I. Preparation principles of low cement castables
The preparation of low cement refractory castables should take "reducing cement content, optimizing microstructure, and improving high-temperature performance" as the core goals, and strictly follow the following four principles to ensure that it has both workability and service stability:
1. Particle grading optimization principle
Particle grading is the basis for determining the bulk density, porosity and strength of castables. It must follow the "closest packing theory" and usually adopt a three-level or four-level grading design:
1) Coarse aggregate (5-15mm): accounts for 30%-45%, mainly plays a skeleton support role, and raw materials with good chemical stability and low thermal expansion coefficient (such as high-alumina bauxite, corundum) must be selected to avoid structural cracking due to volume changes at high temperatures;
2) Medium aggregate (1-5mm): accounts for 20%-30%, fills the gaps between coarse aggregates, improves material fluidity, and must match the composition of coarse aggregates to reduce interface reactions;
3) Fine powder (0.074-1mm): accounts for 15%-25%, further filling the gaps between aggregates and improving density. The particle size of fine powder needs to be controlled within a reasonable range. Too coarse powder will easily lead to loose stacking, while too fine powder will increase water demand;
4) Micro powder (<0.074mm): accounts for 5%-15%, including mineral micro powder (such as silica fume, alumina micro powder) and cement clinker micro powder. It is the key to achieving "low cement". Through the ball effect of micro powder and the reaction with volcanic ash, the cement dosage is reduced and the strength is improved.
2. Principle of precise control of cement dosage
The cement dosage (mainly aluminate cement) of low cement refractory castables is usually ≤8% (mass fraction). A balance must be struck between "reducing dosage" and "ensuring construction and early strength":
(1) Minimum effective dosage: Determine the cement dosage according to the purpose of the castable (e.g. 5%-7% for high-temperature kiln linings, 7%-8% for low-temperature pipelines), avoiding excessive dosage that will result in the formation of low-melting-point calcium aluminates (e.g. CA6, C12A7) at high temperatures, which will reduce refractoriness;
(2) Synergy with micropowder: Through the pozzolanic reaction of silica fume and alumina micropowder, they combine with cement hydration products (e.g. CAH10, C2AH8) to form stable C-A-S-H gel or columnar CA6 crystals, compensating for the strength loss caused by the reduction in cement dosage.
3. Principle of balance between water demand and fluidity
Water demand directly affects the density, porosity and workability of castables and needs to be controlled within the lowest reasonable range (usually 5%-8%):
(1) Reduce water demand: By adding high-efficiency water reducers (such as polycarboxylic acids and naphthalene series), the attraction between particles is reduced, high fluidity is achieved at low water content, and the formation of through pores after excessive water evaporation is avoided;
(2) Fluidity adaptation: Adjust the fluidity according to the construction method (such as pumping and vibration). The expansion of pumped materials needs to be ≥250mm, and the expansion of vibrated materials needs to be ≥200mm. At the same time, avoid excessive flow that causes aggregate stratification.
4. Volume stability control principles
Low cement refractory castables are prone to volume changes during heating due to phase change and decomposition of hydration products. This requires raw material selection and additive control:
(1) Raw material expansion compensation: Add an appropriate amount of expansion agent (such as kyanite and sillimanite, which convert to mullite at high temperature and expand by 10%-15%) to offset the shrinkage caused by the decomposition of cement hydration products (such as CAH10 decomposing into C2AH8 at 100-200℃, with a volume shrinkage of about 10%);
(2) Microstructure optimization: Through micropowder filling and crystal directional growth (such as CA6 columnar crystal interweaving), the loose structure during heating is reduced and the volume stability is improved. It is usually required that the linear change rate after firing at 1100℃ is controlled within ±0.5%.
2. Key raw materials affecting the performance of low cement castables
The performance of low-cement castables (refractory, strength, thermal shock stability, and corrosion resistance) is determined by the chemical composition, mineral structure, and particle size distribution of the raw materials. The core raw materials can be divided into five categories:
1. Refractory aggregate: determines the refractory foundation and skeleton strength of the castable.
Refractory aggregate accounts for 60%-75% and is the "skeleton" of the castable. Its performance directly determines the refractoriness and high-temperature bearing capacity of the castable:
(1) High-alumina aggregate (Al₂O₃≥70%) Composition and performance: Main components are corundum and mullite, refractoriness ≥1770℃, compressive strength at room temperature ≥100MPa, compressive strength at high temperature (1400℃) ≥50MPa; Applicable scenarios: Linings of medium and high temperature kilns (such as cement rotary kiln firing zone, metallurgical heating furnace), it is necessary to avoid excessive impurities (such as Fe₂O₃, TiO₂) to prevent the formation of low melting point phases (such as FeO·Al₂O₃, melting point 1250℃);
(2) Corundum aggregate (Al₂O₃≥90%): Composition and performance: Mainly α-corundum, dense structure, refractoriness ≥1850℃, corrosion resistance (such as resistance to molten steel and slag corrosion) is better than high-alumina aggregate; Applicable scenarios: Ultra-high temperature environment (such as the iron trough of steel blast furnace and non-ferrous metal smelting furnace), it is necessary to control the aggregate particle size distribution to avoid excessive coarse aggregate causing a decrease in thermal shock stability.
2. Refractory micropowder: The core micropowder for achieving "low cement" and performance
improvement accounts for 5%-15%, which is the key to distinguish low cement refractory castables from ordinary castables. It mainly includes:
(1) Alumina micropowder (Al₂O₃≥99%, D50=1-5μm): Mechanism of action: reacts with cement hydration products to form CA6 crystals, improving high-temperature strength; fills the gaps between aggregates and reduces porosity (can reduce apparent porosity from 18% to below 12%); Performance impact: Increasing the amount of micropowder can improve refractoriness, but excessive amount (>15%) will increase water demand and needs to be used with water reducer;
(2) Silica fume (SiO₂ ≥ 90%, D50 = 0.1-0.5μm): Mechanism of action: It has high pozzolanic activity and reacts with Ca(OH)₂ produced by cement hydration to form C-S-H gel, which improves early strength. Its spherical particles can reduce internal friction in the material and improve fluidity. Precautions: The amount of silica fume used must be controlled (usually 3%-8%). Excessive use will cause the castable to form a large amount of low-melting-point glass (such as CaO-SiO₂-Al₂O₃ glass, melting point <1400°C) at high temperatures, reducing corrosion resistance.
3. Binder: The key to ensuring workability and strength development.
The binder of low-cement castables is mainly aluminate cement, supplemented by chemical bonding of fine powder. Its performance affects the setting time and strength of the castable:
(1) Aluminate cement (CA-50, CA-70): Composition and characteristics: CA-50 contains 50%-60% CA (monocalcium aluminate), has a moderate setting time (initial setting ≥45min, final setting ≤10h), and high early strength (1d compressive strength ≥20MPa); CA-70 contains ≥70% CA, has higher early strength, but sets faster and needs to be used with a retarder; Performance impact: The CaO content in cement directly affects the refractoriness. For every 1% increase in CaO, the refractoriness decreases by about 15-20℃. Therefore, cement with a low CaO content (CA-70 CaO≤22%) should be selected;
(2) Setting retarders/accelerators: Setting retarders (such as citric acid and tartaric acid, added at 0.05%-0.2%) extend the setting time and are suitable for long-distance transportation or large-volume pouring. Setting accelerators (such as Li₂CO₃ and CaCl₂, added at 0.01%-0.05%) shorten the setting time and are suitable for low-temperature construction environments (such as winter construction). However, excessive use should be avoided, as it may reduce strength.
4. Water reducer: The core additive for balancing water demand and fluidity
Water reducer is the key to achieving "low water and high flow" in low cement refractory castables. The addition amount is usually 0.1%-0.5%:
(1) Polycarboxylic acid water reducer: Advantages: High water reduction rate (up to 30%-40%), good slump retention (loss of expansion within 1 hour ≤ 20mm), good compatibility with aluminate cement, and will not cause excessive retarding; Performance impact: It can reduce water demand by 2-3 percentage points, increase the compressive strength of the castable after firing at 1100℃ by 15%-20%, and reduce the apparent porosity by 3-5 percentage points;
(2) Naphthalene water reducer: Features: Medium water reduction rate (20%-25%), low price, suitable for scenes with low fluidity requirements; Note: The dosage needs to be controlled. Excessive dosage (>0.5%) will cause the castable to delaminate or lose strength.
5. Functional additives: regulating volume stability and special properties
(1) Expansion agent (kyanite, sillimanite): Function: at high temperature (1100-1400℃), it converts into mullite, expands by 10%-15%, offsets the shrinkage of the castable and avoids cracking; dosage: usually 3%-5%, excessive dosage will lead to excessive volume expansion and generate internal stress;
(2) Anti-explosion agent (metallic aluminum powder, addition amount 0.1%-0.3%): - Function: during the heating process (200-600℃), it slowly oxidizes to generate Al₂O₃, releases a small amount of gas, discharges the free water inside low cement refractory castables, and avoids the structure from bursting due to the rapid evaporation of water at high temperature;
(3) Thermal shock stabilizer (silicon carbide, silicon nitride, added amount 5%-10%): Function: Utilize the low expansion characteristics of silicon carbide (thermal expansion coefficient 4.5×10⁻⁶/℃) and silicon nitride (thermal expansion coefficient 3.2×10⁻⁶/℃) to reduce the thermal stress of the castable and improve the thermal shock stability (usually the number of water-cooled thermal shocks can be increased from 10 times to more than 20 times).
