About Low Cement Refractory Castables

Low cement refractory castable refers to a type of refractory castable that uses calcium aluminate cement as a binder, with a CaO content of less than 2.5%. In other words, the amount of calcium aluminate cement added is about 1/2 to 1/3 of that used in ordinary calcium aluminate cement castables. Castables with a CaO content generally in the range of 1.0% to 2.5% are called low cement castables, while those with a CaO content of less than 1.0% are called ultra-low cement castables.

Unlike ordinary refractory castables, the matrix of low and ultra-low cement castables is made by replacing part or most of the calcium aluminate cement with ultra-fine powder (with a particle size of less than 1 μm) that has the same or similar chemical composition as the main material of the castable. Additionally, a small amount of dispersant (water-reducing agent) and a certain amount of delayed-setting coagulant are added during formulation.

Because ultra-fine powder is used to replace calcium aluminate cement, low and ultra-low cement castables have the following advantages:

The lower CaO content reduces the formation of low-melting eutectic phases in the material, thereby improving refractoriness, high-temperature strength, and slag resistance.

The water demand of the castables is only 1/2 to 1/3 that of ordinary refractory castables, resulting in lower porosity and higher bulk density.

After casting and curing, fewer cement hydrates are formed, which avoids the problem of strength loss due to the destruction of hydration bonds during heating and drying. Instead, with increasing heat treatment temperature and progressive sintering, the strength gradually increases.

Compared with ordinary calcium aluminate cement castables, low cement and ultra-low cement castables differ significantly in their setting and hardening mechanisms. Ordinary refractory castables primarily rely on the hydration of calcium aluminate minerals to generate CAH₁₀, C₂AH₆, C₃AH₈, and alumina gel, which provide bonding through hydration-referred to as hydration bonding.

Low cement castables exhibit both hydration and cohesive bonding, while ultra-low cement castables rely mainly on cohesive bonding. The principle of cohesive bonding is as follows: an electrolyte is added that slowly dissociates ions opposite in charge to the surface charge of the particles. These counter-ions are adsorbed onto the particle surfaces, reducing the ζ (zeta) potential. When adsorption reaches the isoelectric point, coalescence occurs immediately. With the aid of van der Waals forces, the particles then bond together.

For example, in alumina-silica castables with added SiO₂ ultra-fine powder, the condensation and hardening process occurs as follows: when SiO₂ ultra-fine powder is mixed with water, due to its high reactivity, it interacts with the water to form colloidal particles. These particles become negatively charged due to the dissociation of surface Si–OH groups into Si–O⁻ and H⁺. The negatively charged particles gradually adsorb Al³⁺ and Ca²⁺ ions released during the hydrolysis of calcium aluminate. As this adsorption continues, the ζ potential of the particle surfaces decreases. When the system reaches the isoelectric point (i.e., when the particles carry no net charge), particle coalescence occurs, resulting in cohesive bonding.

The coagulation and hardening mechanism of ultra-low cement refractory castables is primarily based on cohesive bonding, with the cement acting as a delayed setting promoter.

For example, when SiO₂ ultra-fine powder is added, it forms gel particles upon contact with water. During the hydration process of calcium aluminate, Al³⁺ and Ca²⁺ ions are slowly released and adsorbed by these gel particles, resulting in a decrease in the zeta (ζ) potential. When the system reaches the isoelectric point-i.e., when the surface charge of the particles is neutral-agglomeration and hardening occur.

In addition, the active surface of SiO₂ can form silanol groups (Si–OH), which, during the drying process, undergo dehydration and bridging to form a siloxane (Si–O–Si) network structure. At the same time, the adsorption of Al³⁺, Ca²⁺, and Na⁺ ions from the electrolyte can also contribute to the condensation and hardening process.

The preparation principle of low cement refractory castables involves the proper selection of particle size distribution, types and amounts of ultra-fine powder, and additives.

The refractory aggregate and powder are made from clay clinker and high-alumina bauxite clinker with good sintering behavior. The maximum particle size of the refractory aggregate is generally 10 mm, depending on the thickness of the engineering lining. The typical particle size distribution is as follows:

10–5 mm: 30%–40%

5–3 mm: 20%–30%

3–0.15 mm: 30%–40%

In general, when formulating low cement refractory castables, a single raw material is rarely used. Instead, a three-level or four-level particle size distribution is applied to achieve maximum packing density. The amount of refractory aggregate used is typically 68%–72%.

Refractory powder is usually prepared from second-grade, first-grade, or special-grade high-alumina bauxite clinker. The powder should be de-ironed after grinding, and its fineness should be ≥90% passing either 0.09 mm or 0.044 mm. It can be used alone or blended in specific proportions. Additionally, fine powders of alumina, white corundum, or brown corundum may be mixed in to enhance the matrix performance of low cement refractory castables. The total amount of refractory powder used is typically 18%–25%. Among this, if high-grade materials are added, their proportion is usually 5%–10%.

There are various types of refractory ultrafine powders. Among them, active SiO₂ powder (i.e., silica fume) is the most commonly used both domestically and internationally. Other types include α-Al₂O₃ powder, Cr₂O₃ powder, and SiC powder. The dosage typically ranges from 2% to 12%.

The combined dosage of refractory powder and ultrafine powder generally falls between 28% and 32%. It should be noted that different types of low cement refractory castables require different ultrafine powders and dosages. Each formulation has an optimal amount, which must be determined through testing. Based on experience, the typical optimal dosage ranges from 4% to 10%.

In low cement refractory castables, additives-mainly high-efficiency water-reducing agents and dispersants-are also used. Common types include NNO, MF, NF, JN, SM, humic acid, citric acid, tartaric acid and its salts, sodium tripolyphosphate, sodium hexametaphosphate, and boric acid. These are usually added in amounts ranging from 0.03% to 1.0%.

When selecting admixtures, they should be compatible with the ultrafine powder, and factors such as material source, ease of use, and cost should be taken into account. Generally speaking, organic admixtures offer better water-reducing and dispersing effects than inorganic admixtures.

Additionally, expansion agents, such as bluestone and silica, may be added to produce slight expansion in the castable at high temperatures.

After familiarizing yourself with these basic preparation principles, many varieties of low cement refractory castables can be formulated. For example, a castable consisting of 70% bauxite clinker aggregate (8–0 mm), 20% bauxite clinker powder of the same grade, 5% 625 CA-50 cement, 5% silica fume, 0.2% sodium tripolyphosphate, and 6.5% water exhibits a drying compressive strength of 20 MPa, a compressive strength after firing at 1450 °C of 72 MPa, and linear shrinkage of 0.01%.

Using the above proportions, the addition of 0.03% tartaric acid and 0.05% borax or 0.3% sodium humate can further improve the performance of the low cement refractory castable. The water content is generally kept below 7%, with lower amounts preferred.

The batching composition of low cement and ultra-low cement castables includes refractory aggregate, refractory powder, an appropriate or small amount of calcium aluminate cement (used as a delayed hardener), and trace amounts of dispersant (high-efficiency water-reducing agent).

The particle size distribution requires multi-level batching, divided into ranges of 5–3 mm, 3–1 mm, 1–0.2 mm, 0.2–0.074 mm, 0.074–0.044 mm, and micron level. The proportions of these different particle sizes can be calculated according to the Furnas batching curve.

Low cement and ultra-low cement castables can be used at higher temperatures than ordinary refractory castables made from the same base materials. Clay-based and high-alumina low cement castables can be applied as inner linings for various heat treatment rotary kilns, such as heating furnaces and even-heat furnaces.

Mullite-based low cement castables can be used for the linings of various high-temperature burners and for wrapping water-cooled pipes in heating furnaces. Corundum and chrome-containing corundum low cement refractory castables can be used as linings in certain parts of steel refining equipment outside the furnace. Examples include: the full lance used in jet metallurgy, the lining of the immersion pipe in RH or DH vacuum degassing systems, the inner lining of IF induction furnaces, and high-temperature wear-resistant linings in catalytic cracking reactors used in the petrochemical industry.

Low cement castables with an Al₂O₃–SiC–C composition can be used as linings in high-alumina iron troughs, blast furnaces, iron pretreatment furnaces, iron discharge channels, and iron ore pretreatment furnaces.

Similarly, Al₂O₃–SiC–C low cement castables can be used for the linings of iron discharge trenches, iron discharge tanks, and integrated powder-injection guns for iron pretreatment.

Low cement castables with MgO–Al₂O₃ or Al₂O₃–MgO–Al₂O₃ compositions can be applied as the inner linings of steel drums and steel discharge tanks.

Low cement SiC refractory castables are characterized by a low coefficient of linear expansion, high thermal conductivity, high strength, and excellent abrasion resistance. They have been successfully applied in thermal equipment such as power generation boilers, non-ferrous metallurgical furnaces, and incinerators, delivering good performance.

These castables use silicon carbide (SiC) with a purity greater than 97% as the refractory aggregate and powder. Ultra-fine SiO₂ powder and metallic silicon are added as antioxidants. CA-70 cement is used as the binder, and a polyphosphate-based water-reducing agent is included as an additive.

The key material properties are as follows:

SiC content: 85%

Bulk density after drying at 110 °C: 2.5 g/cm³

Compressive strength at 110 °C: 45 MPa

Flexural strength at 110 °C: 9 MPa

Linear change after firing at 1000 °C: –0.2%

Compressive strength after firing at 1000 °C: 107 MPa

Flexural strength after firing at 1000 °C: 24 MPa

Linear change after firing at 1450 °C: +0.3%

Compressive strength after firing at 1450 °C: 130 MPa

Flexural strength after firing at 1450 °C: 54 MPa

Thermal conductivity at 400 °C: 12.2 W/(m·K)