Structure and phase composition of ceramics based on pre-treated municipal incinerator bottom ash

In recent decades it has been demonstrated that some of the traditional raw materials in the ceramics industry could be successfully replaced by various industrial wastes. Sometimes, when the chemical composition of the waste is suitable, it is even possible to simply mix the waste with clay materials and to obtain new ceramics without feldspars, with higher crystallinity and improved mechanical properties. However, the sintering process and structure of these materials are very different from those of traditional clay-silica-feldspar ceramics. This is probably one of the reasons why they are not yet recognised as a viable alternative. The aim of this work is to investigate and explain the differences in phase composition and structure of six ceramics, based on bottom ashes from municipal incinerators, which form different amounts of amorphous phase, anorthite and pyroxene. For a better understanding, the viscosities of the residual melts at the corresponding sintering temperatures are also evaluated. It is shown that in the compositions richer in silica, the sintering temperatures are lower and the viscosity is significantly higher. As a result, notwithstanding of the high amount of formed glassy phase, the sintering is incomplete. At the same time, in the ceramics enriched with CaO and Al2O3, which form 40-45 wt% crystalline phase, the sintering temperatures are higher, but the viscosity is lower and the water adsorption is near zero. It is also clarified that in these compositions partial phase formation occurs on cooling, which increases their closed porosity.