Improving the refire line variability of low cement refractory castables (primarily suppressing abnormal shrinkage/expansion at high temperatures and ensuring volume stability) requires addressing three core dimensions: raw material optimization, additive regulation, and process control. Specific methods are as follows:
1. Optimizing Raw Material Selection
The high-temperature stability of raw materials directly determines the refire line variability trend. Prioritizing the following two aspects:
1. Aggregates and Powders: Use low-shrinkage/pre-burned raw materials. Aggregates should preferably be pre-burned corundum, mullite, high-alumina bauxite (≥90% Al₂O₃), and other high-temperature stable materials. Pre-burning can eliminate the raw material's inherent firing shrinkage, preventing excessive overall castable line variability due to secondary aggregate shrinkage at high temperatures. For powders, avoid using uncalcined aluminum hydroxide or low-grade clay to reduce decomposition/sintering shrinkage at high temperatures.
2. Controlling Binder Amount. The binder (aluminate cement) in low cement refractory castables contains CaO, which reacts with Al₂O₃ at high temperatures to form calcium aluminates (such as CA₆), accompanied by a certain volume change. The cement content should be controlled to 3%-6% (by total weight). In combination with silica fume and ultrafine alumina powder, this reduces cement reliance and minimizes calcium aluminate formation.
2. Adding Targeted Functional Additives
Specific additives can be introduced to directly offset shrinkage or stabilize high-temperature structures:
1. Anti-shrinkage agents (expansive agents) such as kyanite, sillimanite, and andalusite (3%-8%) can be added. These minerals slowly decompose at high temperatures to form mullite, which expands by approximately 1.5%-3%. This precisely offsets the castable's sintering shrinkage, keeping the resintering line change rate within ±0.5% (1500°C x 3h).
2. High-temperature stabilizers, such as chromium oxide and zirconium oxide (1%-3%), can inhibit the formation and migration of low-melting-point phases (such as anorthite and glass) in low cement refractory castables, reducing structural collapse at high temperatures and preventing abnormal expansion or contraction.
3. Strictly Control Construction and Heat Treatment Processes
Process defects can amplify variations in the refire line and require key control measures:
1. Ensure construction density. Control the amount of water added during mixing (typically 5%-7%) to avoid excessive moisture, which can increase porosity (pore shrinkage at high temperatures can exacerbate overall line variations). Use mechanical vibration (vibration frequency 2000-3000 times/minute) to ensure low cement refractory castables density of ≥2.6g/cm³ and reduce internal porosity.
2. Optimize curing and heating regimes
a. Curing phase: Curing at 20-25°C and relative humidity ≥80% for 24-48 hours ensures full hydration of the cement and the formation of stable hydration products (such as CAH₁₀ and C₂AH₈). This prevents dehydration at high temperatures later, which can lead to structural loosening.
b. Heating phase: Develop a "low-temperature, slow-rise, high-temperature, hold" curve. In the low-temperature range (≤300°C), the heating rate should be ≤5°C/h (to expel free water and crystallization water); in the medium-temperature range (300-800°C), the heating rate should be ≤10°C/h (to avoid rapid decomposition of hydration products and cracking); and in the high-temperature range (1000-1500°C), the heating rate should be 2-4 hours to promote uniform sintering and minimize volume fluctuations.
