What are the causes of underfiring in sintered corundum-mullite bricks, and what are the corresponding solutions?
Release time:
2025-12-16
【Summary】 The occurrence of underfiring in sintered corundum-mullite bricks is primarily due to improper control of the firing regime and the influence of raw material characteristics.
The occurrence of underfiring in corundum-mullite bricks after firing is primarily attributable to improper control of the firing regime and the influence of raw material characteristics. Underfiring manifests as incomplete densification of the brick body, resulting in high water absorption and insufficient mechanical strength. The underlying causes and corresponding solutions are outlined below:
Cause Analysis
1. Insufficient firing temperature: Corundum-mullite bricks require liquid-phase sintering at a high temperature of 1600–1650℃. If the temperature falls below this range, the mullite crystals will not grow sufficiently, and the amount of liquid phase will be inadequate, resulting in high porosity and low density of the green body. For example, mullite ceramics containing 10% flux have a narrow sintering range of only 20–30℃; even slight temperature fluctuations can easily lead to underfiring.
2. Insufficient holding time: Liquid-phase sintering requires sufficient time to promote particle rearrangement and dissolution-precipitation. If the holding time is inadequate, mullite crystals will not grow sufficiently, the glass phase will not fill adequately, and the green body structure will remain loose.
3. Differences in Raw Material Reactivity: The transformation of alumina crystal forms in raw materials (e.g., γ-Al₂O₃ → α-Al₂O₃) is accompanied by a 13% volume shrinkage. If the raw materials are not pre-calcined, uneven shrinkage during firing can easily lead to cracking or localized overfiring.
Solution
1. Optimize the firing process: Employ small-section tunnel kilns or uniform thermal equipment, and strictly control the heating rate and holding time. For example, holding at 1600℃ for 4 hours can promote the full growth of mullite crystals and reduce porosity.
2. Raw Material Preprocessing: The alumina raw material is pre-calcined at 1450℃ to complete the phase transformation; the raw material is finely ground to reduce particle size variation and enhance sintering activity.
3. Add a mineralizing agent: Introducing a mixture of 1–2% CaO, MgO, or alkaline oxides can reduce the sintering temperature by 70–150℃, promote the formation of a liquid phase, and improve sintering density.
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