How to Choose High-Alumina Insulating Bricks Without Making Mistakes: The Role of Mullite and Corundum Phases Explained

How to Choose High-Alumina Insulating Bricks Without Falling into Common Mistakes

If your furnace is failing prematurely—cracking after thermal cycles, sagging at high temps, or shedding material—you’re not alone. In fact, over 90% of industrial users overlook the critical role of main crystal phases like mullite and corundum in refractory brick selection.

Why Your Furnace Dies Early: 3 Silent Killers

Most plant engineers focus on price or brand, but real failure starts beneath the surface:

• Thermal shock resistance: Poorly formulated bricks crack under rapid heating/cooling—especially in ceramic kilns cycling from room temp to 1200°C+.
• Long-term operating temperature: A brick rated for 1650°C may soften beyond 1500°C if its microstructure lacks stable phases.
• Thermal expansion mismatch: If the brick doesn’t expand evenly with the lining, it’ll spall off—common in steelmaking converters where molten metal contacts the lining.

The Science Behind It: Mullite = Reinforcement Steel, Corundum = Heat Armor

Think of mullite (3Al₂O₃·2SiO₂) as the reinforcing rebar inside concrete. It forms a rigid network that prevents crack propagation during thermal stress. Corundum (Al₂O₃), meanwhile, acts like a heat shield—it stays solid up to 1800°C and resists chemical attack from slag.

In a real-world case from a tile manufacturer in Italy, switching from a low-mullite brick (≤30%) to one with 45–50% mullite cut furnace downtime by 40%. Why? Because the new brick could survive 300+ thermal cycles without visible cracking—a direct result of better phase stability.

Quick Diagnostic Tips: What to Check Before You Replace

You don’t need lab equipment to spot poor fit:

1. Check for fine网状裂纹 (network cracks)—these indicate repeated thermal fatigue due to insufficient mullite.
2. Look for surface spalling—this means the brick’s expansion coefficient doesn’t match the furnace wall.
3. Measure actual service life vs. expected lifespan—many bricks fail before reaching their rated 1650°C continuous use limit.

These signs aren’t just warnings—they’re your first clue that you’re using the wrong brick for your process curve.

What Makes Our High-Alumina Brick Different?

We don’t just sell bricks—we engineer solutions:

• ≥48% Al₂O₃ content for consistent performance across industries.
• Up to 1800°C maximum service temperature—tested per ISO 18897 and ASTM C1549 standards.
• Low thermal conductivity (~0.6 W/m·K at 1000°C) for energy savings up to 12%.
• Engineered mullite-corundum composite structure to minimize spalling risk.

This isn’t theory—it’s what we’ve seen in over 500 installations worldwide, from European ceramic kilns to Middle Eastern steel plants.