We found that some of the sintered paving bricks look the same, but after cutting, it can be seen that it gradually turns black from the outside to the inside. Sending this brick to the tunnel kiln and burning it again will turn into normal brick. Why does this happen? What measures can be taken to prevent black spots in paving slabs?
This reason is generally because the tunnel kiln is not sufficiently burned, and some impurities in it will not be adequately filtered, especially when some iron ions exist alone and are not oxidized and restored, they are black. Once you put it back in the kiln and burn it again for ample oxidation recovery, it will become normal.
Of course, in this case, the secondary sintering is a waste of energy, so it is best to complete the primary sintering to prevent the cost of secondary sintering. Most of the reasons are because the sintering time is too short. The kiln car was pushed out without giving a long time for satisfaction.
Solution: It is recommended to change the sintering time budget, especially for large paving slabs, it is necessary to conduct sintering experiments in advance to avoid showing a black heart. After all, the black heart cannot be seen, but the hardness and strength of this paving slab is unqualified. If it is used, it may cause a sinking situation, which will pose a safety hazard.