Causes and avoidance of firing-related damage Part 2: Strength-reducing reactions in the heating-up zone from 500 to 800° C

Part 1 of this contribution dealt with diffusion-dependent reactions in the heating-up zone at ≤ 500° C (Zi Brick and Tile Industry International 11/2013). Now, Part 2 looks into the firing of bricks in the temperature range 500 to 800° C. Expansion and, to some extent, shrinkage processes take place within that range of temperatures. Solid-state reactions with strength-reducing effects on the freshly forming body also occur within that same range.

4 Expansion and shrinkage processes

In a tunnel kiln firing process for heavy clay products, there should be no mutual contact between the wares to be fired, and all exposed surfaces within and around the setting should be uniformly heated and cooled. The rate-determining material thicknesses are those of the thinnest, unperforated, solid bricks and clinkers/slips, the greatest continuous material thicknesses those of perforated facing bricks, and the maximum thicknesses those of roof tiles. Since both major outside surfaces of the bodies participate equally in the exchange of heat with the...

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