Performance Comparison Between Unshaped Refractories And Shaped Refractory Products

When selecting the optimal technical route, a similar density can be achieved for both unshaped refractory materials and fired refractory products. However, the key distinction between these products lies in their microstructures. For refractory castables, controlling the rheology of the mixture is crucial. This is why the particle size distribution is precisely regulated, and ultrafine powders or even nanoparticles are incorporated. A microporous structure is the most typical characteristic of unshaped refractories. In dense fired bricks, the typical pore size ranges from 20 to 25 μm, with some special products reaching as small as 5 μm. For castables, even after firing, the median pore diameter is usually no larger than 1 to 2 μm. A comparison of the pore size distributions of two typical refractory castables and refractory bricks (fire clay and high-alumina types) is presented in Figure 1.

Corundum C (d50 = 0.7 μm), Corundum B (d50 = 18 μm), Fire Clay C (d50 = 1.2 μm), and Fire Clay B (d50 = 2.5 μm) - these structural differences are reflected in various performance properties. The microporous structure significantly enhances the material's strength and improves its thermal shock resistance, as demonstrated by its increased ability to resist the formation and propagation of cracks under sudden temperature changes. Additionally, it leads to a reduction in thermal radiation at high temperatures. Compared to fired refractory products with similar composition and porosity, the thermal conductivity of castables is reduced by 20% to 30%, as shown in Figure 2.

HA-high-alumina type (B for refractory bricks, C for castables) and FC-fire clay type (B for refractory bricks, C for castables). The microporous structure of castables helps prevent slag penetration in corrosive environments, while also enhancing their resistance to corrosion from molten substances, particularly slag and metals. Unlike fired products, castables have excellent plasticity, allowing the refractory linings to relieve stress through their own deformation without sustaining damage. Phosphate-bonded castables are particularly effective in this respect (Figure 3).

Certain issues may arise when using unshaped refractory linings-problems that do not occur with shaped refractory products. It is important to note that manufacturers supply unshaped refractories in semi-finished form (e.g., dry mixes), which require further on-site processing to create a finished refractory lining. Failure to strictly follow the manufacturer's guidelines during mixing, such as incorrect moisture addition or improper application methods, can lead to such issues. When applying unshaped refractories, it is crucial to allow adequate heating and drying time, and to use a high enough temperature to ensure complete dehydration of the binders. When using traditional hydraulically bonded castables, it is important to account for the strength loss that occurs at medium temperatures. In the medium temperature range (250–600℃), hydraulic bonds gradually decompose because ceramic bonding has not yet formed. Similarly, good volume stability is important for castables, as the matrix undergoes shrinkage at high temperatures.

Comparison of Unshaped Refractories and Shaped Products in Applications

The market share of unshaped refractories has been steadily increasing across almost all application fields. However, certain sectors remain dominated by shaped products, with functional refractories being a prime example where shaped refractories continue to lead. For instance, functional refractories control molten steel flow during continuous casting. In other application areas, high-quality unshaped refractories have yet to be developed, making shaped products the preferred choice.

Shaped refractories are generally used in most applications involving alkaline refractory linings, such as dolomite bricks, magnesia bricks, magnesia-carbon bricks, and magnesia-chrome bricks. These include converter linings, ladle slag line bricks, electric arc furnace walls, the burning and transition zones of cement rotary kilns, and linings for non-ferrous metal smelting furnaces (copper, lead, zinc, etc.). Often, traditional practices play a decisive role, which explains why shaped products are still used for furnace linings designed according to conventional standards.

In recent years, unshaped refractories have been used not only to fabricate new linings but also for the repair and maintenance of in-service refractory linings. Various application methods for unshaped refractories include vibrated and vibration-free (self-flowing) casting, gunning, ramming, and pressing. Some of these methods have become-and will continue to be-the primary techniques for applying unshaped refractories. Conventional uses of unshaped refractories include the bottoms of traditional electric arc furnaces (alkaline refractories), taphole clay, blast furnace iron troughs, and a variety of repair and maintenance materials. In some cases, complex-shaped products have been replaced by unshaped refractories, leading to composite linings, with waste incinerators being a typical example.

Unshaped refractories essentially form seamless linings that take on a fixed shape during service. A prime example of the expanded use of castables is the development of unshaped refractory linings for ladles. In principle, ladle linings can be made from either unshaped refractories or shaped products, with high-alumina (neutral) or alkaline materials as the base. Unshaped linings made from castables are mainly limited to high-alumina materials, while shaped products can be either neutral or alkaline.

Extensive research has been conducted to develop alkaline castables, but there have been no reports of their successful application in ladles to date. The primary challenge is the high hydratability of magnesia, which requires the use of new binders.

In shaped products, carbon is added to the composition to mitigate certain adverse properties of magnesia bricks, such as high thermal expansion and poor slag corrosion resistance. However, the optimal carbon dosage in magnesia castables has not yet been determined, which remains a key factor limiting the widespread and successful use of MgO-C castables. Despite this, with the development of high-performance low-cement and ultra-low-cement castables, unshaped linings have gained an increasingly important role in ladle applications. High-alumina castables, along with ladle lining renewal technology, have become widely adopted.

After a newly built ladle is put into service for a single cycle, the inner surface of the lining can be cleaned mechanically, and a new layer of castable can be poured onto the damaged surface. This process can be repeated multiple times. Compared to the traditional method of building entirely new ladle linings, ladles using the lining renewal technology consume only 40% to 50% of the material, resulting in a 50% to 60% saving in refractory lining materials.

The advantages of unshaped refractories over brick linings can be summarized as follows: reduced labor and time for lining construction, and improved ladle utilization. Fewer ladles in service also lead to lower refractory consumption and costs.

Unshaped refractories play a crucial role in furnace maintenance by enabling large-scale repairs with minimal downtime-even during furnace operation in some cases. Systematic repair of linings can significantly extend the service life of furnaces. A typical example is the repair of alkaline linings (MgO-C bricks) in oxygen converters. Regular gunning with alkaline mixtures, combined with precise control over slag formation and slag splashing for furnace protection, can greatly prolong the life of the lining and reduce refractory consumption to less than 1 kg per ton of steel produced.

When choosing between unshaped refractory linings and traditional brick linings, the advantages of unshaped refractories-such as faster construction (enabled by new binders) and reduced drying and heating times-are crucial considerations. In most cases, raw materials account for the largest portion of the final product cost (up to 60%). Given the significant impact raw materials have on product performance, using low-cost raw materials is not always feasible. From this perspective, unshaped refractories offer undeniable economic benefits, thanks to the dominant role raw materials play in their composition. Continuous improvements in quality, along with faster and simpler construction processes, ultimately lead to more economical usage.

It is expected that the market share of unshaped refractories, relative to shaped products, will continue to grow. This trend will be driven by ongoing research and development of new unshaped refractories, innovations in construction and laying technologies, and advances in precast refractory technology.