.738 W·(m·K)⁻¹), the bulk density is reduced by 25.8%, resulting in a lighter bulk density. The thermal conductivity is reduced by approximately 57%, leading to superior insulation performance. Furthermore, the developed lightweight insulating castable exhibits high low, medium, and high temperature strength. With increasing temperature, the mechanical properties of the castable remain relatively stable without significant fluctuations. It is evident that the rational design of various additives in the formulation significantly improves the flexural strength, compressive strength, and resistance to mechanical vibration of the lightweight castable. The room temperature compressive strength of the castable reaches 24.5 MPa, far exceeding the mechanical properties of ordinary lightweight insulating castable refractory. It not only meets the requirements for demolding and handling but also fully satisfies the requirements for direct use in the furnace after baking at 300℃ using the furnace rollers. The low linear shrinkage rate after various temperature treatments indicates good volume stability.

What Causes Cracks to Appear in High-Alumina Refractory Castables After Furnace Drying?

March 28, 2026 by rsrefractorycastable

High-alumina castables refer to refractory castables with an Al₂O₃ content greater than 48%. They are characterized by high cold and hot strength, good wear resistance, thermal shock resistance, spalling resistance, and good volume stability at high temperatures. They have a wide range of applications, including cement kiln heads and tails, cyclone preheaters, tertiary air ducts, grate coolers, refining furnace covers, blast furnaces, and heating furnaces.

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Performance and Application Advantages of High-Alumina Castables

Traditional lightweight high-alumina castables, due to their large pore size, have high thermal conductivity and can only be used in low-temperature applications (≤1200℃). According to conventional refractory principles, if a material can form a closed, circular microporous structure, its thermal conductivity can be significantly reduced. To improve the performance of lightweight high-alumina castables, high-performance lightweight high-alumina castables with lower thermal conductivity, higher strength, and greater volume stability have been developed by adding pore-forming agents.

Scientific formulation further enhances the high-temperature strength and thermal stability of this series of castable refractory materials, effectively controlling the calcium oxide content and reducing the low-eutectic phase, thereby improving refractoriness, high-temperature strength, and slag resistance. This series of castables is mainly composed of high-alumina refractory raw materials, employing new micronized powder technology and highly efficient composite chemical additives. It features high load softening temperature, long service life, and convenient construction, and has a significant effect on improving insulation, reducing heat loss, and lowering ambient operating temperature.

Suitable for heating furnaces, soaking furnaces, heat treatment furnaces, rotary kilns; linings for various high-temperature burners, water pipe wrapping linings for heating furnaces, components for ladle refining equipment in molten steel, and high-temperature wear-resistant linings for petrochemical catalytic cracking reactors; linings for blast furnace tapping troughs, blast furnace tapping channels, and integral powder spraying guns for molten iron pretreatment. It can also be used to fabricate large precast blocks and furnace linings for rapid construction.

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What are the causes of cracks in the refractory castable after furnace drying?

Furnace drying is a crucial step after the construction of high-alumina castables but before operation, directly determining the future service life of the kiln. During furnace drying, proper preparation is essential, the drying time must be carefully controlled, and the kiln’s temperature rise curve must be operated under strict conditions. Improper furnace drying operations may lead to engineering quality accidents such as castable cracking.

Under normal circumstances, after the furnace drying process, the kiln can operate normally after passing the engineering acceptance test, and the castable will achieve its performance within the operating temperature range. There are three main reasons for cracks in the castable after furnace drying:

The amount of water added during wet mixing of the high-alumina castable was not properly controlled. Adding too much water results in insufficient bonding strength, leading to castable detachment. Adding too little water will also significantly weaken the performance of the castable. Therefore, the amount of water added must be strictly controlled according to the instructions.
Insufficient curing time during the curing of the high-alumina castable results in insufficient strength of the formed castable. Generally, the curing time for castable is 24 hours at higher temperatures. In colder winters, the curing time is 48 hours. If the temperature is low, a small amount of accelerator may be added to speed up the setting process.
The temperature rises too quickly during furnace drying. The boiler temperature rise curve must be carefully controlled during furnace drying. During furnace drying, construction workers typically develop a drying curve based on the boiler’s specific conditions. Strict temperature control is crucial during drying to prevent rapid heating and cooling. After drying, if minor or inconspicuous cracks appear in the castable refractory, they should be promptly inspected and repaired with the same type of refractory. However, if larger cracks appear or if refractory detachment occurs, construction workers must develop a solution and repair the damage with new castable refractory.

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High-alumina Unshaped Refractories

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

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Selection of Refractory Mortar for Laying Refractory Bricks

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③ Accelerator

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High-Performance Alkali-Resistant Castables for Cement Kilns

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