Formation Mechanism of Black Core in Sintered Red Brick Using Incinerated Ash of Sewage Sludge

Abstract

Characterization of black core in a brick body obtained by sintering the incinerated ash of sewage sludge was conducted to clarify its formation mechanism. The ash was pressed into a rectangular body under 100MPa and the body was fired at 1000°C. Under some firing conditions, the formation of porous black core accompanied by extraordinary expansion occurred in the brick body. TG and ESR analyses revealed that iron oxide in the black core was mainly in the form of FeO. The mass-spectrometric analysis of gases released during heating the ash in an oxidizing atmosphere revealed that CO₂ was the only gaseous product and it had to be brought about from the residual carbon. These results indicate that Fe₂O₃ in the ash was reduced to FeO by this carbon. Furthermore, it was assumed that cavities in the body were formed due to the blockade of CO and CO₂, formed at the reduction of Fe₂O₃ to FeO, by the densely sintered surface.